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MAJOR SPONSORSHIP for the 14th AGC has been provided by
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SPONSORING COMPANIES & ORGANISATIONS Aberfoyle Acacia Resources Ltd AGSO BHP Minerals Cyprus Amax Australia Corporation + Cyprus Amax Indonesia Corporation Gnomic Exploration Services PL Homestake Gold of Australia Ltd James Cook University North Limited Normandy Mining Limited Pasminco Exploration Perilya Placer Dome Exploration, Asia Pacific Region Queensland Department of Mines and Energy RGC Exploration Rio Tinto Exploration SRK Consulting Townsville City Council
Geological Society of Australia
ABSTRACTS Number 49
GEOSCIENCE FOR THE NEW MILLENNIUM
Australian Geological Convention Townsville, 6-10 July, 1998
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14'^ Australian Geological Convention, Townsville, July 1998
ISSN 0729-011X © Geological Society of Australia Incorporated 1996 Copies of this publication may be obtained from the Geological Society of Australia Incorporated, 301 George Street, Sydney, NSW, Australia 2000
Example citation for papers in this volume: England. R., Taube, A., and Messenger, P., 1998. Hurgledurgles as a guide to ore at Mount Morgan: a rational explanation. Geological Society of Australia Abstracts No 49, 131.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
GEOLOGICAL SOCIETY OF AUSTRALIA INC Executive Committee 1996-1998 President Past President Vice President Hon. Secretary Hon. Treasurer Hon. Administrative Officer Hon. Promotions Officer Councillor of Executive Division Hon. Editor-AJES Hon Editor-TAG Proxy for Vice President
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David Denham David Groves Bob Henderson Virginia Passmore Richard Blewett Roger Shakesby Bradley Opdyke David Tilley Tony Cockbain Brenda Franklin Phillip Blevin
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
We've come a long way.
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Over 70 years of mining, metallurgical operations, and development at Mount Isa, Queensland, Australia
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
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Ore deposit formation Exploration geochemistry and geophysics Mineral mapping technology Minescale geophysics 3-D visualisation and modelling Coal mine geology and engineering Mine OQuipment and automation Mining technology Mine safety Environmental impacts of mining Dr Bruce Hobbs - Chief Floreat Park Laboratories - Private Mail Bag Wembley - Western Australia 6014 Ph +61 8 9333 6361 Fax +61 8 9383 9324 b.hobbs@dem.csiro.au URL: http://www.dem.cslro.au
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
CONTENTS Abstracts (in alphabetical order offirstauthors)
Author Index
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
SEQUENCE STRATIGRAPHY OF THE ROPER GROUP (MESOPROTEROZOIC), NORTHERN McARTHUR BASIN Steve Abbott Department of Earth Sciences, James Cook University of North Queensland, Townsville, QLD 4811
The Mesoproterozoic Roper Group is a cyclic, predominantly marine, siliciclastic succession which consists of mudstone- and siltstone-rich intervals in alternation with cross-stratified quartz arenite. Minor facies include nonmarine redbeds and paralic pisolitic ironstone. The Roper Group has a preserved areal extent of at least 145,000 in McArthur Basin and the adjacent subsurface Beetaloo Sub-basin. The thickness of the preserved succession ranges from ca. 2,000m in the Roper River area of northern Mc Arthur Basin to a maximum of 5,000 m inferred for the Beetaloo Sub-basin. The preliminary results presented her are based mainly on drillcore obtained by BMR in the mid 1980's as part of a program to test the petroleum potential of McArthur Basin. Graphic logs from stratigraphic drillholes BMR Urapunga 4, 5 and 6 (Sweet & Jackson, 1986), together with new ganmia log data collected by the current NABRE project, were imported into LogPlot® software for interpretation. The sandstone units (up to 80 m thick in the study section) typically consist of medium bedded, medium grained, trough cross-stratified quartz arenite. Granule-rich horizons are present and molds of mud rip-up clasts are locally abundant. Ripple marks are present on parting surfaces. Palaeocurrent orientations are predominantly unidirectional, although reversed directions are present. Indicators or subaerial exposure are absent and the Roper Group sandstones are regarded as deposits of a tide-dominated shoreface. The units of mudstone, siltstone and fine sandstone (up to 325 m thick in the study section) contain a spectrum of facies representing deposition on a storm dominated shelf The deepest and most distal facies consists of laminated black mudstone deposited below storm wave-base. The spectrum passes through several interlaminated and interbedded siltstone and sandstone facies which display an association of storm derived structures which include hummocky cross-stratification, toolmarks, and gutter casts, and were deposited between storm and fair-wather wave-base. Swaley cross-stratified fine sandstones represent deposition within a storm-dominated lower shoreface. Jackson et al. (1988) divided the Roper Group into six depositional sequences I) Mantangula and Limmen Sandstone, Jl) Mainoru, Crawford and Arnold formations, J2) Jalboi Formation, Hodgson Sandstone and Munyi Member, K) Corcoran and Bessie Sandstone formations, L) Velkerri and Moroak formations, and M) Kyalla and Bukalorkmi formations. Sequences range from < 200 m to several hundred metres in thickness and ideally consist of a regressive sanding- and thickening-upward succession of storm-dominated shelf facies capped by tidal crossbedded sandstone. Sequences may be separated by relatively thin intervals of paralic pisolitic ironstone facies, and rarely, non-marine red-beds; both are associated with erosional surfaces which are regarded as sequence boundaries. The study section contains the lower five of the sequences of Jackson et al. (1988) as well as two additional sequences based on subdivision of sequence Jl. The lack of obvious maximum flooding surfaces, the presence of conformable contacts between some sequences, and the presence of erosional surfaces at the base of some tidal sandstone units belies the ostensibly simple sequence stratigraphic architecture of Roper Group sequences. Recent work, particularly in the Cretaceous interior seaway of North America and the Pliocene of New Zealand has shown that regressive sanding- and thickening-upward successions, such as those which dominate Roper sequences, prograde across the shelf from highstand, through falling sealevel, to lowstand. Changes in facies composition and sequence architecture associated with each sealevel phase, including the presence or absence of erosional surfaces at certain stratigraphic levels, allow recognition of highstand, regressive, forced-regressive and lowstand systems tracts, respectively. In intracratonic ramp settings, such as the McArthur basin during Roper Group time, distinction between these systems tracts may be subtle. Nonetheless, elements of each are recognised in the study section. Jackson, M. J., Sweet, I. P. & Powell, T. G. (1988) Studies on petroleum geology and geochemistry, middle Proterozoic, McArthur Basin Northern Australia I: Petroleum potential. The APEA Journal, 28(1), 283302. Sweet, 1. P. & Jackson, M. J. (1986) BMR stratigraphic drilling in the Roper Group, Northern Territory, Bureau of Mineral Resources Record, 1986/19.
GEOLOGICAL SOCIETY 0 ¥ AUSTRALIA, ABSTRACTS No. 49 Australian Geological Comention, Townsville, Jkdy 1998
COLD WATER DOLOMITE (RENISON, TASMANIA, AUSTRALIA): IMPUC ATIONS FOR NEOPROTEROZOIC PALAEOCLIMATE Moh^mm^q H, and Prasada C Rao' ^Department of Geology, University of Mashhad, Mashhad, Iran ^School of Earth Sciences, University of Tasmania, GPO 252-79, Hobart, Tasmania, Aiistralia,7001
Neoproterozoic carbonates and siliciclastic sediments, at Renison in western Tasmania, occur withm tlie central part of the Ehmdas Trough, a north-south trending eugeosyncline. Neoproterozoic Era was a time of intensely cold climate, with extensive low latitude ice sheet and glacio-marihe deposits (Williams, 1993). Extensive glacio-marine deposits of Neoproterozoic age occur across central Australia. The association of dolomite with tillite and the presence of glacial erratics in these sediments, indicates cold water conditions throughout most cf Neoproterozoic time. In Tasmania, thick diamictite units and the presence of dropstones ill lamiiiated dolosiltite in the Smithton Basin, which has been correlated with the Renison sequence, has been recentiy reported by Calver (1995). In the Renison Mine Sequence, four different diamictite horizons have beai recognised. Tasmania was at a palaeolatitude of -20° N at ~ 720 Ma and near the equator at 580 Ma, corresponding to the Sturtian and Marinoan glacials respectively. The carbon isotope chemostratigraphy gives an age range of between 570-800 Ma for the Reniscm carbonates (Adzd^i, 1997). Several petrographic and geochemical features of the least-altered dolomicrite suggest a synsedimentary marine origin. The cold water dolomites formed at Renison and many parts of the world during Neoproterozoic, as indicated by heavy values of dolomite from many regions, relative to a Neoproterozoic seawater value cf about -6%o (Rao, 1997). The calculated palaeotemperature of seawater during the Neoproterozoic, considering 8w = -6%o and the least-altered dolomite values between to -l%o, indicate that the seawater temperatures were around -2 to 7° C using the calcite-dolomite enrichment factor of 3%o (Rao, 1997).. Taking the heaviest value of +0.3%o of Tasmanian Proterozoic dolomite gives a seawater temperature of about 0^ C. REFERENCES Adabi, M.H. 1997. Application of carbon isotope chemostratigraphy to the Reniison dolomites, Tasmania: a Neoproterozoic age. Australian Journal of Earth Sciences 44, 767-775. Calver, C.R. 1995. Ediacarian isotope stratigraphy of Australia. PhD thesiis^ Macquarie University,, 328, Rao, C.P., 1997. Sedimentary Textures: cold, cool, warm and hot, carbonates, 110^ Lome Crescent, Howrah, Tasmania, Tas 7018, 128p. Williams, G.E. 1993. History of the Earth's obliquity. Earth Science Review M, 11-45.
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
MULTI-SCALE STRUCTURAL ANALYSIS OF THE STARRA IRONSTONE HOSTED AU-CU OREBODIES AND SELWYN REGION, NW QUEENSLAND Nicole S. Adshead-BeU School of Earth Sciences, James Cook University, Townsville, Queensland 4811
Fabric development, faulting and fluid flow are inextricably linked, and thus a framework outlining the style and relative age of all deformation and alteration features must be a fundamental aspect of any exploration model in metamorphic terranes. The structural geology facet of most exploration models is commonly generated by interpreting and integrating field mapping and/or remotely sensed data. This approach may be effective in producing a regional scale exploration model, but often has limitations when planning exploration programmes at the all-important prospect scale. Microstructural analysis compliments field mapping when defining the complex tectonic history that is frequently preserved within and surrounding adjacent mineral deposits. Multi-scale structural analysis around the Starra orebodies has been pivotal in determining the structural controls and timing of mineralisation at both the local and regional scales. The controversial Starra ironstone hosted Au-Cu deposit (total reserve of 7.4Mt @ 3.8g/t Au and 1.9% Cu) occurs within multiply deformed and metamorphosed Paleoproterozoic rocks of the Eastern Fold Belt, Mt Isa Inlier. Previous genetic studies focused on mineral paragenesis, whole rock geochemistry and mesoscale structural mapping around the deposit. However, these separate studies lacked a regional perspective. Initial underground reconnaissance mapping, within the Starra orebodies, showed no evidence for mesoscale structural controls of mineralisation, such as development of large scale folds and faults. The polyphase nature of the deformation and lack of an obvious folding/faulting control of mineralisation highlighted the need for a detailed microstructural analysis of the Starra orebodies. Microstructural and textural analysis of the Starra orebodies indicates that the ironstone hosted Au-Cu mineralisation and associated alteration assemblage were emplaced late in the deformation history (syn D4-D6) as a result of shear controlled iron metasomatism. The opposing shear senses of D4 and D6 (west-side-up and eastside-up, respectively) set preferential geometries, which provide an excellent mechanism for dilatation (and associated fluid flow), brecciation and emplacement of mineralisation. Mapping has delineated late granite pods within 100m of the mineralised ironstones localised within the Starra high strain zone. Timing evidence suggests these granites were emplaced coeval to the syn D4 ironstones. The possible genetic association of the ironstones and late granite is supported by published S^^O stable isotope data, which suggests that a magmatic fluid source is most probably responsible for the ironstones and associated mineralisation (Rotherham, 1997). The integration of microstructural analysis of the orebodies with structural mapping (both underground and regionally) infers that large scale controls of mineralisation were the onset of the Starra and Selwyn high strain zones and the emplacement of late stage (syn-D4) granites. Structural analysis of a 1:100 000 aeromagnetic dataset provides critical insight into the nature of the magnetic highs in the Selwyn area. The magnetic highs are predominantly NNE trending elliptical bodies that show no evidence of folding. The broad parallelism between the Starra mineralised ironstones and magnetic highs in the Selwyn area suggest that these magnetic bodies may have resulted from the same processes i.e. structurally controlled with fluid sourced from the abundant late stage granites in the region. This study has demonstrated the importance of multi-scale structural analysis in determining the deposit and regional scale structural controls and timing of mineralisation in a multiply deformed and metamorphosed terrane. The generation of an exploration model based on this technique would focus on areas preserving N-NE trending magnetic bodies emplaced late in the deformation history and localised in zones of high strain (i.e. shear zones, faults) that are spatially associated with late stage granites.
REFERENCES Rotherham, J.F. 1997. Fluid conditions of post-metamorphic ironstones and mineralisation at the Starra Au-Cu deposit, Cloncurry District. Geological Society of Australia, Abstracts 44, 62. Acknowledgements: Selwyn Mine personnel are thanked for logistical support.
GEOLOGICAL SOCIETY OF AUSTRALL\, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
MINERAL DEPOSIT DATABASE AND METALLOGENIC EVALUATION OF THE TECTONOSTRATIGRAPHIC UNITS OF THE NORTHERN TERRITORY Masood Ahmad Northern Territory Geological Survey, Department of Mines & Energy, P. O. Box 2901, Darwin, N. T. Australia
The Narthem Territory Geological Survey's Mineral Deposit Database (NTMINDEP) contains informatioi on over 2500 mineral occurrences. Although modification and updating of this database continue, the present paper aim<t to use the database to analyse and categwise various mineral occurraices in a teaonostratigraphic framework. The late Archaean (cf 2500 Ma) basement to the Palaeoproterozoic Qrogenic Domains does not contain any significant mineralisation except minor uranium anomalies in the Nanambu Complex and seme gold bearing intCTsections in drill holes in the Dirty Water Metamorphics in the Woolner area. The Palaeoproterozoic Oogenic Domains (>2000-1870 Ma) were subjected to widespread and diachronous Barramundi Orogeny (1850-1870 Ma). These Domains provided the bulk of the mineral commodities mined in the Northern Territory. The most productive amongst these are the Pine Creek, Tennant Creek and The Granites-Tanami inliers. The succeeding period of late to post orogenic igneous activity and sedimentaticm has been termed the Transitional D(Mnains (1850-1730 Ma). Although the Transitiaial Domains contain oa\y a few important mineral occurrences the igneous intrusives emplaced during this period are largely responsible the bulk of vein type mineral deposits within the Qrogenic Domains. The unconformably overlying and comparatively less deformed Platform Cover (late Palaeoproterozoic to Neoproterozoic) also contains some significant mina-al occurrences. The predominant mineral commodities include Au, Cu, Sn, U, Pb, Mn, W, Fe, Zh and Al, industrial minerals, heavy mineral sands and semi-p-ecious stones. Uranium is dcxninant in the early Palaeoproterozoic (Rhyacian?), which hosts some world-class uranium deposits; small uranium occurrences are also noted in the late Palaeoproterozoic (Statherian) and in the Phanerozoic. Most gold is within the middle Palaeoproterozoic (Orosirian), which also contains the bulk of vein-type Sn, W, Ta and base metal occurrences. Stratabound/stratiform base metal occurrences are common in the Statherian. Most Mn and Al occurrences are within the Phanerozoic. Most uranium deposits fall into three types: (a) unconformity-type stratabound deposits within carbonaceous and pelitic rocks (eg Rum Jungle and Alligator River regions), (b) Sandstone hosted reduction front type deposits (e.g. Westmoreland, Angela and Bigrlyi) and (c) minor vein type mineralisation in a variety of rock types including granite, basalt, dolerite, acid volcanics and vein quartz. The world-class uranium deposits are spatially related to Archaean granite and gneiss and the unconformity with late Palaeoproterozoic basal arenites of the North Australian Platform Cover. A few common faaors which probably genetically link the gold deposits together are host rock lithology, metamorphism and granite type. Most gold deposits are hosted within greywacke-siltstone-shale sequences metamorphosed to lower greenschist fades and subsequentiy intruded by granites with I-type characteristics. These granites have caused superimposed contaa metamorphism and there is a strong spatial association of gold deposits, with a majority located within the contact metamorpAic aureole. In the Pine Creek Inlier, the traid of the vein-type gold deposits is significantly different from that of the tin and base metal veins, suggesting discrete mineralisation events. About 77% of the mineral occurrences are within the Palaeoproterozoic Qrogenic Domains of the North Australian Craton and mainly comprise vein-type Au, Sn, Cu, Pb and Zn as well as unomformity-type uranium deposits. Most of these mineral occurraices are associated with strata older than the Barramundi Qrogeny (18801850 Ma) and these could be ascribed to syn- to post-OTOgenic granites. The overlying Neath Australian Platform Cover contains about 15% of mineral occurrences; mainly sediment-hosted base metal deposits and a few small uranium deposits. The Proterozoic Qrogenic Dexnains of the Central Australian Mobile Belts and the ovwlying Central Australian Platform Cover have only about 15% of the mineral occurrences, mostly mica and other industrial minerals and a few Cu, Au and Sn deposits. The paucity of mineral occurrences in the Central Australian Mobile Belts could be due to (a) deeper erosion levels, (b) dissipation of mma-al constituents due to repeated and jM-olonged periods of tectonic activity (betweenl880 and 300 Ma), (c) lack of focused and intensive exploration and (d) lack of appropriate metallogenic, geological and geophysical data/datasets.
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
ORGANIC GEOCHEMISTRY OF NEOPROTEROZOIC MICROBIAL MATS, CENTRAL AUSTRALIA Khaled A1 Arouri. Malcolm R. Walter and Gunther C. O. Bischoff School of Earth Sciences, Macqaurie University NSW 2109
Neoproterozoic formations of the Australian Centralian Superbasin contain superbly-preserved organic matter in the form of biofilms or microbial buildups, the origin of which as well as their palaeontological and sedimentological implications are being investigated. In the Officer Basin and Amadeus Basin, several hundred meters-thick successions of marine laminated and graded-bedded calcareous mudstones, dolomitic siltstones and sandstones contain microbial mats in association with acritarchs. Wavy microlamination, rip-up flakes and other microbial features are clearly evident. The mats appear either as amorphous, homogenised organic sheets without internal structures, or consist of filaments and densely accumulated smooth spherical entities (1 jim in diameter) which may be interpreted as coccoidal bacteria. High magnesium calcite or dolomite crystals are closely associated with these botryoidal structures, suggesting a microbial involvement in their formation. The Bitter Springs Formation (Amadeus Basin) is well-known for its abundance of stromatolites which were deposited predominantly in a shallow marine environment for the major part of the section, and a lacustrine setting in its uppermost portion. There, filamentous cyanobacteria are associated with rare acritarchs, in good agreement with the biomarker distributions which are dominated by monomethyl and dimethyl branched alkanes and the extreme dominance of hopanes over steranes. Alternating episodes of oxic/anoxic conditions prevailed, with the presence of irregularly branched acyclic isoprenoids indicating occasional contribution of extreme halophiles including methanogens. The younger (mid-Marinoan) Tanana Formation in the Officer Basin contains more acritarchs and less microbial mats, with a considerable organic carbon content (TOC = 0.1 - 0.3%; Hydrogen Index = 20 - 583) of less thermal maturity (Tmax - 438 'C) than that of the Bitter Springs Formation. Biomarker distributions indicate a joint venture of at least two microbial communities: (1) cyanobacterial mats, characterised by abundant 3,7dimethyl alkanes; and (2) algal mats, characterised by abundant C27 and/or C29 steranes, suggesting a chlorophycean algal input with some contribution from rhodophyte algae. Enhanced abundance of the 4methylsteranes may be related to inputs from prymnesiophytes and/or dinoflagellates. The association of these algal biomarkers with abundant spinose acritarchs may suggest a related biological affinity. An unassigned series of dimethyl alkanes is also encountered abundantly in the acritarch-rich algal mats; their exact identification is still underway. Minimal diasteranes and low pristane/phytane ratios are consistent with a deposition under mild redox potential.
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
A GEOLOGICAL APPROACH TO SUSTAINABLE RESOURCE MANAGEMENT Dhia AI Bakri The University of Sydney, Orange Agricultural College, PO Box 883, ORANGE NSW 2800
In this paper, the author not only argues that geology and related earth science processes provide an effective framework for assessing sustainability, but also asserts that sustainable land resource management is imlikely to succeed unless geological factors are carefully considered. The bio-physical system is the foundation for all socio-economic developments. We now know that these developments can only be viable when the natural resources they depend on are sustainable. The concept of ecological sustainable development (ESD) has been one of the underlying themes of environmental plaiming in Australia and overseas. Plarmers and policy makers grapple with the challenge of reconciling the competing objectives of long-term sustainability and short-term socio-economic demands. Sustainable resource management must account for interactions of a whole host of bio-physical factors (e. g. climate, geology, soil, water, fauna and flora) and the socio-economic factors (e. g. cost, market, politics, culture and technology) at local, national and international levels. This task is hugely complex and, since the dynamics of the system over time must also be assessed, the challenge appears unmanageable. To date, most approaches to assessing sustainability have concentrated on investigating the multiplicity of the effects that arise from these interactions. The more a land resource model focuses on describing effects, the more complicated, expensive and, arguably, irrelevant it becomes. Such an assessment carmot explain what makes the whole system behave as it does, because it only describes the symptoms, and not what creates and drives the system. To safeguard ecosystem functions, and to develop useful policies and practices in land resource management, we must understand the causes of resource degradation. What are the important and unchanging foundations to the teeming mass of detailed effects? Within any given land resource system, the causal factors are climate, time and geology. These three factors are primarily free from the effects of other biophysical factors and human interventions, and are essentially stable. The other biophysical factors, such as soil, water, fauna and flora, are dependent on the interaction of the causal factors, and are subject to alteration from anthropogenic activities. If we focus our assessment of land resource systems on the causal bio-physical factors, we can obtain critical data on cause and effect relationship within the system. The author argues that within a single climate zone, geology is the most critical cause of the inherent characteristics and natural constraints of the bio-physical system. (Geology is here used in its broad meaning, which includes geology, geomorphology, geochemistry, geophysics, and hydrogeology). By studying the geology of a land resource system, we should therefore be able to develop models that explain, predict and diagnose problems and phenomena related to land resource sustainability. In this paper, case studies from Ausualia and the Middle East demonstrate how geology influences resource characterisation and land degradation, focusing on soil formation, water quality and quantity, and agricultural and related management issues. Geological principles relevant to resource sustainability are outlined, and the paper proposes approaches and a methodology for applying geological principles to sustainable land resource management. Given that geology determines many of the characteristics and behaviours of bio-physical systems, it is remarkable that, to date, investigations in sustainable resource management have largely overlooked the geological approach.
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
S-CONTENT IN AUSTRALIAN SUB-CONTINENTAL LITHOSPHERE: PRELIMINARY RESULTS. Olivier Aiard^ Jean-Pierre Lorand^ William J. Powell^ Suzanne Y. O'Reilly', William L. Griffm' 'GEMOC National Key Center, School of Earth Sciences, Maquarie University, Sydney, NSW 2109 ^Museum National d'Histoire Naturelle de Paris, Lab de Min6ralogie, ESA CNRS 7058; 61 rue Buffon, 75005 Paris, France
Sulfides are minor but common phases in mantle rocks. They are thought to be the main mineral host in mantle rocks for highly siderophile elements (HSE). Therefore, sulfide mineralogy and S concentration are key factors for understanding the behaviour of PGEs and the Re/Os isotopic system in the mantle. Previous studies have shown drastically different S contents for the two main types of upper mantle samples, i.e. orogenic massifs and xenoliths. The massifs have S contents in the same order of magnitude as estimated for the MORB source («200 ppm) and correlated with major element contents. In contrast, peridotite xenoliths brough up by alkali basalts generdly have low S contents (<50 ppm), poorly correlated with fertility indexes (FI#; e.g., AI2O3, CaO). Some authors have postulated that these features are due to an overprint by magma transport and weathering processes. However this interpretation is still strongly debated. Four mantle xenolith suites from Eastern Australia have been investigated: Mt Quincan (Atherton Province, Qld), Wallabadah Rock (Liverpool Province, NSW), Allyn River (Barrington Province, NSW), and Mt Gambier (Newer Volcanics Province, SA). Mt Quincan xenoliths are mainly Iherzolite (7>cpx%>19X but a few are ol-rich (75>01%>97). Mt Gambier peridotites display more depleted characteristics, with occurrence of harzbugite and cpxpoor Iherzolites (0>cpx%>9); two are cpx-rich with 14 and 25% of cpx. Xenoliths from Wallabadah Rocks range from harzbugites to Iherzolites (4>cpx%>15) and Allyn River samples are mainly Iherzolitic (cpx%~16). Mt Gambier xenoliths have higher sulfide modal abundances than Mt Quincan samples. Isolated blebs fully enclosed in olivine, spheroidal bodies up to 400 |im in diameter at spinel-silicate junctions, small droplets along fluid inclusion trails or in vermicular intergrov^s at the silicate grain boundaries and sulifide veins penetrating the silicates are the main sulfide occurrences observed in Mt Gambier xenoliths; weathering is not uncommon but of very limited extent (0-30 %). In contrast, Mt Quincan xenoliths contain only a few sulphide blebs (50-300 )im), generally highly weathered (30 to 90 %) per polished thin section. Wallabadah peridotites have very variable sulfide habits and abundances, and the degree of weathering is variable (up to 50%). Allyn River peridotites show very unusual numerous large interstitial sulfides with angular shapes and 'jagged' rims. The common sulfide assemblage is typical of mantle rocks, i.e. pentlandite, monosulfide solid solution (mss) - both Ni poor (=10%) or Ni rich (==30%) and chalchopyrite/isocubanite solid solutions. This assemblage is well preserved in Mt Gambier xenoliths. On the other hand, the sulfides recognizeable in Mt Quincan xenoliths are Ni-poor/Fe-rich mss and pyrrhotite. This difference may reflect both the Ni-poor olivine chemistries of host peridotites and the fact that Ni-rich MSS are less weathering -resistant. Sulfur contents were measured by iodometric titration at the Museun National d'Histoire Naturelle de Paris. The results conform with the petrographic study. S content for Mt Quincan xenoliths lies in narrow range between 10 to 28 ppm. Wallabadah peridotites show S contents ranging from 22 to 130 ppm and some contents as high as 344 ppm were found in Allyn River peridotites. In Mt Gambier peridotites the S concentration varies from 10 to 131 ppm. Mt Quincan S contents do not show any correlation with FI# in contrast for Mt Gambier and Walabadah samples that display a good correlation with FI#. Cu content ranges from 5 to 15 ppm and in all the xenolith suites Cu is well correlated with FI#; Cu contents are higher in Mt Quincan samples than in Mt Gambier xenoliths. The low S contents and the lack of correlation between S and FI# in Mt Quincan peridotite are typical of subcontinental xenoliths. As Cu is still correlated with FI#, and given the weaAering features observed, the low and scattered S distribution is ascribed to post-entrainment processes. However, the good correlation between S content and FI# for the Mt Gambier and Wallabadah suites allow us to estimate the S content Primitive Upper Mantle (PUM), using estimated content of AI2O3 and CaO contents in PUM. This method yields a [SJpu^ around 150 ppm, which is 50 to 100 ppm lower that the other estimates. The unusually high S of Allyn River samples is more problematic, as their S contents are even higher than earlier estimates for the PUM value; therefore this feature may suggest that S has been added by metasomatism. This hypothesis is supported by the trace element pattern of the cpx, which suggests metasomatism by a volatile-rich small-volume melt. These four xenolith suites give us the opportunity to look at the effect of several processes (melting, metasomatism, weathering) upon the HSE and to try to answer question such as whether the lower and more variable PGE contents of mantle xenoliths are due to the perturbed S contents of the xenoliths, or do they reflect differences within the subcontinental mantle? Preliminary results show that the Mt Gambier xenoliths, where S contents appear undisturbed, have higher Au and Ir contents than those of Pyrenean peridotites and PUM, even though thefr S contents are lower.
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
CRUSTAL AGES DETERMINED BY SHRIMP DATING OF ZIRCON FROM GRANULITE XENOLITHS, NEW ENGLAND OROGEN, CENTRAL COASTAL QUEENSLAND Charlotte M. Allen^ and Ian S. Williams^ Key Centre for the Geochemical Evolution and Metallogeny of Continents, Geology Department, Australian National University, Canberra, ACT 0200 ^Research School of Earth Sciences, Australian National University, Canberra, ACT 0200
Measuring the ages of zircons from the lower crust is the most direct means of determining the age structure of the continental crust. Somewhat less direct information can be obtained by studying the ages of inherited zircons in plutonic rocks. Granites are imperfect sampling media in that inherited zircon can be dissolved in some compositions. In an effort to understand the crustal structure in central coastal Queensland, we have done a reconnaissance study of zircons from three granulite xenoliths collected from Miocene mafic volcanic rocks originally studied by Griffin et al. (1987). These lower crustal xenoliths have been interpreted to be residues of melting events. Our results are compared to what we know about crustal ages from granites. Two gamet-chnopyroxene granulite xenoliths from the northern tip of the Bowen Basin (Redcliff Vale and Mt St Martin) yielded only a few zircons. The Redcliff Vale sample yielded 9 zircons, and core ages of: -1.84 Ga, --1.40 Ga, Late Proterozoic (-^00 Ma), Early Palaeozoic H 7 5 and 420 Ma), mid Palaeozoic HOO-260 Ma), and Early Cretaceous 10 Ma). The size of the zircons made distinct rim analyses impossible. The Mt St Martin sample yielded only 4 zircons: three cores yielded ages of-280-270 Ma, and an Early Cretaceous one. In contrast, a two-pyroxene granulite from 150 km west of Noosa Head (Brigooda Basalt) yielded more than 100 zircons, but only two age populations. Seven rim analyses yielded one age population with a mean of 221.0+6.6 Ma (95% confidence). Core ages (n=9) varied from that of the rim to a statistically distinct older one. The distinctly older cores (n=5) yielded a mean age of 241.6+5.5 Ma (95% confidence). The xenolith from the Brigooda Basalt is from an area dominated by Triassic plutonism (K-Ar mineral ages; Gust et al., 1993). Three periods of igneous activity were distinguished: 250-230, 230-220, and 220-210 Ma! This granulite may be meh residue formed at 240 Ma (zircon cores) that was metamorphosed at 220 Ma (zircon rims) and brought to the surface in the Miocene. The two gamet-clinopyroxene xenoliths record a more complicated crustal history but one consistent with known geology. I-type granites and related volcanic rocks east and west of the northern tip of the Bowen Basin range in crystallization age (SHRIMP, zircon) from 308 to 278 Ma (Allen et al., in press; Black, 1994). Ten inheritance ages have been identified but only in volcanic rocks west of the Basin (2434 to 691 Ma; Black, 1994). East of the Basin, Triassic (--240 Ma) and Early Cretaceous igneous rocks are known ranging in age from 145-100 Ma (Allen et al., in press). The Early Paleozoic and Late Proterozoic ages are more difficult to tie to plutonic events; they may be mixed ages. On the other hand, Siluro-Devonian intrusions are known in the New England Orogen, and in the Georgetown Inlier. Proterozoic rocks are common north and west of the Bowen Basin but this is the first direct evidence that lower crust of Proterozoic age underiaid the northern tip of the Basin Basin (New England Orogen) in the Miocene. REFERENCES Allen, C.M., Williams, I.S., Stephens, C.J., and Fielding, C.R. (in press) Granite Genesis and Basin Formation in an Extensional Setting: the magmatic history of the northernmost New England Orogen. Australian Journal of Earth Sciences. Black, L. (1994) U-Pb zircon ion-microprobe ages from the northern Drummond Basin, northeastern Queensland. AGSO Record 1994/34. Griffin, W.L., Sutheriand, F.L., and Hollis, J.D. (1987) Geothermal profile and crust-mantle transition beneath east-central Queensland: volcanology, xenolith petrology and seismic data. Journal of Volcanology and Geothermal Research 31, 177-203. Gust, D. A., Stephens, C.J., and Grenfell (1993) Granitoids of the northern NEO: their distribution in time and space and their tectonic implications. NEO Conference Proceedings, UNE, Armidale, p. 565-572.
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
TERTIARY EVOLUTION AND PALAEOGEOGRAPHY OF THE LAKE EYRE BASIN AND THE DRAINAGE DTVTDE WITH THE SOUTHERN MARINE BASINS. NeviUeF.AUey Mineral Resources Group Primary Industries and Resources South Australia GPO Box 2355, Adelaide South Australia 5001
Following deposition of the Winton Formation in the Cenomanian and up until the Late Paleocene, weathering (including silcrete development) and erosion appear to have prevailed for a large part of central Australia. During the Late Paleocene, tectonic subsidence in northeastern South Australia produced the large Lake Eyre Basin, in which episodic fluvial and lacustrine sedimentation took place. Three main phases of deposition, largely controlled by tectonism and palaeoclimatic changes, are preserved in the basin and the palaeochannels which drained into it: in the Late Paleocene to Middle Eocene sandstone, carbonaceous clastics and conglomerate were deposited; from (?) latest Oligocene to Miocene clay, fine sand and carbonate, with lesser conglomerate; and the ?Pliocene to Quaternary sand and sandy clay. From the Late Paleocene to Early Eocene the basin supported rainforest growing in megathermal to mesothermal conditions. By the Middle Eocene vegetation had changed to localised riparian rainforest in valley bottoms and more open sclerophyllous woodland in the hinterland, indicating monsoonal conditions. For most of this early Tertiary interval deposition was by broad braidstreams flowing northwards. A drainage divide had been established between the Lake Eyre Basin and Eucla-Pirie and St Vincent depocentres. The often postulated connections with either the Murray Basin or the Torrens and Pine Basins are entirely conjectural. Widespread weathering and silicification of early Tertiary and Mesozoic sediments occurred in the Late EoceneOligocene. The silcrete sheets were folded into broad domes during the Oligocene. In the Miocene, scattered elements of megathermal rainforest still existed, but open woodland was far more extensive. Strong climatic seasonality and elevated temperatures promoted the deposition of thick dolomites in giant alkaline lakes. The lakes and the diverse vegetation were home to a rich aquatic and terrestrial fauna. Drainage appears to have been endorheic. At this time continuing tectonism in the Lake Eyre Basin probably led to some stream rejuvenation along the southern basin margin and southwards retreat of the divide at least with the Eucla Basin. By the Pliocene, climate was semiarid and vegetation mainly open Casuarinaceae-dominated woodland with very rare stands of rainforest confined to edaphically suitable sites. Drainage patterns and the drainage divide with the southern marine basins were probably similar to the present. Silicification and ferruginisation of sediments were widespread (possibly from Late Miocene onwards), followed by folding and faulting, particularly marginal to the southern ranges.
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
LEAD ISOTOPE EVOLUTION OF MINERALISATION IN THE PROTEROZOIC THROSSELL GROUP, WA Bruce Anderson^ J. Bruce Genunell^ David Nelson^, David Sharp^ ^Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, TAS 7001 ^Geological Survey of Western Australia, 100 Plain St, Perth, WA ^WMC Resources Ltd (Nifty Copper Operations), P.O. Box 7001, Cloisters Square, WA 6850
The Neo-Proterozoic Throssell Group is part of the Yeneena Supergroup in the Paterson Orogen of Western Australia. Numerous styles of mineralisation have been reported in this region including the epigenetic Nifty sedhosted Cu deposit, the low-grade disseminated epigenetic Maroochydore sed-hosted Cu deposit, Warrabarty PbZn carbonate replacement prospect, and the unconformity-associated vein-type Kintyre uranium deposit. The source of metals in these deposits is unknown. New galena Pb isotope data from the Nifty Cu deposit and other regional prospects have been combined with existing galena data to further constrain the source of Pb. Previous Pb isotope studies have been conducted on the Nifty Cu deposit (GSWA pers. conmi.), Marroochydore Cu deposit (GSWA, pers comm., McKnight, 1992; Reed, 1996) and Warrabarty Pb-Zn prospect (GSWA, pers comm.. Smith, 1996). Pb isotope ratio data from these Throssell Group deposits and prospects plot as a linear trend with Warrabarty at the least radiogenic end and Maroochydore at the most radiogenic end. Data from the Nifty Cu deposit plots towards the least radiogenic end of the trend. The Throssell Group linear trend suggests mixing between two Pb sources. Potential sources of Pb are 1) Rudall Complex, 2) Pilbara Craton, or 3) internally sourced Pb leached from Throssell Group sediments. The Rudall Complex Pb signature is poorly defined as shown by initial Pb-Pb ratios of K-feldspars seperates. Galena Pb isotope data from deposits in the Pilbara Craton hes on a Pb evolution trend similar to growth curve of Cumming and Richards (1975). The least radiogenic Warrabarty data plots on the extension of this growth curve. Whole rock Pb isotope data from Throssell Group sediments form a field that encloses the Maroochydore galena Pb data. A source-mixing model is proposed where Pb from a magmatic source (|i=9.88) is mixed with crustal Pb (M.=10.55). The position of deposits and prospects along the linear trend suggests that the Pb in the Warrabarty Pb-Zn prospect is dominated by magmatic Pb (Pilbara source) and that Pb at the Maroochydore Cu deposit has a crustal source (Pb leached from sediment with a Rudall Complex provenance). Pb in the Nifty Cu deposit appears to have a mixed source. Similar mixing models have been proposed previously, however these models have interpreted the Throssell Group linear trend to be a secondary isochron. The secondary isochron model requires all deposits to have formed at the same time, however textural evidence indicates that Warrabarty mineralisation occurred Pre-D2 (Smith, 1996) and that Nifty (Anderson et al., 1996) and Maroochydore (Reed, 1996) formed syn-Dz- The differences in the timing of mineralisation suggests that the Throssell Group Pb isotope linear trend is not a secondary isochron but represent a mixing trend between crustal and magmatic sources of Pb. REFERENCES Anderson, B.R., Dare, P., Berry, R., and Gemmell, J.B., 1996: The Nifty copper deposit - geology and structure. Geological Society of Australia, Abstracts No. 44, p2. Cumming G.L. and Richards, J.R., 1975: Ore lead isotope ratios in a continuously changing Earth. Earth and Planetary Science Letters, 28, 155-171. McKnight, R., 1992: Constraints on the origin of the Broadhurst stratabound Cu mineralisation, with emphasis on stratigraphic setting and timing of mineralisation. Unpublished BSc(Hons) thesis. Reed, A., 1996: The structural, stratigraphic and temporal setting of the Maroochydore copper prospect, Paterson Orogen, Western Australia. Unpublished PhD thesis. University of Western Australia, Perth, Australia. Smith, S.G., 1996: Geology and geochemistry of the Warrabarty carbonate-hosted Zn-Pb prospect, Paterson Orogen, Western Australia. Unpublished PhD thesis, University of Tasmania, Hobart, Australia.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Towmville, July 1998
THE HYDRODYNAMICS AND GEOCHEMISTRY OF MVT ORE GENESIS IN SOUTHEAST MISSOURI Martin S. Appold' and Grant Garven^ ' Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, USA 21218 ^Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001
Huge carbonate-hosted Pb-Zn-Ba-F ore districts occur in the Mississippi Valley region of the United States. In southeast Missouri, over 900 million tonnes of ore exist in Upper Cambrian and Lower Ordovician carbonate strata that blanket the crystalline ridges and knobs of the Precambrian St. Francois Mountains on the Ozark dome. Rich deposits of galena and sphalerite are concentrated in a reef dolomite facies of the Cambrian age Bonneterre Formation. The largest known deposits of lead in the Earth's crust precipitated along the Bonnterre's Viburnum Trend where metal-bearing fluids were focused by pinchouts of the underlying Lamotte Sandstone and by collapse breccias withm the Bonneterre Dolomite. Past geologic studies have documented fluid temperatures between 90' and 120°C, salinities up to 35 weight % NaCl, and a transient history of paragenesis associated with ore mineralisation. The age of ore formation in southeast Missouri has not been established unambiguously but many agree that ore mineralisation was associated with the AlleghanianOuachita orogeny of Late Pennsylvanian-Earl Permian time. Ore districts within the fold-and-thrust beh of the Appalachian Mountains probably formed during early phases of thrusting and compression but the enormous MVT ores of the U.S. midcontinent region formed during erosion of the tectonic beh which lead to broad emergence ofthe foreland basin and the onset of gravity-driven flow systems. These flow systems persisted for millions of years until erosion lowered the topographic relief Coupled mathematical models for brine migration and geochemical reactions during flow are described here to illustrate the complex yet elegant control of basm hydrodynamics on the giant MVT ore system in southeast Missouri. Several lines of evidence suggest that deep basinal brines were driven northward out ofthe adjacent Arkoma foreland basin during uplift and mountain building along the Appalachian-Ouachita orogen. Metals were acquired in the basal sandstones and fractured basement and transported hundreds of kilometres through both carbonate and sandstone aquifers. Ore formation was concentrated in southeast Missouri because of the regional flow patterns, aquifer pinchouts, and favourable conditions for chemical deposition of Pb-Zn sulphides due to fluid mixing and cooling. Until recently, the mechanisms thought to cause ore mineralisation have remained untested in a hydrodynamic sense and only a few have been theoretically evaluated with geochemical reaction path type calculations based on thermodynamic modelling. Numerical simulations, based on the finite element method, test steady and transient flow models for brine migration across the entire sedimentary basin. We solve the partial differential equations governing simultaneous fluid flow, heat transfer, and chemical mass transport for saturated porous media. We assume the basin is fully saturated with a basinal brine and that eventually this brine is displaced by topography-driven flow. The flow regime extends for 700 km (south-north) and it includes about 5 km of Paleozoic sediments resting on several kilometres of Precambrian basement which is assumed to have a low permeability. Hydrologic simulations are constructed first for the basin-scale to show regional flow patterns, subsurface temperature, and fluid salinity during uplift of the foreland basin. Next, we can "zoom" on parts ofthe basin profile and construct district-scale models to show more details of flow and transport as they are affectedby permeability heterogeneity and basement structure. Our mathematical models suggest that ore formation is constrained hydrologically to time scales less than a few million years, otherwise brines resident in the foreland basin are "flushed" by meteoric recharge across the Permian confining layer and the heat pulse associated with basin-scale fluid flow arrives too late to elevate brine temperatures along the flow path and establish thermal conditions recorded by fluid inclusion data. Geochemical flow models zoom in on these predicted regional flow patterns to simulate coupled inorganic reactions and hydrologic controls on ore formation in the Viburnum Trend. We show examples of reactive flow to test geochemical models of ore deposition: 1) metal and reduced sulphur transport in a regional brine that deposits ore while cooling, and 2) metal transport in a regional brine which mixes with local fluid containing reduced sulphur within the Viburnum Trend. The pros and cons of both scenarios are discussed in light of the hydrodynamic setting.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE RELATIONSHIP BETWEEN EPITHERMAL AND PORPHYRY CU-AU MINERALISATION IN THE DIZON DEPOSIT, PHILIPPINES Joel S. Aquino^ and Gregg W. Morrison^ ^RTZ Exploration, 296 Far East Asia Village, Visayez Ave, Quezon City ^Cyprus Gold Australia, 7 Maiy St., Townsville, Queensland 4810
Epitheraial mineralisation hosted in alsic (advanced argillic) alteration, is popularly thought to be oogenetic with adjacent porphyry copper mineralisation in many major districts (eg. Lepanto-Far South East, Phil; El Salvador, Chile; Nena-Frieda, PNG). In the Dizon deposit, open pit mining has produced a 420m cross-section through the alteration boundary and exposed geological relationships that bear on the timing and genesis of porphyry and epithermal mineralisation in such a system. Dizon is a gold-rich porphyry copper-gold deposit (140mt @ 0.43% Cu & 0.93 ppm Au) hosted in an Early Miocene andesite-diorite stratovolcano. It is related to a Pliocene (3 Ma) multi-phase stock of diorite-quartz diorite composition that is itself part of a group of stratovolcanoes lying on a NNE trending volcanic arc above the Manila trench. The porphyry Cu-Au-Mo mineralisation has the form of a classic carapace on the stock and consists of a vein and minor fracture stockwork with a weak preferred NNE orientation. The stockwork consists of laminar and seam quartz veins with chalcopyrite-bomite-molybdenite-magnetite-native gold. It is hosted in potassic (secondary biotite) and SCC alteration zones and is extensively overprinted by alsic alteration in its upper half. The alsic alteration carries no mineralisation of its own at this point, but the rock is still ore because the quartz in the porphyry veins has protected the enclosed sulfidesfromleaching during alsic overprint. The alsic alteration is part of a blanket that covers 50km^ and at least three discrete porphyry systems in the Dizon area. In the mine itself, there is a sharp base to the blanket which clearly overprints the potassic, SCC and propyhtic alteration zones. There are also extensions of the alsic alteration to depth along a prominent group of WNW trending faults that cut and displace the porphyry orebody. These structures host pods of silica-alunite with minor enargite-covellite mineralisation at shaUow levels, and Au-barren basemetal (Pb-Zn-Ag-Cu) epithermal veins at depth. The WNW structures also localise hydrothermal breccias and dacite dikes very similar to the bodies associated with the large Pua diatreme that cuts off the southern side of the orebody. This seems to be a separate and discrete igneous event related to disruption of the arc during movement on the Philippines Fauh in the period 1.8 to 1.1 Ma. Fluid inclusion and isotopic data suggest the porphyry system formed from high salinity magmatic fluids at >650 to 350' C. It was eroded by at least 1km before the alsic blanket developed. The epithermal system was formed by the boiling of low salinity fluid at 390 to 130T at less than 300m below the present upper surfece of the blanket. The Pb in the epithermal system is from a more enriched source than that in the porphyry system. The simplest explanation of the relationships, chemical differences and fluid data, is that the porphyry and epithermk systems are related to two discrete magmatic-hydrothermal events and two discrete igneous episodes. The alsic alteration is not genetically related to the porphyry system, but part of a much more extensive blanket formed by degassing of younger dacitic intrusions, localised by a prominent set of cross-cutting faults. The broader relationship between the dacitic and dioritic intrusions is not entirely clear. They could be two completely separate igneous events, or early and late phases of one evolving magmatic complex. The narrow time interval (approx. 700,000 years ) and 1km of erosion between the dioritic and dacitic igneous events is similar to developments in many active island arc stratovolcanoes, including Mt Pinatubo, which is only 5km from Dizon. The discovery of Dizon stemmed from prospecting of part of the porphyry mineralisation exposed in a down-cut segment of the alsic blanket. The presence of relics of the porphyry mineralisation, particularly laminar and seam quartz veins is the best guide to the underlying porphyry system elsewhere in the alsic blanket.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
A DYNAMIC MODEL FOR GOLD MINERALISATION IN THE WESTERN LACHLAN FOLD BELT, CENTRAL VICTORIA ^ Penni$ C, Frank P. Bierlein', David A. Foster^, Sue Keay^and Neal J. McNaughton^ ^ Minerals Industry Research Institute, University of Ballarat, PO Box 663, Ballarat, Victoria 3353 Australian Geodynamcs Cooperative Research Centre, Department of Earth Sciences, UTrobe University, Bundoora, Victoria 3083 ^ Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Centre for Strategic Mineral Deposits, University of Western Australia, Nedlands, Western Australia, 6907
Until recently, the geochronological framework of the western Lachlan Fold Belt (LFB) was only very broadly constrained by the age of the sedimentary succession, and approximate timing of deformational events and magmatism. More accurate knowledge of the relative and absolute timing of gold mineralisation in the structural zones of central Victoria (Stawell, Bendigo-Ballarat and Melbourne Zones), critical for constraining the source of gold and the formulation of ore genesis models, has generally been lacking. However, the findings of a number of published geochronological studies carried out in the last two years demonstrate that subduction-related transitory and diachronous deformation occurred across the LFB, progressing from west to east and ranging in age from the Late Ordovician to the Late Silurian. This was followed by granitic magmatism from the Late Silurian to the Early Devonian. Deformation continued through to the Middle Devonian, and is best exemplified by folded Late Silurian to Early Devonian strata of the Melbourne Zone. A later, more extensive pulse of magmatism occurred during the Middle to Late Devonian in central Victoria. Gold mineralisation in central Victoria was episodic and is both genetically and spatially associated with crustal heating linked to regional metamorphism and plutonism. In the Stawell and Bendigo-Ballarat Zones, the majority of gold mineralisation is thought to have formed synchronously with regional metamorphism and thrusting between 450 and 420 Ma, with remobilisation and/or secondary phases of mineralisation occurring during subsequent intrusion of felsic dykes and plutons. This interpretation is based on ^Ar/^^Ar analyses of phyllosilicates from a large number of deposits and detailed investigations of hydrothermally altered metasedimentary wallrock immediately adjacent to lode structures. These data are frirther supported by recently published SHRIMP U-Pb zircon ages of felsic dykes interpreted to post-date main-stage gold mineralisation at Ballarat West and Stawell (Magdala), and by 400 - 370 Ma SHRIMP U-Pb zircon ages of felsic plutons in central Victoria, which are inferred to post-date initial gold mineralisation by as much as 80 m.y. years. A second phase of gold mineralisation, often accompanied by elevated Sb, W, Mo and Cu, is associated with both Late Silurian and Middle to Late Devonian magmatism. These occurrences, best exemplified by many of the Woods Point deposits, Fosterville and the Wonga deposit at Stawell, clearly post-date the emplacement of dykes and thus are spatially (if not genetically) related to melt generation at depth. This later, magmatic-associated phase of gold mineralisation is economically subordinate to the earlier, metamorphic-associated phase of gold deposition in the Stawell and Bendigo-Ballarat Zones, but tends to be the dominant style in the Melbourne Zone. ^Ar/^^Ar dates of sericite from gold-bearing veins associated with the Woods Point dykes and other dykes/stocks in the Melbourne Zone range from 383 to 370 Ma, with a concentration of ages at 374 Ma. These dates overiap, and are slightly younger than recent, unpublished emplacement ages for the Woods Point dykes, which range from 378 to 376 Ma. Field relationships, supported by published geochronological data, preclude a single gold mineralisation "event" in the western LFB. Instead, we envisage the diachronous development of metamorphic-associated gold coincident with regional metamorphism and the final stages of structural deformation in central Victoria. Gold mineralisation during this phase of orogenic development occurred predominantly in two chronological "windows', at 460-440 Ma and 420-400 Ma, whereby the ore fluid was derived from a large, homogenous crustal reservoir with respect to lead. This phase was followed by a later, magmatic-associated style of gold mineralisation during the Late Silurian to Early Devonian and again during the Middle Devonian, when crustal thickening and shortening during the ongoing (diachronous) consolidation of the LFB led to extensive melt development in the lower crust and resulted in widespread magmatism throughout central Victoria.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
PROTEROZOIC OLARY DOMAIN, SOUTH AUSTRALIA: REGIONAL AND LOCAL SCALE HYDROTHERMAL ALTERATION AND MINERALISATION Payl M, Ashlgy^ and Ian R. Plimei^ ^ Division of Earth Sciences, University of New England, Armidale, NSW 2351 ^ School of Earth Sciences, University of Melbourne, Parkville, Victoria 3052
The geology and mineralisation of the Palaeoproterozoic Willyama Supergroup (WS) in the Olary Domain (OD) in South Australia have been investigated in the collaborative "Olary Mapping Project", involving regional mapping and integrated research projects. The WS sequence in the OD has regional correlations with that in the adjacent Broken Hill Block, although there are differences in detail. It contains metamorphosed sedimentary, volcanic and intrusive rocks, interpreted to have been developed in an intracontinental rift, with initial intercalated terrestrial, lacustrine/sabhka and marine sequences being succeeded by deeper marine/lacustrine sequences. The lower part of the OD sequence is dominated by migmatite and quartzofeldspathic and psammopelitic composite gneisses (Composite Gneiss Suite). It grades into, and may simply be a more melted equivalent of the overlying Quartzofeldspathic Suite. The latter contains A-type metagranitoids and co-magmatic felsic metavolcanic rocks, psammopelitic schist and composite gneiss, finely laminated albitite and minor iron formation. The Quartzofeldspathic Suite grades up-sequence into the Calcsilicate Suite, in which laminated calc-albitite and minor calcsilicate and Mn-rich rocks are found. In turn, there is an up-sequence transition into the Bimba Suite, dominated by calcsilicate rocks and marble, with minor Fe(CuZn) sulphides, pelite and albitite. The Bimba Suite is overlain by a regionally sharp contact with the Pelite Suite, composed of pelite and psammopelite, with local graphitic facies, psanmiite, tourmalinite and manganiferous iron formation. It is interpreted that the WS sequence in the OD was largely deposited at -1710-1690 Ma. Several intrusive suites occur in the OD and there have been at least five deformation and metamorphic events. Temporal relationships have been investigated by zircon U-Pb and muscovite Ar-Ar geochronology. A-type granitoids were emplaced and rhyolitic volcanic rocks erupted at -1710-1700 Ma. The WS sequence may have been deformed prior to intrusion of mafic igneous masses at ?~1670 Ma. Small I-type granitoid bodies were intruded into the central OD at -1640-1630 Ma. A major episode of deformation and amphibolite grade metamorphism occurred in the Olarian Orogeny at -1600 ± 20 Ma, with subsequent emplacement of voluminous S-type granitoids and associated pegmatite bodies. Regional-scale retrograde metamorphism and alteration followed episodically between -1580 Ma and -1500 Ma, with further thermal perturbations during the Musgravian Orogeny at -1200-1100 Ma. Mafic dyke emplacement at -820 Ma was a precursor to development of the Adelaide Geosyncline and at least two episodes of low grade metamorphism and deformation occurred between -500-450 Ma during the Delamerian Orogeny. Regional and local scale hydrothermal alteration has episodically affected much of the OD sequence, as well as some intrusives. Sodic alteration of quartzofeldspathic rocks may have been initiated during diagenesis, and was followed by high-temperature (-450-600°C), oxidising and saline fluid interaction, for which there is little supporting evidence of a direct magmatic link; instead, fluids may have been largely metamorphically derived, with significant influence from precursor evaporitic and oxidised sedimentary sequences. Widespread Na(Fe) metasomatism (mainly albite ± Fe oxides, pyrite) and local potassic alteration (K-feldspar, biotite) of quartzofeldspathic rocks has occurred, with strong Fe-metasomatism of iron formations and some albitites (magnetite, hematite, minor pyrite) and CaFe-metasomatism of calcsilicate rocks and ass9ciated breccias, marble and quartzofeldspathic rocks (clinopyroxene, CaFeAl garnet, scapolite, epidote, actinolite). Several styles of mineral deposits are recognised in the OD, including early, syn-sedimentary or diagenetic types, various hydrothermal deposits related to intrusives and metamorphic/alteration events, and late, weathering-related types. Syngenetic and/or diagenetic deposit styles are represented by stratiform laminated to massive and disseminated Fe (Zn) sulphides in calcsilicate, marble and pelitic rocks of the Bimba Suite, iron formations (including baritic types in the Quartzofeldspathic Suite) and Mn-enrichments in the Calcsilicate and Pelite Suites. Manganiferous iron formations in the Pelite Suite are closely analogous to iron formations associated with the Broken Hill ore bodies. Epigenetic mineral deposits are represented by various types of replacements in calcsilicates, marble and iron formation, and vein/stockwork systems, generally with a Cu(AuCoZnMo) signature, and rare metal pegmatites and UThREE deposits related to the -1600 Ma S-type granitoids. The replacement and vein/stockwork types are interpreted to have formed during prograde and retrograde metamorphic and deformation events; they include several "stratabound skam" systems associated with the Bimba Suite, Fe oxide-rich replacements of iron formations and vein/stockwork systems and disseminations in the Quartzofeldspathic Suite. Supergene oxidation and diagenetic processes from the Mesozoic to Recent has led to Cu(CoAu) enrichment deposits and redox-controlled U and Au deposits, especially to the north of the outcropping OD. Exploration potential for stratiform/stratabound, sediment-hosted base metal deposits, and epigenetic CuAu in lithologically and structurally favourable sites is considered high.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE TASMAN OROGENIC SYSTEM IN NORTH QUEENSLAND: KEY ELEMENTS John H. C. Bain^ John J. Draper^, Ian W. WithnaU^, Douglas E. Macken2ie^ Lance P. Black^ Robert J. Bultitude^, Laurie J. Hutton^ Ian P. Rienks^, Peter Welhnan^ 1 Australian Geological Survey Organisation, GPO Box 378, Canberra, A.C.T., 2601 2 Department of Mines & Energy,GPO Box 194, Brisbane, Q, 4001
CRUSTAL PROVINCES A synthesis of North Queensland geology (Bain & Draper, 1997a, b) provides a different view of the nature and development of the northernmost portion of the Tasman Orogenic Zone (TOZ) and with it implications for understanding the Tasmanides overall. The northern TOZ developed on, and interacted with the northeastern margin of the Proterozoic Northern Australian Craton. The TOZ has been substantially influenced by the nature and disposition of several crustal provinces created in the Proterozoic and modified in the Palaeozoic. Metasedimentary rocks are confined to three distinct provinces - Hodgkinson and Broken River, and the igneous dominated Thalanga. In the Late Cambrian-early Ordovician and mid Ordovician, granites of the Macrossan Province intruded the Thalanga Province and adjoining areas of basement, mostly in the Charters Towers Region. During the Silurian and Devonian, widespread granite emplacement (Pama Province) occurred west and south of the Hodgkinson and Broken River Provinces. Dominantly felsic magmatism (Kennedy Province) spread throughout the TOZ and adjacent Proterozoic provinces during the Carboniferous and Early Permian. Principal concentrations occur in a broad band (Townsville-Momington Island Belt) that traverses the TOZ and extends far into the Northern Australian Craton, and geochemically distinct subprovinces correspond with most of the underlying metamorphic provinces. Small remnants of intracratonic basins scattered through the northern TOZ make up two groups separated by a mid Carboniferous unconformity. The earlier group formed in a complex structural setting after the Pama but before the Kennedy Province. The younger group is mostly peripheral to NQ but is intrinsically linked to it by derivation and structural and tectonic events, and is associated with the Kennedy Province. Mineralisation, apart from some gold linked with the Pama, and VHMS in the Thalanga, is dominantly associated with the Kennedy Province MAJOR SUTURES The main structural dislocations in NQ — a major fault system (Palmerville Fault System -PFS) that extends more than 1000 km from the Bowen Basin to near the tip of Cape York, and several large subparallel (eg Battle Camp/Russll-Mulgrave) and transverse (Star River, Clarke River and Burdekin River) faults — constrain the main sedimentary provinces and intracratonic basins, and have influenced the distribution of CarboniferousPermian igneous activity. These are major crustal sutures and many exhibit long, complex histories. Architecture of the Hodgkinson and Broken River Provinces suggest that substantial sinistral displacement has occurred along the northwesterly trending segment of the PFS, probably before the Carboniferous. INTERPROVINCE RELATIONSHIPS The Hodgkinson, Broken River and Thalanga Provinces are faulted against and partially underlain by the Proterozoic Etheridge Province to the west, and by the latest Proterozoic or earliest Palaeozoic Barnard and Cape River Provinces to the east and south. To the southeast, the New England Orogen is faulted against the Barnard, Cape River and Hodgkinson Provinces. Two remnants of the Thalanga Province are separated by the younger Broken River Province and components of the three igneous provinces. TECTONIC ENVIRONMENTS The sediments of the Hodgkinson and Broken River Provinces (mixed carbonate and siliclastic sediments, chert and basalt, but mostly siliclastic turbidite) were probably deposited in extensional continental back-arc basins. Back-arc extension is also the most likely environment for the formation of the mostly non-marine intracratonic basins (e.g., Burdekin, Clarke River & Drunmiond). The Pama Province has probably formed from deep crustal melts resulting from underplating by mantle-derived mafic magmas during crustal extension, rather than from direct subduction derived magmas. The Kennedy Province is also essentially the result of crustal melting in an extensional (or transtensional), possibly back-arc, tectonic environment. REFERENCES Bain, J.H.C., & Draper, J.J., 1997a: North Queensland Geology. AGSO Bulletin 240; Queensland Geology 9. Bain, J.H.C., & Draper, J.J., 1997b: Adas of North Queensland Geology. AGSO and DME.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
PORPHYRY-EPITHERMAL CONNECTIONS £.Max Baker KM. Baker & Associates Pty. Ltd, P.O. Box 176, The Gap, Brisbane 4061, Australia.
The concept of a magmatic connection between intrusive-hosted mineralization and spatially associated epithermal mineralization as outlined Sillitoe, (1989) has become widely accepted and applied throughout the exploration community. To date, the strongest evidence of a direct "magmatic connection" between porphyryand epithermal-type mineralization comes from Lepanto (Phihppines), where Arribas et. al. (1995) have demonstrated that high-sulfidation epithermal Cu-Au mineralization and underlying porprfiyiy Cu-Au mineralization occurred contemporaneously over a period of 300 ka. The mineralization at Lepanto occurs with a diatreme-flow-dome complex, which developed over a period of apjM-oximately 2 my. However, high resolution "^^Ar^^Ar dating in the Porterillos District, (Chile), by Marsh et. al., (1997) demonstrated that the low-sulfidation epithermal-style mineralization and associated porphyry stock are 5 my. older than the mineralization in a nearby porphyry-type deposit which was previously interpreted as the magmatic source for the epithermal miner^ization. The duration of mineralization and magmatism, in the Porterillos district, spans over 14 m.a., which Marsh et. al., (1997) attribute to the porphyry stocks representing protuberances above a batholith that was kept hot from below by repeated additions of mafic magma. Therefore, recognizing just how direct the "magmatic connection" is for a particular situation is a critical fiactor in exploration m the porphyry-epithermal environment. The spatial relationship between epithermal- and porfAiyry-type mineralization is likely to be simplest where there is a direct temporal/genetic link between the two styles of mineralizatioa This is more hkely in the shallower levels of porphyry-type mineralization development i.e. eroded remnants of stratovolcanos or within diatreme complexes, which are relatively short hved (<2-3 Ma.). In this shallow environment telescoping will further enhance the spatial relationship tetween porphyry-type and peripheral epithermal mineralization. In the deeper levels of the porphyry-type environment i.e., above tetholiths, magmatism may last more than 8 Ma, and consequently overprinting b^ successive pulses of porphyry emplacement and mineralization will result in less predictable spatial relationships. Spatial and overprinting relationships are also likely to be more complex within structurally permissive zones where both intmsives and hydrothermal fluids will tend to be focused throughout the entire magmatic episode. In the search for concealed porphyry-type mineralization associated with outcropping or known epithermal mineralization, the followingfieldcriteria are considered geological indicators. The most obvious yet are often overlooked indicators include; clasts of porphyry-type mineralization within associated volcanic and breccia bodies, and quartz stockwork veining overprinted by pervasive argillic/advanced argillic alteration. Areas of extensive advanced argillic alteration (high-sulfidation) associated with peripheral or overprinting low-sulfidation epithermal mineralization, particularly whereflow-domes,diatremes and porphyry stocks are evident, should also be considered very prospective terrain for porphyry-type mineralization. Evidrace that hydrothermalfluidshave passedfromporphyry-type mineraHzing system out into the surrounding rocks, such as the presence of proximal skams, lower-temperature replacement bodies and overprinting of the porphyry by epithermal-type mineralization, are favorable indicator of the right environment for peripheral epithermal mineralization occurring peripheral to porphyry systems. "REFERENCES''. Arribas, A.Jr. Hedenquist, J.W. Itaya, T. Okada, T. Concepcion, R.A and Garcia, J.S.Jr. 1995. Contemporaneous formation of adjacent porphyry and epithermal Cu-Au deposits over 300 ka in northern Luzon, Phihppines. Geology, April 1995; v. 23; no. 4, 337-340. Marsh, T.M. Einaudi, M.T. McWilhams, M. 1997. 40Ar/39Ar Geochronology of Cu-Au and Au-Ag Minerahsation in the Porterillos District, Chile. Econ. Geol., v. 92, No. 7/8, 784-806. SilUtoe, R.H., 1988, Gold Deposits in Western Pacific Island Arcs: The Magmatic Connection. Econ. Geol., Monograph 6. 274-291.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14'^ Australian Geological Convention, Townsville, July 1998
CONTAMINANT FLOW IN RIVER BANKS: KING RIVER, TASMANIA Simon F. Baker and Dr. D.C. "Bear" McPhail, Department of Earth Sciences, VEPS, Monash University, Clayton, Victoria 3168
River banks containing contaminated sediment such as mine tailings can release acid and metals into river water and characterising the groundwater flow through such sediment banks is necessary in understanding mass loading. Mine tailings from massive sulphide ores may be highly reactive due to pyrite oxidation when subjected to weathering processes. Low-pH groundwater from tailings may react with porous media of an underlying aquifer, resulting in changes to physical and chemical properties of the aquifer, which in turn may affect fluid flow and contaminant transport. This has important consequences for determination of the environmental impact, or mass loading of acid and elements, on a river system, principally because simplifying assumptions are often made such as the sediment bank is homogeneous and isotropic.
In addition, the
stratigraphy and flow boundaries of river banks often have to be assumed in order to model the flow accurately. Groundwater flow is estimated in a typical sediment bank 800m long and up to 130m wide on the King River in western Tasmania. This bank is important because it is contaminated with mine tailings from historical mining activity at the Mt. Lyell Copper Mine in Tasmania. Groundwater flow is estimated using an array of 34 minipiezometers installed in lines perpendicular to the river and nests to study possible vertical flow. The piezometers were used to measure hydraulic head, hydraulic conductivity and sample groundwater for chemical analysis. Measurements and samples were taken during summer and winter seasons in order to study seasonal differences in flow and water composition. Hydraulic conductivities range between 8.0x lO.g m/s and 9.6 x 10'^ m/s and hydraulic gradients vary between 8x10'^ m^m in summer to 6.0x10'^ m/m in winter. Measured pH varied from 3.8 to 6.25, and samples of groundwater containing high levels of dissolved solids (TDS > 2mg/L) correlated with highly reduced waters (Eh < lOOmV). A horizontal high-permeability layer approximately 5 cm to 10 cm thick, 2 metres deep and unknown areal extent was located in several parts of the bank. In addition, hard-pan layers of unknown permeability were also observed in some places. Both observations mean that estimated flow calculations are uncertain. Other methods, such as geophysical ones, may provide us with better information on which to base flow models. Geophysical techniques can allow more accurate determination of groundwater flow by detecting changes in stratigraphy, the position and geometry of the water table (unconfmed aquifers) and contaminant layers. Selfpotential (SP), resistivity (AR) and ground penetrating radar (GPR) techniques were applied to improve understanding of the fluid flow, geochemistry and contaminant transport within the bank. Self-potential gradients during summer varied laterally up to 20 mV/m, correlating with salinity gradients and pH, but not hydraulic head. There is a strong correlation between vertical SP gradients and water-unsaturated and -saturated zones, with changes of up to 150mV observed in the transition between zones. The SP values may be a result of spontaneous potentials due to redox reactions of pyrite, or through electro-kinetic effects of fluid flow through porous media. Resistivity was measured by vertical electrical soundings using Wenner arrays.
The results
suggest changing contaminant concentrations with depth, areas of less contaminated water below the reactive zone (i.e., where pyrite oxidation and dissolution leads to acid generation). GPR was used to detect the location and geometry of the water table and stratigraphy. It was effective in detecting the boundary between waterunsaturated and -saturated zones; however, the high electrical conductivity of acid-rock waters limited signal penetration to a depth of a few metres. The results are promising, but further work in the field and laboratory is indicated, particularly in developing understanding of electro-kinetic SP effects in porous media, important in understanding contaminant transport and resistivity.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
THE BINGHAM DISTRICT - PRODUCT OF A GIANT HYDROTHERMAL CELL ABOUT A LARGE GOLD-RICH PORPHRY COPPER DEPOSIT Geoffrey H. Ballantyne Kennecott, 961 Matley Lane, Reno, Nevada 85902, USA
The Bingham mining district in northern Utah is zoned about the Bingham porphyry copper for which production plus reserves total 3,105 nodllion tons with an average grade of 0.73% Cu, 0.013 oz/t Au, 0.093 oz/t Ag and 0.043% M0S2. Bingham occurs in the easternmost mountain range of the Basin and Range province far inland from the site of Eocene subduction. The porphyry orebody occurs within and adjacent to 38 to 37 Ma subalkalic monzonite and quartz monzonite porphyry stocks which intrude folded and thrust-faulted quartzites and subordinate limestones of Pennsylvanian age. Volcanic rocks are absent from the inmiediate vicinity of the orebody but are exposed only 2 km to the east and a volcano probably overlay the orebody when it was forming. Fluid inclusion studies suggest at least 2.5 km of erosion since ore deposition. The copper orebody has the form of an inverted cup with a diameter of -1.5 km and is zoned outward from a bomite-rich inner zone, through a chalcopyrite-dominant zone, to a pyrite-chalcopyrite zone. Most molybdenite occurs in a shell that mimics the shape of the copper orebody but has a smaller diameter and overlaps the inner edge of the copper shell. High-grade copper-gold skams occur where limestone lies within 500 m of the outer edge of the copper shell and are developed over at least 1.5 km vertically. Lead-zinc-silver-gold replacement and vein deposits have been mined from a markedly NNE-elongate halo about the porphyry copper orebody. This halo extends beyond the pyrite halo around the porphyry deposit. Massive lead-zinc mineralization becomes less conmion with increasing distance from the porphyry but has been found up to 7 km from the center of the copper deposit. The Melco and Barneys Canyon sediment-hosted gold deposits lie 7 to 9 km north of the porphyry deposit within broad, annular Au and As geochemical zones that surround the porphyry copper and extend beyond the outer edge of the lead-zinc halo. Recent work at Brigham Young University indicates that volcanic rocks preserved to the east of Bingham include an older suite of latitic agglomerates that are comagmatic with the Bingham intrusions and a younger suite of latites that are chemically distinct. Small volumes of mafic lavas in the volcanic sequence include nepheline minettes that contain olivine and phlogopite phenocrysts, and abundant normative nepheline and leucite. Unaltered minette flows have been dated at 37.82 +/- 0.14 Ma suggesting they are coeval with shoshonite and quartz latite dikes, dated at 37.74 +/- 0.11 to 37.72 +/- 0.09 Ma, within the Bingham orebody. Degassing of primitive minette magma may have contributed some of the sulfur and metals in the Bingham deposit. A Re-Os study of four molybdenite samples from Bingham by John Chesley and Joaquin Ruiz at the University of Arizona yielded a weighted-mean age of 37.0 +/- 0.27 Ma. The molybdenite ages are indistinguishable from ^Aif'^Ai ages recently determined for hydrothermal biotite and sericite from the center of the orebody by William Parry and others from the University of Utah. A preliminary study of gold paragenesis in the porphyry copper indicates that most gold occurs as particles less than 25 microns across, alloyed with less than 15 wt. % silver. Gold particles occur on the boundaries of chalcopyrite and bomite grains and as inclusions within those minerals. Gold recovery to flotation concentrate is -68%. Identifiable silver minerals are rare at Bingham but flotation recovery is -81%. The good recovery and the response of concentrate samples to cyanide and nitric acid leach tests suggest that most silver is present in solid solution in copper sulfide minerals. No genetic relationship has been established between the Bingham porphyry and the Melco and Barneys Canyon sediment-hosted gold ore bodies to the north. The gold deposits lie far beyond the talc-tremolite boundary, the most distal silicate alteration boundary recognized around the porphyry, and fission track or oxygen isotope studies that might establish paleothermal gradients between the porphyry and the gold deposits are lacking. However, Au and As distributions and the shapes of the pyrite and lead-zinc halos around the porphyry copper all suggest that ore fluids may have ascended through the now-eroded rock column above the porphyry then flowed down-gradient to the NNE, depositing gold in reactive formations 7 to 9 km from the up-flow zone.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
AN ALICE SPRINGS (380 MA) AGE FOR A PROGRADE AMPHIBOLITEFACIES SHEAR ZONE IN THE STRANGWAYS METAMORPHIC COMPLEX, ARUNTA BLOCK Michel Ballevre^ Andreas Moller^'^ & Bas J. Hansen^ ^ G6osciences Rennes (UPR CNRS 4661), University de Rennes I, F-35042 Rennes Cedex, France ^ Dept. of Applied Geology, University of NSW, Sydney, NSW 2052 ^ Max-Planck-Institut fur Cheniie, Postfach 3060, D-55020 Mainz, Germany
Foliated gamet-bearing amphibolites occur within the West Bore shear zone, cutting through granulite-facies gneisses of the Strangways Metamorphic Complex. In the amphibolites, large euhedral garnets (up to 3 cm) occur within fine-grained recrystallised leucocratic diffusion haloes of plagioclase-quartz. The garnets and their haloes include a well-developed vertical foliation, also present in the matrix. This foliation is the same as that cutting through the unconformably overlying Heavitree Quartzite. The textures indicate late to post-kinematic growth of the amphibolite facies mineral assemblages. All mineral assemblages record an arrested prograde reaction history. Noteworthy is the growth of garnet at the expense of hornblende and plagioclase, and the breakdown of staurolite-homblende to give plagioclase-gedrite. These dehydration reactions indicate increasing P-T conditions during metamorphism, and suggest heating towards the end of a period of intense deformation. Temperature estimates for the gamet-amphibolite and related staurolite-homblende assemblages from the shear zone are about 600 °C. Pressures are estimated at about 4-5 kbar. A Sm-Nd isochron gives an age of 381 ±7 Ma for the peak metamorphism and associated deformation. This age determination confirms that amphibolite-facies conditions prevailed during shear zone development within the Strangways Metamorphic Complex during the Alice Springs Orogeny. These temperature conditions deviate significantiy from those expected at this depth assuming a normal geothermal gradient, and indicate that the Alice Springs Orogeny was associated with significant crustal thickening in this part of the Strangways Metamorphic Complex.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
ORIGIN OF PODIFORM CHROMITE ORES BY MAGMA MINGLING Chris Ballhaus Dept. of Mineralogy, University of Tubingen, Wilhelmstr. 56, 72074 Tubingen (Germany) Podiform chromite deposits are an interesting and pooriy understood rock type in the upper mantle. The ores occur as rootless irregular bodies in the lithospheric portions of ophiolite complexes, usually situated near the petrologic MOHO. The major phases in the ore are magnesian chromite and primitive mantle olivine (> F090). Orthopyroxene, although common in the lithospheric host, is conspicuously absent in and around the ore bodies. The ore-forming processes themselves are poorly understood. To enrich chromium from 200 to 500 ppm Cr in a typical basaltic mantle melt, to -- 50 percent in a viable ore, we need to have chromite as the only phase on the liquidus. One way to achieve this is magma mixing. In the simple basaltic analogue Mg0-Cr2(^-Si02, the silicate-oxide cotectic is so strongly curved that hybrids of two cotectic melts will mix across the chromite-only saturation field, i.e. will have chromite only on the liquidus. The absence of orthopyroxene suggests that one participating melt endmember is lower in silica activity than the olivine-orthopyroxene peritectic in the lithosphere. One of the most enigmatic features still waiting to be explained are the ore textures. Most podiform chromite deposits contain portions of nodular ore in which rounded chromite aggregates are set in a matrix of forsteritic olivine (leopard ore). Suggested origins of these textures include (1) mechanical coalescence of chromite grains to rounded nodules in turbulent magma conduits, (2) the existence of immiscible oxide melts under mantle conditions, and (3) some unspecified form of hydrothermal activity in the mantle. None of these models seems convincing. If simple coalescence, why do we not encounter nodular textures in stratiform chromite deposits as well? If the textures were hydrothermal in origin, where is the context of the ore textures to the presumed hydrothermal activity and what was the nature and origin of the fluids? If the nodules were crystalline equivalents of immiscible oxide melts, how to obtain the necessary temperatures to melt refractory chromite without causing wholesale lithospheric melting? In this contribution I suggest that the nodular ore textures are due to magma mingling. They form when droplets of two olivine-chromite-saturated mantle melts mingle, yet do not mix instantaneously because of intrinsic viscosity, density, and temperature differences. Before full mixing occurs the two melt fractions - initially dispersed as droplets in each other - are selectively replaced by olivine and chromite. To show how such selective nucleation/replacement mechanisms may operate in nature, I performed melting experiments in the immiscible system Si02-Al203-Fe0-K20 to which I added excess chromite (FeCr204) component (Ballhaus 1998). This system has a wide miscibility gap along which fayalitic depolymerized melt coexists stably with polymerized siliceous melt. As such, it is ideally suited to serve as a "structural analogue" for a mingled magmatic system in the mantle. Although both experimental melts were oversaturated with chromite, chromite only nucleated in the fayalite-rich melt fraction, because here the crystal/melt interfacial energy was lowest In less than three days at 1150®C, all excess chromium in the bulk system was found concentrated as cumulus chromite in the fayalitic melt droplets, producing textures on a microscale not unlike leopard ore in podiform chromite deposits. Applied to the mantle, podiform chromite deposits may outline magma conduits in the lithosphere in which basaltic melts from different pressure regimes are pooled. The nodular ore textures form when low-silica, lowviscosity, high-pressure melt mingles as droplets with more siliceous and more viscous low-pressure liquid in a common magma chamber. All melt fractions involved are saturated with olivine and chromite, however chromite nucleates with more ease in that melt fraction in which the crystal/melt interfacial energy is lowest, which is the low-silica melt. If chromite crystallization is fast relative to the tendency toward complete mixing, delicate minghng textures may be preserved. The richest chromite ores will form where the melt fraction nucleating chromite is small relative to the ambient melt.
REFERENCES Ballhaus C. 1998. Origin of podiform chromite deposits by magma mingling. Earth Planet. Sci. Lett., in press.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
PGE ENRICHMENT PROCESSES IN THE MERENSKY REEF Chris Paul Sylvester^ and David J. Ellis^ ^R.S.E.S. and ^Geology Department, ANU, Canberra ACT 0200 ^Present address: Dept. of Mineralogy , University of Tubingen (Germany) It is cx)nventionally thought that the noble metals of the Merensky reef were enriched by equilibration of magmatic sulfide liquid with primitive silicate melt To satisfy the observed enrichment factors, is was deemed necessary that the ratio of silicate to sulfide melt (i.e. the R-factor) was large, for Merensky-type deposits of the order of 100000 (Campbell et al. 1983). The problem with this model is that the Cl-normalized noble metal spectrum of the sulfide concentrate of the Merensky reef is nearly identical with the metal spectrum of the silicate parent melt, as if the PGE sulfide-silicate partition coefficients were all identical. We know this is not the case. To understand the enrichment processes we need to know how the noble metals distribute among the phases. For this purpose we have analysed sulfides, oxides, and silicates of Merensky samples with the Laser-ICP-MS microprobe. Standards used were synthetic Fei.^S doped with known amounts of a single PGE-chloride, in the concentration ranges expected in natural Merensky sulfides. The standards were equilibrated at 800®C and 1 GPa for three days in a piston cylinder press. Following annealing, aliquots of each standard were analysed with conventional solution ICP-MS to determine actual noble metal concentrations. The standards were then checked with the Laser probe for spatial PGE homogeneity. Our analyses allow us to calculate which noble metal can be accommodated in solid solution in sulfides and which metal has to form discrete PGE phases. Accordingly, Os und Ir are held easily in solid solution, since average Os and Ir concentrations in pyrrhotite and pentlandite are very close to bulk sulfide Os-Ir abundances. Ru is slightly lower in sulfides than in the bulk sulfide concentrate, suggesting that some Ru would have to occur as discrete phases. This is especially true at low fs2 when the solubility of PGE in sulfides is negligible even at high temperature. Rh is largely accommodated in solid solution in pentlandite. Some Pd and all available Pt have to form discrete PGE phases. While Pt is rejected by all sulfides, Pd is fractionated by pentlandite; however, modal amounts of pentlandite in the ore seem insufficient to accommodate all Pd. An interesting result is the common presence of PGE micro-inclusions. The most notable inclusions encountered are microalloys with the PGE combination Os-Ir-Pt. Sizes of these microalloys do not exceed 0.1 }4,m in diameter, and discrete (micron-sized) PGE phases with that combination are not known from the Merensky reef Interestingly, Os-Ir-Pt inclusions occur in all sulfides including chalcopyrite, i.e. irrespective of whether the host sulfide contains these elements in solid solution. The origin of the micro-inclusions is still uncertain. They cannot be exsolutions from the phases in which they occur, because they have also been found in phases that not contain Os, Ir, or Pt in solid solution. It is possible that they are exsolutions from high-temperature precursor monosulfides. The most interesting and currently explored option, however, is that they are magmatic in origin. If they are magmatic, then the silicate melt of the Merensky reef would have been oversaturated with respect to the refractory PGE (Os, Ir, Pt) at the time sulfide saturation. This would imply that the processes leading to stratiform PGE enrichment in layered intrusions are essentially independent of sulfide saturation, i.e. would be the consequence of bottom crystallization of the footwall cumulates below the Merensky reef Our model can explain why the PGE spectrum in the sulfide concentrate of the Merensky ore does not follow the experimentally determined sulfide-silicate partition coefficients - because the role of the sulfide melt was to capture and "immobilize" a pre-existing stratiform PGE anomaly in the stratified magma chamber that was unfractionated with respect to parent melt PGE ratios. Our model can also explain the broad correlation between the average ore grades and the thickness of the footwall cumulates which the R-factor model ignores - because the PGE of the Merensky reef originally formed part of the magma column below the ore horizon. Our model effectively rules out that the Merensky sulfides have ever seen, let alone equilibrated with, large magma volumes from the chamber above the later reef (the R-factor model). Our model survives a mass balance calculation. The sulfide concentrate at Rustenburg contains ~ 500 ppm PGE + Au. The magnesian basaltic suite in the periphery of the Bushveld Complex, chosen to represent the primitive silicate parent melt, contains - 35 ppb total PGE + Au. If we assume that sulfides constitute, on average, one volume percent of the total recovered Merensky ore, then one metre stoping width houses the PGE content of - 130 metres of parent silicate melt. This is significantly less than the average thickness of the cumulate pile between the Merensky reef and the UG-2 chromitite seam.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
STRUCTURAL CONTROL OF METAHYDROTHERMAL MINERALISATION IN THE SOUTHERN NEW ENGLAND OROGEN R.G. Barnes, R.E. Brown, J.W. Brownlow, H.F. Henley, and W.J. Stroud. Geological Survey of New South Wales, Department of Mineral Resources, PO Box 65 Armidale, NSW 2350
The Southern New England Orogen (SNEO) contains over 5000 mineral occurrences mapped and classified during metallogenic and mineral potential studies of north eastern NSW by the Geological Survey of NSW. The SNEO is remarkable for the variety of mineral deposit types and concentrations of specific minerals. Major concentrations of Sn, Mo, W, Au and Sb characterise the SNEO. Only now, using GIS, have the more subtle relationships between structure, geological setting and resultant metallogeny become apparent. The SNEO has several broad classes of mineral deposits. These include placers (which have historically been the major producers); granite-related deposits (mcluding Sn, W, Mo, Au and basemetal deposits which show unequivocal spatial, geochemical and isotopic relationships with granitoids); and quartz vein deposits (predominantly Au±Sb) which are not spatially related to granitoids. The quartz vein Au±Sb±W deposit type is the subject of this paper. We refer to this class of occurrence as the metahydrothermal deposit type, which is largely equivalent to low-sulfide quartz vein deposits, or slate belt Au deposits found world wide. Over 1 500 metahydrothermal deposits occur in the SNEO and the Hillgrove mineral field, which contains the region's largest operating mine, is the best example of the type. Mining at this field since 1878 has produced over 25 t of Au. It is also Australia's major exporter of antimony, with a total production of over 48 000 t of stibnite concentrate. This mine is currently in a major development phase. Metahydrothermal deposits (Au, Sb, Au±Sb, W, Au±Ag, Pb±Zn±Cu and Hg) are widespread throughout the SNEO and occur in mining districts and as scattered individual occurrences. The deposits show remarkable similarities of mineralogy and structural setting. Most occur as sub-vertical veins showing internal hydrofracturing textures. Open space filling is common. Faults or shears locally control all deposits. Major structures appear to have had a regional control on the general location of clusters or belts of these deposits. For example, 20% of the region's metahydrothermal Au occurrences are closely associated with the New England Suture, in the area from north of Bingara to Nundle. This structure separates subduction complex rocks in the east from ophiolites and forearc rocks in the west. The suture has been present throughout the geological evolution of New England and represents a major fluid pathway active since the Late Permian. Aubearing fluids may have been sourced from Tamworth Zone ophiolitic basement or crustal metasediments. In other places, the relationship with major structures is less evident, but again, strong spatial associations suggest the presence of deep fracture systems controlling regional fluid flows. For example, there is a concentration of deposits in a zone about 30 km wide around the margins of the Nambucca Block. These deposits include major concentrations at Hillgrove, Enmore-Melrose, Bear Hill, Coramba-Orara, Wild Cattle Creek and Magword. We suggest that thrusting of the Nambucca Block onto and into the Central Block resulted in crustal thickening and fracturing, especially along the block margins. This thickening caused increased regional metamorphism, generating metahydrothermal fluids which channelled along deep structures but ultimately deposited in tensional features lying above or adjacent to major structures. The major structural features themselves, as indicated by current geological mapping, are not the sites of major deposition. Particular crustal levels and consequently, emplacement temperature, may be responsible for the presence of the Sb-only deposits occurring in and immediately adjacent to the Nambucca Block. Mixed Au-Sb are more distant but still peripheral to the boundary of the Nambucca Block. Crustal thickening together with crustal elevation may be implicated in this process. Other metahydrothermal deposits in the SNEO are more enigmatic. Some may be related to buried granites (e.g. at Copeland), whereas others may be derived from as yet unidentified crustal fractures. In other areas such as Dalmorton, Sb is essentially absent and no major structures have been identified. Individual deposits are oriented almost randomly but deposit clusters are oriented in bedding parallel (and regional structure parallel) and transgressive corridors which may have tapped underlying regional features. At Copeland and Upper Hunter, deposits occur in transgressive shears with recent evidence by others suggesting the possibility that magmatic fluids have contributed to mineralisation. At Weabonga, a stockwork-like clustering suggests a possible localised heat source. Acknowledgement Publication is with the permission of the Director General, NSW Department of Mineral Resources.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
GEOCHEMICAL AND Nd ISOTOPE CONSTRAINTS ON SOURCES OF NEOPROTEROZOIC SEDIMENTARY ROCKS OF CENTRAL-SOUTHERN AUSTRALIA Department of Geology and Geophysics, University of Adelaide
Nd isotope data from Neoproterozoic sedimentary rocks of the Adelaide Geosyncline, southern Australia, and to a lesser extent, the Amadeus Basin, central Australia, imply a significant component of younger primitive source material than the presently exposed Early Proterozoic/Late Archean Gawler Craton and Proterozoic Arunta/Musgrave terranes, respectively. Initial e^^ values for Adelaidean and Amadeus sedimentary rocks range from -4 to -14, while the Gawler Craton values at Adelaidean time were -12 to -22, and the Arunta/Musgrave values ranged between -8 and -13. Yet, zircon geochronological evidence from both regions does not support the proposed existence of a widespread younger source terrane. The lack of younger detrital zircons in the Adelaidean sequence (as determined by the Pb-Pb Kober evaporation technique) suggests a mafic component, which did not supply a detrital zircon population to the sediments. It has been proposed that a widespread ca 800 Ma flood basalt province, formed as a result of mantle plume activity, provided the younger mafic source material required to substantially shift the initial eNd values of the Adelaidean and Amadeus sedimentary rocks away from the exposed basement values. Field evidence for this widespread flood basalt province exists in the 800 Ma Gairdner Dyke Swarm within the Gawler Craton and the Amata dyke suite in the Musgrave terrane of central Australia. To test the theory of a sedimentary mixing model between basement terrane and basaltic province to explain the anomalous z^^ values of the sedimentary rocks, Sm/Nd mixing calculations are made in order to evaluate their suitability as end-member components, and estimate the relative amounts involved of each component. An average Nd isotope composition of the Gairdner and Amata suites is used as one end-member (13.4 ppm Nd, £(800 Ma) = 3.1) and Gawler Craton and Arunta/Musgrave average Nd compositions as the other (30 ppm Nd, £(8oo Ma) = -15). The results suggest that up to 70% of a Gairdner-like component is required to explain the values of up to -4 for the Adelaidean samples. Physical sedimentary mixtures on this order should be reflected in the Sm/Nd ratios of the sediments, with isotopic Sm/Nd ratios of up to 0.1500 predicted by the mixing model. The isotopic Sm/Nd ratios of even the most anomalous Adelaidean and Amadeus sedimentary rocks are only slightly higher than the averages of their Proterozoic/Late Archean sources (0.125 compared with 0.115). Analysis of major element oxide data also does not indicate substantial contribution of a mafic provenance. Other geochemical criteria may indicate mixing with a mafic component (eg., Cr/Th vs. LREE/HREE, Th/U vs. Th, and initial 8Nd vs. Th/Sc), particularly for the Adelaidean sequence, but not to the extent required by the Nd isotope data. To further investigate the paradox highlighted by the results of the Sm/Nd mixing calculations, several finegrained samples from the Adelaidean and Amadeus sequences (including those with the most anomalous Nd isotope compositions) and one coarse-grained sample from the Adelaidean sequence, whose Nd signature indicates a local basement provenance, were washed in HCl to separate exchangeable Sm and Nd from those portions of the elements fixed in crystallographic sites in the minerals. Each component, leachate and residue, was analysed separately for its Nd isotope composition. Mass balance constraints dictate that the Sm-Nd isotope data for the leachate, whole rock (WR) and residue fall on a line on an isochron diagram. The leachates of the fine-grained samples in all cases have a higher Sm/Nd ratio than the WR, and a higher initial value at age of deposition, while the residues have a lower Sm/Nd ratio and lower initial e^id value. Mixing lines formed by the leachate, residue and whole rock for the fine-grained samples yield ages on an isochron diagram that are intermediate between the average depleted mantle model ages of the older felsic source terranes and the proposed 800 Ma mafic source province. We suggest that those phases available to dissolution during leaching, i.e., soluble minerals and adsorbed ions, reflect a preferential chemical mixing, rather than a physical mixing, of the more primitive Nd isotope signature of the mafic basalt province with the basement terrane. Treatment of the coarse-grained sample from the Adelaidean sequence resulted in no separation of leachate and residue from the WR, and it appears these components are not reflecting different sources of Nd and Sm, as is the case for the fine-grained samples.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
A GEOPHYSICAL TRANSECT ACROSS NORTHERN TASMANIA Tim J. Barton Australian Geodynamics Cooperative Research Centre, Australian Geological Survey Organisation, Canberra, ACT 2601
A crustal scale transect based on deep seismic reflection profiling, regional aeromagnetic data and satellite derived gravity data has been used to define the crustal architecture along the north coast of Tasmania. The seismic and the aeromagnetic data were acquired by AGSO as a part of a multidisciplinary study for a NGMA (National Geoscience Mapping Accord) project between the Commonwealth and the Tasmanian State Governments. Two offshore deep seismic lines combine to provide images of the crustal features across the entu-e north coast of Tasmania from the Palaeozoic Eastern Tasmanian Terrane consisting of the Mathinna Beds across the postulated Tamar Lineament and into the Proterozoic Western Tasmanian Terrane Rocky Cape Block. Many of the structures evident in the seismic data may be interpolated onshore on the basis of magnetic and gravity features observed and correlated with surface geology. The Moho is interpreted as the base of the reflective lower crust which displays significant topographic relief across the transect. From the eastern limit of the transect the Moho increases in depth from -24 km off the northern tip of the Devonian Blue Tier Batholith, which is underlain by a west dipping event down to -40 km south of Cape Barren Island Granite, to -33 km in the region of the Tiers Lineament. In the far-eastern segment of the transect the Moho is overlain by major fault structures in the near surface which are also identified on the basis of magnetic and gravity data by Gunn et al. (1997) and have underlying west thrusting structures. Further to the west underlying the Boobyalla Sub-basin there are mid-crustal east thrusting structures. This sub-basin is bounded on its western extent by a major east dipping fault along with further sub-basin extensions of the Durroon Basin. Offshore of the north-western extent of the Mathinna Beds a series of east dipping folded thrust structures from 4 to 10 km in depth extent are observed. To the north of the River Tamar some faulting is present, however as suggested by Leaman (1994) there is no evidence of the postulated Tamar Lineament. Underneath this sequence, which is adjacent to the Badger Head Block, are a number of strong reflections down to the Moho. The Forth Block is underlain by relatively non-reflective crust and is bounded to the west by volcanics extending to the north of the Housetop Granite. The Proterozoic western Tasmanian rocks of the Arthur Lineament overlie east dipping events in a weakly reflective mid-crust with a major east dipping discontinuity through the Moho. The remainder of the Rocky Cape Block is characterised by a weakly reflective midcrust with the Moho at -30 km. The western most extent of the transect has an east dipping mid-crust overlying a west dipping Moho at -33 km. At the western end of the transect the Three Hummock Island granite appears to extend to - 6 km in depth. The interpreted Moho depths across the transect are broadly consistent with crustal models based on refraction profiling (Rawlinson et al., 1998) undertaken in a co-incident experiment with the seismic acquisition. REFERENCES Gunn, P. J., Mackey, T. E., Yeates, A. N., Richardson, R. G., Seymour, D. B., McClenaghan, Calver, C. R. & Roach, M. J., 1997. The basement elements of Tasmania. Exploration Geophysics, 28(1/2), 225-231. Leaman, D. E., 1994. The Tamar fracture system - does it exist ? Australian Journal of Earth Sciences, 41, 73-74. Rawlinson, N., Collins, C. D. N., Semenova, T. O. & Houseman, G. A., 1998. Crustal architecture from seismic refraction data along the north and east coasts of Tasmania. Australian Geological Survey Organisation, Record 1998/2. Acknowledgements: TJB publishes with the permission of the Executive Director, AGSO. This paper is a component of MSc work being undertaken by the author at the Department of Earth Sciences, Monash University, Clayton, Vic. 3168
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
TEMPORAL AND SPATIAL DEFORMATION PARTITIONING DURING TRANSPRESSION, AND GOLD-TELLURIDE MINERALIZATION AT KALGOORLIE Roger Bateman', Andrew Owen^, Campbell McCuaig^ and Barry Drummond^ 'Kalgoorlie Consolidated Gold Mines PMB 27 Kalgoorlie WA 6430 ^Australian Geodynamics CRC, Australian Geological Survey Organization GPO Box 378 Canberra 2601 ^Steffen Robertson Kirsten Consultants PO Box 943 West Perth WA 6872
INTRODUCTON The Golden Mile has been mined for over 100 years, and its contained gold makes it the most valuable cubic kilometre of the Earth. It consists of 850 lodes characterized by breccias and cavity-fill vein textures in several structural styles, and with very variable gold-telluride mineralogy. It has traditionally been view^ed as having developed late (2629 Ma) in the structural history of the Yilgarn, together with most other deposits in the Yilgam, and in a left-lateral strike slip regime. Observations from domain-scale seismic reflection studies to mine-scale structural work suggest that the lodes were formed through alternating shortening and right lateral strike slip during D3 transpression. This, among other features of the deposit, singles out the Golden Mile as a very distinctive style of mineralization in the Archaean. STRUCTURAL EVOLUTION The earliest event identified in the Kalgoorlie terrane was north-south extension, forming discrete basin into which the volcaniclastics were deposited at 2685-2675 Ma. This event cannot be much older than D1 thrusting: Woods (Honours thesis, Curtin Uni, 1997) SHRIMP dated the Golden Mile Dolerite at 2675±2 Ma. The dolerite is folded in the Kalgoorlie anticline that was part of thrusting along the Golden Mile Fault. Woods also dated intrusive dacite dykes (2679±3 Ma) that postdate this folding. This constrains D1 thrusting to 2677 Ma, effectively indistinguishable from the age of extension. The Golden Mile Fault has been imaged by recent seismic reflection work done around Kalgoorlie. The D2 deformation consisted of SW over NE reverse faulting and folding, It is possible that mineralization began at this stage. There are some lodes at the northern end of the mine area that dip westward relatively shallowly, and are dislocated by the fault sets that form some of the lodes in the Golden Mile. These lodes may have formed while stratigraphy was still been subhorizontal. The importance of subhorizontal stratigraphy is that the lodes would also have been subhorizontal, and the intense dilatation of the lodes was not opposed to a l . D3 deformation consists of a transpressional regime, with partitioning of shortening and simple shearing deformation through both space and time. Faults and lodes, in both the Eastern and Western Lodes, all make an anastomosing pattern, enclosing lozenge-shaped blocks in both the Eastern and Western lodes, and are all broadly synchronous. However, the caunter and east-dipping faults may be marginally older than the right-lateral faults, or movement on late right-lateral faults continued later than on the others. The overall strain regime is bulk shortening, indicated by fault offsets with vertical and lateral extension. Strain was taken up along the right-lateral and caunter faults as lateral extension or "escape" structures, while the east-dipping faults accommodated vertical extension, during NE-SW D3 compression and lode formation and deformation. Many D3 structures merge into the basal decollement that is clearly seen, but the Bardoc Fault continues through it. This faults likely served as a major fluid conduit. Right lateral strike-slip faulting occurred along late right-lateral faults: the Australia East and possibly also the Golden Pike faults. This represents bulk shortening changing into right-lateral transpression. Late bulk shortening deformation took place with vertical extension by reverse movement along conjugate faults dipping moderately both east and west, offsetting lodes in the Western Lodes. In the Eastern Lodes, this event was responsible for the development of some bonanza lodes such as the Oroya shoot. D4 consisted of a final right-lateral strike-slip phase along faults trending north, dated by ^^Ar-'^^Ar at 2605 Ma on sericite in quartz stockwork ore. Gold mineralization in quartz stockworks is quite distinct from that at the Golden Mile, and is clearly younger. CONTROLS ON MINERALIZATION The degree and longevity of mineralization at the Golden Mile may be pardy explained by the apparent alternation between shortening and transpression through several stages of deformation, providing repeated opportunities for major dilatation. Another important implication of this work is that lode formation may have commenced before the stratigraphy was steeply tilted. In this situation, tectonic compression and dilatation are not working against each other. It is likely that optimal fluid chemical conditions were equally important for the formation of the Golden Mile. Acknowledgements: The seismic reflection work was done through the Australian Geodynamics CRC. All work was funded by KCGM. Published with permission of KCGM, Homestake Gold of Australia, and Normandy Mining Ltd.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14'^ Australian Geological Convention, Townsville, July 1998
CATHODOLUMINESCENCE AND GEOCHEMICAL PROPERTIES OF KIMBERLITIC AND LAMPROITIC ZIRCONS: APPLICATION TO DIAMOND EXPLORATION Belousova, E.A^ Griffin,
1 GEMOC National Key Centre, School of Earth Sciences, Macquarie University, NSW 2109, Australia. CSIRO Exploration and Mining, P.O. Box 136, North Ryde, NSW 2113, Australia
Zircon is a minor mineral in kimberlites, and is recognised as a member of the low-Cr suite of mantle-derived megacryst minerals. The frequent occurrence of zircon in kimberlites suggests that, by finding specific characteristic features of kimberlite zircons, it may be used as an indicator mineral during diamond exploration. However, the morphology of the kimberlitic zircon grains does not show clearly defined features peculiar to the zircon of kimberlites. Therefore, cathodoluminescence (CL) microscopy and laser ablation microprobe (LAM) ICPMS analysis were used to study the internal structure and chemical composition of zircon crystals from kimberlites of South Africa, Russia, Botswana and Australia. Zoning revealed by CL ranges from fine oscillatory to broad homogeneous cores and overgrowths predominantly in bluish colours; yellow CL colours are much less common. Samples or zones with yellow CL have higher U, Th, Y, and REE than those with blue-violet CL. We suggest that variations in the concentrations of a range of trace elements lead to different amounts of lattice defects, creating the possibility for different levels of direct excitation of luminescence centres, and therefore different CL colours. All crustal zircons studied so far exhibit only dull yellow CL colours. LAM-ICPMS data show that kimberlite zircons have distinctive trace element patterns, with well defined ranges for REE, Y, U, Th and P. Low U contents (commonly 6-20 ppm) and REE contents (Z REE < 50 ppm), as well as chondrite-normalised REE patterns with low and flat HREE, are characteristic of kimberlite zircons and distinguish them from crustal zircons. Yakutian kimberlitic zircons (Russia) are represented by 39 grains from 19 kimberlite pipes in 8 kimberlite fields. The trace element signatures of the Yakutian zircons divide them into two well defined groups, belonging to on-craton and off-craton kimberlite fields. On-craton zircons originate from areas with thick Archaean lithosphere and low geotherms required for high diamond prospectivity. The trace element patterns of on-craton zircons are similar to those of South African and Australian kimberlitic zircons. Furthermore, the trace element abundances and the slope of the trace element patterns decrease towards the inner part of the Archaean craton. Zircons from the off-craton fields, in contrast, have higher concentrations of almost all trace elements. South African kimberlitic zircons are the best represented group; 67 grains from 13 different kimberlite pipes have been studied. Their averaged abundances of heavy REE, Y, Sn, Hf, Mn, Ti and Pb are slightly higher than those of kimberlitic zircons from on-craton fields of Yakutia and Australia. However, the averaged trace element pattern of Southern African zircons is distinct (lower and flatter) from those of zircons from off-craton kimberlite pipes of Yakutia and old Jwaneng zircons (Botswana). Australian kimberlitic zircons are represented by 20 grains from two small kimberlite bodies: Orroroo, South Australia and Pteropus, Kimberley. The Orroroo kimberlite pipe has a low diamond content, while Pteropus is barren. Cathodoluminescence colours range from yellow through pink to bluish and dark violet. Trace element data are very similar for both pipes and are similar to data obtained for other kimberlite localities. Because lamproites are also known to be potentially diamondiferous rocks, lamproitic zircons from Argyle, Australia and the Kirovograd Block of the Ukrainian Shield have been used for comparison. Trace element data indicate that all studied zircons from lamproitic rocks appear to be crustal-derived.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
Sm-Nd EVIDENCE FOR MID-PALAEOZOIC REGIONAL AMPHIBOLITE FACIES METAMORPHISM IN THE STRANGWAYS RANGE, CENTRAL AUSTRALIA Betina Bendall, Martin Hand and John Foden Dept of Geology and Geophysics, University of Adelaide, Adelaide, South Australia 5005
The internal geometry of the Arunta Inher in central Australia is strongly controlled by an anastomosing system of E-W and NW-SE trending crustal scale shear zones that are defined by zones of retrograde schist which crosscut the Palaeo and Mesoproterozoic structural and metamorphic features in the inlier. These shear zones are up to 2 km wide and contain greenschist to mid-amphibolite facies schistose assemblages that formed at depths between 10 and -22 km. The major shear zones are linked by networks of smaller zones and have accommodated up to 10 km of vertical displacement during dominandy south-directed thrusting. Although there is a growing consensus that the majority of the shear zones throughout the Arunta Inlier accommodated significant shortening during the Devonian-Carboniferous Ahce Springs Orogeny (ASO) (-400-300 Ma), the timing of die peak Palaeozoic metamorphism is still uncertain. Two main reasons contribute to this uncertainty. Firstly the vast bulk of mid-Palaeozoic isotopic ages have been obtained from thermochronometers which have closure temperatures significandy lower than the peak temperatures reached in the some of the major shear zones. This is particularly the case for parts of the SE Arunta Inlier such as the Strangways Range. Secondly the recent recognition (Mawby et al this volume, Miller et al this volume) that regional granulite facies metamorphism and deformation in the Harts Range region immediately east of the Strangways Range occurred in the early Ordovician, introducing the possibility that the major shear zones formed in the Ordovician and not in the Devonian-Carboniferous as would be the case if they were associated exclusively with the ASO. In this scenario the mid-Palaeozoic ages from low-T thermochronometers could reflect regional cooling and/or reactivation of early Palaeozoic shear zones. In the Strangways Range the shear zones crosscut Palaeoproterozoic granulites and are defined by schist belts that typically contain kyanite-mica assemblages in metapelites and homblende-clinozoisite assemblages in metabasic rocks. Less commonly metapelitic assemblages contain euhedral garnet in association with staurolite, and in aluminous metabasic rocks gamet-homblende ± staurolite assemblages are locally found. P-T estimates based on the garnet-bearing assemblages are about 600°C and 6 kbar. Garnet-bearing samples from 3 locations in the Strangways Range have been dated using Sm-Nd mineral isochrons in an attempt to better constrain the age of the peak assemblages in the shear zones. In all samples garnet shows either little major element zonation or growth zoning typical of amphibolite facies recrystallisation, and occurs as a euhedral phase between 4-15mm in diameter in non reactive contact with the enclosing schistose fabric. In two samples from approximately 5 meters apart in the Winnecke region in the southern Strangways Range gt-st-bi-wr isochrons yield 312 ± 18 Ma (MSWD=0.8) and 322 ± 6 Ma (MSWD=0.7). In both cases garnet has a significantly higher Smi/Nd ratio than the other phases resulting in pseudo two point isochrons. However if the samples are combined a well spread array yields 332 ± 7 Ma (MSWD=1.3). In the Pinnacles Bore region a gt-st-bi-wr assemblage produces an isochron of 318 ± 24 Ma (MSWD=0.6), while in the Yamba Schist zone in the NW Strangways Range gt-hbd-st assemblages give 379 ± 30 Ma (MSWD=0.09). These Yambah Schist samples show an unusual inversion in Sm-Nd compositions in which hornblende is preferentially enriched in Sm with respect to garnet, resulting in a narrow spread in Sm/Nd compositions and a relatively large error. In all likelihood the inversion in composition between garnet and hornblende reflects the presence of small inclusions of REE-bearing phases in the garnet. Since the closure temperature of garnet-bearing Sm-Nd mineral-whole rock systems is likely to be > 650^^0 the ages are interpreted to represent crystallisation ages of the peak assemblage in the shear zones and confirm that regional amphibolite facies metamorphism occurred in the SE Arunta Inher during the ASO. These results have important implications for the nature of the ASO in the SE Arunta Inlier: (1) Exhumation from depths > 20 km in parts of the orogen began as late as the mid-late Carboniferous (see also Mawby et al this volume), suggesting that for much of it's duration the ASO was associated with relatively modest denudation, (2) the majority of sediment shed from the orogen is not stored in the currentiy preserved foreland basin since it largely accumulated in the Devonian, (3) Significant channelised rehydration of relatively anhydrous crust occurred at depths of > 20 km.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
SEDIMENTOLOGICAL PERSPECTIVE ON A SEISMIC VELOCITY PROBLEM: COMPOSITIONAL VARIATIONS IN MID-MIOCENE CHANNEL SEDIMENTS, OFFSHORE GIPPSLAND BASIN, SE-AUSTRALIA Thomas Bemecker^ 2 Alan D. Partridge^ and John A. Webb^ 1 Department of Earth Sciences, LaTrobe University, Bundoora VIC 3083 2 now at: Petroleum Development Unit, Department of Natural Resources & Environment, PO Box 500, East Melbourne VIC 3002
Under ideal conditions, seismic velocity in a sedimentary sequence increases consistently and proportionally with depth. However, it is common to find inversions in the velocity/depth gradients which may vary laterally, and either create structural artefacts (seismic "pull-ups") or move laterally the position of structures being mapped on seismic sections. These problems are evident in the Gippsland Basin, where a number of dry exploration wells were drilled over seismic anomalies and early wells on the giant oil fields were poorly located because of seismic velocity problems. These problems are related to a Mid-Miocene channel cutting period during the deposition of the Oligocene to Recent carbonate dominated Seaspray Group, which overlies the reservoirs at the top of the Latrobe Group. The Mid-Miocene channels eroded up to 300m into a sequence of marly wackestones and calcareous mudstones. The channels are filled with generally coarser wackestones and packstones, characterised by higher velocities than the underlying carbonates and also show considerable lateral velocity gradients. The concentration of these higher velocity sediments at channel bases and the rapid lateral changes of velocity away from the main channel axes has lead to the development of linear "High Velocity Zones" (HVZ), which produce distinctive two-way time pull-ups on seismic sections. Although the nature of the problem is now well imderstood, the inability to successfully predict the magnitude of the lateral velocity gradients continues to lead to the drilling of unsuccessful exploration wells. In order to investigate the causes of velocity changes in the sediments, a sedimentological study was undertaken examining available core from selected exploration wells, located in the hydrocarbon producing Central Deep of the basin. Cores were collected in regular intervals from below, within and above the HVZ. Detailed microscopical analyses were carried out recording allochem abundance and assemblage, grain-size variations as well as the distribution of diagenetic minerals. The variations in sediment composition were compared with wirelme-logs. In all wells there is a clear petrological difference between samples below and within the HVZ. Below the HVZ are matrix-dominated marls (clay-rich mudstones and wackestones); within the HVZ the sediments are wackestones-packstones with a significantly higher percentage of bioclastic debris. In addition, the sediments within the HVZ are coarser grained, containing fine to medium sand-sized bioclastic fragments, while below the HVZ most bioclasts are silt-sized. Apart from grain-size variations, the clay-content in the sediments appears to have a strong influence on velocity variations. ODP-results have previously shown that clay-rich marls contain flat, water-filled pores which are oriented parallel to bedding. Because seismic waves travel more slowly in water than sediment, the presence of water-filled pores slows waves travelling perpendicular to bedding. Thus, calcareous claystones are often characterised by slower velocities and this can be identified in sediments below the HVZ, which are marls with a low allochem content. However, with increasing depth, compaction will compress and eventually destroy the pores, causmg an increase in seismic velocity with depth, which is evident from the Seaspray Group. Not only are velocity variations influenced by primary sediment composition, but diagenetic products appear to be important as well. Calcite is the dominant cement, but dolomite, glauconite and siderite also occur. In all samples from underneath the HVZ, ferroan calcite cement infilled chambers and cavities within bioclasts, and the fine-gramed bioclastic debris is also composed of ferroan-calcite. By contrast, sediments within the HVZ are dominated by non-ferroan or slightly ferroan calcite. Coarser-grained channel sediments contain a higher percentage of cement, reflecting greater primary porosities; moreover, these sediments appear to host dolomite and glauconite. While it is uncertain how the ferroan/non-ferroan calcite ratio effects velocity variations, it appears that the destruction of pore-space by cementation has an influence.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
AN INFRARED STUDY OF THE HYDROXYL SITES IN OLIVINE Andrew J. Berrv and Hugh St. C. O'Neill Research School of Earth Sciences, Australian National University, Canberra, ACT 0200
The water content of the depleted upper mantle is estimated to be -200 ppm, and that of the primitive mantle to be -2000 ppm. This is based on water concentrations in MORB and the inferred composition of the mantle residue, together with the width of the 410 km seismic discontinuity. The existence of hydrous fluids, and the importance of hydrous mineral phases, in the bulk mantle, is dependent upon the amount of water (strictly hydrogen) that can be structurally accommodated in nominally anhydrous minerals such as olivine and pyroxene. The olivine water content is further in^x)rtant for understanding physical and chemical properties of the mantle, including viscosity, electrical conductivity, ionic diffusivity, and partial melting. Water in minerals is most easily detected by O-H (hydroxyl) stretching vibrations in infrared spectroscopy. Free H2O, such as in fluid inclusions, gives rise to a single, broad band, whereas H2O or OH as part of the crystal structure produces sharp peaks. The integrated peak areas for a range of hydrous minerals and glasses have been used to calibrate infrared spectra for hydroxyl content. All infrared based determinations of mantle hydrogen content use this calibratioa In olivine, hydrogen is expected to occupy point defect sites arising from, for example, a Mg vacancy. The applicability of the calibration to hydroxyl groups of this type is uncertairL Indeed, recent nuclear magnetic resonance (rmu) studies indicate that the solubility of water in olivine could be an order of magnitude greater than indicated by infrared spectroscopy. A quantitative infrared calibration requires the number and nature of the Itydrogen sites to be understood. Spectra may contain over a dozen sharp peaks corresponding to hydroxyl stretches. Factor group analysis can be used to calculate the number of bands expected for a particular site symmetry. In the case of multiple protonation of a single defect, synmietry can be used to predict coupling interactions giving rise to multiple peaks. Shifts in peak position with decreasing temperature provides evidence of coupling with low frequency modes. The preserKe of charge conq)ensating vacancies or interstitials can produce pseudolocalised vibrations, which appear as a series of equally spaced bands. Features such as this are common in the spectra of olivines. A full theoretical treatment of defect vibrational modes is being developed. Experimentally, polarised single crystal studies allow the orientation of the OH dipole to be determined. With reference to the crystal structure this can indicate the hydroxyl site. For example, a strong polarisation parallel to [100] can only be crystalographically e?q)lained by hydrogen occupying a Si vacancy, and bonded to 03 at the apex of a Si04 tetrahedron The interpretation of spectra from natural samples, however, is limited by the uncertainty of the formation conditions. This can be overcome by armealing samples under controlled pressure, temperature, oxygen fugacity, and silica activity, to generate characteristic defects which are retained on quenching. Hydrothermal treatment subsequently traps Itydrogen at the defect sites. The resulting spectra can then be analysed in terms of the expected defect structure. Oriented single crystals of San Carlos olivine (Mgi.8Feo.2Si04) are being used in this work. The effect of iron on the spectra is uncertain. This is being investigated by the synthesis of pure forsterite at 15 kbar and 1400"C. Individual crystals are large enough to be examined using an infrared microscope, ensuring that other phases do not contribute to the spectrum. Previous work has ascribed equally spaced bands to a single vibrational mode, occurring at different energies due to the statistical variations in iron occupancy- of neighbouring sites. Our spectra indicate the presence of these bands in the iron free system. Synthetic forsterite samples prepared at various silica activities display preferential enhancement of bands in particular regions of the spectrum. This result has not been previously reported. A high silica activity is expected to produce Mg vacancies on the Ml site (based on calculations of defect site energy), while a low silica activity should favour Si vacancies. The band dependence on expected vacancy type, together with the polarisation data, allow us torationalisethe origin of the various infrar^ peaks. As a calibration of peak energy with envirorunent, iron free clinohumite (Mg(0H)2-4Mg2Si04) and chondrodite (Mg(0H)2-2Mg2Si04) have been synthesised at -850"C and 30 kbar. The hydro^^l stretching energies in these materials can be correlated to bands in forsterite to help defme hydrogen bonding distances and constrain the defect site geometry.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
IS THE PALEOPROTEROZOIC MOUNT ISA BASIN A RESULT OF ASYMMETRIC EXTENSION ? Peter Betts Australian Geodynamics Cooperative Research Centre Department of Earth Sciences, Monash University, Clayton, VIC 3168
INTRODUCTION The deformed remnants of the Paleoproterozoic Mount Isa Basin extend from the eastern margin of the Leichhardt River Fault Trough to the Murphy Tectonic Ridge in the far NW terrane. Initial basin evolution began during the Mount Isa Rift Event (MIF^), the fourth major Paleoproterozoic extensional event to affect the Mount Isa terrane. The MIRE began before -1710 Ma and had ceased by -1655 Ma. Thereafter a sag basin evolved with intermittent periods of syn-depositional faulting. Basin development was interrupted at -1595 Ma by the compressional Isan Orogeny. The occurence of the MIRE rift system throughout the Leichhardt Rift and the northern Mount Isa terrane suggests that crustal extension occurred over a widespread region of the Western Fold Belt. The extensional fault architecture displays marked variation in orientation between the N-S oriented Leichhardt Rift (Leichhardt River Fault Trough) and the Northern Mount Isa Terrane (NMIT). In the Leichhardt Rift, the half graben are bounded by both N-S and E-W trending normal faults which formed during the earlier Leichhardt and Myally Rift Events respectively. In the mildly inverted (<25 % shortening) NMIT the rift system is characterised by SE thickening half graben that are bounded by NW dipping normal faults and NW striking transverse faults. Variations in the fault orientations reflect the reactivation of pre-existing N-S and E-W oriented normal faults along the Leichhardt Rift. Normal faults in the NMIT are orthogonal to the inferred NW-SE stretching direction during the MIRE.
ASYMMETRIC EXTENSION Terrane scale analysis of the Mount Isa Basin reveals a pronounced asymmetry in the distribution of syn-rift sequences and magmatic provinces. These observations have been used to apply an asymmetrical lithospheric extension model where the locus of crustal stretching is offset from the maximum sub crustal lithospheric stretching. This model explains the basin geometry during and after rifting, the location of MIRE magmatic regions, and the shift in the basin depocentre during the post-rift evolution of the Mount Isa Basin. The surficial effects of extension were focussed along the eastern Leichhardt Rift, with -3-5 kilometres of fluvial to shallow marine clastic sediments accumulating. The absence of magmatic rocks throughout the Leichhardt Rift is interpreted to indicate that a heat source associated with an upwelling asthenosphere and/or mafic underplating was relatively insignificant, and by inference, that stretching of the sub-cnistal lithosphere was minor. Consequently, tectonic subsidence associated with a thinned crust and the development of rotational tilt blocks was not counteracted by the thermal uplift effects associated with the upwelling asthenosphere and/or underplating, thus resulting in overall subsidence during the MIRE. In contrast, significant magmatism occurred along the western and north western margin of the Leichhardt Rift, resulting in the extrusion of bimodal volcanics and the emplacement of dome forming laccoliths between -17101655 Ma. Magmatism is interpreted to reflect the locus of the significant sub-crustal lithospheric thinning, asthenospheric upwelling, and perhaps mafic underplating. Magmatic regions occupied a palaeogeographic high during the early stages of the MIRE. Uplift due to thermal expansion associated with mafic underplating and asthenospheric upwelling may have counteracted similar amounts of subsidence caused by crustal thinning, resulting in a relatively stable uplift/subsidence history in this region. This is reflected in the sedimentological record by the preservation of a thinner syn-rift stratigraphy (750-2000 metres), the abundance of fluvial, coarse sandstones and conglomerates (Bigie Formation and the lower Surprise Creek Formation), and by the absence of several rift sequences compared to the eastern Leichhardt Rift. During the post-rift evolution of the Mount Isa Basin the basin depocentre shifted from the Leichhardt Rift to the NMIT. There was also a major change in style of sedimentation from siliciclastic to dominantly carbonate facies. The post-rift stratigraphy is thickest and the depositional history was longer lived (-1655 Ma to -1595 Ma) compared to the Leichhardt Rift with up to 11 kilometres of post-MIRE sequences accumulating. This shift is interpreted to reflect relaxation of the thermal anomaly which resulted in increased lithospheric subsidence above the point of maximum sub-crustal lithospheric thinning.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
CONTRASTING CHARACTERISTICS OF PROTEROZOIC AND PHANEROZOIC MAJVTLE TYPES: PRELIMINARY RESULTS. Eloise E. Beyer GEMOC National Key Centre, School of Earth Sciences, Macquarie University, Sydney, NSW 2109.
Fundamental differences in the composition of Archean and Phanerozoic lithospheric mantle have been well documented over the last decade, but little attention has been paid to the nature of Proterozoic mantle and how it compares with mantle of different ages. To establish a basis for comparison of Phanerozoic and Proterozoic mantle in Australia sampling was carried out at several localities across Tasmania. Sampling was carried out on either side of the Tasman Line, which delineates the surface expression of the younger Palaeozoic Tasman Fold Belt in the east from the older Proterozoic Central Australia Mobile Belt in the west, to determine any differences in mantle composition beneath the two tectonic regimes. Lithosphere mapping represents a methodology which allows the construction of geological sections of the subcontinental lithospheric mantle (SCLM) and follows the evolution of the lithosphere through time (O'Reilly and Griffin, 1996). The basis of lithosphere mapping in Tasmania is the direct evidence for the petrology and geochemistry of the upper mantle provided by xenoliths entrained in Tertiary basalts. Results from in-situ microanalysis for major element abundances in spinel-bearing xenoliths and their constituent minerals make it possible to determine the distribution of temperature with depth; the depth to the crust-mantle boundary; and the stratigraphic distribution of rock types Electron microprobe analyses have been carried out on xenoliths from two localities; Blessington (east of the Tasman Line) and South Riana (west of the Tasman Line). The mantle wall-rock xenoliths from both localities are exclusively Iherzolites ± spinel. EMP data has delineated two distinct geochemical populations (I0W-AI2O3 vs high-Al203 in clinopyroxene) in xenoliths from Blessington. AI2O3 content in the I0W-AI2O3 group ranges from 2.5-4.0 wt% while the high-Al203 group shows a range of 4.0-6.0 wt%. Modal clinopyroxene in the lowAI2O3 group is 2.1-2.7 compared with 4.8-13.6 in its high-Al203 counterpart. This dichotomy is also reflected in olivine composition. The magnesium numbers (mg#) for olivines from the I0W-AI2O3 group are uniformly high (91.5-91.7) and all lie in the Proterozoic field of a Boyd diagram; modal olivine ranges from 54.9-82.4%. The high-Al203 group display much lower mg# (89.6-90), have a more restricted range in modal olivine (44.555.0%), and plot in Boyd's Phanerozoic field. Preliminary work on South Riana has revealed two main xenolith types. The majority of South Riana samples are fine-grained and cpx-poor (<3.0%) with rarer coarse-grained cpx-rich (>15%) xenoliths. AI2O3 content of clinopyroxene is significantly higher in the coarse-grained samples though both types overlap the high-Al203 group of Blessington. Mg# for South Riana samples ranges from 89.2-90.3 placing them in the Phanerozoic field of a Boyd diagram. Modal olivine is higher in the fme-grained group (72.6-78.9% compared to 60.865.0%). Temperature calculations from EMP data indicate that the two Blessington groups have originatedfromdifferent thermal regimes indicating sampling from different depths. The I0W-AI2O3 group shows a range in temperature from 850-900''C compared to the high-Al203 group which has a temperature range of 925-975°C. This difference in temperatures is also seen in the two South Riana groups again indicating that sampling had occurred at different levels.. The fme-grained samples range in temperature from 880-900°C whereas the coarsegrained group are considerably hotter at 990-1020°C. The absence of garnet-bearing xenoliths at the two localities prevents direct pressure calculations. An approximate depth of origin can be found by plotting known temperatures on the southeastern Australian geotherm. The I0W-AI2O3 group of Blessington and fme-grained xenoliths of South Riana give an estimated depth of 30-35km. This depth is inferred to be the distance to the crust-mantle boundary at these two localities. In contrast the high-Al203 group of Blessington appears to have originated from depths between 40-45km and the coarse-grained South Riana groupfromdepths of 45-52km. REFERENCES O'Reilly, S.Y. and Griffm, W.L. 1996. 4-D lithosphere mapping: methodology and examples. Tectonophysics 262, 3-18.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
TIMING OF MINERALISATION AND HYDROTHERMAL ALTERATION IN THE BALLARAT GOLDFIELD Frank P. Bierlein^ Dennis C. Ame', Stafford McKnight^ and David A. Foster^ Minerals Industry Research Institute, University of Ballarat, PO Box 663, Ballarat, Victoria 3353 ^Australian Geodynamic Cooperative Research Centre, Department of Earth Sciences, LaTrobe University, Bundoora, Victoria 3083
Until recently, very few comprehensive studies of wallrock alteration associated with turbidite-hosted mesothermal gold deposits had been undertaken, leading to the common assumption that interaction of hydrothermal ore fluids with meta-sedimentary host rocks is minimal. Over the last two years, a number of investigations designed to provide a comprehensive description of wallrock alteration in central Victoria have been initiated in order to develop a set of criteria useful for exploration. In general, these investigations indicate an enrichment in CO2, S, As, Au and K2O as quartz reefs are approached, whereas SiOj and NajO are often depleted. Samplesfromrecent underground developments and deep diamond drilling collected along traverses away from the mineralised zones have been analysed by both major and trace element geochemistry, petrography, quantitative XRD, and SEM. The most obvious manifestation of wallrock alteration in the vicinity of central Victorian gold deposits is a distinct bleaching in the immediate vicinity of quartz veining (ie. up to -100m). Petrographic examinations and quantitative XRD analysis indicate that this bleaching is due to a pervasive sericitisation of feldspars within the metasedimentary host rocks, and the development of a variety of carbonate phases. Sulphides are best developed within a few 10s of metres of the main quartz vein(s). Arsenopyrite laths occur closest to the main quartz reefs, whereas pyrite porphyroblasts can extend frirther. Both phases indicate sulphidation of the host rock with the addition of As, at the ppm level, and Au, at the ppb level usually. Petrographic examinations also indicate that wallrock adjacent to major gold deposits, such as in the Ballarat goldfield, have been subjected to several generations of hydrothermal alteration. The prolonged nature of wallrock alteration is indicated by different generations of carbonate spots, some of which show evidence of stretching in the cleavage plane, and others which crosscut the regional foliation. Multiple generations of sericite growth are also inferred, although these are difficult to distinguish petrographically. In this communication, we report ^Ar/^^Ar results of separates and whole-rock samples from the Ballarat West and the Ballarat East goldfields and discuss the findings in view of detailed alteration studies and the existing geochronological data base. Samples from the Ballarat West goldfield record apparent ages of 460 - 467 and 436 ± 7 Ma, as well as minor argon loss at 395 ± 4 and 370 ± 2 Ma, whereas the apparent ages from samples at Ballarat East can be bracketed in terms of 440 - 455 and 391 - 414 Ma age groupings. Minor remobilisation of gold took place during the the emplacement of a felsic porphyritic dyke in the Ballarat West goldfield at 367 ± 1 Ma. Field relations and detailed petrographical studies suggest that the observed plateau ages and ascending-step patterns of hydrothermally altered meta-sediments are not caused by detrital K-bearing phases. These data suggest that mesothermal lode-gold mineralisation in the Ballarat goldfield occurred during two or more broad chronological 'windows', with initial and substantial ore deposit formation developing in the Late Ordovician between 440 and 455 Ma, and thus closely associated with the occurrence of peak metamorphism, folding and thrusting. Repeated reactivation of existing structural weaknesses during the Silurian and Early Devonian due to continuing development and consolidation of the Lachlan Fold Belt as well as extensive magmatism, resulted in further vein development and additional ore mobilisation and/or remobilisation of pre-existing mineralisation. The results presented herein are in agreement with findings for other major gold deposits in central Victoria and further constrain the history of deformation, metamorphism and mineralisation in the western subprovince of the Lachlan Fold Belt.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
ORIGIN OF MESOTHERMAL GOLD MINERALISATION IN CENTRAL VICTORIA; IMPLICATIONS FROM PB ISOTOPES Frank P. Bierlein^ and Neal J. McNaughton^ Minerals Industry Research Institute, University of Ballarat, PO Box 663, Bailarat, Victoria 3353 Centre for Strategic Mineral Deposits, University of Western Australia, Nedlands, Western Australia 6907
Victoria is commonly considered one of the most important slate-belt gold provinces, having produced nearly 2500 tonnes or more than 2 percent of the world's total gold production. The vast majority of these substantial quantities have been mined from a number of mesothermal goldfields located in one of three metallogenic zones which define the central Victorian region, namely the Stawell Zone, the Bendigo-Ballarat Zone and the Melbourne Zone. Gold deposits in each of these three zones display differences in mineralogy, age of host rock, and relationship to intrusive rocks. The origin of gold mineralisation in central Victoria is still poorly understood and the question remains as to whether there has been a single phase or multiple phases of gold mineralisation and whether the auriferous fluids are metamorphic or magmatic in origin. Likewise, the source of gold remains a subject of controversy. The recognition of extensive wallrock alteration haloes as well as structural information and differences in C, O, S and Pb isotope compositions between veins and host rocks point to the external derivation of the hydrothermal fluids from a deeper-seated reservoir. However, the nature and age of the basement terrane inferred to underlie the Lachlan Fold Belt remains undetermined. Various hypotheses have been put forward including derivation of the isotopically uniform fluids from Ordovician upper crustal rocks, while others argued in favour of a Middle Proterozoic mafic source. More recently presented alternatives envisage the lower crust to comprise a combination of two contrasting components, namely Cambrian greenstones and Ordovician sediments, whereby large-scale homogenisation of the ore fluids would have occurred by means of equilibration with a isotopically variable crust under rock-buffered conditions prior to fluid migration into shallower levels of the crust. In this paper, we present new Pb isotope data from three major gold deposits of distinct association located in central Victoria, and utilise existing information in an attempt to constrain the relationships between Pb isotope fingerprints and the origin of ore-bearing fluids. The Ballarat West goldfield is not spatially associated with granite intrusions, but several porphyritic dykes occur within close proximity of the auriferous structures. The Poverty Reef at Tamagulla occurs just outside the contact aureole of the Tamagulla pluton but field relations suggest (re-) mobilisation of ore metals during emplacement of the pluton. Mineralisation in the Maldon area is located within the contact aureole of the multi-phase Harcourt Granite and there are several generations of porphyritic dykes associated with the reef systems. The new Pb isotope data from these major mesothermal lode gold deposits support a model whereby the metals derived from a large reservoir with a long residence time in the crust below the Palaeozoic Lachlan Fold Belt. The Pb isotopic ratios of least radiogenic samples from these deposits are in close agreement with published Pb signatures for turbidite-hosted gold deposits, and for Devonian granites, elsewhere in the Lachlan Fold Belt. Despite their spatial distribution and variations in the geological setting, the Pb signatures point to the extraction and transport of metals from a crustal source area by long-lasting, large-scale hydrothermal systems, resulting in the prominent homogenisation of Pb isotopic ratios. The enduring interaction between large hydrothermal systems and an extensive crustal source reservoir were a vital pre-requisite in the formation of the Victorian gold province. The usefulness of Pb isotopes in mineral exploration and the prospectivity assessment of a given prospect has long been recognised and it should not be surprising that ore fluids throughout the central Victorian gold province (and in fact, the Lachlan Fold Belt of southeastern Australia) are characterised by relatively homogenised Pb signatures. In this regard, the prospectivity of Victoria is analogous to world-class ore provinces elsewhere, such as the Archaean Yilgam Block in Western Australia.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
STRATEGIC MINERAL EXPLORATION IN DEVELOPING COUNTRIES - a case study from Indonesia iRob Bills; ^ David Barr; ^ Graham Begg 1 Manager WMC Indonesia, 2 Senior Geologist WMC Melbourne, 3 Senior Geologist WMC Perth
WMC entered Indonesia in mid 1996, primarily in recognition of its geological potential following the outcome of an international study of some 36 countries. At that time the Busang discovery in Kalimantan was still a 60moz wonder and the euphoria of this discovery had overflowed into every aspect of exploration. Many junior mining companies and tenement speculators were clamouring for properties in Kalimantan, apparently little concerned for the long term ramifications of entering into unrealistic and often unsustainable deals. Clearly some of these companies lacked expertise or had no intention of mineral exploration. Others however were serious and capable explorers but like everyone else in Indonesia at that time, had to compete for properties and geoscientific staff. Was the WMC entry into Indonesia at this time good strategy or just bad timing? Although obviously a difficult period, taken over the longer term both the strategy and the timing were sound and are only now being fully realised. The WMC strategy comprised of a clear and stable purpose, consisting of discovering "world class" deposits of copper and gold, and in this respect probably similar to other major companies operating in the country. The other key strategic ingredient however was a perceived ability to create more "value" by applying better area selection criteria. The jury is still out on just how successful this has been but the overall strategy in the face of considerable pressure to "follow the pack" has thus far stood the test of time. Although the underlying driving force has been a technical approach to area selection, the increasingly complex plethora of other issues including such things as a corporate code of conduct has at times retarded progress but is now providing a sound long term operating framework. Robust area selection is paramount to adding value as it is now becoming increasingly difficult and expensive to prospect ground in the more traditional manner. The challenge therefore is to more effectively identify undiscovered mineral endowment by the integration of exploration models and targeting methodologies within the constraints of existing datasets. Our approach in Indonesia ( and Asia) has been to develop a Cu - Au model at three scales, namely the arc scale(< 1:1m), the province scale( 1:500,000 to 1:250,000) and, deposit scale (> 1:250,000). The model is deliberately broad to include variations on a common theme and extends to Cu-Au porphyry deposits (eg Batu Hijau, Grasberg, Ok Tedi, North Parkes, Kahnakyr, Santo Tomas II etc); skam CuAu ( Ertsberg and parts of Ok Tedi), high sulphidation Cu-Au ( Tampakan, Nena, Lepanto, Pueblo Viejo) and, intrusion related Au-Ag (Porgera, Murantau, Kochibulak). Based on the results to date, the level of confidence in the model and its apphcation is relatively high, providing that the targeting is underpinned by systematic data compilation, uniform and good quality primary datasets and a flexible methodology which takes into account the data type, scale, integrity and tectonic setting of the respective arcs. Thus for Indonesia where the datasets are relatively poor, it was necessary to obtain a uniform coverage of primary data to enhance our targeting capability and because of cost and climatic constraints, regional data such as Scansar Radarsat was chosen. Although the area of the targets identified from the apphcation of the model at the arc and province scale are generally large, integrated systematic empirical exploration has been very effective in quickly reducing the area of interest. Large target areas are not only preferred where data precision is questionable but also empirical exploration at this scale ensures that any variations on the porphyry or epithermal theme are caught in the exploration net. So in addition to continually refming the exploration models, effort has also been directed into empirical exploration methods such as integrating real time alteration mapping with textural and laboratory based studies of porphyry and epithermal lithocaps. The emphasis at the deposit scale has been to estabhsh a framework to effectively record and interpret the fingerprints of the various deposit types rather than rely on a specific set of static and rigid model derived parameters. Acknowledgments: The senior author gratefully acknowledges the assistance in the field and constructive feedback provided by the WMC Philippines Exploration team, in particular Frazer Tabeart, Maylene Gutierrez, Noli Nunez, William Domasig and, Dennis Francisco, thus ensuring the model evolved in a practical and applied manner. Also thanks to M.Fu for capably assisting with the silica cap decrepitation study and to the Exploration Manager of Africa/Asia, Jens Balkau for not only supporting the many field visits but also enthusiastically embracing the results of the targeting.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
USE OF SEDIMENTS TO DETECT ENVmONMENTAL IMPACT ON THE FLUVIAL SYSTEM Gavin. F. Birch'. Evette Robertson\ Stuart E. Taylor ^Environmental Geology Group, School of Geoscience, The University of Sydney, NSW, 2006, Australia.
Sediments have been used to detect sources of contaminanation in the Upper Parramatta River catchment and to establish the eflfects of land use practises on the fluvial environment. Analysis of size-normalised (<62.5 ^m) sediment show substantial enrichment of Zn (95x), Pb {19x) and Cu (12x) relative to pre anthropogenic background in aquatic sediments adjacent to industrialised areas and moderate elevations of these metals in completely urbanised subcatchments. Diffiise sources contribute minor metal loadings to fluvial sediment in even in forested subcatchments. Organochlorine/pesticide sediment concentrations closely follow heavy metal trends due to a common mixed industrial base. Eflfects-based criteria suggest adverse biological impact is possible in streams flowing through the industrial centres. This interpr^ation is supported by sequential extraction results which show a moderate proportion of total heavy metals, especially Zn, is associated with the more bioavailable and mobile exchangeable/adsorbed chemical phase of these aquatic sediments. High total suspended solids in water downstream of one of the industrial centres and high Cu and Pb particulate concentrations suggest contaminants are exiting the catchment, mainly in the soHd phase.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
SMALL-SCALE SPATLU. AND TEMPORAL VARLVNCE IN THE STUDY OF CONTAMINATED AQUATIC SEDIMENTS Gavin F. Birch^ Stuart Taylor^ and Carsten Mal&ai^ ^Environmental Geology Group, The University of Sydney, NSW, 2006.
To adequately interpret results of contaminant sediment analyses (heavy metals, organic contaminants and nutrients) in aquatic systems, it is important to have an understanding of the spatial and temporal variance associated with the data. If small scale-spatial variance is excessive, regional trends may be of limited value. Similarly, if temporal trends are large, the results of a single synoptic survey are probably questionable. Multiple samples at each location and repeated sampling over a predetermined period are required to address such problems. However, very few contaminant studies include such information because of the resources - financial, human and analytical - required often make such an approach exorbitant and beyond the reach of researches. A knowledge of the behavior of contaminants and of sedimentary processes in the aquatic environment as well as, many small scale spatial and temporal studies have led to the establishment of an intemal protocol to minimise the strict statistical requirement of acquiring repetitive data at each site. In most cases, the majority of contaminants are associated with the fine Section of the sediment and thus variance in small-scale spatial and temporal contaminant data is largely texturally driven in whole sediment analyses. If the diluent effect of the coarse fraction can be reduced, small-scale temporal and spatial variance is also reduced. There are many normalisation methods, but a common technique is to analyse a specific size range of sediment, usually the mud (< 62.5 Dm) fraction. Although some diluent remains in this fraction (silt), size normalization greatly improves data compatibility. Many small-scale temporal and spatial investigations undertaken by the Emironmental Geology Group are examined to assess the magnitude of such variance and to gain an understanding of the relationship between this type of variance and the sedimentological environment. Statistically designed marine studies offshore Sydney indicates small-scale spatial variability in contaminant concentrations in sediments is substantially less than regional trends. Considerable work has also been undertaken in the estuarine environment to understand the relationship between small-scale spatial variability- and sediment texture. Because the estuarine environment is largely depositional and afifected by low energy processes, sedimentary contaminant variability^ is low. This is especially the case for extensive deep mud basins and tidal flats. However, even in more dynamic areas e. g. intertidal areas, small-scale spatial variation is low on a sizenormalized basis. Sediment variability is most pronounced in the fluvial system and even size-normalized data are inconsistent, probably due to marked changes in the amount and composition of the silt and clay fraction of the sedimentSeveral studies have been undertaken to determine temporal change in sediments associated with the commissioning of the deepsea sewage outfalls adjacent to Sydney. Although temporal variability in contaminant data is high in the marine environment, it is not to a level, which is statistically significant. Considerable work in the sandy lower estuary environment and in shallow, muddy ofifchannel embayments of estuaries conclude that small-scale temporal variability is low. However, a temporal study undertaken in the intertidal and fluvial environments, showed temporal variation in contaminant concentrations to be greater than small-scale spatial variation. Because variance is consistent for geologically uniform areas, an altemative approach to repetitive sample analysis at each site, is to undertake small-scale spatial and temporal studies in discrete sedimentological environments in the study area after initial sediment characterization has been completed. Conceptually, this approach is similar to the geochemical practice of determining precision by repeat analysis of only a representative number of samples in a batch. The relationship between data variability and sedimentological environment can also be used to better design sampling programmes by matching sample density to the variability of geological environment. The whole issue of spatial and temporal variability brings into question the often-excessive cost and effort made in reducing analytical precision and accuracy in contaminant investigations.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
ABSTRACT: INTERPRETATION OF AEROMAGNETIC DATA IN THE BROWSE BASIN USING THE DISCRETE WAVELET TRANSFORM Robert T. Bird^ Thomas A. Ridsdill-Smith^ R. Dietmar Muller^ and Mark Pilkington'^ 'Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia, Nedlands, Western Australia 6907 "Centre for Strategic Mineral Deposits, Depanment of Geology and Geophysics, and Department of Mathematics, University of Western Australia, Nedlands, Western Australia 6907 ^Department of Geology and Geophysics, University of Sydney, New South Wales 2006 "^Geological Survey of Canada, 615 Booth Street, Ottawa, Ontario K1A 0E9, Canada
Aeromagnetic data are essential to mineral resource exploration and the application of these data to petroleum exploration is on the rise. The magnetic method's increased use coincides with significant technological advances in the acquisition, processing, and interpretation of magnetic data. For example, the production and display of horizontal and vertical derivatives of the magnetic field are conmion and quantitative interpretation methods, such as Euler deconvolution and analytic signal calculation, incorporate these derivatives. Typically, derivative calculations are conducted in the Fourier domain although the resulting data may suffer from the corruption of high-frequency noise. Calculation of derivatives higher than first order will accentuate this noise. Therefore, subsequent use of these derivatives may be compromised. Alternatively, we use the discrete wavelet transform (DWT) method of Ridsdill-Smith and Dentith (1998) which provides an efficient means for the calculation of horizontal and vertical derivatives by the diagonalization of these operators in the wavelet domain. The algorithm is stablized in the presence of noise with wavelet de-noising techniques. The results may be incorporated into subsequent applications. In particular, we are interested in quantitative methods that retrieve magnetic source positions and depths (e.g., Euler deconvolution) thus providing additional constraints on geologic interpretations. To illustrate this approach, we analyze high resolution aeromagnetic data from the Browse Basin, offshore of Australia. REFERENCE Ridsdill-Smith, T.A., and Dentith, M.C., 1998. The wavelet transform in aeromagnetic processing. Submitted to Geophysics. Acknowledgements: Data for this study was provided by World Geoscience Corporation Limited.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14* Australian Geological Convention, Townsville, July 1998
ABSTRACT: TESTING THE RODINIAN SUPERCONTINENT FIT USING CONTINENTAL POTENTIAL FIELD DATA: A NEW PROJECT Robert T. Bird^ and Walter R. Roest^ 'Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia, Nedlands, Western Australia 6907 -Geological Survey of Canada, 615 Booth Street, Ottawa, Ontario K1A 0E9, Canada
Compelling geologic and palaeomagnetic evidence has been presented by a number of researchers suggesting that the palaso-continents of East Gondwanaland (Austraha, Antarctica and India) and Laurentia (North America and Greenland) were once juxtaposed in the Proterozoic supercontinent Rodinia. It has also been suggested that this supercontinent included cratonic blocks from Siberia, South China and North China, among others. Controls on palaeogeographic reconstructions before Pangea are limited, however, and proposed models for the fit of the continents are not well constrained. In recent years, many of the Rodinian constituents have been surveyed with aeromagnetic or gravity methods (or both) and continent-wide compilations of aeromagnetic and gravity measurements have allowed large-scale interpretation and correlation within old cratonic blocks. These data also reveal geophysical signatures of major basement features that may correlate across reconstructed continental boundaries. Therefore, the principal objective of this new study is to test the proposed Rodinia models independently with potential field data. We plan to compile, analyze and compare the signatures of available aeromagnetic and gravity data across the proposed palaeo-continental boundaries of Rodinia. The final results will constrain palaeogeographic maps developed to test the predictions of the supercontinent hypothesis. In addition, the results may have important economic implications such as in the search for the missing extensions of the rich Mt. Isa—Broken Hill metallogenic belt.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
U-PB ZIRCON DATING OF THE PALAEOPROTEROZOIC ETHERIDGE GROUP, GEORGETOWN REGION: IMPLICATIONS FOR REGIONAL CORRELATIONS Lance P. Black\ Ian W. Withnalll Peter W. Gregory^ and John H.C. Bain' ' Australian Geological Survey Organisation, GPO Box, 378, Canberra, ACT 2601 ^Geological Survey of Queensland, GPO Box 194, Brisbane, Queensland 4001 ^ Consultant, 72 Tristania Way, Mt Gravatt East, Queensland 4122
The Etheridge Province in the Georgetown region of north Queensland is the most easterly extensive occurrence of Proterozoic rocks in northern Australia. It is of considerable importance to our understanding of the evolution of the continent. However, in spite of enormous advances over the last 25 years in our knowledge of the geology of the province, the depositional age of the its main component, the Etheridge Group, has remained imknown, mainly because of a dearth of interbedded felsic rocks suitable for isotopic datmg. However, a minimum estimate of 1550 Ma was provided by SHRIMP zircon dating of granitic rocks which intrude, and felsic volcanics that overlie, the metasedunents. Important new constraints on the depositional age of the sequence that are reported here have been provided by datmg earlier generations of intrusive rocks and felsic leucogneiss. Mafic rocks were emplaced into the Etheridge Group in at least two pulses, one coincidingwith the extrusive Dead Horse Metabasalt and a second as sills in the Lane Creek Formation, in about the middle of the Group. Dating the later pulse should constrain the time of deposition of the middle part of the Etheridge Group, because there are no sills above this interval. A leucogabbro band in a layered sill from the Ironhurst area yielded abundant, 1655.9 ± 2.2 Ma cogenetic zircon. No zircon of this origin was found in mafic granulite from the Einasleigh Metamorphics, the oldest part of the Etheridge Group. The zircon in that rock grew at 1554 ± 4 Ma during metamorphism late in the main regional D2-M2 event, which was also a time of widespread granite emplacement. Zircon from another amphibolite in the Einasleigh Metamorphics, at Stockmans Creek, has 1674.9 ± 3.3 Ma igneous cores and 1553 ± 3 metamorphic overgrowths. The Stockman Creek amphibolite cuts a granitic gneiss, indicating the presence of an early felsic intrusive episode not previously recognised in the Georgetown Province. Similar granitic gneiss has been recognised in at least two other localities. Junction Creek and near the.Kaiser Bill copper mine. All of the gneisses appear to be of I-type, and one is trondhjemitic in composition. SHRIMP analyses of zircon from these granitic gneisses yield crystallisation ages of 1684.2 ± 2.1 and 1695.8 ± 1.5 Ma, respectively, and provide a minimum age of about 1695 Ma for the lower part of the Einasleigh Metamorphics. Leucogneiss layers, a volumetrically minor component within the Einasleigh Metamorphics, have also been studied on the premise that they might be tuffs or at least epiclastic rocks derived from a relatively juvenile volcanic source. They have no known equivalents in the lower grade parts of the Etheridge Group to the west. Most of the SHRIMP dates for leucogneiss from the central part of the province reflect inheritance from older provenances ranging between 1800 and 2900 Ma in age. However, two samples from the eastern part of the province yield considerably younger zircon. A sample of leucogneiss from SW of The Lynd contains zircon composed of 1705 ± 7 Ma cores and 1564 ± 6 Ma rims. Another sample from S of Lyndhurst gives similar results, with 1699 ± 12 Ma cores overgrown by 1567 ± 4 Ma rims. The latter are interpreted to have grown from local partial melts within the gneisses during the onset of the D2-M2 event. The interpretation of the zircon core ages is dependant on the origin of the rocks. If they are tuffs or epiclastic sediments (possibilities that are favoured by geochemical evidence),these rocks should be no older than about 1700 Ma. However, the leucogneiss is strongly deformed and the field relationships are not well constrained. It is therefore possible that they were dykes, in which case the dates would only provide a mmunum age. In summary, the data suggest that the lower half of the Etheridge Group was deposited between about 1700 and 1650 Ma. Assuming uniform sedimentation rates, deposition may have continued to about 1600 Ma. The new data indicate that the Etheridge Group is broadly contemporaneous with sedimentary sequences that host the stratabound base-metal deposits of the Mount Isa Inlier (--1670 Ma), Broken Hill Block (-1690 Ma) and McArthur Basin (-1690 Ma).
39
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
EVOLUTION OF THE BUNGLE BUNGLE RANGE, EAST KIMBERLEY D.H.Blake Australian Geological Survey Organisatioii, GPO Box 378, Canberra, ACT 2601
The present landscape of the East Kimberley is dominated by two contrasting groups of landfomis: those of the Sturt Plateau, within the Sturt Creek drainage system, in the southeast, and those of the East Kimberley hills, mainly within the Ord River drainage system, to the northwest and north. The Sturt Plateau is a mostly flat to gently undulating plain developed on Proterozoic and Archaean bedrock overlain by a veneer of Cainozoic alluvial, colluvial, aeolian, residual, and chemical sediments, including laterite and black soils. Scattered hills and ridges of resistant Proterozoic sandstone rise above the general level of the plain. The Sturt Plateau extends eastwards, across the Tanami Desert, to central Australia. The East Kimberley hills, which include the Bungle Bungle Range, are characterised by steep-sided hills, ridges, cuestas, and dissected plateaux, all developed on Proterozoic and Palaeozoic bedrock; bevelled summit levels define an ancient land surface that can be correlated with the present day Sturt Plateau. The Bungle Bungle Range represents a gently undulating plateau, 400 - 600 m above sea level, that has been deeply incised (maximum 200 m) by tributaries of the Ord River. This incision has resulted in the formation of the spectacular gorges and clusters of steep-sided beehive-shaped hills that characterise the range. The remnants of the summit plateau within the range are part of the Sturt Plateau land surface. The range is formed of flat-lying to gently tilted sandstone and conglomerate of the Devonian Mahony Group. These rocks, the youngest rocks preserved in the Palaeoproterozoic Ord Basin, were covered by an unknown thickness, probably several thousand metres, of younger sediments before deposition in the Ord Basin ceased about 300 Ma ago with the onset of tectonic activity involving open folding, faulting and uplift. This major tectonic event, the last to involve folding and major faulting in the East Kimberley region, may be correlated with the 300 Ma main phase of the Alice Springs Orogeny of central Australia. Subsequently, the region appears to have been relatively stable tectonically, and subjected to subaerial erosion throughout the Mesozoic and Cainozoic. Vast amounts of rock were stripped from the region during this period, including the kilometres-thick sequence of Palaeozoic rocks deposited on top of the Mahony Group, prior to incision of the Sturt Plateau land surface. The Devonian sandstone forming the steep-sided beehive-shaped hills for which the Bungle Bungle Range is famous is extremely friable, readily crumbling to a powder, and is pale grey to white on fresh (unweathered) surfaces. The beehives, though, show a prominent subhorizontal banding, parallel to bedding, with dark grey bands alternating with orange bands. Each band is a few metres thick and consists of several sandstone beds. The dark grey bands are coated by cyanobacteria and the orange bands by iron oxide. Although only a few millimetres thick, these coatings protect the sandstone, which otherwise would be highly susceptible to wind and rain erosion, to the extent that the beehives remain as relatively stable landforms. The effects of cavernous weathering are clearly evident on the beehives where the protective coatings have been damaged. The grey and orange banding is not related to obvious differences in the nature of the sandstone, and is attributed to a delicate balance between the local climate and slight variations in physical properties of groups of sandstone beds. The Sturt Plateau land surface is preserved 30 km to the east of the Bungle Bungle Range, across the Ord River, on White Mountain. Here, about 300 m above sea level, it is overiapped by fluvial sediments capped by chert containing non-marine gastropods and marine foraminifera {Ammonia beccarii) indicating an age no older than Miocene. Since the deposition of the chert, probably in a coastal lagoon, a relative lowering of sea level by about 300 m has resulted in the Ord River and its tributaries cutting down into Sturt Plateau land surface to form the present-day landscape. This phase of renewed erosion possibly began in the late Miocene (at about 5 Ma), when regional upUft associated with the start of subduction of the Australian plate beneath the southeast Asian plate to the northwest coincided with a well documented worid-wide lowering of sea level by about 200 m. Evidently, the landforms of the Bungle Bungle Range, and of the East Kimberley hills in general, are less than about 20 Ma old, and may be the result of erosion during the last 5 million years.
40
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
REVIEW OF NEW DATA AND INTERPRETATIONS FOR THE PALAEOPROTEROZOIC OF THE EAST KIMBERLEY, WESTERN AUSTRALIA D.H. Blake, D.M. Hoatson and L.A.I. Wybom Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601
Results of geological mapping in the East Kimberley since 1990 by the Australian Geological Survey Organisation (AGSO) and Geological Survey of Western Australia include new digital datasets and maps, two contrasting tectonic models for the Palaeoproterozoic, and important implications for mineral exploration. The main data sets are field sites, rock descriptions, structures, geochronology, geochemistry, and mineral deposits, all of which are linked to a SITES form in AGSO's Field Geology database, and airborne magnetic and gammaray spectrometric, gravity, and Landsat data. The SITES form incorporates a unique site ID number (= observation/sample site) and its geographic coordinates. The data have been used to prepare new geological maps being released as digital data sets, on-demand hard copy maps, and lithoprinted maps. New field and geochronological data show that the Palaeoproterozoic sedimentary, volcanic and intrusive rocks of the East Kimberley range in age from -1915 Ma to -1800 Ma and were regionally deformed and metamorphosed at -1865 Ma, -1835 Ma, and possibly also 1850 Ma. The rocks are confined to the northnortheast trending Halls Creek Orogen, which is subdivided into western, central and eastern zones. The three zones are separated from one another by major sinistral strike-slip faults. No stratigraphic or intrusive units older than 1840 Ma can be confidently correlated across the bounding faults. The western zone of the orogen is characterised by -1870 Ma low- to high-grade metasediments (mainly turbiditic) and by little deformed or metamorphosed 1860-1850 Ma felsic volcanics and granites and -1855 Ma layered mafic intrusions. The central zone is dominated by 1865-1840 Ma sedimentary (mainly turbiditic) and bimodal volcanic rocks which were deformed and metamorphosed at -1835 Ma, by 1832-1810 Ma granite and gabbro, and by several layered mafic-ultramafic intrusions of which three have been dated: at 1856 Ma, 1845 Ma, and 1830 Ma. The eastern zone consists predominantly of 1915-1905 Ma bimodal volcanic rocks and overlying 1880-1845 Ma sedimentary (partly turbiditic) intercalated with mafic, felsic, and alkaline volcanic rocks; these were first deformed and metamorphosed (mainly low-grade) at -1835 Ma, during the main phase of the Halls Creek Orogeny. Some of the post 1830 Ma granites in the central zone extend short distances into the two adjoining zones. Clastic sediments and basaltic volcanics of the Kimberley Group extended across all three zones at -1800 Ma. One of the two tectonic models for the Palaeoproterozoic of the Halls Creek Orogen involves an ensialic setting. Extension, crustal thinning, and bimodal magmatism, represented in part by coeval, but not comagmatic, granite, gabbro and layered mafic-ultramafic intrusions, continued from before 1915 Ma to about 1840 Ma, with no generation of oceanic crust. This was followed by convergence, but not necessarily any subduction, at -1835 Ma. Subsequent extension was accompanied by bimodal plutonism from about 1832 Ma to 1810 Ma. In the other model the orogen represents convergence of three terranes, represented by the western, central, and eastern zones, separated by oceanic crust. Granites were generated in the western and central zones during and after northwestward subduction of oceanic crust. The central zone collided with the western zone at -1850 Ma, and the eastern zone collided with the combined central and western zones at -1820 Ma. Implications for mineralisation in the East Kimberley include: A, the layered mafic-ultramafic intrusions of the Halls Creek Orogen, unique in the Palaeoproterozoic of Australia, are prospective for several styles of platinum-group-elements, Cr, Ni, Cu, Co, and Au deposits; B, the granites are mainly unfractionated, in contrast to those associated with economic mineralisation elsewhere in the Proterozoic; C, alkaline volcanics in the eastern zone are prospective for rare earth elements and associated metals and also for gold; D, volcanogenic massive sulphide mineralisation is associated with subaqueous fractionated felsic volcanics in the central and eastern zones, but not with felsic volcanics in the western zone as these are subaerial and unfractionated; and E, no unconformity-type U deposits are known in basal sandstone of the Kimberley Group, in contrast to similar sandstone of comparable age in the Pine Creek region, perhaps because of a lack of mineralising oxidised saline brines.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE YARROL FORE-ARC BASIN: A COMPLEX SUITE OF VOLCANIC FACIES AND ALLOCHTHONOUS LIMESTONE BLOCKS Paul R. Blake. Glenn A. Simpson, Barry G. Fordham and Mark A. Hayward Geological Survey of Queensland, PO Box 194, Brisbane, Queensland 4001
The Upper Devonian to Lower Carboniferous volcanic and sedimentary rocks of the Yarrol Province have been re-examined with an emphasis on their lithofacies. Previous subdivisions have tended to be largely time units recognised over restricted parts of the provmce. The new approach not only results in units which are more readily mapped over the entire Province but also allows the components of the Yarrol fore-arc to be more sensibly differentiated. The units proximal to the arc are called the Mount Hoopbound Formation (formerly Dee Volcanics and Boulder Creek Grit) and Balaclava Formation (formerly Pond Formation). Both of these formations were deposited mainly in a shallow marine environments and are dominated by volcaniclastic sedimentary rocks, with lesser ignimbrite, siltstone, and lava flows. The Mount Hoopbound Formation is typified by andesitic volcaniclastics including conglomerates which are commonly up to cobble and boulder in grainsize. The Balaclava Formation is composed of rhyolitic volcaniclastics, and is usually finer grained than the Mount Hoopbound Formation, with conglomerates typically only in the granule to pebble range. Deposits in the fore-arc basin which are distal to the arc are assigned to the Mount Alma Formation and are dominated by siltstone, fine-grained sandstone, and granule to pebble conglomerate. The siltstone and finegrained sandstone are usually thinly interbedded, and commonly rhythmically interbedded. These rocks contain only rare plant stems and sparse trace fossils, but thin section observation reveals abundant microfossils which are possibly radiolarians. Ripple laminations are rare, but soft sediment deformation features are prevalent. The conglomerates are commonly poorly sorted, some fine upwards, and typically contam abundant rip-up clasts. Rare conglomerates contain sparse brachiopods, crinoids and corals. As deposition of the Mount Alma Formation commenced in the distal parts of the Yarrol fore-arc, material, including blocks of limestone, was eroded from the pre-Late Devonian volcanic-dominated units. The allochthonous nature of these blocks of limestone was originally recognised by the difference in fossil ages. Conodonts and corals from the limestone blocks indicated an Early Devonian age, but brachiopods collected from conglomerates at the base of the formation are Late Devonian in age. Therefore, this revealed that there are Early Devonian limestones within a Late Devonian sequence. Detailed work around some of the limestones in the Bracewell area revealed that the bedding in the limestones was not parallel to the bedding of the surrounding sediments, supporting the theory that they are allochthonous blocks. These allochthonous limestone blocks occur around the Bracewell, Mount Etna, and Boynedale areas. In the Bracewell area, the coral and conodont faunas within the blocks, with ages close to the Lower Devonian-Middle Devonian boundary, are identical with those within the Mount Holly beds, strongly suggesting that this unit is the source. Similarly, the presumed allochthonous limestones around Boynedale would appear to have been derived from the Calliope beds based on similar conodont dates. In the Mount Etna area, the limestones are Lower Devonian, and given their considerable extent, their most likely source is the Mount Holly beds to the south, or possibly the Craigilee beds to the west. The size of the allochthonous limestone blocks is highly variable. They occur as pebbles and cobbles in conglomerate, and others form outcrops tens to hundreds of metres long. The rocks around the Mount Etna limestone mine show the same relationships, therefore this limestone, which is 2.8 km across, is also interpreted to be an allochthonous block. In the central Yarrol area, the units proximal to the arc are separated from the distal unit by the Early to Middle Devonian Capella Creek Group, and in the southern part of the Calliope Range, the Mount Alma Formation and the Balaclava Formation crop out within about 2 km of each other, but are still lithologically distinct. This suggests that the Capella Creek Group may have formed a palaeotopographic high during the Late Devonian, separating the two environments. In its western extent, the Mount Alma Formation possesses beds which appear to have received some volcanically-derived material similar to that forming the Balaclava and Mount Hoopbound Formations, but mostly the Mount Ahna Formation appears to reflect a distinct provenance.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14'^ Australian Geological Convention, Townsville, July 1998
A RE-EVALUATION OF MINERALISED ORDOVICIAN INTRUSIVES IN THE LACHLAN FOLD BELT: IMPLICATIONS FOR TECTONIC AND METALLOGENIC MODELS. Phillip Blevin ARC National Key Centre for the Geochemical Evolution and Metallogeny of Continents Geology Department, Australian National University, Canberra ACT 0200
Ordovician porphyry-style Cu-Au mineralisation in the central Lachlan Fold Belt is related to intrusive complexes of diverse compositional character. The intrusive suite at Copper Hill is medium-K calcalkaline in character, those at Goonumbla and Rain Hill are transitional from high-K calcalkaline to "shoshonitic" (in terms of K2O versus Si02), while the intrusive complex at Cadia is "shoshonitic". The intrusive suites at Cargo and Lake Cowal appear to be medium-K calcalkaline in character. Surprisingly, of all these suites examined in this study, only that at Cadia is actually "potassic" in the strict sense (i.e. molecular K/Na > 1). All the intrusive suites are silica near-saturated to saturated, with magmatic quartz being more abundant (and appearing paragenetically earlier) in the medium-K suites. Plagioclase phenocrysts in the more K-rich suites are commonly mantled by alkali feldspar, a result of peritectic reactions during crystallisation, and not high temperature K metasomatism. Pyroxene compositions are typical of calcalkaline arc rocks in terms of Ca-FeMg end member components and Al-Ti relationships. Mineralogical evidence for alkaline and/or silica undersaturated magmatic affinities are absent from all the intrusives. Incompatible trace element abundances are significantly lower than for other "shoshonitic" rock suites from various tectonic settings, and most closely match those from arc related medium- to high-K calcalkaline rocks. The relative abundances of incompatible trace elements are proportional to the K content of each suite with the exception of Rain Hill. K2O contents in the K-rich suites increase rapidly with increasing Si02 compared to changes in K2O content in the medium-K suites. Internal differentiation within the suites is controlled by fractional crystallisation and cumulate processes dominantly involving plagioclase and clinopyroxene. Low to very low Ni contents in all suites are consistent with early olivine removal. Eu anomalies do not increase significantly despite plagioclase fractionation, presumably due to high magmatic /O2 conditions. The Ordovician intrusive suites (including similarly aged granitoids) are chemically distinct from granitoids of Silurian and Devonian age in having lower Rb, Y and HREE, and higher Sr and K/Rb at any given Si02 value. The systematic relationships between petrography, mineralogy, rock composition and inter-element ratios argue persuasively against any one of these features (e.g. quartz phenocrysts, graphic groundmass textures, low K contents etc) being due to post crystallisation alteration, metamorphism etc. The mineralised Ordovician intrusive suites show a systematic progression in petrographic and compositional character from rocks of indisputable medium-K calcalkaline affinity to high-K calcalkaline affinity, and finally to relatively more enriched rocks. Given their relationship to calcalkaline rocks, these enriched equivalents would be better termed as "very high-K calcalkaline" instead of "shoshonitic". The use of the terms "shoshonite" or "shoshonitic", even if correctly applied using lUGS classification criteria, obscures the genetic relationship between such rocks and their contemporaneous, less K- and incompatible element-enriched calcalkaline equivalents. These results have implications for metallogenic and tectonic models for the LFB. Porphyry Cu-Au style mineralisation is commonly associated with arc-related calcalkaline magmatism, and the Ordovician of the LFB provides no exception in that regard if indeed contemporaneous subduction was present. The occurrence of CuAu mineralisation in suites of diverse compositional character suggests that models linking the metallogenic potential of these suites to incompatible element enrichment processes requires critical re-evaluation. Similarly, models arguing against the existence of contemporaneous subduction during the Ordovician on the grounds that "normal" calcalkaline rocks are absent need to be reassessed. Given the above results for the intrusive components of the Ordovician System in NSW, a contemporaneous subduction scenario cannot be discounted on these grounds. Acknowledgments: This work was undertaken as part of AMIRA project P425. Project sponsors are thanked for their support and their generous permission to publish.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
POTENTIAL APPLICATIONS OF LAM-ICP-MS TECHNOLOGY IN ECONOMIC GEOLOGY: A PRELIMINARY STUDY OF MOLYBDENITE AND PYRITE. Phillip Blevin^ and Simon Jackson^ ARC National Key Centre for the Geochemical Evolution and Metallogeny of Continents ^Geology Department, Australian National University, Canberra, ACT 0200 ^ School of Earth Sciences, Macquarie University, Sydney, NSW 2109
The laser ablation, microprobe (LAM)-ICP-MS offers enormous potential in advancing trace element studies of minerals and materials through significantly improved detection limits for in situ analysis, and the ability to generate profiles through minerals by progressive ablation. Molybdenite (M0S2) and pyrite from several ore deposits were examined to ascertain trace element occurrences and abundances, evaluate problems relating to inclusions and zoning, characterise the suitability of M0S2 samples for Re-Os dating, as well as exploring more general applications in ore deposit studies. The results of the M0S2 analytical campaign are reported on here. The sulfides were analyzed in 100 micrometer thick sections using a UV laser ablation microprobe (LAM)-ICPMS at Macquarie University. In the absence of a suitable sulfide reference material, standardisation was performed using a silicate reference material, NIST SRM 610, with a nominal concentration of 500 ppm of most elements. Differences in ablation yield between analyses were corrected by internal standardization using Mo (M0S2) and Fe (pyrite) concentrations, which were calculated from stoichiometry or had previously been determined by electron microprobe microanalysis. This protocol is expected to give semi-quantitative results. Indeed, averaged Re contents determined by this method agreed well with those obtained by previous workers using M0S2 separates from the same deposits. Data were collected using a time-resolved data acquisition protocol, allowing signals to be examined for assessment of zoning and to be selectively integrated to minimize contributions to the signal from inclusions and host minerals. The elements analysed were Si, Ti, Cr, Fe, Ni, Cu, Zn, As, Se, Zr, Nb, Mo, Pd, Ag, Sn, Sb, Te, W, Re, Au, Pb and Bi. Samples were sourced from numerous East Australian occurrences as well asfromoverseas porphyry Mo and porphyry Cu systems. TRACE ELEMENT BEHAVIOUR Re, W, Se, Te, Bi and possibly Sn were found to comprise important lattice substitutions in M0S2. This is evidenced by the high degree of correlation of these elements with Mo. Zonation, particularly of Bi, was present in some instances. More commonly, Bi correlates with either Pb, or with Se and Te. This is consistent with the occurrence of discrete inclusions of galenobismutite and cosalite (Bi-Pb), ikunolite (Bi-Se), joseite and tetradymite (Bi-Te), bismuthinite and native bismuth. Transient correlations of Sb and Cu with Bi suggests the presence of bismuthinite-stibnite and bismuthinite-aikinite solid solutions. Ablation of M0S2 crystals oriented both parallel and perpendicular to the c axis suggest that most inclusions occur along cleavages, but an exsolution origin for some of the inclusions cannot be ruled out. Au is present as isolated Au inclusions and in Bi-bearing inclusions. Pd also reports in some samples. High Sn registrations at the commencement of ablation runs represent surface contamination during polishing. DEPOSIT AND METALLOGENIC STUDIES M0S2 from chalcophile dominated (porphyry Cu-Au) deposits have higher Re, and lower Bi, W and Sn contents and Se/Te ratios than M0S2 from lithophile (Mo, W, Sn) dominated deposits. Re and W contents are sensitive indicators of the degree of compositional evolution of the related igneous rocks, and thus provide a useful genetic fingerprint. The systematic variation of Re contents in M0S2 from different deposit types has implications for understanding the occurrence of Mo and Au in otherwise lithophile dominated ore deposits (and related granitoids) that are also highly depleted in other siderophile and chalcophile elements. In particular, the results imply that Re becomes uncoupled from Mo during magmatic differentiation. Significant Au deposits are known to be related to these igneous rocks. ASSESSMENT OF MOLYBDENITES FOR Re-Os DATING A suggested test for assessing M0S2 samples for Re-Os dating involves comparing their Re contents with their structural polytype. "3R" M0S2 has been assumed to grow via a screw dislocation mechanism triggered by incorporation of Re. By inference, low Re "3R" M0S2 crystals are inferred to have lost Re subsequent to crystallisation, and therefore potentially may yield unreliable Re-Os dates. Results of this study suggest that W and Bi may also occur as lattice components within M0S2. The degree of structural mismatch of Bi, W and Sn within the M0S2 lattice are on par or higher than that for Re, thus the presence of these elements during crystallisation should trigger 3R polytype formation. Under these circumstance there should be no (a priori) reason to doubt the veracity of dates obtained from low Re, "3R" M0S2 samples obtained from lithophile dominated deposits. 44
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
M"" Australian Geological Convention, Townsville, July 1998
IGNEOUS RELATIONSHIPS AND ZONING CHARACTERISTICS OF THE >1.4 KM DEEP GOLD-BEARING KIDSTON BRECCIA PIPE, NORTH QUEENSLAND, AUSTRALIA Renato Bobis', Edward Gary Rowe^ and Gregg Morrison' '60 Bumda St., Kirwan, Townsville 4817 ^77 Armstrong St., Hermit Park, Townsville 4812 '7 Mary St., West End, Townsville, 4810
The deep drilling program (> 10 kms) to test gold mineralisation below the present orebodies at the Kidston breccia pipe and the deeper mining operations have provided a 1.4 km vertical exposure of the pipe and revealed unequivocal paragenetic relationships between igneous phases, brecciation and gold mineralisation. The various intrusive phases, mainly rhyolitic in composition, span pre- to post-breccia development and occur as coalescing rhyolitic plugs and dykes (both confined to, and cutting the pipe) and as clasts within the breccia pipe. A coherent pre-breccia intrusive occurring below the Wises orebody (350m below surface) generated its own potassic and phyllic alteration and hosts Mo, As, Cu and gold stockwork mineralisation and irregular tourmaline veins and cemented breccias. There is an apparent textural zonation inwards, from a margin of sparse porphyry to heteregeneous porphyry to a core of'crowded' porphyry. Careful documentation and reconstruction of igneous and matrix composition within the pipe pointed to the main stage breccia to be related to a 'normal' quartz feldspar porphyry phase. A post-breccia intrusive phase, intimately associated with the main gold mineralisation event, consists of a quartz feldspar porphyry dyke with branching arms. The final magmatic phase is the intrusion of andesite dykes which cut mineralisation. Post-breccia pipe modification is manifested by the vertical zonation of mineralisation styles, vein-alteration mineralogy and metal distribution. The uppermost zone is barren (carbonate-pyrite-pyrrhotite), followed by 200250m thick Au-basemetal sulphides domain consisting of sheeted vein and cavity infill mineralisation (i.e.,orebearing zone). This is followed by a zone of pyrrhotite-dominated cavity infill mineralisation, and then a lower sheeted vein zone starting at around 700 meters below surface and hosting pyrrhotite, base metal sulphides, molybdenite, fluorite, bismuth, wolframite and scheelite. The ore deposits at Kidston are confined between an inferred breccia roof and the disc-shaped quartz feldspar porphyry sills. High grade orebodies occur at the structural intersections characterised by subhorizontal and moderate to steep fracture sets. Higher grades occur at the intersection of these two sets in dilational zones in the northeast and southwest comers of the pipe. The mineralogical and element zoning patterns reflect an upward declining thermal gradient from a polymetallic system with a Mo (W, Cu) core. The evolutionary model for the principal gold mineralisation includes: (i) the Kidston intrusives represent volatile-rich apophyses of the much larger magma chamber; the predominance of synbreccia 'normal' porphyry throughout the pipe (increasing abundance towards the center and with depth) suggest that exsolution and crystallisation caused overpressuring of the system, ultimately leading to brecciation.(ii) magmatic fluid flow and fluid confinement in the pipe was essentially coeval with postbreccia quartz feldspar porphyry intrusion; (iii) heating and overpressuring of fluids in equilibrium with basemetal sulphides-Au in the upper portion of the pipe; (iv) decompression effects as the magma chamber feeding the postbreccia quartz feldspar porphyry dyke and sills collapse leading to the principal gold precipitation; (v) post mineralisation modification including andesite dyke intrusion, normal faulting and erosion.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THERMAL EVOLUTION OF THE CENTRAL HALLS CREEK OROGEN, W.A: FIELD, PETROLOGICAL AND GEOCHRONOLOGICAL CONSTRAINTS Simon Bodorkos^ Peter A. Cawood^ and Nicholas H.S. Oliver^ ^Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth WA 6845 ^Economic Geology Research Unit, School of Earth Sciences, James Cook University, Townsville QLD 4811 The tectonothermM evolution of many Proterozoic orogenic belts remains poorly understood, often due to a paucity of good-quality geochronological data. This complicates the placement of absolute "time-pins" in a geological history, and low-precision data place few constraints on the duration of individual thermal events. The Palaeoproterozoic (-1920-1800 Ma) Halls Creek Orogen of northern Australia provides a well-exposed collisional interface between the Kimberley and North Australian cratons, and comprehensive regional U-Pb SHRIMP zircon geochronology has recently been carried out as part of the AGSO-GSWA Kimberley-Arunta Mapping Accord, augmented by our own work. The central part of the belt comprises well-exposed high-temperature, low-pressure (HTLP) metasedimentary rocks (mostly pelitic to psammopelitic gneisses and migmatites) which have been heavily intruded by layered mafic bodies and voluminous tonalitic and granodioritic plutons. Peak metamorphic temperatures are in the range ~650-800°C at pressures of 300-400 MPa (Thomett 1986). Evidence for significant mantle-related magmatic activity commencing at -1880 Ma is contained in three huge layered mafic intrusions emplaced in the middle crust in the interval 1860-1855 Ma (Page et al. 1995a). Maximum depositional ages of the metasedimentary sequence are defined by detrital zircon suites with U-Pb SHRIMP ages of 1865 ±2 Ma (Page et al 1995a) and 1864 ±4 Ma (Bodorkos et al, unpubl. data). A pretectonic, strongly foliated and recrystallised garnet-bearing leucocratic granite (U-Pb SHRIMP age 1850 ±2 Ma; Page et al 1995b) intruded the sedimentary layers, producing a ~ 100-metre wide pyroxene-homfels contact aureole in the host metacarbonates (despite its low solidus temperature) which is recognisable despite subsequent regional HTLP metamorphism, indicating that the host rocks were cold (<350°C) at the time of granitoid emplacement. Timing of peak metamorphism is constrained by a conventional U-Pb age of 1845 ±3 Ma (Oliver et al, unpubl. data) from monazites in stromatic migmatites, and an identical U-Pb SHRIMP zircon age of 1845 ±4 Ma (Bodorkos et al, unpubl. data) from metamorphic overgrowths on detrital zircons in an upper-amphibolite facies psammopelitic gneiss. A layered mafic body (U-Pb SHRIMP age 1841 ±3 Ma; Trudu & Hoatson 1996) intrudes the sequence, cross-cutting the regional gneissosity (S2) and triggering large-scale re-melting of pelitic wallrocks. Repeated mafic magma injection has resulted in sheath- and vein-type leucosomes which contain zircon overgrowths with U-Pb SHRIMP ages as young as 1835 ±3 Ma, suggesting that mafic magmatism has played a key role in prolonging elevated temperatures in the host rocks over an interval as long as 10 million years. Nearby, intrusion of the large Mabel Downs Tonalite (U-Pb SHRIMP age 1832 ±3 Ma; Page et al 1995b) followed immediately. The pluton was emplaced under mid-amphibolite facies conditions, synchronous with D3 shearing and folding structures which overprint S2 fabrics. From the same suite, the Sally Downs Supersuite (U-Pb SHRIMP zircon age 1821 ±4 Ma; Page et al 1995b) was emplaced synchronous with macroscopic open F4 folding. The limbs of these folds are truncated by post-1820 Ma retrograde shear zones. This proposed thermal history features protracted HTLP metamorphism at the exposed mid-crustal level, with wallrock temperatures remaining above ~5(X)°C throughout the interval -1850-1820 Ma. The prolonged elevation of the crustal geotherm is a likely response to a major mantle melting event 30-40 million years earlier, resulting in the emplacement of voluminous layered mafic bodies in the mid-crust -10 million years prior to peak metamorphism, combined with subsequent repeated mafic magma injection. Potential thermal models for processes resulting in voluminous mafic plutonism coupled with protracted HTLP metamorphism include asthenospheric upwelling resulting in convective lithospheric thinning, detachment or delamination and mafic underplating of the lower crust. REFERENCES Page R.W., Hoatson D.M, Sun S. & Foudoulis C. 1995a. High-precision geochronology of Palaeoproterozoic layered mafic-ultramafic intrusions in the East Kimberley. AGSO Research Newsletter 22, 7-8. Page R.W., Tyler I.M. & Blake D.H. 1995b. Geochronology of magmatism and high-grade metamorphism, Kimberley region, W.A. Australian Conference on Geochronology Abstracts 3, 25. Thomett J.R. 1986. Evolution of a high-grade metamorphic terrain in the Proterozoic Halls Creek Mobile Zone, Western Australia. PhD thesis, University of Western Australia, Perth (unpubl.). Trudu A. & Hoatson D. 1996. Depths of emplacement of Precambrian layered intmsions in the East Kimberley, AGSO Research Newsletter 25, 10-12.
Tyler I.M. & Page R.W. 1996. Palaeoproterozoic deformation, metamorphism and igneous intrusion in the Central zone of the Lamboo Complex, Halls Creek Orogen, Western Australia. Geological Society of Australia Abstracts 41, 450.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
BIOFILMS AND THEIR ROLE IN CARBONATE PRODUCTION: AN EXAMPLE FROM THE COORONG LAGOON, SOUTH AUSTRALIA Yvonne Bone^ and Joachim Scholz^ 'Dept of Geology and Geophysics, University of Adelaide, South Australia, 5005 - Universitaet Hamburg, Geologisch-Palaeontologisches Institut/Bryozoentnim (Coll. E. Voigt), D-20146 Hamburg, Germany
Calcareous benthic invertebrates are the major producers of carbonate sediments in the marine realm in all environments. There are two major methods whereby these organisms reproduce: (1) by various types of budding processes (both forward or self-overgrowth) or (2) by sexual reproduction. The former leads to extremes in spatial competitiveness whereas the latter allows for the dispersal of the species into new areas. This presentation highlights a vital element, missing in the literature - namely the role of biofilms in these processes. Epizoic microbial mats (biofilms) play a major role in producing a substrate that is suitable for colonisation by the motile larvae of many diffrerent epibenthic invertebrates. Previous experiments show that a totally inert substrate is not readily sensed as a potential settlement site and so is passed over by the chemo-sensory invertebrate larvae. It is true that eventually an inert surface will be colonised, but this only occurs once all favourable sites are already occupied or simply by chance. There is a marked preference for settlement sites on surfaces that are bioactive. The living invertebrates themselves potentially provide ideal settlement surfaces, but most of them have developed successful physical and/or chemical anti-fouling mechanisms whereby they are sensed as being unacceptable. The less sophisticated micro-organisms lack these anti-fouling mechanisms. Our work has focussed on bryozoans, particularly those using an encrusting architecture for their colonial growth, i.e. laminations or sheets. These forms use a number of different budding mechanisms whereby the zooids at the growing margin of the colony "creep" across new substrate or self-overgrow, thereby becoming multi-laminar. Settlement of their larvae and the larger, more primitive and longer lived cyphanaute larvae of species such as the Coorong Lagoon bryozoan, Conopeum aciculata, in particular, are released into the ambient waters, where they must search for a suitable substrate. The Coorong Lagoon studies show that a variety of phototrophic and autotrophic bacteria, diatoms, microalgae and fungi are intimately associated with C. aciculata. This assemblage is always present wherever new colonies are found, at the growing edges of old colonies and coating pre-existing layers. As to be expected, the species composition is typical of an hypersaline environment. Four major genera of cyanobacteria dominate, namely Pleurocapsa, Myxosarcina, Leptolyngbya and Microcoleus. Some of the benthic cyanobacteria also produce large volumes of extracellular slimes which are extremely viscous polysaccharide solutions. These result in a tough "slime mat" that not only provides an attractive settlement site or overgrowth site, but also acts as a stabilising agent and as a sediment trap, thus enhancing the accumulation of carbonate and other particles. Scanning Electron Microscopy has revealed the presence of these fine biofilms between the layers of bryozoans, in the Coorong Lagoon C. aciculata build-ups and in similar marine forms in the Great Australian Bight, e.g. Celleporaria sp. Scholz has previously shown that non-settled panels are sterile areas. It is anticipated that further studies will show that the presence of biofilms is a critical requirement in the colonisation of substrates, thereby elevating them to the status of being a major player in the production of carbonates.
47
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
CUSTOMISED ARCVIEW MEETS LAW OF THE SEA PROJECT REQUIREMENTS Irina Borissova', Robin Gallagher^, Philip A. Symonds' ^Australian Geological Organisation, GPO Box 378, Canberra, ACT 2601 ^GlSolutions, GPO Box 2237, Canberra ACT 2601
Over the past two years the Australian Geological Survey Organisaion's (AGSO's) Law of the Sea project has been developing new strategies for handling large amounts of diverse digital data traditionally managed within in-house specialised applications such as Petrosys and Geoquest. The work of the project is based on analysis of geophysical data collected along ship tracks including bathymetry, gravity, magnetics and seismic. There are nine areas around Australia where seabed jurisdiction of the 'legal' Continental Shelf could be extended beyond 200 nautical miles provided the claim is justified according to the terms set out in the 1982 United Nations Convention on the Law of the Sea (UNCLOS). For each of the areas there is digital data from about a hundred surveys, and the number of records in each survey varies from a few thousand to about one hundred thousand. The data necessary to support Australia's Continental Shelf claim need to be processed, interpreted and presented to the recently established UN Commission on the Limits of Continental Shelf within the next six years. The claim has to be supported by bathymetric profiles across the continental slope showing foot of continental slope (FoS) picks. Where additional proof is required to justify FoS picks, bathymetric profiles may be accompanied by corresponding seismic sections and/or gravity and magnetic profiles. In some areas sediment thickness derived from seismic data may also be used to extend the Continental Shelf claim. When the claim is presented to the UN commission it is important to be able to quickly access any particular crossing of the continental slope, to get attribute information on each point on the profile and to evaluate positional accuracy of the data, in respect to the navigational systems used on the surveys. These requirements are difficult to meet interactively without using a GIS application. Here we present the results of customising Arc View to enable us to produce an integrated system, where profiles for all geophysical parameters have attribute information available for each data point. Seismic sections are hotlinked to corresponding profiles. Parts of survey lines representing selected profiles can be viewed on a map together with the picked FoS points. General survey information obtained from AGSO's OZMAR ORACLE database can be added to Arc View tables enabling queries to be made regarding availability of geophysical data and navigational systems. The module has been built as an Arc View extension and could have more generic use. Viewing parts of surveys lines as profiles, and the ease of tagging any point on the profile and correlating it to available spatial information such as gravity and magnetic images, and regional geology, makes this an attractive tool in any geological interpretation based on geophysical survey data.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
TECTONIC SETTING AND BASIN DISTRIBUTION OF THE SOUTHERN LORD HOWE RISE REGION Irina Borissova^ and Philip A. Symonds' ^Australian Geological Organisation, GPO Box 378, Canberra, ACT 2601
During the last five years the Australian Geological Survey Organisation (AGSO) has used its vessel RA^ Rig Seismic to acquire over 7000 km of deep-seismic data (16 second record length) in the southern Lord Howe Rise region. This data set consists of 3200 km of 1992 seismic lines over the area between 32° and 38° south, including two regional transects across the whole province, from the Tasman Basin to the South Norfolk Basin; and a series of 1996 lines over the southwestern margin of Lord Howe Rise (LHR) and the New Caledonia Basin (NCB), that were acquired during collaborative AGSO/New Zealand Ministry of Commerce Law of the Sea surveys in the LHR - Norfolk Ridge (NR) region. These are supplemented by older seismic data such as that collected by United Geophysical in 1970, Mobil in 1972, and the Bundesanstalt fiir Geowissenschaften und Rohstoffe in 1979. This study mainly focuses on interpretation of the 1992 data, but uses the 1996 and older data to correlate tectono-stratigraphic events throughout the whole LHR-NR-New Zealand region. The results of the seismic interpretation were incorporated into a GIS enabling basin locations and style, major faults, and the distribution of volcanic features and planated basement to be readily compared to satellite gravity data. This approach provided a means of extrapolating the major tectonic elements away from the sparse seismic data set, and enhanced our understanding of basin development and distribution in the region. LHR is a large continental block which separated from eastern Australia during the period of Late Cretaceous (83 Ma) - Paleocene (52 Ma) seafloor spreading that resulted in the opening of the Tasman Basin. The early opening of the Tasman Basin was a relatively complicated process involving a number of microplates, and occurred in a stepwise- fashion by means of rift propagation from south to north. Intracontinental extension commenced in the region in the Late Jurassic-Early Cretaceous forming the southern Australian margin rift system and initiating the Otway, Bass and Gippsland basins. During the Late Cretaceous (CenomanianCampanian), NE-SW oriented extension related to rifting and left-lateral movement between Australian and LHR produced a broad extensional terrane throughout the LHR-NR region that may have been up to 700 km in width. As continental breakup occurred near the western edge of this extensional terrane the eastern Australian margin is a narrow, largely basin-free region, whereas the western part of the LHR is underlain by a zone of horst and graben structures up to 300 km in width. A range of extensional basins are present with sediment fill varying from 500 to 3000m. The earliest rift-basin fill is likely to be fault controlled terrestrial clastics and coal measures, with the overlying section deposited predominantly in shallow marine and upper bathyal environments. Breakup adjacent to the southern LHR was associated with intense magmatic activity represented by the 96 Ma rhyolites intersected at DSDP Site 207; by probable seaward-dipping reflector wedges beneath parts of the western margin of LHR; and by the large volume of volcanogenic sediment that shed westwards into the Otway, Bass and Gippsland basins from 120-95 Ma. On LHR, deformed and intruded syn-rift sediments are associated with volcaniclastics and lava flows. Heating and underplating associated with this magmatic episode may have been responsible for uplift and sub-aerial erosion of the eastern part of the southern LHR and much of the Challenger Plateau. Some of the syn-rift basins are almost totally eroded, and in those that are preserved sediment thickness rarely exceeds a few hundred metres. The NCB to the east of the southern LHR was probably initiated during the mid-Cretaceous, and commonly contains up to 4000 m of sediment. A significant episode of compressional tectonism that deformed the preNeogene section of the NCB is also represented by thrusting and fault reversal along parts of the adjacent margin of the LHR. This event appears to culminate with the onset of convergent tectonism along the eastern margin of the Australian plate in the Late Eocene to mid-Oligocene. Comparisons with adjacent explored basins (Bass Strait and Taranaki Basin), palaeogeographic considerations, and the dredging of potential source rocks on the flanks of the West Norfolk Ridge indicate that parts of the southern LHR-NCB-NR region may have long-term petroleum potential.
49
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE POORAKA FORMATION - A WIDESPREAD QUATERNARY REGOLITH UNIT IN SOUTH AUSTRALIA. R.P. Bourman,' P. Maitinaitis J.R. Pnescott^ & A.P. Belperio^ ^School of Environmental and Recreation Management, Faculty of Engineering and Environment, University of South Australia, Warrendi Road, The Levels. South Australia, 5095. 2 Department of Physics and Mathematical Physics, The University of Adelaide, South Australia, 5005. ^Formerly Mines and Energy Resources South Australia, PO Box 151, Eastwood, South Australia, 5063. Currently Minotaur Gold, la Gladstone St, Fullarton, South Australia, 5063. The Pooraka Formation is a distinctive alluvial deposit flanking the Mount Lofty-Flinders Ranges and the Gawler Ranges in South Australia. It comprises reddish-brown coloured sediments with variably developed calcareous pedogenic horizons with clay-rich red brown earth B horizons and occasional bleached A-horizons. It is associated with river terraces and alluvial fans. Most detailed study of the Pooraka Formation has occurred in the Adelaide area. The Pooraka Formation bears consistent stratigraphic relationships with the underlying Keswick Clay/Hindmarsh Clay or the Taringa Formation, and overlying Holocene, grey/black alluvial sediments. Until recently the Pooraka Formation was considered to range in age between 50,000 to 20,000 years. It was regarded as younger than the Last Interglacial, Anadara-boaring Glanville Formation. However, at Victor Harbor fill-top river terraces developed on the Pooraka Formation along the Inman and Hindmarsh Rivers grade to a shoreUne at ca + 6 m above present sea level. Furthermore, the Anadara shells interfmger with the terrestrial Pooraka Formation far upstream, demonstrating that the Anadara, last interglacial deposits (Glanville Formation) and the Pooraka Formation are intercalated coastal and terrestrial equivalents, suggesting a similar last interglacial (ca 125,000 year) age. Subsequent thermoluminescence dating of the Pooraka Formation sediments at Dry Creek and Walkerville, on the high terrace of the River Torrens, returned numerical ages consistent with a last interglacial age. The luminescence data have thus demonstrated viability as an independent means of testing the hypothesis that the Pooraka Formation extends back to the Last Interglacial. Thus the Pooraka Formation spans a far greater time interval than previously realised, extending back as far as the Last Interglacial, an age that provides the appropriate palaeo-climatic conditions and palaeo-environments for fluvial sedimentation. Close to the coast the unit is graded to a shoreline higher than the modem one, while inland the wetter, interglacial conditions would have favoured aggradation of sediments as opposed to drier glacial conditions that would have facilitated dissection. This timing would also ensure sufficient time both for the build up of extensive deposits of the Pooraka Formation over large areas, including much of the city of Adelaide, and time during glacials and interstadials for erosion of the Pooraka Formation to develop the extensive terrace system of the River Torrens. The revised age of the Pooraka Formation has implications for landscape evolution, archaeological prospecting and palaeomagnetic prospecting for the so-called 'Blake Event' in the southern hemisphere, as well as for the antiquity of the Diprotodon in the Adelaide area. River terraces formed on the Pooraka Formation and deposited within valleys cut into it have resulted from episodes of incision and filling; incision reflecting drier glacial conditions and filling, wetter interglacial times. The terraces are filltop or fillstrath and converge or diverge downstream depending on proximity to the shoreUne and the influences of faulting and climatic change. Some terrace sequences grade to the same local base level: they are almost certainly a consequence of climatic change, whereas close to the coast, relative sea level changes have dominated over climatic changes, and the terraces diverge downstream. Streams with long courses to the coast reduce in discharge across low coastal plains and this is reflected in terraces which converge, merge and overlap, with the younger grey-black sediments completely covering the older units. Some of the crossover points are marked by known faults and may be potential indicators of faults in other areas.
50
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
APPLICATION OF SEQUENCE STRATIGRAPfflC BASIN ANALYSIS TECHNIQUES TO SEDIMENT-HOSTED MINERAL EXPLORATION: THE ELIZABETH CREEK PROSPECT, NORTHWEST QUEENSLAND, Barry R. BradshawV Deborah L. Scott\ Andrew A. Krassay\ Peter N. Southgate^ ^Australian Geological Survey Organisation, GPO Box 378 Canberra, ACT 2600
Three years of integrated structural and sequence stratigraphic investigation in the North Australian Basins Resource Evaluation project (NABRE) is providing a new chronostratigraphic fiamewoik for prospective rocks cf Palaeoproterozoic age in northern Australia. In the upper McNamara Group these studies have led to a better understanding of host sediments for the Century zinc deposit. It is now possible to understand the regional setting and geometry of basins that developed in upper McNamara Group time. NABRE investigations clarify the geometry of the sub-basin hosting the Century deposit, the architecture of sediments contained therein, and establishes its relationship to the basement template. By combining new architectural stratigraphic information with previous models for mineralisation, a better prediction of subsurface mineral plays is permitted. The Century zinc deposit occurs within a sub-basin of the upper-most Lawn Hill Formation (H4s) in the upper McNamara Group. Local thickening of the H4s interval from 100 m to 300 m in the sub-basin provides the accommodation space for deposition of the Century-host carbonaceous shales. Gamma-ray and lithology logs obtained from outcrop sections of H4s outside of the Century-age sub-basin provide a sequence stratigr^hic template for this stratigraphic interval. Interpretation of a petroleum industry seismic grid to the north of Century has identified Century age strata in the area imaged by the seismic profiles. The H4s interval is correlated fiom outcrop sections to four petroleum wells and their associated seismic profrles. On seismic sections the H4s interval is dominated by a thin, highly reflective seismic unit; the high reflectivity is generated by dolomitic intervals within a siltstone-dominated package. In most seismic profiles, the H4s interval shows minor growth across wrench feult systems within narrow grabens. Thickening of H4s strata across these growth faults is probably insufficient to produce the accommodation space for Century-host carbonaceous shales. However, in the southeastern part of the seismic grid, the H4s interval thickens to the south fiom 115 m to 575 m into the Elizabeth Creek Fault zone. Thus, a Century age sub-basin (the Elizabeth Creek sub-basin) with near identical basin geometiy and sediment architecture is present in the southeastem seismic grid. Geopotential and seismic data provide the fiameworic for deep structural interpretations beneath the McNamara Group. The Elizabeth Creek sub-basin occurs adjacent to a deep seated fault system rooted in basement that displays multiple phases of reactivation and fracturing. Reconstmction of flattened seismic sections shows a similar series of tectonic events to those associated with the Century zinc deposit, namely: 1) formation of the Elizabeth Creek subbasin and deposition of host carbonaceous shales at about 1595 Ma; 2) deep burial of the H4s strata beneath a cover of at least 1500 m of the remaining Lawn Hill Formation between 1595-1585 Ma; 3) regional folding and faulting through the sub-basin during the Isan Orogeny between 1585-1500 Ma. The basement fault system and intercormected younger reactivation structures may have acted as conduits for the movement of mineralising fluids into the deformed Elizabeth Creek sub-basin during the Isan Orogeny. Seismic data confirm the interconnectivity cf the various aged tectonic events. The Elizabeth Creek sub-basin is believed to be a prospective area with the potential to host a Centuiy-equivalent zinc deposit. Indirect evidence for possible mineralisation in the southeastem seismic grid includes: 1) elevated base metal values, particularly zinc, in a carbonaceous silty-shale interval beneath H4s (based on cutting fiom petroleum wells); 2) anomalously low resistivity values (ie high conductivity) through this carbonaceous silty-shale interval in one well (Beamesbrook#l) drilled adjacent to the sub-basin; 3) anomalously high reflectivity^ in a seismic profile through the Elizabeth Creek sub-basia The area of potential economic recovery fiom the Elizabeth Creek sub-basin is limited to a 7 km long zone extending northeast fiom the Elizabeth Creek Fault where post-depositional folding has unroofed overlying strata and bought potential host-rocks to within 350 m of Mesozoic (Carpentaria Basin) cover. Thus a potential new zinc prospect, 'the Elizabeth Creek Prospect \ is predicted to occur beneath cover based on integrated seismic, drill core, outcrop, and well log sequence stratigraphic analysis.
51
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
TOWARDS THE FORMALIZATION OF SEQUENCE STRATIGRAPHIC UNITS Albert T. Brakel Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601
Contrary to the belief of some, the formalization of rock units is not confined to those conforming to lithostratigraphic principles. Already the Australian Stratigraphic Index (soon to be re-named the Geological Units Database to reflect the fact that it contains a wealth of information on units besides mere names) contains igneous geochemical suites. The rapid growth of sequence stratigraphic studies makes it essential that these units be included also, but this requires the formulation of principles for classifying and naming them. Sequence stratigraphy is the study of the large-scale architecture of sedimentary successions, involving the integrated interpretation of stratal patterns (from seismic sections, wells and outcrops), lithofacies and depositional environments. Units of genetically-related strata, bounded by surfaces of erosion, non-deposition, or their correlative conformities, are interpreted within a chronostratigraphic framework to deduce the fill history of a basin. Although sequence stratigraphic units have been around in the literature for the best part of two decades, no scheme has emerged so far that has achieved consensus of even part of the geoscientific community. Almost everyone in this field has used ad hoc, temporary and informal ways of referring to them. The most common ways are alphabetic or numeric: Sequence A, B, C, ., or 1, 2, 3, or I, II, IH, Other schemes makes use of the geological age: Pzl, Pz2, Pz3, Cambrian 1, Cambrian 2, Cambrian 3, etc. Although a clear hierarchy of first, second, third, and even fourth order sequences can be recognized in some areas, most workers do not specify what order of sequence they are dealing with. All this makes it difficult to enter such units in databases, where the power of a database can be used to gain new insights. Because a name such as Sequence 1 or Cambrian 1 is not unique, databases so far have had to record as part of the identity of the unit its informal name, and the basin or even part of the basin where the name was used. If two or more nomenclatures have been used for the same succession, the authors and dates of publication have to be specified as well. Obviously we can't go on like this forever, and GSA's Stratigraphic Names Committee and AGSO, as well as the international ISSC Working Group on Sequence Stratigraphy, are working towards a scheme for formalizing these units. The first questions to resolve are what is a sequence unit, and what hierarchy of units has the greatest practical value. A sequence has been defined as the package of sediments bounded by unconformities and their correlative conformities (sequence boundaries). The sequence boundaries represent time Unes within a basin succession, and the enclosed sequences can therefore include all facies from the onshore area, through the littoral, shelf and shelf margin zones, and out into the deep water parts of the basin. This makes a sequence unit fundamentally different from a lithostratigraphic unit. Lithostratigraphy and sequence stratigraphy are useful for different purposes, and the tool to use depends on the job to be done. It also stands to reason that type sections are meaningless in sequence stratigraphy, where lateral facies changes ensure that the vertical successions of lithologies are dramatically different from place to place. As the defining parameters are the sequence boundaries, only these can have a specified type locality. Because there are different orders of sea levelfluctuation,both eustatic and non-eustatic, a hierarchy of sequence units can be generated. A scheme used by Haq et al. (1988) makes use of cycles, supercycles, supercycle sets, megacycles, and megacycle sets. Others have preferred parasequences, sequences, supersequences and megasequences. There are also variations of these. It needs to be decided what the hierarchy should be, and how to define each step in it. In the naming of sequence units, each unit must be able to be referred to uniquely. Sequence unit names must also be distinctively different from lithostratigraphic names. Above all, using the geographic components of existing lithostratigraphic unit names for sequence units of different vertical extents, as has been done by some authors in the Hamersley Basin, is a recipe for confusion, and must be avoided. The advantages and disadvantages of various possible naming schemes will be discussed, including a scheme that links sequence stratigraphic nomenclature to lithostratigraphic nomenclature without the pitfalls of the nomenclature that has been used by some in the Hamersley Basin. It is highly desirable that national guidelines be developed quickly before a larger number of "formal"-style sequence units are published, each set up according to different principles. The Stratigraphic Index and Stratigraphic Names Committee welcomes any ideas or comments about how sequence stratigraphic units should be classified, defined, and named. 52
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
THE PAST IS THE KEY TO THE PRESENT: REMEMBERING CLARKE AND STUTCHBURY D.F. Branagan Department of Geology & Geophysics, University of Sydney
1998 marks the bicentenary of the births of two important pioneers of Australian geology, Rev. W.B. Clarke (1798-1878) and Samuel Stutchbury (1798-1859). Clarke, trained in geology at Cambridge University under Adam Sedgwick, arrived in New South Wales in 1839 and became established as the local authority on geology. Stutchbury, who had visited Sydney in 1825-26, experienced in mining and geology through practical work, arrived as official government mineralogical surveyor in 1850. In the five years of Stutchbury's survey (1850-55) they worked independently. At the beginning, Clarke and political friends attempted to undermine Stutchbury's appointment, claiming he was a "mere museum curator". Later, when Stutchbury's credentials as an acknowledged consultant on mining geology in Britain were made known, Clarke was somewhat conciliatory. Both contributed to Queensland geology, following the earlier work by Ludwig Leichhardt. Stutchbury in particular carried out mapping from the New South Wales border (as it became) covering mainly the coastal strip as far north as Gladstone. This work was carried out in the eighteen months between mid 1853 and the aid of 1854. Stutchbury's work on the coal measures along the Brisbane River below Ipswich was particularly influential in opening up this industry in Queensland. He also paid attention to the need for water on the Darling Downs, considering both the surface and possible artesian supplies. His journey north towards Gladstone took him into relatively undeveloped country under possible attack by Aborigines. Clarke went into Queensland only briefly, in mid 1853, following his examination of the Clarence River district, as part of his search for gold deposits on behalf of the government, but at the time contributed little to mapping. Although Clarke's time in Queensland was very brief he continued to gather information about its geology in the following years, particularly in connection with the coal measures and their possible age, and gold occurrences, establishing and maintaining contacts with Queensland geologists, and publishing a number of papers between 1865 and 1876, the last on the deep oceanic depression off Moreton Bay. Both Clarke and Stutchbury became involved in following up Leichhardt's discovery of fossil remains along the Condamine River in 1844. Mr. Turner found bones from King's Creek, Darling Downs in 1847, which were sent to the Australian Museum in Sydney and examined by Clarke, Leichhardt and Wall, curator of the Museum. They proved to be the remains of a Diprotodon. Both Stutchbury and clarke located further specimens during their surveys in 1853. The 1850s reports of Clarke and Stutchbury, through the efforts of P.P. King, were published in both Colonial and British Parliamentary papers, and through the latter became probably better known to European geologists than to their Australian counterparts. While the work of both was acknowledged, to some extent, by the naming of fossils and the use of their maps and reports by geologists such as R.L. Jack and R. Etheridge, Edgeworth David and E.C. Andrews, later generations are familiar only with the name of Clarke, and know little of Stutchbury. However the reports and maps of these pioneers still repay attention. Stutchbury's mapping (in particular Queensland) was incorporated in the first geological map of Australia, published by R.Brough Smyth in 1875. Clarke was remembered by Leichhardt, who named the south west branch of the Burdekin River for him. Like other pioneers, Clarke and Stutchbury deserve to be remembered for the basic groundwork they laid, on which later generations of geologists have raised the present edifice. Their monuments are their field reports and maps. Neither trained others to follow their traditions, Clarke because he was a confirmed individualist, Stutchbury because he did not stay long enough. Establishing a continuing tradition was left to A.R.C. Selwyn and his Geological Survey of Victoria from whence sprang C.D.Aplin, R. Daintree, who began the work of the Geological Survey of Queensland, and R. Etheridge Jnr., who contributed so much to Queensland palaeontology, in association with R.L. Jack, another important pioneer.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE ESSENTIAL INGREDIENT FOR A NATIONAL GIS? THE GEOLOGICAL UNITS DATABASE! Catherine E. Brown'. Lesley A. I. Wyborn' and Murray S. Hazell' 'Australian Geological Survey Organisation (AGSO), GPO Box 378 Canberra, ACT 2601
What does a national, variable scale GIS require? • National standards • A way of uniquely identifying each of the units portrayed on any map at any scale • A way of showing different levels of detail at different scales • Automatic generation of map legends • Links to related information held in databases eg sample localities, age data, unit descriptions & definitions, sections, type section locality, photos, geochemistry, bibliographic references) • Easy access to current information The Geological Units Database can provide, or provide links to, most of this problem-solving data now. Let me show you how accessible it is and what you might do with the data. First, a very short history. The Geological Units Database is the next stage on from the Stratigraphic Names Register and database (GEODX), which has been around, in various forms, since 1949. Many geoscientists thought the Register only catered for bedded units defined on lithostratigraphic principles, but all kinds of names and unit definitions were always recorded (including granites!) The 'database' started as several linked card files and we are still adding this pre-1969 'legacy data' to the present Oracle database as staffing allows. A major restructure and upgrade in 1995 led to the recording of more geological details and to improving links with other AGSO databases eg OZROX, ROCKCHEM, GWATER, STRATA. More links are planned, especially with OZCHRON. In July 1995 the Stratigraphic Names Database was the first AGSO database to go live for querying on the Internet through AGSO's World Wide Web pages. It often gets more than 100 WWW queries a day now, as well as inquiries by phone, fax, email and letter. Public access to the computer database is available through the services section of AGSO's World Wide Web server (http://www.agso.gov.au). You can search by geological unit name or through the bibliography. We have national standards for stratigraphic names because Australia follows the International Stratigraphic Guide. The GSA's Stratigraphic Names Committee is currently also considering guidelines for various other ways of dividing the rock record such as suites based on igneous geochemistry and sequence stratigraphic subdivisions. Each unit name in the Geological Units Database has a unique numerical identifier (stratno). Name changes and misspellings can be followed through the former name and replaced by fields. More links are being found as the 'legacy data' is added. We are also developing an unnamed units table so that all polygons in a digital map can get a unique identifier from one source. This step is essential for joining more than one map into a seamless coverage. We hope to start populating this table from various existing digital and digitised maps. Different levels of detail can be shown in the GIS by simply using the parent field and the FLATSTRAT view to choose anywhere in the hierarchy between Member and Supergroup or Granite/Volcanic and Supersuite., depending on the scale of the map required. For an even broader view, the geological province can be used. The digital map legend need only store the stratno as the geological units database is an authority table that can automatically supply a wealth of information including maximum and minimum ages, hierarchical links, units overlying and underlying at the type section location, and through links with other databases, geochronological ages, descriptions of particular samples, pictures of fossils, geochemical data etc. In the foreseeable future unit definitions will also be available, including type section locations and descriptions. By simply maintaining the unit descriptions in a relational database, and just porting out the details to the GIS, when required, the digital data can stay current with minimum effort. New work results in many updates in the Geological Units Database each year. From March 1997 to March 1998 there were 1218 names added and 3776 records updated. These updates should be automatically reflected in the GIS next time it is requested, without the large amounts of manual labour required in the past.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
CALCAREOUS EPIPHYTES: THEIR SIGNIFICANCE AS CARBONATE PRODUCERS 'Kirstv M. Brown. 'Yvonne Bone and ^Noel P. James 'Dept. of Geology and Geophysics, University of Adelaide, South Australia. 5005 ^Dept. of Geological Sciences, Queens University, Kingston, Ontario K7L 3N6, Canada
The definition of what is an epiphyte is somewhat controversial. For this study epiphytes are circumscribed as organisms which are wholly confined to living attached to the surface of plants. This is opposed to epizoans, which attach to the surface of other animals, and epibionts which use biota, i.e. plants or animals, as their substrate. The extent of epiphytic species in the marine realm is still unclear. Particular epiphytes, for example biyozoans, secrete calcareous skeletons which, when the host plant dies and decays, detach and become part of the detrital sediment. In general, previous studies have not identified the extent to which epiphytic skeletal material contributes to overall carbonate sediment production. Epiphytes have a global distribution, but their significance varies widely, hi warm/tropical waters they are relatively insignificant contributors due to the abundance and relatively rapid growth rate of other organism such as corals. However, m temperate and cool water environments such as the southern Australian margin, epiphytes are often locally one of the dominant contributers to the accumulating carbonate sediments. The recognition of epiphytes, especially those that are species specific, can infer the presence of marine flora, most of which are not normally preserved in the rock record. Other physical attributes that can arise from the recognition of epiphytes are indications of temperature and depth. Temperature can be inferred from their relative abundance in cool/temperate waters. Depth ranges can be reconstructed due to the restriction of the hosts to the photic zone. This can be further subdivided on the basis of whether the host is an alga or a grass, as the latter inhabit a shallower depth range. However, these relationships are only qualitative. A potentially more quantitative use of epiphytes as a paleoenvironmental indicator is their isotopic record. Preliminary results show that calcareous epiphytes that use low Mg-calcite, e.g. Hornera foliacea (bryozoan) precipitate their skeletons at or close to ambient seawater isotopic composition. Their abundance in the modem cool/temperate water environments and the fossil record suggest that they could be used to fortify the results obtained from brachiopod isotope data. Other epiphytes such as calcareous algae use high Mg-calcite and do not secrete their skeletal material in isotopic equilibrium with their ambient sea water. They do however, provide a source of carbonate for cement during diagenetic alteration. The current research involves identifmg the present day epiphytes and their host substrates on the Lacepede and Bonney shelves and the adjacent Gulf St. Vincent and Spencer Gulf, South Australia, and aims to establish the contribution of theu* calcareous skeletons to the recent carbonate sediments. It is also addressing the problem of post-mortem transport of the host substrate whereby the epiphyte is a passive passenger, frequently resulting in its eventual deposition in an environment completely differentto that in which it lived.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
CAINOZOIC TECTONICS AND LANDFORM EVOLUTION OF THE COAST AND ADJACENT fflGHLANDS OF SOUTH EAST NEW SOUTH WALES M.C. Brown. 51 Debenham St, Mawson, A.C.T., 2607
Mid Tertiary non-marine sediments and basalt and deeply-weathered old landscape remnants occur in several areas of the coastal zone between Bendalong and the Victorian border in south east New South Wales. The sediments and basalts, as with similar sediments and basalts of inland plateaux near the Great Divide, are preserved mostly in palaeo valleys with palaeo relief up to several hundred metres. Near Pambula, Moruya, and Ulladulla, mid-Tertiary sediments and basalts in N-S trending palaeo valleys dip gently seaward at angles around 1 degree. The dip is close to the eastward slope of the offshore seismic basement and also close to the average slope between the coastal zone and old land surfaces of the inland plateaux. These relationships lend strong support to the idea proposed in several publications of C.D. Oilier and C.F. Pain that the coastal zone and offshore seismic basement have been downwarped relative to the inland plateaux - an idea previously based largely on landforms and apparent anomalies in drainage patterns. The simple coastal downwarp hypothesis can not be applied in the area between Brooman, on the Clyde River, and the coast. In this area the coastal zone has been tectonically lowered by Cainozoic faulting. Steep slopes on the west side of the Clyde River, between Cockwhy Ridge and Cockwhy Creek, and between the Murramarang Range and the coast, are eroded fault scarps. The two fault blocks between the Clyde River and the coast are gently back-tilted to the west. Most of the tectonic lowering of the coastal zone and offshore seismic basement appears to be post mid-Tertiary. This suggests that marine sediments of the offshore sediment wedge are likely to be not older than Miocene. There could also be older Tertiary non-marine sediments and perhqjs basalts in palaeo valleys at the base of the section. Some previous authors have concluded that this part of south-east New South Wales has been tectonically stable since the Permian, and that the differences in elevation between old land surface remnants of the coastal zone and the inland plateaux are entirely erosional in origin. This conclusion is based mainly on what I believe to be fallacious arguments about the stratigraphy and structure of Permian and Triassic sedimentary rocks around Nowra and Ulladulla, and also on incomplete and partly erroneous observations on the form of the erosion surface beneath midTertiary basalts and sediments of the Ulladulla area. The structure and stratigraphy of bedrock below an erosion surface is irrelevant to the question of whether or not there has been any gentle regional tilting or warping since that surface was formed. Comparisons between oxygen isotope ratios of weathering profiles from the highlands and coastal zone have also been used as evidence for long-term tectonic stability. I believe that the isotope data can be interpreted in ways entirely consistent with Cainozoic tectonic lowering of the coastal zone. The magnitude and style of Cainozoic tectonism in the coastal zone is similar to that in the adjacent highlands clearly documented in regional and detailed studies in the highlands by many authors.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
APPLICATION OF ARTIFICIAL NEURAL NETWORKS TO PROSPECTIVITY ANALYSIS IN A CIS ENVIRONMENT: A COMPARISON WITH STATISTICAL AND FUZZY LOGIC METHODS FOR AU AND SN DEPOSITS OF THE TENTERFIELD AREA, NSW Warick M. Brown'. Geoffrey R. T. Taylor^ Jusma^dy^ David I. Groves\ Carl M. Knox-Robinson' ^Department of Geology and Geophysics, University of Western Australia, Nedlands, Western Australia 6907 ^Department of Applied Geology, University of New South Wales, Kensington 2052
Geographic information systems (GIS) are emerging as an important technology in mineral exploration as a means of integrating, displaying, and disseminating large multi-source geoscientific datasets. Despite this, relatively little research has been done on the methodology of metallogenic analysis or how to use GIS to select favourable areas for exploration in a region. The process of combining maps containing different geoscientific datasets (e.g. geology, geophysics, geochemistry, and remote sensing data) to produce a single map depicting areas ranked according to their potential to host deposits of a particular type is the focus of this study. Existmg methods have some important limitations. Empirical statistically-based methods, e.g. weights of evidence (Bayesian probability) are not applicable in poorly-explored areas or in areas containing few known deposits. These methods require a statistically significant number of occurrences to estimate the relative importance of geological evidence associated with the deposits. Conceptual methods, such as index overlay or fuzzy logic, have the advantage that they can be applied to areas containing few known deposits but they rely on a deposit expert to make subjective assessments. The mineral systems approach developed by AGSO is a conceptual methodology that involves identifying parameters considered essential for ore formation, and translating these parameters into mappable criteria or attributes that may be extracted from a regional-scale GIS database. All these conceptual methods depend on applying a deposit model to decisions about which geological parameters might be significant. The problem here is that new deposit types are still being discovered, and models for well investigated deposit styles are constantly being revised in the light of new discoveries. Artificial neural networks (ANNs) offer a new approach to the problem of mineral prospectivity mapping. Although a large body of literature describes the application of ANNs to pattern recognition and classification of remote sensing data, there are very few documented geological applications in the public domain. Despite this, ANNs have potential in this area, because they can; 1) extract underlying patterns in a dataset, 2) require no statistical model for the data, 3) do not require pre-existing knowledge (e.g. a deposit model), 4) can operate at acceptable accuracies when the data quality is poor or some data are missing, 5) are flexible, and can be retrained when new data become available, and 6) are suitable for use on large mixed datasets. This pilot study compares the effectiveness of back-propagation-trained MLP neural networks with the weights of evidence and fuzzy logic methods for estimating prospectivity. For this purpose, a small GIS database containing primary and placer tin and gold deposits from the Tenterfield area, N.S.W (Tenterfield 1:100,00, sheet 9339), and the GeoDips raster-based GIS software package (Jusmardy & Taylor) were chosen since prospectivity maps based on the weights of evidence and fuzzy logic methods could be prepared using in-buih functionality of GeoDips. The database consists of the thematic layers; geology, airborne magnetics and radiometrics, faults, and deposit occurrences. Prospectivity maps using the artificial neural network approach were prepared, using simulation packages available in open domain (PDF and SNNS) and data obtained from the GeoDips thematic layers. Neural networks were trained using a training dataset comprising half of the area and deposit points and a test dataset comprising the remaining area. The effectiveness of the methods were compared quantitatively by measuring the success rates in 'predicting' the known deposits in the test data area, and taking into account the proportion of the total area represented by the prospective area containing those deposits. Initial results demonstrate the potential of ANNs to be used effectively to estimate mineral prospectivity, and show that success rates are comparable to the fuzzy logic method. Both ANN and fuzzy logic methods appear to be more successful predictors of prospectivity than the Bayesian probability method. Acknowledgements: WMB wishes to thank Dr J. Leach, Department of Geomatics, University of Melbourne for his assistance in initially formulating this study.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
GEOCHEMISTRY AND GEOCHRONOLOGY OF THE MARLBOROUGH OPHIOLITE: IMPLICATIONS FOR THE TECTONIC HISTORY OF THE NORTHERN NEW ENGLAND FOLD BELT Michael Bruce. Yaoling Niu, Terrence Harbort and Rodney Holcombe Department of Earth Sciences, The University of Queensland, St Lucia, Queensland 4072
The New England Fold Belt (NEFB) extends for -2000 km along the eastern margin of Australia and is a tectonic collage of mid-Palaeozoic subduction/accretion elements, late Palaeozoic extensional basins and structures all overprinted by a major Permo-Triassic contractional event. At the northern end of the NEFB thrusting associated with this latest event has exposed a dismembered ophiolite (Marlborough ophiolite), the largest (-700 km^) complex of this type known in Australia. The ophiolite is tectonically interleaved with rocks of the mid-Palaeozoic accretionary complex (-370-340 Ma). An ophiolitic component has previously been recognised within these accretionary units to the south. An apparent ophiolitic component also crops out on South Percy Isle which appears to be separated from the Marlborough ophiolite by the Stanage Fault Zone, an approximate north-northeast trending oblique shear zone with dextral strike slip offset. Previous interpretations have been that these are elements of oceanic crust underplated by basal accretion during subduction and then exhumed during subsequent events. Geochemical and age data suggest that the Marlborough ophiolite is a fragment of normal oceanic lithosphere formed at an ancient oceanic ridge, which is at least 170 Ma older than the accretionary complex, and has been intruded at least once by mafic dykes with island arc signatures. These observations are interpreted to indicate that the eastern margin of Australia evolved from an Island arc/Back-arc system to an Andean-style convergent margin. Despite the structural complications and metamorphic overprinting, residual harzburgitic mantle rocks (partially to entirely serpentinised) and crustal lithologies such as basalts and gabbroic cumulates are readily identified in the ophiolite. Unaltered cores of relic igneous chromium spinel from both the harzburgites and dunite-chromite pods display a limited range in Cr/[Cr+Al] = 0.3-0.4, which overlaps the data array defmed by abyssal peridotites formed beneath ocean ridges. Within the partially serpentinised harzburgites, igneous olivine has a composition of F090.91 and orthopyroxene ranges in AI2O3 from 2.9 to 3.7 wt%. Minor clinopyroxene has a Cr203 content of 0.9-1.3 wt%. All these are typical of normal oceanic peridotites. The crustal cumulates and entrained pockets of primary liquid compositions of the ophiolite contain geochemical signatures of depleted average N-MORB. Whole-rock and mineral separates from one of the gabbros give a Sm-Nd isochron age of 561 ± 2 3 Ma (2a error) and a z^^ = 8.7 ± 0.2. Some apparently younger (unconstrained geochronologically) mafic dykes, that both cut the harzburgites and form tectonically emplaced slices within the fault-bounded ophiolite blocks, show Island Arc geochemical signatures. Still younger [251 ± 78 Ma (2a), = 6.8 ± 0.3 whole rock Sm-Nd isochron] mafic dykes with 'enriched' incompatible trace element signatures also exist and appear to be broadly coeval with the Permo-Triassic contractional event. The enriched signatures may have resulted from contamination by continental crustal material. Preliminary studies of ultramafic lithologies from elsewhere in the northern NEFB indicate diverse origins. Residual spinels in serpentinites of the accretionary North D'Aguilar block have Cr/[Cr+Al] = 0.38-0.45, which appears consistent with a spreading ridge origin. Despite variable alteration high chrome spinels (Cr/[Cr+Al] = 0.6) are recorded from serpentinites along the Yarrol fault. Serpentinite domes from the Bowen Basin contain Cr spinel with Cr/[Cr+Al] = 0.55. Serpentinised harzburgites, previously assumed to be displaced remnants of the Marlborough ophiolite, crop out on South Percy Isle, -150 km north of Rockhampton. These harzburgites contain residual spinels with Cr/[Cr+Al] = 0.8, suggesting that they may have formed in a supra subduction zone environment, and are therefore of a different origin than that of the Marlborough ophiolite. This observation suggests that the Stanage Fault Zone may have had a long history, and is tectonically far more significant than a simple transcurrent displacement.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
THE ^OZMAR' MARINE GEOSCIENCE DATABASE - AGSO's LARGEST SPATIAL SYSTEM Cameron Buchanan, Phil Symonds & Rod Rvbum Australian Geological Survey Organisation, GPO Box 378, CANBERRA CITY ACT 2601
Most marine geoscience data is collected as a series of points along the path of the survey vessel. Bathymetry, gravity and magnetics are generally handled via a time-based sampling scheme — usually at one minute intervals. Seismic data, on the other hand, is distance based, the shot points being equally spaced as far as possible. Traditionally these two types of data are handled as separate computer files, and the seismic people have no interest in what happens outside of the seismic lines. Acoustic swath data (sidescan sonar) extends some distance either side of the ships path, but is still related to that path. Only satellite imagery and gravity, and the topography derived from them are truly aerial in coverage. AGSO's Law-of-the Sea Project has the ambitious task of trying to collate all the existing marine geoscience data into a comprehensive picture of the sea floor covering an enormous area — from Madagascar to New Zealand, and the equator to the coast of Antarctica. This represents a formidable volume of data scattered amongst hundreds of computer tapes from many sources. What became apparent from an early stage, was that the traditional file-based methods of handling marine data were not up a task of that magnitude, and that a properly constructed and fully attributed information management system was required. With that thought in mind, about 18 months ago we began to construct a prototype relational database called 'OZMAR' which was designed to take much of the data required by the Law-of-the-Sea Project. We also believe that OZMAR can eventually house all of AGSO's marine data, including bottom observations, cores and samples of various sorts. Central to OZMAR is the concept of a survey (or 'cruise') which starts the moment a ship (or 'platform') leaves port and continues until the ship arrives back in port (not necessarily the same port) at the end of the stint at sea. A 'SURVEYS' table is the natural starting point for OZMAR. For most of the survey the ship takes bathymetric, gravity and magnetic observations which are sampled to one-minute intervals. This sampled information gives rise to the POINTS table that records the date, time, latitude and longitude of each sample point. Because the instrumental conditions may vary during a survey, it is necessary to introduce an intervening table we have called SEGMENTS, which defines parts of the survey instrumental conditions remain constant. This can also be used to restrict parts of a survey, or tag those parts in national, as opposed to international waters. Although one could group bathymetry, gravity and magnetics in the POINTS table, by using separate tables for these entities we allow for the possibilty of several sets of data at the one point. For example, we can include raw data and recalculated results, or data from several different instruments. Other OZMAR tables allow us to record the datum used, navigation methods, instruments, and observational methods. Separate tables exist for seismic lines and shotpoints, but these are linked to segments. Provision has been made for swath metadata. Our initial attempts to establish OZMAR were somewhat frustrated by lack of RAID disk space on the Oracle database server then in use. Nevertheless, we were able to load 102 surveys totalling over 3.5 million points with their associated depth, gravity and magnetic data, and we also proved the ease with which data could be extracted from OZMAR on a geographical basis, as well as using other criteria. With a new, much faster Oracle server our storage restrictions have now been lifted and we plan to load the complete dataset (566 surveys) within the next few months — i.e., by the time this paper is presented. The completed OZMAR should have over 40 million points and some tens of gigabytes of data. We also hope to install Oracle's 'Spatial Cartridge' which will allow for much faster and more versatile spatial queries. It will also automatically partition the POINTS table on a geographic basis as the t^ible grows, and will free the performance of the system from any negative correlation with database size. Real-time spatial metadata queries on AGSO's Web site will become a practical proposition. With such a large spatial database it is essential we make use of the best technology available to properly handle such a valuable information resource. Without such a database system, existing GISs are incapable of seamless operation on such a dataset.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
NEW PERSPECTIVES ON THE PALAEOECOLOGY OF THE TE AUTE LIMESTONE COMPLEX, HAWKES BAY, NEW ZEALAND John St J. S. Buckeridge UNITEC, P.O.BOX 92025, Auckland, New Zealand
The Te Aute lithofacies limestone complex lies on the east coast of New Zealand's North Island. It is comprised of a series of uniform to poorly bedded calcarenites and coquinas of late Neogene age. The complex was deposited off-shore, to the east of the current land mass, and during the Quaternary was structurally accreted to the mainland as a series of longitudinal prisms. This paper reviews the current interpretation of the depositional environment, which implies a facies change in the limestone from shallow water in the west, to deeper water in the east. Primarily utilising barnacle palaeobathymetry, it is shown that one of the oldest and most westerly limestones, (the mid Pliocene Titiokura Limestone), was deposited in deep water. Living barnacles have clearly defined bathymetric ranges and temperature tolerances. When barnacles are abundant in fossil remains, especially of late Cainozoic age, they are often useful palaeoenvironmental indicators. The barnacle fauna of the Titiokura Limestone is characterised by the presence of the deep water balanomorph Pachylasma. Species of Pachylasma are widely distributed in the living shelf fauna, being found in middle to outer shelf environments, although on very rare occasions specimens may be found living in waters as shallow as 55 metres. On the basis of the abundant and comparatively well preserved plates of Pachylasma then, it may be inferred that the Titiokura Limestone accumulated in moderately deep, off-shore conditions. This interpretation is confounded however, by the presence, in the same horizons, of species of an exclusively intertidal balanomorph, Epopella. Whilst it is apparent that the bathymetric ranges of some taxa change through time, all known species of Epopella are demonstrably intertidal to uppermost subtidal. They are also characteristic of temperate waters. Sedimentological observations suggest that the Titiokura limestone is a mixed thanatocoenosis that accumulated in the mid to outer shelf environment. Shallow water elements were introduced as components within submarine avalanches and slurry deposits. Intertidal temperature regimes were probably similar to those currently existing along the New Zealand coastline.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
INVESTIGATING THE ORIGIN OF SALT IN THE UPPER BURDEKIN USING SPATIAL ANALYSIS Elisabeth N. Bui and Christopher J. Moran CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601
The objective of the research presented here was to develop and test a methodology for assessing whether the distribution of saline outbreaks is random or related to particular landscape or geological features. The study area was the upper Burdekin river basin which corresponds roughly with the Dalrymple Shire in north Queensland. Soil samples were used in conjunction with a digital elevation model to examine the landscape distribution of soluble salts in the region. The spatial relationship between soil and groundwater salinity was tested by comparing the point patterns of saline soils and saline bores. The possibility that certain geological units could be sources of salt was tested. The relationship between distance downhill from a geological unit and the occurrence of saline points was compared with the relationship expected if the saline points were randomly distributed with respect to the same units. Distance along flow direction from geological map units to the saline points was derived by converting point and polygon data to grids and calculating distance along flow direction by image dilation procedures conditioned on elevation. More saline points than would be expected at random appeared close to the margins of Cambro-Ordovician and Triassic geological units. The Cambro-Ordovician units, extending approximately east-west across the middle of the study area, comprise the Seventy Mile Range Group. This group evolved in a back-arc basin environment (Henderson, 1986) which suggests that some of its lithologies could be sources of the CI in soils as many arc magmas have a high CI content (Hedenquist and Lowenstem, 1994). Salinity around Lake Buchanan whose geochemistry precludes salt of atmospheric origin (Chivas et al., 1986)appears associated with Triassic sediments (TRs). REFERENCES Chivas, A.R., De Dekker, P, Nind, M., Thiriet, D., and Wasson, G. 1986. The Pleistocene paleoenvironmental record of Lake Buchanan: an atypical Austrahan playa. Paleogeogr., PaleoclimatoL, Paleoecol. 54:131152. Hedenquist, J.W. and Lowenstem, J.B. 1994. The role of magmas in the formation of hydrothermal ore deposits. Nature 370:519-527. Henderson, R.A. 1986. Geology of the Mt. Windsor subprovince-a lower Paleozoic volcano-sedimentary terrane in the northern Tasman orogenic zone. Aust. J. Earth Sci. 33:343-364.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
CONSTRAmXS ON ACROSS-STRIKE FLUID FLOW DURING POLYMETAMORPfflSM IN ALPINE CORSICA FROM STABLE ISOTOPE PROFILES I.S. Buick^. 1. Caitwri^t' and J.A. MUler' ^Department of Earth Sciences, La Trobe University, Bundoora, Vic. 3083, Australia Apartment of Earth Sciences, Monash University, Clayton, Vic. 3168, Australia
In coUisional orogens fluids may be evolved at various times during subduction and subsequent exhumatioa Some fluid escj^s from the subducting slab at relatively higji cnistal levels. However, much is probably liberated at depth during high-pressure/low-temperature (HP/LT) metamorphism, or during subsequent exhumation under greenschist-facies conditions. Alpine Corsica contains a series of nappes that were emplaced during the collision of the European continental plate with a Tethyan subduction conq)lex during the late Cretaceous/early Eocene. The Schistes Lustr^ nappe contains the dismembered relicts of a Tethyan ophiolite, oceanic metasediments and thrust slices of obducted European continental basement, and was metamorphosed at high pressures, most typically to blueschist grade. Major compressional structures were reactivated as extensional shear zones during the Oligo-Miocene, associated with a greenschist-facies overprint and locally extensive veining. In this p ^ r we constrain geometry of syn-metamoiphic fluid flow in the Schistes Lustres Nappe using stable isotope profiles across marble-metabasalt or -schist contacts. Stable isotope profiles were measured across interlayered calcite-rich marble and metabasalts and/or calcitebearing micaceous schists at localities that equilibrated at either blueschist- or greenschist-facies conditions. Irrespective of grade the cores of marbles of greater than 1-2 m width preserve oxygen isotope profiles characterised by plateau regions with isotopic values that are httle reset from sedimentary precursors (5^^0(Cc) = 23 lo 26 %o) and steep boundary layers where values decrease to values close to those of the adjacent metabasites and micaceous schists at marble contacts (-'12-17%o). The S^^O values of the metabasites/schists locally increase towards the marble contact, resulting in sigmoidal boundary layers that are commonly tens of centimetes to, at most, 2-3 metres wide, and that are centred at lithological contacts, or less commonly, occur tens of centimetres into the maible layers. Carbon isotope values are little reset over the same distance. The contact between marbles and metabasalts represents a sharp initial discontinuity in across which subsequent isotopic exchange has taken place, as shown by the boundary layer oxygen isotope gradients. The s h ^ of the isotopic profiles is qualitatively similar to that which results from diffusion-dominated isotope exchange between two semi-infinite bodies in a system in which local isotopic equilibrium has been maintained, suggesting that litde or no fluid flowed across strike during either HP/LT metamorphism or the subsequent greenschist overprint. The boundary l^ers could have been formed within periods of 5x10^ to 6x10^ years, which is similar to that estimated in studies of diffusion-dominated boundary layers and is reasonable given the timescale of metamorphic events (-10^-10^ years) during the Alpine orogeny. At greenschist grade, small (< Im wide) maible layers do not anywhere preserve unreset oxygen isotope values, but instead develop asymmetrically peaked profiles that are associated with the development of grain size reduction in greenschist-facies calc-mylonite zones at maible-met^asalt/schist contacts. For these marbles, the mid-points of boundary layers are displaced into the maible on both sides, and may reflect the edges of channels oriented along the maible-metabasalt contacts along which water-rich fluid flowed during shearing. The isotopic conqx)sition of this fluid could have been either a) in isotopic equilibrium with the silicate rocks, or b) intermediate between that of the silicate rocks and marble, and the initially sharp channel boundaries would have subsequently been quickly smoothed out by diffusioa The stable isotope data presented here argues against a major across-strike component of fluid flow diuing Alpine polymetamorphism on Corscia Most fluid flow probably occurred along strike, in channelways controlled by permeability contrasts between interlayered rocktypes,in shear zones or in locally abundant vein networks.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
COLOUR IN MARINE SEDIMENTS Liz Campbell and Yvonne Bone Department of Geology and Geophysics, University of Adelaide, Adelaide, South Australia, 5005.
It is customary to record the colour of the sediment at the time of collection of the sample in any marine geoscience research. This is done whilst the sediment is fresh and still bathed in its sea-water medium. The colour is primarily derived from the inert sediment grains. Decay of any contained organic matter will start soon after collection. The resultant prolific explosion of various bacteria will lead to the formation of a reducing environment and the production of black meta-stable u-on sulfides. This results in a rapid and dramatic change in the colour of the sediment, and the colour recorded at this stage would be very different to that recorded immediately after collection of the sediment. Colour is of vital importance to marine organisms living in the photic zone, e.g. for camouflage, defense and attracting a mate. These colours rapidlly fade after death, e.g. brightly coloured sponges' giving a whitish to light buff colouration to the sediment. The colours of some marine biota, however, do persist, e.g. the bryozoan lodictyum phoeniceum retains its vivid magenta colour for many years,' but these are the exceptions. Thus, the brilliant colours seen in the living marine biota are not reflected in the colour of the accumulating sediments. The overall colour of marine sediments, however, is mainly controlled by the ratios of the contained calcareous skeletal fragments, early marine diagenetic minerals (e.g. glaucony), organic degradation by-products and terrigenous components. The last, with a wide range of possible colours, can bias the observed colour by quite small contributions, e.g. the presence of garnet placers imparting a pinkish hue. Kemper (1995), attributed colour changes in the Aptian of northwest Germany to palaeoclimatic variations. Li^t-coloured layers represent sediments laid down during warm periods with an adjacent arid mainland and correspond to times of high sea level and transgression. An increased supply of dark-coloured sediments, rich in clay, were deposited during cold periods in a humid climate. The dark clays are absent in the marginal areas of the basins as they were deposited during times of glacioeustatic sea level lows. Most of the southwestern and southern continental margins of Australia are dominated by cool-water bryozoan-dominated carbonate sediments deposited on high energy open shelves. Yet even in this high-energy, mixing environment, there is a distinct colour variation from one facies to another, from greenish to whitish to brovmish to tan, etc. A high percentage of dark reddish-brown relic material occurs in many of these sediments. What will happen to the colour of these sediments with time? And why are they the colour they appear to be? Are there links with palaeoclimatic conditions? This research endeavours to determine the role colour and colour patterns can provide ui assisting in the unravelling of the controls on the formation of accumulations of sediment through time. To do this, we must standardise our interpretation of "what colour is that"? Hue, value and chroma, the three aspects of the Munsell Chart, are being used to characterise the colour of the sediments. This reduces, but does not entirely preclude, personal interpretations. REFERENCE Kemper, E. 1995. The causes of the carbonate and colour changes in the Aptian of NW Germany. Neues Jahbuch fur Geologic und Palaontologie 196, 275-289.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE INTERNAL STRUCTURE AND SEDIMENTOLOGY OF THE ESK TROUGH, SOUTHEAST QUEENSLAND Lorraine M. Campbell. Rodney J. Holcombe & Christopher R. Fielding Department of Earth Sciences, University of Queensland, Qld 4072, Australia
The Esk "Trough" is a NNW-SSE trending structural entity in southeast Queensland which extends over a distance of some 260 km, has a maximum width of 35 km (typically about 25 km) and is approximately 6500 km^ in area. This feature is defined by the surface exposure of the Toogoolawah Group of Early to Middle Triassic continental sediments and intermediate volcanics (predominantly volcanic sediments). The outcrop of these Triassic rocks is largely confined between the Devonian-Carboniferous North D'Aguilar and Yarraman Blocks on the eastern and western sides respectively. At its southwestern extent the "Trough" is unconformably overlain by Late Triassic-Early Jurassic Moreton Basin sediments. At its southern end it is bounded on both sides by sequences of Permian marine sediments and intermediate and felsic volcanics - the Cressbrook Creek Group to the west and the Northbrook Beds to the east. The relationship of these Permian units to the "Trough" is faulted and unconformable and they are faulted against the Palaeozoic basement blocks. The Toogoolawah Group, in the southern and central part of the "Trough", and in ascending stratigraphic order, is traditionally defined as consisting of the Bryden Formation, the Neara Volcanics and the Esk Formation which are exposed respectively from east to west. New data suggest that the stratigraphic order of the Toogoolawah Group is incorrect, with the Neara Volcanics being the youngest unit. The Bryden Formation, found as isolated outcrop areas along the south-eastern side of the "Trough", and the Esk Formation, found continuously exposed along most of the western side of the "Trough", are very similar in character and composition and structurally appear to be lateral equivalents. The Toogoolawah Group reaches a maximum of around 5000 m thick within the central part of the "Trough" with the basal sedimentary package (Bryden Formation and Esk Formation) having a maximum thickness of about 2000 m and no stratigraphic top of the Neara Volcanics is exposed. Similarities in fossil fauna and petrology between the Permian Cressbrook Creek Group and the Northbrook Beds suggest that they may be one rock unit which is laterally continuous beneath the Toogoolawah Group. The Toogoolawah Group has most commonly been equated in the literature with the Esk Trough, a graben or a rift valley. On both margins of the "graben" the unconformable base of the Toogoolawah Group is exposed and in neither case corresponds to, or sedimentologically is close to, a basinal fault. On the eastern side there is an angular unconformity with the Early Permian Marumba Beds and a nonconformity with the Late Carboniferous Claddagh Granodiorite. On the western side there is an unconformity with an altered pillowed basalt unit of possible Permian age and an angular unconformity with the Maronghi Creek Beds of the Yarraman Block (E. Willey, pers. comm.). The steeply dipping marine Marumba Beds, exposed beneath the eastern unconformity, were heavily oxidised by subaerial exposure prior to accumulation of the non-marine Bryden Formation. The implication of this is that Hunter-Bowen thrusting, which initiated in the Late Permian, had, by the Early Triassic, produced an exposed fold-thrust highland which provided the source area for the basal Bryden Formation conglomerates which are a base of slope deposit. The Toogoolawah Group was deposited in a more extensive basin than its current "Esk Trough" exposure indicates. There are sub-horizontally dipping outliers of Neara Volcanics overlying deformed basement rocks in the North D'Aguilar Block to the east. At the unconformity with the Claddagh Granodiorite, the Neara Volcanics consist of fine grained to medium grained sandstones, mudstones and pebble conglomerates, typical of fluvial deposits (higher in the sequence a greater volume of, and larger clast conglomerates occur). The depositional basin of the Toogoolawah Group was more regionally extensive than the "Esk Trough" but overlies a deformed, Early Permian rift sequence (Marumba Beds). Faults related to this Early Permian rift likely persisted and controlled Late Permian thrusting as well as later deformation in the Middle Triassic and postTriassic times. Thus the Esk Trough, as a graben structure, refers to an Early Permian feature, not a Triassic structure. The Toogoolawah Group Triassic sequence has a similar geometry and is stratigraphically comparable to the Triassic succession on the eastern side of the Bowen Basin and thus may be part of the Hunter-Bowen foreland basin system.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
QUATERNARY FORAMINIFERA AND RELATIVE RIVERINE AND OCEANIC INFLUENCE WITHIN THE HOLOCENE ESTUARY OF THE RIVER MURRAY, SOUTH AUSTRALIA. John H. Cann^. Elizabeth J. Bamctt^ and Robert P. Bourman^ ^School of Engineering (Applied Geology), University of South Australia, Levels Campus, Warrendi Road, Mawson Lakes, S A 5095 ^Mawson Graduate Centre for Environmental Studies, University of Adelaide, S A 5005 ^School of Environmental and Recreation Management, University of South Australia, Levels Campus, Warrendi Road, Mawson Lakes, S A 5095
The study area includes the lower estuaiy of the River Murray, in the vicinity of the town of Goolwa, and the Coorong Lagoon, southeast of the mouth of the River Murray. The Coorong Lagoon is the most recent of a series cf coastal lagoons that have fornied at times of high Quatemaiy sealevel. During successive interglacial sealevel maxima, near shore coastal beach-dune barriers have resulted in the concomitant isolation of shallow water backbarrier lagoons of restricted circulatioa The modem barrier, the Younghusband Peninsula, facing the Southem Ocean, extends from the mouth of the River Murray, close to Goolwa, some 300 km towards Kingstoa Regional uplift and sediment infilling has modified the lagoon progressively in the southeast to a series of saline lakes and wetland swan:q)s. The River Murray debouches to the Southem Ocean through the coastal barrier, the Sir Richard Peninsula to the northwest and the Younghusband Peninsula to the southeast. The outlet setting is geomorphically dynamic and it is known that the river mouth has migrated in both directions since European settlement. In modem historic times, when the river flow has been minimal, the outlet has temporarily closed (e.g. 1981). Thus, within the lower estuaiy of the River Murray, there is potential for variable environmental interaction of the saline waters of the Southem Ocean versus the fresh water flow of the river. Assemblages of foraminifera within the Coorong Lagoon are conunonly dominated by the euiyhaline species Ammonia beccarii, together with fewer numbers of Elphidium articulatum. In contrast, the inner shelf marine environment is characterised by a more diverse assemblage in which Discorbis dimidiatus, E. crispum, Kmacelliforme and various cibicidid species are known to predominate. These fauna, as fossils, therefore have potential to allow discrimination between sediments formed under primarily lagoonal influence as opposed to those deposited in a mainly marine setting. 1.57 m of sediment was recovered in a core from an intertidal, samphire, saltmarsh flat several km southeast of the river mouth and sampled for foraminifera at 10 cm intervals. Foraminifera within the uppermost 30 cm of saltmarsh sediment were almost exclusively Trochammina inJJata, but at 40 cm there was a transition to subtidal muddy sands in which coastal lagoonal and inner shelf species were approximately equally represented. Thus, in the subtidal environments of the core site it can be argued that the influence of open marine and lagoonal waters were equivalent for the time of sedimentation. Another core recovered 3.00 m of sediment from the shallows of the River Murray near Goolwa. This core was also sampled for foraminifera at 10 cm intervals and the relative Sundance of A, beccarii + E. articulatum was compared with that of D. dimidiatus + E. crispum -f E macelliforme + other species. These data revealed that maximum estuarine influence occurred during deposition of sediments at core depths of 120 cm and 220 cm, and maximum oceanic influence when sediment at 290 cm was deposited. Radiocarbon ages derived from analysis of organic matter determined that the sediments were of middle Holocene age, and these dates were later refmed using AMS radiocarbon age determination on estuarine foraminifera and ostracods. While the reasons for the variable oceanic versus lagoonal/estuarine influences within the lower estuary of the River Murray remain uncertain, it is clear that likely factors include the relative quantities of fresh water that were delivered to the lower reaches of the river, and the extent to which the mouth of the river was obstmcted by sedimentation The methods enq)loyed in this investigation have potential for elucidating palaeogeographic and palaeoclimatic histories of such estuarine settings.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
CONTEMPORARY AND FOSSIL QUATERNARY FORAMINIFERA, PALAEOENVIRONMENTS AND PALAEOSEALEVELS OF NORTHERN SPENCER GULF, SOUTH AUSTRALL^. JohnH. Cann^. Antonio P. Belperio^ and Colin V. Murray-Wallace^ ^School of Engineering (Applied GeologyX University of South Australia, Levels Campus. Warrendi Road, Mawson Lakes, S A 5095 ^Minotaur Gold NL, la Gladstone Street, Fullarton, S A 5063 ^School of Geosciences, University of Wollongong, NSW 2522
The Study area is the northern part of Spencer Gulf within an area delineated by the cities of Port Pirie, Port Augusta and Whyalla, and including the narrow estuarine extension northwards of Port Augusta. The region is subject to a pronouriced Mediterranean climate. Summer water temperatures are high and salinity is significantly higher than that of the Southern Ocean. Maximum water depth is little more than 20m, these deeper areas forming relatively narrow elongate channels. Surrounding subtidal areas shallow upwards to broad intertidal and supratidal flats, with acconq)anying zones of vegetatioa Seagrasses grow thickly within most subtidal and intertidal areas. Along much cf the coast there are well developed mangrove woodlands, beyond which occur extensive supratidal samphire and other salt-tolerant plant communities; maximum tidal range is >4 m. Cyanobacterial mats grow on the otherwise bare inner sand flats, and within some of the mangroves. Most sediments within these settings are bioclastic carbonates. Benthic foraminifera occur in all the marine environments of the northern gulf and are particularly abundant in the seagrass meadows. A transect from the western coast to the edge of the deeper channel revealed an assemblage dominated by three species, in order of abundance, Nubecularia ludfuga, Peneroplis planatus and Discorbis dimidiatus, with less abundant, but consistently present Cribrobulimina mixta, Spiroloculina spp., Vertebralina striata, Massilina milletti, Triloculina spp., Elphidium crispum and E. macelliforme. Adjacent to the coast, but confined to a narrow zone within and close to mangrove woodland, Trochammina injlata. Ammonia beccarii and E. articulatum were all commoa A corresponding transect from the eastem side revealed a very similar, but not identical distribution of species. In hypersaline waters of the estuary north of Port Augusta, A. beccarii and E. articulatum were consistently present, and T. injlata occurred in association with sporadic mangroves. Ammobaculites barwonensis and Nonion depressulus also appeared to berepresentativeof this setting. In waters m between Port Pirie and Wtyalla there is a clearrelationshipbetween the numerical distribution of species and the depth of water. Species that are abundant in shallow subtidal seagrass meadows, e.g. D. dimidiatus, are less common in deeper water, and in contrast A/, milletti reaches maximum numbers and is the most common species in the deepest water. Quinqueloculina lamarckiana exhibits a similar, but less obviousrelationshipwith water depth. A core whichrecovered4 m of sediment from the deepest part of the Port Pirie to Whyalla transect was sampled fer fossil foraminifera at 10 cm intervals and at selected horizons fossil molluscs were t^en for either radiocak>on or amino acid racemisation age determinations. On the basis of the derived ages, the Holocene-Pleistocene boundary was determined to be at 180 cm and the underlying interval to ca. 250 cm was assigned to oxygen isotope stage 3. Shell from the base of the core yielded an AAR age which corresponded to oxygen isotope stage 7. The ^stribution of fossil foraminifera within the core can be interpreted to reveal the record of sea level changes. The lowermost part of the Holocene interval is dominated by Miliolinella labiosa, ahnost to the exclusion of all other species. Extreme palaeoenvironmental stress is signified by the essentially monospecific assemblage, which is interpreted to have lived in shallow, lacustrine-like waters at the onset of the postglacial transgression into the northem gulf. As the transgression proceeded, M milletti became established and increased in abundance to maximum numbers (at 90 cm) thus signifying sealevel stability. This maximum water depth is equally signalled by Q. lamarckiana. The gradual decrease i n M milletti and Q, lamarckiana upcore from 90 cm is consistent with an isostatic uplift of the northem gulf area of >4 m, with consequent fall in relative sealevel, in response to eustatic loading of the post glacial transgression. The stage 3 interval, 250-180 cm is dominated by N. lucifuga, thus signifying that waters of this transgression were never more than a few metres deep at the core site. The interval 350-250 cm is assigned to oxygen isotope stage 5e, the last interglacial, both on the basis of its stratigraphic position and on the occurrence throu^out the interval of Marginopora vertebralis, which is an established indicator species for sediments of this age in southern Australia. The relative abundance of M. milletti and Q, lamarckiana in these last inteiglacial sediments confirms that sealevel then was similar to that of today. For the lowermost stage 7 interval, 400-350 cm, a transgressive signal is registered by the reducing numbers upcore of the shallow water indicators, N. lucifuga and Peneroplis sp. and the corresponding increase in a species which favours deeper water, E, macelliforme. However the top of thisrniithas been eroded and palaeosol development has precluded foraminiferal analysis at 360 cm.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14^'' Australian Geological Convention, Townsville, July 1998
WERRIE BASALTS AND ALUM MOUNTAIN VOLCANICS: GEOCHEMICAL SIGNATURES AND GEODYNAMIC IMPORTANCE Graziella Caprarelli^ ^Department of Environmental Sciences, UTS, PO Box 123, Broadway, NSW, 2007
INTRODUCTION The Werrie Basalts and Alum Mountain Volcanics of NSW erupted during the crustal thinning and rifting which followed the end of Devonian-Carboniferous subduction along the eastern coast of Australia, and which had its expression in the Late Palaeozoic magmatic activity of the New England Fold Belt. Subduction resumed in the Late Permian some hundred km east of the previous arc. The Early Permian was a period of geodynamic transition, which needs to be thoroughly understood to be able to evaluate the overall dynamics of the Late Palaeozoic eastern Australian convergent margin. Because of their eruption during this transitional period, the the Early Permian volcanics are a valuable tool to study the establishment of the arcs and the evolution of the eastern margin of Australia. However, because of their widespread alteration, which modifies the chemical and isotopic abundances of elements of petrogenetic significance, not enough importance has been given in the past to the Early Permian volcanics. This work proposes that the Early Permian volcanics can provide useful geochemical information in spite of their alteration, if particular care is placed into defining the geochemical systems more resistant to weathering. Their geodynamic importance is also outlined. SAMPLE DESCRIPTION AND METHODOLOGY The volcanics are Lower-Middle Permian in age and outcrop in the Tamworth Belt. Rocks belonging to the Werrie Basalts are also present in drill-holes in the Sydney-Gunnedah Basin. Samples of Werrie Basalts from outcrops and drill-holes and of Alum Mountain basalts, andesites and rhyolites, were collected and studied for their chemical and isotopic compositions. Major and trace elements abundances were determined by XRF and INAA techniques (University of Adelaide, SA; Bequerel Laboratories, NSW). Hydrogen and oxygen isotopic ratios and ^^Sr/^^Sr and ^''^Nd/^'^^Nd ratios were determined by mass-spectrometry (CIS, NSW). Complementary petrographic examination and electron microprobe work were also carried out (UTS; GEMOC, Macquarie University, NSW). EFFECT OF WEATHERING ON GEOCHEMICAL SIGNATURES OF VOLCANICS To use the Early Permian volcanics in geodynamic reconstructions their chemical and isotopic compositions need to be unmodified since extrusion. Subaerial alteration processes affected all the rocks. The first task was to account for the effect of weathering of the samples. Petrographic and EPMA work revealed that calcite, quartz, chlorite and saponite are widespread. A thorough study of the geochemical effects of alteration on the rocks allowed to conclude that: (1) the oxygen isotopic composition is unmodified by weathering; (2) trace element abundances have been modified in the Alum Mountain Volcanics more than in the Werrie Basalts, where alteration was mostly isochemical; (3) calculated initial ^^Sr/^^Sr ratios are significant only in the more mafic lithotypes, because of the non-conservative effects of alteration on the Rb/Sr system; (4) the REE were unfractionated, although their overall concentrations were enhanced by the intense carbonation of the volcanics; (5) as a consequence, the initial ^"'^Nd/^'^'^Nd rations can be considered to be unmodified by weathering. GEODYNAMIC IMPLICATIONS The geochemical study of the samples of Early Permian volcanics of the Sydney-Gunnedah Basin and Tamworth Belt demonstrates that the rocks are sub-alkaline. The isotopic composition of oxygen and particularly the initial Sr and Nd isotopic ratios of the more mafic lithotypes (s^d as high as +7.8) indicates that they have a depleted mantle signature, which contrasts with that of the Carboniferous and Late Permian igneous rocks of the New England Fold Belt. This is particularly clear in an -esr diagram, where the Carboniferous and Late Permian rocks plot along a horizontal trend indicative of subduction, and the Early Permian volcanics plot along a different trend towards and within the field of MORBs. The geochemical signature of the Early Permian volcanics indicates that the crustal thinning and rifting of the lithosphere in the transition period between the end of subduction and its re-establishment eastwards was related to partial melting of depleted upper mantle.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
ADVANCES IN PETROLEUM SYSTEM EVALUATION OF THE OFFICER BASIN, WESTERN AUSTRALIA
Carlsen, G. M., Apak, N.S., Ghori, A., Grev. K.. Stevens, M.K, Geological Survey of Western Australia, 100 Plain Street, Perth, Western Australia
Empress 1A was cored to further define the petroleum source potential of Neoproterozoic and Cambrian rocks in the Officer Basin. Source rocks are the only remaining physical element of the petroleum system to be proven. Temporal elements of the petroleum system require further definition. We are defming the chronostratigraphic framework of the basin through biostratigraphy, isotope chemostratigraphy, SHRIMP, and K - A r dating techniques. The occurrences of stromatolites and associated fossil assemblages provide regional correlation and ages for Neoproterozoic rocks. The application of SHRIMP, and K-Ar has provided ages for samples from Empress 1 A. Current studies on the Browne Formation include Strontium/Bromine, Carbon/Carbon and Oxygen isotope analysis following observation of depositional features indicating a lacustrine environment. Structural analysis of dipmeter data indicates a stable slowly subsiding platform on the southwestern margin of the Yowalga Sub-basin. Geochemical analysis of samples from Empress lA has located 7 samples with measured total organic carbon content greater than 0.5%. The indicated thermal maturity of these samples is presently marginally mature to mature. Apatite fission track analysis and reflectance analysis indicates that the Neoproterozoic rocks of the Yowalga Sub-basin achieved maximum thermal maturity during the latest Neoproterozoic to early Paleozoic (600 to 300 Ma). A second thermal event identified during the Eocene (40 Ma) may indicate a period of hot fluid migration which could be associated with the migration of hydrocarbons. Results to date prove reservoirs at bed and bed set scale with porosity greater than 15 % and permeability of hundreds of millidarcies. Bed set to parasequence scale shale occurs in the Hussar, Kanpa and Steptoe formations in Empress 1A proving effective seal rocks. The close association of laminae scale source rocks with good quality reservoirs and sealing horizons provide all of the necessary physical elements for a petroleum system in the Yowalga Sub-basin. Future exploration will focus on defming structurally influenced depositional trends between Kanpa lA and Empress lA where accommodation space has been created along the transition between basin margin and deeper water environments. The search for source rocks in the Officer Basin depends upon knowledge of the depositional settings, which requires palinspastic reconstructions of the current northern margin of the Officer Basin. Currently planned field studies, geophysical surveys and stratigraphic tests will focus on the northern margin of the Waigen Sub-basin.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE PALAEOHYDROGRAPHY OF THE PACIFIC DEEP WESTERN BOUNDARY CURRENT SYSTEM
'Carter. R.M., ^Carter L. & 'McCave N. 'School of Earth Sciences, James Cook University, Townsville, Q4811 ^ e w Zealand Oceanographic Institute, NIWAR, Wellington, New Zealand ^Department of Earth Sciences, University of Cambridge, United Kingdom
The circulation of cold, deep water is one of the controlling factors in Earth's climate. Forty percent of this water enters the world ocean through the Southwest Pacific Gateway as the Deep Western Boundary Current (DWBC), which flows north out of the southern ocean along the east side of the Campbell Plateau-Chatham Rise-Hikurangi Plateau, east of New Zealand. The volume transport of the DWBC is about 20 x 10^ m^ s'^ (Sv), which comprises c.40% of the total input of deep water to the world ocean. The magnitude of this flow, and the low temperature of the water involved, are major determinants of the oceanography of the world ocean and of the global heat balance. Thus imderstanding the evolution of the DWBC is fundamental to understanding planetary ocean hydrography. The evolution of the DWBC system, and of related circum-Antarctic currents, has taken place since 32-25 Ma, when plate movements created the first oceanic gaps south of Australia and South America. An excellent stratigraphic record of these events occurs in onland South Island, and in offshore waters east of New Zealand. The genesis of a regionally widespread 29 Ma mid-Oligocene unconformity (Marshall Paraconformity) may relate to the inception of Western Boundary Current activity as much as to global sea-level change, and sediment drifts of Oligocene to Recent age are widespread in sediments deposited in a variety of water depths since the middle Oligocene. Since about 10 Ma, additional terrigenous sediment from uplifting mountains along the New Zealand plate boundary has been transported through deep-sea channel/fan systems, delivered into the path of the DWBC, entrained northwards as major current drifts, and finally consumed by subduction at the same plate boimdaiy after a transport path of some 3,500 km. In this way, sediment from the New Zealand Southern Alps on top of the Australian Plate arrives back beneath the same plate in the Hikurangi-Kermadec subduction zone, after a brief, schizophrenic sojourn in this world unsure as to whether it was a turbidite or a contourite, and uncertain whether it will return in a new cycle as tephra or new crust. The outstanding requirement for achieving enhanced understanding of the mid-late Cenozoic evolution of Pacific palaeoceanography is for stratigraphic and high-resolution time-series data from the intermediate to deep ocean parts of the DWBC system. Such data can only be obtained by ocean drilling, which will be gathered during ODP Leg 181 to be accomplished off eastern New Zealand in August-September, 1997.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
REGIONAL-SCALE HYDROTHERMAL FLUID FLOW AND ORIGINS OF PBZN-AG MINERALISATION AT BROKEN HILL, AUSTRALIA: CONSTRAINTS FROM OXYGEN ISOTOPE GEOCHEMISTRY Ian Cartwright Department of Earth Sciences, Monash University, Clayton 3168, Australia
Elucidating the origins of mineralisation is important for our overall understanding of geological processes, and is essential for mineral exploration programs. However, in metamorphosed or deformed terrains, the origins of mineralisation may be difficult to determine. The Broken Hill region of New South Wales, Australia, hosts major Pb-Zn-Ag mineralisation. Although studied for over a centur>, there is no consensus as to the origins of the mineralisation, mainly due to the granulite facies metamorphism and deformation that affected the region. Mineralisation models have included: exhalative, syndiagenetic, metamorphic, and post metamorphic. Here, new oxygen isotope data are presented that suggest that the orebody is exhalative and was formed as part of an extensive hydrothermal fluid flow system that persisted post its formation. The Broken Hill inlier comprises deformed and metamorphosed Proterozoic rocks of the Willyama Supergroup that are commonly interpreted as detrital sediments with minor carbonates and interlayered rhyolitic to basic volcanic rocks. The volcanic rocks, and associated intrusives, were probably formed at -1690 Ma. The Broken Hill region underwent metamorphism at-1600 Ma. Temperatures increased southward, with andalusite + muscovite (-500 °C), sillimanite + muscovite (580-680 °C), sillimanite + K-feldspar (680-760 ''C) and two-pyroxene (760-800 °C) metamorphic zones defined. At higher grades, pelitic and quartzofeldspathic rocks underwent partial melting, with subsequent retrogression fluxed by fluids exsolved from crystallising melts. Later shear zones also caused locallyintense retrogression. Most Pb-Zn mineralisation occurs within high-grade Broken Hill Group rocks that comprise pelites, calcareous rocks, and rhyodacitic volcanics. The orebody comprises stacked lenses of galena + sphalerite with rhodonite + fluorite + quartz + garnet + calcite. The orebody is associated with quartz + gahnite + garnet rocks that have beenvariousy interpreted as exhalatives , skarns, and garnet sandstones. Metasedimentary and metavolcanic rocks at Broken Hill, Australia, show regional-scale lowering of values from 12-14%o to 9-10%o with values as low as 7%c within a few hundreds of metres of the Pb-Zn-Ag orebodies. Coexisting minerals preserve oxygen isotope fractionations that are appropriate for regional metamorphic temperatures. This, and the absence of any correlation between values and intensity of retrogression precludes significant isotopic resetting occurring after the regional metamorphism. Stable isotope resetting by large-scale fluid flow during the regional metamorphism is unlikely as the fluid would have had to flow across stratigraphy and major structures. The association of low values with the orebodies suggests a link with mineralisation. The values are similar to those recorded in exhalative deposits where convective circulation of ocean water has occurred (eg Kuroko). However, the overall scale of resetting (tens of kilometres) is much larger than that around typical exhalative deposits and has dimensions similar to those of large igneous-hydrothermal systems. A two-stage fluid flow model is proposed. In Stage 1, mineralisation around hydrothermal vents at, or close to, the ocean floor at -1690 Ma produced localised low values. Base metal mineralisation in hydrothermal settings is common, and the quartz + garnet + gahnite rocks plausibly represent siliceous exhalative rocks that are commonly found associated with mineralisation in these settings. In Stage 2, large-scale, hydrothermal circulation caused regional-scale resetting of the oxygen isotopes and also other large-scale changes such as in the geochemistry of the Hores Gneiss. The heat source for both events could have plausibly been intrusive rocks (such as the Alma Gneiss) that were emplaced at around the same time as the volcanism. Preservation of the low values through the regional metamorphism implies generally little pervasive fluid flow at that time, which is often the case in high-grade regional metamorphic terrains.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
METEORIC FLUID FLOW THROUGH ALICE SPRINGS AGE SHEAR ZONES, REYNOLDS RANGE, CENTRAL AUSTRALIA: IMPLICATIONS FOR REGIONAL TECTONICS AND CRUSTAL FLUID FLOW Ian Cartwright\ Ian S. Buick^ David D. Lambert\ David A. Foster^ ^Department of Earth Sciences, Monash University, Clayton Vic. 3168, Australia ^School of Earth Sciences, La Trobe University, Bundoora Vic. 3083, Australia
Rocks in shear zones often undergo significant mineralogical, chemical, or isotopic changes as a result of the action of fluids. In many metamorphic terrains, shear zones formed after the peak of regional metamorphism and provide a record of late- or post-metamorphic fluid flow. The sources of fluids in these late shear zones are often unclear. In general, rocks that have undergone high-grade metamorphism will not produce fluids in later lower-grade events, and crystallising melts are a plausible source of fluids only during the initial stages of cooling. Hence, it is likely that at least some of the fluids in shear zones will be sourced from outside the terrain (e.g. the surface, the mantle, or underthrust unmetamorphosed rocks). The Reynolds Range is a multiply deformed and metamorphosed Proterozoic terrain within the Arunta Inlier of central Australia. It consists of two stratigraphic associations. A suite of pelitic and semipelitic metasediments, the Lander Rock Beds, together with early granitic plutons, form the basement to metasediments of the Reynolds Range Group (which include metamorphosed quartzites, pelites, marls, and marbles). The Reynolds Range Group rocks are intruded by later granitic plutons. An early metamorphic-deformation event, Dl-Ml, affected the basement rocks. Following the deposition of the Reynolds Range sediments there was a period of contact metamorphism at -1.78 Ga. Regional metamorphism (M2) occurred at -1.6 with M2 grades varying from greenschist (-400 °C) in the northwest to granulite (~74Q±40 °C) in the southeast at a relatively constant pressure of 400-500 MPa. The Reynolds Range is cut by major NE-dipping thrusts and shear zones, some of which extend into the deep crust. These major structures are interconnected by smaller shear zones. The grade of the assemblages in the shear zones are locally as high as amphibolite facies, with kyanite and staurolite or garnet and sillimanite developed in sheared metapelites. In the Southeast Reynolds Range, metre- to tens-of-metre wide amphibolite facies (550-600 °C, 500-600 MPa) shear zones cut metapelites, while metre- to hundreds-of-metre wide greenschist facies shear zones (350-450 300-400 MPa) cut metagranites. Rb-Sr and "^^Ar-^^Ar data suggest that both sets of shear zones formed in the 4(X)-300 Ma Alice Springs Orogeny, with the sheared granites yielding well-constrained ^"Ar-^^Ar ages of -334 Ma. The sheared rocks commonly have undergone metasomatism implying that the shear zones were the pathways of significant fluid flow. Altered granites within greenschist facies shear zones are transformed to white mica + quartz ± chlorite schists. These rocks have gained Si and K and lost Ca and Na relative to their unsheared counterparts, suggesting that the fluid flowed down-temperature (and hence probably upward) through the shear zones. Timeintegrated fluid fluxes estimated from silica addition are 10^-10^® moles/m^ (-lO'^-lO^ mVm^). The sheared granitic rocks locally have values as low as 0%o that are much lower than the values of the unsheared granites (79%o), and lower than other rocks in this terrain. In common with low rocks in general, we consider that the fluid flowing through these shear zones was meteoric. The flow of meteoric fluids into the deep crust may have been driven by topography, convection, and/or seismic activity accompanying the Alice Springs deformation. By contrast, sheared staurolite + kyanite-bearing pelites in amphibolite facies shear zones have similar values (6-8%o) to those of their unsheared counterparts. The fluids infiltrating these shear zones may have been derived from crustal sources, or have had their oxygen isotope signatures reset due to fluid-rock interaction. The infiltration of meteoric fluids into the ductile crust has important implications for tectonics. Possible mechanisms for fluid infiltration include: seismic pumping, overthrusting over unmetamorphosed sediments, or topographically induced fluid flow. This study shows that the surface is a viable source of fluids for the retrogression of metamorphic terrains.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
CONSTRAINING EXPLORATION MODELS FOR ARCHEAN GOLD MINERALISATION, YILGARN CRATON - WHAT ARE THE MISSING LINKS? Kevin F. Cassidv T. Campbell McCuaig ^ and David C. Champion ' ' Australian Geological Survey Organisation, Canberra, 2601, Australia ^ SRK (Australasia), 25 Richardson Street, West Perth, Western Australia 6005, Australia
Holistic exploration models for ore deposit formation require detailed knowledge of four essential components: (1) geometry of the mineralised system, (2) alteration and geochemistry of the system, (3) relative timing of fluid flow and mineralisation through structures, and (4) the regional stratigraphy, and structural, intrusive and meta-morphic history of a host terrane, and where components (1) through (3) fit within this evolutionary framework. Appropriate theoretical models generally employ a mineral systems approach where the ore deposit is considered as only one part of a complete regional-scale mmeral system. In such models, emphasis is placed on process rather than just the product. However, in practice, realistic geological processes are complex and usually incorporate several physical and chemical processes with complex interdependencies. Therefore, both empirical and theoretical considerations are required to achieve practical models for ore deposit formation. The first three essential components of a holistic model for gold mineralising systems in the Yilgam Craton of Western Australia are relatively well understood. Many features of gold mineralisation are common at most deposits (e.g., fluid and ore chemistry, strong structural control), indicating that they form part of a coherent genetic group. However, what features are important at the terrane- or even craton-scale; for example, can the gold mineralising events be linked to the tectonic evolution of individual terranes or the craton? Current models for the formation of Archean gold mineralisation in the Yilgam Craton do not incorporate a sufficiently detailed and precise understanding of the critical geological processes that were responsible for localising a deposit. Historically, research emphasis was on obtaining the stratigraphic and structural features of the Yilgam Craton. In the past twenty years, this emphasis has shifted to studies of gold mmeralisation at the deposit-scale, resulting in significant advances in understanding the various physical and chemical processes involved in the formation of a deposit. However, recent advances in the geochronology of gold mineralisation of the Yilgam Craton show that there was more than one gold mineralising event in the eastern part of the craton (e.g., Yandal belt). Several researchers also invoke 'early' mineralisation linked to an extensional phase of terrane development. Better geophysical and geological mapping has also shown that there are several major problems in our understanding of the geological history at the terrane- and craton-scale. In combination, there is a need to gain a better understanding of the stratigraphic, stmctural, intmsive and metamorphic history at the greenstone-belt or terrane-scale studies. What are the key geological features of the highly mineralised eastem Yilgam Craton? Although there are a number of features that are well understood, there are several major problems with our current geological knowledge of the highly-mineralised Kalgoorlie Terrane: (1) There is a common lower mafic/ultramafic stratigraphy across major stmctures. What was the role of synvolcanic stmctures during their extmsion? (2) There are common mismatches for the upper parts of the stratigraphy (Black Flag Beds and above). What is the absolute timing of this sequence and what role did extension and compression have in their development? (3) What is the absolute timing of granitoid 'doming' along the westem part of the terrane? Is it related to north-south extension or compression? (4) What constraints do the granitoids give on the cmstal thickness and heat flow throughout the terrane during granitoid emplacement? Did granitoid emplacement continue to ca. 2630 Ma throughout the terrane? (5) Metamorphism is poorly understood and probably better explained in terms of greenstone-belt-scale stmctural and thermal events rather than a single craton-wide tectonic or magmatic event. Can additional constraints be placed on the absolute timing, and conditions of metamorphism? A better understanding of these features is essential to unravel the geological and mineralisation history of the Kalgoorlie Terrane. Future research should provide a balance between deposit- and terrane-scale studies, with terrane-scale studies concentrating on aspects of the stratigraphic, stmctural, intmsive and metamorphic history of a terrane. In particular, improved knowledge of the role of granitoids and metamorphism in the hydrothermal mineralisation process is essential. Evidence for multiple mineralising events, including potential significant 'early' mineralisation, necessitates the unravelling of a terrane's evolution to effectively analyse the mineral potential of early stmctures. A better understanding of tectonic history will enable improved holistic models for gold mineralisation and potentially improved predictive capability to enable such models to become genuinely effective exploration tools.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE LACHLAN MARGIN, VICTORL\: THE MOYSTON FAULT, A NEWLY RECOGNIZED TERRANE BOUNDARY Ross A. Cavlev & David H. Taylor Geological Survey of Victoria. PO Box 500, East Melbourne, Victoria 3002
Recent GSV mapping of Western Victoria has resulted in a greatly improved understanding of its structural evolution and resolved the nature of die boundary of the Lachlan Fold Belt (LFB) with the older Adelaide Fold Belt (AFB) to the west. The Moyston Fault is now recognized as the suture zone between the two. The fault is the largest thrust recognized so far in the Victorian LFB and occurs just to the east of the Grampians as a major east-dipping structure. It separates areas with very different geological histories, and sharply delineates the western limit of the Victorian goldfields. The older rocks to the west have affinity with the South Australian AFB. To the east, LFB rocks from at least mid-crustal levels have been emplaced upwards and westwards onto the AFB along the Moyston Fault during the Silurian. The western LFB is comprised of Cambrian oceanic volcanics overlain by Cambrian to Ordovician quartz-rich turbidites, of which only the upper (sedimentary) part is widely exposed between Heathcote and Ararat. Its wide expanse, uniformly tight folding, cleavage and faulting, and uniform low greenschist metamorphic grade all suggests a horizontal enveloping surface. Only in the immediate hangingwall of some of the major meridional, listric thrust faults are parts of the lower volcanic sequence brought to the surface. Near Ararat the nature of the LFB changes markedly across the west-dipping Coongee Fault. The region between the Coongee Fault and the strike-parallel Moyston Fault 15 km further west (the Moomambool Subzone of the Stawell Zone; MS) is characterized by fundamental changes in structural, metamorphic and geophysical character, and represents the exposed midcrustal levels of the LFB emplaced upwards and over the AFB to the west. Approaching the Moyston Fault from the east, successively lower levels of polydeformed volcanic and sedimentary rocks are exposed across numerous large, closely spaced, subparallel, west- and east-dipping thrust faults (Cayley & Taylor, in press). The metamorphic grade of the MS progressively increases from low greenschist facies, adjacent to the Coongee Fault, to amphibolite facies in the hangingwall of the Moyston Fault. The volcanics progressively alter to homblende-gametquartz-plagioclase schist, and overlying turbidites become coarse quartz~muscovite-(gamet) schist. Structural intercalation of these units is fractal. Mylonites are ubiquitous, especially in the western parts of the MS. These changes reflect an approximately 45° easterly dip to the overall enveloping surface across this region, directly related to the orientation of the underlying Moyston Fault. This bounds the MS abruptly on its western side, running through Moyston, Londonderry, and just west of Mt Drunmiond (Cayley & Taylor, in press). Amphibolite grade schist of the MS in the hangingwall of the Moyston Fault is thrust over weakly deformed, overturned, very low-grade sediments and volcanics of the AFB. Geophysical evidence, such as the higher overall gravity and total magnetic intensity of the AFB extends east beneath the Moyston Fault as far as the Coongee Fault, interpreted by us to reflect the extent of AFB crust beneath the LFB. East of the Coongee Fault there is no evidence of pre-LFB crust at depth. The coincidence of the eastern margin of the MS—which marks the abrupt change in overall dip of the regional LFB enveloping surface—together with the first geophysical indications of AFB crust at depth, and the geometry of the Moyston Fault is strong evidence for the westward emplacement of a wedge of deforming LFB rocks from mid-crustal levels onto a rigid, east-dipping crustal scale ramp of AFB crust, acting as the backstop/indentor in a classical convergent setting. The numerous west-dipping thrusts within the hangingwall (MS) are truncated by the Moyston Fault, and must therefore have developed outboard, most likely in the position of the Coongee Fault. Here, successive antithetic faults (backthrusts) acconmiodated space problems created by the abrupt steepening of the LFB enveloping surface where west-moving LFB material encountered the AFB ramp at mid-crustal levels. As the leading thrust of the deforming Lachlan orogen, the Moyston Fault is likely to have remained continually active throughout the formation of the western LFB, accumulating the largest displacement and longest-lived movement history of any exposed fault in the western LFB. REFERENCE Cayley, R.A. & Taylor, D.H. (in press). Ararat 1:100 000 geological map report. Geological Survey of Victoria Report 115.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE STAWELL GOLDFIELD, VICTORIA: REGIONAL SETTING AND NEW DISCOVERY POTENTIAL Ross A. Cavlev & David H. Taylor Geological Survey of Victoria. PO Box 500, East Melbourne, Victoria 3002
The Stawell Gold Mine is Victoria's biggest gold mine (90 000 oz/yr). Recent GSV mapping and geophysical interpretation of the surrounding region, combined with detailed analysis of company data, explains the regional setting of the mine and identifies areas with similar prospectivity to the north and south (Cayley & Taylor, in press). The geology of the mine area and immediate surrounds has been described in detail by many recent workers. Although very complex, there is excellent 3-D geological control with numerous underground exposures and diamond drillholes. The regional setting of the mine sequence has, however, remained somewhat enigmatic—it is truncated against the Stawell Granite to the south, and obscured by poor exposure and cover to the north and west. Our work identifies the east-dipping, east-side-up Pleasant Creek Fault (PCF), just west of the mine sequence, as the regional scale structure responsible for emplacing the mine sequence to its present position near the surface. The Coongee Fault to the east is an important secondary control. Extensions of mineralised Stawell structures have been identified to the south of the Stawell Granite where, despite thin cover and easy access, exploration has been limited. Improved understanding of the regional setting and geophysical character of the mine sequence has also allowed for a well constrained interpretation of the northern strike extensions beneath shallow cover, generating new exploration targets in this region. The Stawell mine sequence and the PCF occur in the Moomambool Subzone, the strongly deformed hangingwall of the east-dipping Moyston Fault. The Moyston Fault sharply delineates the western limit of the Victorian goldfields, and is now recognised as the terrane boundary between the Lachlan Fold Belt and the older Adelaide Fold Belt to the west (see Cayley & Taylor, 1998, this volume). The PCF, clearly visible on magnetic images as the western margin of a magnetic volcanic-dominated package, can be seen to converge with and link into the Moyston Fault northwest of Stawell, and must therefore share the same geometry—a synthetic hangingwall step-off structure. The mine sequence at Stawell consists of a gently north-plunging antiformal culmination (the Magdala Antiform) of thrust-stacked Cambrian oceanic volcanics overlain by volcanic sediments which pass up into quartz-rich Cambro-Ordovician turbidites. These were typically altered to schist during intense NE-SW shortening in the Silurian. The transition from early ductile to later brittle deformation (which hosts mineralisation) in the mine sequence may reflect changing pressure conditions during a single protracted deformation, as the mine sequence was brought from -15 km depth towards the surface along the PCF. The Central Lode System (CLS) at Stawell is developed in shears along the faulted volcanic-sediment interface on the west flank of the Magdala Antiform, and links into the sub-parallel, mineralised Stawell Fault to the west. Together these structures enclose a west-facing belt of turbiditic sediments. This distinctive belt also occurs south of the Stawell Granite, enclosed by the Copes Hill and Cathcart faults in the Garden Gully region. The Copes Hill Fault separates turbidites to the west from a deformed, broadly antiformal belt of faulted metavolcanics to the east, and is therefore the direct equivalent of the CLS. Further south, this fault hosts the historically important Eaglehawk Reef Limited exploration has yet to rigorously test these prospective turbidite/metavolcanic contacts. The mine mineralisation is truncated by the northeast-dipping South Fault which underiies the Madgala Antiform and CLS and truncates the Stawell Granite contact aureole. Our mapping shows that the South Fault links westwards into the PCF as parallel northeast-dipping mineralised quartz reefs, and eastwards into the east-dipping Concongella Fault as an anastomosing fault network which skirts, then cuts the Stawell Granite. The South Fault is therefore a mineralised regional-scale linking fault, connecting the PCF and the Concongella Fault, partly reactivated and mineralised during post-intrusion north-south shortening. The post-granite mineralising event in the Stawell region elevates the prosepectivity of reactivated portions of these, and related faults. REFERENCE Cayley, R.A. & Taylor, D.H. (in press). Ararat 1:100 000 geological map report. Geological Survey of Victoria Report 115.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
DIAGENETIC TIMING OF FERROAN DOLOMITES AND SIGNIFICANCE FOR MINERALIZATION AT THE GEORGE FISHER ZN-PB-AG DEPOSIT, MT ISA. Ltigy H. Chapman School of Earth Sciences, James Cook University, Townsville, Queensland, 4811.
Mineralization at the George Fisher deposit (107Mt @ 93g/t Ag, 5.4% Pb and 11.1% Zn) occurs as stacked, stratiform lenses hosted by rhythmically laminated, pyritic carbonaceous siltstones and separated by barren, thickand thin-bedded mudstones of the ubiquitously fine-grained, carbonate-rich Proterozoic Urquhart Shale. Studies of base metal sulphides at the deposit have identified two temporally distinct styles of sphalerite mineralization. Earliest sphalerite occurs with pyrite, as fine-grained, mm-scale, layer-parallel replacements of ferroan dolomite and ankerite. These Fe-rich carbonates represent one of the earliest known generations of carbonate at the deposit, and formed prior to the deformation of the sedimentary sequence. Textural and geochemical data show that they precipitated from relatively hot and focussed fluids quite distinct from those related to the syndeformational system associated with Mt Isa copper mineralization. Thus, at least two hydrothermal events affected rocks in the vicinity of the George Fisher deposit. The major rock constituents of least mineralized examples of rhythmically laminated siltstones and mudstones include anhedral, very finely crystalline ferroan dolomite and ankerite mosaic (30-80%), detrital silt-sized quartz (20-50%) and silt-sized dolomite (<10%). Quartz and dolomite typically occur as isolated grains in ferroan dolomite and ankerite. Key textural associations that are uniformly developed throughout the various sedimentary facies include selective ankerite alteration rims on dolomite grains, absence of ankerite rims on detrital quartz grains and consistent paragenesis of ferroan dolomite and ankerite. These relationships indicate that the Fe-rich carbonates in these sediments represent a widespread authigenic component that formed in part by selective replacement of a preexisting carbonate matrix or cement. Whole rock ferroan dolomite-ankerite has an isotopic range of = 18.0 to 18.5%o SMOW, and = -3.5 to -2.6%o PDB. These values are consistent with the formation of Fe-rich carbonates from fluids at elevated temperatures and/or fluids with relatively low data indicate that the bulk of the carbon was inherited from a carbonate precursor with minor organic-derived carbon. Fe-rich carbonates typically form from reduced formation waters during deep burial of a sedimentary pile, and exhibit lower and ratios compared with sedimentary and early-diagenetic carbonates. Although the Fe-rich nature of earliest replacement carbonates at George Fisher is consistent with their precipitation from reduced fluids during late diagenesis, the values of diese carbonates are depleted relative to those typically reported for burial dolomites. and of Fe-rich carbonates from George Fisher are depleted relative to examples of Urquhart Shale distal to mineralization (cf. Waring, 1990), although the Fe-rich nature of the carbonates is apparently uniform (cf. Neudert, 1983). Furthermore, syndeformational carbonate veins and alteration at George Fisher display greater depletion and define a separate fluid trajectory. This indicates that the isotopic signature of early Fe-rich carbonates reflects an episode of fluid migration at the deposit prior to Cu mineralization in the region. Relationships to distal Urquhart Shale suggest that fluid temperatures were locally elevated at the George Fisher deposit compared to those fluids responsible for the precipitation of distal Fe-rich carbonates which have a more primary carbonate signature. Therefore the volumetrically significant Fe-rich, and isotopically anomalous, replacement carbonates at George Fisher appear to represent a syndiagenetic alteration associated with locallyintroduced hydrothermal fluids. If this is the case, it is possible that these carbonates formed as part of an evolving mineralization system that was responsible for the introduction of in situ pyrite and earliest sphalerite mineralization at the George Fisher deposit. REFERENCES Neudert, 1983. A depositional model for the Upper Mount Isa Group and implications for ore formation. PhD Thesis, Australian National University, Canberra, Australia. Waring, 1990. Genesis of the Mt Isa Cu ore system. PhD Thesis, Monash University, Melbourne, Australa. AcknQwledgements This work was published with the permission of MIM Ltd. and benefitted from discussions with Mark Hinman.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
I-TYPE GRANITES OF THE LACHLAN FOLD BELT AND THEIR SOURCE ROCKS B.W. Chappell ARC Key Centre for the Geochemistry and Metallogeny of the Continents (GEMOC), Department of Geology, ANU, Canberra, ACT 0200
I-type granites comprise a little less than half of all the granites of the Lachlan Fold Belt. They occur in association with S-type granites throughout the central parts of the belt, but alone or with very minor amounts of such granites in the eastern and western parts. The I-type granites cover a wide range of compositions but their study is simplified by a subdivision into suites that share mineralogical, chemical, isotopic, and textural features. A broader and less precise subdivision is made into supersuites. Compositional differences between suites are thought to correspond to analogous differences between source rocks. Variations within suites resulted in some cases from fractional crystallisation, but more generally were the product of differing degrees of separation of a partial melt from the residual unmelted material (restite). Rocks of the Boggy Plain Supersuite make up close to 10% of the I-type granites. These range from cumulate gabbros to highly fractionated monzogranites and that diversity was the result of fractional crystallisation. The higher temperatures of these granites and their high Sr and Ba contents have lead to the suggestion by D. Wybom that their source rocks were related to plutonic equivalents of the Ordovician shoshonite volcanic rocks that are prominent in the northern part of the belt, and which share a similar geographic distribution. Some other I-type suites of the belt, such as Jindabyne, are relatively mafic. However, more often there is a prominent felsic component, that was produced as a partial melt from quartz- and feldspar-bearing rocks in the crust. Progressively more mafic rocks of those suites contain increasing amounts of a restite component. There is at most a very minor component in these rocks of direct mantle derivation, so that chemical and isotopic compositions have the potential to provide valuable data on the nature of the deep crust. Both the I- and S-type granites of the Lachlan belt have a range of compositions for radiogenic isotopes, with a small amount of overlap. McCulloch & Chappell (1982) interpreted such data in terms of distinct I- and S-type sources, noting that granites have other compositional features that preclude their formation from mixtures of isotopically more primitive and more evolved components. In contrast. Gray (1984) proposed a mixing model to account for the Sr isotopic compositions, and Keay et al. (1997) interpreted the isotopic data in terms of a three-component mixing model. However, such models are not consistent with chemical compositions of the granites in the Bega Batholith, the source of much of the isotopic data. From east to west, granites of that batholith are isotopically more evolved, which would be consistent with an increasing sedimentary component in the source rocks, and with the decreasing Na and Sr contents of the granites, and by inference their source materials. However, Ca increases in abundance westwards as the rocks become more isotopically evolved, which is not consistent with increasing amounts of a sedimentary component, particularly one that resembles the exposed Ordovician turbidites. If mixing was an important process in producing the source materials of the I-type granites of the Bega Batholith, then it must have involved components, both igneous and sedimentary, that are not exposed at the surface. It is more likely that mixing was of secondary importance and that the different isotopic and chemical components, or suites, of the Bega Batholith, correspond to distinct source rock compositions, of various ages, in the deep crust. Further work should enable those deep components to be better defined and contribute to a better understanding of the remote lithosphere of the Lachlan Fold Belt. REFERENCES Gray, C.M., 1984. An isotopic mixing model for the origin of granitic rocks in southeastern Australia. Earth and Planetary Science Letters 70, 47-60. Keay, S., Collins, WJ. & McCulloch, M.T., 1997. A three-component Sr-Nd isotopic mixing model for granitoid genesis, Lachlan fold belt, eastern Australia: Geology, 307-310. McCulloch, M.T. & Chappell, B.W., 1982. Nd isotopic characteristics of S- and I-type granites. Earth and Planetary Science Letters 58, 51-64.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
COMPOSITIONAL VARIATION IN THE S-TYPE GRANITES OF THE LACHLAN FOLD BELT AND ITS CAUSES B.W. Chappell' and A.J.R. White' ^ ARC Key Centre for the Geochemistry and Metallogeny of the Continents (GEMOC), Department of Geology, ANU, Canberra, ACT 0200 'Victorian Institute of Earth and Planetary Sciences (VIEPS), School of Earth Sciences, University of Melbourne, Parkville, Vic 3052
S-type granites of the Lachlan Fold Belt are always peraluminous or oversaturated in Al. This implies that they were derived from a similarly Al-oversaturated source, which therefore must have been through at least one cycle of weathering. The S-type granites range from mafic cordierite- and biotite-rich granodiorites and tonalites, to very felsic, and sometimes strongly fractionated, monzogranites. The variation from mafic to felsic compositions cannot be due to the direct mixing of felsic crust-derived melts with mafic melts, because suites of S-type granites become more peraluminous as they become more mafic. Compositional variation within the S-type granites results from a variety of factors and processes. (a) degree of weathering of the source rocks. Rocks of the Cooma Supersuite are of restricted occurrence and were derived from the most strongly weathered materials and are therefore lower in Na and Ca than the other S-type granites. The component suites of the Bullenbalong and Koetong supersuites, for example, were derived from less-weathered source rocks, while source rocks of the Wyangala Supersuite were only mildly weathered. Granites that were derived from a particular source composition are placed in suites; differences between source rocks (and suites) are related to weathering rather than the addition of mafic rock, as some have suggested. (b) retention of variable source-rock compositions. The more mafic rocks of the Bullenbalong Suite have increasing Zr contents with increasing Si02. In magmatic rocks saturated in Zr, that can only be due to the retention of a corresponding compositional feature from the source sedimentary rocks, so that the igneous rocks represent sedimentary rock compositions that were mobilised by partial melting. (c) restite fractionation. In the more felsic rocks of the Bullenbalong Suite, and for the mafic rocks (<69 % SiOi) of the Koetong Suite, for example, variation resulted from the separation of varying amounts of restite from a more felsic melt. (d) cumulate rocks formed by fractional crystallisation. For several reasons, the most mafic S-type granites cannot be cumulates. However, there are rocks in which the accumulation of feldspar has resulted in Sr contents significantly higher than other rocks in that suite. Such rocks are complementary to (e). (e) rocks formed from liquids produced by fractional crystallisation. More felsic granites produced by the equilibrium crystallisation of liquids resulting from the fractional crystallisation of precipitated minerals, are found in the Koetong Suite and some other felsic suites. These include rocks produced by extreme fractional crystallisation, such as the Interview Suite of western Tasmania. (f) late stage hydrothermal alteration. Almost all of the most felsic S-type granites have compositions close to the Tuttle & Bowen minimum, so that crystal-liquid equilibrium, either during partial melting or fractional crystallisation, dominated in their formation. However in some rocks, e.g. the Ardlethan Granite, there is sufficient departure from such compositions to show that some hydrothermal alteration was superimposed on dominant fractional crystallisation. These mechanisms of producing variation have some important implications. For example, many mafic S-type granites are very close to source-rock compositions, which enables the development of that source to be modelled, and the fractionation of the granites from that composition to be examined in detail. At the other extreme of composition, the most highly evolved felsic granites are the result of extended fractional crystallisation ± hydrothermal alteration. This is important for understanding the origin of felsic granites in areas where more mafic granites are not exposed, such as Tasmania and Cornwall.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
THE STRUCTURAL DEVELOPMENT OF THE QINLING OROGEN: IN THE DABIE REGION USING THE HISTORY OF MOVEMENT PRESERVED IN PORPHYROBLASTS Aigen Chen School of Earth Sciences, James Cook University, Townsville, Queensland 4811
In multiply deformed and metamorphosed terranes, early matrix deformation fabrics can be partly or completely destroyed. However, it has been found that linear indicators of movement (e.g. Foliation Intersection Axes) can be preserved in porphyroblasts from the effects of subsequent orogenesis. In the Dabie Region of the Qinling Orogen in central China, about 90 samples around three macroscopic folds all contain garnet and albite porphyroblasts with simple to complex inclusion trails. Up to three foliations (SpS3), with undisputed truncations between them can be observed. These early fabrics have no relation to the matrix foliation which has also been folded in the later deformation event S5 with axial-plane parallel crenulation. A young shallowly dipping crenulation cleavage (S^) is observed both in outcrop and under the microscope. In several samples high pressure blue schist minerals have been found as inclusion trails. These high pressure minerals have been the focus of debate concerning orogenesis in the Dabie Region. Unfortunately, numerous tectonic models associated with either the exhumation of high pressure rocks and/or the plate collision process are still disputable, depending on different approaches. A significant problem is the lack of detailed documentation of microstructure, particularly those preserved in porphyroblasts, because it is these porphyroblasts which also preserve the relics of some high pressure minerals! The method used in this study is the measurement of early lineations preserved in garnet and albite porphyroblasts by the means of thin-sectioning samples at 10° intervals. This three dimentional approach is independent of the porphyroblast-forming process, i.e. rotation or non-rotation. It also has the advantage over measurement on the inclusion trail orientation which is workable only for simple inclusion trails in 2 dimension. So the results obtained by using this method have great potential for solving the problem surrounding the mechanism of spiral inclusion trail development-an enigmatic issue which puzzled structural geologists for decades. A total of 109 FIA were recorded from 85 samples, including 60 samples containing one FIA 24 samples have either two or three set of FIA and one sample (b5-9) in which 4 set of FIA have been documented. These FIA can be divided into 4 set based on their orientation. The mean vector of these 4 set of FIA are 4.4® (25 FIA data), 56.8° (23 FIA data), 283.3°(46 FIA data) and 328.3° (15 FIA data) respectively. The relative timing for these FIA set can be determined on the basis of truncations between inclusion trails, relationship between matrix foliation and inclusion trails, different generations of porphyroblast growth (different phases or different stages of same mineral) and particularly the core-to-rim or core-medium-rim changes recorded in a same sample. The FIA succession in the Dabie Region is, from older to younger, NE-SW, SEE-NWW, N-S and SSE-NNW. It is noteworthy that only the last set of FIA (i.e. the SSE-NNW one) are generally parallel to the axial planes of the'three sampled macroscopic folds while all three earlier set of FIA lie oblique to them. Yet the orientation of these earlier set of FIA do not change across a range of younger structures. Interestingly enough is the SEENNW set of FIA , which has the largest number of FIA data, has a FIA orientation parallel to the overall trend of the Qinling Orogen rather than the subsequently generated folds. This coincidence, combined with other evidence may imply FIA, as linear indicators of movement, can be preserved in porphyroblasts from the effects of subsequent orogenesis in the Dabie Region. The FIA orientations and their relative timing also suggest a possible connection between FIA and plate motion between North and South China Blocks in the period ranging from 240 Ma to 140 Ma. That means , during this period, the collision between North and South China blocks initiated in the easternmost section along a NE-SW trending Suture Zone as indicated by the earliest NE-SW trending FIA set between 240-200 Ma. Further collision took place along SSE-NWW trending Suture Zone which preserved the second set of FIA with SSENWW orientation between 200-160 Ma. Finally the collision may take place along a nearly N-S trending zone which produced N-S trending FIA during the time of 160-140 Ma. The high pressure blue schist minerals mainly appear in the core of garnet porphyroblasts. They correspond to the earliest fabrics preserved in the porph>Toblasts, implying these high pressure minerals formed during the NESW trending Suture Zone. Microstructural anatomy for some multicored porphyroblasts favoured a non rotation mechanism of porphyroblast development. Some interlocked porphyroblasts provided an important reference framework which can be used to compare with other isolated porphyroblasts. The similarity of inclusion trail orientation between these interlocked porphyroblasts and unlocked ones obviously support non rotation for these unlocked porphyroblasts during subsequent deformation. 78
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
STRIKE-SLIP-INDUCED LOCAL COMPRESSIONAL DEFORMATION IN THE ARCHAEAN GREENSTONE BELTS, NORTHEASTERN YILGARN CRATON, WESTERN AUSTRALIA She Fa Chen^ Songfa Liu^ and Walter K. Witt^ 'Kalgoorlie Regional Office, Geological Survey of Western Australia, P.O. Box 1664, Kalgoorlie, WA 6430 ^Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 ^Sons of Gwalia Ltd., 16 Parliament Place, West Perth, WA 6005
N- to NNE-trending folds and reverse faults in the Laverton, Duketon, Yandal, and Mt Kilkenny-Welcome Well areas, are markedly anomalous with respect to the regional NNW structural trend of the northeastern Yilgam Craton, Western Australia. The dominant NNW tectonic grain is defined by regional faults, elongate granitoid bodies, macroscale folds, and extensive axial planar foliation. Some NNW-trending regional faults preserve evidence for early reverse movement during a regional shortening stage, followed by sinistral strike-slip movement during a transpressional stage. The N- and NNE-trending structures are predominantly situated within antidilational jogs bounded by the NNW-trending strike-slip faults and at their terminations. Antidilational jogs are local compressional structures generated by strike-slip faulting on stepover boundary faults. The Laverton, Duketon and Yandal antidilational jogs, with a rhombic geometry and a width of 30-50 km, comprise three major structural elements. (1) NNW-trending, right-stepping, sinistral strike-slip faults form the northeastern and southwestern boundaries of the antidilational jogs. These include the Claypan and Hootanui Faults in the Laverton antidilational jog, the Hootanui and Lulu Faults in the Duketon antidilational jog, and the Ninnis Fault and part of the Keith-Kilkenny Fault Zone in the Yandal antidilational jog. (2) Internal N- and NNE-trending macroscale folds and reverse faults are truncated by the NNW-trending boundary faults at a moderate to high angle. These N- and NNE-trending structures are well developed in the Laverton and Yandal antidilational jogs, and locally in the Duketon antidilational jog. Within the Laverton antidilational jog, some internal reverse faults have an arcuate geometry, and the deflection of these faults against the NNW-trending Hootanui Fault suggests sinistral strike-slip on that fault. During the transpressional stage, polymictic conglomerate was deposited southeast of Laverton, in a compressional basin bounded by the N- and NNEtrending Lawson and Kelly Faults. (3) A N-trending transpressional fault extends diagonally across each antidilational jog. It links the NNW-trending boundary faults and transfers sinistral movement from one boundary fault to the other. One example is the sinistral Laverton Fault within the Laverton antidilational jog. Polymictic conglomerate is locally present along this fault. The Ockerbury Fault, a zone up to 1 km wide of strongly sheared rocks within the Yandal antidilational jog, is another example. In the Mt Kilkenny-Welcome Well area, there are five macroscale N- and NNE-trending folds, with a wavelength of 3 to 10 km. These are the Kilkenny Syncline, Rio Tinto Syncline, Corkscrew Anticline, Welcome Well Syncline and Benalla Anticline, generally plunging to the S and SSW. Their limbs are truncated by N- and NNE-trending reverse faults. These N- and NNE-trending structures are truncated by the NNW-trending Kilkenny Fault at a high angle. Although the Keith-Kilkenny Fault Zone that contains several NNW-trending faults, extends further to the NNW towards the Perseverence-Wiluna areas, the Kilkenny Fault within the fault zone is terminated in the Mt Kilkenny-Welcome Well area. In addition, granitoid intrusion late in the fault zone history may have formed a buttress that impeded northerly transport of greenstones. It is suggested that the local compressional deformation, induced by sinistral strike-slip faulting on the Kilkenny Fault, has produced the Nand NNE-trending, macroscale folds and reverse faults in the Mt Kilkenny-Welcome Well area. It is concluded that regional ENE-WSW shortening in the northeastern Yilgam Craton is responsible for the development of NNW-trending reverse faults, upright folds and axial planar foliation. Reverse movement on the boundary faults of the Keith-Kilkenny Fault Zone resulted in the deposition of polymictic conglomerate and greywacke in the 3-8 km wide Pig Well graben. During subsequent E-W transpression, some pre-existing reverse faults were reactivated as sinistral strike-slip faults. Local compression generated by sinistral strike-slip on the NNW-trending faults produced thie N- and NNE-trending structures within the antidilational jogs and at fault terminations.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
LATE DEVONIAN TO EARLY PERMIAN SEQUENCE STRATIGRAPHY AND OIL -GAS ASPECTS OF THE TARIM BASIN, NORTHWEST CHINA Z. 0 . Chen and G. R. Shi School of Ecology and Environment, Deakin University, Rusden Campus, 662 Blackbum Road, Clayton, Vic 3168 Australia
Tarim Basin, occupying area of 572.700 sq. km., is situated between the Tianshan and Kunlun fold belts of northwestern China. It is also bounded by the Aerjin Mts in the east. This basin develops a relative complete sequence through the Pre-Cambrian (Proterozoic) to Quaternary. The geological investigation and petroleum exploration in recent years show that the Tarim Basin is the largest petroliferous basin with rich petroleum resources in China. The reserve for oil and gas is up to 19.1 billion ton, which account for one-fifth of the total reserve in China (Gu, 1997). The main oil-bearing strata are concentrated within the Late Devonian to Early Permian rocks (Chen, 1997). The facies analysis and sequence stratigraphic correlation of the outcrop sections and well logs permit the recognition of 15 sequences, corresponding to the third -order cycles, from the Upper Devonian, Carboniferous and Lower Permian rocks. Of these sequences. Sequences 1 to 5 are referred to the Supersequence I, reflecting a second-order cycle dated as the Famennian of the Late Devonian to the Early Carboniferous (Chen, 1995, 1997). Sequences 6 to 10 are assigned to the Supersequence 11 of the Bashikirian to Moscovian of the Late Carboniferous, which is defined by fusylinids Profusulinella zone and Fusulina -Fusulinella zone. Sequences 11 to 13 are attributed to the Supersequence HI of the Kassimovian to Gzhelian of the Late Carboniferous, and the Asselian -Artinskian of the Early Permian, marked by the appearance of fusulinids Triticites zone, Sphaeroschwargerina zone and Eoparafusulina zone. This supersequence is the topmost marine depositional package in the central desert areas of the basin. Supersequence IV comprises sequences 14 and 15, which are indicative of the Kungurian age of the Early Permian in accordance with the correlation of conodont and brachiopod faunas. These sequences were only accumulated along the southwestern and northwestern margins of the basin. Each supersequence boundary is characterized by a prominent regional unconformity, and is often associated with the occurrence of the reservoir, while the mudstone of the maximum flooding surface condensed sections (mfscs) are significant source rocks or cap rocks. The largest reservoir is the deltaic sandbody of the Famennian Donghetang Formation of the Upper Devonian, which forms the Lowstand System Tract (LST), and appears at the basal part of the Supersequence 1. The gypsum-bearing mudstones of the Visean of the Early Carboniferous often form the Highstand System Tract (HST) and are excellent caprocks across the basin. The Visean black shales of the Transgressive System Tract (TST), distributed in the southwestern margin of the Tarim Basin, are potential source rocks as well.
Gu, J.Y. 1997. Carboniferous sequence stratigraphy and oil and gas in Tarim Basin, Northwest China. In Stratigraphy, ed. by Naiwen Wang & J. Remane. Proceedings of the 30th International Geological Congress. Utrech, the Netherlands, Tokyo, Japan, 11, 61-66. Chen, Z.Q. 1995. Late Devonian to early Carboniferous outcrop sequence stratigraphy from Bachu area of Tarim Basin with discussion on C/D boundary. Acta Palaeontologica Sinica 34 (4), 475-487. Chen, Z.Q. 1997. Late Devonian to Early Carboniferous Sequence stratigraphy of the Tarim Basin, Xinjiang, NW China. Palaeoworld 7,177-196.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
HEAVY METAL CONCENTRATIONS IN PORT KEMBLA OUTER HARBOUR B.E. Chenhall, B.G. Jones and A. Goodfellow School of Geosciences, University of Wollongong, Wollongong, NSW 2522
Samples from the heavily industrialised Port Kembla Outer Harbour, south of Sydney, New South Wales, were investigated to determine trace element concentrations in the sediment as well as possible sources. Copper, lead, zinc, arsenic, gold, nickel, selenium and antimony were identified as major trace metal contaminants. These elements attained maximum concentrations in the southern portion of the Outer Harbour adjacent to the Southern Copper Smelter. Based on chemical analysis of dusts and fume generated by various industries in the Port Kembla area, together with statistical analysis of the trace metal data, the Southern Copper Smelter is the most likely source for antimony, selenium, copper, gold and arsenic; BHP for cobalt and iron; with HHP and Southern Copper being responsible for zinc, lead and nickel contamination. Microscopic examination of the sediments indicates that industrially-derived particulate matter, including coal, coke, graphite and siliceous slag fragments is abundant. These particulates are suggested to make a significant contribution to trace metal loading of the sediment. The trace metal pollutants probably have entered the harbour via both direct atmospheric deposition and water-borne mechanisms. In the souther n part of the harbour a plume of sediment trace metal contamination, of probable water-borne origin, is associated with drainage systems originating from the copper smelter. Pockets of sediment in this area are highly contaminated with Cu, Pb, Zn, As and Se (present in concentrations exceeding Dutch C criteria). Alkaline, reducing conditions, together with bacterially-mediated processes probably favour trace metal retention in the sediment. Trace metal-sediment grain size relationships indicate that the silt and clay fractions are associated with the highest concentrations of trace metals. However the relatively weak positive correlations between the finer sediment fractions and trace metal concentrations suggest that other parameters such as organic matter content are important in tree metal retention within the sediment column. In the environment of Port Kembla Harbour, coarse (>63 micron) trace metal-bearing particulate matter, known to be present in the sediment may also contribute to the lack of clearly defined relationships between sediment grain size and trace metal content. Port Kembla Outer Harbour is the focus of a current plan involving creation of new ship loading facilities via dredging and relocation of the spoil to form reclaimed land. Previously trace metal contaminated spoil has been dumped at sea, a practice which has now been discontinued. Effective management of Port Kembla Outer Harbour sediments requires that the sediment remain under anoxic, alkaline conditions to avoid the possibility of trace metal release. In a pilot program, a dredge spoil disposal technique called "sub-sediment deposition" has been employed in the Outer Harbour as the first stage of land reclamation. This technique is based on sub-aqueous sealing of the contaminated material with uncontaminated sediments to stop leaching. It appears that the dredge disposal site experiment was successful and further disposal in this manner should be encouraged.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14'^ Australian Geological Convention, Townsville, July 1998
SOURCE ROCK IN A SHALLOW CARBONATE RAMP SUCCESSION, MIDDLE DEVONIAN KEG RIVER FORMATION, ALBERTA, CANADA Nancy Chow' and Lavem D. Stasiuk" 'Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada. Email: n_chow@umanitoba.ca -Geological Survey of Canada, 3303 - 33rd Street N.W., Calgary, Alberta T2L 2A7, Canada
Sedimentology and organic petrology of Devonian organic-rich carbonate deposits in western Canada have provided new insights into the first-order depositional controls on hydrocarbon source rocks. Organic facies, as defined by incident light microscopy and maceral assemblages, have been correlated with depositional facies and cycles to recognize the role of organic productivity on the accumulation of organic matter in shallow-marine carbonate environments. The results of this integrated study should stimulate interest in areas with shallowmarine successions previously considered to have poor source rock potential. The Keg River Formation in northern Alberta is a succession of carbonates and siliciclastics that were deposited in the Elk Point Basin during Eifelian and Givetian time. The formation includes an areally widespread Lower Keg River ramp unit and an Upper Keg River platform unit fringing the Peace River Arch along the western flank of the Elk Point Basin and extending eastward into the La Crete sub-basin. In the outer part of the platform near the Senex oil field and in the La Crete sub-basin, the formation consists of three genetic successions: a lower ramp sequence and an upper ramp sequence, which together comprise the Lower Keg River Member; and an overlying shelf and basin sequence with isolated reefs comprising the Upper Keg River Member. The lower ramp sequence, 4-12 m thick, is a shoaling-upward cycle that represents the initial open-marine incursion into the Elk Point Basin. The sequence is of relatively uniform composition throughout the basin, consisting of basal, massive to laminated mudstones overlain by brachiopod-crinoid wackestones to floatstonesrudstones. The upper ramp sequence is also a shoaling-upward cycle and consists of a lower bituminous succession that grades upward into a nonbituminous carbonate succession. In the proximal ramp position, the bituminous succession (~21m thick) consists of seven or eight sub-cycles, each composed of bioturbated, argillaceous mudstones and dolomitic shales ("green shales") overlain by skeletal wackestones and packstonesfloatstones. The source of terrigenous clastic sediment was probably the Peace River Arch to the west. Toward the basin, the green shales grade laterally into bituminous laminites which, in turn, coalesce into a metre-thick laminite (bituminous marker) in the distal ramp position. The bituminous marker can be recognized throughout the La Crete sub-basin and is composed of finely laminated, bituminous layers and carbonate-rich layers containing abundant stylolinids and nektonic ostracods. The bituminous marker is overlain by an upward shoaling sequence, ~5 m thick, of brachiopod-crinoid wackestones-floatstones grading up into oncoid packstonesfloatstones. The bituminous marker is the potential hydrocarbon source rock (TOC values of 5-18%) in the La Crete sub-basin and is interpreted to be a condensed section deposited under anoxic conditions in water depths of only 20-40 m. Episodic, high organic production in near-surface waters along a shallow ramp was the first-order control on the accumulation of organic matter in the upper ramp sequence of the Lower Keg River Member. Unequivocal evidence for high productivity in the bituminous marker and coeval, platformal green shales is the extensive occurrence of "algal bloom" organic facies. These facies are defined by (1) algal akinete cells, which reflect stressful paleoenvironmental conditions commonly attributed to algal bloom episodes and (2) organic-rich laminae, containing abundant large and thick-walled Prasinophyte alginites indicative of algal blooms, interlaminated with organic-lean laminae, containing smaller "non-bloom" Prasinophytes. In the distal ramp, die high rate of planktonic productivity led to the development of an anoxic zone in the underlying waters, at times virtually throughout the water column, and preservation of organic matter. TOC content was also enhanced by very low rates of pelagic sediment dilution. In the proximal ramp, high rates of sediment dilution due primarily to terrigenous clastic sediment shed off the Peace River Arch, combined with oxygenated conditions due to surface circulation, resulted in green shales with low TOC content and reduced source rock potential. Acknowledgments: This research was funded by NSERC, The University of Manitoba, Natural Resources Canada, PanCanadian Petroleum Ltd., Home Oil Co. Ltd. and Gulf Canada Resources Ltd. Thanks to Jack Wendte for thought-provoking discussions about the Devonian of western Canada.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
GEOCHEMICAL PARTITIONING OF HEAVY METALS IN BOTTOM SEDIMENTS OF THE BRISBANE RIVER ESTUARY M.W. Clark Centre for Coastal Management, Southern Cross University, P.O. Box 5125, East Lismore, NSW
Using sequential analysis and grainsize normalisation the geochemical partitioning of Cu, Cd, Pb, Zn, Ni, Cr, Co, As, Mn and Fe in bottom sediments in the Brisbane river estuary are investigated. Geochemical partitioning of metals within the sediments show that sulphides play an important role in controlling metal species. Copper is most strongly controlled by sulphide, followed next by Pb, Cr, Ni, Co, and Zn, where only about 50% of the metal load is controlled by sulphide precipitation. This sulphide domination of bound metals, however, does not hold for manganese where speciation is dominated by the exchangeable/porewater phase, or for cadmium; cadmium concentrations are low and small variations in concentration cause large variations in the species plots. The speciation of Zn, Pb, Ni, Co and Mn are unusual because their speciation plots contains significant proportion of carbonate bound metal; carbonate may be a byproduct during sulphate reduction in the presence of reducible iron. The sulphide domination of trace metal speciation, the significant proportions of carbonate phases, and the high mobility of the managanese in Brisbane River estuary sediments is consistent with the sediment redox, where low Eh conditions support precipitation of metal sulphides and promote manganese mobility in the form of Mn^^. Despite low Eh conditions analysis of sulphur carbon environmental plots, and degree of pyritisation data suggest that bottom sediments are in contact with well oxygenated waters and are not detrimental to the health of benthic organisms.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE P-T PATH OF EOCENE ECLOGITES AND BLUESCHISTS IN NEW CALEDONIA: CONSTRAINTS FROM CALCULATED MINERAL EQUILIBRIA IN Ca0-Na20-Fe0-Mg0-Al203-Si02-H20.
OsfiffcSiLL-Clarkfi^ Christopher J. Carson' and Roger Powell^ 'Department of Geology & Geophysics, University of Sydney, Sydney, NSW 2006. ^School of Earth Sciences, University of Melbourne, Parkville, VIC 3052.
High-P rocks of the Pam Peninsula, northern New Caledonia are divided into three zones: (1) an uppermost ferroglaucophane-lawsonite zone of Cretaceous to Eocene metasediments and metavolcanics of the Diahot terrane that experienced peak conditions involving P=7-9 kbar and T=400±58°C; (2) albite-epidote-omphacite zone Diahot terrane rocks that experienced blueschist facies conditions of P=12.6±1.2 kbar and T=570±36°C; (3) lowermost metabasic eclogites of uncertain age that form the Pouebo terrane, which experienced high-P conditions of P=23.9±3.0 kbar and T~600°C. Barroisite and glaucophane-bearing eclogite occurs as metre to kilometre-scale pods in coarse-grained hydrous mineral-rich "glaucophanite" formed during hydration and decompression of the Pouebo terrane. Metamorphism and deformation were consequent to 44-51 Ma Eocene convergence, when sedimentaiy and ophiolitic nappes were thrust over the eclogites in a SW direction; white mica ages constrain metamorphism to have ended by 37±1 Ma. Large steps in metamorphic grade are coincident with SW-dipping and NE-dipping faults that separate the three zones and were formed during two stages: (1) comparatively slow uplift and hydration of the Pouebo terrane before it was juxtaposed with the albite-epidoteomphacite zone at P~14 kbar; (2) comparatively rapid uplift of both the Pouebo terrane and the albite-epidoteomphacite zone to form a domal core of eclogite flawed by significantly lower grade rocks to the SW and NE.
To constrain the metamorphic and mineralogical evolution of the Pouebo terrane, a petrogenetic P-T grid has been calculated for the model system Ca0-Na20-Fe0-Mg0-Al203-Si02-H20. Calculated P-T and P- and TJf(H20) pseudosections for bulk compositions appropriate to these lithologies are presented to illustrate the dependence of mineral assemblage on P, T, and rock composition. The eclogites experienced a clockwise P-T path that reached P =19 kbar and T ~600°C. The eclogitic mineral assemblages are preserved because reaction consequent upon decompression consumed the rock's fluid. Extensive reaction during decompression occurred in rocks that saw fluid influx during exhumation of the Pouebo terrane.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE PRECOMPRESSION SIZES AND SHAPES OF LEPTOPHLOEUM CASTS H.T. Clifford' and M.R. Pemberton^ 'Geology Department, Queensland Museum ^Mathematics Department, University of Queensland
Theoretical analyses of the variation observed in the widths of leaf-cushion bases around the perimeters of Leptophloeum casts have shown that the patterns of variability may be used to predict cast shapes, as seen in section, prior to their compression. Results indicated that before compression most casts were elliptical in transection which accords with the hypothesis that the plants possessed large herbaceous stems which were initially circular in section but assumed an elliptical outline when they collapsed onto the ground and their centres rotted away. Infilling of these hollow stems produced internal casts which became compressed with burial. That the stems were herbaceous is supported by the following observations. None of the casts studied tapered or bore branch-scars. Furthermore, on larger casts the leaf-cushion bases are neither separated nor show any evidence of being laterally stretched both of which conditions are typically associated with secondary growth. Simple geometrical procedures based on the ratio of the minimum to the maximum widths of the casts have been developed to determine their precompression sizes and shapes as seen in transaction.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
M"" Australian Geological Convention, Townsville, July 1998
A STRUCTURAL OVERVIEW OF THE INTRACRATONIC PETERMANN OROGENY IN THE NORTHERN TERRITORY Dorothy Closed Ian Scrimgeour\ Martin Hand^, Thomas Flottmann^ and Christine Edgoose' 1 Northern Territory Geological Survey, P.O. Box 2655, Alice Springs, NT, 0871. 2 Department of Geology and Geophysics, University of Adelaide, SA, 5005
The Petermann Orogeny is a late Neoproterozoic to early Cambrian (-560-520 Ma) intraplate event in central Australia that has exposed an almost complete structural section over a length scale of ~ 100km from the lower to upper crust. In the deepest parts of the orogen, exposed south of the Woodroffe Thrust in the Mann Ranges, domains of high strain dextr^ strike slip and north vergent dip slip deformation alternate on a kilometre scale. These structures formed at approximately 13 kbar and 750°C and are consistent with an overall dextral transpressional setting. Further north, high-strain domains associated with partial melting formed at -10 kbar and 700°C and show a similar structural style to the deepest parts of the orogen. North of the Woodroffe Thrust, the orogen consists of a partially migmatised granitic complex that was metamorphosed at 6-7 kbar and 630-650°C. The metamorphic grade decreases northward toward the Petermann Ranges where the granites are overlain by the lower units of the Amadeus Basin. The major structural feature in the Petermann Ranges is the Piltardi Detachment Zone (PDZ), a 2-5 km thick intensely deformed detachment horizon consisting of interleaved basement and cover that carried a crustal-scale basement nappe northward toward the Amadeus Basin. Metamorphic assemblages in the detachment zone indicate the lower parts of the cover sequence were buried to >15 km. The basement nappe has an outcrop area of -5000 km2, and is interpreted to be >5 km thick. The exhumed deep crustal mylonitic fabrics exposed south of the Woodroffe Thrust presumably represent the root zone of the nappe. The vast majority of stretching lineations in the PDZ indicate north-directed transport, but major shear zones 10-20 km to the south accommodate predominantly strike-slip movement, again suggesting the orogen developed in a transpressive regime. The detachment horizon was originally dipping south but has been undercut by later thrusts that have rotated it into a north-dipping position so that in the field, mesoscale shear indicators show normal movement. However the presence of a giant basement slab in the hanging wall of the cover-bearing detachment confirms it is a folded thrust surface. The continuation of shortening represented by the secondary thrusts produced an antiformal culmination (the Pottoyu Dome) beneath the detachment with an amplitude of -15 km and a strike length of -125 km. Northdirected movement along the Woodroffe Thrust occurred late in the development of the orogenic belt. The outboard part of the orogen in the Northern Territory is represented by relatively low strain structures in the Bloods Range that formed at greenschist or lower grade. Folds are generally upright and are locally carried forward along basal detachments. To the north of the Bloods Range, sequences in the Amadeus Basin apparently show little evidence of deformation during the Petermann Orogeny. Intensity of deformation and metamorphic grade also decreases to the east, towards the Kulgera region. To the southeast of the Petermann Ranges, a series of quartzite peaks known as the Olia Chain, forms a distinct structural terrain. This region is characterised by north- to northwest-vergent, mostly thin skinned deformation associated with at least three generations of progressive fold-thrust structures that increase in grade to migmatite southward toward the Woodroofe Thrust. Occasionally the detachments ramp down into the basement resulting in the formation of km-scale recumbent basement cored isoclinal folds during the shortening. The structural trends in the Olia Chain are truncated by the detachment that carries the major Petermann basement nappe, suggesting the Olia Chain structures represent a relatively early phase in the evolution of the Petermann Orogen. The amount of shortening associated with the exposed section in the Northern Territory is difficult to estimate. However, a generalised restoration of the Petermann nappe into a basement position suggests significant shortening was associated with its emplacement (-80 km?). Similar shortening is also implied by the exhumation of - 1 3 kbar rocks along the -30° dipping Woodroffe Thrust. Given the existence of major detachments in the Olia Chain that predate the emplacement of the Petermann nappe and the intensity of the structures that refold the nappe, we conservatively estimate that shortening is >125 km. It is important to realise this region only represents the northern side of the orogen, and that major shortening may have also occurred in the core of the orogen and along the southern margin of the Musgrave Block. Although speculative, we suggest the total shortening during the Petermann Orogeny may be greatly in excess of the above estimates. Given the intraplate setting of the deformation, this poses intriguing geodynamic problems.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
1997 ERUPTION OF McDONALD ISLAND (SOUTHERN INDIAN OCEAN) : NEW TRACE ELEMENT AND Th-Sr-Pb-Nd ISOTOPIC CONSTRAINTS ON HEARD-McDONALD ISLAND MAGMATISM Kenneth D. CollersonL Marcel Regelous^ Robyn Frankland^ and J. Immo Wendt^ Graeme Wheller ^Dept. of Earth Sciences, University of Queensland, Brisbane, Q 4072 Australia ^Volcanex International Pty. Ltd., Blackmans Bay, TAS, 7052, Australia
McDonald and Heard Islands (HI) lie on the Kerguelen Plateau (at 72° 35'E - 73°45'E and 53°2'S - 53°15'S) ca. 4000 km SW of Australia and represent the site of contemporary volcanism caused by the Kerguelen-Heard plume (Barling et al., 1994). HI is dominated by basanite, basalt and trachybasalt. Volcanism commenced at 1.2 Ma and continues at Mawson Peak on Big Ben to the present day. Lavas on Big Ben, Laurens Peninsula and Azorella Peninsula are chemically distinct and appear to have been derived from different magma chambers (Collerson et al., in prep.). The McDonald Islands, 44 km west of HI, are composed of extensively eroded laminated tuffs, kaersutite phyric phonolite domes and pelagic limestone. In view of their extremely eroded character, and K-Ar age dates of ca 36±3 ka and 79±3 ka (Clarke et al., 1983), McDonald Island volcano was believed to be extinct. However, in March 1997, while en route to HI, from Antarctica, "SRV Aurora Australis" sailed within 4 nautical miles of McDonald Island. Plumes of steam were being emitting from a number of point sources and one fissure system on the steep northern face of the island. These fumarolic features indicated that the island had experienced a recent eruption. Unfortunately due to logistical constraints it was not possible to land. Major element and ICPMS trace element analyses of fresh pumice from beaches on Laurens Peninsula (HI) are strongly alkaline with elevated incompatible element abundances and the following trace element ratios: Ce/Pb (4.24 - 4.56); Nb/Ta (19.67-20.18); Zr/Nb (8.17 - 8.24); Nb/U (18.12 - 18.85) and TWU (3.67 - 3.70). Although similar to McDonald Island lavas (Barling et al., 1994), the pumices are generally more evolved, suggesting derivation in an extensively fractionated magma chamber. The geochemically highly evolved character of the pumices together with TIMS Th isotopic data: (250Th)/(232Th) = 0.846 - 0.889 and (238U)/(232Th) = 0.846 0.852, suggest the magma resided in a strongly fractionated magma chamber for a long time prior to eruption. Fresh pumices have the following Sr, Nd and Pb isotopic compositions: 87Sr/86Sr (0.70558 to 0.70580), 143Nd/144Nd (0.512532 to 0.512564), eNd (-1.44 to -2.07), 206Pb/204Pb (17.752 to 18.161), 207Pb/204Pb (15.500 to 15.568), and 208Pb/204Pb (37.904 to 38.478). These are within error of values reported for McDonald Island phonolites by Barling et al., (1994), viz., 87Sr/86Sr (0.70573 to 0.70596), 143Nd/144Nd (0.512531 to 0.512554), eNd (-1.64 to -2.09), 206Pb/204Pb (18.12 to 18.229), 207Pb/204Pb (15.499 to 15.507), and 208Pb/204Pb (38.332 to 38.443). By contrast, weathered pumices from HI have isotopic compositions similar to HI magmas: 87Sr/86Sr (0.70484-0.70563); 143Nd/144Nd (0.512722-0.512734); 206Pb/204Pb (18.752-18.824); 207Pb/204Pb (15.597-15.614); 208Pb/204Pb (39.165-39.239) and are unlikely to have been derived from McDonald Island. Isotopic compositions of the fresh pumices are different from other potential young volcanic sources in the southern hemisphere, e.g.. South Sandwich Islands, Marion Island, lies Crozet and the Ross Sea Igneous Province. They are thus interpreted to have been derived from McDonald Island, supporting the field observation of recent volcanic activity and clearly showing that the island is not extinct. As Heard and McDonald Islands are Australian territory, Australia now has two active volcanos (Big Ben on HI and Samarang Hill on McDonald Island). Strongly leached pumice residues are well correlated in Pb-Pb space and define a 835±11 Ma (2a) secondary isochron with |il = 8.21. This late Precambrian age indicates the presence of Gondwana sub-continental lithospheric mantle beneath the southern Kerguelen Plateau. Kerguelen-Heard EM-1 type magmatism thus involves interactions between plume and ancient lithospheric mantle reservoirs. REFERENCES Barling, J., Goldstein, S.L. & Nicholls, I. A. 1994 Geochemistry of Heard Island (Southern Indian Ocean): Characterization of an enriched mantle component and implications for enrichment of the Sub-Indian Ocean mantle. J. Petrol., 35, 1017-1053. Clarke, I., McDougall, I. & Whitford, D., 1983 Volcanic evolution of Heard and McDonald Islands, Southern Indian Ocean. In: R.L. Oliver, P.R. James and J.B. Jago, (Eds.) Antarctic Earth Sciences, Australian Academy of Sciences, 631-635.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
ALTERATION AND MINERALISATION IN THE MOONTA-WALLAROO DISTRICT OF THE EASTERN GAWLER CRATON, A COMPARISON WITH THE SOUTHERN CURNAMONA PROVINCE. Colin H.H. Conor. Minerals Division, South Australian Department of Primary Industries and Resources
The intent of this paper is to highlight the prospectivity of ~1750-~1650Ma volcano-sedimentary rocks for CuAu-Pb-Zn mineralisation, especially where they are coupled with -1600—1500 Ma granites. Yorke Peninsula forms the easternmost exposure of the Gawler Craton. In the 1950's, airborne-magnetic searching by SA Dept Mines over the Wallaroo-Moonta region of northern and central Yorke Peninsula (Moonwall hereon) for buried iron-ore deposits, led to the discovery of iron-oxide, alkali-feldspar, carbonate bodies; these were designated as 'skams' during the long exploration partnership between NBH & WMC between 1960 and 1985. On a broader scale quartz-albitites attest to region-wide alteration in the -1750 Ma Wallaroo Group. In the Moonwall case, and like the Cloncurry district, much of the 'skam' alteration overprints silicic rocks like metasiltstone, quartz-albitite and even granite. Locally the alteration is so intense that the formal name Oorlano Metasomatite is used where 'skamification' has produced large bodies of rock lacking an identifiable precursor. The 'skams' generally show a persuasive spatial relationship with -1600-1575 Ma Hiltaba Suite granite bodies; however the temporal relationship between 'skams' and regional albitisation is not knovm although metamorphic parageneses suggest the latter to have had an extensive history, possibly partly diagenetic. The 'skam' assemblage is a high temperature one comprising various combinations of albite, microcline, scapolite, diopside, amphibole, biotite, magnetite, haematite, sulphides with minor components such as apatite and titanite (ie. Na, K, Ca, Mg, Fe, S). These minerals form extensive bodies of ironstone, calcsilicate, alkalifeldsparite and carbonate. There is minor later stage alteration to epidote and chlorite and locally large zones of chalcocite-bearing kaolin-rock. Although alteration is massively replacive there are localities where the ingress of metasomatic components can be observably related to penetrative conduits. Examples of such conduits are faults, foliation, fold axial plane stmctures, veins, susceptible strata, igneous contacts and breccia bodies; the breccias are interpreted to have formed hydrodynamically in dilating structures. Evidence supports the broad synchroneity of granite intmsion, alteration and tectonism. Although no syngenetic mineralisation is proven in the Wallaroo Group, it is suspected. Possible contenders are extensive low-grade sphalerite in graphitic schist east of Kadina, metamorphosed sulphidic iron-formation in the Wallaroo Mines area and copper (gold) mineralisation in meta-calc-pelite near Alford and Wheal Hughes; associated with the Cu-Au examples are flat, bedding-parallel, albite 'concretions' (ie. informally 'Moonta biscuits'). Pyrite, and locally pyrrhotite, are paragenetically the earliest formed epigenetic sulphide phases and commonly coexist with magnetite and biotite as matrices to breccias or along other stmctures. Chalcopyrite and sphalerite tend to be late and associated with carbonate. All Cu-Au mines in the Wallaroo-Moonta field are stmcturally controlled. The same was found to be the case with the recently closed Poona Mine and Wheal Hughes deposits; the ore in these was emplaced in NW dipping, reverse faults developed within ductile, high strain zones within the potassic+haematitic-altered, meta-volcanic Moonta Porphyry. Apart from Cu-Au, other relatively abundant, potentially economic elements include: Mo, Zn, Pb, Co, U, Ce. It is possible that the alteration seen in the Moonwall District is but a small portion of a much larger system which included the northem Gawler Craton, the Stuart Shelf basement containing the Olympic Dam Cu-Au-U deposit, portions of the Cumamona Province, and the Cloncurry District, which at 1600-1500 Ma was possibly proximal to the Wemecke Breccias of Canada. Geological elements tying Moonwall to the Olary and Broken Hill domains of the Cumamona Province include: similar depositional ages, mafic and felsic volcanics (Moonwall -1750 Ma, Olary -1710 Ma, Broken Hill -1690 Ma) interdigitated with fine-grained metasediments (including quartz-albitite, iron formation, calcsilicate, graphitic sediment), an intense thermal perturbation at -1600Ma (Moonwall - syntectonic A and I-type Hiltaba Suite granites; Olary - metamorphism and development of S-type granites; Broken Hill - granulite-facies metamorphism), high-temperature Na, K, Ca, Mg, FeO alteration, Cu-Au mineralisation in metavolcanics or metasediments (including even 'Moonta biscuits'!).
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
LATE CARBONIFEROUS FRESHWATER TRACE FOSSILS FROM THE JERICHO FORMATION, ALPHA, CQ Alex G. Cook Queensland Museum, PO Box 3300, South Brisbane QLD 4101.email: alexC@qm.qld.gov.au
Exquisitely preserved trace fossils are found within quarry exposures of the Jericho Formation, southeast of Alpha, central Queensland. The claystone units represent periglacial lake deposition during the Late Carboniferous (Stephanian) within the Galilee Basin. These trace fossils are part of an ''Umfolozid' assemblage, common to Gondwanaland for this time, but also known from Europe. Other Australian penecontemporaneous records of such trace fossils are those from Tasmania and New South Wales. Ichnofossil morphology varies according to substrate viscosity at time of formation, size of animal creating the trace and the mode of locomotion being adopted by the animal. All traces discussed were made by an unidentified arthropod, possessing up to ten locomotory appendages and a furcate telson.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
GASTROPODS FROM THE LATE DEVONIAN OF THE CANNING AND BONAPARTE GULF BASINS, WA. Alex G. Cook Robert B. Blodgett ^ Michael R. House ^ & Thomas Becker ^ 'Queensland Museum PO Box 3300 South Brisbane ^ Department of Zoology, Oregon State University, Corvallis, Oregon,USA,97331-2941 ^ Department of Geology, The University of Southampton. 509 5NH United Kingdom. ^ Museum fur Naturkunde, Humboldt Universitat zu Berlin, Axel Springer Str. 54A Berlin, 10117, Germany.
The Frasnian is viewed as a time of great cosmopolitanism for gastropods worldwide, with specific and generic links throughout the Old World Realm strong. Unfortunately this view is not based on data, rather than on the "woeful ignorance". Gastropods from the Frasnian in Western Australia represent the most diverse fauna of Frasnian gastropods hitherto known, with eight taxa (six new) from the Bonaparte Gulf Basin (Westwood Member of the Hargreaves Formation) and in excess of twenty taxa from the Canning Basin (Pillara Formation, Virgin Hills Formation, and Sadler Limestone). These include several new genera of gyronematines, new eotomariid genera, and new species of bellerophontids, murchisoniids, platyceratids and subulitids. Famennian taxa in Australia have previously only been recorded from moulds in erratics from White Cliffs, New South Wales. The Famennian record in Western Australia consists of a modestly diverse, but poorly preserved fauna from the Virgin Hills Formation, of at least seven taxa, only one of which is specifically common with local Frasnian gastropods. The Canning Basin gastropod faunas, in part, consist of material immediately predating and postdating the F-F boundary and thus provide an insight into faunal turnover of the group during this time interval,
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
OXIDISED (SULFATE-BEARING) BRINES - THEIR IMPORTANCE FOR THE FORMATION OF AUSTRALIAN PROTEROZOIC Pb-Zn (SEDEX) DEPOSITS David R. Cooke. Stuart W. Bull, Ross R. Large and Peter J. McGoIdrick Centre for Ore Deposit Research (CODES), University of Tasmania, GPO Box 252-79, Hobart, 7001
INTRODUCTION Few would argue that the brines responsible for stratiform sediment-hosted Pb-Zn (SEDEX) deposits are derived from their host sedimentary basins. Because reduced lithologies host mineralisation, it is generally assumed that the base metal transporting fluids were also reduced (e.g. Goodfellow et al., 1993). However, in thick sedimentary successions, the composition of the basin-fill (oxidised vs. reduced sediments) must control the chemistry of the mineralising brines. In the case of the northern Australian SEDEX deposits, we believe that the ore-forming brines were oxidised ( > I^H^S )» ^ originally proposed by McGoIdrick and Keays (1990). McARTHUR & MOUNT ISA BASINS - SOURCES OF OXIDISED (SULFATE-BEARING) BRINES The Australian Proterozoic SEDEX deposits occur in reduced sediment packages that are often a minor overall component within thick sequences of oxidised clastic rift sequences and evaporitic carbonate platform deposits (e.g. McArthur Basin: HYC - Barney Creek Formation; Mount Isa Basin: Mount Isa & Hilton - Urquhart Shale). In the case of the McArthur Basin, the carbonate lithologies, original sulfate-bearing evaporites and hematitic sandstones that dominate the stratigraphy underlying the HYC deposit must have buffered the basinal brines to oxidised, near-neutral conditions. We believe that the carbonaceous pyritic sediments of the Barney Creek Formation, in combination with the anoxic bottom waters in which they were deposited, were a highly effective redox trap. They were the first major reductant that oxidised basinal brines would have encountered as they migrated up through the McArthur Basin stratigraphy due to seismic ruptures along the Emu Fault System. GENETIC CLASSIFICATION We propose a two-fold subdivision for SEDEX deposits, based on fundamental differences in the chemistry of the mineralising brines. The type of sedimentary basin from which the ore fluids are derived, and the lithologies contained within the basin, control these differences in fluid chemistry. The results of our chemical modelling show that there are two discrete brine types capable of transporting Zn and Pb; reduced acidic brines and oxidised brines. We propose that Selwyn-type deposits (e.g. Paleozoic deposits of the Selwyn Basin, Sullivan, Rammelsberg) precipitate from reduced (RS-predominant), acid brines; and McArthur-type deposits (e.g. McArthur River, Mount Isa, Hilton, Century) form when oxidised (SO,'-predominant), near-neutral fluids precipitate their base metal load. Acidic, reduced Selwyn-type ore fluids will be produced from sedimentary basins dominated by siliciclastic turbidites and carbonaceous shales. Barite is a conmion precipitate in these systems, because reduced fluids are capable of transporting significant Ba"* (< 1 ppm). McArthur-type ore fluids will be generated in basins dominated by oxidised clastic and evaporitic carbonate sediments, together with minor carbonaceous shales. Because the solubility of barite in oxidised brines is low (< 1 ppm), it is not precipitated during McArthur-type ore formation. Temperature decrease and dilution (fluid mixing), addition of H,S and pH increase can all be important for deposition of Zn and Pb from Selwyn-type ore fluids. In contrast, reduction and/or addition of H^S (via fluid mixing or interaction with earlier-formed pyrite) will be the important processes for sphalerite and galena deposition from McArthur-type ore fluids. Interaction with reduced S in the trap environment is probably the key to ore deposition. Temperature change will be unimportant in McArthur-type systems. REFERENCES Goodfellow, W.D., Lydon, J.W., and Turner, R., 1993. Geology and genesis of stratiform sediment-hosted (SEDEX) zinc-lead-silver sulphide deposits, in Kirkham, R.V., Sinclair, W.D., Thorpe, R.L, and Duke, J.M., (Eds.), Mineral Deposit Modeling. Geol. Assoc. Can., Special Paper 40, 201-252. McGoIdrick, P.J. and Keays, R.R., 1990. Mount Isa copper and lead-zinc-silver ores: coincidence or co-genesis? Econ. Geol., 85, 641-650.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
INTEGRATING CHEMOSTRATIGRAPHY WITH OTHER STRATIGRAPHIC TECHNIQUES TO RESOLVE NEOPROTEROZOIC CORRELATIONS IN THE KIMBERLEY REGION OF NORTHWESTERN AUSTRALIA. Maree Corkeron Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, Nedlands, WA, 6907.
Stratigraphic correlation of Precambrian strata has long been impeded by poor biostratigraphic and geochronological control. The world wide preservation of glacially derived sedimentary rocks associated with enigmatic 'cap dolostones' with Neoproterozoic ages was recognised by Harland (1965) and provided potential marker horizons for global stratigraphic correlation during this time. While such correlation proved to be more complicated than was first hoped, these glacigenic strata serve as a broad framework for Neoproterozoic division. Furthermore, investigations into associated carbonate rocks has led to the development of stable and unstable isotope (C, O, Sr) chemostratigraphy as a tool in yielding more robust means of correlating strata on the local, regional and global scale. The carbon and strontium isotopic compositions of seawater varied systematically through the Neoproterozoic (Kaufman et aL, 1997) and this tenor provides a useful framework for geochemical stratigraphic analysis of Neoproterozoic sedimentary successions. When combined with other tools such as biostratigraphy, lithostratigraphy, sequence stratigraphy and geochronology, the potential to resolve many correlation problems is greatly enhanced. The integration of chemostratigraphy, biostratigraphy and lithostratigraphy as correlation tools is being applied to Neoproterozoic rocks in the Kimberley region of northwestern Australia. Here, three geographically separate successions have had conflicting correlations proposed on lithostratigraphic grounds and alternate methods of resolving the correlation are required. Lithologically, the successions contain the enigmatic association of both glacially derived strata and a distinctive thin pink overlying dolostone unit. Other carbonate strata are also significant components of the successions. Stable isotope and elemental analysis have been undertaken systematically up section on these carbonate units in an attempt to fmd specific trends in which can be correlated between the Kimberley successions, and then compared with trends determined from central and South Australian successions, and then globally. Negative excursions of about -5 permil in cap dolostones above both the Walsh and Landrigan Tillites supports the correlation of these units. A negative is also recorded in the cap dolostone above the Moonlight Valley Tillite and although of smaller magnitude, supports the lithostratigraphic correlation of this formation with the Walsh and Landrigan Tillites. Similar negative trends observed in the Marinoan cap dolostones of central and South Australia supports the proposal that the Walsh, Landrigan and Moonlight Valley tillites are Marinoan Glaciation equivalents. results are less definitive from carbonate strata of the Egan Formation (also containing glacigenic strata) and Boonall dolomite although similar trends of a positive to negative to positive curve are observed in both formations. This parallel trend, together with biostratigraphic control using stromatolites, supports the correlation of the Egan Formation and Boonall Dolomite. REFERENCES Harland, W. B., 1965. Critical evidence for a great infra-Cambrian glaciation. Geologische Rundschau, 54, pp 45-61. Kaufman, A. J., Knoll, A. H., Narbonne, G. M., 1997. Isotopes, ice ages, and terminal Proterozoic earth history. Proceedings of the National Academy of Science, 95, pp 6600-6605.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
CAINOZOIC LANDSCAPE EVOLUTION IN COASTAL NORTHERN NEW SOUTH WALES: VOLCANISM, FLUVIAL DEVELOPMENT AND GEOMORPHIC INHERITANCE. $t(?phgn J, Cottgr''^ and John V. Smith' Centre for Coastal Management, Southern Cross University, Lismore, NSW, 2480 ^Present address: Centre for Landscape Evolution and Mineral Exploration, University of Canberra, ACT, 2616
INTRODUCTION The identification of Eocene basaltic lavas along the coastal plain in northern New South Wales has important implications for the understanding of the landscape evolution of the local region as well as contributing towards the knowledge of the uplift history of the Great Escarpment in eastern Australia. This Eocene volcanic landscape is partly superimposed by the subsequent Oligocene - Miocene volcanism associated with the Tweed shield volcano. The geomorphology of the study area (from 29°S, 152°45'E to the coast and north to the NSW-Qld state border) is explicitly controlled by these volcanic events. Moreover, the volcanic landscape has an inheritance imposed by the pre-volcanic topography. The contemporary fluvial system has developed within these structural and topographic controls, but possesses many geomorphic features directly related to the prevolcanic drainage network. This study highlights the importance of detailed geomorphological investigations to elucidate the processes of landscape evolution that have persisted throughout the Cainozoic period. VOLCANISM Two major eruptive episodes have been discriminated within the volcanic sequence in northern New South Wales based on K/Ar dating, palaeomagnetism, whole rock geochemistry and electron microprobe analysis of the constituent mineral assemblages. Eocene basaltic lavas (at least 41 Ma) were erupted onto an irregular, south easterly dipping palaeo-surface in the south eastern portion of the study area. Initially, these lavas were ponded between palaeoridges infilling the basement depressions, and can be observed in the contemporary landscape as the plateau surfaces east of Lismore. In places, late-stage analcime + zeolite dykes and sills have intruded the lava flows but are abruptly discontinuous at erosional surfaces. The Late Oligocene - Early Miocene volcanism, associated with the Tweed shield volcano, partially overlaps with the earlier volcanic sequence. Initially, lavas flowed towards the south. However, a topographic depression existed between the two volcanic sequences as indicated by a laterally extensive but shallow (< 3m thick) bentonite clay deposit. The formation of this deposit by in situ alteration of volcanic tuff from the rhyolitic episode of the Tweed volcanic event, provides a clear indication of the existence of swamp or lake environments bordering the northerly extent of the Eocene volcanism. A fmal basaltic episode that draped the land surface is indicated by extensive thick lava flows infilling stream channels. Uplift associated with the intrusion of the central gabbroic stock (Mt Warning) of the Tweed shield volcano was restricted to its immediate vicinity and cannot be extrapolated to the entire volcanic surface. However, contact metamorphism of the Palaeozoic metasediments due to the emplacement of a granitic intrusion (Mt Nullum) adjacent to Mt Warning has a more regional impact by forming persistent topographic highs in the eastern portion of the study area. All the above geological components have impinged upon the fluvial evolution within this volcanic landscape. FLUVIAL DEVELOPMENT Pre-volcanic drainage patterns were controlled by the south easterly dipping palaeo-surface. The existence of this surface predates any uplift associated with the volcanism and represents a relict landscape element along the margin of the Clarence - Moreton Basin. Eruption of the Eocene lavas disrupted the palaeodrainage patterns forming extensive swamp and lake environments. The palaeo-Wilson's River was diverted along a topographically lower region between the Eocene lavas and the palaeo-surface. Whilst subsequent basaltic and rhyolitic lavas from the Tweed volcanic event draped the land surface, the south easterly dip was maintained and the extensive lake and swamp environments bordering the Eocene lavas prevailed. Moreover, whilst the base of the rhyolite lavas attest to an uneven topography, fluvial erosion of associated tuff deposits indicate that the major palaeo-drainage networks were maintained. The overlying basaltic lavas have been extensively eroded by rejuvenated stream gradients, but have exhumed the earlier drainage networks. The landscape evolution is flmdamentally controlled by the pre-volcanic topography despite extensive volcanism in the region since the Eocene. The implications of this stability for the Cainozoic landscape evolution in northern New South Wales are discussed.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
PETROGENESIS OF THE RHYOLITIC LAVAS FROM THE TWEED VOLCANIC PROVINCE IN NORTHERN NEW SOUTH WALES: A GEOCHEMICAL AND MICROPROBE STUDY. Stephen J. Cotter*'^ and John V. Smith^ 'Centre for Coastal Management, Southern Cross University, Lismore, NSW, 2480 ^Present address: Centre for Landscape Evolution and Mineral Exploration, University of Canberra, ACT, 2616
INTRODUCTION Fractional crystallisation, assimilation, replenishment and mixing of basaltic andesite within a upper crustal magma chamber is proposed as the mechanism of genesis of rhyolite lavas within the Oligocene - Miocene Tweed volcanic provmce in northern New South Wales. The rhyolite lavas comprise 10% by volume of the initial lava pile occurring between two basaltic sequences. A detailed microprobe study of the mineralogy of the rhyolite lavas, inclusions within perlitic obsidian carapaces, lithic tuffs and underlying basaltic lavas provides the basis for assessing the petrogenesis of these lavas. This data is supplemented by major, trace and rare earth elemental analyses obtained by Xrf, LA-ICP-MS and INAA methods for whole rock and selected mineral phases of these lavas which highlight a comagmatic link between eruptive events. The understanding of the petrogenesis of the rhyolite lavas has important implications for the volcanic stratigraphy in this portion of the Tweed volcanic province. WHOLE ROCK GEOCHEMISTRY Aphyric (crystal-poor) and porphyritic (crystal-rich) two-feldspar rhyolites form distinct lava domes within the study region (Smith and Houston, 1995). At the whole rock scale, there is negligible geochemical differences between the two rhyolite types with the lavas exhibiting depletion in most elements. Moreover, if the underlying basaltic andesite, hawaiite and andesite lavas are included, the K2O concentration is positively correlated with increasing Si02, emphasising an important link between the basaltic lavas and the fractionating magma chamber. This relationship has not been recognised in presumably contemporaneous rhyolite lavas from Southern Queensland. Further, when recast into the Oliv-Cpx-Plag-Qtz pseudotemary, the basaltic lavas demonstrate a calc-alkali fractional path towards silica enrichment as crustal material is assimilated into the melt. Finally, the trace element and REE normalised diagrams illustrate characteristic and appropriate enrichment/depletion patterns for a fractionating anatetic melt. The level of extreme fractionation, particularly in Rb and K, that is observed in the southern Queensland rhyolites is not achieved in the New South Wales silicic lavas. Hence, at the regional volcanic province scale, the petrogenesis of the rhyolite lavas is clearly disconcordant with a simplistic shield volcano model but rather, the geochemical dissimilarities reflect distinctive isobaric conditions during magma genesis resulting in diverse mineral assemblages between lava domes. MINERALOGY The crystal-rich rhyolite lavas differ significantly from the crystal-poor lavas. Co-existing Fe-Ti oxides and a Mg-rich (phlogopite) biotite phase are characteristic of the crystal-rich lavas. Only the titanomagnetite and a Ferich biotite phase is observed in the crystal-poor lavas. Fe-Ti oxide geothermometry for the crystal-rich lavas provides equilibrium temperatures between 700 and 840° at, to slightly below QFM stability. This is slightly below plagioclase-melt and biotite-meh equilibrium temperatures indicating a crystallising sequence of sanidine^olivineforthopyroxene, ilmenite, ti-poor phlogopite, titanomagnetite compared to a potential sequence of sanidine^olivine^orthopyroxene, Fe-biotite, titanomagnetite for the crystal-poor lavas, amphiboles are conspicuously absent in all samples. Early precipitation of the biotite phase depletes the melt proportionally in Fe^"^ and thereby promotes titanomagnetite crystallisation. In comparison, the titanomagnetite phase is not recorded in the rhyolite lavas from southern Queensland, indicating equilibrium conditions were significantly below QFM stability. The importance of Mg and Fe partitioning within groundmass biotite microlites as the essential driving mechanism for controlling melt redox conditions is documented. The interplay between biotite and titanomagnetite mineral geochemistry for the rhyolite and rhyodacite lavas between distinct crystal-rich and crystal-poor lava flows necessitates isobaric adjustments to occur within an essentially closed-system magma chamber under anhydrous conditions, rather than continued assimilation of wall rock. The petrogenesis of the rhyolite lavas in this study is compared with other silicic lavas from East Australian Cainozoic volcanic provinces. REFERENCES Smith, J.V. & Houston, E.C. 1995. Structure of lava flows of the Nimbin Rhyolite, northeast New South Wales.
Australian Journal of Earth Sciences, 42, 69-14.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Comention, Townsville, July 1998
CRUST-MANTLE MIXING IN THE I-TYPE JINDABYNE SUITE: A Nd-Sr-O ISOTOPE STUDY SoniaCousins^"'". W. J. Collins^ M. T. McCulloch^ and Anita Andrew^ ^Department of Geology, The University of Newcastle, Callaghan, NSW 2308 ^Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 ^Centre for Isotope Studies, CSIRO, North Ryde, NSW "•"Current address: Geological Society of Australia, 7th Floor, 301 George Street, Sydney, NSW 2000
The petrogenesis of I-type granitoids of the Lachlan Fold Belt has long been considered a major controversy of Australian geoscience. The I- and S-type classification scheme of Chappell and co-workers implies that these two contrasting granite types are derived from partial melting of distinct crustal sources. Individual I- or S-type suites are characterised by distinctly linear trends on Harker variation diagrams, and, according to the restite model, the chemical variation is ascribed to restite unmixing. However, another possibility, supported by the published hyperbolic eNd-Srj array for Lachlan Fold Belt granitoids, is that they formed by crust-mantle mixing. If the genetic classification into I- and S-types is adopted, a paradox exists because the contrasted chemical types imply melting of distinct crustal sources, but the hyperbolic isotopic array suggests a mixing system between two specific end-members. The objective of this project was to determine the relation between the granitoids of the 410-420 Ma I-type Jindabyne Suite and the adjacent Blind Gabbro, which falls on the same linear variation trends as the suite. TTie Jindabyne Suite is important because it forms part of the Kosciuszko Batholith, where Iand S-type granitoids were originally defined. Seventeen samples from the Jindabyne Suite, including the arc-derived Blind Gabbro, were analysed for Rb-Sr and Sm-Nd isotopic compositions. The samples, from high-alumina gabbro (48.7% Si02) to felsic granodiorite (72.5% SiOj), form a linear array in eNd-Sri space (sNd -1.8 to -7.3, Srj 0.704895 to 0.707459), which falls within the published hyperbolic trend for Lachlan Fold Belt granitoids. The array, ranging from more mantlelike values for the gabbroic rocks to more crustal-like values for the granitoids, suggests crust-mantle mixing. Ten samples were also analysed for O isotope compositions. All are enriched relative to the mantle 8*^0 value of 5.7 +- 0.3%o, ranging from 6.83%o to 8.84%o. Bi-variate radiogenic-stable isotope correlation diagrams indicate that the samples range from mantle-like values for the gabbroic rocks to more crustal-like values for the granitoids, suggesting crust-mantle mixmg. The isotopic results are supported by major and trace element whole rock geochemistry, petrography and chemistry of primary mineral phases. On trace element spidergrams, the most mafic Blind Gabbro sample has a distinctive arc-like signature (with LILE-enrichment and a negative Nb anomaly), but all other samples have signatures that resemble continental crust. Petrographically, all rocks within the suite share many features: plagioclase is the dominant mineral phase and calcic cores exist in all rock types, regardless of bulk chemistry; cellular plagioclase, mafic clots and poikilitic phenocrysts are common features; ferromagnesian minerals (pyroxenes, biotite and amphiboles) display smooth variation from the most mafic to the most felsic rock types. The Jindabyne Suite cannot be the resuh of restite unmixing, as there is significant isotopic variation between individual samples. Similarly, the isotopic variation rules out an exclusive role for crystal fractionation. Simple two-component mixing accords well with the isotopic data, but the end-member chemical and isotopic compositions suggested previous studies are not appropriate. Rather, the results from this study strongly suggest that the Jindabyne Suite appears to have a specific crust-mantle mbc of unique end-member compositions. For example, all samples, including the gabbros, are too radiogenic to have been derived directly from depleted mantle, indicating that some enriched mantle source - such as subcontinental lithosphere, or perisphere - must be envisaged. Given the distinctive arc-like signature and evolved isotopic composition of the Blind Gabbro, and their association with coeval tonalite granitoids, the inferred tectonic environment for the Siluro-Devonian magmatism is a magmatic arc.
95
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
CONTROLS ON MINERALISATION AT THE AWAK MAS GOLD DEPOSIT, SULAWESI, INDONESIA Simon Cox', Robert Smillie^ Andrew Allibone', Didik Setyono^ Nick Archibald\ Bill Power^, Kevin Alexander^ 'SRK Consulting, P.O. Box 250, Deakin West, ACT2600 W Masmindo Eka Sakti, Menara BCD 6th Floor, Jalan Jend Sudirman Kav. 26, Jakarta 12920 and. Level 5, Australia Place, 15 William St., Perth, WA6000 'Fractal Graphics, 39 Fairway, Nedlands, WA6009
The Awak Mas gold deposit lies in the foothills of the Latimojong Mountains of southwest Sulawesi. The deposit was discovered by rock-chip sampling following regional stream sediment sampling in late 1988. The in-situ geologically constrained global resource estimate for the deposit is 27.8 million tonnes at 1.89g/t gold, containing 1.69 million ounces at a 0.7 g/t cut off. Awak Mas is hosted in schistose metasediments, metavolcanics and metadolerites of the Cretaceous Latimojong Formation. Gold mineralisation occurs in quartz vein stockworks and breccias that contain minor pyrite. Alteration associated with the mineralisation is albitic, locally pyritic, with minor carbonate and a peripheral chlorite-albitequartz assemblage. Gold occurs as 5-50 micron grains concentrated along the margins of pyrite grains and as inclusions within pyrite. Quartz veins and breccia infill are characterised by crystalline textures. Crustiformcolloform, chalcedony and moss textured veins are absent. Open space mineral infills that contain quartz, pyrite, chlorite, and clay are restricted to minor late fractures. Traces of vuggy rhombohedral adularia and bladed calcite indicative of throttling under hydrostatic conditions are restricted to these minor late fractures. Alteration assemblages and mineralised veins overprint ductile fabrics that developed during accretion of the Latimojong Formation. Alteration assemblages are similar to the metamorphic (albite-chlorite-carbonatequartz±lawsonite±actinolite) assemblage implying that the mineralising fluid was buffered by the metamorphic assemblage. Mineralisation is localised along oblique-normal faults, extensional shears and within fracture arrays that developed during east-west directed extension. Wider zones ofmineralisation are localised within discrete fault-bounded blocks where the earlier foliation is in a more favourable orientation for extensional reactivation during oblique-normal movement on the bounding faults. Similar structural controls on mineralisation have been mapped at several prospects withm 5km of Awak Mas. Mineralisation occurred during a period of localised extension following Paleocene-Oligocene accretion and high pressure/low temperature (lawsonite-albite facies) metamorphism of the host rocks. Textural evidence for widespread throttling in the form of various non-crystalline forms of vein silica and adularia that would be expected if the deposit developed in an epithermal environment are absent. Circulation of hydrothermal fluids occurred in response to the regional high geothermal gradient associated with the rapid uplift of the Latimojong Mountains. Fluids are inferred to have been of metamorphic and/or meteoric origin although the relative proportions are not known. Cross-cutting relationships, radiometric dating and superimposition of likely fluid temperatures on fission track uplift curves imply gold mineralisation occurred between 8-6Ma.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
ORE GENESIS IN FRACTURE-CONTROLLED HYDROTHERMAL SYSTEMS: PERCOLATION THEORY APPROACHES Stephen F Cox ^'I Jean Braun^ Mark A Knackstedt^ 'Department of Geology, The Australian National University, Canberra, ACT 0200 ^Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 ^ School of Petroleum Engineering, The University of New South Wales, Kensington, NSW 2052
Percolation theory provides powerful insights for understanding ore deposit localisation in cases where the architecture of fluid flow in hydrothermal systems is controlled by permeability distribution within linked networks of active faults, shear zones and associated fracture systems. The evolution of permeability during deformation is controlled by a very dynamic competition between deformation-induced porosity-creation processes and porosity-destruction processes. Localisation of deformation in faults and shear zones leads to flow localisation, with large-scale flow systems forming when active faults and shear zones link to create percolation networks. Broad regions of fluid focusing develop around the upstream (deeper) segments of active shear networks and fluid discharge regions develop in the downstream (shallower) parts of these systems. The length scales of fluid flow are controlled by connectivity among elements of an active fracture network and between depositional sites and potential sources of metals and fluids. Progressive deformation promotes increasing connectivity among fractures, faults and shear zones. The onset of fluid flux at scales sufficient to link fluid sources and potential ore deposition sites occurs at a percolation threshold which is dependent on factors including strain, fracture geometries and relative rates of fracture growth and nucleation. Critical strains required to reach the percolation threshold can be very low. The architecture of flow within fracture networks is influenced by the relative proportions of backbone, dangling and isolated structures in the network. We illustrate flow patterns and the geometry of fluid focussing and discharge regions around permeable fault systems by the use of two-dimensional finite element models of steady state fluid flow in and around simple, high-permeability fault networks that are embedded in a less permeable medium. Using numerical modelling, we also illustrate the complexity of flow paths in fracture networks in both two and three dimensions. The relative proportions of isolated, dangling and backbone elements change as a function of the total strain and fracture density. For systems just above the percolation threshold, the backbone is a very small proportion of the total fault population and flow is localised on a few structures linking fluid sources and sinks. Particularly in the mid- to upper-crustal seismogenic regime, rapid changes in permeability distribution in fault networks due to episodic fault slip, aftershock sequences and interseismic hydrothermal sealing can lead to sudden changes to the location and architecture of flow backbones. For hydrothermal systems well above the percolation threshold, fluid flow is distributed over a larger proportion of the fracture population. Near the percolation threshold in 3D networks, high fluid flux occurs along tortuous flow paths ("red bonds") which have limited along strike continuity. We apply these principles to examine the distribution of gold deposits in Archaean mesothermal systems and to explore the relative importance of backbone and dangling sites for influencing localisation of gold deposition within percolation networks. Fluid-rock reactions are favoured at fluid discharge sites in downstream segments of fault networks, especially in dead-end structures. Backbone elements are high flux structures which present limited opportunities for fluid-rock reaction except near their downstream termination. However, they are sites of potentially large fluid pressure fluctuations during seismogenic behaviour. Fluid mixing reactions are favoured at both the upstream and downstream terminations of dead-end elements where fluids focus into, or discharge from faults. Clustering of ore deposits adjacent to major shear zones is interpreted in terms of fluid migration through "red bonds" and into dangling fault elements where effective fluid-rock reaction or mixing may occur.
97
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
PAPER ONE DAY, GIS THE NEXT... CONVERTING PAPER AND DIGITAL RECORDS INTO THE SOUTHEAST QUEENSLAND GEOLOGICAL GIS Len C. Cranfield. John S. Tuttle, Garry Pascoe and John Greig Geological Survey of Queensland, GPO Box 194, Brisbane, Queensland 4001
ABSTRACT Geographic Information System (GIS) technology provides much greater scope to update geological map data. The Queensland Department of Mines and Energy's (DME) experience with this technology led to the proposal of a project to generate a geological GIS over the Southeast Queensland Region, including the SEQ 2001 planning region. This paper sets out the project's progress and describes the process of translating multiple, disparate geological data sets into a corporate GIS asset. The primary data set underpitming the Southeast Queensland project is a seamless digital geological map of the region. This current update of the area's geology has been digitally compiled from: • a combination of field mapping and investigation of mineral prospects • published 1:100 000,1:250 000 and other geological paper map data from various sources • field notebook information from previous Geological Survey 1:250 000 and 1:100 000 mapping projects and resource studies • borehole information • interpretations from geophysical (radiometric, magnetic and gravity) images and satellite imagery. Significantly, the seamless map objective has led to the resolution of ambiguous interpretation differences between geological units and boundaries, in different map sheet areas. In addition, digital geological map symbols have been generated from published maps and existing data stored in DME's corporate Minerals and Energy Resources Location and Information Network (MERLIN) Geoscience and Resource Data Base (GRDB). The result is a unique map legend for the Southeast Queensland Region including the SEQ 2001 area, and the ability to produce: • updated 1:100 000 geological maps m digital and hard copy format, • a seamless digital geological and hard copy map of the region. Additional feature attribute data have been compiled for the seamless digital geology enabling derivative themes to be produced and this enhancement has facilitated the development of the GIS package for Southeast Queensland. The impending release which covers the SEQ 2001 planning area, includes information on mineral occurrences, mineral deposits, industrial minerals and extractives resources, whole rock geochemistry and radiometric age dating. The GIS environment will allow spatial modelling techniques to be applied to some data sets to create further derivative themes eg mineral potential. Additional changes to the to the seamless map presented by Cranfield & others (1997) are also incorporated as a resuh quality assurance testing and field mapping and investigation of mineral prospects undertaken in 1997 on the Esk, Goomeri and Nanango 1:100 000 Sheet areas. Field work completed in 1997 over the Kingaroy and Murgon 1:100 000 Sheet areas and proposed in 1998 over part of the Jandowae and Boondooma 1:100 000 Sheet areas will be released towards the end of the year.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
MELT EXCLUSIONS: A NEW TOOL FOR MONITORING VOLATILES AND CHALCOPfflLE METALS IN EVOLVING MAGMA SUITES Anthony J Crawford. Vadim S Kamenetsky, Rohan Wolfe and Leonid V Danyushevsky (All at Centre for Ore Deposit Research, School of Earth Sciences, University of Tasmania, GPO Box 252-79 Hobait Tasmania)
INTRODUCTION Studies of fluid inclusions have provided considerable new information about the compositions, pathways and temperature histories of fluids involved in ore genesis. In particular, fluid inclusions have demonstrated that magmatic fluids are the main ore-forming fluids in porphyry Cu-Au deposits, and have strongly implicated magmatic-derived fluids as a significant component in high-sulphidation epithermal deposits. Considerably less certainty surrounds the role of a magmatic-derived fluid in Kuroko-type Cu-Pb-Zn VHMS deposits. Fluid inclusions provide a window into the fluid environment usually at T<500°C. Most magmas, however, are effectively frozen by SOQOC, leaving a temperature gap of -300^C between the magmatic fluid record, and hydrothermal (metal-transporting) fluids subsequently trapped as fluid inclusions. Complementary to fluid inclusions, melt inclusions can provide key information about magmatic volatile contents that wholerock geochemical studies cannot offer. Melt inclusions are small (usually «100|Lim) portions of magma trapped in phenocrysts during crystal growth, and they may be simple (melt plus shrinkage bubble) or compound (melt + daughter crystals + bubble). Continued cooling of the magma after trapping of a melt inclusion may lead to overgrowth of the host mineral on the walls of the melt inclusion, decreasing the volume of the melt inclusion and strongly depleting the remaining melt in the host mineral component. Heating with visual control using a heating stage microscope, can melt back the overgrowth material, and effectively reconstruct the melt inclusion composition to that of the magma at the point of trapping. We have built a modified and upgraded version of the heating stage apparatus first designed by Prof AV Sobolev at Vemadsky Institute of Geochemistry in Moscow, which provides optical control of homogenisation and phase transformations in an f02-controlled environment. Melt inclusions are heated until the instant of disappearance of the shrinkage bubble, which is taken to be equivalent to the point of trapping of the melt inclusion. The melt is rapidly quenched from this homogenisation temperature using a blast of He (because of its very low thermal inertia), and invariably produces a crystallite-free glass. Such quenched glassy inclusions can then be exposed by polishing, and analysed by various microanalytical techniques, including electron microprobe, FTIR, laser Raman spectroscopy and laser ablation ICP-MS. AN APPLICATION - PORPHYRY CU DEPOSITS A major target of our research has been to examine the evolution of volatiles and metals during crystallisation of magmas that host economic porphyry Cu-Au deposits, and similar, but barren, intrusions in known porphyry districts. Abundant fluid inclusion data document the coexistence of hypersaline liquid (brines) and vapour in porphyry deposits. Most metals partition preferentially into the brine, from which they may be precipitated to form ore. High temperatures and salinities of ore-forming fluids suggest that there should be a continuum from originally low-volatile silicate melt through hydrous silicate melt and hydrous saline melts to late-stage waterrich fluids. Extensive fluid inclusion research has been unable to address the early, high-temperature stages of magmatic fluid exsolution from porphyry deposit magmas. In contrast, the melt inclusion record of intrusive porphyries (mineralised or barren) is untapped, apart from a few studies which documented silicate/salt melt immiscibility in granites. We have commenced this attack with a melt inclusion study of the Balut dyke from the Dinkidi porphyry Cu-Au deposit in Luzon. This dyke is a late stage transitional alkaline monzodiorite pegmatite strongly implicated in the Cu-Au mineralisation at Didipio. Fresh magmatic hornblende and clinopyroxene phenocrysts preserve numerous melt inclusions. Inclusions contain different proportions of silicate glass, silicate daughter crystals (e.g., mica and amphibole), opaque minerals (e.g., haematite, magnetite, and chalcopyrite), salt crystals (e.g., Na and K chlorides and sulfates), a vapour bubble and liquid H2O. Preliminary ICP-MSdetermined bulk compositions have significant quantities of Na, K, S, CI, and precious- and chalcophile metals. These inclusions record the latest stage in the evolution of such magmas, when an immiscible metal- and volatile-rich salt-silicate component separates from the cooling magmatic system and evolves into hydrothermal hypersaline brines and solutions. Further studies on this occurrence, and several other major porphyry Cu related intrusive rocks are in progress, and should provide important new constraints on the late magmatic evolution of fluids, brine segregation, and partitioning of the chalcophile and base metals. An optimal outcome from this work would be the demonstration that 'fertile' potentially mineralising magmatic systems have certain diagnostic compositional features clearly differentfrombarren systems.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Comention, Townsville, July 1998
PETROGENESIS AND GEODYNAMIC SIGNIFICANCE OF THE BASEMENT COMPLEX OPHIOLITE, PENTECOST ISLAND, VANUATU ISLAND ARC Anthony J Crawford^ Richard C Price^ and Roland Maas^ 1: Centre for Ore Deposit Research, School of Earth Sciences, UTasmania, GPO Box 252-79, Hobart, TAS 2: School of Science and Technology, UWaikato, NZ. 3: VIEPS, LatrobeU, Bundoora The Vanuatu Island Arc is unusual in the western Pacific in that it lies above an eastward-subducting IndianAustralian plate, rather than typical westward subducting Pacific oceanic crust. Along most of the Vanuatu arc, subducting oceanic crust (North Loyalty Basin) is of Eocene age (-40-50 Ma), and is presumably backarc crust behind the Eocene D'Entrecasteaux - Loyalty intra-oceanic arc, although this has yet to be demonstrated conclusively. The central section of the Vanuatu arc consists of three belts of islands: an active Central Chain of volcanoes, an Eastern Belt composed of the long, narrow meridional islands of Maewo and Pentecost, and a Western Belt consisting of the large islands of Espiritu Santo and Malakula. The Western Belt islands, and the Eastern Belt, were uplifted during collision -3Ma with the submarine D'Entrecasteaux Zone, an Eocene intra-oceanic arc that emerges further south as the New Caledonia - Loyalty Islands Ridge. This collision is on-going, sliding north along the trench at 3-4 cm/yr. Between the uplifted Western and Eastern Belt islands is the down-flexed North and South Aoba Basins, believed to be floor^ by oceanic crust. Western Belt islands contain large areas of Early Miocene volcanics and high-level intrusive rocks, recording the early history of the Vanuatu arc when it was part of the Vitiaz arc system above west-dipping subducting Pacific Oceanic crust. A reversal in the polarity of subduction, usually considered to be triggered by collision of the Ontong Java Plateau with the Solomons arc 6-lOMa, affected magmatism in the Vanuatu arc. Late Miocene volcanics on the Eastern Belt islands of Maewo and Pentecost are mainly arc tholeiitic compositions, and are generally considered to be the earliest manifestations of arc magmatism above the current east-directed subduction zone, post-subduction reversal but before collision of the D'Entrecasteaux Zone. Also exposed on southern Pentecost Island in the Eastern Belt, in apparent high-angle fault contact with the Late Miocene lavas and volcaniclastics, is a complex and poorly exposed sequence of mafic and ultramafic rocks, constituting a disrupted ophiolite, termed the Basement Complex. Good geological evidence suggests that emplacement of this ophiolite was directly related to the D'Entrecasteaux Zone collision ~3Ma. More problematic however, is the age and tectonic setting of eruption of the ophiolitic rocks, particularly the ophiolitic basaltic lava sequence known as the Batmar Metalavas. Is the ophiolite early North Fiji Basin backarc crust upon which the post-subduction reversal modem arc was built, is it Eocene crust of the North Loyalty Basin and basement ot the proto-Vanuatu arc (now exposed as Early Miocene lavas in the Western Belt), or is it exposed oceanic crust of the Aoba Basin (and if so, what age and composition is this)? To address this problem, and clarify the tectonic evolution of the Vanuatu - North Fyi Basin - North Loyalty Basin region, we are completing a detailed petrological - geochemical -geochronological study of the Basement Complex ophiolite, based on samples from the New Hebrides (Vanuatu) Geological Survey collections in Port Vila. Mineral compositions from the cpx-poor Iherzolitic ultramafic tectonites preserved as slices above amphibolitized mylonitic sole rocks are similar to those in mid-ocean ridge-type peridotites, and unlike the very refractory peridotites exposed in supra-subduction zone forearc-type ophiolites. Olivine compositions range to F091J, but are typically Fo90-91, and chromites are relatively aluminous, with Cr/(Cr+Al) values mainly from 35-50. Ultramafic cumulates and gabbros are opx-poor. The dolerites and basalts are low-K tholeiitic, with -1.2% Ti02 at 8% MgO, LREE-depleted REE patterns, and slight negative Nb anomalies. In sum, the Basement Complex rocks are petrographically and geochemically most typical of early backarc basin crust, and are clearly not part of the Late Miocene arc magmatic sequences on the Eastern Belt islands. Preliminary Pb isotopic data indicate that the ophiolitic metabasalts do not share the typically 'Indian' low ^^^Pb/^^Pb (<18.3) of the North Fiji Basin backarc basin basalts, and probably preclude an early North Fiji Basin crust source for the ophiolite. The Basement Complex basalts have 206p5/204p|5 values very close to published values for drilled basaltic basement of the North Loyalty Basin, and on this basis at least are more reasonably equated with these Eocene lavas. Ar-Ar dating of best-preserved ophioUtic avas and dolerites is in progress to test this correlation. If correct, this correlation indicates that the oceanic basement of the Aoba Basin is Eocene backarc basin crust, and that the Western Belt Early Miocene arc magmatism was probably constructed either on 40-50Ma backarc basin crust behind the Loyalty -D'Entrecasteaux arc, or on the rifted-off remnant arc from this sytem isolated during North Loyalty Basin opening. Regional geodynamics of the Vanuatu arc system and adjacent older arcs will be evaluated on the basis of this new data. 100
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
LANDFORMS, SEDIMENTS, DEFORMATION AND UPLIFT AT THE LEADING EDGE OF A CONVERGENT MARGIN: GEOLOGY OF THE LAE URBAN AREA, PAPUA NEW GUINEA Keith A W Crook^*^ and Keyu Liu^ 1 Hawai'i Undersea Research Laboratory. University of Hawaii, Honolulu HI 96822, USA ^ Earth Sciences, James Cook University, Townsville QLD 4811,
Lae, Papua New Guinea's second city, lies on the leading edge of the South Bismarck plate convergent margin, which is over-riding the Australian plate along the Ramu-Markham Collision Zone. Reconstruction of the Holocene deformation and uplift history of the Lae urban area is based on examination of landsc^^ elements, sedimentary facies, and geological structure, supplemented by data from geotechnical and water boreholes, shallow reflection and refraction seismic surveys and resistivity probes. Lae straddles two structural units of contrasting geomorphic and sedimentologic character, separated by the Bumbu Fault, a normal fault down-thrown to the NE, that has been active during the Holocene. The Mt Lunaman block, west of the Bumbu Fault, which was tilted NE-wards at or before 8000 yrs BP, has been uplifted episodically during the Holocene and probably earlier, at 4.8-8.8 m/ka. This block is bounded on the W by the Dowsett Fault, a steeply NE-dipping thrust, that effectively forms the plate boundary at which the Pleistocene Leron Fm was thrust over contemporary f ^ delta gravels at 870±110 yrs BP. The Busu block, east of the Bumbu Fault, is a fan delta, the foreset slope of which terminates offshore at the leading edge of the plate boundary - the Markham Canyon. The subaerial depositional surface of the fan is unmodified tectonically, except at the mountain front and near the Bumbu Fault, but the fan slope (0.5®) is anomalously low, possibly due to back-tilting of the leading edge of the plate boundary. Uplift rates are poorly constrained at <2.3 m/ka. During the middle and late Holocene, the Bumbu River flowed across the Mt Lunaman block, creating erosional and gravely depositional landforms. However, the most recent uplift ca. 250-300 years ago, which left +5 m shoreline traces on both blocks, caused avulsion of the river into its present course along the Bumbu Fault. The villagers at Buko Settlement, on the east bank of the Bumbu at its mouth, retain an oral history of this river avulsion.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
EXPERIMENTAL EVIDENCE SUPPORTING PORE-FILLING MODELS FOR GAS STORAGE BY COAL Peter J Crosdale Coalseam Gas research Institute, School of Earth Sciences James Cook University, Townsville, Qld. 4811 email: Peter.Crosdale@jcu.edu.au
The fundamental mechanism of gas storage by coal generally accepted by the geological community is that of monolayer adsorption. The main reason for this broad acceptance is that the adsorption isotherm is easily and adequately modelled using the theory of Langmuir, which was formulated to describe monolayer gas adsorption on mica, glass and platinum. However, coal is usually thought to be a microporous solid. Industrial chemists working with activated carbons and other microporous solids have long recognised that monolayer (Langmuir) and multilayer (BET) adsorption models are inappropriate for these materials and prefer to use volume filling models such as described by Dubinin - Radushkevich (D-R) and Dubinin - Astakhov (D-A). For pure gas adsorption isotherms, all four type of models give good fits (less than 4% relative error). The D-A model gives the best fit, but this is expected as the model is optimised to the experimental values. Adsorption and desorption isotherms using a gas mixture of 50% CH^ and 50% COj have been determined on coals from South Bulli andDartbrook Collieries (NSW). Coals are crushed to -0.212 mm and brought to an equilibrium moist state prior to the analysis. Dead volume of the adsorption bomb is evaluated using helium. The bombs are evacuated to less than 20 mbar, after which a known volume of the gas mixture is introduced at a variety of pressure steps up to 6 MPa. The system is allowed to equilibrate until the pressure becomes constant. A sample of the free gas is taken at each pressure step. Isotherms are constructed for the total adsorbed gas, adsorbed CH4 and adsorbed COj using data from the drop in pressure during adsorption and the change in composition of the free gas. The experimental results show that the isotherm for the total adsorbed mixed gas falls approximately midway between the pure CH4 and pure CO2 isotherms. However, analysis of the adsorption isotherms of the individual gases shows that the CH4 component follows a trend which approximates the pure CH4 isotherm, while the increase in adsorbed COj is approximately linear and unrelated to the pure gas trend. Modelling using a monolayer or multilayer approach would predict the opposite results. Here, a dynamic equilibrium exists between the adsorbed and non-adsorbed phases, in which the larger adsorption coefficient of COj would see it preferentially adsorbed under equilibrium conditions. Pore filling models, however, would permit preferential adsorption of CH4 as the rate of diffusion is inversely proportional to the molecular weight. More rapid diffusion of the lighter CH4 into the micropores allows it to occupy the bulk of the adsorption sites. Mixed gas desorption isotherms have also been evaluated. The total gas desorption isotherm again falls approximately midway between the pure gas end members with only a small hysteresis. However, analysis of the gas components shows them to again behave independently, with a very strong hysteresis. Methane is rapidly desorbed while carbon dioxide is preferentially retained. This result is expected because of the larger coefficient of adsorption for carbon dioxide, and would be predicted using either monolayer or pore filling models. Acknowledgments This work has been supported by the Australian Coal Association Research Programme (ACARP), Shell Coal and the Australian Research Council (ARC).
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
PETROGRAPHY AND GEOCHEMISTRY OF TERTIARY COALS FROM THE ZEYA - BUREYA BASIN, FAR EASTERN RUSSIA Peter J Crosdale and Ken J Woolfe Coalseam Gas research Institute, School of Earth Sciences James Cook University, Townsville, Qld. 4811 email: Peler.Crosdale@jcu.edu.au
The Zeya - Bureya Basin covers 1200 km^ north of the Amur River, with its southern boarder centred around the town of Blagoveshchensk. The basin's coal resource is estimated at greater than 10"^ t and includes the most economically significant coal deposits of Far Eastern Russia. The coal-bearing sequence is 200 to 300 m thick and was deposited during the Palaeocene to Miocene in fluvial environments. Coal deposits at Raichikhinsk, Yerkovtsi and Cergeyevka have been investigated. The Raichikhinsk and Yerkovtsi deposits are Palaeocene in age while Cergeyevka is probably late Miocene. Samples were collected from open cut mines, which were active at Raichikhinsk and Yerkovtsi and abandoned at Cergeyevka. Lithotype logging was performed and small block samples collected to represent each lithotype. At Raichikhinsk approximately 4 m of coal was measured, representing most of the seam section; at Yerkovtsi 0.5m from the middle of a 5 m seam was investigated; at Cergeyevka a random sample from the top most portion of an 8 m seam was collected, with additional material from the basal portion supplied by the Amur Research Centre, Blagoveshchensk. Macroscopic logging indicated the Palaeocene coals could be characterised using brightness profiles traditionally used for bituminous coals. They were dominated by dull lithotypes, with large amounts of evenly distributed fiisain and minor thin (1-2 mm) fusain bands. No strong vertical profile trends were observed. Three thin (3-5 cm) tuffs were also noted. The Cergeyevka samples were of medium - dark brown coal lithot^q^es and some xylite. A total of 30 coal and 3 tuff samples were collected. Coals were analysed for proximate, ultimate and maceral analyses (Table 1), and reflectance of humotelinite. Blue light excitation fluorescence was used to identify liptinite components. Some bright coal samples were specifically targetted for rank evaluation. Table 1 Ranges of geochemical and petrographic data. Number of geochemical samples : Raichikhinsk = 8; Yerkovtsi = 3; Cergeyevka = 1. Number of petrographic samples : Raichikhinsk = 23; Yerkovtsi = 4; Cergeyevka = 3. Locality
Raichikhinsk
Inherent Ash Moisture (%) W
Volatile Fixed Volatile Matter Carbon Matter (%) (%) (%,daf)
C (%)
H (%)
N (%)
S (%)
0 (%)
Huminite
Lipt- Inertinite inite (o/o,mmf)
n.d.
0.0- 0.08.0 96.3 0.0- 0.00.8 89.9 0.0- 0.00.8 22.8
Yerkovtsi
32.2 40.7 35.2-
1.3 8.1 2.2 -
23.7 32.1 22.0-
29.9 36.9 31.8-
37.7 - 62.2 - 3.5 - n.d. 49.2 70.0 5.1 38.5 - 62.8- 3.5 - n.d.
n.d.
26.12.733.1 100.0 30.11.6-
Cergeyevka
39.1 58.0
7.6 22.6
28.3 15.8
35.2 3.6
46.8 81.4
n.d.
32.7 25.7
65.7 64.4
5.0 7.9
n.d.
100.0 76.4100.0
Petrographic and geochemical results indicate the Raichikhinsk and Yerkovtsi deposits are hard brown coal whilst the Cergeyevka deposit is a soft brown coal. Geochemical parameters have a large range and are strongly influenced by coal type. Volatile matter (daf) is strongly correlated to H/C and total huminite. Van Krevelen plots show Raichikhinsk and Yerkovtsi are kerogen type III to type IV, consistent with their huminite to inertinite petrography. Cergeyevka is kerogen type II suggesting a strong algal influence, but this was not confirmed by fluorescence observations. Maceral analysis shows the coals from Raichikhinsk and Yerkovtsi to be dominated by inertinite group macerals, especially semiflisintie and inertodetrinitne. The paucit>' of liptinite is confirmed by fluorescence observations. Maceral composition is unusual for Tertiary coals but shows strong similarities with many Permian coals e.g. Blair Athol (Bowen Basin) or Oaklands Basin coals. The Cergeyevka coals are compositionally similar to many other Tertiary coals, but the small number of samples and very high ash content (dry basis) may not be representative. This project was undertaken as part of an initiative between the Cambridge Arctic Shelf Programme (CASP) and the The Russian Academy of Sciences, Far East Branch, Amur Research Centre, Division of Regional Geology and Hydrogeoiogy, Blagoveshchensk.
103
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
TECTONIC EVOLUTION AND EXPLORATION POTENTIAL OF THE GAWLER CRATON, SOUTH AUSTRALIA SJJDMr, CM. Fanning ^ M.C. Fairclough^ ^Mines and Energy South Australia. GPO Box 2355, Adelaide, SA 5001 ^Research School of Earth Sciences, ANU, Canberra, ACT 0200 ^Merritt Mining NL, 77 Thomas St. Subiaco WA 6008
Interpretation of newly acquired high-resolution aeromagnetic data, accompanied by reassessment of corresponding integrated geological data, establishes a complex Archaean to Mesoproterozoic tectonic history for the Gawler Craton in South Australia. A key outcome is the recognition of major tectonic events between 1540 and 1565 Ma and at -1650 Ma that postdate the Kimban Orogeny. The northwest part of the craton has been subjected to high-grade metamorphism at these times, whereas in the central part of the craton coeval deformation is focused withm major shear zones. The proposed new name for this orogenic advent is the Kararan Orogeny, derived from the major crustal feature the Karari Fauh. Deformation during the Kararan Orogeny was essentially contemporaneous with Hiltaba Suite and Gawler Range Volcanics magmatism and is a critical factor in the emplacement of copper-gold mineralisation. We propose that deformation associated with the Kararan Orogeny is related to continental collision between the eastern proto-Yilgam Craton, in the north west, and the central Gawler Craton-East Antarctic Craton (the Mawson Continent) in the south. The Fowler Orogenic Belt is one of the most intensely deformed regions within what is a broad collisional zone, and together with adjacent less-deformed crust may host mantle-sourced Proterozoic nickel deposits. Although recognition of the Kararan Orogeny is significant to our new understanding of the craton, there is a long and complex record of events that commenced in the late Archaean. Significant stratigraphic units within the craton and then- subsequent tectonothermal history are briefly summarised. Newly available data have been mcluded as far as possible following on from MESA's publication o f ' The geology of South Australia - The Precambrian in December 1993. It is well known that the Gawler Craton is host to the world-class Mesoproterozoic Olympic Dam copperuranium-gold deposit. The craton also has significant potential for the discovery of additional copper-gold deposits related to acid volcanism and/or late-stage high crustal level granitic intrusives associated with altered and deformed Archaean and Palaeoproterozoic hosts. This potential is now being realised with application of the cost effective calcrete sampling technique to detect anomalous gold. The discovery, in 1996, of shear-hosted gold in the central Gawler Craton within the Yarlbrinda Shear Zone has intensified exploration in those regions where Hiltaba Suite and Gawler Range Volcanics intrude shattered basement. Exploration expenditure for Archaean gold has increased significantly following discovery of the Challenger gold prospect in June 1995. Gold mineralisation occurs in complexly deformed Archaean quartz-feldspargamet-cordierite gneiss. Many other calcrete-hosted gold anomalies remain to be tested within poorly exposed Archaean Mulgathing Complex that also comprises layered ultramafic sills and exposed komatiites.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
CARBONATE SEDIMENTS: STREAKY BAY, SOUTH AUSTRALIA. Richard Daniel'. Chris von der Borch\ Noel James^ and Yvonne Bone'. 'Department of Geology and Geophysics, University of Adelaide, South Australia, 5005. ^School of Earth Sciences, Flinders University, Bedford Park, South Australia, 5042. ^Department of Geological Sciences, Queen's University, Kingston, Ontario, K7L3N6, Canada.
Streaky Bay is a large embayment on the West Coast of Eyre Peninsula, South Australia. It is partially protected from high wave attack and swells from the Southern Ocean by substantial headlands at the mouth. Islands and extensive shallow near-shore tidal deltas attenuate wave energy within the bay. Proterozoic felsic intrusives occur north and south of the bay as basement whereas the headlands are composed of Pleistocene Bridgewater Formation aeolianites. Sediments are predominantly biogenic in origin. Extensive sampling and analysis of sediments collected on transects across the bay show distinct patterns in both component and grainsize distribution patterns. The sediments are mixtures of: 1) ubiquitous Holocene biogenic particles (--60%); 2) unidentifiable abraded bioclastic grains, localised to sand banks and spits (-20%); 3) calcarenite or relict carbonate grains, restricted to areas where the Bridgewater Formation is exposed (--10%) and 4) Proterozoic rock fragments, especially in the region of Olive Island (-10%). Significant Holocene biogenic sand-size components are fragments of, in decreasing order of abundance, molluscs, biyozoans, foraminifersand calcareous red algae. Minor contributions are fragments of echinoderms, brachiopods, various crustaceans, serpulids, corals and sponge spicules. Molluscs, whilst present in significant quantities throughout the bay, are most abundant in the basinal and deeper tidal-delta areas (>75%). Foraminifers are concentrated in the central sea-grass meadows flanking the banks and tidal deltas and in the central area of the bay, where brown algae are prolific. The highest percentage and diversity of bryozoans, including rare vagrants, occurs centrally at the bay entrance where diverse suitable substrates occur. Calcareous red algae are ubiquitous in all the sediments collected. Diatoms are common in the fine sand- to mud-sized sediment fraction. Sediments are supplied from the entrance to the bay, from around the headlands and probably from deeper off-shore shelf areas. These sediments have formed extensive spits abutting the north and south margins of the bay. Material has also accumulated across the inner half of the floor of the bay, forming two elongate sand banks approximately 9 km in length. Large tidal deltas occur between and adjacent to these sand banks. The sea-floor of the deeper outer areas are hard substrates of eroded Bridgewater Formation, with isolated mobile sand waves dispersed across the sea floor. Sediment here is mostly derived from the nearby islands and isolated subaqueous highs. The grainsize distribution of the sediments ranges from gravel to mud size. Gravel-size sediments (up to 40%) occur at the mouth of the bay whereas only minor amounts of gravel (<5%) occur within the bay. Localised concentrations of gravel are the result of biogenic activity, skeletal fragments and erosion of the bay floor. Coarse to very coarse sands are present in high percentages (up to 80%) at the mouth, centre and central tidal delta regions of the bay. The grain size of the sediments from the sand banks and the tidal deltas is predominantly medium to fme sand, with minor areas extending to gravel size (<10%). Mud is present (up to 50%), along with medium to very fine sand, in the comparatively deep-water of the inner marginal basins. These are protected from the effectsof the open sea by the sand b a i i s and tidal deltas. The fine sandy muds are characterised by silt-size grains. The calcium carbonate content of the mud is >95% and consists of low and high magnesium calcite and aragonite.
105
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
H2O GEOCHEMISTRY IN DEPLETED TO MODERATELY ENRICHED MIDOCEAN RIDGE MAGMAS Leonid V. Danvushevskv\ Rodey Batiza^, Stephen M. Eggins^ ^ Department of Geology, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001 ^ Department of Geology & Geophysics, 2525 Correa Rd., University of Hawaii, Honolulu, HI 96822, USA ^ ARC Key Centre for Geochemical Evolution and Metallogeny of the Continents (GEMOC), Department of Geology, and Research School of Earth Sciences, Australian National University, GPO Box 4, Canberra, 2601
We present and discuss new determinations of H2O, REE, and major and minor oxides in -220 samples from five suites of nfiid-ocean ridge basalt (MORB) from the northern East Pacific Rise, the South East Indian Ridge adjoining the Australia-Antarctic Discordance (AAD), the Mid-Atlantic Ridge at 26^S, and a Pacific ODP Site 896A. This study is different from previous studies of H2O variation in normal, and enriched MORB in that for most samples, all analyses (H2O by the Fourier Transform infrared spectroscopy, major elements by electronmicroprobe, and trace lements by laser ablation inductively-coupled plasma mass spectroscopy) were carried out on the same small region of the same glass chip. In terms of the variation of relative H2O (H20/other element) contents with indices of mantle heterogeneity such as La/Sm or K/Ti, we find that ~ 80% of samples from all areas belong to a 'main trend' of variation. We interpret the main trend as the average result of melting multi-component, heterogeneous mantle. The 'average' geochemical behavior of H2O within the main trend is intermediate between La and Ce. However within individual suites, H2O can be La-like, K-like and even Sm-like during petrogenesis. Along the main trend, H2O is La-like at La/Sm< 0.6 and is somewhat between Ce and Nd at La/Sm> 0.6, a phenomenon that may explain the kinks in the main trend for H20/La and H20/Ce vs. La/Sm plots. This behavior could be due to a compositional dependence of the H2O distribution coefficient. We show that H20/Ce values are not constant in ail glasses within a region, as was previously thought (Michael, 1995), but vary systematically with changing La/Sm. Different regions have different isotopically distinct enriched components, though all have surprisingly similar relative H2O contents. We agree with Jambon and Zimmermann (1990) that the enriched components represent metasomatized, not primitive, mantle, with metasomatic components probably derived ultimately from recycled oceanic crust. Suites comprising the main trend also exhibit regional shifts in relative H2O contents that seem to reflect differences in both the enriched and depleted mantle components. Relative H2O contents in glasses from the AAD and MAR are affected by local petrogenetic factors, resulting in correlations between relative H2O and La/Sm oblique to the main trend. Glasses affected by the local factors usually plot within the main trend, but may be displaced beyond it in the extreme cases. Within the AAD, such a local factor is identified as a variable proportion of garnet in the source of the lavas, causing H2O to be Sm-like in its incompatibility. In the MAR glasses, the local factor may be assimilation of altered oceanic crust, and H2O behaves K20-like. In addition, glasses with anomalously high relative H2O contents (compared to the main trend) were found in almost every area. The source of the high-H20 melts may be the main participant in magma genesis in some areas. In general, high relative H2O contents are present in extremely depleted lavas, implying their independence from the enriched components. The reasons for higher H2O appear to be different in deferent areas. An important conclusion from our results is that H2O in MOR magmas is a very sensitive petrogenetic indicator within suites of cogenetic MORB. H2O contents are a powerful tool providing additional insights into MORB magma genesis. REFERENCES Jambon A. & Zimmermann J.L. 1990. Water in oceanic basalts: evidence for dehydration of recycled crust. Earth Planetary Science Letters 101, 323-331. Michael P.J. 1995. Regionally distinctive sources of depleted MORB: Evidence from trace elements and H2O. Earth Planetary Science Letters 131, 301-320.
106
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
REPEATED FELDSPAR ALTERATION EVENTS DURING THE CONSOLIDATION HISTORY OF THE McARTHUR GROUP Garry Davidson Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania, 7001 Low to moderate temperature alkali metasomatism characterises some shallow evaporitic sedimentary facies and their adjacent sedimentary successions. In the McArthur Group of the Proterozoic McArthur Basin, Northern Territory, feldspathised beds are common in both carbonate- and shale-dominated units, and larger scale Kmetasomatism is identified by the regional aeroradiometric response of some units, specifically the Stretton Sandstone, and the Lynnott, Barney Creek and Tooganinie Formations. Feldspathic alteration of several styles is particularly focussed on former felsic tuffaceous rocks, as evidenced by Ti/Zr values that are distinct from the host sediments, as well as euhedral zircons, resorbed phenoocryst fabrics, and uncommon vitriclast relics. These are divided into primary airfall and slump-resedimented varieties. McArthur Group feldspar alteration comprises several styles: (1) trough margin albite + K-feldspar alteration along at least some parts of the Emu Fault Zone, for instance HYC and the Glyde Sub-basin; At HYC this alteration is zoned from microcline-dominant near to the Emu Fault, to albite-dominant LO-3.5 km away from the fault (Logan 1979). S^^Owholerock of the HYC beds has strong linear correlations with AI2O3, Si02 and [Na20+K20], permitting modelling of the replacement process as equilibrium precipitation of quartz and feldspar from a low temperature meteoric fluid (32±27° C) (Davidson, in press). The beds also have REE patterns that are distinctly Eu-depleted, which has implications for the low temperature leaching and transport of Eu. (2) ore-related alteration at the HYC Zn-Pb-Ag deposit, which overprinted trough margin alteration (1) in places, adding disseminated ankerite, adularia and base-metal sulphides to some previously feldspathised beds; (3) early diagenetic K-feldspar metasomatism that was associated with descending/advecting brines, that were sourced from overlying evaporite sequences, and transmitted through porous clastic sediment zones, such as the Donnegan Member of the Lynott Fm, and the overlying Yalco Fm. The main evidence for this mechanism is the confinement of the style to clastic portions of the carbonate package beneath recognised evaporite zones, and the petrographic evidence of porosity filling by K-feldspar, as well as wholescale K-feldspar replacement. This style is the most widespread stratabound alteration. Speculatively, the brine-generation process responsible for this alteration may have also played a part in the generation of base metal deposits in the basin (Davidson 1998). (4) extensive inversion-related K-feldspar metasomatism of volcaniclastic beds, producing whole rock Si02/Si02+Al203 of 0.75-0.94, and hematite-bearing microcline-quartz±calcite assemblages in transgressive vein and replacement zones. McArthur Group alkali metasomatism occurred at several stages in basin development. Trough margin metasomatism, and alteration associated with base metal formation at HYC, occurred during very shallow burial, such that volcanic glass was directly replaced by feldspar. Metasomatism of clastic sequences beneath evaporites by descending/advecting brines may have occurred over shallow to deep burial diagenesis, evidenced by a change in style at some sites from porosity filling to vein generation. Lastly, the u-ansgressive basin-centre replacement zones may have developed during basin inversion (1640-1430 Ma), evidenced by a typical association with late steep fracture systems, and a mineralogic uniformity over a large area. This multiplicity of feldspar events in the unmetamorphosed McArthur Basin sounds a cautionary note to the interpretation of feldspar alteration in more deformed equivalent terrains such as the Mt Isa Eastern Succession zones. REFERENCES Davidson G.J. in press. Feldspar metasomatism along a Proterozoic rift margin— 'smoke' around a base metal 'fire' (HYC deposit, Australia), or a product of background diagenesis? Bull GeoL Soc. America. Davidson G.J. 1998. Alkali alteration styles and mechanisms, and their implications for a 'brine factory' source of base metals in the rift-related McArthur Group, Australia. Aust. J. Earth Sci. 45: 33--49. Logan R.G. 1979, The geology and mineralogical zoning of the H.Y.C. Ag-Pb-Zn deposit, McArthur River, Northern Territory, Australia [M.Sc thesis]: Australian National University, 187 p. Acknowledgements: The financial support of AMIRA project P348, and the supply of sanfples from the HYC deposit by MIM Exploration, is gratefully acknowledged.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE EVOLUTION OF SE PNG Hugh L Davies Geology UPNG, University NCD, Papua New Guinea; hdavies(2)upng.ac.pg
The Papuan peninsula and islands comprise a mountainous core of metamorphic rocks bounded northeastwards by a major ophiolite complex. The metamorphic rocks are broadly zoned from higher grade in the NE to lower grade in the S and SW as follows: • amphibolite with enclaves of eclogite in the northern D'Entrecasteaux Islands; • amphibolite (western Misima); • blueschist to greenschist fades (Emo metabasites in NE Owen Stanley Range; parts of the SucklingDayman massif; parts of Port Moresby hinterland); • greenschist facies (much of the remainder of the Owen Stanley Range; southern D'Entrecasteaux Is; eastern Misima and remainder of Louisade Archipelago); • prehnite-pumpellyite facies (parts of Suckling-Dayman massif and Port Moresby hinterland); and • zeolite facies (south of the Suckling-Dayman massiOIn addition, high temperature granulites occur discontinuously as an aureole at the base of the ophiolite (Lus and McDougall, this volume). Parts of the protolith of the metamorphic rocks and the ophiolite are Latest Cretaceous in age (Maastrichtian age from planktonic foraminifera). Metamorphism was effected in the Early Cainozoic in the footwall of a NEdipping subduction system. The high grade metamorphic rocks of the northern D'Entrecasteaux Islands represent the deeper parts of the subducted slab (subducted to depths >75 km) and the lower grade metamorphics represent shallower parts of the same slab (with exceptions, see mention of Oligocene convergent tectonics below). Early Cainozoic arc-continent collision was followed by uplift and cooling of the metamorphic rocks. These rocks yield a wide range of isotopic ages, probably reflecting a long history of uplift and cooling. The oldest wellfounded ages are from the metamorphic aureole of the ophiolite (Lus & McDougall, this volume). Uplift intitally may have been in response to continued transpressional(?) convergence after collision. The evolution of SE PNG through the Paleocene to Oligocene is complex and not well understood. By the MidEocene arc polarity had reversed with the result that arc-related Mid-Eocene volcanics (Eia volcanics) were deposited on the ophiolite. Also in the Mid-Eocene, the metamorphic rocks partly emerged to become a source of schist clasts in bioclastic limestones near shore (Mafulu beds, Tapini area) and a probable source of fine siliciclastic sediments deposited far from shore (Port Moresby beds). Late Eocene(?) convergence and stacking of the Port Moresby beds was followed by Early(?) Oligocene emplacement of gabbro, presumably in an extensional environment (Sadowa Gabbro), and a further convergent event in the Late Oligocene or Early Miocene, approximately synchronous with the docking of the Eastern Papua Composite Terrane to the Australian craton (AusIMM GEM 97 conference proceedings, pp. 61-66). In this convergent event the Eocene sediments and Oligocene gabbro were metamorphosed to prehnite-pumpellyite facies and the gabbro locally to greenschist-blueschist facies. The Solomon Sea Basin opened in the Late Eocene or Oligocene (magnetic anomalies interpreted to indicate 3824 Ma) and the Trobriand Trough S-dipping subduction system became active in the Late Oligocene or Early Miocene. By Late Oligocene or Early Miocene the intermediate and higher grade metamorphic rocks that now form the D'Entrecasteaux Islands had partly emerged and formed a substrate for shallow marine sedimentation. Uplift of the metamorphic rocks of the Owen Stanley Range, the D'Entrecasteaux Islands and Misima Island accelerated in the (Latest Miocene-) Pliocene and Quaternary, driven by thermal effects and extensional tectonism associated with the opening of the Woodlaric Basin. From the results of a GPS-based study of present-day plate motions, Tregoning et al. (JGR, 1998) deduce that crustal extension in the SE part of the Papuan peninsula should give way to left-lateral strike-slip and convergence in the NW part of the peninsula, west of 148°E. Neotectonic features of the area around 148®E are compatible with this (Buna 1:250 000 Geological Series Map and Explanatory Notes; Geological Survey of PNG, 1998). Extensional processes in continental crust immediately east of the D'Entrecasteaux Islands will be investigated by Ocean Drilling Program Leg 180 in mid-1998.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
OSTRACOD DISTRIBUTION ON THE LACEPEDE AND BONNEY SHELVES: THEIR RELATION TO SEDIMENTARY FACIES AND LOCATION ON THE CONTINENTALSHELF AND SLOPE, Miles Davies^. Yvonne Bone^ and Ben McHenry^ I Department Geology and Geophysics, University of Adelaide, South Australia, Australia 5005. 2 Division of Natural Sciences, South Australian Museum, South Australia, Australia 5000.
Sediments on the cool-water, wave dominated Lacepede and Bonney shelves of the southern passive margin of Australia consist of Holocene and late Pleistocene siliciclastic and carbonate grains. The Lacepede Shelf (and to a lesser extent the Bonney Shelf) has been divided into 5 separate zones which have a characteristic range of sedimentary facies. The ostracod distribution patterns across the Lacepede Shelf upper section of the slope, (40m to ~350m), show a dynamic relationship between ostracod assemblage, sedimentary facies and location on the slope. The same relationships are not as clear on the Bonney Shelf, due to a patchy ostracod distribution, but differences can still be seen. Ostracod specimens are rare on the Lacepede Shelf at depths less than --70-100m depending on the amount of down-welling. Species that do appear in shallow water samples include Foveoleberis brevirostra and Foveoleberis sp.l, Neonesidea chapmani and Paranesidea sp.l. Well sorted and reworked siliciclastic and carbonate material are the dominant sediments. This high energy environment and constant re-working is believed to be a key factor in the distribution of ostracoda. Ostracods become more abundant at -lOOm on the Lacepede Shelf - the shelf break region, below wave base. Quartzose bryozoan^ivalve sands and robust bryozoan sands are characterised by adult and late stage instars of Foveoleberis spp. and Neonesidea spp. with lesser species including Cytheralison cosmetica, Neobuntonia sudaustralis and Hanaiceratina balcombensis. Autochthonous vs allochthonous classification of the adult specimens is dubious. Some of the specimens are glassy in nature and lack carapace abrasion, (characterising negligible transportation if any), whereas others are opaque and show abrasion. This is either a product of the high energy conditions or a product of significant transport. Which of these two is dominant still debatable. Ostracod populations on the upper shelf slope, (130 to 180m), are more diverse although the overall fauna is still dominated by Foveoleberis spp., Paranesidea spp. and A^eonesidea spp.. Robust bryozoan sands and muddy bryozoan sands dominate the sedimentary facies. Middle slope assemblages, (-180 to 330m), are dominated by autochthonous Actinoleberis spp., Cytheropteron spp., Occulocytheropteron spp. This assemblage is also characterised by the first appearance of autochthonous Parakrithella sp. This assemblage also has an increasingly high diversity of allochthonous material increasing with depth, with mid to late-stage instars of Xestoleberis spp. and Paradoxostoma spp. being common. Assemblage changes in samples deeper than ~330m are hard to gauge due to the limited study material but faunal changes are more subtle being largely defined by the first or last appearance of certain species and the percentage of allochthonous fauna in the sample. Bythocypris spp. appear at a succession of depths in the middle slope assemblage as well as this assemblage. They appear as one of the few autochthonous species. The narrower Bonney Shelf is influenced by strong upwelling currents which seem to effect the ostracod populations dramatically. Assemblages are dominated by autochthonous Cytherella spp., Argilloecia spp., Callistocythere spp. and Loxoconcha spp.. While near-shore, (-40-50m), and an upper shelf slope, (130 to 180m), assemblages exist, there is an absence of specimens between these depths. The recognition of ostracod distributions in modem environments such as the Lacepede and Bonney Shelves can provide vital information for the interpretation of depositional environments of Cainozoic cool-water carbonates.
109
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
CHARACTERISTICS AND POSSIBLE ORIGINS OF DIAMONDS FROM WELLINGTON AND BEVGARA, NSW R. M. Davies^. S. Y. O'Reilly^ and W. L. Griffin^' ^ ^ ARC National Key Centre for the Geochemical Evolution and Metallogeny of Continents (GEMOC), Macquarie University, NSW, 2109 2 CSIRO Division for Exploration and Mining, P. O. Box 136, North Ryde, NSW, 2113
The eastern margin of Australia hosts diamond deposits that occur in paleodrainages beneath Tertiary basalt, and in present day alluvial systems. Neither identified source rocks, nor the traditional suite of indicator minerals is recognised in association with the diamonds. Their occurrence overlying Phanerozoic basement terranes of the Tasmanide Orogen, raises the possibility that they may have a younger eastern Australian lithospheric origin, thus potentially identifying new environments for diamond formation and preservation. This study investigates the nature and origin/s of alluvial diamonds from Wellington, NSW and 350 km north at Bingara, NSW, by characterising the diamonds on the basis of their morphological features, mineral inclusions, carbon isotopes, internal structures, nitrogen contents and nitrogen aggregation states. Diamonds at Wellin^on and Bingara are of two types, here termed group A and B. At Wellington, group A diamonds dominate in the ratio of 4:1, while at Bingara, the population is essentially of the B type. Surface features of all diamonds are characterised by polished forms that have been strongly rounded by resorption. Etch features are similar to those of diamonds from kimberlite and lamproites, ie indicative of diamond transport to the surface in a magma. Unique to group B diamonds are irregularformswithfrostedpits and strong deformation features. Mild abrasion is evident on most stones, and radiation damage is more common in the group A diamonds, suggesting different alluvial histories for the two groups. The compositions of syngenetic mineral inclusions indicate that the group A diamonds formed in a dominantly peridotitic mantle volume; a small number of stones contain eclogitic inclusions. Olivine (Fo 93) is the dominant inclusion. Rare pentlandite and chromite also occur. The group B diamonds have only eclogitic inclusions with the exception of one diamond containing olivine (Fo 89); the inclusion suite includes a wide range of diopside - omphacite clinopyroxenes (3 to 40% Jd), Ca-rich gamet (61 - 83% Gr), coesite, sphene and molybdenite. Sphene and molybdenite have not previously been recognised as syngenetic inclusions in diamond. Furthermore the extremely calciimi-rich compositions of the garnets and many of the clinopyroxenes are unique. Carbon isotope measurements for group A diamonds range between S^^C -10%o and -3%o, withm the world range for peridotite diamonds, and suggest a derivation from a mantle carbon source. Group B diamonds are ^^Cenriched (S^^C = -5%o to +3%o), a signature that may suggest a crustal origin for the carbon. Internal growth features of the group A diamonds are characterised by planar octahedral layers that suggests growth in mostly stable conditions. In 25% of these diamonds, intermediate zones are truncated by one or more resorption episodes with overgrowths of octahedral layers. All diamonds show a late resorption episode that probably occurred in the emplacing magma. Nitrogen contents of group A diamonds are generally high (250 to 2500 atomic ppm). Corresponding nitrogen aggregation states indicate two sub-groups: the main group show a range between 6 to 42% laB; and a small group of stones has low nitrogen contents (< 900 at. ppm) but high laB aggregation states (44-95%), indicative of longer mantle thermal maturation histories, and/or greater plastic deformation. Low-nitrogen group B diamonds show evidence of rapid and unstable growth contemperaneous with defomiation in the form of non-planar growth fecets and displacement and brecciation of structures that have been rehealed, with episodes of diamond precipitation, and show subsequent overgrowths. In these diamonds, central structures are nitrogen rich (ca 1000 ppm), and rim zones are nitrogen poor (<100 ppm). Group B diamonds with high nitrogen contents have homogeneous structures. The high nitrogen B diamonds appear to be an early growth stage, as do the low nitrogen diamond cores. B diamonds show high degrees of nitrogen aggregation, relative to nitrogen contents with mixes of laA to laB types. The characteristics of the group B diamonds are consistent with formation in a subducted oceanic plate. Rodingitisation of basaltic dikes in peridotite prior to subduction, or mixing with Mg-carbonates, followed by subduction could account for the calcium rich inclusion suite, as well as the heavy carbon isotopic signatures. A subduction origin may link the formation of these diamonds to the New England Orogen. The group A diamonds may be derived from a more conventional source such as cratonic Proterozoic lithosphere of central Australia, or underlying eastern Australia at some time before 200 Ma. 110
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
STYLE AND CONSEQUENCES OF THE PERMIAN-TRIASSIC HUNTER-BOWEN OROGENY IN NORTHERN AUSTRALIA Brett K. Davis^ Robert A. Henderson^ and Ricardo G. Zucchetto^ ^School of Earth Sciences, James Cook University of North Queensland, Townsville, Queensland, 4811
The Hunter-Bowen Orogeny represents a protracted period of intense Permian-Triassic tectonism, the effects of which are widely developed in eastern Australia. We present here the results of detailed regional structural analysis of the northern Tasman Orogenic Zone, in particular the Hodgkinson Province, and present new structural and geochronological data that document the progressive development of structural relationships during evolution of the Hunter-Bowen Orogeny in northern Australia. Permian lithologies in the northern Tasman Orogenic Zone have been systematically examined, and integrated with observations from over 3000 field locations, structural and petrological data from more than 6000 thin sections, and 12 new isotopic dates of Permian assemblages. In particular, we have focussed on a number of Permian granite supersuites in the Hodgkinson Province that display crystallization ages spanning more than 30 million years. The Hodgkinson province has been affected by four major deformation events (D1-D4). D2-D4 have produced regional-scale structures that are recognised on the basis of penetrative fabrics and folds which show clear, regionally consistent overprinting relationships. D3 and D4 are products of Hunter-Bowen Orogeny and were contemporary with phases of granite emplacement as shown by the relationships of S3 and S4 microstructures to porphyroblasts which grew in contact metamorphic aureoles adjacent to granite plutons. A close spatial relationship, together with strong geochemical similarities for D3 and D4 granites, suggest that they were closely associated in time. Dating of granites of D4 association show that this deformational phase represents the main phase of Hunter-Bowen Orogeny. D3 fabrics are best developed in zones of enhanced metamorphic grade, are typically subhorizontal or gentle inclination, and are associated with recumbent folds. This deformation is attributed to a tectonic phase immediately preceding the main contractional phase of the Hunter-Bowen Orogeny. D4 is attributed to the main compressional phase of the Hunter-Bowen Orogeny and involved crustal shortening with the imposition of upright structures and the widespread emplacement of granite. Province-scale distribution of D4 structures shows a fundamental break across an inferred major crustal discontinuity, termed the Desailly Structure, which trends approximately ESE-WNW. This structure is interpreted to represent a zone of diffusely partitioned deformation that has influenced foliation development, cleavage trend line orientations, and the geometries of Permian plutons. D4 structures are well developed across the breadth of the Hodgkinson Province to the north of the structure but restricted to rocks in the eastern sector south of the Desailly Structure. To the north of the Desailly Structure, D4 features can be divided into zones which display consistency in style and intensity of structural development, particularly foliation morphology. Composite S 2 - S 4 features are common to all zones and are a product of the accommodation of D4 deformation by favourably oriented D2 structures. D4 foliation intensity is much more marked in the vicinity of the Permian granitoids and is interpreted to be a function of the additional heat and fluids supplied by magmatism synchronous with D4 shortening. Gold mineralization in the Hodgkinson Province is likely a product of Hunter-Bowen deformation also, and mineralization displays structural relationships consistent with emplacement during D4.
111
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
COMPOSITION OF PRIMARY MELT AND FLUID INCLUSIONS IN OLIVINE PHENOCRYSTS FROM LOMBOK, INDONESIA: IMPLICATIONS FOR MAGMA GENESIS AND METAL ENRICHMENT Fernando Della-Pasqua^ Khin Zaw^, Terry Memagh^, Chris Ryan^ and Rick Vame^ ^School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, TAS. 7001, Australia ^Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, TAS. 7001, Australia ^Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia ^CSIRO Division of Exploration and Mining, Box 136, North Ryde, NSW 2113, Australia
Melt and fluid inclusions from a suite of primitive samples on the island of Lombok, Indonesia were investigated to 1) define the chemical affinities of the parental magma and 2) assess the role of CO2 and trace element composition on magma genesis and metal enrichment. The Sunda-Banda arc is characterised by a wide compositional range of mafic magmatism. However, although basalts are common in this arc, picrites are not and instead, the most primitive rocks occurring on this arc are ankaramites (Mg^<73). Ankaramites are highly porphyritic and typically contain abundant phenocrysts of diopsidic clinopyroxene, minor olivine and traces of plagioclase. Olivine phenocrysts in ankaramite from Lombok are the earliest crystallising silicate phase. Thus, primary inclusions in these phenocrysts preserve information about the prevailing magmatic conditions during the early stages of crystallisation. Four types of such inclusions were found: L(melt), S(solid), V(vapour), and a combination of the three (L±V±S). S-type inclusions consist mostly of chromian spinel (Cr^56-74). Very rarely however, immiscible fluid bubbles are accidentally attached to tfiese Cr-spinel crystals and become trapped by a growing olivine phenocryst to form S+V+L primary inclusions. A DILOR MICRODIL-28® Laser Raman Spectroscope (LRS) at the AGSO, Canberra, was used to scan for N2, CO2 and CH4 fluid species in the fluid bubble of S+V±L type inclusions. These analyses confirm the trapping of a C02-rich fluid phase by the growing olivine (Fo88_9o) phenocrysts. However, N2 and CH4 were not detected. The shrinkage bubbles within melt inclusions were also investigated by LRS. These shrinkage bubbles form due to a change in density, and thus volume, inside the inclusion during cooling which also leads to an increased volatile pressure within the melt inclusion. As a shrinkage bubble forms, volatile species dissolved in the trapped melt may partition into the shrinkage bubble, which may or may not approach a vacuum, depending on the original concentration of volatiles in the melt. It is interesting to note that CO2 was only detected in S+V±L inclusions but not in the shrinkage bubble within melt inclusions, possibly as a result of their small volume (diameter <1 |im) and/or low concentrations. Melt inclusions (L-type) in magnesian olivine phenocrysts are typically crystalline. These inclusion were homogenised, using a heating stage, and then quenched to a homogeneous glass to determine their major and trace element composition. Electron microprobe analyses of homogenised melt inclusions indicate that the ankaramites magma formed by the aggregation of silica undersaturated melt fractions. Preliminary Proton Induced X-ray emission (PEXE) microanalysis of the crystalline melt inclusions showed elevated concentrations of Cu, S and Ca, and significant Ni and Se. The homogenised melt inclusions showed trace Sr and Y, but very low concentrations of ore elements (Cu - 40 ppm). No Cu was observed in the vapour-rich inclusions. The presence of S+V±L type inclusions in olivine phenocrysts is of fundamental significance for magma genesis because: 1) it indicates that, at least locally, some zones of the magma were fluid saturated during early stages of crystallisation, 2) it supports a mechanism where ankaramitic melts are generated as primary melts by mantle partial melting in the presence of CO2, 3) it suggests that a CO2 fluid phase must play an important role in the genesis of primitive ankaramitic magmas, 4) the presence of such immiscible fluid at magmatic temperatures (>900°C) provide a potential medium for the early partitioning of metals into a volatile phase within the host magma, 5) with increasing degree of crystallisation the exsolution of this fluid phase is likely to increase as magma evolves to more felsic composition, and 6) demonstrate the presence of mantle derived CO2. In summary, these results suggest that the ankaramite magma was saturated in C02-bearing fluids early in its evolution. This interpretation can be extended to other ankaramite suites such as Ulakan (Bali) and Merelava (Vanuatu arc), where similar S+V±L primary inclusions are found. Combined melt and fluid inclusion data may be used to ascertain the mineral potential and the barren/fertile nature of magma types.
112
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
NEGATIVECARBONISOTOPEEXCURSIONSATTHE CRETACEOUS/TERTIARY ANDPALAEOCENE/EOCENEBOUNDARffiSrUMQUEEVENTSORAN FOR PRE-TRIASSIC CARBON ISOTOPE EXCURSIONS? Gerald R. Dickens^ 'School of Earth Sciences, James Cook University, Townsville, Queensland 4811
Earth history is punctuated by apparently brief time intervals when the of the shallow ocean decreases by > 2 %o. These negative excursions occurred at the Palaeocene/Eocene (p/e) Boundary, at the Cretaceous/Tertiary (K/T) Boundary, and at several times prior to the Triassic. The excursions are important because they can be used as time markers, and because they necessitate upheavals in the global carbon cycle. The pre-Triassic excursions are of growing interest to Earth science studies in Australia; this presentation examines causes of rapid excursions with an emphasis on our understanding of the K/T and p/e boundaries. Four general mechanisms have been proposed to explain pronounced and rapid negative anomalies in the shallow marine carbon reservoir: (1) surface productivity declines precipitously to decrease the flux of ^^C from the shallow ocean carbon reservoir to the deep ocean carbon reservoir (e.g. Zachos et al, 1989); (2) overturn of a stagnant deep ocean carbon reservoir with h i ^ concentrations of CO2 increases the flux of ^^C to the shallow ocean carbon (e.g. Knoll et al., 1996); (3) impact of a volatile extraterrestrial body adds ^^C to the entire global carbon reservoir (e.g. Wilde and Quinby-Hunt, 1997); (4) thermal dissociation of oceanic gas hydrate and subsequent release and oxidation of CH4 adds ^^C as CO2 to the entire global carbon reservoir (e.g. Dickens et al, 1995). Unlike pre-Triassic time intervals, causes for excursions at the K/T and p/e boundaries are constrained with deep ocean records. The -2 %o decrease in the of the shallow marine carbon reservoir across the K/T boundary is coincident with an antithetical increase in the of the deep ocean carbon reservoir. In contrast, the -3 to -4 %o decrease in the of the shallow marine carbon reservoir across the p/e boundary is coincident with an -2 to -3 %o decrease in the of the deep ocean carbon reservou*. The K/T boundary is associated with redistribution of carbon isotopes within the ocean whereas the p/e boundary is associated with addition of ^^C to the entire ocean. The excursion at the K/T boundary is best explained by a substantial decrease in surface productiviW: chemical and faunal evidence indicates that productivity diminished at the time of the excursion. The C excursion at the p/e boundary is best explained by massive input of CH4: significant deep ocean warming immediately preceded the excursion, and deep ocean carbonate dissolution and O2 deficiency (as expected with CH4 oxidation) were contemporaneous with the excursion. Importantly, neither excursion can be explained by a redistribution of carbon isotopes via ocean overturn. On the basis of mass balance considerations, explanations for pre-Triassic negative excursions have stressed models involving ocean overturn (Knoll et al., 1996a,b). Although compelling evidence to support or refute such models has not been forwarded, if these speculations are correct, there is no analogy between pre- and postTriassic negative excursions. Profound changes in carbon cycling at the K/T and p/e boundaries must therefore be regarded as two truly unique events in the evolution of our Earth. However, Knoll et al. (1996bX in making mass balance calculations to dismiss CH4 release as a viable explanation for pre-Triassic negative 8 C excursions, used an estimate for the mass of the global gas hydrate reservoir that is too small by several orders of magnitude. In sunmiary: at least two mechanisms must be invoked to cause observed pronounced and rapid negative excursions in the geological record. Although multiple arguments can be forwarded to exclude ocean overturn as a cause of post-Triassic excursions, there is no good reason to dismiss massive CH4 releasefromoceanic hydrates as a cause of pre-Triassic excursions. REFERENCES Dickens G.R., O'Neil J.R., Rea D.K. & Owen R.M. 1995. Paleoceanography 10: 965-971. Knoll A.H., Bambach R.K., Canfield D.E. & Grotzinger J.P., 1996a. Science 273: 452-457. Knoll A.H., Bambach R.K., Canfield D.E. & Grotzinger J.P., 1996b. Science 274: 1549.. Wilde P. & Qumby-Huint M.S., 1997. Paleogeog. Palaeoclim. Palaeoecol 132: 47-63. Zachos J.C., Arthur M.A. & Dean W.E., 1989. Nature 337: 61-64.
113
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
GNALTA GROUP, FAR WESTERN NSW: A LATE CAMBRIAN TECTONIC MELANGE? Nicholas G. Direen^ & Anthony J. Crawford^ 1 CODES-SRC, University of Tasmania, GPO Box 252-79 Hobart TAS 7001 Australia
The Gnalta Group outcrops over an area of 600 km^ far western NSW. All exposures of the Group lie between two multiply reactivated, high-angle regional faults, the Lawrence and Mt Wright Faults. The mapped Gnalta Group comprises a mixture of Late Neoproterozoic alkaline basalts and serpentinite melange (Mount Arrowsmith volcanics), Early Cambrian calc-alkaline volcanics and fossiliferous limestones (Mount Wright Volcanics, Cymbric Vale Formation), Middle Cambrian fossiliferous shales (Coonigan Formation) and a ?Late Cambrian ultramafic complex (Macs Tank Ultramafic Complex). Nowhere does the Gnalta Group interfinger with rocks of equivalent age (Teltawongee beds); it is unconformably overlain by Early Ordovician deposits of the Mootwingee Group. Previous studies of the Gnalta Group have all emphasised the disrupted nature of the rocks at surface, ascribing this to "cryptic faulting and folding". Our analysis of new aeromagnetic and radiometric data, together with ground magnetics, gravity traverses and mapping indicates that the Gnalta Group is not a coherent stratigraphic package, and may never have been. Aeromagnetic data showing excellent continuity elsewhere in the Koonenberry region are highly contorted and discontinuous over the Gnalta Group. This accords with the observation that the Gnalta Group rarely presents continuous outcrops greater than a few tens of metres in dimension, and has juxtaposed non-related lithologies with few observed stratigraphic relationships. At most locations structural data show the Gnalta Group strikes NNW, concordant with the regional mid-Palaeozoic tectonic strike. Despite this, the regional NNW trending cleavage is conspicuously absent from Gnalta Group rocks and only two mesoscale fold closures have been recognised. We have interpreted this data to suggest that the Gnalta Group was dismembered prior to reorientation during regional mid-Palaeozoic folding. Most of the mapped volcanics in the Gnalta Group have been excluded on the grounds of Neoproterozoic age and alkaline geochemistry. This leaves unexplained the intimate, apparently interfmgered, relationship of these older volcanics (the Mount Arrowsmith Volcanics) with younger, calk-alkaline varieties (Mt Wright Volcanics). Also unexplored is the role of the Macs Tank Ultramafic Complex. This steeply east-dipping fault slice is correlated with tholeiitic volcanics in the ?Late Cambrian Ponto beds. These younger rocks which abut the Gnalta Group along the Mt Wright Fault to the west and south, have reached amphibolite grade and show an east-dipping schistosity indicating east-over-west tectonic transport. Despite the close affinity between the Macs Tank Complex and the Ponto beds, the Macs Tank Complex includes pods of the calc-alkaline Mt Wright Volcanics of the Gnalta Group. To the south, in the same structural position as the Macs Tank complex, lies a previously undescribed volcaniclastic chert interfingered with both Cymbric Vale Formation and Ponto beds. This unit has been highly deformed, and may represent a tectonic matrix. We interpret all of this evidence to indicate a latest Cambrian to early Ordovician compressional event in which Late Cambrian oceanic rocks (Macs Tank, Ponto beds) were thrust westwards over the rifted continental basement of Gondwana (Mt Arrowsmith Volcanics, Teltawongee beds). Caught up in this event was a fragment of a Lower to Middle Cambrian intraoceanic island arc (Mount Wright Volcanics, Cymbric Vale and Coonigan Formations). This fragment probably once represented a separate although it is no longer a coherent body. The outboard margin of this fragment is marked by serpentinite melanges on the Mt Wright thrust, while later normal faulting obscures the inboard margin. A diachronous collision event may explain a spread of SHRIMP ages in the Ponto beds ranging from Middle Cambrian to ?Early Ordovician. Acknowledgements: The authors publish with the permission of the Geological Survey of NSW.
114
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
CHEMICAL EVOLUTION OF GROUNDWATER IN THE MURRAY BASIN: GEOCHEMISTRY AND MASS TRANSFER MODELLING *S.S. Dogramaci'^ A.L. He^czeg^ & Y. Bone' 'Department of Geology & Geophysics, The University of Adelaide, Adelaide, SA, 5000 ^CSIRO, Land and Water and. Centre for Groundwater Studies, Private bag No.2, Glen Osmond Rd. SA, 5064
The concentrations and types of total dissolved solids (TDS) in groundwater play a major role in the suitability of water for its intended use. A knowledge of whether the chemical composition of the groundwater is caused by natural processes or induced by human activities is a prerequisite in many groundwater management issues such as potential exploitation, sustainable yield and groundwater contamination. The chemical evolution of groundwaters in the Murray and Renmark Group aquifers of the south-western Murray Basin were studied using solute tracers, stable isotopes of water molecules 5'®0), strontium isotopes ^^Sf) and mass transfer modelling. These methods can provide an insight into possible mechanisms that control the chemical composition and salinity of groundwater in the Murray and Renmark Group aquifers. The regional Murray and Renmark Group aquifers of the southeastern Murray Basin extend over approximately 60,000 km^ of eastern South Australia and western Victoria. The unconfined Murray Group Aquifer is separated from the underlying Renmark Group Aquifer by a 20m to 30m thick aquitard. As the region has no surface drainage, groundwater is an important water resource for agriculture and stock use. The groundwater salinity of the Murray and Renmark Group aquifers increases along the hydraulic gradient from (fresh) -500 mg/1 at the southeastern basin margin to (saline) --23,000 mg/1 at the northern part of the study area near the River Murray. Two hydrochemical zones are delineated in the study area: Zone A is characterised by fresh to brackish groundwater with a TDS concentration range of -500 to 5000 mg/1. This zone covers approximately 80% of the study area extending in a northerly direction for 300km from the basin margin in the south. Zone B is characterised by saline groundwater that has a TDS concentration ranging from 5,000 mg/1 to -23,000 mg/1 in both aquifers. Typically groundwater of Zone B occurs in the north and northwestern part of the study area. The values and Br/Cl and Na/Cl ratios indicate that groundwater in the Murray Group Aquifer is of meteoric origin. The geochemical mass balance calculations suggest that the current chemical composition in the Murray Group Aquifer in Zone A is produced by combination of: evapotranspiration of rain water prior to recharge, equilibrium with carbonate minerals and Na-Ca exchange reaction under soil gas pCOj o f a t m . Similarly the ^Sr/^'^Sr ratio can be used to divided the groundwater of the Murray Group Aquifer into two distinct groups. The groundwater that has relatively less radiogenic ^Sr/®^Sr ratio is located in hydrochemical Zone A and the groundwater that is characterized by more radiogenic ^Sr/^'^Sr ratio is located in hydrochemical Zone B. The higher salinity and radiogenic ^Sr/^^Sr ratio of groundwater in Zone B can be explained by the mixing of relatively more saline and radiogenic groundwater of the underlying Renmark Group Aquifer via upward leakage. This is supported by the hydraulic head distribution of the Renmark Group Aquifer which shows potential for upward leakage in Zone B. A mixing model using Sr concentration and ratio suggests that the groundwater of the Murray Group Aquifer in Zone B is derived from three sources: (i) upward leakage from the Renmark Group Aquifer, (ii) local recharge, and (iii) laterally flowing groundwater of the Murray Group aquifer. The Na and CI ion concentrations, along with values of groundwater of the Renmark Group Aquifer, also reveal the dominance of evapotranspiration of soil water during recharge. This is a major process which causes the groundwater salinity to rise above that of rainfall. The variable amount of water loss during evapotranspiration in the recharge area causes variation of CI and other major ion concentrations along the inferred flow line in the confined Renmark Group Aquifer. In addition to evapotranspiration, the mass balance and mass transfer calculations suggest that carbonate mineral dissolution, Na-Ca exchange and back reaction of Na to form secondary clay minerals control the chemical evolution of groundwater. Current address: Water and Rivers Commission, Hyatt Centre, 3 Plain Street, East Perth, WA 6004. 115
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
TECTONIC SETTING IMPLICATIONS FROM GREYWACKE PROVENANCE DATA FROM THE HODGKINSON PROVINCE Jan Domagala'. Robert J. Bultitude', Paul JT. Donchak', Barry G. Fordham\ John S. Jell^ and Mark Fanning^ ' Geological Survey of Queensland, GPO Box 194, Brisbane, Queensland 4001 ^Department of Earth Sciences, University of Queensland, St Lucia, Queensland 4067 ^ Research School of Earth Sciences, Australian National University, Canberra, ACT 2600
The mid-Palaeozoic Hodgkinson Province (-^SOkm long by ~150km wide) represents the northernmost exposures of the Tasman Fold Belt System of eastern Australia. The rocks in the province are variably deformed, steeply dipping and are dominated by turbiditic greywacke and mudstone, with variable amounts of limestone, chert, and basalt. Provenance data indicate the mainly quartz-rich greywackes of the Mulgrave Formation (?Lower Ordovician), were derived from a continental block and deposited in an extensional, passive margin setting. The Upper Ordovician Mountain Creek Conglomerate which unconformably overlies the Mulgrave Formation marks a significant influx of volcanic derived sediments The quartz-poor greywackes indicate a transitional-arc provenance of felsic to intermediate composition. A dated dacite clast (~455±5Ma) from the conglomerate records an apparent subduction related volcanic event approximately 10-15My prior to deposition of basal limestone (Richmondian) in the formation. The Silurian to Lower Devonian Chillagoe Formation is characterised by extensive carbonate ramp accumulations deposited under relatively stable conditions over a period of about 30My. Voluminous basalt in the northern part of the formation together with apparent growth faults in the south are indicative of extensional tectonics. The rate of extension appears to have increased rapidly towards the end of the Chillagoe Formation time when increased siliciclastic sedimentation locally eroded and eventually smothered the carbonate accumulations. This roughly coincided with the intrusion of the Cape York Peninsula Batholith (pooled age '-407Ma) to the west, which presumably resulted in hinterland uplift and increased sediment supply, and the opening of the Hodgkinson Basin. The siliciclastic sediments of the Lower to Upper Devonian Hodgkinson Formation were deposited mainly in an extensive submarine fan system. Two distinct petrofacies which represent different provenances have been identified. The majority of greywackes, dispersed throughout the basin, were sourced from a continental block, whereas greywackes in the Cooktown and, to a lesser extent, Bellevue areas were sourced, in part, from a magmatic arc. The source of the latter can be inferred from zircon dating of a conglomerate containmg volcanic clasts south-west of Cooktown, where a rhyodacite? clast yielded a SHRIMP age of '-465±21Ma. This age is consistent with derivation from the volcanic-arc? source of the Mountain Creek Conglomerate (-^SStSMa) and/or volcanolithic sediments in the Mountain Creek Conglomerate. The continental block sediments were most likely sourced from the Cape York Peninsula Batholith and Proterozoic metamorphic rocks of the Ethridge Province. Such a source is consistent with SHRIMP dates no younger than 420±14Ma for zircons from the matrix of the conglomerate south-west of Cooktown. Except for the submarine basalts scattered through the formation, evidence for penecontemporaneous volcanism is restricted to the presence of fine volcanic shards in a mudstone north-west of Cooktown. These shards may have been derived from arc-related volcanism east of the present coastline, or from a more distal source unrelated to the tectonic setting of the Hodgkinson Basin. Assuming the sediments of the Hodgkmson Formation reflect the tectonic setting instrumental in the formation of the Hodgkinson Basin, the composition of the greywackes indicate the previously proposed rifted continental margin model is the most appropriate. The proposed tectonic settings for the units of the Hodgkinson Province are as follows: • extensional passive margin for the ?Lower Ordovician Mulgrave Formation; • compressional convergent margin (subduction complex) for the source of sediments of the Upper Ordovician Mountain Creek Conglomerate; • extensional (fairly slow) passive margin for the Silurian to Lower Devonian Chillagoe Formation; and • extensional passive margin for the Lower to Upper Devonian Hodgkinson Formation.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
SEQUENCE STRATIGRAPHY OF THE MOUNT ISA GROUP, NORTH-WESTERN QUEENSLAND ^ Jan Domagala:^. Bruce McConachie^ Martin Nuidert^ and Rod Page"^ ' Department of Mines and Energy Queensland, GPO Box 194 Brisbane, 4001 Santos Ltd, South East Asia, PO Box 138 Lutwych, Brisbane, 4030 Queensland University of Technology, School of Natural Resource Sciences, GPO Box 2434, Brisbane 4001 "^Austalian Geological Survey Organisation, GPO Box 378, Canberra ACT 2600
The Palaeoproterozoic Mount Isa Group crops out discontinuously over an area of approximately 300km by 60km mainly within the north-trending Leichhardt River Fault Trough of the Western Fold Belt. The sequence is approximately 4 000m thick and includes (from the base) Surprise Creek Formation, Warrina Park Quartzite, Moondarra Siltstone, Breakaway Shale, Native Bee Siltstone, Urquhart Shale, Spear Siltstone, Kennedy Siltstone and Magazine Shale. Major event-related surfaces (A, B, C, D, and E) and their respective accommodation packages are recognised in detailed measured sections at Oxide Creek, Crystal Creek, Paroo Range, Leichhardt West, Mount Isa and Hilton and in core from the Mount Isa area. These event surfaces and accommodation packages, which are readily recognised in the carbonate dominated, basin margin, ramp sequences of the McNamara Group, are less apparent in the dominantly fme-grained, siliciclastic, basin-like sediments of the Mount Isa Group. Gamma logs of measured sections and core, combined with limited SHRIMP zircon dating, proved invaluable for identifying and correlating many of the surfaces in the monotonous sequences of the Mount Isa Group. Detailed work on surface A and the basal Surprise Creek Formation in the Mount Isa area is presented by Pidgeon & others (this volume). Event surface B generally lies towards the upper part of the Warrina Park Quartzite, although in some locations the Warrina Park Quartzite has been completely eroded. The surface is marked by a change in the stacking pattern and locally by the occurrence of desiccation cracks and a significant change in provenance inferred from the appearance of coarse micaceous-feldspatholithic sandstone which contrasts with the underlying quartz-rich sandstone of the Warrina Park Quartzite. The dirty basal sandstones pass rapidly into the overlying monotonous mudstone sequence of the Moondarra Siltstone. Detrital zircons from a bed at the maximum flooding surface of event B in the Paroo Range section yielded an age of '-1668Ma. This age represents a maximum age for the flooding surface in the lower Moondarra Siltstone and partly constrains the timing of the B event surface. Event C m the Oxide Creek and Leichhardt West sections is defined by a sandstone sequence which appears to represent a significant basinward shift in facies. A significant increase in accommodation space is recorded between C and D surfaces in the westernmost Leichhardt West measured section. This increase in accommodation space is attributed to increased fault movement against the basement rocks of the KalkadoonLeichhardt Block, which lay immediately to the east. Event D occurs in the lower part of the carbonate dominated Native Bee Siltstone. In the Leichhardt West section it is marked by an influx of mixed siliciclastic/carbonate density current deposits. The sourcing of siliciclastic sediments from the Kalkadoon-Leichhardt Block is inferred from rare westward directed palaeocurrent indicators. Minor sequence boundaries and maximum flooding surfaces have been identified in the Urquhart Shale, however, the next major surface (E) occurs towards the top of the Mount Isa Group where a decrease in the carbonate content and a corresponding increase in siliciclastic sediments mark the transition from Spear Siltstone to the Kennedy Siltstone and Magazine Shales.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE AGE OF DEPOSITION AND FIRST DEFORMATION OF THE PALAEOPROTEROZOIC WDLLYAMA SUPERGROUP, BROKEN HILL, AUSTRALIA. Tony Donaghv'. Mike Hall', George Gibson' & Allen Nutman^ 'Australian Geodynamics Co-operative Research Centre, VIEPS Department of Earth Sciences, Monash University, Clayton, VIC 3168. ^Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601. ^Research School of Earth Sciences, Australian National University, Canberra, ACT 0200.
New results from integrated SHRIMP zircon U/Pb geochronology and structural mapping raise questions about the previously accepted stratigraphic and structural model for the granulite-facies Palaeoproterozoic Willyama Supergroup of Broken Hill. The previous interpretation (Willis et al 1983) discounts the possibility of intrusive relationships between key meta-igneous stratigraphic marker horizons and the surrounding metasedimentary sequences. However, lithologies have been rotated into apparent conformity during three generations of high-strain deformation and original boundary relationships are often ambiguous. Debate surrounds: (i) the nature of the protoliths of many key meta-igneous marker horizons; (ii) the time of deposition of the sedimentary sequence that surrounds them; and (iii) the nature of the depositional environment of the Willyama Supergroup. This is driven by an effort to understand the ore-forming processes of the giant Palaeoproterozoic Broken Hill Pb-Zn-Ag ore body; whether it is syndepositional or epigenetic. Within the Thackaringa region, the Alma Gneiss - a key quartzofeldspathic stratigraphic marker of the Thackarinp Group, is clearly a granitoid intrusive into surrounding stratigraphically distinctive Na-metasomatised quartz-albite metasediments. Furthermore, the metasediments are well-foliated and partially melted pre-syn incorporation into the granitoid margin. Zircons from the Alma Gneiss yield a single magmatic age of 1693 ± 10 Ma. A nearby mafic intrusive into similar Na-metasomatised quartz-albite metasediments yields a similar magmatic ^^^Pb/^^^Pb age of 1690 ± 11 Ma. The mafic body clearly transgresses well foliated and partially molten metasediments and was initially thought to belong to a much younger generation of mafic intrusives (ca 500-800 Ma). The quartz-albite metasediments yield complex detrital zircon patterns with a maximum deposition age of 1700-1710 Ma, correlating with similar lithologies in the nearby Olary region of South Austraha. The previously accepted Willyama Supergroup deposition age is 1690 ± 5 Ma (Page & Laing 1992), derived from SHRIMP dating zircons of the Hores Gneiss, a key meta-igneous quartzofeldspathic marker horizon of the Broken Hill Group interpreted as a metavolcanic (Willis et al. 1983, Page and Laing 1992). However, ca. 1690 Ma ages reported here of both felsic and mafic intrusives cross-cutting partial melt and tectonic fabrics places the metasediments of the Thackaringa Group at sufficient depth within the crust for melting and ductile deformation synchronous with this previously accepted deposition age. Hence, the possibility arises that either; (a) there is a substantial time gap between deposition of the Thackaringa and Broken Hill Groups, allowing time for deformation of the Thackaringa Group lithologies prior to deposition of the Broken Hill Group; or (b) the Thackaringa and Broken Hill Groups are one continuous sequence and the Hores Gneiss of the Broken Hill Group is part of the same intrusive event as the Alma Gneiss, rather than an extrusive metavolcanic. This has important ramifications for mineral exploration within the Broken Hill area, as the current model for the formation of the Broken Hill ore body is thought to be syndepositional - synchronous and stratigraphically equivalent to the Hores Gneiss. The onset cf deformation within the Broken Hill-Olary region is generally inferred to immediately precede granulite fecies metamorphism at ca 1600-1590 Ma (e.g. Page and Laing 1992). Onset of deformation within the Thackaringa region predates this by ca 100 My. It is likely that wide spread granitic plutonism and mafic intrusion at ca 1690 Ma accompanied a major orogenic event, typical of many major intrusive events throughout the geologic record. REFERENCES Page, R. W. & Laing, W. P., 1992. Felsic metavolcanic rocks related to the Broken Hill Pb-Zn-Ag orebody, Australia: geology, depositional age, and timing of high-grade metamorphism. Economic Geology, 87, 2138-2168. Willis, I. L., Brown, R. E., Stroud, W. J. & Stevens, B. P. J., 1983. The early Proterozoic Willyama Supergroup: stratigraphic subdivision and interpretation of high to low-grade metamorphic rocks in the Broken Hill block. New South Wales. Journal of the Geological Society of Australia, 30, 195-224. Acknowledgments We thank Rio Tinto Exploration Pty Ltd for financial support for this project and permission to publish.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
CLASSIFICATION OF DELTAIC PLAINS BASED ON FRACTAL DIMENSIONS AND SEDIMENTARY ARCHITECTURE OF DISTRIBUTARY CHANNEL SYSTEMS Y u Dong and Xiuqin Lu Dept. Geology of Huainan Institute of Technology, Huainan , Anhui, 232001 , P.R. China
Dividing types of distributary channel systems Based on genesis , distributary channel systems (DCS) are subdivided into river and tide- dominated DCS . Taking into consideration the internal sedimentary architecture of channel fill and sand distribution patterns or geometric structure of drainage systems , the former is further divided into sinuous , branched and anastomostic systems , and the latter into tide-branched and tide-anastomostic DCS . Characteristics of fractal geometry of DCS Controlled by physical factors of environment, DCS reveal a certain characteristics of geometric structure and factal geometry . Fractal dimensions of five types of DCS are calculated by means of fractal geometry . Results show that there is self-similarity in a distributary channel system , and fractal dimensions calculated vary from l.o46 to 1.763 . In general the sinuous has simple geometric structure and low fractal dimension that is below 1.30 ; the anastomostic , the branched and the tide-anastomostic have complex geometric structures and high fractal dimensions that are above 1.40 . Fractal dimension of the tide-branched is between above tow groups . In low deltaic plain , fractal dimensions of DCS are closely related to physical parameters of environment , especially to the gradient of offshore and average power of wave . Low fractal dimension corresponds to the great gradient of offshore and high average power of wave. Table 1 indicates the relationship between the fractal dimensions of DCS in low deltaic plains and environmental parameters . table 1 . Different types of DCS and their fractal dimensions vs. environmental parameters Distributary channel Fractal dimensions Currents Average power Gradient of systems (DCS ) of wave offshore (mVs) (%) (j/s) Sinuous DCS Low <1.30 3316.95 89.8 18.1 Branched DCS High >1.40 6419.5 0.468 5.3 Tide-branched DCS Middle 1.30-1.40 4936.6 3.737 5.8 Tide-anastomostic DCS High >1.40 23529 0.390 1.45 Types of deltaic plains and combinative modes Since characteristics of DCS reflect environmental parameter , types of deltaic plains can be defined by types of DCS . The paper has defined five types of deltaic plains , including delta plains with sinuous , anastomostic , branched , tide-branched and tide-anastomostic distributaries . Delta plains of Rhone , Zhujiang and Mekong belong respectively to deltaic plain with sinuous , anastomostic and tide-branched distributaries . At least five types of combination modes of delta plains are found : type 1 deltaic plains with sinuous anastomostic— branched distributaries ; type 2 deltaic plains with sinuous — branched distributraries ; type 3 deltaic plains with sinuous tide-anastomostic distributaries ; type 4 deltaic plains with sinuous — tide-branched distributaries ; type 5 deltaic plains with single sinuous distributaries . Type 1 is found in Permian Coal Measures of North Anhui. Mississippi deltaic plain is a typical example of type 2 . 119
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
TENNANT CREEK PROVINCE: A STRIKE-SLIP OROGENIC TERRAIN. Nigel Donnellan^ Robert S. Morrison^ and Kelvin J. Hussey^ ^Northern Territory Geological Survey, PO Box 2655, Alice Springs, NT 0871 ^WMC Resources Ltd., St. Ives Gold Mine, Kambalda PO, Kambalda, WA 6442
At the present level of exposure the Tennant Creek province comprises a turbiditic flysch succession, the Warramunga Formation, which together with granite and granodiorite (Barramundi Igneous Association; BIA) were respectively deposited and intruded penecontemporaneously with the Barramundi Orogeny (locally c. 1.861.84 Ga^). Extrusive, predominantly subaerial volcanic rocks; rhyolitic and rhyodacitic ignimbrite, lava and tuff; and associated volcaniclastic and clastic sedimentary rocks (Flynn Subgroup; 1.84-1.82 Ga) are also included in the province although they were not deformed during the Barramundi Orogeny. The Warramunga Formation (-1.86 Ga) is a polydeformed sub- to lowermost-greenschist facies succession of (volcani)lithic and sublithic arenite, wacke and siltstone; terrigenous mudstone and argillaceous banded ironstone ('haematite shale'). Sm/Nd CR model ages^ are < 2.45 Ga and eNd at 1.86 Ga >-4.9. This Formation hosts the so-called Tennant Creek type', massive ironstone associated, gold-copper-bismuth mineralisation with ~140t of gold produced to the end of 1996. The Warramunga Formation was folded during the Barramundi Orogeny (Dl). These folds are moderate to tight, and upright with E-W to ESE-WNW trending fold axes and an axial planar slaty cleavage. SI was reactivated c. 1830-1790. S2 (NW-SE) and S2' (NE-SW) are (orthogonal) crenulation, locally fracture or slaty, cleavages in the Warramunga Formation. This deformation (D2/D2') (Strangways Orogeny?) affects both the Warramunga Formation and the Rynn Subgroup, and produced upright, concentric dome and basin folds arranged en echelon throughout the Tennant Creek Inlier; and particularly well-exposed in the Davenport province. F2/F2' folds in the Warramunga Formation are well-exposed on the mesoscale. Intrusive representatives of the BIA ('early' granites; 1.86-1.83 Ga) are syntectonic biotite-bearing granite and granodiorite, and minor tonalite (e.g. Tennant Creek Granite and Mumbilla Granodiorite); with seriate to porphyritic and 'incipient' rapakivi textures, interstitial mafic minerals, and blue, opalescent ovoid quartz. However, the reddish-grey colour, rapakivi texture, abundant and varied enclave population as well as their association with shallow quartz-porphyry intrusions, rhyolitic ignimbrite and dolerite are more typical of anorogenic granites. These granites have Sm/Nd DM model ages <2.6 Ga and eNd at 1.85 Ga > -4.5. These granites could be derived from cnistal melting of Archaean or earliest Palaeoproterozoic biotite and hornblendebearing tonalite (TTG) together with admixed sedimentary, and juvenile mantle-derived material. There are at least two episodes of mafic magmatism in the Tennant Creek province. One episode is essentially contemporaneous with the syntectonic granites of the BIA and comprises intrusions of dolerite and gabbro. There is a second episode of dolerite, quartz-diorite and diorite intrusion. The latter episode overlaps in time with predominantly felsic volcanic activity in the Rynn Subgroup, and mafic/felsic volcanic rocks in the Ooradidgee Subgroup in the Davenport province. Post-tectonic ('late') granites are represented in the Tennant Creek province by the two-mica, peraluminous S-type Warrego Granite (-1700-1650), and the 'Gosse River East Granite' (1712 Ma). There is a late episode of lamprophyre intrusion (-1685 Ma). Sedimentation in the Tennant Creek province was initiated in response to movement about a dextral shear with a releasing bend. The ongoing history of sedimentation and deformation (Barramundi and Strangways Orogenies) in the Tennant Creek and Davenport provinces relate to ongoing dextral shear, and inversion within this system. At the time of the Barramundi Orogeny the locus of subduction was to the south of the Arunta Inlier where it has been well-substantiated and documented by previous authors. It is suggested that possible subduction in the vicinity of Tennant Creek may have ceased or been stalled before onset of the Barramundi Orogeny. However, the precise stratigraphic, structural and magmatic relationships between the Tennant Creek and Arunta Inliers have yet to be resolved. Only then could we to determine if the Barramundi Orogeny at Tennant Creek represents either the accretion of a discrete arc or a distal manifestation of that further to the south? ^The age constraints quoted in this abstract are largely based on single crystal U-Pb SHRIMP dates and are the work of D. Compston (1995). These ages independently corroborate the stratigraphy of the authors which was based on field relationships. ^ e Sm-Nd isotopic analyses were performed at the Isotope Analysis Facility, University of Adelaide, and we are grateful to Dr. J. Foden.
REFERENCES Compston D. 1995. Time constraints on the evolution of the Tennant Creek Block, northern Australia. Precambrian Research 71, 107-129.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE SUB-SEAFLOOR REPLACEMENT ORIGIN OF THE ORDOVICIAN HIGHWAY-REWARD VHMS DEPOSIT, MOUNT WINDSOR SUBPROVINCE, NORTHERN QUEENSLAND MarkO.IP9Ylg' and David L. Huston' ^ Centre for Ore Deposit Research, University of Tasmania, Hobart, Tasmania 7001 ' AGSO, Canberra, Australian Capital Territory 2601
The literature on volcanic-hosted massive sulfide (VHMS) deposits emphasises formation on the seafloor by accumulation of sulfides precipitated from exhaling hydrothermal fluids. There is increasing evidence to suggest that sub-seafloor sulfide accumulation by open space filling and replacement of volcanic and sedimentary facies may be more important in the genesis of some VHMS deposits. There are few detailed descriptions of subseafloor replacement deposits in modem and ancient volcanic successions. An integrated volcanological, sedimentological and ore deposit study of the Cambro-Ordovician Seventy Mile Range Group has provided insights into the characteristics of Highway-Reward type sub-seafloor replacement deposits. The Cu-Au Highway-Reward massive sulfide deposit is hosted by a silicic submarine (below storm wave base) syn-sedimentary intrusion-dominated volcanic centre. Detailed mapping of phenocryst populations and contact relationships indicates the presence of more than thirteen distinct porphyritic units in a volume of 1x1x0.5 km. Peperitic upper margins suggest that at least 75% of the rhyolites to dacites were emplaced as small (<350 m diameter) syn-sedimentary sills and cryptodomes. The intrusions have steep margins and are separated from neighbouring intrusions by thin (0.2 to 30 m) disrupted intervals of siltstone, sandstone turbidites, polymictic lithic breccia, and non-welded pumice- and crystal-rich breccia. Evidence for extrusion of magma onto the seafloor is limited to a single, partly extrusive cryptodome. The Highway and Reward pyrite-chalcopyrite pipes occur within, but close to, the steep margins of cryptodomes within the host succession. The pipes are discordant to local bedding and contain relict domains of intensely altered rhyolite to dacite. Pyrite±quartz stringer veins occur below the pipes, and on some sections also occur in sericite-quartz-altered lithofacies above the pipes. Near-surface pyritic ores have oxidised to form a gossanous zone. The pyrite-chalcopyrite pipes are surrounded by a halo of sphalerite-galena-barite-rich mineralisation. Within this envelope there are four styles of mineralisation: (1) strata-bound pyrite-sphalerite-chalcopyrite-barite lenses; (2) disseminated sphalerite within sericite-quartz-chlorite-altered rhyolite at the top of the pipes; (3) massive and semi-massive pyrite-sphalerite-chalcopyrite±barite at the margins of the pyrite pipes; (4) veins of sphalerite±galena-barite in altered volcanic facies along the margins and tops of the pipes. Massive sulfide mineralisation is enclosed within a discordant alteration envelope which extends from at least 150 m below the orebodies to over 60 m above the Highway pipe. A quartz-sericite±pyrite zone is centred beneath the pipes and on some sections extends into the hanging wall lithofacies. Small domains of intense chlorite±anhydrite alteration occur within the footwall quartz-sericite±pyrite zone and along some margins of the pipes. Quartz-sericite±pyrite aheration gives way laterally and vertically to domains of sericite-chlorite±quartz and chlorite-sericite alteration. Beyond the hydrothermal alteration halo, rocks of rhyolitic to dacitic composition contain various assemblages of feldspar, calcite, sericite, chlorite, epidote, quartz and hematite. The available evidence is consistent with syngenetic accumulation of the pyrite-chalcopyrite pipes and sphalerite-rich mineralisation. Most of the massive sulfide ores formed by sub-seafloor replacement of rhyolite to dacite and volcaniclastic units because: (1) the mineralisation is hosted by intrusive or mass-flow emplaced units; (2) discordant and strata-bound ores contain relic patches of coherent facies or precursor volcanic particles; (3) peperite and massive sulfides are not mixed, unplying sulfide deposition postdated emplacement of the enclosing succession; (4) pyrite pipes are discordant to bedding; (5) there are replacement fronts passing from strata-bound sphalerite-rich ores into discordant pyrite-pipes; (6) zones of strong quartz-sericite-alteration and pyrite veining extend into the hanging wall without any abrupt break in intensity. At Highway-Reward, deformation, disruption of bedding, resedimentation, dewatering and low-grade metamorphism of the host succession accompanied emplacement of the intrusions. The resulting patterns of permeability and porosity are interpreted to have focussed hydrothermal fluid flow within the fractured glassy margms of the cryptodomes. An advancing pyrite-chalcopyrite front progressively replaced the rhyolite to dacite intrusions, peperite and sediment. A partly extrusive cryptodome formed a barrier to ascending hydrothermal fluids, promoting sub-seafloor ponding of the fluids and replacement within and below its peperitic base. Lower temperature fluids which diffused from the margins of the pyrite-chalcopyrite pipes deposited a broad halo of sphalerite-galena-barite mineralisation. Strata-bound ores formed by replacement of a porous and permeable pumice breccia bed above the main Reward pipe.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
DEPOSITIONAL CONTROL OF BLACK SHALES IN THE PROTEROZOIC CANDLOW FORMATION, NORTH QUEENSLAND John J. Draper Geological Survey of Queensland, GPO Box 194, Brisbane, Queensland 4001
Twelve factors are identified as controlling the formation and characteristics of black shales: geological setting, productivity and anoxia, microbial mat formation, organic matter type, sedimentation rate, water depth, salinity, water turnover, latitude, climate, geological age, diagenesis/metamorphism. The geological setting does not dictate whether a sequence will be organic rich or not, but the geological setting places constraints on the thickness and richness of organic matter that can accumulate (Table 1). The trace element composition of black shales is controlled predominantly by oxia/anoxia, water depth, deposition rate, and water turnover. Organic matter type is crucially important in petroleum source rocks and has an hnpact on trace element distribution. Geological age imposes limitations on some factors e.g. lack of burrowers in the Proterozoic does not favour aeration of the sediments. Burial and metamorphism impacts largely on the carbon content and its nature, due to maturation and loss as hydrocarbons. C/S ratios need to be used with caution in older rocks because of maturation effects plus other factors. Table 1. Summary of organic carbon values versus thickness of organic rich sediments in various settings Setting Thickness(m) Max Corg % Silled basin moderate 7 Upwelling moderate - thick 26 Transgressive shelf thin 30 Epieric sea/sag thin - moderate 30 Progradational submarine fan moderate - thick 1 Evaporative (coastal) thin - moderate 5 Lake moderate - thick 34 Coal basins thin - moderate >75 Thin = metres; moderate = tens of metres; thick = 100s to 1000s metres Higher levels of trace elements such as Mo, V, U are favoured by anoxic bottom waters as are the sulphide elements such as Cu, Pb and Zn. Slow deposition and persistence of anoxia is required for significant enrichment. Under freshwater conditions the lack of water turnover appears to limit any enrichment irrespective of bottom water oxia/anoxia. The Candlow Formation is part of the Palaeo- to Mesoproterozoic Etheridge Group in the Georgetown Region, north Queensland. The Candlow Formation is a fme grained unit with black shales (carbonaceous mudstones), mudstone, siltstone and mudclast sandstone; the Stockyard Creek Mudstone Member comprises 300 m of black shale. The unit is metamorphosed to greenschist facies. A notable feature of the black shales is the high organic carbon values - values of up to 9 % for greenschist facies rocks translate to 20 % or more original organic matter. These rocks would have had many of the characteristics of oil shales. Geochemical data indicate that the depositional environment did not favour enrichment of trace elements in the black shales. Marine bottom water anoxia seems unlikely. The C/S ratios do not support marine sea floor anoxia and neither do the anoxia indicators like Mo, Ni, V, Ni/Co, V/Cr and U. On the other hand Mn values do not indicate widespread bottom water oxia. Geochemistry and outcrop and core logs suggest that deposition occurred in a large lake or inland sea with the sediment derived from a weathered, fme grained provenance. A variety of mud/silt-dominated fluvial, delta plain, delta front and lacustrine environments are present. Mudclasts in the mudclast sandstones were derived by erosion of microbial mat deposits. Black shales formed from the microbial mats would have formed under oxic waters. Increased subsidence resulted in the formation of lacustrine deposits of the Stockyard Creek Mudstone Member; black shales formed from plankton rain and under anoxic bottom waters. The Candlow Formation is prospective for sediment-hosted lead-zinc(-silver^ deposits based on comparison with Mount Isa-Lawn Hill-McArthur Basin deposits. The Candlow Formation would appear to have no potential for sediment-hosted stratiform copper deposits, but may have potential for syn-deformational Mount Isa style copper deposits. The formation has potential for carbonaceous-hosted or carbon-associated gold deposits.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
PROTEROZOIC ROCKS OF NORTH QUEENSLAND John J. Draper'. John H.C. Bain^ Ian W. Withnall', Richard S. Blewett^ Lance P. Black^ Robert J. Bultitude', Peter WelIman^ Douglas E. Mackenzie^ and the GSQ-AGSO North Queensland Project Team ' Geological Survey of Queensland, GPO Box 194, Brisbane, Queensland 4001 ^ Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601
The following is a summary from a report on the Geology of North Queensland (Bain & Draper, 1997a,b). PALAEOPROTEROZOIC TO MESOPROTEROZOIC The oldest rocks in North Queensland are probably the unexposed Kowanyama and Ciaraville Provinces which were cratonised during the Barramundi Orogeny (1880 and 1850 Ma). Along the western margins of North Queensland, the resulting crust was rifted several times between 1880 and 1520 Ma forming the Mount Isa and Keer Weer Provinces. The oldest of the Proterozoic provinces east of the Kowanyama Provmce is the Etheridge Province which appears to onlap the Kowanyama and Ciaraville Provinces. It comprises two subprovinces: Forsayth (south) and Yambo (north). The oldest rocks are in the Forsayth Subprovince and are amphibolite (-1670 Ma) and leucogneiss and deformed granite (-1695 Ma) in the Einasleigh Metamorphics. Many of the sedimentary rocks in the Forsayth Subprovince were deposited before --1650 Ma. The sediments in the Yambo Subprovince were deposited after '-1640Ma and may be contemporaneous with post 1650 Ma sediments in the Forsayth Subprovince. The Forsayth and Yambo Subprovince sediments were deposited in a continental rift and epicontinental sea with accompanying mafic lavas and dykes. The sediments in the Yambo Subprovince were metamorphosed and intruded by mafic and felsic rocks at -1585 Ma and 1575 Ma. Deformation occurred in the Forsayth Subprovince prior to deposition of fluvio-deltaic sediments in the western Forsayth Subprovince; this deformation could correspond to the 1585 and 1575 Ma events in the Yambo Subprovince or a possible younger event (-1560-65 Ma). Metamorphism and intrusion of felsic rocks dated at -1550 Ma affected the whole of the Forsayth Subprovince, and possibly the Yambo Subprovince. At die same time, the Croydon Province formed to the west of the Forsayth Subprovince. Felsic volcanic rocks were deposited in a large, subaerial, intracontinental cauldron and intruded by felsic granites. Fluviatile sediments overlying the volcanics may be of Neoproterozoic to Cambrian age. Detrital zircons of -1585 Ma from the Yambo Subprovince occur in the Savannah Province. Sediments were deposited in an intracontinental setting and mafic sills were intruded in the lower part of the sequence suggesting rifting. Major deformation and felsic intrusion may have occurred at -1550 Ma. Minor gold, tin and base metals are found in rocks of this age. The large Croydon Goldfield is hosted by Proterozoic rocks, but the age of mineralisation is open to interpretation. NEOPROTEROZOIC AND CAMBRIAN Events between 1550 Ma and the Late Cambrian are not well defmed. Ultramafic rocks in the Halls Reward Metamorphics have a minimum age of -1300 Ma, but the significance of these ultramafic rocks is uncertain. The Silurian to Devonian granites in the southeast of the Ravenswood Batholith have depleted model ages of 1120 - 1230 Ma and contain detrital zircon of-1100 Ma. Detrital zircons of -1130 Ma occur in the Iron Range Province and detrital and magmatic zircons of -1145 Ma occur in the Cape River Province. The Cape River Province contains metamorphosed sedimentary rocks. Associated mafic rocks have geochemical characters consistent with an extensional intraplate setting perhaps on a passive margin where less mature source rocks were rifted away during the Rodinian breakup. Some of the immature sedimentary rocks were derived exclusively from an adjacent, 'Grenville' age source. The rocks were deposited after 1145 Ma and prior to 460 508 Ma (?Delamerian Orogeny). Deformation and regional metamorphism also occurred between about 500 460 Ma. The Iron Range Province contains metamorphosed sediments deposited after 1130 Ma. Possible sponge spicules suggests a Palaeozoic age. The sediments are of marine origin and the presence of mafic intrusives suggests rifting. The age of deformation and metamorphism is unknown. The presence of iron formation and calc-silicates in both the Iron Range and Cape River Provinces and the presence of similar mafic rocks in both provinces suggest a common depositional setting. A minimum age for the Barnard Province is provided by intrusion of granite at - 486 Ma preceded by major deformation and metamorphism. No maximum age is available. Fine grained metasedimentary rocks, and mafic and ultra mafic rocks are present; this combination suggests a passive margin, rifting setting like that of the Iron Range and Cape River Provinces. Neoproterozoic to Early Palaeozoic rocks contain uneconomic iron deposits and minor gold. REFERENCES Bain, J.H.C. & Draper, J.J., 1997a: North Queensland Geology. AGSO Bulletin 240; Queensland Geology 9. Bain, J.H.C. & Draper, J.J., 1997b: Atlas of North Queensland Geology. Australian Geological Survey Organisation and Geological Survey of Queensland.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville. July 1998
THE AUSTRALIAN NATIONAL SEISMIC IMAGING RESOURCE (ANSIR) THE EARTH SCIENCES' MAJOR NATIONAL RESEARCH FACILITY Barry J. Drummond ANSIR, Director, Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601
The Australian Geological Survey Organisation (AGSO) and the Australian National University have signed an agreement with the Commonwealth Government to establish and manage a Major National Research Facility (MNRF) in seismic imaging of the Earth. The facility is called the Australian National Seismic Imaging Resource (ANSIR). In establishing MNRFs, the Commonwealth recognises that major scientific and technological advances are increasingly dependent on researchers having access to very expensive equipment and facilities beyond the resources of most institutions to finance. Therefore the Commonwealth has funded the establishment costs for seven new MNRFs, of which ANSIR is one. In developing the national science and technology infrastructure in this way, the Commonwealth is taking a strategic step towards keeping Australia at the leading edge of key technological developments and so enhancing its scientific and industrial competitiveness. The Commonwealt has provided $5 million for the construction and purchase of a suite of seismic recording equipment and vibratory energy sources. The recording equipment will include a state-of-the-art seismic reflection recording system, and stand-alone data loggers, some capable of recording short-period data and others with broad-band capability. The equipment will have the capability of recording energy from vibratory sources, explosions, and earthquakes, so once the facility is established it will be capable of imaging the Earth at a range of scales. The establishment of this MNRF marks a milestone in Australian science as the Earth sciences join other scientific disciplines in having access to major research infrastructure. Experience overseas (eg., Canada, USA, France, Germany) has shown that carefully targeted research in seismic imaging can provide the 'glue' to unite many of the Earth-science disciplines into true multidisciplinary 'big' science. ANSIR will help address a longrecognised need for Australian researchers to have access to state-of-the-art geophysical equipment. The need was highlighted in the Australian Geoscience Council report Towards 2005: A prospectus for research and research training in the Australian Earth sciences' (National Board of Employment, Education and Training, AGPS, 1992), which stressed the importance of modem instrumentation and facilities for research in the Earth sciences and the importance of cross-fertilisation of research programs involving shared access to major instrumental facilities. ANSIR's Objectives are similar to those set for other MNRFs: that is, to 1) create a centre of excellence in the field of seismic imaging of the Earth with the capability of pursuing world class research and training in that field; 2) promote the Facility and its role widely with the aim of • encouraging use of the Facility; and • promoting the MNRF Program; 3) conmiercialise Intellectual Property in such a manner as to ensure that the maximum benefit accrues to Australia, including • Australian industry, • the Australian envirormient and • the Australian economy generally; and 4) seek opportunities to enhance the Facility. AGSO and ANU joindy will operate the facility, but to fully meet the objective they have been set they need to encourage its use by researchers from other Australian and overseas research institutions who have project operational funds. The facility has an independent Management Board, and an Access Committee which will vet scientific proposals to ensure that they are of high merit. Merit is the only criterion for determining priority for access to the facility. Now that the management infrastructure is in place to oversee the purchase and construction of the equipment, attention has turned to developing a forward research and educational program that is both broadly based and focused on the needs of the education and industry stakeholders. The facility is expected to be ready for commissioning later this year.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
DEEP SEISMIC PROFILING: A TOOL FOR UNLOCKING CONTINENTAL ASSEMBLY AND LITHOSPHERIC DYNAMICS Barry J. Dnimmond and Bruce R Goleby AGCRC / ANSIR cA Australian Geological Survey Organisation, P.O. Box 378, Canberra, ACT, 2601.
Early deq) cmstal seismic data from Central Australia suggested that the region consists of two different pieces of crust that joined along the Redbank Zone early in its evolution. The predominant style of deformation in this area is 'thick-skinned'. 'Thin-skinned' faulting occurs only in the Amadeus Basin, and occurred during the latest deformation episodes related to the Alice Springs Orogeny. In contrast, in the Cobar Basin in NSW, deep seismic reflection profiling imaged a major mid crustal detachment that appears to have controlled the formation of the basin. This dichotomy of crustal scale tectonics observed in several thousand kilometres of deep seismic profiling across Australia, in which thick-skinned tectonics was mostly foimd in older terranes and thin-skinned tectonics was mostly found in younger terranes, suggested that perhaps tectonic styles were controlled by different rheologies, with those in older terranes generally more uniform and brittle than those in younger provinces. It may also have reflected different spatial dimensions to plate tectonic processes at different times in Earth history, perhaps with larger mantle convection cells now than in the past. However, recent re-processing of data from the Archaean Eastern Goldfields province of the Yilgam Block has revealed a range of structures with demonstrable different styles, ranging from thin skinned continental assembly to late thick skinned deformation. The middle crust appears to have been thickened and assembled by a large series of crustal duplexes, which are a 'thin-skinned' process. The duplexes are west directed, and have their tops at what are now middle to upper crustal levels. Their bottoms link into what appears to have been a ductile zone in the bottom 25% of the crust. Back thrusts can be mapped through the poorly reflective upper crust, forming "crocodiles" in the seismic reflection data; these are often seen in crustal sections of orogens, and interpreted to imply that the upper crustal deformation was detached from mid-lower crustal processes. These structures formed early in the life of the crust. Subsequently, major crustal-scale faults, eg., Ida Fault, Bardoc Shear, acted as 'thick-skinned' structures, cutting the entire crust. The Ida Fault dips east and spears to have adopted the boundary between two older duplex structures as its course through the crust. The Bardoc Shear probably reactivated an older back thrust. These faults probably had strike-slip as well as normal and oblique-slip movements. The Yilgam data therefore suggest crustal assembly by 'thin-skinned' processes followed by 'thick-skinned' processes. A similar history might be inferred in the Eastern Succession at Mount Isa. Early west-directed thrusting of the supracrustals occurred on shallow detachments or 'thin-skinned' episode; they were subsequently cut hy older, steeper faults, which link downwards into a structure interpreted to be the boundary between two former platelets. Near-vertical province-scale strike-slip faults were the last major style of deformation. Seismic data often highlight the youngest structures. However, as we improve the fidelity of our images, we are able to unravel older events. Then we see early 'thin-skinned' thickening, followed by 'thick-skinned' deformation. Is this is correct, then the data from Cobar suggest that the western Lachlan Fold Belt has undergone its thin-skinned thickening process but may not have reached the stage where the crust has become thick and strong enough to respond to stresses through thick skinned processes. However, we have not been able to find evidence of the primary tectonic events which caused the first crust to form in any of the data studies so far, nor to differentiate early oceanic and continental crust.
Acknowledgments: The above projects have all been undertaken by either by AGSO or the AGCRC. This paper is published with the permission of the Executive Director, Australian Geological Survey Organisation and the Director, Australian Geodynamics CRC.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
CRUSTAL STRUCTURE IN EASTERN TASMANIA - AN EXAMPLE OF AN INVERTED PASSIVE MARGIN? Barry J. Drummond\ RJ. KorsdiS A.V. BroW, TJ. Barton^ ^Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601 ^Tasmanian Geological Survey, PO Box 56, Rosny Park, TAS, 7018
The Tyennan Block is interpreted as the Proterozoic core of Tasmania. Seismic reflection data collected offshore south of Tasmania show the block to be mostly non-reflective, implying a monotonous lithology with few boundaries. Occasional apparently east dipping weak reflections are interpreted as faults which penetrate to deep crustal levels. The Moho is sub-horizontal at l i s two-way-time (TWT), or about 33-35 km deep. The eastern boundary of the Tyennan Block dips gently east, and forms the basement to younger sedimentary cover rocks of the Jubilee Element. The monotonous non-reflective nature of the Tyennan Block continues farther east, on a seismic profile subparallel to the eastern coastline. It can be traced in its pristine form as far north as the eastern Tasman Peninsula, but farther north it appears to be broken into several crustal-scale, tilted fault blocks. Each fault block is bounded above and below by south dipping reflections which link into the Moho, mosfly at significant inflections in Moho topography. These fault blocks now lie at mid-crustal levels, and are overlain by a non-reflective crust typical of many parts of Palaeozoic basement in Australia, and interpreted in this part of Tasmania as the probable southernmost limit of the Matiiinna beds. Still farther north, along the northern third of the eastern coastline of Tasmania, the lowermost crust has numerous strong reflections which can be interpreted as packets resembling apparently north directed thrust duplexes. The sole faults to the duplexes link into the Moho, mosfly at inflection points in Moho topography, as for the tilted fault blocks farther south. The tops of the thrust duplexes lie below a set of weak, sub-horizontal reflections which forms the floor to a non-reflective upper crust correlated with the Mathinna beds. Reflections weaker than those in the lower crust extend upwards to the south, and are interpreted as back thrusts. Seismic refraction models (Rawlinson, et al., 1998) show a two layer crust along both the eastern and northern coasts, with upper and lower crustal thicknesses consistent with those of the non-reflective upper aust and reflective lower crust interpreted in the seismic reflection data. The seismic velocities in the upper crust and lower crustal layers are consistent with an upper crust derived from lithified clastic and granitic rocks and a lower crust that might contain significanfly more mafic rock. The geometry of the reflections from the seismic lines to die south and east of Tasmania are reminiscent of an inverted passive margin. In this model, the Tyennan Block would have formed a continent, with ocean to the east or northeast. Crustal extension occurred through block faulting inboard and massive crustal attenuation farther seaward. The strongly reflective crust in the northern part of the profile along the eastern coast would be extended continental crust that accumulated passive margin sediments and perhaps volcanics and oceanic aust, which was all later inverted. Crustal shortening would appear to have been most extensive in the north, where the reflective lower crust is thickest and the Moho is bowed down. The non-reflective upper crust through this region consists of highly folded Mathinna beds and granitic intrusions. When combined with data from a transect along the northern coast (Barton et al., this volume), the apparent southern dips on thrust duplexes in the lower crust, and northern dips on inferred faults in the upper crust, translate to real dips more to the southwest and northeast, respectively. This implies a mass transfer direction from southwest to northeast in the lower crust, and an opposite sense on backthrusts in the upper crust. REFERENCES Barton, T.J. 1998. A geophysical transect across northern Tasmania. Geological Society of Australia, Abstracts, this volume. Rawlinson N., Collins C. D. N., Semenova T. 0. & Houseman G.A. 1998. Crustal architecture from seismic refraction data along the north and east coasts of Tasmania. Australian Geological Survey Organisation, Record, 1988/2. Acknowledgements: B JD, RJK & TJB publish with the permission of the Executive Director, AGSO.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
GEOCHRONOLOGY OF THE TELFER GRANTTOroS: ZIRCON AND TITANITE U-PB SHRIMP DATA Janet M. Dunphy and Neal J. McNaughton Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, University of Western Australia, Nedlands, WA, 6907
SHRIMP U-Pb studies on accessory mineral phases such as zircon have often been used to place constraints on the age and evolution of geological samples. Commonly, zircons with relatively low U contents are analysed, as a correlation between high U concentrations and Pb loss and/or high common Pb has frequently been noted. This is due to the degeneration of the zircon structure as a result of radioactive decay of the U, which causes the grains to become metamict and susceptible to mobility of the daughter products (Pb). In some circumstances, however, no suitable low-U grains can be found, resuhing in poor data quality, which translates into large uncertainties in the calculated ages. Under such conditions it may be more useflil to turn to other co-existing accessory phases such as titanite, monazite and/or xenotime in order to extract more meaningful data. Titanite commonly contains trace amounts of U and its structure generally excludes initial (common) Pb, hence making it ideal for U-Pb radiometric dating. In this paper we will present SHRIMP U-Pb data for both zircon and coexisting titanite from several granitoids of the Telfer district (Paterson Province, Western Australia). Granitoids in the Telfer region of Western Australia were intruded synchronous with the latter stages of the Paterson Orogeny. Approximately 20% of the Telfer district is underlain by granites which have been classified into two main groups on the basis of geological field relationships, petrology, geochemistry, relative timing and Pb isotopic composition (Goellnicht, 1992). The Mount Crofton suite consists of generally undeformed, late- to post-tectonic,fractionated(>71 wt% Si02), magnetite-bearing granites which have a Pb-Pb isochron age of 678 ± 36 Ma (Goellnicht, 1992). The Minyari suite has an indistinguishable Pb-Pb age of 643 ± 34 Ma and consists predominantly of lessfractionated,ihnenite-bearing granites (Goellnicht, 1992). In an effort to better resolve the temporal evolution of the Telfer region, we analysed several samples from both the Mount Crofton and Minyari suites using standard U-Pb SHRIMP methods. A summary of the results and previous zircon studies is presented in Table 1. Table 1. Summary of published and preliminary U-Pb SHRIMP results (206Pb/238U age in Ma) for the Telfer granitoids. Minyari suite Porphyritic biotite syenogranite: M21 (this study) Porphyritic biotite monzogranite: M34 (this study) Biotite monzogranite (Nelson, 1995) Mount Crofton suite Biotite monzogranite: CG-3 (this study) Biotite monzogranite: CG-2 (this study) Biotite monzogranite: C-85b (this study) Biotite monzogranite (Nelson, 1995)
zircon U (ppm) 629±8 115-2235 621+8 109-7720 633±13 170-4008
titanite U (ppm) 632±5 66-221 645±4 97-426 not analysed
scatter 126-2880 scatter 353-8375 not yet available 621±13 221-9564
654±8 76-269 not yet available 640±8 69-224 not analysed
Both the Mount Crofton and Minyari suites are highly fractionated (Goellnicht, 1992) and their zircons have correspondingly high U-contents and accompanying Pb-loss from radiation-damaged areas of some zircon grains. Coexisting titanite, in comparison, has significantly lower U-contents and does not ^pear to have suffered Pbloss, giving consistent and concordant age data which is generally older than the zircon data. These results suggest titanite may be a better mineral than zircon to use for SHRIMP geochronology in U-rich granites. Although inherited titanites may also explain the older titanite data, analyses for each sample form a single age population, suggesting they are not of xenocrystic origin. The data therefore may suggest that the crystallisation age of the granites is best determined by the titanite data, with the zircon data providing a minimum age. The overlapping ages for samples from the Minyari and Mount Crofton suites suggests they are coeval. This result, coupled with the geochemical and isotopic studies of Goellnicht (1992), suggests that they were derived at the same time from different sources, and that these sources must have already been juxtaposed at ca. 654-632 Ma. References: Goellnicht 1992, PhD thesis UWA (unpubl); Nelson 1995, GSWA Record 1995/3.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
STEREODIGITAL MAPPING AND VISUALIZATION OF GEOLOGICAL STRUCTURES David W. Dumev and Andrew J. Fountain^ School of Earth Sciences, Macquarie University, NSW 2109. ('now at WMC Resources, Kambalda, WA 6444.)
This paper describes work on two themes: (1) stereodigital acquisition of geological and structural data from aerial photographs and (2) spatial visualization and analysis of those data applied to areas of folded strata. Stereodigitization is the process of acquiring and archiving three-dimensional georeferenced data from a photograph stereopair using a digitally-interfaced stereoscope. Advantages for geological mapping include: • full control by the geologist over selection of air-photo information, at optical resolution, • accurate georeferencing, hence direct transferability to maps and other projections, as well as compatibility with other georeferenced data sources, • applicability to remote or inaccessible areas. A system designed for geological and structural mapping has been developed at Macquarie University, using a Qasco SD4 stereodigitizer with QuickBasic software enhancements. Information can be recorded for attitudes of bedding and fold hinge-lines, faults, formation and surficial boundaries, streams and culture. Data are acquired as sets of points in map-coordinate space and converted to planes (least squares and minor axis methods), lines (vectors) or 3D arcs. The integrity of digitized bedding planes and fold hinge-lines can be checked against ground truth, visual inspection of air photos and goodness of fit criteria. These data are subsequently filtered and downloaded for display and analysis. Arc data can be combined with measured structures or can be mapped in their own right, depending on aims and nature of the terrain. (Bedding measurements require areas of good exposure and moderate relief) Map composition and visualization are currently implemented on a PC with the Surfer desktop package. Planimetric information is exportable to Arclnfo. Structural attitudes and formation contacts can be visualized or analysed three-dimensionally by a variety of methods: • stereographic analysis (attitudes), • projection onto cross-sections (attitudes and contacts), • tilting for profile viewing 'down-plunge' (contacts), • map analysis (attitudes). Spatial analysis of bedding-plane attitudes in map view has been carried out by a new method called the gradients method which assists visualization of 3D fold structure. This requires conceptualization of a folded surface independent of the topographic surface. The assumption currently used to achieve this is that measured attitudes are independent of vertical postion and therefore project onto the conceptual fold surface vertically above or below the sample locations (applicable only to upward-facing beds in upright to steeply inclined folds). Gradients in a chosen direction are computed from local bedding attitudes. Gridding, contouring and shading then provide a pseudo-illuminated image of the fold structure in which structural highs, lows and dips can be visualized, analogous to a sun-lit land surface. Transverse gradients (across the folds) emphasize positions and slope directions of fold limbs and cross-sectional shape of the folds. Longitudinal gradients (along the folds) show directions and amount of fold plunge. Thirdly, data components of different kinds (zero contour of transverse gradient, derivative of tranverse gradient, value of longitudinal gradient) can be combined to produce maps the fold crest- and trough-lines and their plunges. These techniques are demonstrated for an area of folded Upper Devonian red beds at Mount Kangarooby (Gooloogong), New South Wales, which was investigated in a recent Honours project by AJF. Acknowledgement? Partial support for this project was provided by a Macquarie University Research Grant and the NSW Land Information Centre. We thank B. Donnelly, P. Fletcher and R. Golberry for technical advice on the equipment.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
STRESS AND FLUID CONTROL ON DECOLLEMENT IN COMPETENT LIMESTONE David W. Dumev'. Antonio Teixell^ and Maria-Luisa Arboleya^ ^School of Earth Sciences, Macquarie University, Sydney NSW 2109, Australia ^Departament de Geologia, Universitat Aut6noma de Barcelona, 08193 Bellaterra, Spain
Decollements in thin-skinned fold-and-thrust belts are normally accounted for by localization in lithologically weak horizons, such as evaporites, and shales, which can generate high fluid-pressure. The Larra thrust of the west-central Pyrenees is one of the exceptions to this rule where d^collement is located wholly within a nonporous, impermeable and competent limestone formation. The thrust forms the floor of a thrust system of hectometric-scale imbrications developed beneath the northern extension of the Eocene synorogenic Jaca basin. It runs for about 17 km in Upper Cretaceous limestone before climbing into overlying shale and turbidite units and shows a cummulative displacement of 5 km, determined from hangingwall imbricates. The fault rock at the d^collement is a dense stack of mainly bedding-parallel calcite veins with variable internal deformation by twinning and recrystallization. Veins developed as extension fractures parallel to the maximum stress, cemented by cavity-type crystal growth. Crack-seal microstructure, slickenlines and shear indicators (other than oblique stylolites) are lacking. Channelled high differential stress in the strong limestone suggests that veins were generated as compressional, en-echelon crack arrays in a fluid-weakened horizon. The fluid must have been derived from a hinterland source by migration along fracture porosity created by the decollement, probably at pressures approaching (but not exceeding) lithostatic pressure. Ductile deformation, although conspicuous, cannot account for the kilometric displacement of the thrust. Displacement was mostly accommodated by slip on water sills constituted by linked, open cracks from different arrays. A model of stress reorientation, related to cyclic, transitional brittle-ductile differential-contraction, shear, relaxation and cementation at a rheological step in the limestone, is proposed as a mechanism for episodic propagation and movement of the decollement. The model accounts for the peculiar microstructural character of the fault zone and for alternating sequences of bedding-parallel shortening (leading to crack dilation) and bedding-parallel shear (leading to decollement slip). In this model, hangingwall imbrication is a consequence of, rather than a trigger for, slip on the decollement.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
MAGNETITE SKARN AT THE BIGGENDEN MINE, SOUTHEAST QUEENSLAND Man$<?Mr Edraki and Paul M.Ashley Division of Earth Sciences, University of New England, Armidale, NSW 2351
This Study presents mineralogical and geochemical information on, and discusses the genesis of, the Biggenden magnetite deposit, which is an important source of high-quality magnetite for the coal washing industry, and which historically produced gold, copper and bismuth. Skam at the Biggenden mine has replaced rocks of the early Permian Gympie Group, comprising intercalated basaltic/andesitic flows, tuffaceous and fragmental rocks of sedimentary/volcanic provenance, siltstone, and limestone. Assemblages consisting of plagioclase, hornblende, diopside (± scapolite) in the mafic homfelses and quartz, muscovite, biotite, plagioclase and cordierite in the metasiltstones are consistent with hornblende homfels grade of isochemical contact metamorphism produced by the intrusion of the adjacent Triassic Degilbo granite. The development of skam has followed contact metamorphism and has replaced all rock types. Skam mineralogy indicates that there was an evolution from prograde through to various retrograde assemblages. Prograde garnet (Ad2o-75Gr24-7oSp2loPyo.2-0.5)» clinopyroxene (Di6o-95Hd5^o)> magnetite and scapolite formed initially. Magnetite ore, consisting dominantly of magnetite, calcite and gamet, is part of the prograde assemblage. Epidote and Cl-bearing hastingsite or ferropargasite are the early retrograde minerals and are followed by chlorite, calcite, actinolite, quartz and sulphides. The main sulphide minerals are pyrite, chalcopyrite and bismuthinite, with traces of arsenopyrite, molybdenite, cobaltite, sphalerite and stannite, along with rare native bismuth and gold. Late-stage retrogression is indicated by development of nontronite, calcite and quartz. Skam formation has been accompanied by desilication of host rocks, introduction of Ca, Fe, Mn, Mg, Sn, W, Mo, Bi, Cu, Zn, As, Au and S, and probable volume reduction. Fluid inclusions in prograde skam calcite and gamet yield high homogenisation temperatures (500°-550°C) and high salinities (--30 equivalent wt % NaCl). Fluid inclusions in the sulphide-stage quartz and calcites homogenise between 300® and 400°C and have lower salinities. The and values of carbonates show the typical trends of depletion characteristic skam deposits, which results from infiltrating magmatic/meteoric waters. The water/rock ratio has been calculated at 5-10 during skam formation. Theoretical isotopic fractionation curves for the infiltration process imply that the limestone was the main contributor for carbon in skam calcites. The calculated of the metasomatic fluid in equilibrium with skam calcites at 500°C is close to that calculated for the fluid in equilibrium with the granite and anhydrous skam silicates. Coexisting quartz and biotite in the granite yield a temperature range from 513° to 560°C, based on the measured ^^O fractionation between these minerals. Oxygen isotope values for gamets have a restricted range and the values of fluids in equilibrium with gamet and amphiboles are slightly higher than the range which is found for fluids in many skams related to felsic magmatic rocks. Light oxygen and hydrogen isotopic values for the fluids in equilibrium with retrograde chlorite, indicate that the retrograde fluids were from a high-latitude, meteoric source (consistent with the interpreted palaeolatitude of southeast Queensland in the Triassic. The values for skam sulphides and the lead isotope data for two galena samples are consistent with a magmatic source for the mineralising fluid, with the latter data also showing that mineralisation took place in the late Triassic. The scarcity of fracture systems, veins and endoskams, imply that there has been little fluid infiltration across the Degilbo granite contact as exposed. However, the current isotopic results and the fact that there has been a considerable introduction of the granitophile elements to the site of ore deposition, emphasise the role of this pluton in ore genesis. We consider that the Degilbo granite is in fact the uhimate source for the ore fluids, but that the fluids were derived from a deeper level than is currently exposed (Edraki & Ashley, 1997). REFERENCES Edraki M. & Ashley P. M. 1997. New geological interpretations on the Biggenden magnetite skam. Southeast Queensland. Geological Society ofAustralia, Abstracts 44,28. Acknowledgments: The authors wish to thank Dr Anita Andrew and technical staff of the Centre for Isotope Studies, CSIRO, for the isotopic analyses. This study has also benefited from discussions with Dr Nick Stephenson.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
HURGLEDURGLES AS A GUIDE TO ORE AT MOUNT MORGAN: A RATIONAL EXPLANATION R. England^ A. Taube^and P. Messenger^ 1. Consulting Geologist, Townsville 2. Perilya Mines N.L., Mount Morgan 3. Great Central Mines Ltd, West Perth
The Mount Morgan Au-Cu ore body and its host rocks occupy a roof pendant in the Late Devonian Mount Morgan Tonalite. The ore body consists of massive pyrite with stringers of quartz, chalcopyrite and gold. Recently discovered sub-ore grade stratabound mineralisation some 600 m SE of the old open cut consists of banded and massive sulphides with underlying stringer minerahsation. Spotted rocks form an envelope around the pipe-like ore body and the newly discovered "Car Park" and "Slag Heap" mineralisation and have been used in exploration as an indicator of the proximity of ore. Almost all of the spots are thoroughly retrogressed to assemblages with quartz, albite, chlorite, clay-mica and pyrite. The origin of the spotting has been so obscured by retrogression that the rocks have been known locally, for about 25 years, as "hurgledurglite" (Taube, 1986). The spotting is developed in tuffaceous siltstones, fine-grained intrusive felsic porphyry, coarse mass-flow or debris-flow deposits and in minor strongly chloritic zones. The spotting in a strongly chloritic zone has been identified unequivocally in one sample as completely pinitised cordierite with perfectly preserved sector twinning. This sample also contains fresh green spinel rimmed with hogbomite, both containing Za Hogbomite is a rare Al-Fe-Mg-Zn oxide mineral found mostly in the foot-wall alteration zones of metamorphosed massive sulphide deposits. This and nearby samples (about 200 mfi-omthe Mount Morgan Tonalite) also contain completely chloritised coarse columnar Ca-poor amphibole. Fresh magnesian cummingtonite has been probed in two samples of spotty altered intrusive felsic porphyry. The weak to strong chloritic/phyllic/sihceous spotting in these rocks was almost certainly also cordierite, commonly with abundant fine inclusions of quartz. The assemblage of cordierite and Ca-poor amphibole shows that the upper stability limit of fairly magnesian chlorite + quartz (c. 500° at 0.5-1 Kb) was exceeded. The envelope containing cordierite and Ca-poor amphibole probably formed by contact metamorphism of preexisting chloritic alteration in which Ca was depleted and Mg enriched Chloritic alteration common in the foot walls of VMS systems, is generally ascribed to reaction of various rock types with sea water. The pervasive retrogression may be due to continued percolation of sea-water after the emplacement of the Mount Morgan Tonalite. The spotted "hurgledurglite" zone enveloping the orebody has not previously been interpeted as a halo of metamphosed VMS-style magnesian alteration, although high-T contact metamorphic assemblages have been describedfi-omthe area. The present interpretation supports the VMS model for Mount Morgan (Taube, 1986). It indicates that the intrusion-related model of Arnold and Sillitoe (1989), in which the ore fluid is derived from the Mount Morgan Tonalite, may require separate origins for the Au-Cu mineralisation and its massive pyrite host. REFERENCES Arnold G.O. & Sillitoe R.H. 1989. Mount Morgan gold-copper deposit, (Queensland, Australia: evidence for an intrusion related replacement origin. Economic Geology v. 84, 1805-1816. Taube A. 1986. The Mount Morgan gold-copper mine and environment, (^eensland: a volcanogenic massive sulphide deposit associated with penecontemporaneous faulting. Economic Geology v 81,1322-1340 Acknowledgements: The authors acknowledge the permission of Perilya Mines N.L. for pubhcation.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
3D SEDIMENTARY AND STRUCTURAL MODELS, MORANBAH COAL MEASURES, BOWEN BASIN, AUSTRALLV Joan Esterle & Guy LeBlanc Smith CSIRO Exploration and Mining, 2643 Moggill Rd, Pinjarra Hills, Brisbane, Queensland 4069 Email: j.esterle@dem.csiro.au and g.leblanc-sniith@dem.csiro.au
A 3D structural and sedimentary model was developed from mine-scale data for a series of mines in the Moranbah Coal Measures along the northwestern margin of the Bowen Basin. Permian coal is extracted from two 6-12m thick regionally extensive seams that are hosted in gently east-dipping volcanolithic sediments that are weakly folded and faulted. Data was sourced from drillholes (>1500), 2D-seismic, and highwall mapping (>15km). A logical-letter coding system was employed to constmct the spatial model in Mincom Minescape software. This model provides a framework for the prediction of lithological and structural variability and recognition of patterns in orientation, geometry and frequency of occurrence that can be assessed for impact on mining conditions for the Goonyella Middle (GMS) and Lower (GLS) seams. The lower interval from GLS to GMS is siltstone dominated with 1 to 10m thick intercalated sandstone sheets and minor, thicker sandstone channels and is characterised by departure and reattachment of a thin rider seam. This sequence is interpreted as comprising large splay and bay or lake-fill deposits. The upper iaterval from the GMS to the P Tuff (a regional time-marker horizon) is sandstone dominated. It comprises an architecture of amalgamated channel-sandstone bodies, generally between 40 to 60m thick and 2 to 4km wide, which trend southwards in a belt across the leases. In cross-section, the sandstone bodies are lenticular and contain a variety of cross-stratification structures, but they are more massive (structureless) towards the core of the bodies and thin and interfmger along their flanks with weaker interbedded siltstones and mudstones. Sandstone bodies predominantly show vertical genetic stacking patterns. The architecture of the sedimentary units suggests a series of trunk distributary channel fill sandstones with margins characterised by thinner but widespread lobate and sheet-like sandstones characteristic of overbank and splay deposits. The thickness distribution of the P Tuff reflects the underlying remnant channel system. Stratigraphically up section, the inter-seam sequences become thinner and less sandstone dominated up to the Goonyella Upper (GUS) seam. Above the GUS, thick amalgamated sandstone channels again become predominant. Three principal types of small-scale faulting affect strata: thrusts (<35° dip reverse-slip), normal (steep dip-slip) and shears (steep lateral-slip). Generally, normal and thrust fault trends intersect or abut at right angles. Shears are difficult to pick and appear to be steep and have an oblique relationship to thrust trends. Two main types of shearing are interpreted: right-slip shears trending approximately 45 degrees; and left-slip shears trending 315 degrees. These trends align with major basement shear trends mapped in the Drummond Basin. Tectonic movement along these shears throughout geological time is thought to be the progenitor of all structural features described above, as well as creating the accommodation space for the deposition of thick channel-fill sandstones. These shear faults are thus interpreted as structures with a long history of reactivation that have been through phases of shear, extension and even compressional movements. BHP Coal and Shell Coal Australia kindly provided data and site access for this research and their support is gratefully acknowledged.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
LASER ABLATION MICROPROBE ICP-MS OF NOBLE METALS IN IMMISCIBLE SULFIDES Noreen J. EvansBrent I. A. Mclnnes'-^ and Simon Jackson^ ^CSIRO, Exploration and Mining, P.O. Box 136, North Ryde, NSW 2113 ^GEMOC School of Earth Sciences, Macquarie University, North Ryde, NSW 2109
During a 1994 cruise of the RV Sonne high-K calc-alkaline basalts (shoshonites) were dredged from 1400 m water depth on the eastern flank of the Tanga volcano, Tabar-Lihir-Tanga-Feni island arc, east of New Ireland, Papua New Guinea. The glassy margins of the vesicular flows contain spherical, 15-60^m diameter Cu-rich sulfide globules. The spheres have not ciystallized sulfide or oxide minerals which indicates they are quenched inmiiscible Cu-Fe-O-S liquids, trapped as the basaltic melt became sulfide saturated during eruption. Previously, we reported proton and electron microprobe analyses on these sulfides and their glassy host which allowed calculation of sulfide-silicate partition coefficients (Mclnnes and Evans, 1996). Herein we present recently acquired laser ablation microprobe ICPMS data for the platinum-group elements which gives us greater insight into details of the metal distribution. The sulfide inclusions were analyzed in a 100 micrometer thick section using a UV laser ablation microprobe (LAM)-ICP-MS at Memorial University of Newfoundland, Canada. Standardization was performed using a rapidly quenched NiS fire assay button, prepared from a starting mixture doped with PGE to give a nominal concentration of 200 ppm. Differences in ablation yield between analyses were corrected by internal standardization using sulfur (S), which had previously been determined by electron microprobe microanalysis. Data were collected using a timeresolved data acquisition protocol, allowing signals to be examined and selectively integrated to minimize contributions to the signal from the minerals hosting the sulfide inclusions. In our case, the contribution of metals from the glass is negligible. A NiS fire assay button of reference material SARM-7 was analyzed several times to check precision and accuracy of the technique. Except for Ir and Os, which were near or below detection limit, precisions (Isigma) were better than 20% and accuracies within 10% of certified values. TABLE 1. Results of (LAM)-ICP-MS for 10 different sulfide globules (in ppm). Sample Ni Ru Pd Os Ir Pt Au Ag- Au/Ag oc09a04 488 284 0.39 1.18 0.46 138 80.4 49 1.6 oc09a05 2182 0.91 13.9 1.34 0.14 1.65 1.59 225 7E-3 4204 oc09a06 0.68 238 0.67 bid 204 62.8 50 1.3 oc09a07 2192 0.39 11.7 bid 0.12 bid 0.41 29 0.014 oc09a08 3759 0.06 45.4 bid bid 1.43 17.0 20 0.85 oc09a09 940 0.44 1.8 bid bid 0.09 0.01 128 7.8E-5 oc09al0 1738 0.06 4.2 bid bid 0.07 0.53 2.0 0.3 oc09all 1220 0.45 71.4 bid 0.06 6.21 7.48 121 0.06 oc09al2 1719 1.23 94.6 0.34 bid bid 8.93 31 0.3 oc09al3 186 0.19 27.4 bid bid 3.34 7.82 29 0.3 mean 1371 0.33 30.5 0.77 0.15 3.1 3.3 39 stnd dev(lCT) 2.57 2.81 5.27 1.87 2.33 18.8 14.9 3.6 bid = below limit of detection; mean and standard deviation forPGE, Au and Ag detennined on log normalized concentrations calculated without detection limit filtering. *semi-quantitative due to uncertainty of Ag recovery during NiS fire assay.
We have previously reported elevated Cu and Pd contents in these sulfides, but the current data (Table 1) shows that the levels of all PGE, Au and Ag are extremely high. This result indicates the sulfide liquids are capable of removing vast quantities of metal from the magmatic system. There is significant variability in metal content from grain to grain. For example, Pt content ranges from 203ppm to less than 70ppb. The presence of Pd, Ag and Pt spikes within the signal peak for individual grains implies heterogeneous distribution of PGE. We suspect PGE tellurides may account for the presence of spikes as proton microprobe data revealed 93 ppm Te in the sulfides. The individual signal plots show good correlation between the Cu, Pd Ag and Pt responses, suggesting these elements may be somehow associated in the sulfides. REFERENCES: Mclnnes, B.I.A. and Evans, N.J. Cu- and Pd-rich Immiscible sulfide liquids in submarine shoshonitic lavas from the Tabar-Lihir-Tanga-Feni Island Arc, Papua New Guinea. 13th AGC, Canberra, 1996.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
LAND DEGRADATION AND THE EARTH SCIENTIST W Ray Evans Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601
Land degradation issues loom large as global society demands that resource usage should be sustainable. A larger proportion of the world's population on a daily basis faces severe food, fibre and water shortages. This is further compounded by decreasing amounts of arable land and volumes of unallocated fresh water. Large scale efforts are now either in place or being planned to better manage the worlds natural resources. Central to the issue of sustamability is the notion of present usage rates balanced against the forecast needs of future generations. That is, sustainability implies a planning t i m e f i ^ e that is exceptionally long compared with other issues. Whether various resource bases are effectively renewable or not and on what timescale further complicates forecasting. Land degradation is widespread throughout Australia and is found in many forms including; soil acidity, soil structure decline, soil nutrient loss, soil sodicity, wind and water erosion, salinity, and rising watertables. Most of these forms are increasing in their spatial occurrence and severity. In most cases the different forms usually co-exist. It is estimated that the various forms of degradation account for about $2 billion in lost production annually, an un-quantified loss in capital, and a similar un-quantifiable loss in bio-diversity. In fact, most commentators would suggest that Australia's agricultural systems are not sustainable in their current configuration. The earth sciences has an important supporting role to play in the management of these problems. The areas where the earth science discipline can contribute are; through our deductive approach; our understandings of time scales and the issues of equilibrium; our reading of the landscape; and our ability to regionalise and map across spatial scales. The main discipline areas of relevance would include regolith studies, quaternary geology, hydrogeology, earth surface geochemistry, and regional geophysics (radiometrics and EM). These disciplines have to be brought to bear on the problem within a very complex framework of institutions and community-based management bodies who are not necessarily focused on the longer term issues so central to the attainment of sustainability over the time frames required. This has to be done also within an environment of contestable research and investigation funding - the primary client, in most cases, not having the resources to fully solve the problem. This raises the question: are we best positioned to be able to respond, are we ever going to be able to respond with our current knowledge. The future challenges for us as a discipline lie in our ability to become multidisciplinary outside our traditional areas of endeavor, especially the integration of the earth sciences with agricultural science and resource economics; and to be client focused, competitive and contestable. However, it is not always clear who the clients are, and whether they understand the issues. Equally, it is not always clear that the clients have the time and the money to make the right decisions.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
GEOLOGY OFF EAST TASMANIA INDICATED BY HIGH-RESOLUTION SEABED IMAGING Neville Exon', Peter Hill', Jock Keene^ and Stuart Smith-' 'Australian Geological Survey Organisation, Canberra 2601 ^Department of Geology & Geophysics, University of Sydney, Sydney 2006 'Scripps Institution of Oceanography, La Jolla, California 92093-0223.
Much of the sea bed off eastern Tasmania and in Bass Strait, in water depths of 2000-4000 m, was recently surveyed by AGSO using the SeaBeam 2000 multibeam sonar system of the RV Melville. Information from this and other sources provides an improved understanding of how the margin first developed and was eroded and sedimented. The continental shelf is 20-30 km wide; the margin as a whole is 70-100 km wide, giving way to the abyssal plain at 4000-4200 m. The offshore continental margin of eastern Tasmania lay within East Gondwana for most of the Cretaceous, with the Lord Howe Rise welded to Australia. Heating and uplift in the Eastern Highlands at about 95 Ma must have been accompanied by a period of stretching and rifting of continental crust, which gave way to the opening of the Tasman Sea in the Late Cretaceous at about 80 Ma. The margin subsided as the Tasman Sea opened, and initial erosion of the highlands deposited substantial quantities of terrigenous sediment on oceanic crust, well into the Early Tertiary. The multibeam mapping shows that from 42®30'S to 41°S, two physiographic provinces are separated by a north-trending, coast-parallel, steeper slope at 2000-2500m. Small canyons meander on the sedimented upper slope, but follow basement structures across the unsedimented lower slope. The deeper, outer province (25004000 m) forms the continental margin out to the foot of slope, which is roughly the continent-ocean boundary. The foot of slope was markedly offset during continental breakup, with the rift direction NW-NNW, and the transfer direction NE-ENE. However, the steep slope at 2000-2500 m has much smaller offsets; perhaps both because transfer faults die out landward, and because a Miocene carbonate platform has built outward. Off central east Tasmania (42°30'S to 41°40'S), consisting of the Late Carboniferous to Triassic Tasmania Basin and the Devonian granites of the Freycinet Peninsula, faults trend 30-45° and 320-330°. Further north (41°40'S to 41°S), off the northeast Tasmanian block of Early Palaeozoic turbidites and Devonian granites, faults trend 70-80° and 330-340°. East of the Fumeaux Islands (41°S to 40°15'S), geologically similar to the northeast Tasmanian block, canyons are larger than those in the south, suggesting that they were fed by larger rivers, and they are generally structurally controlled. The area is geologically complex, with old deformed rocks, intrusions and extrusions, and structures trending 0°, 15° and 90°. The area northeast of the Fumeaux Islands (40°15'S to 39°S) is much less complex, probably consisting of shallowly dipping Late Palaeozoic and Mesozoic sediments. Canyons are small. Faults trend 330° and 15-40°, and there are two major embayments at the foot of the slope. On the lower continental slope there has been little post-breakup sedimentation, probably because the early flood of terrigenous sediments bypassed the margin down canyons and there was little terrigenous input thereafter, and also because eddies from the East Australian Current inhibited pelagic sedimentation. Pre-rift, rift and break-up structures and rocks are all exposed in places. On the shelf and upper slope, reflection seismic profiles show that the margin has 500-1000 m of Late Cretaceous to Cainozoic strata on pre-breakup basement rocks. These strata can be divided into three sequences: an upper sequence of well-bedded shelf sediments, presumably latest Miocene to Recent in age; a middle sequence of highly reflective, irregulariy bedded sedimentary rocks which crop out in places and consist of Miocene limestones; and a lower indeterminate sequence beneath an unconformity assumed to be Oligocene. Nothing is known of the lower sequence, but Pat Quilty and Andrew Telfer (1994) have described Early Miocene bryozoal calcarenite, Early-Middle Miocene chalk, and Late Miocene micritic limestone, from the middle sequence on the upper slope in present water depths of 800-1200 m. These foram-rich limestones were probably all laid down on the shelf, suggesting sinking of about 1000 m since their deposition. Hugh Jones and Peter Davies (1983) carried out a detailed sampling program on the shelf Quartz sands with shelly material dominate the inner shelf, whereas the outer shelf consists either of muddy quartzose or calcareous sediment, or of bryozoan sands and gravel that are probably largely relics from lower Pleistocene sea level. Such sediments may make up much of the well-bedded upper seismic sequence.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
MANTLE MELTING AT 1 AND 1.5 GPA AND THE PETROGENESIS OF MORE: IMPLICATIONS FOR DETERMINING THE COMPOSITIONS OF HIGH-PRESSURE MANTLE MELTS USING DIAMOND AGGREGATES AND NATURAL MINERAL MIXES. Trevor J. Falloon^ David H. G^een^ Leonid V. Danyushevsky\ and Ulrich H. FauF ^School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001 Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200
The experimental determination of liquid compositions in Iherzolite as functions of pressure and temperature provides constraints on mantle dynamics and magma genesis. In this paper, we present a detailed evaluation of the use of natural mineral mixes as starting material in peridotite melting studies at 1 GPa. As an example we have chosen to test the data obtained by Baker and Stolper (1994) on a Iherzolite composition (MM-3) presented as a potential source for Mid-Ocean Ridge Basalts (MORB).This study is the most fully documented published melting study using natural mineral mixes. We have tested the Baker and Stolper (1994) data in three ways: (1) we have defined the liquidus phases and conditions of the partial melt compositions obtained by Baker and Stolper (1994); (2) we have reacted these partial melt compositions with a fine grained synthetic starting mix of MM-3 composition; and (3) we have performed additional melting experiments at 1 and 1.5 GPa using the synthetic mix of peridotite MM-3. Our results demonstrate that only the highest temperature experiment of Baker and Stolper (1994), performed at 1390°C, approached an equilibrium melt of peridotite MM-3 composition and that lower temperature experiments have not reached equilibrium, retaining residual unreacted minerals and metastable melt compositions. The degree of disequihbrium increases progressively with decreasing temperature. Disequilibrium is attributed to the lack of reaction of the natural mineral mix and to disequilibrium melting reactions of the metastable, relatively coarse grained mineral mix. Other contributing factors include disequilibrium caused by the use of a diamond aggregate trap. We also present peridotite melting experiments using the mineral mix KLB-1 at IGPa which has been used as a starting material in the mantle melting study of Hirose and Kushiro (1993). Our results demonstrate that the mineral mix KLB-1 fails to equihbrate even after ~340hrs at temperatures of 1280-13(K)°C. We present reversals of the IGPa peridotite melting experiments of Hirose and Kushiro (1993) at 1300 and 1350"C. Our reversals demonstrate that the mineral mix/diamond aggregate trap technique used by Hirose and Kushiro (1993) has also failed to produce equilibrium melts of a mantle peridotite composition. It is reconmiended that data from peridotite melting studies utilizing natural mineral mixes be used with reservation and that natural mineral mixes are not a suitable starting material for such studies. The use of diamond aggregate for separation and trapping of the melt phase compounds rather than solves the problems inherent in the use of natural, mineral mixes. The evaluation of experimental methods enables us to present the melting trend for MM-3 at 1.0 GPa and 1.5 GPa and compare this with similar melting trends for both more fertile (MORB-Pyrolite) and more refractoiy (Tinaquillo Lherzolite) Iherzolite compositions. Our new data provide confirmation that primitive MORB glasses have compositions consistent with batch melting (or re-equilibrated pooled melts) of Iherzolite at moderate to high degrees of melting at 0.8 to 2.2 GPa, implying mantle potential temperatures in Mid-Ocean Ridge settings of upto Tp-1450X. REFERENCES Baker M. B. & Stolper E. M. 1994. Determining the compositions of high-pressure melts using diamond aggregates. Geochimica et Cosmochimica Acta 58, 2811-2827. Hirose K. & Kushiro I. 1993. Partial melting of dry peridotites at high pressures: determination of compositions of melts segregated from peridotites using aggregates of diamonds. Earth and Planetary Science Letters 114, 477489.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
EXPLORATION TECHNOLOGIES TO ENTER THE 21ST CENTURY Ross Fardon Ross Fardon and Associates Pty Ltd., 9 Mariner Street, Manly West, Qld. 4179
Prediction is always difficult so the paper is based on a letter from a field camp in 2010 to take advantage of hindsight. The big breaks by then were in electrical geophysics, instant mineralogy, and airborne gravity, but the incremental advances in computer power made all data available instantly in the field, or at any place on earth during conference hook-ups. An unexpected consequence of the ability to map geophysical rock facies to a depth of a kilometer, was the need for more field work, to ground truth the interpretations. Airborne geophysics of all types had been flown mainly by unmanned craft for several years, and was backed up by array electromagnetics on the ground. Every explorer could do far more than the ER-Mapper people at present, without significant training. All data including air photos were digitised, registered and available at voice or finger tip at any scale. Geochemistry had gone a long way with partial extraction from primary mineral surfaces, and mobile metal ions had found a wealth of deposits in the world's deserts. Metal prices maintained their long historic decline, and looked like doing so for another lifetime. Seismic was a big factor in understanding all of the cratons, and the phenomenal unravelling of the whole crust had led to more giant deposits, looming like a whale in the deeps. The difference between geologists, geophysicists and geochemists was fast vanishing as every explorer had to evaluate all of the data in front of her. New data was so abundant and integrated that the scientists had to be expert, and on-going training took about 20% of the time of explorers in most significant companies. Companies were doing more in-house research, but also more pre-competitive collaborative work. The transformation from now to then could be summed up as like a dozen technologies as good as magnetics in the 1990's, all maturing together - and then some.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998 REGOLITH HYDROLOGY OF THE DUNDAS TABLELANDS - AN INVESTIGATION OF SOILWATER LOGGING AND SALINITY Jonathon Fawcett & Peter Dahlhaus, University of Ballarat, P.O. Box 663, Ballarat Victoria 3350
Soil salinisation and soil water logging are a major problem facing graziers of the southeastern Dundas Tablelands, Victoria. In conjmiction with the Bulart Land Management Group, an intensive investigation of the regolith hydrology was condurted to establish a useful predictive model for land management options. The work was funded through the National Landcare Program. The regolith of the Bulart area has developed a distinctive catena profile (5 - 30 metres thick) consisting of a duplex soil with abundant pisoliths underlain by a zone dominated by sub-horizontal iron-rich mottles and concretions. Underlying this is a lower mottled zone which grades into a pallid, kaolin-rich horizons overlying the fresh rhyolite. Using a technique developed by the CSIRO Soils Division, large diameter (30cm) intact soil cores were extracted at two sites that represented the geomorphology of the Bulart region. At each site four cores were taken to establish the hydrological variation in the regolith topsequence. The cores were encased in PVC pipe and carefully prepared for testing using hot - wax sealing to prevent side leackage. The hydrological tests were designed to simulate a variety of field conditions. Bottom-up saturation of the cores provided for standard laboratory permeameter test of saturated hydraulic conductivity. Rainfall simulation tests of vaiying intensity and duration allowed seasonal hydrology to be studied. Ponding and draining enabled quantitativeflowprofiles to be calculated for the various soil horizons. Piezometric measurements and drainage at intervals through the core provided information on transmission of water throughout the profile. The investigation of the hydraulic properties of the upper regolith has identified the importance of preferred patii flow and the ability of the regolith profile to transmit water vertically and horizontally. Prior to a seasonal wet period (eg. autumn), unsaturated conditions allow water to flow vertically downwards through the preferred paths (macropores). Once water entering the regolith, as a result of precipitation, reaches the saturated zone of the lower mottie zone (water table) water in the macropores backfills such that saturated lateral flow commences between connected preferred paths. Whiletiievolume of water draining from the upper regolifli (especially the pisolitii horizon) is greater than that which is being drained due to groundwater discharge into the underlying Rocklands Rhyolite, the macropores within the profile remain saturated. Once the saturated macropores are filled totiieupper regolith, lateralflow(interflow and throughflow) commences in the upper regolith, especially through the pisolitii horizon, which has a much higher saturated hydraulic conductivity than in the rest of the regolitii. Thick regolith development on the plateau provides large water holding capacity with preferential paths for water movement to C horizon. Perched water in the A2 (E) horizon of the solum. Saturation provides head for lateral water movement down slope. Solum Thinner regolith on erosional slopes. Break of slope seepage.High volume of lateral flow.
Upper mottled zone Lower mottled zone Weathering rock 15 to 30 m depth
Thicker alluvial soils, waterlogged by combined interflow, surface runoff and saline groundwater discharge
Very slow groundwater movement through fractured rhyolite
Saline groundwater discharge
The spatial continuity of preferred patiis varied according to the slope angle. On the lower parts of the steeper slope, macropres were less connected, possibly due to soil creep or illuviation. The capacity of the lower slopes to transmit the interflow and throughflow is often exceeded by mid winter. At this time break-of-slope seepage and water-logging of higher slopes results. This model does not support the adoption of groundwater recharge control metiiods to mitigate salinity. Control of soil waterlogging would be more appropriate.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
REGIONAL MINERALIZATION MAPPING IN WESTERN AUSTRALIA Kenneth M. Ferguson, Lee Y. Hassan, and Roger W. Cooper Geological Survey of Western Australia, 100 Plain Street, East Perth, Western Australia 6004
CONCEPT The Geological Survey of Western Australia has initiated a series of regional mineralization syntheses to enhance and promote the mineral prospectivity of a number of large, geologically coherent regions or terranes. Targeting both greenfields areas, of relatively poorly known potential, and well known, more established, mineral provinces, the studies draw on a wide range of publicly available data, with the aim of fostermg exploration activity, and increasing and diversifying the State's resource inventory. ELEMENTS In each study area the most recent 1:100 000 and 1:250 000 scale geological mapping by the GSWA, has been used to provide a reliable, up-to-date geological framework. For the first time, these studies incorporate the vast amount of valuable information contained in open-file, statutory, exploration company reports. These are used to construct a detailed digital spatial index (SPINDEX) to exploration methodology, linked to a summary of results for each exploration project. Open-file reports, in combination with all other available published sources, including resource information, are also used to assemble a database of mineral occurrences (WAMIN) in each study area. Occurrences are defined as an accumulation in bedrock or regolith of an economic mineral exceeding an agreed concentration and size, ranging from a mineralized outcrop or drill intersection to an operating mine. Many occurrences are field checked and accurately located, some by differential GPS. The geological mapping, and data from SPINDEX and WAMIN, are combined with Landsat TM and regional geophysical layers to provide a basis for intelligent assessment of mineral potential for all relevant deposit models, in a form that is capable of being updated at appropriate intervals. PRODUCTS Each completed study incorporates a report containing details of the mineralization models and styles, descriptions of major deposits, and an assessment of mineral controls, trends and potential; a 1:500 000 scale mineralization map; and a CD version of the ArcView and Arc Explorer GIS database containing all the geological, geophysical, exploration, resource, and mineral occurrence datasets. MAJOR OUTCOMES Studies to date have been completed in southwest Western Australia, the north Eastern Goldfields, and the Proterozoic Bangemall Basin. Further studies are under way in the west Pilbara and Kimberley regions. In southwest Western Australia one of the new findings was the recognition of the potential for epithermal gold deposits along the Darling and Dunsborough faults, at the intersections of these structures with major northwesttrending structures. Gold potential is also seen on the sheared contact zone of the Boddington and Lake Grace terranes, and in the Archaean metamorphic complexes. Further occurrences of Greenbushes-type tin-tantalumlithium pegmatite are likely in the Donnybrook-Greenbushes shear zone. Potential also exists for PGE, nickel, chromium, vanadium, base metals, iron ore, diamonds, silica sand, and kaolin and other industrial minerals. In the north Eastern Goldfields, potential is high for further discoveries of mesothermal gold in areas of low strain, dilational brittle and brittle-ductile structures, marginal to regional-scale, north-northwesterly trending, high strain ductile shear zones within greenstone belts. This is supported by recent major discoveries in the Yandal and Duketon belts. Detailed analysis of basal komatiitic lava flow field facies in the Agnew-Wiluna greenstone belt indicates further nickel potential. Potential for VMS base metal deposits in the felsic volcanics of the area is limited both by the shallow-water host depositional environment, and by the lack of the LIL(large ion lithiphile)-enriched type rhyolite and dacite suite associated with VMS mineralization in the Teutonic Bore deposit. In the Bangemall Basin, the combination of structural and stratigraphic aspects of the basin development have contributed to the distribution of fault-fissure related copper mineralization and to the potential for complex hydrothermal systems of the type responsible for the enigmatic, clastic-hosted Abra deposit.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Towmville, July 1998
COMPARISON OF THE CENTRAL ANDES WITH THE SILURIAN-DEVONIAN LACHLAN FOLD BELT, SOUTHEASTERN AUSTRALIA Christopher L. Fergusson School of Geosciences, University of Wollongong, NSW 2522, Australia
One of the most puzzling features of the Lachlan Fold Belt is the exceptional width of the zone of intense deformation, which is over 700 km in an east-west extent across the Victorian sector of the fold belt. This fact alone has even led some to question the role of plate tectonics in the evolution of the belt. Most convergent margins are considerably narrower than the Lachlan Fold Belt that in any case does not contain the full spectrum of forearc elements. The Central Andes is an example of a convergent margin where the system is up to 800 km across from the coast inland. A foreland fold-thrust belt with east-verging thin-skinned thrusting is exposed at the eastern part of the Central Andes in Bolivia. In the Altiplano and eastern Cordillera of the Central Andes, west-vergent thrusts are well developed and synthetic to the Andean subduction zone. Cainozoic deformation has been accompanied by significant tectonic erosion in the forearc as is well documented off the coast of Peru. The extent of deformation in the Central Andes is reminiscent of that in the Lachlan Fold Belt although there are still many significant differences between these systems. The Central Andes has developed within crust of normal continental thickness and Precambrian crystalline basement probably underlies much of the belt. Cainozoic tectonics in the Central Andes has resulted in a present-day crustal thickness up to double normal continental crust. In contrast the Silurian crust of southeastern Australia developed from a type of Cambrian oceanic crust (infant arc crust of normal oceanic thickness) and crustal thicknesses probably never far exceeded normal continental crust. The widely developed shallow marine environments that occurred in the Silurian-Devonian interval in the Lachlan Fold Belt show this. In contrast to the Central Andes, no antithetic Silurian-Carboniferous foreland fold-thrust belt occurs along the craton-ward margin of the Lachlan Fold Belt. It has been suggested that remnants of the subduction zone associated with the Lachlan Fold Belt in the SilurianDevonian interval may be preserved in the New England Fold Belt. Evidence for subduction in the Early Palaeozoic of the southern New England Fold Belt is indicated by the presence of Cambrian supra-subduction zone ophiolites, Ordovician blueschists and a Cambrian-Ordovician subduction-related volcanic succession. In situ Silurian - Early Devonian rocks appear to be largely absent from the southern New England Fold Belt, but a well developed mid to Late Devonian subduction-related volcanic and marine succession is present (Tamworth Group). The existence of Ordovician blueschists in the western part of the accretionary subduction complex is taken as evidence that the subduction zone has been in more-or-less its present tectonic location. By the Early Carboniferous major growth of the subduction complex was occurring. No Silurian to mid Devonian subduction complex is recognised, although Silurian rocks are incorporated in the Late Devonian - Carboniferous subduction complex. The lack of a subduction complex of Silurian to mid Devonian age, and even the lack of volcanic rocks and their derivatives of this age, could reflect tectonic erosion occurring along the subduction zone. In this respect tectonic erosion was probably synchronous with major deformation and crustal development in the Lachlan Fold Belt as has been the case for the Cainozoic development of the Central Andes. The tectonic development of the Lachlan Fold Belt in the Silurian to mid-Devonian reflects a convergent margin setting with widespread contractional deformation. At the same time much of the Lachlan Fold Belt was characterised by basinal deposition with deep- to shallow-marine and subaerial settings with accompanying silicic to mafic volcanism. Deep-water sedimentation occurred in the volcanic Hill End Trough and the nonvolcanic Melbourne Trough with areas of deep-water sedimentation in the Cobar Basin and elsewhere. In contrast, the Cainozoic history of the Altiplano in the Central Andes was dominated by fluvial sedimentation accompanied by silicic (ignimbrites) and mafic volcanism. The settings in both the Silurian-Devonian Lachlan Fold Belt and the Cainozoic Central Andes are considered similar in that they both represent sedimentation during crustal thickening rather than crustal extension. Many authors have inferred that significant extension must have occurred during the Silurian - Early Devonian history of the Lachlan Fold Belt despite its well documented orogenic history. At the broadest scale deformation and sedimentation have been synchronous in both the Lachlan Fold Belt ("Lachlan Orogeny") and the Central Andes. Within regions, such as eastern Victoria and elsewhere, distinct phases of deformation are recognisable and attributable to the traditional orogenic scheme (e.g. Benambran, Bindian, and Tabberabberan). Deformation patterns in the Lachlan Fold Belt are complex but are dominated by intense to moderate shortening with abundant upright folds and associated fault systems. Significant parts of the Lachlan Fold Belt have an easterly vergence (western Victoria, eastern New South Wales) which would have been synthetic to the adjacent subduction zone as occurs in the Central Andes. 140
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
NEOPROTEROZOIC AGE FOR DEFORMATION AND METAMORPHISM IN THE ANAKIEINLIER, CENTRAL QUEENSLAND: IMPLICATIONS FOR OPENING OF THE PACIFIC OCEAN AND TIMING OF THE DELAMERIAN-ROSS OROGENY Christopher L. Fergusson\ Tim J. G^een^ C. Mark Fanning^, Paul F. Carr', Ian W. Withnall^ and Simon B.S. Crouch^ ^School of Geosciences, University of Wollongong, NSW 2522 -Research School of Earth Sciences, The Australian National University, GPO Box 4, Canberra, ACT 2601 ^Geological Survey of Queensland, GPO Box 194, Brisbane Queensland 4001
In central and southern Queensland a major part of the Tasmanides, the Thomson Fold Belt is largely overlain by a cover consisting of the Mesozoic Great Australian Basin and underlying Middle to Late Palaeozoic sedimentary basins. Significant exposures of the Thomson Fold Belt occur in the Anakie Inlier of central Queensland. A jointmapping project in collaboration with mineral dating of metamorphic monazite at RSES using the SHRIMP has established that the Anakie Metamorphic Group is of Neoproterozoic age. Herein, we discuss implications of the new understanding of Neoproterozoic deformation in the Anakie Inlier. The Anakie Metamorphic Group consists of pelitic, psammitic and graphitic schist, greenstone and quartzite, with less abundant marble, calc-silicate rock and serpentinite. A Neoproterozoic depositional age has been inferred for the Anakie Metamorphic Group on the basis that deposition must have predated Neoproterozoic deformation and metamorphism. Lithological layering is the only widely developed primary feature apart from rare planar lamination, graded bedding, and relict sedimentary and igneous textures. Due to the extent of pervasive deformation, and the elimination of nearly all sedimentary structures needed to determine younging, stratigraphic relationships have not been determined and stratigraphic thicknesses are not well established. The association of rock types and the widespread lithological layering is consistent with the precursor of the Anakie Metamorphic Group being a predominantiy sedimentary succession composed of shale and quartz-rich to lithic sandstone with some intercalated mafic volcanic rocks and/or shallow intrusions. The extent of deformation and metamorphism precludes determination of depositional environments, but a rift-related deep-marine continental margin setting similar to that inferred for parts of the Adelaide Trough seems the most feasible setting. Three deformations are recognised within the Anakie Metamorphic Group; the most intense is D2 that has largely obscured Di. Di developed a strong Si defined by a mineral preferred orientation. Fi are only recognised as complex refolded hinges on the map-scale. The D2 deformation formed the main schistosity in the rocks and is gendy dipping over much of the Anakie Inlier. Abundant tight folds occur at micro- to macroscopic scales. Mineral lineations and pressure fringes on magnetite indicate east-northeast stretching. The regional low-angle nature of D2 is a common feature of deeper levels in many orogenic belts and is commonly attributed to a thrust-type convergent setting. D3 has caused significant reorientation of D2 structures and formed upright dome and basin patterns evident in lithological units throughout the inlier with northeasterly to easterly, broad to open folds. Metamorphism in the Anakie Metamorphic Group is mainly of greenschist facies with pervasive recrystallisation. Mineral assemblages include biotite-muscovite-chlorite in schist and epidote-actinolite in greenstone. A belt of amphibolite facies with andalusite-gamet and traces of staurolite occurs in the far-western part of the inlier (Eastern Creek) to the west of Clermont. The staurolite-bearing rocks preserve relics of locally higher-grade metamorphism that has been almost obliterated by the pervasive greenschist to amphibolite event associated with the intense second deformation. SHRIMP U-Pb monazite ages from these rocks appear to indicate two peaks of metamorphism. An older higher temperature (> 600°C) metamorphic peak occurred at 583±10 Ma which we interpret as the time of formation of most monazite grains. We associate this event with the higher-grade metamorphism and the Di deformation. Some monazite grains record an -540 Ma age and our interpretation is that this is the timing of the major D2 event and associated metamorphism. Previously published K/Ar muscovite analyses record an age of -500 Ma that we regard as cooling of the D2 event with some older K/Ar ages reflecting the longer duration of this metamorphic event. This new interpretation has significance for two major tectonic problems. First, it indicates that convergence involving regional deformation and metamorphism was active at 580 Ma along the Palaeo-Pacific continental margin of Gondwanaland. This implies that the Pacific Ocean was undergoing subduction at this time and is consistent with the suggestion that the Palaeo-Pacific Ocean formed by breakup of Rodinia at -700 Ma as inferred from the Delamerian Fold Belt in South Australia. Second, it indicates that orogenic events in northeastern Australia, which were previously correlated with the Delamerian Orogeny, extend over a similar time range to that of the Ross Orogeny in Antarctica.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
FACIES ARCHITECTURE OF THE UPPER BURDEKIN RIVER OF NORTH QUEENSLAND, AS REVEALED BY GROUND-PENETRATING RADAR Christopher R. Fielding'. Jan Alexander^ and Robert McDonald^ 'Department of Earth Sciences, University of Queensland, Qld 4072, Australia ^School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK ^Terradat Geophysics UK Ltd, PO Box 319, Cardiff CFl 3UJ, UK
The Burdekin River is one of a class of streams draining sub-humid, tropical parts of northeastern Australia that are characterised by up to 2-4 orders of magnitude fluctuations in discharge. In the study area near Charters Towers, discharge is 0-20 m^ s'^ through most of any year, but rises rapidly to peak discharges of >20000 m^ s'\ Such major flow events are triggered by rainfall from tropical cyclones that form typically between January and April of any year, but which are otherwise highly unpredictable. Until recently, these rivers have been largely ignored, and their behaviour and deposits are poorly documented. Here we present some of the results of a major, inter-disciplinary study of a reach of the Burdekin River, and concentrate on the three-dimensional lithofacies architecture of the channel deposits across three bends of the river (Dahymple, Brigalow and Big Bend). The distribution, texture, composition and bedforms of surface sediment were mapped. Topography was surveyed following successive flow events to assess channel bed changes using a Geodimeter Total Station. Subsurface deposits were examined by drilling, in natural exposures, cutbanks and gully walls, and in shallow pits and trenches. Subsurface facies architecture was also investigated by two Ground-Penetrating Radar (GPR) surveys. The river is typically 300-400 m wide and 5-25 m deep in the study reach, and during much of any year flow is restricted to a narrow, misfit channel, leaving most of the river bed exposed. The river bed can be divided into areas of sand and gravel flats, and bars of varying geometry and size (including point, lateral and braid bar forms). Within the channel, alluvium is up to 10 m thick, and rests on an irregular basement surface. The lower bar and bed deposits are composed of very well sorted, coarse to very coarse sand, and gravel, arranged in erosively bounded, cross-bedded stratasets. Red/brown mud layers up to 0.1 m thick are preserved locally within these sediments. Low-lying parts of the river bed become colonised by linear groves of the paperbark tree Melaleuca argentea, which withstand high stage flows, and nucleate bar growth. In situ, reclined Melaleuca trunks are evident both at the subsurface and in the subsurface. The surface of the upper parts of bars and banks are silt to very fme sands with ripple cross-lamination and abundant comminuted plant debris. The large-scale architecture of three bends as revealed by GPR is grossly comparable to the point bar studied by Bridge and others (Sedimentology 42, 1995, 839-852), but there are important differences. The distribution of sedimentary structures as interpreted from radar facies shows a strong zonation relative to position on the point bar. Adjacent to the thalweg, deep, narrow scours are filled by large-scale cross-bedding that passes updip into planar stratification with lesser, smaller-scale cross-bedding. Comparison between 1996 and 1997 surveys indicates that the bankfull event of March 1997 involved wholesale re-organisation of the alluvium, with scouring at least locally to the bedrock interface. The 1997 record shows a transition upward through the section from large-scale cross-bedded strata into planar-bedded sediments, whereas the 1996 section shows a prominent, laterally persistent truncation surface separating large-scale cross-bedded facies from the overlying facies, the upper strata interpreted to reflect reworking of the top section by successive, more modest flow events between the previous bankful event (March 1991) and March 1997. The lower bar locally shows development of unit bars, which are internally composed of trough cross-bedded strata truncated by a surface-parallel, concordant radar facies that is interpreted as arising from reworking. An inner bank bench at Big Bend has a similar internal structure to the unit bar preserved in the same area. The upper bar is draped by a surface-parallel, concordant radar facies that corresponds to interlayered silt/very fine sands and coarser sand beds and which drapes down across the inner bank bench at Big Bend. Significantly, although dipping reflectors recording lateral accretion are preserved within the alluvium, their low angle of inclination and irregularity would make identification in the rock record difficult. The possibility therefore exists that deposits of rivers such as the Burdekin preserved in the ancient might be misinterpreted as braided river facies.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
SURFACE EXPOSURE DATING AND EROSION STUDIES USING COSMOGENIC RADIOISOTOPES, AT THE ANSTO ANTARES AMS FACILITY David Fink. Greg Elliot and David Child ANTARES-AMS, PHYSICS DIVISION, AUSTRALIAN NUCLEAR SCIENCE AND TECHNOLOGY ORGANISATION, PMB 1, Menai, NSW, 2234, Australia.
Earth scientists have long sought an analytical technique based on radiometric methods that would quantify both temporally and spatially, the chronology of surface landforms and associated denundation rates. Such a technique is now developing based on the in-situ production of long-lived cosmogenic radioisotopes ^^e (Ti/2=1.5Ma), ^^Al (O.TMa) and^^Cl (0.3Ma) by cosmic ray bombardment of exposed rocks, surfaces and within the first meter or so of the Earth's crust. Production of these cosmogenic radionuclides (CRN) takes place predominately in the upper atmosphere. In contrast, the rate of in-situ production in surface rocks is extremely low - a few tens of atoms per gram per year. However, the ultrasensitive technique of Accelerator Mass Spectrometry (AMS) (Tuniz et al 1998), with a detection limit of 100,000 atoms, can now be used to readily measure this telltale signal. Meaningful measurements can be made with as little as --100 grams of rock sample exposed at sea-level for times as short as a 5-10 ky. The build up over time of in-situ radioisotope concentrations can be utilised as radiometric clocks to elucidate an "exposure history" of a rock surface or landform (Nishiizumi et al, 1993). Parameters characterising the exposure history are effective exposure and burial age, erosion rate, depositional and uplift rate. Alternatively, if exposure has been sufficiently long for the in-situ signal to reach equilibrium, only an estimate of the long-term erosion rate (averaged over a few half-lives) can be determined (Bierman, 1994). Generally, the method is applicable for exposure periods upto a few million years and erosion rates of the order of 0.1 to 10 mm/ka. This in-situ model has been applied in a variety of geomorphic contexts: glacially polished bedrock and moraine boulders, meteorite impact craters, alluvial fans, beach terraces, volcanic eruptions, desert sands and paleoseismic events.. In parallel with a successful ^^C AMS program, routine measurements of ^^Al, and ^^Cl have been demonstrated at the ANTARES AMS facility at the Australian Nuclear Science and Technology Organisation (ANSTO). With this capability, ANSTO is co-ordinating a comprehensive program in the application of in-situ CRNs within our region which is based on strong universit}/ collaborations in the Earth Sciences and with the Australian Antarctic Division. Projects in the study of^Quatemary glacial chronology in the Southern Hemisphere target three regions exhibiting distinct glacial features - Tasmania (Central Plateau and West Coast Ranges), Antarctica (Larsemann Hills and Prince Charles Mts.) and New Zealand (Southern Alps, Fiordland and Mt. Ruapehu). As the antiquity of the Australian landscape generally precludes the direct application of in-situ exposure dating, we have focused our investigations here to a study of estimating erosion rates of tectonically stable surfaces, desert sand-dune mobility and turnover rates, and formation processes of coastal escarpments and flared granite inselbergs. Where applicable, thermoluminescence ages and apatite fission track dendundation rates will be used to compliment cosmogenic in-situ results. A fully equipped geochemistry laboratory, funded by ANSTO, for the difficult task of sample processing and preparation of AMS targets has been completed and is operated by two geochemists. The presentation will briefly outline principles of the in-situ cosmogenic model, describe operations at the ANTARES AMS facility and give our first results of ^^Cl and exposure ages in Tasmania. It will conclude with a pictorial view of some sampling sites. REFERENCES Tuniz, C., Bird R., Fink D. and Herzog G, 1998, ''Accelerator Mass Spectrometry: Ultrasensitive Analysis for Global Science", CRC-Press, Florida, USA Nishiizumi,K., Kohl,C.P., Shoemaker, E.M., Amold,J.R., Klein,J., Fink,D., Middleton,R., Lal,D., 1993. Role of in-situ cosmogenic nuclides and ^^Al in the study of diverse geomorphic processes. Earth Surface Processes and Landforms 18, 407. Bierman,P.R., 1994.Using in-situ produced cosmogenic isotopes to estimate rates of landscape evolution : A review from the geomorphic perspective, J.Geophys.Res 99(B13), 885.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
CRUSTAL ARCHITECTURE IN THE EASTERN LACHLAN OROGEN FROM DEEP SEISMIC PROFILING AT WIDE-ANGLE AND NEAR-VERTICAL INCIDENCE p. M. Finlavson^ l J. H. Leven^ ^ D. W. Johnstone^•^ R. J. Korsch^'^ & R. A.Glen^'^ ^Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601 ^Australian Geodynamics Cooperative Research Centre ^NSW Geological Survey, PO Box 536, St. Leonards NSW 2066
Although there is general agreement that the Lachlan Orogen developed into continental crust in eastern Gondwana during the early Palaeozoic, there is still debate about how it evolved and what now forms the substrate to the exposed rocks. The orogen has been described as forming at a convergent continental margin from mid-Cambrian to Early Silurian times, before being subjected to back-arc extension, felsic volcanism, and granite emplacement until the mid-Devonian. Recent geochemical data favours an intra-oceanic island strc setting for Lachlan Ordovician volcanics resting on a ?Cambrian oceanic crust and some authors favour a model for the whole Lachlan Orogen related to (possibly multiple?) subduction zones at the margins of eastern Gondwanaland. Other authors have suggested hot-spot volcanism as the mechanism for the Ordovician volcanics. Others, using geochemical arguments, prefer a model involving delayed heating, melting and thinning connected with a subcontinental lithosphere. An Andean model has also been proposed as a possible tectonic analogue for eastern Lachlan Palaeozoic history. The need to constrain tectonic models and evolutionary history within the Lachlan Orogen has become more important as exploration fof world-class ore deposits in the Ordovician volcanics continues. Seismic profiling techniques provide some of the 3-D images required to constrain tectonic models at a scale appropriate for geological mapping. Such methods were applied during 1997 to examine crustal architecture of the eastern Lachlan Orogen both across and parallel to the regional meridional grain in the region around Orange. Both near-normal incidence and wide-angle seismic reflection profiling methods were used. The WNW trending LTZ is thought to be a significant Ordovician-Silurian crustal-scale feature, possibly influenced orebody development. The 105 km of reflection profiling (3 lines) investigated N-S trending structures from the Cowra Trough eastwards onto the Ordovician Molong Volcanic Belt and into the mid Silurian-Devonian Hill End Trough. It also investigated WNW trending structures associated with the Lachlan Transverse Zone (LTZ), an Ordovician-Silurian crustal structure that influenced orebody localisation. Wide-angle seismic methods investigated the P-wave velocity (compositional) differences throughout the crust along a 350 km N-S line from the Ordovician sediments and Siluro-Devonian granites in the south to the Ordovician volcanics in the north, within the Molong-Wyangala Structural Zone. The principal seismic phases recorded during the wide-angle profiling included refracted phases through the upper, middle and lower parts of the crust and upper mantle, and reflected phases from intra-crustal and Moho boundaries. Preliminary interpretation of data at six shot sites indicates that there are significant P-wave velocity (compositional) and geological features within the crust. • There are P-wave velocity increases at 8-15 km depth and at 18-22 km depth (??sites of compositional change and/or detachment/decollement surfaces). • The Moho depth is at 43-44 km (orogens world ave. 46 km) with an underiying upper mantle velocity of about 8.02 km/s (orogens world ave. 8.01 km/s). • Upper crustal velocities are 5.92-6.28 km/s at 2-15 km. depth. • Middle crustal velocities are 6.28-6.58 km/s at 15-22 km depth. • Lower crustal velocities are 6.5-7.27 km/s at depths greater than 22 km. • Velocities of 6.5-6.8 km/s (??mafic granulite, amphibolite, diabase, greenschist facies basalt) are not evident at depths of less than -17 km. • Velocities of 6.8 km/s and greater (??anorthosite, gabbro, homblendite, mafic garnet granulite, eclogite) are not evident at depth less than 24 km. • Some data require low velocity zones in the upper and middle crust (??thrust zones, tectonic emplacement). The seismic reflection profiling shows some significant features in the Orange Bathurst region. For example, seismic Line 2 shows prominent reflectors from 1 to 2.5 s two-way time (3-7 km) corresponding to the top of the Ordovician volcanics dipping eastward under the Mumbil Shelf and Hill End Trough or to a detachment surface within the Ordovician volcanics.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE AEROMAGNETIC CONNECTION: A NEW LOOK AT THE GONDWANA GEOLOGY OF ANTARCTICA AND AUSTRALIA Carol Finn^ Detlef Damasked Tim Mackey^ and David Moore^ ^ U. S. Geological Survey, MS 964, Denver Federal Center, Denver, CO 80225 2 Bundesanstalt fur Geowissenschaften und Rohstoffe, Stillweg 2, D-3000, Hannover, 51 GERMANY ^ Australian Geological Survey Organization, GPO Box 378, Canberra, ACT 2601 "^Geological Survey of Victoria, P.O. Box 500, East Melbourne, Victoria 3002
INTRODUCTION The connection of new aeromagnetic data from Victoria Land, Antarctica with existing data from southeastern Australia, provides a new view of the Gondwana geology and plate tectonic setting of the regions. This aeromagnetic connection allows the mapping of previously unidentified sub-ice geologic units in Antarctica and provides constraints on plate tectonic reconstructions and early Paleozoic convergent margin processes of part of eastern Gondwana. MAGNETIC SIGNATURES High-amplitude, broad-wavelength positive anomalies are associated with mafic middle to Late Proterozoic Gawlor igneous rocks buried under Adelaidean sedimentary rocks on the Stuart and Spencer Shelves. The same type of anomalies occur over ice-covered central Victoria Land, and are interpreted to be due to similar rocks. Exposed and buried gabbros and diorites in the vicinity of the Delamerian orogen and Padthaway Ridge east of the Gawlor rocks, produce distinctive linear and circular magnetic highs over a 200-km wide area that can be traced into the Ross orogen, Victoria Land. East of these regions are distinctive linear belts of magnetic highs with amplitudes of approximately 400 nT. In Australia, the northwest-trending belt is 400-km long, 20-40 km wide and lies over the Murray Basin, northwest of the exposed Cambrian oceanic Stavely volcanic rocks. The correlative belt, 100 km long, in Antarctica can be observed trending northwest over the northern part of the Rennick graben and Bowers volcanic rocks which are equivalent to the Stavely. The sources of the highs are not exposed, but drilling in Australia, encountered 20-m thick layers of magnetic volcanic rocks correlated with the Stavely volcanics that crop out further south and very magnetic boninites. In Victoria Land, the exposed 2500 m-thick Bowers volcanic rocks produce circular, high-frequency anomalies with amplitudes of only about 25 nT, similar to those over the exposed Stavely belt. This suggests that the volcanic rocks are not the primary sources of the high-amplitude linear positive anomaly belts in either location. Instead, we interpret that the anomalies are primarily related to the boninites, which may compose ophiolites, obducted fragments of oceanic crust, that may underhe the Bowers and Stavely terranes. East of these highs, over the Stawell, Australia and Robertson Bay, Antarctica, Cambrian to Ordovician turbidites is a magnetic quiet zone broken by narrow, linear magnetic highs with amplitudes of 10-25 nT interpreted to be associated with small slices of mafic and serpentinized oceanic crust commingled with the graywackes. Scattered circular and donut-shaped highs with amplitudes of approximately 40 nT occur over Devonian to Carboniferous intrusions within these domains. CAMBRIAN-ORDOVICIAN PLATE TECTONIC SETTING The characteristic magnetic signatures described above can be used to reconnect Antarctica and Australia into their Gondwana configurations if we assume continuity of these regions during the Paleozoic. The geometry defined by the aeromagnetic data can be used with marine magnetic, satellite gravity and geologic data to refine published plate reconstructions of Gondwana. Comparison of the aeromagnetic signatures from the early Paleozoic Gondwana margin with those characterizing the Jurassic convergent margin of California, provides a framework for models of the Gondwanan tectonic setting. Two general tectonic models can explain the observations from the Gondwana convergent margin: 1) collision tectonics, as defined by the Ross-Delamerian orogenic belts, which formed by collision of an oceanic island arc with the continent and 2) "Cordilleran-type" tectonics, as defined by changes in dip, direction and velocity of a single west-dipping subduction system.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE AGE OF FELSIC MAGMATISM AND LODE-GOLD MINERALISATION EVENTS IN THE LAWLERS AREA, YILGARN CRATON, WESTERN AUSTRALIA I.R. Fletcher, J.A. Mikucki^. N.J. McNaughton, E.J. Mikucki and D.L Groves Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, University of Western Australia, Nedlands 6907 AUSTRALIA ^ Current address: Geological Survey of Western Australia, 100 Plain Street, East Perth 6004
The Archaean Lawlers gold camp in Western Australia contains a number of lode-gold deposits hosted by a range of lithologies, including m^ic metavolcanic, metasedimentary rocks and granitoids. The largest deposit, the Great Eastern, is hosted by the Lawlers Tonalite and younger leucogranites which have been dated at ca. 2665 Ma by zircon and titanite U-Pb geochronology using SHRIMP. Lode-gold mineralisation at the Great Eastern deposit is unusual, compared to most other lode-gold deposits in the Archaean Yilgarn Craton, in that dominant early pyrrhotite-stable shear zone-hosted mineralisation is overprinted by relatively minor hematite-stable mineralisation related to brittle faults at an angle to the mineralised shear zone. Both mineralisation events have produced titanites in proximal alteration assemblages, which have been dated using SHRIMP to yield ages of 2646±25 and 2592±9 Ma, respectively. These results unambiguously indicate that the late mineralisation was not related to magmatism responsible for the granitic host rocks, nor is it likely that the hematite-stable ore fluid evolved from the pyrrhotite-stable fluid during a single hydrothermal event. Further, although the age for the early major mineralisation event is compatible with a craton-wide 2.64-2.63 Ga lode-gold mineralisation event, the younger age indicates significant post-2.63 Ga mineralisation. Some evidence for post-2.63 Ga mineralisation has been inferred from Ar-Ar plateau ages for hydrothermal micas from a number of deposits in the Yilgarn craton, notably from the Mt Charlotte and Wiluna deposits. Although the veracity of these ages as either formation ages or blocking temperate ages is disputed, the 2592 Ma age from the titanites formed during the late mineralisation at Great Eastern is unquestionably related to a discrete younger mineralisation event, and supports recently published evidence from the Big Bell deposit that hydrothermal titanites related to mineralisation formed significantly after the craton-wide main stage of lodegold mineralisation. It is noteworthy, however, that most Yilgarn lode-gold deposits which have reliable isotope age determinations younger than 2.63 Ga are seen to overprint older mineralisation (eg. Mt Charlotte and Golden Mile, Big Bell, Lawlers). The possibility that all these younger mineralisations were formed by remobilisation of gold from pre-existing mineralisation must be seriously considered. Acknowledgements: Special thanks go to Marion Dahl for sample preparation, and Jo Cannon, Merryl Jones and Staff from the former Forsyath Mining Services and Plutonic Resources for logistical support. Zircon and titanite analyses were carried out on a Sensitive High Resolution Ion Micro Probe mass spectrometer (SHRIMP II) operated by a consortium consisting of Curtin University of Technology, the Geological Survey of Western Australia and the University of Western Australia with the support of the Australian Research Council.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
RECOGNITION OF LIMESTONE-CAPPED SEAMOUNTS (EXOTIC TERRANES) WITHIN ACCRETIONARY PRISMS FORMED ALONG THE LATE PALAEOZOIC CONVERGENT MARGIN OF GONDWANA., EASTERN AUSTRALIA Peter G. Flood Earth Sciences Division, University of New England, Armidale NSW 2351
Limestone-capped basic metavolcanics are preserved within the accretionary prisms of the Mid to Late Palaeozoic Hodgkinson and the Late Palaeozoic New England Orogen of eastern Australia. These limestones range in age from Ordovician to Late Carboniferous age. The metavolcanics underlying the limestones display igneous flow textures, variable degree of crystallization, partial chloritization, moderate degree of metamorphism to greenschist facies assemblages, and geochemical and isotopic signatures consistent with an origin related to intra-plate, hot spot, oceanic-island basalts (OIBs). A combination of palaeomagnetic, sedimentological, and faunal evidence indicate that these limestone-capped seamounts formed in a near-equitorial latitude. Some may have travelled many thousands of kilometres before being incorporated within the convergent margin of Gondwana possibly when this landmass was a high latitudes. Such ancient seamounts may be considered exotic terranes. Not only do they provide a temporal constraint on the age of accretion (which is always younger than the ages of radiolarian-bearing cherts, but older than the ages of radiolarian-bearing tuffaceous sediments within the accretionary wedge) they also provide an opportunity to decipher part of the history of hot-spot activity within the Panthalassa Ocean.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE SEARCH, DISCOVERY AND DEVELOPMENT OF AUSTRALIAN Ca-TYPE CLINOPTILOLITE: A DECADE OF PROGRESS p. G. Flood Division of Earth Sciences, University of New England, Armidale NSW 2351, Australia pflood@metz.une.edu.au
The occurrence of natural zeolites in altered Carboniferous volcaniclastic sediments of eastern Australia was first recorded in 1958 (Coombs, 1958); however, it has only been since 1987 (Flood, 1987) that exploration companies have been searching in Australia for economic deposits of zeolites. To date, the author has delineated two major zeolite provinces, namely: The New England zeolite province in New South Wales (Flood, 1991), and The Drummond zeolite province in central Queensland (Flood, 1995). Both provinces are closely related (fore-arc and back-arc, respectively) to an ancient (Late Devonian-Early Carboniferous) "Andean"-margin continental volcanic arc. The zeolitic volcaniclastic sedimentary rocks and pyroclastic rocks contain as much as 80% Ca-type clinoptilolite and minor mordenite (Flood, 1995). Hie cation-exchange capacity of the naturally occurring zeolite-bearing rocks generally averages 1.5 meq/g, and stratigraphic thicknesses of 4 to 16 m are common. Several occurrences contain resources in excess of five million tonnes and conmiercial development of six deposits (two in New South Wales and four in Queensland) is underway. Annual production from one deposit now approaches 6 000 tonnes. Run-of-mine material is crushed and screened to a variety of size fractions governed by end-use, such as soil conditioning, odour absorption, stock-feed additives, sewage-ammonia reduction, heavy metal reduction, and water filtration (Flood et al., 1993). The potential of both zeolite provinces is considerable. Volcaniclastic sedimentary rocks constitute a new type of exploration target for economic occurrences of natural zeolites. REFERENCES Coombs, D. S. (1958) Zeolitized tuffs from the Kuttung glacial beds near Seaham, New South Wales: Aust. J, Sci, 21: 18-19. Rood, P. G. (1987) Occurrence and nature of zeolitic tuffs within the Late Carboniferous Currabubula Formation, Werrie Syncline, New South Wales: Geol Surv. NSW Sydney Report 1987/146: 75-81. Flood, P. G. (1991) Prospecting for natural zeolites: An exploration model based upon an occurrence in 300 Ma old rocks near Werris Creek, New South Wales: Trans. Inst. Min. Metall (Sect. B: Appl. Earth Sci.) 100: B9-B13. Flood, P. G. (1995) Australian occurrences of Ca-type Clinoptilolite. In Ming, D.W., Mumpton, F A. (eds) Natural Zeolites '93. Int. Comm.Natural Zeolites, Brockport, New York: pp. 15-21. Flood, P. G., Marx, W. T., Roberts, C. L. (1993) Zeolite production from the Escott mine of Mount Gipps Limited, Werris Creek, NSW. In Woodcock, J.T., Hamilton, J.K. (eds) The Sir Maurice Mawby Memorial Volume. Aust. Inst. Mining Metallurgy Monograph 19: pp. 1431-1434.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
SEISMIC STRATIGRAPHY OF THE STANSBURY BASIN - A HIDDEN FORELAND BASIN OF THE DELAMERIAN OROGEN? GEODYNAMIC AND ECONOMIC IMPLICATIONS. Thomas Flottmann & Peter W. Haines Dept of Geology & Geophysics, University of Adelaide, Adelaide S A 5005 Dept of Applied Geology, University of South Australia, Levels Campus, Mawson Lakes SA 5095
The western margin of the Delamerian Orogen in South Australia is formed by a well recognized fold-thrust belt which presumably provided a significant flexural load. Despite this, the lack of recognised foreland basin in previous studies of the region is somewhat anomalous. Seismic reflection surveys suggest that the Gulf St Vincent area of the Stansbury Basin is filled with four prePermian sedimentary packages. The oldest package is probably of Neoproterozoic age, while the overlying three packages (S0-S2 in ascending order) are interpreted as early Palaeozoic rocks. These inferred early Palaeozoic packages reach a total thickness of up to 6000 m in the east, but are less than 500 m thick in the west. In particular, the middle package (SI) tapers distinctly westward and northwestward. These successions show little internal deformation but are separated by faults from the highly deformed Delamerian Orogen to the east and are also faulted against Early to Middle Cambrian strata that crop out on Yorke Peninsula to the west. Early Palaeozoic reverse movement along faults that may have originated as growth faults during deposition of SO outiasted deposition of package SI. Internal onlap relationships suggest a westward migration of the depocentre of package SI through time. Package S2 is not affected by Delamerian deformation and only shows imprints of Cainozoic deformation, which also affects the overlying Permian and Cainozoic sediments. We interpret the lowest Palaeozoic package (SO) as an equivalent to the Early Cambrian Normanville Group to the east of Gulf St Vincent. The middle and upper packages (SI & S2) are interpreted as deposits of a hitherto unrecognised foreland basin to the Delamerian Orogen and are interpreted to be of Cambrian to Ordovician age. This interpretation suggests that a portion of sediment shed off the encroaching Ross-Delamerian orogen was deposited into a (comparatively small) foreland basin to the west of the orogen. Significant proportions of the eroded sediment were, however, also deposited to the east of the orogen in basins that were later incorporated into the Lachlan Fold Belt. The foreland basin interpretation provides a range of new potential petroleum plays in the Gulf St Vincent region, which is currently at a very early stage of being explored by offshore reconnaissance drilling.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
DEEP SEISMIC PROFILING IN THE BROKEN HILL REGION: IMPLICATIONS FOR STRUCTURAL GEOLOGY IN THE WILLYAMA SUPERGROUP Tanya Fomin. George Gibson, Andrew Owen, Barry Drummond, Kevin Wake-Dyster and David Maidment Australian Geological Survey Organisation, PO Box 378, Canberra, ACT 2601
Seismic reflection data in the Broken Hill region were recorded by AGSO in 1996. The survey of about 130 km total length was designed to improve understanding of (1) the structural geology of the area and (2) the relationships between the main geological features, and thence to provide a new perspective for mineral exploration in the Paleoproterozoic Willyama Supergroup. The results of this survey combined with new structural mapping enabled a fundamental reappraisal of the geological evolutionary model for the region. Several blocks with different reflection patterns were delineated in the reflection data. The seismic character of the Moho varies along the line. On average the thickness of the crust is ~ 40 km. In the western part of the seismic line, a - 3 km thick zone of high reflectivity is recognised at the base of the crust while in the eastern part of the line (corresponding to the central block due to its SE dip) this zone is twice to thick. The central block is characterised by very high reflectivity throughout the whole crust. Strong continuous reflectors correspond to known geological structures such as Mundi Mundi Fault, Mt Frank and Stephen Creek Shear zones. Generally the whole block between the Mundi Mundi Fault and Stephen Creek Shear Zone dips to the south-east and represents a very broad shear zone. The characteristic geometry of fault zones within the central block is repeated. This repeating geometry incorporates both major and minor faults as well as splays off each and running parallel to the main fault at greater depth. The central block is a very good example of seismic imaging of geological structures. The seismic image of the crust to the west from Mundi Mundi Fault and to the east from Stephens Creek Shear Zone is different. Weak reflectivity with flat short reflectors typifies the western block. An apparent rift geometry with rotated fault blocks and asymmetrical sedimentary basins is recognised in the eastern block. Also evident in the eastern part of the section are several sedimentary basins of Devonian or younger age. Several minor NW dipping faults, which seem to have affected the sediments, are obvious in seismic data. This implies that these minor faults were originated later then the main SE-dipping faults and they are syn- or post-depositional. The seismic reflection data do not map the NW-dipping structures well. Several zones of sub-horizontal reflectors were recognised from the seismic data. We interpreted these zones as either sub-horizontal detachments at -10 and -18-24 km depth levels or as those areas where the major shear zones flatten out at depth into major regional decollements. The most significant outcome of the seismic reflection survey is that several major shear zones were imaged the majority of which dip south-east. The seismic data do not constrain the age of the fault zones direcdy but they characterise shapes and most importantly depths of penetration of shear zones. Some of these zones penetrate through the whole crust while others extend to the middle crust only; given their depth of penetration many shear zones may have conducted fluids from deeper levels to the shallow levels and this is an important implication for mineral exploration. Structural mapping indicates that many of the shear zones imaged by the seismic data originated during high-grade metamorphism accompanying the Mesoproterozoic Olarian orogeny. Some of these zones probably formed during the break-up of Rodinia in the Late Proterozoic. These latter ones may have been reactivated at that time or at some later stage of the geological evolution of the region which subsequently underwent the early Palaeozoic Delamerian and the late Palaeozoic Alice Springs orogenies (Gibson et al., 1998).
REFERENCES Gibson, G.; Drummond, B.; Fomin, T.; Owen, A.; Maidment, D.; Wake-Dyster, K. (1998). Structural and tectonic evolution of the Broken Hill region revisited: implications for mineral exploration in the Willyama Supergroup. In: Finlayson D.M. & Jones L. eds. An AGCRC Symposium on Mineral Systems and the Crust Upper Mande of SE Australia, Extended Abstracts. Australian Geological Survey Organisation Record 1998/2(in press).
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
BASIN HISTORY OF THE YARROL BLOCK: STEADY TECTONIC SUBSIDENCE AND REPEATED FOREARC DEVELOPMENT FROM SILURIAN TO PERMIAN Banv G. Fordham. Cecil G. Murray, Mark A. Hayward, Glenn A. Simpson and Paul R. Blake Geological Survey of Queensland, GPO Box 194, Brisbane, Queensland 4001
With conodont dates now available through much of the pre-Namurian of the Yarrol Block\ the stratigraphic control readily available from, in particular, the Carboniferous to Permian can be extended further back in time. In addition, remapping has resulted not only in a stratigraphic scheme applicable throughout the Block but also in more-accurate estimates of stratigraphic thicknesses for the sequence. An ahnost fully marine history for the Yarrol Block can be traced most easily from the Upper Silurian to Carboniferous of the north-west portion (Mount Morgan and Craigilee areas) and from the Carboniferous to Permian preserved in the east in the Yarrol Syncline. This 150 m. y. period accumulated some 13 000 m of volcanosedimentary and carbonate rocks corresponding to 5 000 m of tectonic subsidence (calculations indicative only, based on values generalised to whole units). A major problem in proceeding with analysis of this history concerns the interpretation of the Upper Carboniferous-Lower Permian interval in the Yarrol Syncline. Reexamination of this sequence has, following on from the work of John Roberts, failed to locate evidence there of the regional hiatus found elsewhere in the marine of eastern Australia but, on the contrary, gathered further support for an uninterrupted sequence in the Syncline. The resultant overall pattern for the Late Silurian to Permian of the Block is of regular tectonic subsidence throughout. Although tectonic subsidence appears to have been faster prior to the Carboniferous, the subsequent rates are only marginally lower. This result is somewhat surprising given the rapid deepening implied by the considerable thicknesses previously accorded Silurian-Devonian units such as the Mount Holly beds. However, the thicknesses and depths of deposition of these units appear to have been overstated and, in fact, appear to be similar to those used herein from the north-west of the Block. On closer examination, the history consists of successive episodes of some 30-45 m. y. duration: Late SilurianMiddle Devonian; Late Devonian-very early Toumaisian; early Toumaisian-Visean; and Namurian-Permian. Each episode begins with moderately fast tectonic subsidence, tailing off with increasingly extensive carbonate deposition, though the deceleration in subsidence is highly variable between episodes. The final NamurianPermian episode appears to have been abruptly terminated. Given the moderate rates of tectonic subsidence recorded for each episode, the entire Late Silurian-Permian history of the marine portion of the Yarrol Block would appear to be the sequential development of forearc basins—^without any indication of crustal thinning or, on the other hand, sustained tectonic stability. In the western part of the Block there is ample evidence of erosion and terrestrial deposition in the latter part of this history. Dating control of this facies (the Youlambie Conglomerate) is poor but reexamination of its contacts with underlying post-Visean marine rocks suggests these changes in facies were gradational. Also, the marked variation in relative thicknesses of these facies in a few sections where both are preserved suggest this change to have been highly diachronous. This implies that there was a sustained period of increasingly extensive subaerial exposure in the western Yarrol Block from the Namurian to the Early Permian. After this, tectonic subsidence similar to that maintained in marine parts of the Block appears to have restarted.
' term for the Paleozoic of the Yarrol Province west of the Yarrol Fault
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 M"" Australian Geological Convention, Townsville, July 1998
CHRONOLOGY OF DEFORMATION IN THE LACHLAN FOLD BELT: IMPLICATIONS FOR THE TECTONIC EVOLUTION OF EASTERN AUSTRALIA David A. Foster^ and David R. Gray^ Australian Geodynamics Cooperative Research Centre, 'Department of Earth Sciences, La Trobe University, Melbourne Victoria 3083; and Department of Geology, University of Florida, Gainesville Florida 32611 USA (d.foster@latrobe.edu.au) ^Department of Earth Sciences, Monash University, Melbourne Victoria 3168, Australia
Ar-Ar data from fabric-forming white mica, strongly cleaved slates and phyllites, and micas from granitic mylonites define the timing of metamorphism, deformation and exhumation in the Lachlan fold belt (LFB). Insights gained from these data help constrain the tectonic setting of the Tasman orogen during Palaeozoic time. In the western LFB the result indicate that major deformation ranges in age from 460-420 Ma, in the Stawell and Bendigo-Ballarat zones, and 410-390 Ma in the Melbourne zone, and this deformation migrated eastward. In the central LFB deformation in Tabberaberra zone migrated southwestward from -430-405 Ma, and occurred between >430 and 400 Ma in the Wagga-Omeo complex, with the younger phases related to exhumation. In the northern part of this belt -400 Ma ages record deformation in the Cobar basin. In the eastern LFB Ar-Ar mica dates range from 450-340 Ma. In this belt, ages of 450-440 Ma are yielded from the Narooma complex, 405-390 Ma ages are found along the major thrust faults bounding the higher-grade metamorphic complexes, and 360-340 Ma ages are found in the Hill End and related zones. Results also highlight periods of reactivation and overprinting defprmation. Detailed data from most of the fault zones of the western LFB allows the style and rate of deformation to be defined. Turbidites overlying oceanic crust were deformed by early folding over an eastward-propagating decollement/melange zone. This phase was followed by structural thickening as folds tighten leading eventually to thrusting within zones of intensified deformation. This chronology requires that fabrics were forming in the decollement/melange zone while sedimentation was ongoing in the hanging wall. This style of deformation is characteristic of modem and Cenozoic accretionary wedges where deformation initiates in the decollemont zone beneath a growing sediment prism, followed by wedge thickening due to folding, thrusting, and underplating of underthrusted slices. Our results are consistent with an interpretation where the various belts of the LFB developed within and along the margins of a small Palaeozoic oceanic basin-arc-accretionary complex system along the margin of Gondwana, similar to the present western Pacific region. Accretion occurred by closing of the basin system by subduction-accretion processes and some translation. Classic sutures are not common in these settings, but are represented by major fault and melange zones were deeper level rocks and slices of oceanic crust are exposed. The western LFB is an eastward migrating accretionary complex that developed against a back-stop of the Delamerian orogen between 460 and 390 Ma. The central LFB comprises: 1. a higher grade metamorphic and plutonic complex formed by Ordovician-Silurian metamorphism perhaps due to ridge subduction in a forearc setting, and exhumed at -405-400 Ma due to southward thrusting and erosion, and 2. a Silurian accretionary complex in the southwest. The eastern LFB developed as an Ordovician accretionary complex and arc system that was rifted in the Silurian, contracted in the Late Silurian-Early Devonian, and reactivated at 360-340 Ma.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
MODELING THE FORMATION OF SUB-CONTINENTAL LITHOSPHERIC MANTLE R.A. Frankland', K.D. CoUerson', A. Ewart\ Y.Niu\ P. Kepezhinskas% M.J. Defant^ 'Department of Earth Sciences, University of Queensland, Brisbane, QLD. ^^)epartment of Geology, University of South Florida, Tampa, PL.
The sub-continental lithospheric mantle (SCLM) is petrologically, geochemically and isotopically diverse and heterogeneous. It has acted as a chemical, mechanical and thermal boundary layer that has played a major role in the preservation of the cryptic record of crustal evolution. The evolution of the reservoir has occurred through the complex interaction of chemical and tectonic processes that reflect aspects of the differentiation of the planet. The growth of this reservoir has been previously explained in terms of thickening through cooling, accretion from plume activity, intrusion and underplating, continental collision, and accretion of mantle wedge material. In view of the mineralogical and chemical similarity between non-metasomatized xenolith suites from contemporary island arcs, such as Kamchatka, and depleted harzburgite xenoliths from sub-cratonic Archean lithosphere a genetic relationship may exist. These observations have led us to propose a geochemical model for the orogenic formation of SCLM involving progressive fractional melting of residual mantle within an accreting arc environment. The starting composition for this model is spmel Iherzolite, representing Depleted MORE Mantle (DMM) beneath a back-arc environment. This DMM may be either the result of early earth continental crust extraction (Hofrnan, 1988) or the residue of hot spot melting (Morgan & Phipps Morgan, 1997). The model comprises two stages of melting of this material: 1. Extraction of back-arc basalt resulting in a more depleted back-arc residue 2. This depleted back-arc residue is selectively enriched by slab derived fluid/melt, producing the fertile source material for the arc. This fertile material is then re-melted to produce island arc basalts. The resulting residual arc mantle shows excellent geochemical coherence between large ion lithophile element, transition element and high field strength element abundances with the composition of samples of depleted buoyant harzburgite from the continental lithospheric mantle. The modeled residue is also comparable to recent average compositions determined for SCLM if subsequent metasomatic input is taken into consideration. Hence, the modeled residue is interpreted to represent an initially unmetasomatised SCLM. Compositional variations between arc settings and subsequent metasomatic modification can readily give rise to the observed heterogeneous SCLM sampled by xenoliths. The model accounts for the orogenic accretion of SCLM to a pre-existing nucleus of cratonic lithospheric mantle. The cratonic nucleus may have a different origin, whose high Si - low Fe whole rock content sees to be consistent with it being the residue of komatiite extraction. Komatiite melts are the result of a higher extent and greater depth of melting, leaving a buoyant orthopyroxene-rich cratonic residue. The buoyancy of both of these cratonic and orogenic residues results in the growth of SCLM occurring via vertical and lateral thermal and mechanical accretion processes.
REFERENCES Hofinan, A. W., 1988. Chemical differentiation of the Earth: the relationship between mantle, continental crust, and oceanic crust. Earth & Planetary Science Letters, 90, 3, 297-314. Morgan, J. W. & Phipps Morgan, J., 1997. Two stage melting of a multi-component mantle: a way to generate both ODB and MORE from 'whole mantle' convection. Terra Nova Abstract, 9, 57.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
GEOSCIENCE IN LAND-USE PLANNING - BUNBURY BASALT CASE STUDY M. J. Freeman, Department of Minerals and Energy, lOOPlain St, East Perth, Western Australia 6004 m.freeman@cime.wa-gov.au
The Department of Minerals and Energy, WA [DME] is the State repository of geoscientific data relevant to strategic resource protection and utilisation, and has statutory responsibilities for basic raw materials [BRM] on Crown land. Statutory responsibilities for BRM on private land rest with the Ministry for Planning, although until recently this had been delegated to local government authorities. This delegation led to a number of cases where approvals were granted for both BRM extraction and for a simultaneous incompatible land-use adjacent to quarries. This paper identifies one such case where access to a strategic resource could be denied if local views prevail. Bunbury Basalt is quarried at a site 7 km south of Bunbury, 180 km S of Perth. Quarrying was commenced in the 1930s when the area was farmland remote from housing. At present two quarries extract a total of approximately 400 000 t/a and a third quarry is proposed. Most of the crushed rock is used for applications where any crushed, reasonably strong, crystalline rock would suffice, and sources of these rock are widespread in the Yilgam Craton 15 km to the east. However, because of its relatively homogeneous and high strength, Bunbury Basalt is in demand for a number of particularly high-value products for which it is locally unique. The basalt outflowed at 133 Ma in association with crustal tension at initiation of Australo-Indian rifting. The basalt flowed northwards in major valleys towards the gulf which occupied the then Perth Basin. At least two flows occurred up to the order of 20-40 m thick, separated by several metres of sediment. The flows are evident on aeromagnetic maps from the south coast N for 150 km, under Bunbury and then offshore 50 km W. No source vents have as yet been identified, although several isolated occurrences may indicate former vents which did not produce extensive flows. Near-vertical dykes of basalt have been intersected in oil wells west of the main area of flows and which are not apparent on the regional aeromagnetic data. After extrusion, sediment up to the order of 2 km thick was deposited on the basalt, and the sequence was affected by N-trending faulting and folding. As a result of this sediment accumulation and deformation, the basalt is mostly buried beyond mineable depths. A study completed by DME identified only two sites within about 70 km of Bunbury where the rock is available at potentially mineable depths: at the Bunbury site; and at a second location about 25 km south of this one in an area used for broad-scale and low-intensity agriculture. During the past 20 years, the local Shire has allowed urbanisation adjacent to the quarries. Recent agitation by residents to close the quarries has been accompanied by the formation of a vocal pressure group to achieve this aim. Objections are based on traffic past houses and from perceptions that blasting vibrations are a nuisance and causes structural damage to housing. Applications for housing approvals and subdivisions adjacent to the quarries are still being submitted. When the local Shire refused to renew annual licences to the existing quarries and to refuse rezoning to allow the third quarry to commence, the State Government became involved. At the same time that DME compiled a geoscientific report on the basalt and identified its strategic importance, the extractive licences were renewed by the Minister for Local Government and the rezoning was approved by the Minister for Planning, thereby overruling the local Government decisions. The geoscientific report concluded that the basalt will be very important for many decades and recommended exclusion of incompatible land uses within a buffer zone surrounding the quarries. DME recommended this be 2 km based on previous determinations by other State agencies and the fact that residents complain about vibration at up to that distance. It also recommended determination of an appropriate buffer through measuring vibration levels from the existing operations. At present, funding is being sought for this study which will measure ground and airblast vibrations, requiring close co-ordination with the existing operators to document all parameters pertaining to each blast. Because of the potential for seasonal variations to accentuate the vibration, the study will continue for at least 12 months, will also be measured. The study is expected to define: a series of zones around the quarries in which vibrations will prevent certain developments; construction standards for new buildings within the zones; and perhaps cost-effective means of ameliorating the perceived vibration problems in existing houses near the quarries. This will be a critically important case for demonstrating that the appropriate application of geoscience, in the broad concept, is essential for future planning to enhance the aesthetic and economic well-being of communities.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
THE TEMPORAL VALIDITY OF ECONOMIC GEOLOGICAL MODELS AND IMPLICATIONS FOR LAND-USE PLANNING M. J. Freeman, C J Kojan and A. J. Smurthwaite Land-Use Planning Geologists, Land Access Unit, Dept Minerals and Energy, 100 Plain Street, East Perth, Western Australia 6004 m.^nian@ dme.wa.gov.au;
The Department of Minerals and Energy, WA [DME], administers the State's Mining Act and gives geoscientific advice and information to assist in planning for future land uses to avoid affecting access to potentially mineralised areas, and particularly to avoid the State consenting to developments which could compete with or prevent mining, t o ensure access is retained, the Mining Act requires the Minister for Mines to give his concurrence before any parcels of Crown land can be alienated. In the Eastern Goldfields of WA, sites of active mining and related infrastructure mostly occupy no more than a few tens of hectares. Such areas can be sterilised through poor siting of a housing or commercial subdivision. Interpretations of the prospectivity of ground which has not been thoroughly explored are based on the application of known models of mineralisation to the geology of the sites. At sites where outcrop is good and adequate geological mapping is available, the level of prospectivity can be assessed with confidence. However, the Eastern Goldfields have a deep overburden which consists of soil, transported sediments and deepweathered profiles. This blanket conceal the nature of the underl)dng geology and often prevents the unambiguous application of appropriate economic geological models. Kalgoorlie is one of the most important mining towns in Australia and has recently undergone rapid growth. Sites available for heavy industry near the city are limited, and the need to relocate an explosives storage area in the mid 1980s was instrumental in the State Government initiating a search for a new industrial estate. In co-ordination with DME, six potential sites of 10 km^ were selected within 50 km of Kalgoorlie. At that time [1991] the prime gold mineralisation model thought to be applicable was related to structural zones intersecting mafic and ultramafic volcanics. Three of the sites were chosen for detailed investigation. Of these, one is underlain by granitoid, the second by an extensive conglomerate and the third [White Dam] by finegrained clastic sediments with minor felsic volcanics. Ultimately the site underlain by the granitoid was selected after a small geochemical survey conducted by DME over the site at State expense gave negative results, and a company holding a tenement over the site agreed to excision of the area. Outcrop at the White Dam site is vary sparse, although it was inferred that there was a low level of prospectivity for gold based on the knowledge of rock types and shear zones and interpretations of geochemistry and geophysics.. In 1994, exploration was recommenced at Binduli, about 6 km south of White Dam, by a new tenement holder. Following application of a fresh geological approach, the company discovered several orebodies. This mineralisation is related to shear zones and to felsic volcanics within shale. The White Dam site is along strike from Binduli, and exploration is now proceeding along the strike length of prospective rocks. Additional areas of gold mineralisation have been located extending through the area initially selected as a possible industrial site although no orebodies have yet been defined north of Binduli itself. This case illustrates two issues: [1] that land-use planning requires early input of geological data to for society to make sound decisions; and [2] the difficulty in predicting the possible extent of prospective ground in landuse planning studies prior to a full knowledge of the geology and of the models of mineralisation applicable to sites under consideration. Several industrial sites near Kalgoorlie initially selected by town planners in the absence of geological input were favoured because they were close to the city and their surface morphology was attractive. Town planners can recognise these attributes and can easily bias their area selection through not having the geoscientific background to fiilly understand concepts of "potentially prospective ground" arguments advanced by geoscientists. The White Dam site was not regarded by industry nor DME as highly prospective prior to the locating of Binduli ore because the geological models prevailing did not correlate with the rocks thought to be present at the site. Land-use planning geologists commonly have to argue against developments at sites which appear to have attractive surficial attributes with ideas based on inferences of potential mineralisation. This case clearly demonstrates that geologists should not hesitate to use any appropriate model of mineralisation to protect mineralised ground. Failing acceptance of the models, they should insist on a comprehensive drilling program.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
MICROPLATE TECTONICS EAST OF AUSTRALIA Carmen Gaina^ Diftfmar R Miiller', Jean-Yves Royer^, and Phil Symonds^ 'Department of Geology and Geophysics, The University of Sydney, NSW 2006 ^Unite mixte de recherche 6526 Geosciences Azur, Villefranche sur Mer, France ^Australian Geological Survey Organisation, Canberra, ACT, Australia
We present a new model for the tectonic evolution of the Tasman Sea and Coral Sea based on dense satellite altimetry data and a new shipboard data set. We utilised a combined set of revised magnetic anomaly and fracture zone interpretations to calculate relative motions and their uncertainties between the Australian and the Lord Howe Rise plates from 73.6 Ma to 52 Ma and between the Australian and Louisiade Plateau from 62 to 52 Ma. The early opening of the Tasman Sea cannot be modelled by a simple two-plate system because (1) rifting in this basin propagated from south to north in several stages and (2) several rifts failed. We identified 13 continental blocks which acted as microplates between 90 Ma and 64 Ma. Our model is constrained by tectonic lineaments visible in the gravity anomaly grid and mterpreted as strike-slip faults, by magnetic anomaly, bathymetry and seismic data and, in case of the South Tasman Rise, by the age and affinity of dredged rocks. By combining all this information we derivedfiniterotations that describe the dispersal of these tectonic elements during the early opening of the Tasman Sea. From chron 31 (67.7 Ma) to chron 29 (64.0 Ma) the model implies transpression between the Chesterfield and the Marion plateaus, followed by strike-slip motion. This transpression may have been responsible for the formation of the Capricorn Basin south of the Marion Plateau. Another major tectonic event took place in the Tasman Sea at chron 27 (61.2 Ma), when a counterclockwise change in spreading direction occurred, contemporaneous with a similar event in the southwest Pacific Ocean. Thefiniterotations for the Coral Sea are differentfromthe finite rotations that describe the opening of. the Tasman Sea; this implies the existence of a triple junction between the Australian plate, the Mellish Rise and the Louisiade Plateau. The triple junction has been active during the opening of the Coral Sea. Magnetic anomalies, fracture zones visible on the satellite derived gravity grid and strike-slip faults indicate that extension occurred between the Mellish Rise and the Louisiade Plateau, and transtensional motion occurred between Australia and the Mellish Rise (attached to the Chesterfield and Kenn Plateaus). The configuration of the triple junction from chron 27 to chron 26 was either ridge-ridge-ridge (RRR) or ridge-fault-fault (RFF). At chron 26 (58 Ma) the triple junction had a RFF configuration and migrated southward as the relative motion between the Louisiade Plateau and the Mellish Rise stopped while strike-slip commenced between the Mellish Rise and the Kenn Plateau. The gravity low between the Kenn Plateau and the Mellish Rise is interpreted as a strike-slip fault active from about 57 Ma to 52 Ma. This configuration lasted until seafloor spreading ceased in the Coral and Tasman seas at about 52 Ma. Out model implies extension in the Osprey Embayment that might explain small areas of oceanic crust existent west of the Coral Sea Basin. The western bound^ of the Coral Sea was a strike-slip fault (visible also on the satellite derived gravity grid) with a NE-SW direction, active between 58 and 52 Ma (chron 26 to 24).
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
A GIS FOR THE CAIRNS REGION Paul D. Garrad. Robert J. Bultitude and John S. Tuttle Geological Survey of Queensland, GPO Box 194, Brisbane, Queensland 4001
The Geological Survey of Queensland commenced geological mapping and compilation of mineral occurrence information in the Cairns region in 1982. These activities have generated a wealth of information highly suitable for integration into a GIS (Geographic Information System). The data sets included in this GIS package are: 1) detailed geology and geological attributes (derived mainly from 1:25 000, photoscale compilations), 2) mine information (including deposit classification), 3) geological observations, 4) granite classifications, and 5) structural information. These data sets were generated from ARC/INFO coverages and Oracle tables with analysis performed using ArcView and the Spatial Analyst extension. The integration of these data into a consistent environment has enabled regional and local-scale analysis of a highly mineralised part of Queensland. Using the structural information available from the geology layer it is apparent there is a strong correlation between northwest-trending structures in the Hodgkinson Formation and the distribution of slate-belt gold mineralisation. The development of quartz-stibnite mineralisation peripheral to these centres is also apparent. The localisation of skam and polymetalic vein mineralisation in the Chillagoe-Mungana region is also noteworthy. This area is adjacent to an inflexion in the Pahnerville Fault. The mineralisation appears to be concentrated in dilational zones adjacent to this major structure which has had a long history of episodic movements. The presence of reactive limestone has also focussed some of the mineralising systems. Many of these mmeralising systems are probably related to late Palaeozoic granitic rocks in the area. The association between tin-tungsten mineralisation and S-type granites is apparent if the granite classification layer is interrogated. The distribution of granites and structures can be used to highlight areas of potential mineralisation produced either from fertile granites themselves or from circulating meteoric waters and mineral scavenging processes generated by locally high heat flow due to the intrusion of granites. These hydrothermal fluids may have been concentrated by faults into structural traps. This style of mineralisation has been suggested for the West Normanby Gold Field and Mount Madden area. The use of simple buffering and intersection queries rapidly identifies the known mineralised structures for slatebelt mineralisation or the drainage systems hosting alluvial gold. Known localities of alluvial gold mineralisation, used in conjunction with the digital elevation model for the region enables the catchment areas shedding gold to be delineated. This information combined with the distribution of known reef gold deposits indicates there are several catchment areas in the region which are devoid of knovm hard rock gold occurrences. By adding structural information and granite data the areas with potential for gold mineralisation can be defmed. These examples illustrate the benefits of digital geological information for the rapid assessment of a region's mineral potential using established and new geological concepts and mineralisation models. The use and enhancement of these data sets, in a GIS environment, will increasingly provide a critical advantage in the competitive mineral exploration environment.
Acknowledgements: The authors would like to gratefully acknowledge the significant contibution made by the Graphical Services Unit of the DME in the preparation of the digital data.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
LOWER CRUSTAL FLOW AND ITS EFFECT ON EXTENSIONAL STYLES Gartrell A. P. Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, Nedlands, WA 6907
Geological, geophysical and laboratory evidence all suggest that the continental lithosphere is Theologically stratified. This layering reflects changes in mechanical behaviour and flow process of continental lithospheric rocks as determined by depth-dependant physical (temperature, pressure) and chemical (mineralogy, presence of water) environment. Rheological modelling predicts a range of rheological configurations, depending mainly on geothermal gradient and rock composition. The lower-crust is shown to be very weak under all but the coolest geothermal gradients. At geological strain rates the lower-crust is thought to constitute a channel of rock which is able to flow from regions of high lithostatic pressure to regions of low lithostatic pressure. For example, flow should occur towards the axis of a rift basin as the more competent upper- and middle-crust thin by necking. Flow in the lower crust is thought to be crucial in determining deformation styles during extension (e.g. Melosh, 1990; Wernicke, 1992; Westaway, 1998). In particular, lower-crustal flow has been shown numerically to allow the development of low-angle faults by changing stress conditions in the overlying rock. When the lower-crust flows, it will impart a shear traction at the base of the strong middle-crust. This shear component is shown to rotate the principal stress axes up to 45° from the vertical, thus allowing low-angle faults to develop. A relationship between the magnitude of the shear couple developed the magnitude of pressure of the overburden is shown. A large basal shear stress relative to lithostatic pressure results in a significant rotation of the stress axes, whereas a relatively small shear stress will have little effect on the orientation of stress axes. However, although major fault styles may be implied (low-angle detachment vs high-angle faulting), the numerical modelling only predicts stress conditions and is unable to demonstrate the formation and evolution of faults and shear zones during deformation. Here, analogue modelling techniques from Gartrell (1997) are used to physically demonstrate a relationship between the thickness of rheological layers and rift basin styles. Three layer-crustal systems, comprising a brittle upper-crust (sand), a strong transitional or semi-brittle middle crust (strong silicone putty) and a weak ductile lower-crust (weak siUcone putty) were extended. Localised rift basins developed where the competent upper layers were of sufficient thickness relative to the weak basal layer. Otherwise, distributed (wide rift) extension occurred. Within the group of experiments in which localised extension occurred, two end member boundary fault styles formed; high-angle planar normal faults and listric detachment style faults. Rift basins dominated by high-angle planar normal faults developed in experiments with the greatest thickness of upper- and middle-crust layers. It is interpreted that this result is due to a high overburden pressure and possibly flow away from rift axes. Rift basins dominated by low-angle detachment boundary faults developed when the upper- and middle crust layers where thinner, interpreted to be the result of a greater ratio basal shear stress to overburden pressure. Intermediate styles, where both high-angle and low-angle boundary fault systems developed strongly, formed in experiments with intermediate upper- and midcrustal thicknesses. REFERENCES Gartrell A. P. 1997. Evolution of rift basins and low-angle detachments in analog models. Geology 25, 615618. Melosh H. J. 1990. Mechanical basis for low-angle normal faulting in the Basin and Range province. Nature 343, 331-335. Wernicke B. 1992. Cenozoic extensional tectonics of the U.S. Cordillera, in Burchfiel B. C., Lipman P. W., and Zoback M. L. (eds). The Cordilleran Orogen: Conterminous U.S. Boulder, Colorado, Geological society of America, The Geology of North America G-3, 553-581. Westaway R. 1998. Dependence of active normal fault dips on lower-crustal flow regimes. Journal of the Geological Society, London 155, 233-253. Acknowledgements: The author thanks the Tectonics Special Research Centre (TSRC) for finiancial support to present this work. Further support comes by way of a Minerals and Energy Research Institute of Western Australia (MERIWA) top-up scholarship held by the author.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
COMPUTER SIMULATION OF HYDROTHERMAL FLOWS IN SEDIMENTARY BASINS
Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001
Sedimentary basins are subjected to many forces known to drive large-scale fluid migration and these flow systems are constantly evolving during the dynamics of basin subsidence, deformation, and uplift. Topographic relief is the dominant and often the strongest mechanism for deep fluid flow in continental-type sedimentary basins, just as it is for shallow groundwater systems near the Earth's surface. In the deep subsurface flow, rates of metres per year can be sustained for time scales of millions of years and flows can extend for hundreds of kilometres, most commonly across uplifted foreland basins which were formed by tectonic compression and subsequent erosion of adjacent orogenic belts. Maxunum flow rates of m/yr develop in deep basin aquifers and are focused while rates of mm/yr occur in aquitards but diffused over huge areas. Topographic configuration, permeability heterogeneity, and basin geometry are the principal factors controlling flow patterns. Buoyancy-driven flow cells can develop in both continental and marine sedimentary basins whenever fluid density gradients associated with temperature and salinity affect the fluid dynamics. Flow rates of cm/yr are typical of these hydrothermal flows which appear to be favoured in rifts and sag basins where vertical permeability is accentuated by fracture networks and fluid density gradients are strongest due to high heat flows and/or salt tectonics. Transient flow fields characterise abnormal pressure gradients created during rapid sedimentation, faulting, tectonic dilation, and chemical diagenesis. The flow rates associated with these systems can vary from less than mm/yr during basin compaction up to km/yr during venting by faults. The duration of flow is strongly dependent on permeability and bulk compressibility of the media albeit geologically rapid. All of the above fluid flow systems have a profound effect on geologic processes such as sediment diagenesis, petroleum migration and trapping, ore formation, faulting, and metamorphism that represent the coupling of fluid flow, heat transport, deformation, and chemical mass transport. In recent years mathematical models have been developed to quantify the nature of this coupling which in the past was mostly inferred from observational data. While these mathematical models vary considerably in their complexity and the geologic processes they represent, abnost all are based on basic laws defining energy and mass transport in porous and fractured media. Partial differential equations can be written to express the conservation of fluxes in the hydrothermal system, although limitations always exist in regard to the representation of fractured rock with continuum theory, the scale effectsof permeability, and the availability of physical parameters for modelling studies of specific basins. The theory behind these models is mostly well known: the real challenge for research today is putting the theory into practice such that it has the greatest benefit to broad groups of Earth scientists including exploration geologists using the tools in ore deposit and petroleum studies. Numerical solutions to the flow and mass transport equations are necessary and desirable in most cases to properly represent their physical and chemical coupling and to make predictions of flow patterns in geologically interesting basins. Finite element techniques have been used with great success because of tiieir ability to handle complex geometiy, tensorial properties and boundary conditions, and the flow equations can be solved with efficientdirect inversion of the numerical matrices. Chemical reactions can be incorporated in reactive flow problems with large thermodynamic data bases but substantial supercomputer CPU time is needed because of the high degree of nonlinearity. For three dimensional flows, iterative methods such as the preconditioned conjugate gradient technique are the most suitable for problems of flow in highly heterogeneous sedimentary basins. Hydrothermal flows in fractured basins require treatment with dual-porosity formulations and numerical modelling with special finite elements that can deal with conduit-type flow at the aperture scale of a discrete fault. Several case studies are presented here to illustrate the state-of-the-art in modelling hydrothermal flows in sedimentary basins with the fmite element method. Recent examples include studies on ancient Pb-Zn ore formation in carbonate rocks of Ireland and the Midcontinent USA, hydrothermal systems in continental rifts such as the Dead Sea, Israel and Rio Grande rift, New Mexico, and large-scale convection within the submarine sedimentary basins fnnging the Hawaiian archipelago.
159
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
SOUTHERN AUSTRALIA AND SIBERIA: A COMPARISON OF TWO LITHOSPHERIC TRANSECTS Oliver F.GauP. William L. Griffin^'^ ^^d Suzanne Y. O'Reilly^ ' ARC National Key Centre For GEMOC, School of Earth Sciences, Macquarie University, NSW 2109 ^CSIRO Exploration and Mining, P.O. Box 136, North Ryde, NSW 2113.
Lithospheric transects in southern Australia and Siberia have been constructed using data from mantle xenocrysts brought to the Earth's surface by kimberlitic and basaltic activity. The southern Australian transect extends from the Eyre Penmsula of central South Australia to Jugiong in southeastern New South Wales. This transect encompasses both cratonic and fold belt environments and crosses the Tasman Line, which separates Archean/Proterozoic western Australia from the Phanerozoic accreted terrains of eastern Australia. The Siberian transect also crosses a number of terrains from the Archean Eastern Siberian craton in the south to Proterozoic mobile belts in the north. These two transects provide an opportunity to examine the changes in lithospheric structure and chemistry from cratonic to off-craton environments. Comparison of the two transects can be used to determine what patterns are common to both cratons and what factors are independent of tectonic setting. Samples studied consist of garnet and chromite heavy mineral concentrates and were analysed by electron microprobe (major elements) and proton microprobe or laser ablation ICPMS (minor and trace elements). Temperature and pressure estimates have been made for each garnet grain through the use of the single mineral Ni-thermometer and Cr-barometer (Ryan and GrifFm^ 1996). These data have been used to estimate model conductive paleogeotherms for each locality. Results show a wide range of geotherms from 35 mWm"^ in the Archean of Siberia, through 40-43 mWm-^ in Proterozoic regions of both transects, to in excess of 50 mWm'^ in Phanerozoic eastern Australia. Estimates of lithospheric thickness, based on trace element concentrations in garnet, vary from 240 km in the core of the Siberian craton, through 160-170 km in Proterozoic sections, to -100 km in eastern Australia. Temperature and pressure estimates for garnet also allow the construction of "chemical sections". These are plots of garnet chemistry against depth which provide a useful tool for interpreting changes in a number of variables with depth within the lithosphere. Results from these chemical sections reveal some interesting patterns: • minor rock types are more commonly harzburgitic in Archean sections and wehrlitic in Phanerozoic areas • Cr203 content of garnet is lower in younger sections • a steady trend to higher Y/Ga and lower ZxfY ratios in garnet exists between Archean and Phanerozoic lithosphere • the pattern of metasomatic activity appears to vary little between Archean and Phanerozoic times • Ti02 garnet increases with depth in most sections regardless of age A method for estimating olivine Mg content from garnet chemistry has been developed using inversion of the O'Neill and Wood (1979) gamet-olivine Fe-Mg exchange thermometer. The Ni-temperature of a given garnet grain can be used, in conjunction with the Mg, Fe and Ca content of the garnet, to solve for olivine Mg and Fe content. The Mg content of olivine is an important indicator of the degree to which lithospheric mantle has been depleted through the extraction of basaltic melts. Also, because olivine is the most common mineral in the lithospheric mantle, the Mg content of olivine is one of the main factors in determining the density of mantle rocks. Results of this procedure show an overall decrease in olivine Mg content with depth in most sections. There is also a trend to more Fe-rich compositions in younger areas. Values in the shallow portions of sections range from Mg# >94 in Archean areas to Mg# of 90-91 in Phanerozoic sections. This indicates a lower degree of depletion in younger lithospheric sections and could also account for a density increase of -0.03 g/cc due to olivine chemistry alone. REFERENCES O'Neill, H. StC. and Wood, B. J. 1979. An experimental study of Fe-Mg partitioning between garnet and olivine and its calibration as a geothermometer. Contributions to Mineralogy and Petrology, 70, 59-70. Ryan, C. G. & Griffin, W. L. 1996. Garnet geotherms: Pressure-temperature data from Cr-pyrope garnet xenocrysts in volcanic rocks. Journal of Geophysical Research, 101, 5611-5625.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
ACTIVELY FORMING HIGH SULFIDATION ALTERATION ON THE SEAFLOOR, DESMOS CALDERA, EASTERN MANUS BASIN, PNG J. Bruce Gemmell^ Raymond A. Binns^ and Joanna M. Parr^ ^Centre for Ore Deposit Research, University of Tasmania, Hobait, Tasmania 7001 ^CSIRO, Division of Exploration and Mining, North Ryde, NSW 2113
The eastern Manus Basin (EMB) is a 60 km wide extensional zone of submarine volcanism between two transform faults in a back-arc environment to the north of the New Britain island arc. A basement of BocaieOHgocene arc crust is being thiimed by extension, with rapid sedimentation on the tilted blocks. Present-day submarine igneous activity (basalt to rhyolite) in the EMB has geochemical and isotopic affinities with arc volcanism and is related to northward subduction of the Solomon Sea Plate at the New Britain Trench. The eastem Manus Basin contains three known active hydrothermal zones (PACMANUS, SuSu and DESMOS), and is becoming recognised as a regional-scale modem analogue for volcanic-hosted mineral districts in ancient backarc environments. DESMOS, discoveredby Japanese scientists in 1990, is a mafic volcano located at 3°42.0'S, 15r52.3'E. The volcano contains a 250m-deep, 1.5 by 2.0km caldera, slightly elongated in a north-northwest direction (Gamo et al., 1997). The DESMOS caldera consists of basaltic andesites with a primary mineralogy of plagioclase, olivine, orthopyroxene and clinopyroxene phenocrysts set in a groundmass of glass (Gena et al., 1997). A small but very active hydrothermal field, called Onsen, was discovered on the northern wall of the caldera at 1930 m depth. Extensive silicic andargillic alteration, with disseminated pyrite, is associated with hydrothermal venting but no massive sulfides were observed. In 1995 white smokers were found to be venting extremely acid (pH 2), sulfate-rich fluids between 88® and 120°C. The presence of abundant SO4 in combination with low 6D and 6H2S led Gamo et al (1997) to suggest these fluids were of magmatic derivation. The PACMANUS I cruise in 1991 onboard the RV Franklin collected material via dredging from the Onsen site. Petrologic observations in combination with XRD, PIMA, XRF (majors) and ICP-MS (minor and trace elements) analyses were carriedout on unaltered and altered samples. Altered samples are predominately white to light grey and often contain a rim of lithology-destructive intense alteration surroimding a dark grey-green axe of weakly altered basaltic andesite. Primary silicates and glass have been modified to an advanced argilhc alteration assemblage consisting of quartz, kaolinite, native sulfur, alunite and pyrophyllite. Whole-rock geochemical analyses have been determined for unaltered and altered material. Parent rock composition is tighdy constrained. Mass balance calculations (Gresens analysis) comparing the unaltered and altered samples suggest that all elements, with the exception of Zr, have been mobilised during the intense add alteration. Many of the so-called "immobile" elements (e.g. Al, Ti, Y, Nb) appear to be very mobile. Relative and absolute gains and losses during alteration have been quantified. Most elements have suffered relative loss, however Si, Cu, Ag, Sb, Mo, Bi, As, Se, Ti and Ce have relative gain during alteration. Calculation of absolute gains and losses indicates that only Si andS have experienced significant gains (>5 g/lOOg) while the only components exhibiting significant loss are Al, total Fe, Mg andCa. The advanced argillic alteration assemblage and magmatic character of the vent fluids indicates that the Onsen hydrothermal field is an example of a subaqueous, high sulfidation hydrothermal system. It is now recognised that the same characteristic geological and geochemical features that separate epithermal deposits in subaerial settings into high and low sulfidation types are obsCTved within the family of polymetallic, volcanic-hosted massive sulfide deposits situated in submarine, back-arc tectonic aivironments. REFERENCES Gamo, T., & 18 others 1997. Acidic and sulfate-rich hydrothermal fluids from the Manus back-arc basin, Papua New Guinea. Geology, 2 5, 139-142. Gena, K., Mizuta, T., Ishiyama, D. & Urabe, T. 1997. Geochemical characteristics of altered basaltic andesite by sulfuric-addrich solution from the DESMOS caldera, Manus Basin, Papua New Guinea, JAMSTEC Journal of Deep Sea Research. 13, 269-285.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
A HYDROTHERMAL ORIGIN FOR THE GIANT PALAEOPROTEROZOIC MOUNT TOM PRICE IRON ORE DEPOSIT Geology Department, Hamersley Iron Pty. Ltd.', M.E. Barlev^ A.L. Picka^d^ S. Hageman^ and S. Folkert^ 'GPO BOX A42, Perth, Western Australia, 6001 ^Centre for Strategic Mineral Deposit Research, The University of Western Australia, Nedlands, Western Australia, 6907
Giant iron-ore deposits, such as in the Hamersley Province of northwestern Australia, which may contain more than a billion tonnes of iron oxides, are the world's major source of iron. It is generally accepted that these deposits result from supergene oxidation of host banded iron formation (BIF), accompanied by leaching of silicate and carbonate minerals, and that many formed during Mesozoic and Tertiary lateritic weathering. The occurrence of ore-grade hematite pebbles in 2 billion year old conglomerates has been used to infer that the oldest giant hematite deposits (e.g. Mount Tom Price, Whaleback, Paraburdoo in Australia and Sishen in South Africa) had formed by that time thereby providing key evidence that the Earth's atmosphere was already oxygenrich. We present new paragenetic data that shows that formation of massive hematite ore at Mount Tom Price involved early, h i ^ temperature crystallization of magnetite- and siderite-rich assemblages (accompanied by loss of SiOi). This was followed by development of hematite- and iron carbonate-bearing assemblages, also at higher than ambient temperatures andfluidpressures, with subsequent oxidation of magnetite, crystallisation of further hematite, and leaching of carbonates and silicates as temperatures decreased (during Palaeoproterozoic uplift of the deposit). Further leaching of silicates and carbonates and crystallisation of goethite accompanied Mesozoic to Tertiary weathering. A largely hydrothermal origin for giant Palaeoproterozoic hematite deposits also explains the great depth extent of ore (>400 m down dip), sharp transitions from ore to unmineralised BIF, or pyrite bearing shales, and local cross-cutting relationships between ore and unmineralised BIF observed at some deposits, better than does an origin by burial metamorphism of ore formed by low-temperature supergene weathering. It also remains consistent with existing interpretations that the biosphere contained significant oxygen at the time of iron-ore formation
162
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 N"" Australian Geological Convention, Townsville, July 1998
ROLE OF DEEP-WATER STROMATOLITES IN FORE-REEF DEVELOPMENT & SIGNIFICANCE FOR REEF EVOLUTION, CANNING BASIN, W.A. Annette D. George Dept of Geology & Geophysics, University of Western Australia, Nedlands WA 6907 The diversity of microbial limestones in the upper Devonian reef complexes of the Canning Basin has been known for many years (e.g., Playford et al. 1976). These authors also recognised the widespread development of microbial rocks in the reef complexes (back-reef, reef margin and fore-reef), and significantly, the presence of distinctive non-fenestral, deep-water stromatolites. The growth forms of these deep-water stromatolites have been described, although little work has focused on the biological and ecological aspects of stromatolite growth since. Re-assessment of the sedimentology of the fore-reef strata within a sequence-stratigraphic framework provides an opportunity to focus on the role of the deep-water stromatolites in slope development and what information they provide regarding reef evolution. Several sequences (recording third-order sea-level cycles) have been interpreted from the sedimentology of the Napier Range in the Dingo Gap area (George et al. \991). The forereef succession ranges in age from upper Frasnian (conodont zones 11-13) through to late Famennian (triangularis to expansa Zones). Stromatolite biostromes and small bioherms are common, particularly in uppermost Frasnian to lower Famennian strata. The Frasnian-Famennian boundary lies within a mappable condensed interval which includes stromatolites, intraclastic breccias, heavily burrowed clastic limestones and allochthonous blocks. In the early Famennian strata, thick stromatolites (several metres) grew on the slope and also encrusted the downslope side of allochthonous blocks. The stromatolites are commonly associated with distinctive calcite-cemented intraclastic breccias, and together these units signify low rates of sediment production on the platform. This is highlighted by their alternation with packages of ooid grainstones which reflect times of high sediment production, with abundant sediment supplied to the slope. Remnants of platform limestones are preserved in the Napier Range. The Frasnian platforms have sharply defined, rectilinear margins suggestive of margin collapse. Locally, thick stromatolites (up to six metres) are preserved on the vertical scarp-like margins indicating substantial time elapsed between platform-margin collapse and deposition of abutting slope strata. During this time stromatolites grew on margins which were bypassed by any sediment transported to the slope. The Napier Range is an extensive NW-trending range stretching for at least 140 km. In the northern Napier Range, the fore-reef slope succession in the Barker River area is broadly similar to the Dingo Gap area, and although there are variation in facies, the Frasnian-Famennian boundary interval is lithologically distinct and its position has been confirmed using conodont biostratigraphy. A siliciclastic-dominated unit within the Barker River section contains common stromatolite biostromes. These yield conodonts which place useful age constraints on deposition of the siliciclastic unit, as a similar unit at Dingo Gap is associated with subaerial exposure (George et al. 1997; George & Powell 1997). Above this interval, thin stromatolite biostromes cap small-scale fining-upward cycles in the Barker River section. Typically, development of high-order cyclicity on the fore-reef slope is rare because of the episodic nature of sedimentation and laterally restricted depositional patterns. A prominent feature of the Barker River area is a large stromatolite bioherm. Conodont biostratigraphy indicates that the bioherm ranges from late Frasnian (Zone 13) to early Famennian in age (triangularis Zone), and deep-water corals are also present within the upper Frasnian part of the bioherm. Coeval slope strata drape the margins of the bioherm indicating that the bioherm maintained relief above the adjacent slope and acted as a barrier for sediment bypassing for several million years. Water depths in which the stromatolites grew are still poorly constrained, although depths of at least 50 m are indicated from heights of dip slopes elsewhere in the reef complexes. REFERENCES George, A.D. & Powell, C. McA., 1997. Paleokarst in an Upper Devonian reef complex of the Canning Basin, Western Australia. Journal of Sedimentary Research, 67, 935-944. George, A.D., Playford, P.E., Powell, C.McA. & Tornatora, P.M. 1997. Lithofacies development on an Upper Devonian mixed carbonate-siliciclastic fore-reef slope, Canning Basin, W.A. Sedimentology, 44, 843-867. Playford, P.E. 1980. Devonian 'Great Barrier Reef of Canning Basin, Western Australia. American Association of Petroleum Geologists Bulletin, 64, 814-840. Playford, P.E., Cockbain, A.E., Druce, E.G. & Wray, J.L., 1976. Devonian stromatolites from the Canning Basin, Western Australia. In: Walter, M.R. ed. Stromatolites: Developments in Sedimentology, Elsevier, Amsterdam, 20, 543-563. 163
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Comention, Townsville, July 1998
POST-EARLY CRETACEOUS TECTONISM AND LANDSCAPE DEVELOPMENT IN THE NORTHERN BARRIER RANGES, WESTERN NSW David L. Gibson CRC LEME, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601
The northern Barrier Ranges, 110 km NNE of Broken Hill, include a north-trending 5 x 35 km range of cuestas formed by east-dipping Devonian arenites, along the northwestern margin of the Bancannia Trough, a Devonian depocentre. A mostly narrow zone of covered pediments with local alluvial and colluvial (sheetwash) fans is present immediately east of the curved range-front which defines the eastern limit of outcropping Devonian rocks. The pediments merge downslope with depositional plains over the Bancannia Trough. Rises and low hills mainly on Neoproterpzoic metasediments occur west of the cuestas. The break of slope at the base of the eastern range-front is generally coincident with the surface trace of a poorly exposed east-dipping unconformity between outcropping resistant Devonian rocks, and generally poorly cemented Mesozoic sediments. These sediments, previously undated but considered to be of Eocene and younger ages (most recently Neef et al, 1995), consist of fluvial to shallow marine clayey sandstone, mudstone, and conglomerate with poorly preserved Aptian to Albian microflora. They contain pods and layers of cemented (silcreted and less commonly ferruginised) rock which approximately follow bedding, forming low cuesta-like landforms rising from the pediments. Mesozoic (with probable Neocomian plant macrofossils) and younger sediments are also locally present west of the area of outcropping Devonian rocks, where they unconformably overlie older rocks in scattered silcrete-capped mesas, and low rubbly outcrops. Here, the base of the Mesozoic has a very low dip to the west, and is at a higher elevation than at the eastern range-front. Locally, a veneer of the Mesozoic sediments unconformably overlies the Devonian rocks on the 10-15° slope of the range-front. Palaeocurrents (Neef et al, 1995) indicate a generally northerly slope during Mesozoic deposition. A distinctive lag of well-rounded milky quartz clasts up to 30 cm across and fragments of silcreted and ferruginised sediment is present on parts of the range-front, on some bevelled areas within the ranges, and on part of the undulating area west of the ranges. This lag is compatable with a Mesozoic, but not with a Devonian source, and suggests the former presence of Mesozoic sediments over a wide area, including much of the presentday ranges. The preserved and indicated previous positions of the Mesozoic sediments suggests post-Early Cretaceous monoclinal deformation. Thrusting at depth on a west-dipping fault system, with surface deformation expressed mostly as flexing rather than displacement, is considered likely to have occurred. The configuration of the rangefront at one location suggests that there has been en-echelon offset in the post Early Cretaceous displacement. The locations of two areas of erosional rises extending east from the range front are consistent with warping above splay faults originating at flexures in the main fault planes. Dips in the Devonian rocks steepen from 10-20®E in the west to around 60TE at the eastern range-front (Neef et al, 1995). Unfolding the interpreted monocline indicates that the dip of the Devonian rocks was relatively uniform at about 25°E at the time of Mesozoic deposition. Local variations in the elevation of the base Mesozoic unconformity, and 2 metre boulders of Devonian arenite locally present in the Mesozoic sediments immediately above the unconformity, indicate some local relief prior to deposition. However, the present form of the range results primarily from post-Lower Cretaceous monoclinal folding associated with probable thrusting at depth along the western margin of the Bancannia Trough, stripping of most of the uplifted Mesozoic sediment, and incision of the exhumed Devonian rocks by east-flowing drainage initiated by uplift associated with the folding. Acknowledgements: This study was funded by the Broken Hill Exploration Initiative and CRC LEME. Thanks to Mike McPhail (ANU), David Greenwood (VUT) and Stephen McLoughlan (University of Melbourne) for palaeontological determinations. This abstract is published with the permission of the Executive Director, AGSO, and the Director, CRC LEME. REFERENCES Neef, G., Bottrill, R.S. & Ritchie, A., 1995. Phanerozoic stratigraphy and structure of the northern Barrier Ranges, western New South Wales. Australian Journal of Earth Sciences, 42, 557-570.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
STRUCTURE AND TECTONIC EVOLUTION OF BROKEN HILL INLIER REVISITED IN THE LIGHT OF NEW STRUCTURAL MAPPING AND SEISMIC REFLECTION DATA G. M.Gibson\ Tanya Fomm\ Barry Drummond^, Andrew Owen\ David Maidment^ and Kevin Wake-Dyster^ ^Australian Geological Survey Organisation, PO Box 378, Canberra, ACT 2601
Deep seismic reflection profiling and structural mapping undertaken by AGSO in the Paleoproterozoic Willyama Supergroup as part of the Broken Hill Exploration Initiative are forcing a re-evaluation of existing models for the structural and tectonic evolution of Broken Hill. Contrary to earlier expectations, the major structures in the Broken Hill region dip southeast rather than northwest and accommodate a much greater degree of thrusting, transposition and internal disruption (imbrication) than previously envisaged. Earlier suggestions that the Broken Hill region is underlain by large-scale southeast-verging nappes (Dl) refolded about more upright, tight to isoclinal, northeast-trending D2 folds could not be verified. Rather, the available kinematic evidence from several regionally significant shear zones indicates that tectonic transport was mainly directed from southeast to northwest and thus in the opposite direction to that formerly proposed. Most existing structural models for the Broken Hill region assume continuity of the regional stratigraphy with little or no disruption of the stratigraphic succession by high-temperature shear zones and mylonites. In view of the large number of high-temperature shear zones now recognised both in the field and in the seismic images this proposition may no longer be tenable. Several of these shear zones extend to middle and lower crustal depths, and a particularly prominent set of shears appears to have disrupted the lithostratigraphic package in the Barrier Range where a 20-25 km-wide southeast-dipping imbricate zone is revealed in the seismic data. This imbricate zone has the character of a thrust and fold belt and incorporates structures on which there has been multiple movement with both strike-slip and reverse-slip components recognised. This imbricate zone is also of regional tectonic significance, separating an unexposed western block dominated by sub-horizontal or shallow-dipping reflectors from an eastern block in which an apparent rift geometry (rotated fault blocks, asymmetric sedimentary basins) of uncertain age is preserved. The western margin of this central zone is the Mundi Mundi fault, a crustal penetrating structure which could also constitute the boundary between the Olary and Broken Hill blocks. In contrast, shear zones in the eastern block penetrate to only mid-crustal levels and flatten out at depth into regional decollements at about 10 and 18 km. Previous workers emphasised the retrograde shear zones of the Broken Hill region across which little or no significant displacement was thought to have occurred. These shear zones typically trend northeast or northwest and whilst some probably originated during the Mesoproterozoic Olarian orogeny, others lie parallel to the geophysically-defmed late Neoproterozoic continental margin of Australia, and thus more likely owe their origin to late Neoproterozoic continental rifling and the events accompanying the break-up of Rodinia. Alternatively, the latter may also date from the Olarian orogeny and were simply reactivated at the time of continental rifting. In any event, the northwest-trending Stirling Hill shear zone is intruded by ultramafic dykes thought to be age equivalents of the 827 Ma Little Broken Hill gabbro, and thus must have been active during the onset of sedimentation in the Adelaidean Supergroup (ca. 830 Ma). The earliest Adelaidean rocks were deposited in fault-angle depressions and half-graben which are similarly oriented northwest, raising the possibility that dyke emplacement, shear zone development, and late Neoproterozoic sedimentation are all related and all share a common origin in the events leading up to the break-up of Rodinia. During renewed deformation around 500-520 Ma (Delamerian orogeny), many shear zones were reactivated as northwestdirected thrusts. A late-stage conjugate set of east-west and north-south trending faults with dextral and sinistral offsets respectively overprinting all earlier structures may be of Delamerian or younger (?midPaleozoic Alice Springs orogeny) age.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
N"" Australian Geological Convention, Townsville, July 1998
WOODCUTTERS Pb-Zn-Ag DEPOSIT, NORTHERN TERRITORY: GEOMETRIC, FLUID-INCLUSION AND ISOTOPIC CONSTRAINTS ON GENESIS Alan D. Giles and Brian Marshall University of Technology, Sydney, PO Box 123 Broadway, NSW 2007, Australia
Woodcutters Pb-Zn-Ag deposit lies within Early Proterozoic rocks (carbonate/pelite units with intercalated tourmalinite, banded iron formation and thin tuffaceous layers) of the Batchelor Shelf Sequence. Deformation comprises basin-growth. Early Proterozoic foldingAnetamorphism, and later periods of faulting. Basin growth was accompanied by subvertical, north-trending, pre-Di faulting. Early Proterozoic Di/Mi produced Ntrending upright folds (Fi), cleavage (Si), and elongation structures (lineation, boudinage and quartz-carbonate ladder veins). In the open cut, the hingelines of two N-trending, symmetrical antiforms converge to form a single antiform; plunge variation defines a culmination. Reactivated pre-Dj faults constrained development of the antiformal structure^ both are offset by N"W-stnking late faults. Ore is stratabound within Whites Formation, but fault-related mineralisation extends down into the Coomalie Dolomite, 900m below mine-site. Banded galena(g/i)-sphalerite(5p)-pyrite(pj) (Pb-Zn-Fe) ore comprises bedding-parallel bands of varying thickness (<l->10cm), grainsize and mineral percentages. Discrete bands persist for tens of metres and in places, display grading. Pyritic 'ore' comprises py, sp, gn, asp and minor sulfosalts; it extends down-limb from the Pb-Zn-Fe orebodies and is more finely banded. The Pb-Zn-Fe orebodies have cross-sectional geometries consistent with replacement of carbonate-rich units in antiformal hingezones, whereas the pyritic 'ore' and lower grade pyritic mineralisation either die out down-limb, or persist as fault-related and vein-type mineralisation linking the stratigraphically-controlled orebodies. In places, the linking mineralisation has a laminated fabric that is discordant to bedding and sharply abuts wallrock. The overall deposit comprises stratigraphically-controlled hingezone- and culmination-related replacements, plus 'feeder' mineralisation associated with pre-Di faults. There are six inclusion-populations. Type I solid inclusions in colloform sp pre-dated the syn-D] remobilization of Pb-Zn sulphides. Type II decrepitated secondary inclusions occur in sp and gangue {qtz and dolomite); they pre-date remobilization, because they are preserved in the strained phases but form grainboundary arrays in recrystallised phases. Type III decrepitated growth-zone primaries occur in syn-Di veins. Types IV, V, and VI inclusions overprint all vein-systems and Di/Mi-recrystallized sp and gangue in banded ore. Microthennometiy on the nucleated vapour phases of Types V and VI inclusions yield salinities of 0.8521.8 and 2.03-15.8 wt% NaCl eq., respectively; Th values (uncorrected for pressure) range from 88-270 and 245-320 respectively. values for coexisting gn, sp and py from equigranular ore, py from lamprophyre dykes, and diagenetic py from the country rocks are: 9.0-12.7%o (py in ore), 12.7-14.8%^ (py in pre-Dj dyke), 8.7-10.5%o (py in post-Di dyke), 17.3-36.7%^ (diagenetic pyX 6.8-10.3%c (gn) and 9.0-11.0%c (sp). Temperature calculations on coexisting sulphide pairs demonstrated disequilibrium in the Pb-Zn-Fe ore and most of the pyritic 'ore', but one sample of pyritic 'ore' (py-sp - 384°C, py-gn - 424°C, sp-gn - 446X) has possibly reequilibrated under lower greenschist facies metamoiphism (-'425°C). Orebody-geometiy, ore-layers overprinted by Si, ore-remobilization by pre-Di intrusions, and Si-aligned clasts of colloform sp within a recrystallised-sp matrix, all support ore-emplacement prior to Di/Mi. Sulphides in syn-Di/Mi ^/z-carbonate veins suggest some liquid-state remobilization. Early fluid inclusions were destroyed by Di/Mi and post-Di decrepitation. Late fluid inclusions record post-Di/Mi 'non-genetic' fluid events. 8^S values for py-gn-sp in ore and dykes, compared with values for diagenetic py, suggest different sulphursources: reduction of seawater sulphate is probable for the diagenetic py\ an igneous source (via magmaderived or basement-circulating hydrothermal fluids) could apply to the ore. Metal-source(s) are unresolved, but basin- rather than basement-derivation is favoured. Woodcutters ore was emplaced pre-Di, during late diagenesis or early (pre-peak) metamorphism. Early diagenetic py was most probably overprinted by a structurally-controlled (by pre-Dj faults) and stratigraphically-influenced ^y carbonate layers) mixed-source hydrothermal system with basin-derived metals and basement-derived sulphur. Remobilization accompanied Di/Mi and later fault-reactivation.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
UNIVERSITY GEOSCIENCE IN AUSTRALIA INTO THE NEXT MILLENNIUM Andrew J.W. Gleadow Department of Earth Sciences, La Trobe University, Bundoora Vic 3083
The last decade has seen unprecedented change and restructuring in the Australian University sector with significant and far-reaching impacts on Geoscience departments. The next decade promises to be at least as challenging, and probably more so, if the recommendations of the recent Review of Higher Education Financing and Policy, the so-called "West Report" are implemented. This Report recommends that the nation should now make a commitment to a "decade of managed reform" in the tertiary education sector. For university departments still reeling from the recent changes, this recommendation is unlikely to be greeted with enthusiasm but contains challenges we cannot ignore. The last decade has seen amalgamations and mergers, erosion of operating budgets, restructuring of organisational units, and, most recently, 'downsizing' of staffing levels in most universities. In some cases this has even led to closure of geoscience departments or programs, so that the 29 departments in 1991 are now reduced to 26 in 1998 with still more ad hoc closures likely. The recent Minerals Council of Australia Report Back from the Brink has once again raised the suggestion that even now there are too many geoscience departments and that this represents a fragmentation of effort. Their recommendation that the number should be further reduced echoes the conclusions of some earlier reports, both in Australia and overseas. The number of specifically 'geoscience' departments is, in fact, already smaller than 26 as there has been a trend for many of smaller departments to be absorbed into more broadly-based 'environmental' or similar programs. This trend is likely to continue with small numbers of geoscience staff continuing to work within a multi-disciplinary unit. The number of departments that are still offering a comprehensive geoscience program at undergraduate level is probably already closer to 20 and could decrease, on current trends, to only around 15. On the positive side, the last decade has also seen an emerging trend towards greater collaboration and sharing of resources between neighbouring geoscience departments, and the setting up of regional consortia to coordinate these activities. It has also seen the establishment of a number of Special Research and Key Centres and several Cooperative Research Centres in the earth sciences which have greatly strengthened the discipline and its links to other research agencies as well the mineral and oil industries. These developments are likely to remain as important elements which will help stabilise the university geoscience sector over the next decade. Current issues within the university system include: the generally much smaller size and greater volatility of undergraduate geoscience populations compared to other science disciplines, small staff numbers in many departments, limited exposure of students generally to the geosciences during their secondary schooling, the boom and bust cycle in demand, and often uncertain career paths for our graduates in a globalised industry. At a time when departmental staffing and operating budgets are determined largely, if not entirely, by student numbers these factors can combine to threaten the viability of some geoscience programs. It is important that we think carefully about these issues and arrive at creative and rational solutions which will not only maintain but strengthen the university geoscience sector over the next decade. Many of the recommendations of the 1992 ARC Towards 2005 Review, although never implemented, remain relevant to the present situation and would repay careful re-examination. New technology provides many opportunities for enhancing the effectiveness and range of delivery of our courses and to amplify the capacity for collaborative research. Together these are likely to change the way we work and allow us to operate within significantly different structures from those prevailing today. The way forward will inevitably involve substantial changes and will not be without difficulty. However, not to face up to the issues will invite the imposition of externally derived, and probably mis-conceived, structures which could undermine the future of geoscience education and research in the universities. We need to decide now whether we want to be drivers or passengers in this process. 167
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
TECTONICS AND SEDIMENTATION PROCESSES IN THE SOUTH-EAST PACIFIC: NEW GEOPHYSICAL MODELS Karsten Gohl School of Earth Sciences, Macquarie University, Sydney, NSW 2109
The south-east Pacific region has been the focus of increasing geoscientific interest over the last years. Reasons for this interest are (1) the active geodynamic processes related to the Pacific-Antarctic and EastPacific ridge systems, (2) the complicated plate tectonic kinematics of the region, and (3) the architecture and evolution of the Antarctic continental margin and its glacial-marine sedimentation cycles. The largest source of geodynamic activity stems from the fast-spreading ridge systems. Repeated reorientation of Euler poles and spreading axes created complex plate relations in the south-east Pacific. The southernmost Pacific, however, has been regarded as a region of tectonic inactivity since the Oligocene. New seismic, gravimetric and magnetic data as well as geological sampling show that, even after the end of convergence between the Phoenix and the Antarctic plate, the Antarctic continental rise has been tectonically and magmatically active until recent times. Seismic and gravimetric surveys of a zone of dominant northsouth striking gravity anomalies in the Bellingshausen Sea indicate the existence of an interplate convergence perpendicular to the margin. An accreted sedimentary wedge within an asymmetric basement graben can be clearly identified. Overlying sediments provide an acoustic-stratigraphic record of the vertical basement deformation. Geophysical data from the Antarctic continental margin suggest a continuation of this tectonic feature from oceanic to continental crust. The glacial-marine sedimentation processes along the Antarctic margin and the continental rise are strongly affected by the tectonic dynamics. Orientation and dimensions of basement ridges have controlled sedimentary transport and deposition of material from suspension currents. They form nuclei for the growth of sediment mounds and drifts as observed along the Pacific-Antarctic margin. This presentation provides an overview of the tectonic and sedimentary evolution of the south-east Pacific region based on new geophysical evidence.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
CYCLIC OCEANIC CRUSTAL ACCRETION PROCESSES: EVIDENCE FROM SEISMIC PROFILING OF THE SOUTH-EAST ATLANTIC Karsten Gohl^ Sonke Neben^, Christian Reichert^, Heinrich Meyer^. and Karl Hinz^ ^School of Earth Sciences, Macquarie University. Sydney. NSW 2109 ^Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2. D-30655 Hannover, Germany
Extensive seismic profiling in the South Atlantic has provided evidence of distinct variability in the internal structure of Mesozoic oceanic crust, both in space and time. The data suggest that crustal accretion was pulsed in phases of both distinct and gradational structural and compositional changes. This style of accretion is not known from present day spreading axes but represents an important generation process for the oceanic crust in the Cretaceous South Atlantic. We selected an area in the southern Angola Basin to perform detailed seismic studies of the oceanic crust in order to analyse and quantify the changes in crustal accretion. Seismic reflection and refraction profiles along and perpendicular to crustal flow lines show changes of up to 30% in thickness of the magmatic crust. We can demonstrate the presence of four major crustal categories and several gradual changes within the Cretaceous oceanic crust of the South Atlantic. Crustal category A comprises the prominent wedge of seaward-dipping reflectors (SDRS) beneath the continental margins. These SDRS represent voluminous lava flows subaerially extruded during the early rifting stage. Crustal category B has a small-scale irregular basement and an upper and middle crust which is seismically almost transparent. Coherent reflectivity is low in the lower crust. Crustal category C has a strongly reflective and irregular basement relief The middle crust is seismically transparent, and the lower crust and upper mantle are represented by a thick sequence of reflections dipping toward the spreading axis. Crustal category D is characterised by a flat and smooth basement surface with a sequence of seaward-dipping reflectors beneath. This upper crustal sequence resembles the SDRS of volcanic margins but is thinner. The middle crust is again seismically transparent, while the lower crust exhibits highamplitude reflectivity without a preferred pattern. Our preferred explanation for these isochronous changes in the seismic images of the oceanic crust is that there has been episodic cycles in the accretionary process with periods of alternating high and low magma supply at the South Atlantic spreading center during Cretaceous time. This could either be the result of a pulsing of the nearby Tristan da Cunha mantle plume or variations in the temperature of the MORB source.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
SEISMIC PROJECT "SETARAP": SEDIMENTARY AND TECTONIC EVOLUTION OF THE AGULHAS RIDGE AND AGULHAS PLATEAU, SOUTHEAST ATLANTIC AND SOUTH-WEST INDIAN OCEAN Karsten Gohl^ Michael J. Seargent^ and Gabriele Uenzelmann-Neben^ GEMOC and School of Earth Sciences, Macquarie University, Sydney, NSW 2109 ^Alfred Wegener Institute for Polar and Marine Research, PO Box 120161, D-27515 Bremerhaven, Germany
The Agulhas Ridge and the Agulhas Plateau represent remarkable topographic features in the SE Atlantic SW Indian Ocean region. They have been present since the early opening of the Southern Atlantic and Indian Ocean and play a major role in controlling directions of the Antarctic deep sea current, the Benguela current and the warm Agulhas current since the Miocene. The Agulhas Ridge forms part of the Falkland-Agulhas fracture zone system crossing the Atlantic Ocean and exhibits a steep topographic relief ranging from more than 6000 m to less than 2000 m beneath sea-level. Multiple leaping of spreading centers along the fracture zone and magmatic activities during hot spot crossings have been part of its tectonic formation. Understanding its tectonic evolution is therefore crucial in reconstructing the palaeoceanographic history of the Southern Ocean. The Agulhas Plateau, on the other hand, raises about 2000 m above the surrounding seafloor. It exhibits particular characteristics among the oceanic plateaus as it is believed to consist of Cambrian and Proterozoic continental fragments as well as of large areas of overthickened oceanic crust. Plate tectonic reconstructions infer that the continental part of the plateau was conjugate to the Falkland Plateau and Maud Rise. It had, therefore, a key central position during the initial opening of Gondwana. To adress questions regarding the tectonic history, sedimentation processes, and lithospheric structure, a shallow and deep seismic refraction and reflection survey was recently conducted across the ridge and the plateau using ocean bottom seismometers (OBS), a 2400-m long streamer, and both powerful and highresolution airgun sources. Seismic reflection profiling of the Agulhas Ridge was closely linked to the concurrent palaeoceanographic ODP Leg 177 to provide stratigraphic control at shallow level. In this presentation, we will show preliminary results of the seismic project and discuss possible implications for the tectonic history of the area.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
COMPOSITION OF THE LOWER CRUST OF THE MOUNT ISA INLIER FROM REFRACTION SEISMIC STUDIES AND PETROPHYSICAL MODELLING Alexev Goncharov. Barry Drummond, Clive Collins, & Lesley Wybom Australian Geodynamics Cooperative Research Centre, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601
Refraction and wide-angle seismic data recorded along the Mount Isa geoscience transect (Drummond et al., 1998) provided scope for modelling the petrological composition of the crust in this region. Two onedimensional velocity models - the average model for the whole of the seismic line, and the model for the middle part of the line with a high-velocity anomaly at mid-crustal level - were interpreted in terms of the bulk geochemistry of rocks. To interpret the seismic velocity models we developed petrophysical modelling-based methodology. The key element of this methodology is the construction of the VDRT (velocity as a function of depth and rock type) map for the assumed modem PT-conditions in the crust. The measured seismic velocities are then compared to this map to obtain an estimate of the proportion of various rock types at certain depth ranges. An important feature of our approach is that we treat the crust as a mixture of a limited number of rock types represented by their end-members. The bulk geochemical composition within each type of rock is kept constant and the mineralogical compositions allowed to vary to account for equilibration at the pressures and temperatures likely to have existed when the rock was formed. VDRT map provides a systematic approach to translation of seismic velocities into compositional models of the crust; it also enables probabilistic solution of the problem of non-unique correlation between the seismic velocity and petrology of the rock. At this stage the method considers igneous rocks only. We also developed reference models for the Si02 distribution in the crust of several Precambrian shields and estimated various uncertainties in the solution of this problem. Although substantial differences in composition occur throughout the crust, the averaged Si02 content in the upper 35 km of the analysed crust is very similar and the Australian Proterozoic crust (Table) is representative of these regions in that respect. The apparent differences in seismic models between the shields can mostly be explained by different thermal regimes affecting equilibrium mineralogies. Despite very significant variation in Si02 content in the crust Depth (N), Average Si02 content in the upper along the Mount Isa transect, the averaged Si02 values in the km N km of the crust, % whole-line-average and anomalous-middle-part models merge Mount Isa Australian to less than 1 per cent difference in the lower crust at a depth anomalous whole line Proterozoic of 45 km (about 10 km above the Moho), where the two middle part average average models become indistinguishable (Table). Thus a remarkable 0 72.0 72.0 72.0 balance of high and low seismic velocities along any vertical 10 69.8 71.2 71.5 profile through the crust in this region (Goncharov et al., 20 67.1 70.4 69.1 1998) translates into petrological models, whereby the Si02 30 67.2 68.8 66.5 content averaged down to the lower crustal level is also 40 65.7 66.7 63.4 50 62.6 62.7 60.2 balanced. Geochemical features of the granites in the Mount Isa region imply that they were derived from pre-existing plagioclase-bearing lower crustal sources by partial melting of rocks of tonalite to diorite composition. Thus the dioritic to gabbroic petrological composition determined for the lower crust in the region is in keeping with the inferred origin of the granites. Both Mount Isa models (Table) show more felsic lower crust and more felsic total crust than in any of the reference models. Because of significant effect of uncertainties in modem temperature estimates on the petrological interpretation this conclusion critically depends on what geotherm is used to construct the VDRT maps for reference and regional models. REFERENCES Drummond, B.J., Goleby, B.R.G., Goncharov, A.G., Wybom, L.A.I., Collins, C.D.N, and MacCready, T., 1998. Crustal-scale stmctures in the Proterozoic Mount Isa Iniier of North Australia: their seismic response and influence on mineralisation. Tectonophysics 287 (in press). Goncharov, A.G., Lizinsky, M.D., Collins, C.D.N., Kalnin, K.A., Fomin, T.N., Drummond, B.J., Goleby, B.R.G., & Platonenkova, L.N., 1998. Intra-cmstal "seismic isostasy" in the Baltic Shield and Australian Precambrian cratons from deep seismic profiles and the Kola superdeep bore hole data. In: ''Structure & Evolution of the Australian Continent", Editors J. Braun et al, Geodynamics Series 26, 119 - 138. Acknowledgement: This abstract is published with the permission of the Director, Australian Geodynamics Cooperative Research Centre and Executive Director of the Australian Geological Survey Organisation.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
ENVIRONMENTAL GEOSCIENCE: ESSENTIAL FOUNDATION FOR ENVIRONMENTAL MANAGEMENT Victor A. Gostin Department of Geology & Geophysics, University of Adelaide, Adelaide, South Australia 5005
Australia's increasing population and escalating demands on its resources of water, soils and geology have resulted in environmental problems, conflicting interests, inefficient use, and actions that often require rehabilitation. Many environmental issues include rock/soil/water interactions, and Australian geoscience has much to oSer towards a better understanding, and hence a better management of such environments. Many students of environmental science and some professional environmental scientists, may be unaware of the scope of geological techniques and information. Therefore, one way of educating these students and enlightening the professionals, is to collect a set of articles that provide good examples of geology useful to environmental practitioners. Existing texts generally deal with cool northern hemisphere areas and do not sufficiently address issues that are common in Australia. This paper presents an outline of a book currently being compiled by the author, and titled: GONDWANA to GREENHOUSE Environmental geoscience - an Australian perspective This book seeks to provide a compendium of major environmental issues from around Australia, where geological and related earth science perspectives are essential to enlightened political opinions and sound management practices. It is a necessaiy foundation for any serious student of the environment, a cornerstone for any environmental science course, and a starting point fw many environmental analyses. Contributions to this book come from the Australian Geological Survey Organisation, State Geological Surveys, University academics, and geoscience consultants. Australian geological characteristics relevant to major environmental issues are listed below, and the book is structured around these: 1. A landscape of Gondwanan antiquity, with some surfaces having Mesozoic ages. Such areas have ancient drainage networks, deep weathered regolith, and limited useful water supplies. 2. Two huge interior basins and smaller coastal basins are filled with Mesozoic to Recent sediments. These carry groundwaters that reflect the climates of their recharge areas such that low to medium salinity waters dominate the tropical and cool temperate areas, whereas medium to high salinity waters dominate the arid and semi-arid basins. However, many present day features are inherited from past conditions, so that understanding Quatemary history as well as rates of geological processes is important in assessing the role of such inheritance. The central dunefields, gibber plams, breakaways and (dry) lakes and river systems are all products of changing Quatemaiy climates. 3. Australia also has a long coastal fringe with either a mixed rocky and wave-dominated beach zone or one with estuaries, lagoons, or wide tidal flats. Coral reefs form a significant part in the northeast, and a good record of their growth is now available. Understanding the effects of Quatemaiy sea level history, together with any tectonic activity is important for long-term coastal management. Along the southeastern coasts pollution from cities and industries have reached geological proportions. 4. Urban communities are subject to various geohazards including earthquakes, landslides, and problem soils. Disposal of urban wastes, treatment of mine wastes and landscape rehabilitation have become important issues. In all these matters, geoscience must play an increasing role. Natural background geochemistry, including radioactivity varies widely and needs to be understood in order to determme the significance of human contributions. 5. Finally, Australia's continental shelves are increasingly being used for petroleum related structures, and Australia's incredibly vast marine jurisdiction requires thorough baseline data for seabed management and fcx" monitoring climate change.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE APPLICATION OF DIGITAL TERRAIN AND SATELLITE GRAVITY DATA TO STRUCTURAL MAPPING OF AUSTRALIA'S NORTHERN MARGIN Paul A. Gow CSIRO Exploration and Mining, PO Box 437, Nedlands, Western Australia, 6009 and Australian Geodynaraics Cooperative Research Centre
The northern margin of the Indo-Australian Plate records a complex geodynamic history. This history includes the formation of giant porphyry-related Cu-Au deposits during the Miocene-Recent in Irian Jaya and Papua New Guinea (Grasberg, Porgera, Ok Tedi), and the formation of significant hydrocarbon reserves on the North West Shelf and in the Papuan Fold Belt. This study has been driven by the need to understand the geodynamic evolution of the margin, and in particular is focussed on understanding the regional-scale structural controls on formation of the major Cu-Au deposits. Regional-scale digital terrain models and satellite-derived global marine gravity data have been applied to mapping large-scale structures in New Guinea and offshore northern Australia. The elevation data is a composite dataset with a spacing of 30-arc seconds (approximately 900m), and the gravity data, from the Geosat and ERS-1 satellites, is retrieved from 6-8km spaced profiles. The elevation data provides a regional overview of areas previously mapped with radar imagery, and the data can be interpreted using similar techniques based on morphotectonic elements. Regional-scale fault structures can be mapped in the young New Guinea terrane, with faults recognised as either fault scarps or drainage patterns where erosion has carved a path along the fault trace. The data helps constrain extensions to, and define linkages between, currently mapped faults. Major fold structures can also be identified in the data, particularly in the Papuan Fold Belt. The global marine gravity data have a much coarser resolution, but provide an overview of the style of block-faulting that has occurred in the Timor and Arafura Seas. In particular, fault controls on the geometry of portions of the New Guinea coastline are evident. The combination of both datasets allows recognition and correlation of major fault structures which extend from onshore New Guinea into the north Australian basins. In particular, major NNE-trending faults are evident extending from the Fly Platform into the offshore Arafura Basin, with the largest visible structure extending for over 650km. The sense of movement on the structures appears to be dextral, with a west-down dip-slip component also evident, and expressed as fault scarps with up to 80m relief. Evidence for the extension of these structures north into the Papuan Fold Belt and the sutured margin is unclear, possibly as a result of the greater topographic relief in the highlands masking their expression in the elevation data. Reactivated transfer structures of a similar orientation have been inferred in the Papuan Fold Belt based on structural mapping (Hill, 1991; Mason, 1996), and have been interpreted as controlling the location of MioceneRecent porphyry-related Cu-Au mineralisation in Papua New Guinea (Corbett, 1994). The structures mapped from the elevation and gravity data may represent direct or linked extensions of these inferred transfer faults. REFERENCES Corbett G. J. 1994. regional structural control of selected Cu/Au occurrences in Papua New Guinea. In: Rogerson L ed. Proceedings of the PNG Geology, Mining and Exploration Conference 1994, 57-70 Hill K.C. 1991. Structure of the Papuan Fold Belt, Papua New Guinea, AAPG Bulletin, 75, 857-872 Mason R. A. 1996. Structure of the Western Papuan Fold Belt. In: Buchanan P.G. ed. Proceedings of the Third PNG Petroleum Convention, Port Moresby, 1994
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
ANOMALIES JN PLIO-PLEISTOCENE CYCLOTHEMS, WANGANUI BASIN, NEW ZEALAND: A SEQUENCE STRATIGRAPHIC INDICATOR FOR THE LOCAL FLOODING SURFACE (LFS)
Graham, I.J.. Ditchbum, R.G. & Whitehead, N.E. Institute of Geological & Nuclear Sciences Ltd. P.O. Box 31-312, Lower Hutt, New Zealand
Removal of from ocean waters occurs mainly through scavenging by micron-xized detrital aluminosilicate particles in the top 1500 m of the water column, especially clays and ferric hydroxy-oxides. On settling to the ocean bottom, often as fecal pellets or other aggregates, this accumulates in marine sediments distant from continental landmasses at about L2-1.5 x 10® atoms cm"^ y ', which is an amount equivalent to the atmospheric flux, higher rates of accumulation characterise sediments deposited near landmasses, because of the input of river-derived ^^e. Uniform highstand systems tract siltstones from Plio-Pleistocene sequences from Wanganui, New Zealand, contain a background level of 6-10 x 10^ atoms g'^ of Against this background, major peaks of abundance up to 10 times as great (to c. 64 x 10^ atoms g"occurs in samples from each of the three sequences for which data was collected.The spikes do not coincide precisely with peaks in carbonate content, grainsize, or changes in bulk rock chemical composition. Rather, they appear a little above the base of each sequence, at the location of the local flooding surface (LFS). The LFS represents the level within a sequence where rapid deepening has occurred, causing the shoreline to migrate away from the site of deposition, which is left as a sediment starved substrate on the middle shelf The spikes therefore orginated in response to seafloor sediment starvation during rapid transgression. They are not related directly to geomagnetic changes, ice sheet melting, sediment focusing, or biogenic scavenging, as has been suggested previously for ^^e peaks from other seabed location.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
U/Pb AND Nd/Sm GEOCHRONOLOGY OF OPHIOLITIC ROCKS FROM THE TUMUT SERPENTINITE PROVINCE, SOUTHERN NSW Ian T.Graham, Brenda J. Franklin, Brian Marshall and Graziella Caprarelli Department of Applied Geology, UTS, PO Box 123 Broadway, NSW 2007
Ultramafic and associated rocks from the Tumut Serpentinite Province of southern NSW have been widely discussed in tectonic interpretations of the Lachlan Fold Belt. One of these, the Coolac Serpentinite Belt (CSB), has been variously interpreted as the basal unit of a dismembered ophiolite suite, part of an 'embryonic* ophiolite, a tectonic slice from the underlying Cambro-Ordovician basement, and an allochthonous slice translated eastward from the Cambro-Ordovician basement of Victoria. Much uncertainty over the tectonic setting and significance of the serpentinite belts results from inadequate age constraints. In recent years, U/Pb isotope geochronology has been widely used, with considerable success, to date magmatic zircons from plagiogranites within ophiolite belts. Before this, Nd-Sm isotope geochronology was used to date the intrusive sequences of ophiolites, but this typically yielded a higher degree of uncertainty. Both techniques have been used to constrain the age of formation and emplacement of the CSB and Wambidgee Serpentinite
Belt (WSB) of the Tumut Serpentinite Province (TSP). The CSB outcrops discontinuously for over 56km and is up to 2.5km in width. To the east, it has a complex faulted/intrusive contact with the S-type Young Granodiorite, while to the west it is faulted against metavolcanic and metasedimentary rocks of the Honeysuckle Beds, or is supposedly intruded by the mafic/ultramafic North Mooney Complex. Most of the CSB is composed of relatively unfoliated, partially serpentinised harzburgite. The WSB outcrops discontinuously for over 100km and is up to 2km in width. It has a complex faulted/intrusive contact with the S-type Young Granodiorite, or faulted contacts with the CambroOrdovician Jindalee Beds. It is principally composed of massive and schistose serpentinite. Small bodies of plagiogranite enclosed within schistose serpentinite are widely distributed throughout both belts. They are chemically and mineralogically similar to leucocratic rocks associated with ophiolites worldwide. Silicic magmatic rocks (e.g. the Young Granodiorite) occurring adjacent to the belts are chemically distinct from the plagiogranites. Most of the bodies contain enclaves of metabasalt and amphibolite, are greatly depleted in K2O and enriched in Na20, and are defined as oceanic plagiogranites. Basaltic dykes and gabbroic intrusive masses also characterise both belts and, although metamorphosed to at least the upper greenschist facies, are geochemically akin to basalt and gabbro in ophiolite sequences worldwide. Zircon concentrates were obtained from four separate plagiogranite bodies and one leucogabbro body, distributed along the length of the serpentinite belts. They were subjected to U/Pb isotopic dating using the SHRIMP II ion probe at the ANU. Based on the results, the CSB plagiogranites have a mean age of formation of 401Ma (s.d = 1.4Ma), while the WSB bodies have a mean age of formation of 400 Ma (s.d = 1.2Ma). Nd-Sm isotopic analyses were performed (at Carleton University, Ottawa) on samples of plagiogranite, basalt and gabbro from both belts. The data form a linear array with a good spread in Nd/Sm ratios and a good fit. On the ^^^Sm/^^Nd vs ^^^Nd/^'^^Nd diagram, the isochron obtained yields an age of 399Ma. When an age of crystallisation of 400Ma is applied to the data, the eps Nd chT values obtained range from 3.78 to 7.07, suggesting derivation from a depleted mantle source. The U-Pb and Nd-Sm isotopic data clearly suggest that at least the intrusive sequences of the Coolac and Wambidgee serpentinite belts of the Tumut Serpentinite Province formed at approximately 400 Ma, this being an Early Devonian age, rather than Cambro-Ordovician or Silurian as previously postulated. The formation age falls within the age range of 420-390 Ma assigned to the protracted thermal event that accompanied emplacement of most granite bodies within the Lachlan Fold Belt. Also, when errors are taken into account, it falls within the K-Ar age of biotite from the S-type Young Granodiorite. Geochronological data could not be obtained on the ultramafic rocks, but studies on ophiolites elsewhere suggest that they are not substantially older than the intrusive sequence.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
GENESIS AND EVOLUTION OF PODIFORM CHROMITITES WITHIN THE TUMUT SERPENTINITE PROVINCE, SOUTHERN NSW. Ian T.Graham, Brian Marshall and Brenda J. Franklin Department of Applied Geology, UTS, PO Box 123 Broadway, NSW 2007
Many podiform chromitites occur within the Tumut Serpentinite Province (TSP) of southern NSW: most are hosted by the Coolac Serpentinite Belt (CSB); a few lie in the Wambidgee Serpentinite Belt (WSB). Although many are small and tailings-covered, a few are well-exposed and thereby facilitate detailed geochemical, mineralogical and textural studies. This enables construction of a genetic and evolutionary model that may also apply to podiform chromitites in general. In the CSB, the chromitite deposits are widely distributed within the main harzburgite mass and westem schistose zone. However, despite there being some concentration within the northern part of the belt, they lack defineable system. The distribution of deposits in the WSB is similarly irregular. In both belts, the deposits are tabulate, range from Im to >30m in maximum plan dimension, and have a thin halo of relatively massive, now-serpentinised, chromite-rich dunite. For CSB deposits, this passes outward, through a narrow zone of schistose serpentinite, into relatively unfoliated porphyroclastic harzburgite. Massive, granoblastic rodingite dykes cross-cut the chromitite and contain fragments of the other rock types. The enclosing harzburgite mass is grossly vertical to steeply dipping, whereas the chromitites dip shallowly and are discordant to the harzburgite's primary layering. Inward from the chromite-bearing dunitic wallrock, the deposits comprise densely disseminated chromitite, nodular chromitite, massive polycrystalline chromitite and, uncommonly (e.g. the MLA deposit), a small core of pegmatitic chromitite Chromitite textures can be subdivided into formational and post-formational types. Formational textures dominate some deposits whereas post-formational textures dominate others. At meso-scale, formational textures include chromite-rich and chromite-poor bands of variable grain-size defining layering, nodules of chromite within serpentine, and vein-like bodies of chromitite. At micro-scale, they comprise chromite-olivine pseudo-net texture, progressing into polycrystalline chromitite with sparse mineral inclusions. Postformational textures range from discrete fractures to intense brecciation. The fractures generally lack offsets and therefore have no significant shear components. Other post-formational textures comprise various replacement relationships. CSB chromitites almost comprise a complete spectrum from Cr-rich to Al-rich types. They are typically ophiolitic and low in TiOa. WSB chromitites are ophiolitic and either Al-poor or Al-rich. The wide traceelement variation between various chromitite-types suggests that there is little correlation with major element chemistry. REE concentrations within all chromitites are low. Enrichment in Os, fr and Ru relative to R and Pd, is typical of most ophiolitic chromitites. Chromite grains enclose Al-rich chromite, PGE-bearing nickel sulfides, forsteritic olivine and pargasitic amphibole. Fracture-fill includes nickel sulfides and Ni-Fe alloys, but the former dominate some deposits, whereas the latter dominate others. Most recent models for the genesis of podiform chromitites invoke crystallisation from MORB-type magmas within narrow conduits in the upper mantle. For the TSP, chromitites crystallised from a MORB-type magma in response to changing environmental conditions within narrow cavities in ophiolitic upper mantle harzburgite. We interpret the different chromitite types in terms of the degree of evolution of the MORB-type magma and hence the extent of fractionation of the source. Following chromite-crystallisation, late-stage hydrous melts exsolved from the magma and, coupled with the confining pressure, induced sintering of the chromite grains and development of various pseudo-primary inclusions. The chromitites were then intruded by gabbroic dykes, most likely representing an even more fractionated MORB source. Rodingitisation and serpentinisation occurred during progressive cooling of the chromitites and host rocks; the processes were accompanied by systematic fracturing of the chromitites and remobilisation of their chemical components.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
FORMATION AND TECTONIC SIGNIFICANCE OF THE TUMUT SERPENTINITE PROVINCE, SOUTHERN NSW. Ian T.Graham, Brian Marshall, Brenda J. Franklin and Evan C. Leitch Department of Applied Geology, UTS, PO Box 123 Broadway, NSW 2007
The Tumut Serpentinite Province (TSP) occupies a long narrow tract within the Lachlan Fold Belt of southern NSW. It contains four main serpentinite belts and numerous small serpentinite bodies, separated by metavolcanic and metasedimentary rocks ranging in age from Cambro-Ordovician to Late Silurian. The Coolac Serpentinite Belt (CSB), one of the main belts, has been variously interpreted as the basal unit of a dismembered ophiolite suite, part of an 'embryonic' ophiolite, a tectonic slice derived from the underlying Cambro-Ordovician basement, and an allochthonous slice translated eastward from the Cambro-Ordovician basement of Victoria. Much uncertainty over the tectonic setting and significance of the serpentinite belts results from lack of a combined study on all of the major belts and inadequate age constraints. Nevertheless, an understanding of the development of the belts is crucial to developing a tectonic model for the generation of the Lachlan Fold Belt. The main serpentinite belts range from 23 to 110km in length and 1.25 to 3.5km in width. Most contacts are faulted, although the CSB and WSB (Wambidgee Serpentinite Belt) have more complex faulted/intrusive contacts with the S-type Young Granodiorite. Tectonic inclusions derived from adjacent units occur in the marginal zones of several of the belts. Typically, the belts comprise massive serpentinite or harzburgite, with intemal shear zones consisting of schistose serpentinite. The main foliation in the serpentinite belts has a consistent NNW-SSE trend and is identical to that in the adjacent rock units. It therefore developed either during or after emplacement of the belts. Dyke-like bodies of plagiogranite, gabbro and basalt, and small intrusive masses of pyroxenite and dunite occur within most of the belts. Podiform chromitites occur in the CSB and WSB. The podiform chromitites of the TSP are geochemically, mineralogically and geometrically akin to those found in the mantle sequence of most ophiolites. Plagiogranites and gabbros from the serpentinite belts are geochemically and mineralogically akin to plagiogranites and cumulus and high level gabbros from most ophiolite sequences. Serpentinites from the different belts are geochemically similar to each other and to fresh harzburgite, thereby suggesting derivation from a harzburgitic parent. Basalts from the serpentinite belts are relatively enriched in Na20 and depleted in K2O and are typical ocean floor tholeiitic MORB-type basalts. The mafic rocks (i.e. basalt, gabbro) of the serpentinite belts. Honeysuckle Beds and North Mooney Complex have a similar geochemical signature and are different from those of the Micalong Swamp Complex. Plagiogranites from the serpentinite belts are geochemically distinct from the Young Granodiorite and felsic parts of the Micalong Swamp Complex. U-Pb ion probe study of zircons from plagiogranites and leucogabbros of the CSB and WSB shows that the crystallisation ages approximate 400 Ma, whilst an inherited zircon age seemingly derives from Late Silurian felsic volcanic rocks in the region. Nd-Sm isotopics from intrusive rocks of the two belts are similarly consistent with a crystallisation age of 400 Ma. The error limit of published age data for the Young Granodiorite overlaps the crystallisation age of the plagiogranites and leucogabbros. As the plagiogranites, leucogabbros and other rock types within the serpentinite belts underwent common deformational and metamorphic histories, their of crystallisation-age constrains the ages of deformation and metamorphism of the belts. The serpentinite belts are interpreted as ophiolites of the 'embryonic' type that initially formed within a back arc basin environment in the Late Silurian-Early Devonian. Typical 'ophiolite' stratigraphy did not develop as the back arc basin had a very brief existence. In fact, crystallisation of the MORB sequence and emplacement of the belts onto continental crust, together with metamorphism and deformation, may only have spanned 20 Ma or so. The Late Silurian to Eariy Devonian TSP differed from basins elsewhere in the Lachlan Fold Belt at this time in that a volcanic arc was ruptured by mantle-derived MORB magmas which ascended to the surface. However, their extrusion was short lived and after the Eariy Devonian, the development of the Tumut region differed little from that of other parts of the Lachlan Fold Belt.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
GEOHAZARDS RISK AND THE COMMUNITY Ken Granger Australian Geological Survey Organisation, GPO Box 762, Brisbane, QLD, 4001
Geohazards can be defined as a range of natural earth processes which interfere adversely with human activity. They include phenomena that are the focus of a range of geosciences including geology, geophysics, geomorphology, hydrogeology and geochemistry. They may be acute (in the sense that they kill people or do sudden and spectacular harm, as in the case of earthquakes, landslides and volcanic eruptions) or chronic (they rarely kill people but do significant economic and environmental harm, typically over a long time, as in the case of salinity, reactive clays and contamination). Acute geohazard events in Australian communities have made headlines on many occasions over the past decade. Those that come immediately to mind include: • • • •
the December 1989 Newcastle earthquake, which killed 13 people and caused approximately $4 billion in economic loss - one of Australia's worst natural disasters; the September 1996 rock fall at Cowaramup Bay near Gracetown in WA which killed 5 adults and 4 children; the July 1997 landslide at Thredbo which killed 18.people - Australia's most lethal landslide; and, the many landslides triggered by intense rainfall from former Tropical Cyclone Sid which cause many millions of dollars damage to roads, water supply utilities and buildings in and around Townsville.
Chronic geohazards, by contrast, make the headlines much less frequently, but in reality they account for damage and distress that is several orders of magnitude greater than that caused by acute geohazards. As with the impact of other natural hazards such as cyclones, bushfires and floods, there is a tendency for the community at large to push the experience of geohazard impact out of the conscious mind as quickly as possible. Unfortunately, human memory is much shorter than the return period of such events. The risks posed by natural hazards frequently tend to be ignored, overlooked or understated in activities such as urban planning, urban development, building and infrastructure construction and community education, with the result that when an event does occur, there is unnecessary loss of life and injury, economic and psychological stress and community (political) outrage. Traditionally, the geosciences have focused almost exclusively on the study of the hazard phenomena. The role of doing anything about them tended to be left to the engineering profession or, in the extreme, the emergency services. The increasing attention being paid to 'risk management' across many disciplines, following publication of the standard AS/NZS4360:1995 Risk management, and a parallel shift in emphasis by the Commonwealth Government towards promoting risk mitigation/disaster reduction, and away from disaster relief funding, is shifting the emphasis of the geosciences. The AGSO Cities Project, together with its many operational and research collaborators, is taking a lead in this cultural evolution. The Cities Project undertakes applied research directed towards the mitigation of the risks to Australia's urban communities posed by a range of geohazards. The ultimate objective is to facilitate safe, sustainable and prosperous communities. Priority is being given to the acute hazards of earthquake and landslide. A series of comprehensive pilot studies of geohazard risk in Queensland cities (Cairns, Mackay, Gladstone, Brisbane and Gold Coast), together with more focused work in Newcastle, Adelaide, Botany and Launceston, is being used to develop and demonstrate hazard analysis, modelling, GIS and risk assessment techniques that are appropriate to Australian urban conditions. The decision support tools developed, and the underpinning information compiled, are being made available to local emergency managers, planners, engineers and risk managers to support their risk mitigation responsibilities. Complementary work by other AGSO researchers, employing the Cities Project risk assessment process and philosophy, is being to address the risks posed by some of the chronic hazards such as acid sulphate soils, coastal erosion and ground water contamination. This paper provides illustrated examples of the geohazard risk assessment work conducted under the Cities Project,
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
EXTRACTION TECHNIQUES IN EXPLORATION FOR BURIED MINERALIZATION, CURARA WELL, WESTERN AUSTRALIA D.J. Gray Cooperative Research Centre for Landscape Evolution and Mineral Exploration, CSIRO Exploration and Mining, Private Bag, Wembley, Western Australia 6014
At the Curara Weil Au prospect, about 420 km NNE of Perth, WA., primary and lateritic gold mineralization are overlain by up to 20 m of barren transported overburden. A previous study conducted for CRAE had suggested that mobile metal ion (MMI) extractions of soils located buried mineralization. The site was therefore selected for a series of investigations of this and other partial and selective extractions in delineating buried mineralization. Soil samples were collected from one profile and three traverses in mineralized and background areas and were treated using various commercially available and in-house analytical extraction techniques. The selective extractions involved pH 5 acetate (for carbonate and adsorbed metals), followed by 0.1 M hydroxylamine (Mn oxides) and 0.25 M hydroxylamine ('amorphous' Fe oxides). As expected, most of the extractable Mn is dissolved with the 0.1 M hydroxylamine, with Co and (in part) Ba and Ni. That so few elements are associated with separate phase Mn oxides is surprising, given the accepted capacity of Mn oxides to scavenge metals. Interestingly, extractable Mn correlates with extractable Fe, even though they are dissolved by different reagents, indicating an indirect secondary origin for these two phases (e.g., surface drainage depositing Mn and Fe, or biological effects). Accordingly, other metals (REE, Cu, Pb and Zn) correlate with Mn, even though they are hosted by amorphous Fe oxides. The partial extractions used were 4 M hydrochloric acid (HCl) (15°C / 4 hours), MMI and Enzyme Leach (designed to dissolve amorphous Mn oxides). There was good agreement between the combined results for the selective extractions and the HCl extractions for most elements, despite only a very minor proportion of the total Fe being dissolved, suggesting that these extractions are dissolving specific Fe-rich components, be they separate minerals or disordered mineral surfaces. MMI concentrations were generally lower than for the sequential or HCl extractions, approximate proportions (MMI/HCl) being Co (0.02), Ni (0.1), Cu (0.25), Zn (0.7) and Cd (1), though with similar spatial distribution patterns. The enzyme leach concentrations were lower again (generally by at least 5 times) than MMI for all elements except Co and Ni. Soils overlymg mineralization contain relatively high concentrations of extractable Ba, Be, Cd, Co, Cu, Ni, Pb, REE, U and Zn for most to all of the extractions and for As, Au, Cs, Ga, Hg, Mo, Nb, Sn, Te, Th and T1 for one or some of the extractions. Different techniques were optimal for various elements {e.g., MMI gave the highest contrast for Cu over mineralization, with no enhancement for Co and Ni). The best anomaly to background contrast was obtained from extractable Mn, which exceeds 800 ppm over mineralization, with a background of near zero, whereas amorphous Fe has a maximum of 700 ppm, with a 200 ppm background. The higher concentrations of extractable base metals above mineralization are primarily due to association of these metals with Mn oxides and/or amorphous Fe oxides. The MMI-extractable metals showing high contents over mineralization {e.g., Cu and Cd) were also associated with these phases. There is no known association between these metals and primary or secondary mineralization at Curara, suggesting that this spatial correlation of extractable (and, weakly, total) metals with the buried Au deposit is coincidental. Similarly, there is unlikely to be a direct link between the surface Mn anomaly and the buried Au mineralization 10-20 m below. Surface phenomena, unrelated to the presence of mineralization, such as the present-day drainage, are possible causes. It is considered that partial extractions can only be understood and interpreted correctly if conducted in conjunction with an understanding of locally critical soil phases such as Mn oxides, amorphous Fe oxides, carbonates and any other materials likely to adsorb or otherwise accumulate dissolved ions. Acknowledgements: This research has involved collaboration between CSIRO/CRC LEME and the Mineral industry through AMIRA project 409 and the industry sponsors are thanked for their encouragement and support. CRC LEME is supported by the Australian Co-operative Research Centres Program.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
THE GEOLOGICAL EVOLUTION OF EASTERN AUSTRALIA: GEOCHRONOLOGY AND THE PLATE TECTONIC PARADIGM David R. Grav^ and David A. Foster^ Australian Geodynamics Cooperative Research Centre, ^Department of Earth Sciences, Monash University, Melbourne Victoria 3168, Australia dgray@earth.monash.edu.au ^Department of Earth Sciences, La Trobe University, Melbourne Victoria 3083; and Department of Geology, University of Florida, Gainesville Florida 32611 USA
The Tasman Orogenic Belt provides a classic example of continental expansion by thickening and accretion of oceanic crustal and recycled continental materials. The geological evolution of eastern Australia has involved structural thickening of deep marine sedimentary rocks, largely quartz-rich turbidites, cherts and mafic volcanics during protracted and diachronous deformation along the Gondwana margin from 550 Ma in the west to 220 Ma in the east. Development of successive fold belts including the Delamerian Fold Belt (550-450 Ma), Lachlan Fold Belt (450-340 Ma) and the New England Fold Belt (310-210 Ma) was by outward stepping of a migrating deformation front(s) associated with various subduction systems in a complex, SW-Pacific style setting. Fixist concepts of orogenesis, based largely on unconformities, have to be abandoned in this paradigm. Enlargement of the Australian continent involved the successive accretion of: •imbricated Late Proterozoic and Cambrian turbidites of a foreland fold-and-thrust belt (DFB) associated with W-directed subduction (Flottman et al. 1 9 9 4 ) . High T metamorphism is associated with 5 1 6 - 4 9 0 Ma, largely I type granites, with postkinematic, A-type granites intruding from 497-481 Ma (Turner et al. 1996). •accretion of a very large, subcontinent sized, subduction-accretion complex (western LFB) during Wdirected subduction (from at least 500 Ma through 380 Ma) and involved the closure of a small ocean basin (Soesoo et al. 1997). This large volume of sediment was derived from the DFB (Turner et al. 1996) and from cannibilisation of the structurally thickening wedge as part of the developing western LFB (Foster et al. 1998). •accretion of a high T metamorphic complex (Wagga-Omeo Metamorphic Belt) in a deformed accretionary wedge associated with an E-dipping subduction zone (430-400 Ma); analogous to the Chugach Terrane of Alaska. •accretion of a disrupted Ordovician arc (Ordovician shoshonites of central NSW), forearc sequences (Monaro Zone) and remnant of an accretionary complex (South coast of NSW); developed above W-dipping subduction zone at 440- 450 Ma. The Ordovician arc was pulled apart in the Silurian and contracted in the Carboniferous (360-340 Ma), due to deformation and accretion outboard in the developing NEra. •accretion of an arc-forearc-subduction complex system (NEFB) associated with long- lived, W-dipping subduction. The NEFB records high P metamorphism at 320 Ma, exhumation of the high P rocks at 306 Ma, with intrusion of S-type granites at 260 Ma (e.g. Tia Complex) (Fukui et al. 1993) coincident with intermediate P metamorphism in the Nambucca Block and upright folding in the D'Aguilar and Beenleigh Blocks (Holcombe & Little 1993). Subduction-related magmatism occurred from 380 Ma to 210 Ma (Gust et al. 1993). Most tectonic hypotheses of the Tasman Orogenic Belt regard this long-lived subduction zone (NEFB) to be responsible for the tectonic accretion and evolution of eastern Australia through the Palaeozoic (e.g. Fergusson & Coney 1992). However, surface geological relationships and extraordinarily complex patterns of the timing of deformation in the LFB (Gray & Foster 1997) require three concomitantly operating subduction zones, the innermost responsible for deformation and magmatism of the DFB and the outermost for the deformation and metamorphism of the NEFB. This setting is very similar to modem oceanic microplates, subduction zones, arcs, and accretionary prisms located between the subducting Pacific plate and Asia. Tectonic features and processes were transient and reflected changing accommodation mechanisms to plate motions, and along-strike variations in the orogen are expected to be substantial.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
The Geochemistry of Groundwater and Mine Tailings in a Sediment Bank of the King River, Tasmania Deborah K. Green and D C. "Bear" McPhail Department of Earth Sciences, VIEPS, Monash University, Clayton, Victoria, 3168
The disposal of sulphidic rich mine tailings into the Queen River has contributed to the deposition of 26 highly contaminated sediment banks in the adjoining river system of the King River. Regional groundwater and surface water flow through the sediment banks has the potential to become contaminated through mechanisms of sediment - water interaction. Subsequent discharge of this contaminated water from the sediment banks is contributing acid and metals to the King River, causing the death of all aquatic life in the river system and adversely affecting aquatic life in the Macquarie Harbour. One sediment bank of the King River, Bank D, was chosen for detailed investigation in order to quantify the impact of its groundwaters to the King River in summer and winter. Furthermore the mechanisms of groundwater - sediment interaction were studied in order to understand what factors are controlling the composition of the groundwater discharged into the King River. The major dissolved components of the groundwater are sulphate (29.8 mg/1 to 5459.6 mg/1) and iron (5.1 mg/1 to 2155.0 mg/1), derived from the processes of pyrite oxidation. The dissolved trace metals that are for the most part above recommended limits for aquatic ecosystems are Cu (0.8 |ig/l to 14,530 |j,g/l), Zn ( 13.0 |LLg/l to 1850 )Lig/l), Co (1.1 |Lig/l to 2311.8 ^xg/l) and Pb (0.01 jig/1 to 108 |Xg/l). Speciation of these trace metals is predicted to be dominated by the free ion and sulphate complexes. The mineralogy of the pore sediments in the bank consists predominantly of quartz, with lesser amounts of sericite - quartz lithic fragments and minor felsic volcanics and metasedimentary minerals. Sulphide minerals are predicted to be the major source of the trace metals, in particular Cu, Pb, Zn and Co, to the groundwater. In all cases the sulphides minerals are dominated by pyrite which typically represents less then 1 volume percent of the deeper sediment and up to 10 % of the surface sediment. The sulphide minerals existing in the unsaturated zone of the sediment bank are exposed to atmospheric oxygen, oxidize and dissolve, creating acid and releasing metals. Widespread oxidation of the sulphides is prevalent in this zone, as indicated by the occurrence of iron oxides coating detrital grains and the relatively high concentrations of trace metals in the surface waters. Subsequent infiltration and vertical percolation of the surface water through the sediment bank is predicted to be the likely pathway by which trace metals are incorporated into the groundwater and thereafter discharged into the river. There is evidence, however, that indicates trace metals can be attenuated in deeper sediment before reaching the river, through interaction of the contaminated groundwater with the sediment. Two mechanism of attenuation were identified that are potentially effective in reducing the concentration and subsequent discharge of trace metals (i.e., Cu, Pb, Zn and Co) into the King River. These mechanisms are adsorption and incorporation into precipitating framboidal sulphides. The framboidal sulphide grains are likely the result of sulphur-reducing bacteria active in deeper regions of the sediment bank. The fluxes of trace metals discharged from the sediment bank into the King River were estimated for the two seasons, using the results of a related hydrogeological study (Hooper, 1997). We estimate that acid and metal fluxes into the King River from Bank D are approximately three times larger during winter compared to summer; a likely result of the higher groundwater flux in this season and increased interaction in and flushing of unsaturated sediment. The fluxes of some trace metals discharged from the sediment bank in the winter season are estimated to be 25g/day Cu, 6g/day Zn, 0.2g/day Pb and 7g/day Co. The overall impact of this particular sediment bank to the King River is estimated to be negligible relative to the flux of metals in the river that result from the mine site. The flux of metals in the King River may be up to 3 to 6 orders of magnitude higher then the flux of metals discharged from Bank D. Even when considering all 26 sediment banks of the King River, the river chemistry is likely to be dominated by the waters entering the river system at the mine site; however, the impact of most of the other sediment banks is unknown as is the effect of flood and rain events, which may release high concentrations of acid and metals during short-lived events. REFERENCE Hooper, W.C. 1997. The Hydrogeology of a Sediment Bank on the King River, Tasmania. Unpublished Honours Thesis, Monash University, Melbourne, 139pp.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
MINERALISATION IN THE TASMANIAN SECTION OF THE TASMANIAN OROGENIC ZONE Geoffrey R Green Mineral Resources Tasmania, PO Box 56, Rosny Park, Tasmania 7018
Tasmania has a remarkably diverse range of mineral deposits, many of which are world class, associated with Neoproterozoic and Palaeozoic rocks. Significant ore types include Proterozoic iron ore and magnesite; Cambrian volcanic hosted massive sulphide (VHMS)-gold and ultramafic-related platinum group minerals, chromite and nickel; Ordovician syndiagenetic, carbonate hosted zinc-lead-silver deposits; Middle Devonian slate-belt gold deposits and Late Devonian to Early Carboniferous (D to C) granite related tin-tungsten and silver-lead ± zinc and copper deposits. To date, over 90% of the value of Tasmania's mineral production has come from the VHMS and D to C granite-related ores, but there are large resources in the Proterozoic rocks. Proterozoic mineralisation is best developed in the Arthur Lineament, a NNE striking, 110 km long, zone of highly deformed rocks, with blueschist facies mineral assemblages locally preserved, bounded by lower greenschist to sub-greenschist clastic shelf and turbidite sequences to the west and east respectively. The flanking sequences were thrust over the rocks of the Lineament, probably in the Early Cambrian. Main rock types include psammitic and pelitic metasediments, dolomites and tholeiitic metavolcanics and intrusives, the latter occurring in three subparallel belts. Iron ore is associated with the eastern of these, as stratiform cupriferous magnetitepyrite deposits with a silicate gangue dominated either by antigorite ± talc or tremolite-actinolite ± chlorite. The ores, the largest of which at Savage River is currently being mined, either are pretectonic or were emplaced early in the deformational history. In the Lyons River and Arthur River areas, magnesite is associated with dolomite and minor quartz, pyrite, talc and chlorite as a concordant unit flanked to the west by amphibolite and pyritic siltstone and to the east by quartz schist and quartz-mdca schist. Magnesite in the Main Creek deposit is associated with chlorite-quartz schist with minor dolomite, magnesite and pyrite. The genesis of these deposits is poorly understood, current interpretations are that the iron ores are sedimentary exhalative deposits and the magnesite is a low temperature replacement of dolomite; however it is also possible that both are dolomite replacement styles, related to Proterozoic gabbro or granitoid emplacement. The most important metallogenic event was associated with the deposition of the late Middle to Early Cambrian Mount Read Volcanics (MRV) which formed in a back-arc setting following Early Cambrian collision of ProtoTasmania with an ensimatic island arc. Three world class deposits, the classic Zn-Pb-Au-Cu-Ag volcanic-hosted massive sulphide (VHMS) deposits of Rosebery and Hellyer (as well as the smaller Hercules and Que River orebodies) and the Cu-Au Mount Lyell field, occur within the MRV. The high grade gold deposit at Henty is also believed to have formed as part of a submarine hydrothermal system, although the spectacular gold grade (27 g/t) may be partly a result of redistribution of gold within the mineralised system by dissolution from rocks undergoing ductile deformation and reprecipitation within multiply brecciated massive quartz alteration. The major deposits are grouped within districts sharing a broadly common geological history and have similar sulphur and lead isotopic signatures. Another significant mineralisation style is debris flow deposits containing massive sulphide clasts, the sources of most of which are yet to be discovered. Mesothermal gold deposits, hosted by Ordovician to Early Devonian clastic sequences in north-eastern Tasmania, were emplaced during middle Middle Devonian west-vergent folding and thrust faulting, with the Tasmania Mine at Beaconsfield, a high grade (24 g/t) million ounce producer with similar resources at slightly lower grade, predominant. There is a significant distinction between the bewildering variety of styles of posttectonic D to C mineralisation in western Tasmania and the limited range in the eastern Tasmanian Bassian terrane, a contrast ascribable to the more complex geology and presence of carbonate and, to a lesser extent, ultramafic rocks, in the former region and a higher degree of granite unroofing in the latter. Western Tasmania is the site of the world class carbonate-replacement or distal skam tin deposits of Renison Bell and Mount Bischoff, associated with highly fractionated ilmenite series granites, and the proximal exogranitic scheelite skarns of King Island and Kara, related to magnetite series, I-type granitoids. In contrast in eastern Tasmania, the most important Sn-W deposits are veins adjacent to granite cupolas (Aberfoyle, Storeys Creek) and there are also sizeable endogranitic Sn greisen deposits (Anchor, Royal George). These styles are also present in western Tasmania, but they are relatively insignificant. Historically important Pb-Ag vein deposits, notably those of the Zeehan field, and more recently recognised, sub-economic, skam-hosted base metal and fluorite mineralisation, flank plutons associated with the Devonian tin systems, but do not form significant haloes around magnetite series granitoids.
182
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
TRACE ELEMENT DISCRIMINANTS FOR MANTLE PROCESSES - AN EXPERIMENTAL APPROACH Trevor H. Green, John Adam and Geoffrey T. Nichols, GEMOC, School of Earth Sciences, Macquarie University, NSW 2109
Conspicuous trace element diJBferences are well documented for major types of igneous provinces in the earth's crust. Thus relative to primitive mantle, hot-spot plate centre volcanism (e.g. ocean island basalts) is characterized by large ion lithophile element (LILE), Hght rare earth element (LREE) and high-field-strength element (HFSE) enrichment. In contrast, normal mid-ocean ridge basalts are LILE and LREE depleted with HFSE unchanged, and most volcanics jfrom subduction zone settings show LELE enrichment and similar or depleted HFSE relative to primitive mantle. These overall trace element features reflect mantle source regions that have been modified by metasomatic and/or melting processes, that in turn are controlled by trace element partitioning between fluidtoelt/residual minerals. ]mpoj:iant residual minerals that may significandy affect LILE, REE and HFSE abundances include clinopyroxene (cpx), amphibole (amph) and garnet (gt), whereas rutile is a crucial accessory mineral that may influence HFSE abundances. The e?q)erimental database now available for the partitioning of trace elements between these minerals andfluid,various siHcate melts (basaltic to high-SiOj rhyohtic compositions) and carbonatite allows recognition of potential marker elements or ratios that characterize dijfferent metasomatic or melting events. In general, mineral/fluid Ds are higher than mineral/melt values, but noteworthy exceptions occur. Thus for cpx mineral/fluid Ds for Pb and Ba are similar to mineral/melt values. Cpx/fluidfi-actionatesU/Th more strongly than cpx/melt. Cpx/sihcate and cpx/carbonate melt Ds appear similar, except for Zr and Hf which arefi-actionatedin opposite direction and for HFSE/REE v^ch are higher for cpx/carbonate pairs. For amph Rb and Pb mineral/fluid and mineralAnelt Ds are close in value, but Rb/Ba behaviour is distinctly different for amph/fluid (<1) and each melt (> 1 for carbonate melt, -1 for sihcate melt). Also Nb/Ta isfi"actionatedmore by amph/fluid and amph/carbonate melt than by amph/silicate melt, and HFSE/REE are hi^er for amph/carbonate pairs than for either amph/silicate melt or amph/fluid pairs. For gt/fluid, Ds for Ba and Sr are lower than mineral/melt values, and U/Nb and Pb/Sr will decrease influidsbut will increase in melts throu^gtfi^tionation. Gt/silicate or carbonate melt Ds show very similar behaviour. Although only a small number of rutile/fluid and melt D values is available, the very high D values for HFSE are striking, so that a small volume of rutile may have a major effect on these trace elements. Some results show that rutile/fluid or high-SiOj meltfractionatesNb/Ta in ihc opposite direction to rutile/lower-SiOj melt and to cpx, amph or gt/melt. Thus rutile/fluid or high-SiOj melt fractionation will show a decrease in Nb/Ta compared with an increase in Nb/Ta for all the other mineral/meltfractionatingcases. Also rutile/fluid that is opposite to cpx/fluid, but similar to (though much hi^er than) gt^uid Addition of F to the volatile component causes a marked drop in D values for HFSE, but less effect on LHJE, for amphAnelt pairs. Thus the relatively high HFSE/LILE ratios observed in alkaline magmas may indicate the presence of F in their petrogenesis, if amphibole is an importantfractionatingmineral. Taken together these results indicate that relatively low Rb/Ba and HFSE/REE point to a carbonatitic melt acting as a mantle-modifying metasomatic agent, whereas hi^ U/Th and Rb/Ba and lower U/Nb suggest afluidrole. If rutile/fluid or high-SiOj melt partitioning behaviour exerts an important control on the geochemistry of a mantle source region, then derived magmas may have Nb/Ta < model mantle, whereas if rutile/lower-SiOj melt control (together with cpx, amph or gt) is more significant then Nb/Ta will be > model mantle. Thus recently obtained Nb/Ta data for subduction zone volcanics of 11 to 33 may reflect this contrasting rutile/fluid or melt Nb and Ta partitioning behaviour. An important corollary is that a model cmtinental crustal value of Nb/Ta -- 11 would suggest that any major contribution to the growth of continental crustfromsubduction zone volcanism should comefrommagmas derivedfroma source region previously modified by afluidor high-sihca melt that had equilibrated with rutile, most likely in the subducted oceanic crust Additional evidence for afluidrather than a siHcate meh role may comefromcareful assessment of Rb/La, U/Th and Pb/Sr relative to Nb/Ta. Thefluid/meltpartitioning data summarized here indicate that a negative correlation of these ratios would confirm fluid involvement. However, it is crucial to verify that rutile/fluid and rutile/meh Nb, Ta partitioning data show the contrasting behaviour that preliminary results suggest.
183
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
TRACE ELEMENT CHARACTERISTICS OF MANTLE XENOLITHS FROM THE KERGUELEN ARCHIPELAGO (INDIAN OCEAN) M. Gr^goire^ and S.Y. O'Reilly^ 1 GEMOC, School of Earth Sciences, Macquarie University, Sydney, NSW 2109 The Kerguelen islands are the third largest oceanic archipelago (6500 km^) after Iceland and Hawaii and magmatic activity has extended over 45 Ma. They have evolved from a location near the SEIR to a present-day mtraplate setting. Therefore, the Kerguelen islands present a specific geological setting combining characteristics of the Iceland and Hawaiian regions. Their geodynamic evolution is related to a progressive change in composition of magmatism from tholeiitic to alkaline. Alkali basalts from the Kerguelen islands have entrained many mantle peridotites (harzburgites and cpx-bearing dunites, type I xenoliths) in addition to various other ultramafic and mafic xenoliths (metamagmatic rocks). The harzburgites and the dimites were equilibrated in the spinel peridotite stability field (T = 850-1150 °C). To date no fertile mantle Iherzolite has been found in the Kerguelen archipelago. The harzburgites have been divided into protogranular Cr-diopside-bearing harzburgites and poikilitic harzburgites which contain an interstitial magnesian-augite, sometimes associated with phlogopite and amphibole. Some scarce samples of the two types of harzburgites show an unusual mineral association consisting of feldspar + olivine (2) + Ti-chromite + rutile + Mg-ilmenite + armalcolite + Ca-Cr armalcolite (FORIAC paragenesis). These minerals occur in reaction zones replacing opx and spinel or as thin veins or dykelets cross cutting olivines. Coarse grained dunites (ol > 90 wt%) always contain small amount of cpx and spinel. A lot of dunitic samples also contain phlogopite while amphibole or opx only appear in few of them. Dunites are sometimes observed as wall rocks of magmatic veins (websterites, homblendites, clinopyroxenites). Bulk rock trace element characteristics of type I Kerguelen xenoliths point out the fact that the whole samples have been affected at various degree by metasomatic processes. Most of the samples show LREE-enriched patterns and the few samples which display LREE-depleted patterns have a La/Sm ratio too high to be explain only by depletion processes. Clinopyroxenes from type I xenoliths show large trace-element variations, for example, Sr ranges from less than 0.5 to 371 ppm, Ti from 25 to 8500 ppm and Zr from less than 1 to 325 ppm. The amphiboles and phlogopites coexisting with clinopyroxene (cpx) in a few samples show high amounts of Nb (up to 100 ppm) and Ta (up to 10 ppm) especially in harzburgites. The feldspar of the FORIAC mineral paragenesis is variable in composition, but is alkali-rich (K2O: 1-10.5 wt%). Rutile and Ca-Cr-Zr armalcolite may contain significant amounts of Nb203 (0.30-1.85 wt%), Zr02 (0.15-3.00 wt%) and Ce203 (0.07-1.45 wt%). Trace-element characteristics emphasize the origin of the Kerguelen harzburgites as a result of two main processes: (1) high degree of partial melting (20-30 %), and (2) multiple metasomatic processes. The multimetasomatic events are associated with the activity of the Kerguelen plume and explain the modification of this previously depleted mantle to varying degrees. The main metasomatic event resulting in trace element enrichment of the two types of harzburgite and the crystallization of Mg-augite ± phlogopite and amphibole is probably related to the percolation into the upper mantle of "liquids" ranging from basaltic melts to "carbonatitic melts" and may sometimes correspond to Ti- and alkali-rich, H20-poor fluid. The two-stage process which explains the petrological and geochemical characteristics of Kerguelen harzburgites may be related to the origin and evolution of the Kerguelen archipelago: (i) partial melting is related to formation of the Kerguelen oceanic lithosphere in the vicinity of the South East Indian Ridge, (ii) reaction between the harzburgitic residue and melts related to the activity of the Kerguelen mantle plume in the within-plate setting of the islands. Trace-element features of cpx-bearing dunites are consistent with a formation through reactions between formerly depleted harzburgites and basaltic melts of transitional to alkaline affinities propagating through cracks (reactions opx + liquid 1 —> ol + liquid 2 and opx + liquid 1 ~> cpx + liquid 2). As suggested for cpx-bearing dunites occurring in Iherzolitic massifs, Kerguelen cpx-bearing dunites constitute wall-rocks of the veins and veinlets cross-cutting the upper mantle. The formation of Kerguelen dunites is related to the long-lived Kerguelen hotspot-activity.
184
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
FLUID-ROCK REGIMES FOR VEIN FORMATION: INFERENCES FROM STRUCTURAL ANALYSIS AND STABLE ISOTOPE GEOCHEMISTRY Robert T. Gregory'. David R. G^ay^ and Ian J. Richards' -••Stable Isotope Laboratory, Department of Geological Sciences, SMU, PO Box 750395, Dallas TX 75275-0395, USA MePS Department of Earth Sciences, Monash University, Clayton, Victoria 3168
Veins are the direct products of fluid rock interaction within the crust of the Earth. Vein compositions along with the distribution of veins in space and time provide insights into the physical conditions during vein formation. Inferences on the conditions of vein formation are clearly influenced by the scale of the observations. Water-rock ratio, integrated fluid-flux or reaction progress calculations are extremely sensitive to assumptions about initial fluid composition and the size of the representative volume. Vein formation occurs in at least two very diJBferent regimes at opposite ends of the fluid flux spectrum. Hydrothermal veins associated with cooling plutons and certain types of hydrothermal ore deposits are clearly the result of large scale fluid infiltration and metasomatism. These types d deposits generally occur at shallow levels within the crust where porosity and permeability is sufficient to support fluid flow under near hydrostatic conditions. Veins clearly form in a much wider variety of geologic settings where permeability and porosity are low enough that the rocks behave as nearly closed-systems (low fluid-flux system). We have examined vein formation (low fluid-flux type) in a number of localities: the Lachlan fold beh, the South Island of New Zealand (Otago schist, Caples terrane, and Torlesse Group), Ouachita orogenic belt (Oklahoma and Arkansas), and Saih Hatat high pressure rocks (Oman mountains). In convergent margin settings involving siliciclastic sedimentary rocks, the majority of vein formation occurs during the chevron folding phase with development of slatey cleavage in pelitic layers and incipient cleavage development in psammites. Earliest veins are layer parallel veins formed during the initial stages of shortening. Later veins include en echelon gash veins, saddle reefs, and oblique veins formed after fold limbs have steepened (e.g. the Lachlan fold belt). Vein volumetric abundances (averaged over length scales comparable to the largest continuous vein) are rarely more than a few percent. Rocks with transposition layering (ductilely-deformed) exhibit vein volumes little changed from the volumes inherited from the early chevron fold phase. This suggests that most vein formation occurs on the prograde path before the peak of metamorphism or deformation. During ductile deformation, older veins rotate into the foliation direction. Veins extend (boudinage) parallel to the stretching lineation and chloritic-black walls develop along the margins. These black walls are the result of pressure solution; not from infiltration metasomatism associated with primary vein formation. New vein growth is generally in gash veins with fibre growth parallel to the stretching lineation or in pressure shadows associated with boudin formation. In Oman, these late gash veins are associated with the exhumation of the rocks and form on the retrograde portion of the metamorphic path. Oxygen isotope relationships between veins and host rocks are dramatically different between the origmal chevron folding phase (Lachlan fold belt style) and the ductile deformation phase (Otago Schist style). Veins formed in Lachlan-style settings, typically are more homogeneous in than coexisting host rocks. This is a consequence of the rates of oxygen isotope homogenization in the fluid phase relative to those in the solids. In any representative volume, the mole fraction of fluid is small enough that the time constant for its approach to quasi-steady state is fest compared to that for the coexisting solids. The quasi-steady state fluid \ahie (the long time scale part of the solution to the coupled differential equations that describe these types of systems) can be approximated by simple mixing between two end-member fluids, the external fluid (fluid-buifered system) and the rock-buffered fluid (slowly varying function of time due to exchange in the solids): T) = J
^ ^
rock-buffered
| ^ ^ /
^^
Rock-buffered conditions obtain as u/k-^0; u/k is the dimensionless ratio of the fluid flux rate to the isotopic reaction rate. Our stable isotope data indicate that this condition is satisfied for Lachlan-style vein formation. Veins formed from such fluids are uniform in values over a wide range of formation temperatures. The fluid effectively averages more material than the immediately adjacent material and achieves its target value before any major move towards homogenization occurs in the host rocks. The results in steep arrays in whole rock (y-axis) versus vein plots. In contrast, transposed veins in ductilely-deformed rocks typically exhibit the same heterogeneity as their coexisting host rocks. This indicates that the veins have recrystallized and subsequently reequilibrated their values or that their primary formation occurred under a regime where the length scale of the exchange was even smaller than that for the typical chevron folding phase.
185
GEOLOGICAL SOCIETY OF AUSTP^LIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
DEFORMATION AND METAMORPfflSM OF fflGH P ROCKS, SAIH HATAT, OMAN: IMPLICATIONS FOR THEIR EXHUMATION Robert T. Gregorv^ John Mc. Miller^ and David R. Gray^ ' Stable Isotope Laboratory, Department of Geological Sciences, SMU, PO Box 750395, Dallas TX 75275-0395, USA ^ VIEPS Department of Earth Sciences, Monash University, Clayton, Victoria 3168
High P rocks occur within former continental shelf rocks below the allochthonous Samail Ophiolite within the Saih Hatat domal window of northeastern Oman. In contrast to other parts of the autochthonous Arabian margin exposed in the Oman Mountains, northeastern Saih Hatat suffered high pressure blueschist-to-eclogite facies metamorphism accompanied by intense deformation involving nappe development. The regional fold-nappes evolve by folding and transposition of the high pressure fabrics during a compressional event that occurs while the overall lithostatic pressure is decreasing. These fold-nappes are associated with greenschist facies metamorphism and are cut by lowangle high strain zones which consistently show southwest-over-northeast sense of shear opposite to the sense inferred for ophiolite emplacement. A major crustal discontinuity separates two zones of regional nappes with the upper plate hanging wall rocks at lower metamorphic grade than the footwall rocks. As much as a 13 kbar pressure difference exists between the upper plate (6-10 kbar, carphoUte-bearing assemblages) and lower plate glaucophanebearing eclogites preserved within mafic mega-boudins at As Sifah (>12 kbar, possibly >20 kbar). In some high P terrains, apparent increases in metamorphic grade down structural section and low-grade S-C fabrics are often attributed to extensional exhumation. Even though Oman exhibits these features in common with many high pressure terrains, exhumation was accomphshed during crustal-scale compression with attendant formation of foldnappes during ascent. The upper plate rocks consistof pre-Permian to Jurassic rocks folded into large (10 km scale) recumbent nappe structures which at high structural levels are more cylindrical in form with fold axes at high angles to the NNE-SSW stretching lineation. At lower structural levels, the fold-nappes in the upper plate become more attenuated and sheath-like with increasing strain (X/Z up to 170:1). Fold hinges become coincident with the stretching lineation.. Chlorite-albite assemblages in mafic schist define the axial surface foliation to these upper plate fold-nappes, but relict sodic amphiboles occur as relicts within the albite porphyroblasts. The stratigraphy of the lower plate rocks is similar to the upper plate involving metamorphosed equivalents of Arabian platform sediments, but these rocks have suffered uniformly higher strain (XZ> 25:1). The largest lower plate nappe structures have amplitudes on the order of 5 km and may be parasitic to still larger structures. Fold axes are parallel to the regional lineation which strikes NNE-SSW. Sense of shear indicators yield a transport direction of south over north in the lower plate. Boudins develop over a large range of scales (from the microscale up to km-sized blocks). All of the As Sif^ area eclogite facies metabasalts are preserved in km-scale megaboudins indicating that the progressive deformation continued beyond the peak of metamorphism. Outside the megaboudins at As Sifah, peak metamorphic assemblages associated with the earlier structures are almost completely overprinted by intense deformation that culminates in the formation of the regional recumbent closures. A progressive decrease in metamorphic grade is recorded by assemblages that are consistently oriented around the northeast-southwest stretching lineation. Mafic lithologies show a change from garnet, phengite, sodic amphiboles and clinozoisite to later fabrics are defined by garnet, phengite, sodic/calcic amphiboles, epidote, clinozoisite and albite (+quartz), indicating that pressures must be at least below the jadeite = albite + quartz equilibrium line. Mafic units at the lowest grade have epidote, sodic/calcic amphiboles, albite, actinolite, hematite and chlorite assemblages and arefrequentlyassociated with sodic/calcic amphibole and albitebearing veins that are orthogonal to the stretching lineation. Argon geochronology on white mica populations suggests that exhumation of the lower plate units occurred over a time period of at least 12 million years and perhaps longer. Preservation of the the blueschist and eclogitic assemblages over this time period requires underthrusting of colder footwall rocks under dynamic conditions typical of convergent margin settings. Because the thermal relaxation time of the crust is very short (-10 Ma) thermal reequilibration will destroy the blueschist and eclogitic assemblages. High P regimes require a low geothermal gradient for their preservation. It is not enough to deeply bury a high P/low T metamorphic rock; the low geothermal gradient must be dynamically maintained. It is clear that a convergent margin setting accompanied by underthrusting of colder material is critical not only for the production of high P/low T metamorphic rocks but also for their exhumation. Bringing the rocks to the surface rapidly (adiabatically in the extreme) produces a high T/low P overprint destroying the blueschist and eclogite facies assemblages. In an environment such as Oman, the continued underthrusting of colder footwall rocks enables the eclogites and blueschist to survive their ascent to the surface at rates slower than the thermal diffusivity of the crust.
186
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
NEOPROTEROZOIC STROMATOLITE CORRELATIONS IN THE CENTRALIAN SUPERBASIN Kathleen Grey Geological Survey of Western Australia, 100 Plain Street, East Perth, 6004
A lack of adequate stratigraphic correlation has placed constraints on hydrocarbon and mineral exploration in the Neoproterozoic Centralian Superbasin, especially in the Officer Basin of WA. Stratigraphic relationships of Officer Basin outcrops and their connection to the rest of the superbasin have been hotly debated. A review of stromatolite data, together with outcrop and drillcore sampling, formed part of GSWA's Interior Basins Petroleum Initiative. Stromatolite biostratigraphy has not only allowed isolated Western Australian outcrops to be placed in their stratigraphic context, but also provides Australia-wide correlations. Stromatolites occur in all four Neoproterozoic Supersequences in the Centralian Superbasin, and correlations proposed from stromatolite distributions are supported by palynological data. In WA, Supersequence 1 at the base of the Centralian Superbasin succession contains two assemblages. The older one is dominated by Acaciella australica and Basisphaera irregularis and has been identified in the middle Browne Formation, the Woolnough and Skates Hills Formation, and at about 1400 m in Empress lA. The same assemblage occurs in the lower Loves Creek Member of the Bitter Springs Formation (Amadeus Basin), and probably in the Callanna Group (Adelaide Rift Complex). The upper assemblage in the western Officer Basin is dominated by Baicalia burra, but also contains Tungussia wilkatanna and a conical stromatolite. It outcrops in unnamed carbonates along the Eagle Highway, in the Neale Formation near Neale Junction, and at Constance Headland in the Tarcunyah Group (formerly part of the Paterson Orogen). It occurs in the Kanpa Formation in Hussar 1, in the Kanpa and Steptoe Formations in Empress lA (976 m to 480 m), and in NJD 1. It has also been recorded from the Burra Group in the Adelaide Rift Complex, but there is apparently no equivalent in central Australia, presumably because of major erosion before the Sturtian glaciation. These distributions imply a correlation between the Callanna Group and Bitter Springs Formation, rather than between the Burra Group and Bitter Springs Formation as previously suggested. Unpublished palynology and isotope chemostratigraphy lend support to this interpretation. Several forms occur in Supersequence 2. They appear useftil for basinal correlation, but have fairly restricted geographical distributions. Likewise, most Supersequence 3 stromatolites are known only from a few locations. However, towards the top of the succession, Tungussia Julia occurs in the Wonoka Formation (Adelaide Rift Complex), Julie Formation (Amadeus Basin), and Egan Formation (Kimberley area). A few forms occur in Supersequence 4, but again are known from only a few localities. Australian Neoproterozoic taxa show restricted time distributions. Although the use of stromatolites for biostratigraphy has been disparaged because of supposed environmental controls, individual taxa plot only once on a range chart, even though environments are repeated. Moreover, some taxa are found in a range of environments. This is not what would be anticipated if environmental control were the overriding influence. Features arising as a response to environmental influences can be identified, but an individual taxon retains many of its inherent characteristics. Thus, although Basisphaera irregularis changes from club-shaped to tabular when conditions shallow, the microstrucUire remains constant, and it can still be identified as B. irregularis. Environmentally controlled distribution patterns can also be recognised, for example B. irregularis is rare in sediment-starved carbonate environments, where Acaciella australica is the predominant taxon, but gradually becomes dominant as clastic-sediment input and energy regime increase. Different associations of stromatolites occur in younger high energy environments. To be effective stromatolite correlation has to be based on adequate systematic studies with identification to form level (= species level). The biostratigraphic scheme developed for Supersequence 1 from outcrops and drillholes in the Officer Basin was tested by GSWA's recent stratigraphic drillhole. Empress lA. The drillhole contains over 70 stt-omatolitic horizons, and the two assemblages occurred as predicted from the stratigraphic model. The proposed correlations have implications both for hydrocarbon exploration within the Officer Basin, and for our understanding of the stratigraphic and tectonic setting of the Centralian Superbasin and Adelaide Rift Complex. The emerging biostratigraphic and palaeoenvironmental framework should enhance the development of models for hydrocarbon and mineral prospectivity.
187
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
CHEMICAL TOMOGRAPHY: IMAGING COMPOSITIONAL VARIATION WITHIN THE LITHOSPHERIC MANTLE W.L. Griffin'I C.G. Ryan^ and Suzanne Y. O'Reilly' 1. GEMOC National Key Centre, School of Earth Sciences, Macquarie University, NSW 2109, Australia 2. CSIRO Exploration and Mining, P.O. Box 136, North Ryde, NSW 2113, Australia
The subcontinental lithospheric mantle (SCLM) carries a geochemical, thermal and chronological record of largescale tectonic events that have shaped the Earth's crust, but little of it is accessible to geological mapping. However, with microbeam analytical techniques, garnet and chromite xenocrysts in mantle-derived volcanic rocks can be used to construct realistic geological sections of the SCLM. Trace-element thermometers place each grain, and the information contained in its geochemistry, in a stratigraphic context, and where enough samples are available, we can determine the paleogeotherm, the detailed distribution of rock types with depth, the spatial distribution of fluid-related (metasomatic and anatectic) processes and the depth to the lithosphere-asthenosphere (LAB) boundary within the tectosphere for each section. A series of such "mantle drill holes" can be used to map SCLM geology in 2 or 3 dimensions, and the 4th dimension (time) can be added where multiple episodes of volcanism occur in one region. Paleozoic kimberlites intruding two Archean nuclei witiiin the Sino-Korean craton, separated by the TanLu fault zone, have sampled mantle sections with markedly different bulk composition, rock-type stratigraphy and fluidrelated metasomatic signatures. This finding suggests that the TanLu fault separates two originally distinct terranes within the craton. Tertiary parakimberlites and basalts in the same region sample thin, fertile Phanerozoic-type SCLM, implying the removal or dispersal of the Archean SCLM root. Contouring of data collected from sampling points along a traverse reveals regional trends in SCLM composition and stratigraphy, which can be interpreted geologically. Detailed images of the SCLM along a 1000-km traverse in the Siberian Platform, constructed using garnet data (>4000 analyses from -50 kimberlites) show mantle domains of distinct rock type distribution and composition. These domains correspond to closely crustal terranes mapped from surface geology and geophysics, suggesting that the terrane boundaries are translithospheric, that each terrane (microcontinent) had its own SCLM, and that these survived amalgamation into the Siberian Craton. Along the traverse, the Archean-Proterozoic boundary is clearly defined by changes in SCLM composition and lithospheric thickness. Changes in thickness and composition with time m the northern part cf the traverse are related to Devonian rifting. Detailed mapping in northem Canada reveals an unusual 2-layered lithosphere beneath the central part of the Slave Craton, with a shallow (<150 km) ultradepleted layer and a deeper more normal layer; this structure has been mapped over an area of >9000 km^. Toward the outer parts of the craton the deeper layer, interpreted as a plume head, rises to shallower depth, displacing the ultradepleted material. Proterozoic SCLM is found around the craton margins and along the trace of a major rift zone (Kilohigok Basin), and is mferred to have replaced the Archean SCLM by rifting and asthenospheric upwelling. The Chemical Tomography technique can be combined with geophysical analysis to extend the mapping to areas away from the mantle sampling points provided by kimberlites and other volcanic rocks.
188
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE EAST YILGARN DATA COMPENDIUM- COLLATING A CRATON'S GEOSCIENCE AT 1:100 000 P.B. Groenewald, M. McCabe, M. Painter and F.I. Roberts East Yilgam Custodianship, Geological Survey of Western Australia, P.O. Box 1664, KALGOORLEE WA 6430
The Geological Survey of Western Australia (GSWA) geoscientific data custody is divided according to geological provinces, with that of the eastern Yilgarn Craton as a forerunner in establishing the structure of the new relational databases. As a consequence of its mineral wealth, this 520 000 km^ region has been subject to a century of diverse geoscientific studies, predominantly mineral exploratory in nature, but also including standard GSWA mapping at 1:100 000 and academic research into specific aspects of late Archaean crustal evolution. This enormous volume of information resides mostly in hard copy geological maps of various ages, numerous publications ranging from GSWA explanatory notes for maps to volumes in the international journals, and the many thousands of statutory reports from all tenement holders and prospectors which make up the West Australian Mineral Exploration (WAMEX) microfiche library. Access to this vast and valuable data resource is almost entirely manual at present, requiring human resources to recover information from historic tenement holdings. Any advance in making this data readily available requires development of a major relational database system wherein one may query and analyse any aspect of the geoscience information from spatial, structural, stratigraphical, petrological, geochemical or interpretative angles. Initial development of what is essentially the foundation to a data warehouse has been done through the collation of geological maps to create a seamless GIS spatial database. This was done in ARC/INFO from existing 1:100 000 geological maps, some digitised from hardcopy, others converted from publication versions developed using Microstation graphics. Establishment of the initial database structure was done using fifteen GSWA maps from the central Eastern Goldfields, with due geological edge-matching and standardisation of rock types. A most demanding aspect of this was the development of a map legend suitable for the entire province, which is a structurally complex and poorly exposed region where stratigraphic controls and correlations are sparse, and where the mapping has been done by different organisations with diverse geological approaches and standards of documentation. At present, all other existing map coverage of the Eastern Goldfields is being incorporated at the same scale, while detailed mapping of the remainder is in progress and, simultaneously, the database system is being expanded to include text and visual data. A spatial index to the exploration data will allow linkage to WAMEX. The identity and spatial relations of all WA mineral tenement holdings already exist in a graphical database (TENGRAPH) and it is envisaged that linkage of this to the geoscience data system will be a powerful tool for exploration companies and prospectors. GSWA field mapping and petrographic data are already housed in WAROX, a database to which linkage will be made quite readily. For the purposes of the spatial database, the maps are also being digitised and combined at a scale of 1:250 000 in a somewhat simplified format, with tectonostratigraphic rather than simple lithostratigraphic units being used. This is advanced for the entire East Yilgam, with over 100 greenstone belts and even greater numbers of granitoid bodies identified in the intervening areas. Comparison of the geological characteristics of tectonic units using these two layers should facilitate examination of the differences and correlations between the local greenstone belt stratigraphies, allowing further development on petrogenetic and tectonic models for the Yilgam Craton (Myers, 1997, and citations therein). Similarly, relational study of laboratory data such as petrography, regolith, rock and mineral chemistry, stable isotopes, metamorphic assemblages, fluid inclusion characteristics and geochronology will also become possible as the system develops, allowing progress in understanding of the region and thus in the application of conceptual as well as empirical prospecting activities. Other data available for large parts of the region, such as gravity, airbome magnetic and radiometric sensing, Landsat TM and aerial photography, have already been used in hard copy form for compiling the GSWA outcrop and interpretative maps. Access to all these data types as additional layers in the database may be possible for some users in the future, no doubt leading to development of progressively more accurate solid geology layers, significant advances in conceptual understanding, and greatly enhancing the mineral prospectivity of what is already the most rewarding fragment of Archaean cmst in the world. REFERENCES Myers, J.S. 1997. Preface: Archaean geology of the Eastern Goldfields of Western Australia - regional overview. Precambrian Research, 83, 1-10. Acknowledgement: This abstract pubhshed with the permission of the Acting Director, GSWA.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
NEW DIRECTIONS IN SOIL SURVEY: SOIL LANDSCAPE MODELLING IN BUNDABERG Mike J. Grundy and Ross D. Searle Resource Sciences Centre, Department of Natural Resources, Indooroopilly, Queensland 4068 Department of Natural Resources, Bundaberg, Queensland 4670
Traditional land resource survey is based on the development of implicit conceptual models by land resource surveyors, which link point specific land attributes and a range of spatially distributed environmental data (one or more of geology, topography, climate, vegetation, land use, hydrology, aerial photograph reflectance patterns). Land resource surveyors gradually develop expert know^ledge of their study area. The techniques of survey are thus underpinned by a dynamic and evolving series of local mental models which allow the surveyor to interpolate between sparsely located data collection sites. The Enhanced Resource Assessment project in Queensland is developing new resource assessment approaches which build on this knowledge. With new developments in computerised spatial data and tools, the hitherto implicit modelling process can be made explicit and predictive. Moreover, this modelling process facilitates the prediction of individual attributes rather than classes of soil types and better reflects the natural variability of soil attributes across the surface of the earth. This paper describes the use of these techniques in the prediction of key soil attributes which influence the management of basalt soils in the Bundaberg area. Two explicit models for the distribution of soil attributes were constructed using conceptual soil landscape relationships, analysis of published studies and existing local data and implemented in a computerised raster modelling environment. These models were designed to be the first stage of an iterative modelling approach the models inform the subsequent sampling program and are further refined based on the targeted sampling. The end product is a predictive understanding of soil attribute distribution which can be expressed as a computerised surface of known reliability. The modelling process used a DEM (Digital Elevation Model - 20m grid) produced from digitised contours and drainage vectors, using ANUDEM software. A series of derived landscape attribute surfaces was generated using TAPES-G software and other algorithms. The first model used a hierarchical rule-based approach. Potential explanatory landscape attribute variables derived from the digital elevation model were used in a hierarchical decision tree to determine soil attributes for each 20m cell . The hierarchical approach assumed that different pedogenic processes occur as landscape changes so that the importance of explanatory variables is not constant across the surface. An approximately continuous surface of predicted attributes is produced. The resulting surfaces are testable, readily updated after further sampling and can feed directly into environmental process models or models for land suitability for environmental management. The second modelling approach used a fuzzy rule-based system to classify the raster data sets and thus map the likely occurrence of the soil attributes. Each soil attribute was considered to be a smgle class, and each grid cell was considered to have a degree of membership in each soil attribute class. Membership functions were generated for each soil attribute based on the distribution of values within each raster data set of derived landscape attributes. Rules were generated to allocate membership in all soil attribute classes for each grid cell. For example, an appropriate rule set would be: If landscape attribute slope is high, then the soil attribute, soil wetness, is very low; if profile curvature is highly concave, soil wetness is high. The rules were derived from the specific relationships identified during the development of the hierarchical model. The membership fimctions and rules were codified in fuzzy system software and a final membership in each soil attribute class was generated using fiizzy algebra. Memberships in each soil attribute class were visualised by mapping the output raster datasets. The maps indicate the likely location of specific soil attributes (ie. high membership in an attribute class) and areas of transition (moderate membership in several attribute classes). Both approaches have strengths and weaknesses. The hierarchical approach grew easily from traditional soil survey and therefore has strong learning value, but the crispness of the categories introduced error which was difficuh to accommodate . The use of fuzzy class memberships rather than crisp classes permitted a more continuous representation of gradually changing soil attributes, visualisation of intergrades between classes and provided more robustness in the predictions. A validation data set has been collected independently of the modelling process to establish the reliability of the predictions.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
THE NATURE OF THE BASEMENT TO THE KIMBERLEY BLOCK, NORTHWESTERN AUSTRALIA Peter Gunn and Anthony Meixner Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601
The Kimberley Block of northwest Australia is bounded on its southeastern side by the Halls Creek Mobile Zone and on its its southwestern side by the King Leopold Mobile Zone. The northeastern and northwestern extensions of this area are covered by the Joseph Bonaparte Gulf and the Timor Sea. In its present configuration the Kimberley Block is covered by gently folded and warped Early Proterozoic sediments containing Early Proterozoic basalt flows and dolerite sills. The area is sometimes referred to as the Kimberley Basin. The origin of the Kimberley Block and the nature of its basement has attracted considerable conjecture. Studies of the area have been frustrated by the blanketing cover of the Proterozoic rocks of the Kimberley Basin. The area has recently been surveyed by high quality aeromagnetic surveys and the interpretation of the resultant data provides new insights into the basement of the area. Studies of the aeromagnetic data are complicated by the magnetic effects of the extensive Proterozoic Hart Dolerite and Carson Volcanics whose cumulative thicknesses are the order of several kilometres. These basic intrusives and extrusives, which are known in much of the area of the Kimberley Basin, appear to continue over an extensive area to the south of the Kimberley Basin plus the area covered by the southern Joseph Bonaparte Gulf. This assemblage of sills and volcanics has been interpreted as being a result of a flood basalt phenomenon related to hot spot activity. Igneous feeders to these horizontal sheets have been identified. Upward continuation and stabilised downward continuation filtering of the magnetic data is successful in suppressing the magnetic effects of the Hart Dolerite and the Carson Volcanics thereby allowing the delineation of magnetic features in the basement of the area The initial stage of the basement study was to produce a map of the distribution of magnetic anomaly sources in the filtered datasets. Correlations with gravity data and modelling of magn'i^tic and gravity data established limits on the depths, densities, magnetisations and geometries of the various sources. This process allowed various hypotheses for the geological and tectonic evolution of the area to be examined within the constraints of what is actually present. Several terranes with distinct magnetic and gravity characteristics can be recognised in the basement to the Kimberley Block. The basement to the southeastern half of the Kimberley basin is interpreted to be a type of non-magnetic granitic terrain which, is bounded on its northwestem limit by a northeast trending linear zone of magnetic granitic intrusions. Northwestward of the linear zone of magnetic granites it is possible to distinguish two additional basement terranes whose lithologies are not obvious however their differing rheological characteristics appear to have acted as controls of swarms of northeast trending fractures that are localised over their boundaries. These fractures are filled with reversely magnetised dolerite dykes. A conjugate set of northwest trending fractures contains normally magnetised dolerite. The fractures appear to have controlled Devonian-Carboniferous extension in the Bonaparte Gulf area and may be the result of strain accompanying simultaneous DevonianCarboniferous extension in the Bonaparte Gulf, northeast of the Kimberley Block, and in the Fitzroy Trough southwest of the Kimberley Block. A linear zone of dense, magnetic, northwesterley dipping material under the southeastem edge of the Kimberley block could be due to ophiolitic material associated with a subduction process postulated by various previous workers in the area. Northeasterly trending granites within the Kimberley Block basement may have resulted from such a subduction process. Plots of the geographical distribution of the known kimberlite intrusions in the area show clusterings over the anomalies interpreted as due to ophiolitic material and adjacent to anomalies interpreted as indicating feeders to the Hart Dolerite and Carson Volcanics.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
MAPPING AUSTRALIAN GEOLOGY UNDER COVER: A MODEL STUDY APPLIED TO THE BOULIA AND SPRINGVALE 1:250 000 MAP SHEETS, QUEENSLAND By PJ. Gunn, A. Meixner, T. Mackey and D. Blake Australian Geological Survey Organisation GPO Box 378, Canberra ACT 2601
Eighty five percent of Australia is covered by flat lying sediments, sand, and soil and as a result exploration for mineral resources in non-exposed "basement" rocks such as comprise Archean, Proterozoic and Palaeozoic terranes must rely on remote sensing methods and drilling. The prime questions in assessing the resource potential of non-exposed basement areas are: what rock units occur in the area, what is the resource potential of these rock units; what is the depth to the basement rock imits, and do the rock sequences covering the basement rock units have a resource potential? The Australian Geological Survey Organisation has produced an interpretation of aeromagnetic, gravity and available outcrop and drill information which provides the required answers for the Boulia and Springvale 1: 250 000 map sheets that cover the area where units of the prospective Mount Isa Inlier plunge southward beneath a cover of flat lying Cambrian and younger sediments. A methodology has been developed for this interpretation which could be used as a basis for a nationwide study of "Australia under cover". The prime tools of the interpretation were aeromagnetic and gravity data which map basement imits beneath the cover units. Various processings and enhancements of the geophysical were prepared and combined with geological and available drill information data in a Geographical Information System (GIS). The interpretation was produced as a GIS overlay. Magnetic images map magnetic units and the starting philosophy of the magnetic interpretation was to produce an initial "magnetic source map". The outlines of magnetic sources (together with depth extents and dips where possible) were estimated on the basis of the basic laws of magnetic induction assisted where appropriate by reference to compilations of model magnetic body responses. The magnetic source map can be regarded as a fact map, that could be produced by any competent physicist with no knowledge of geology. The geological stage was reached when, as far as possible, geological identifications were assigned to the magnetic sources and structure and the distribution of non magnetic units was inferred. Geological identifications were simplest to make when control in the form of an overlay of aeromagnetic data with mapped outcrop geology either within or adjacent to the study area was available. Where such control was lacking tentative identifications were made on the basis of source geometry, the relative magnetisation of the source, the character of the magnetic response of the source and by the spatial relationship of the source with other sources. Gravity data which maps relative densities and drill information assisted with source identification and mapping of non-magnetic units. In cases where it was difficult to assign a lithology to a source the source was represented by its outline together with an indication of its relative magnetisation. Later information may allow the source's identification and in any case such a representation shows that it exists. Structure can be deduced from the geometrical relationships of sources. The computation of the depths to magnetic sources was an important component of the economic assessment. The depth to basement study also indicates if the sediment cover is likely to have hydrocarbon prospectivity. The interpretation defined the southern continuation of prospective units of the Kalkadoon-Leichhardt Trough and the Eastern Succession. Volcanic flows were identified in the Cambrian succession of the Georgina Basin and groups of small circular magnetic anomalies having the characteristics of anomalies due to kimberlite intrusions were identified in the area covered by the Georgina Basin sediments.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
TOWARDS BETTER GEOPHYSICAL MODELLING OF THE HAMERSLEY IRON PROVINCE - 1 : MAGNETIC PETROPHYSICS Wanwu Guo, Zheng-Xiang Li. Michael C. Dentith and Christopher McA. Powell Tectonic Special Research Centre, Department of Geology and Geophysics The University of Western Australia, Nedlands, Western Australia 6907
The Hamersley Iron Province of Western Australia contains extensive banded iron formations (BIFs) and large iron deposits. The investigation of magnetic properties of the Hamersley BIFs, iron ores and other rocks is the first step towards a better magnetic anomaly interpretation over the province. Different jfrom any other rock, BIF is characterised magnetically by high susceptibility, strong anisotropy, high natural remanent magnetism (NRM) (or high Q value) and strong self-demagnetisation effects. These factors make the features of a BIFrelated magnetic anomalies very complicated, which are illustrated by our theoretical models. Five himdred and seventy-four oriented samples and some industrial drill cores were collected from the province for petrophysical and palaeomagnetic analyses. Some magnetic petrophysical results obtained so far are summarised below. MEAN BULK SUSCEPTIBILITY (MBS) The five main iron formations of the Hamersley Group all show a similar distribution pattern of MBS - a logarithmic bimodal distribution. This means that each iron formation has two major susceptibility groups. For the Brockman Iron Formation, however, the MBS of the Joffre Member BIFs exhibits a logarithmic normal distribution, whereas that of the Dales Gorge Member BIFs shows a similar distribution to the BIFs in the Hamersley Province as a whole. The Weeli Wolli, Joffre, Dales Gorge and Mount Sylvia BIFs have the highest susceptibility. The Boolgeeda BIF has intermediate susceptibility but it is still higher than that of the iron ores and other non-BIF rocks. The Marra Mamba BIF has the lowest susceptibility, which is even lower than the iron ores and dolerite dykes. Iron ores have intermediate susceptibility. Brockman martite-microplaty hematite ores have slightly higher susceptibility than Marra Mamba martite-goethite ores. Fresh dolerite and Cheela Spring Basalt have variable susceptibility. Other rocks are low in susceptibility. ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS) The Dales Gorge, Joffre and Weeli Wolli BIFs have strong AMS with well-developed bedding-parallel or subbedding-parallel magnetic fohation. Some hematite ores, the Boolgeeda BIF and the Mt McGrath hematite conglomerate have low AMS with recognisable sub-bedding-parallel magnetic foliation. Other rocks, including the badly weathered BIFs of the Marra Mamba Formation, are generally isotropic. NATURAL REMANEIVT MAGNETISATION (NRM) With respect to the current horizontal, the NRM direction of the Joffre, Mount Sylvia and Weeli Wolli BIFs are almost totally scattered. However, a basin-wide northwest-up post-folding NRM direction from the Boolgeeda BEFs, and a northwesterly sub-horizontal post-folding NRM direction from the Marra Mamba and Dales Gorge BIFs, can be recognised. Martite-microplaty hematite ores of the Tom Price, Paraburdoo and Channar deposits, and the Barrett-Lennard hematite conglomerate, have a northwesterly sub-horizontal NRM, while the NRM of the Mount Whaleback ores lies in the northwest direction with a positive inclination around 55®. NRM of Marra Mamba surface martite-goethite ores generally has a northwesterly sub-horizontal Fisher-mean direction, but the difference between the Fisher and vector means is large. Dolerite dykes and Cheela Springs Basalt have a relatively high NRM intensity, but the directions are scattered. NRM of other units has a low intensity and is insignificant for magnetic modelling. Weathering and lightening have greatly affected the magnetic properties of some of the surface samples. Reasonable estimation of the characteristic susceptibility and NRM of BIFs from the data gathered from surface samples remains a crucial problem for achieving better magnetic anomaly interpretation, which is the aim of the next stage of the study. Acknowledgements: The Minerals & Energy Research Institute of WA, BHP Iron Ore, Hamersley Iron Pty. Limited, and Robe River Iron Associates are thanked for supporting this research. Academic support and technical and field assistance, were provided especially by D. Kepert, M. Kneeshaw and J. Ronaszeki (BHP), M. Pal, M. Flis, K. Dettbam, J. Phillips and D. Flynn (HI), T. James, D. Mason and C. Robinson (RRIA), R. Hackney, D. Martin and R. Powell (UWA). WG's PhD study is supported by a Commonwealth Government of Australia Overseas Postgraduate Research Scholarship, and a UWA University Postgraduate Award.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
A TURBIDITE-HOSTED, GOLD(-BISMUTH) QUARTZ VEIN DEPOSIT, UNION HILL MINE, MALDON, CENTRAL VICTORIA Alistair C. Hack\ John A. Mav^ogenes^ Paul O. Hoskin, and Robert J. Scott^ Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 Present addresses: 'Australian Geological Survey Organisation, PO Box 378, Canberra, ACT 2601 ^GEMOC, Department of Geology and RSES, The Australian National University, Canberra, ACT 0200 ^CODES SRC, University of Tasmania, GPO Box 252-79, Hobart, Tasmania, 7001
The Union Hill gold mine, located in Central Victoria, occurs in a high-grade contact metamorphosed succession of Ordovician quartz-rich turbidites. The predominantly gold-bismuth mineralisation is hosted in structurally controlled quartz veins. Mineralised veins are related to a high-angle, west-dipping, reverse fault zone consisting of a series of discontinuous, approximately north west-stepping en echelon fractures (Linscott's- and Eaglehawk reefs). Faulting postdates folding. A reverse displacement of about 30 m is inferred. The fault zone is nonplanar, having been influenced by fold geometry. Mineralisation is concentrated in a dilatant zone that cuts across bedding between the German Anticline-Boundary Syncline fold pair. Complex and deformed vein geometries are common throughout the sedimentary succession but are best developed adjacent to the major reverse fault zones. Structural relations between bedding and veins indicate protracted vein growth during folding. Deformed vein geometries and cross-cutting relations can be easily explained by varying amounts of limb rotation, flexural-slip, and flexural-flow throughout the sedimentary package during folding. A later, relatively minor, strike-slip deformation has also affected the deposit and probably involved reactivation of earher high-angle reverse fault segments. The exact magnitude and the movement-sense of this event are not known but were probably small and involved conjugate faults. Post-regional deformation emplacement of the Harcourt Granite produced a large contact metamorphic aureole over much of the goldfield. At Union Hill, peak contact metamorphic pressure-temperature conditions woe about 1-2 kbars and >500°C, and post-dated quartz veining. The low-temperature gold-bismuth assemblages would have been mobile under these conditions. Original/pre-metamorphism ore textures could not be identified. A SHRIMP U-Pb age of 366db5Ma obtained from zircons from a felsic dyke that cross-cuts the quartz veins is coeval with the Harcourt Granite of 361±7Ma (Richards and Singleton, 1981). This places a minimum age on regional deformation in central Victoria and clearly demonstrates that the Harcourt Granite was not involved in the formation of the mineralised quartz veins. Zircons separated from a mineralised quartz vein yielded a spectrum of ages that are consistent with derivation from the Ordovician host-rocks. A combination of fluctuating high-fluid pressures and shear stress, as predicted by fault-valve behaviour (Sibson et al., 1988), appears to explain geometric and timing relations between veins, faults and mineralisation. Faultvalve action provides potential gold depositional mechanisms and may explain the predominantly quartz veinhosted mineriisation. The relations between fault-controlled quartz-vein structures and mineralisation at Union Hill are consistent with a syn-deformational origin for the gold-bismuth mineralisation. No unequivocal evidence was observed (or has been previously documented) which indicates granite-related bismuth-tungsten-molybdenum-bearing fluids have permeated through the aureole, modifying existing gold-quartz vein deposits, as proposed by earlier workers. Consequently, similar quartz vein-hosted deposits may not be restricted to the turbidite succession within the contact metamorphic aureole around Maldon. REFERENCES Richards J.R. and Singleton O.P., 1981. Paleozoic Victoria Australia; Igneous rocks, ages and interpretation. Journal of the Geological Society of Australia 28, 395-421. Sibson, R.H., Robert, F., and Poulsen, H., 1988. High-angle reverse faults, fluid-pressure cycling and mesothermal gold-quartz deposits. Geology 16, 551-555. Acknowledgments: We would like to thank Alliance Gold Mines for generously providing access to the Union Hill gold mine.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
DEEP SEISMIC REFLECTION AND REGIONAL GEOPHYSICS OF THE HAMERSLEY PROVINCE FORELAND FOLD-AND-THRUST BELT Ron I. Hackney'. Bruce R. Golebjr, Christopher McA. Powell', David A. Hollingsworth^ Peter A. Cawood\ Barry J. Drumniond% Doug Kepert'* and David Mason'' 'Tectonics Special Research Centre, Dept of Geology & Geophysics, University of Western Australia, Nedlands WA 6907 -ANSIR, c/- Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601 ^Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth WA 6001 ^BHP Iron Ore, PO Box 7122, Cloisters Square WA 6850 -'Robe River Iron Associates, PO Box P1224, GPO Perth WA 6001
INTRODUCTION New multi-channel deep-seismic reflection profiles cross the southeastern portion of the early Proterozoic Hamersley Province foreland fold-and-thrust belt in the Pilbara region of Western Australia. Results from surface mapping along with gravity and magnetic data collected in conjunction with the seismic data are being used to provide constraints on geological cross-sections through the province. The seismic work was conducted during July and August of 1997 by AGSO and processing of the data is continuing. The first year of the project has yielded substantial amounts of new information, both geological and geophysical, that builds on the extensive knowledge of the province gained by the participating companies, BHP Iron Ore and Robe River Iron Associates. SEISMIC WORK Seismic work comprises two north-south profiles totalling approximately 135 km. The first profile is located 100 km west of Newman and begins in the relatively undeformed foreland. It continues south across two prospective iron ore regions into the more highly deformed parts of the fold-and-thrust belt. The second line lies 40 km east of Newman and crosses the southern margin of the Hamersley Province onto the exposed Pilbara basement of the Sylvania Inlier. The seismic data have now been processed to brute-stack and, at this early stage, there are parts of the seismic sections where an interpretation of subsurface structure can be made; however, there are other sections where further processing will be required to clarify possible interpretations. Overall, the sections show good midcrustal and deeper reflections and provide an indication of the depth to the basement-cover contact. Tirstarrivar information indicates that there are significant near-surface variations along the traverses. This information will be used to obtain a better image of the top 10 m to 100 m of near-surface geology along the traverses; information particularly useful in any mining operations. GRAVITY WORK Gravity data were collected along the seismic lines at shot-points spaced 240 m apart, using a Scintrex CG-3M gravity meter. Shot point elevations, accurate to approximately 20 cm, were determined using differential GPS. The Bouguer anomaly profiles correlate with expected subsurface geometry on a regional scale and further constrain Hamersley Province geometry. Simple gravity models indicate that the Fortescue Group increases in thickness from - 2 km in the north to almost 5 km in the south. This increase in thickness may reflect stratigraphic thickening during accumulation of the Fortescue Group, thrust stacking, or some combination of these two possibilities. Regional Bouguer anomalies over the Pilbara craton become increasingly negative from north to south. Such a variation reflects the increased crustal thickness required to compensate for the additional load of Hamersley Province rocks on the southern margin of the craton. An increase in crustal thickness is supported by seismic refraction work (Drummond, 1981) and the seismic reflection results from this project. REFERENCES Drummond, B.J., 1981. Crustal structure of the Precambrian terrains of northwest Australia from seismic refraction data. BMR Journal of Geology & Geophysicsy 6, 123-135. Acknowledgements: This project is being carried out by the Tectonics Special Research Centre (University of Western Australia, Curtin University of Technology) and the Australian Geological Survey Organisation, in close consultation with BHP Iron Ore and Robe River Iron Associates. Funding is provided by these companies, the Minerals and Energy Research Institute of Western Australia and the Tectonics Special Research Centre.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
PRE- AND SYN-TECTONIC SEDIMENTATION ASSOCIATED WITH THE DELAMERIAN OROGENY, SOUTH AUSTRALIA: A REAPPRAISAL OF STRATIGRAPHIC RELATIONSHIPS AND TIMING OF EVENTS Peter W. Haines^ and Thomas Flottmann^ 'Department of Applied Geology, University of South Australia, Levels Campus, Mawson Lakes SA 5095 ^Department of Geology and Geophysics, University of Adelaide, Adelaide SA 5005
The Cambro-Ordovician Delamerian Orogeny and the equivalent Ross Orogeny of Antarctica, were fundamental events which affected the Palaeo-Pacific margin of Gondwana, setting the stage for the subsequent development of the Tasman Orogenic System further east. Despite this importance, fundamental details such as the timing of events and relationships between sedimentation and tectonics remain poorly understood. The Delamerian Orogen of South Australia includes a thick deformed succession of Cambrian age. Cambrian rocks also occur as thinner platform cover sequences in adjacent regions. It has been generally assumed that all of the Cambrian successions are pre-tectonic, deposited prior to the Delamerian Orogeny, and that any syntectonic foreland sedimentation has not been preserved. This concept is strongly supported by the prevailing regional correlations. Under this scheme the clearly pre-Delamerian Kanmantoo Group, which has been deformed and metamorphosed in the highest grade southern core of the orogen, is equated with a sequence of predominantly clastic shallow marine to terrestrial red beds which form the upper part of the Cambrian succession elsewhere. This 'red bed package' includes the Billy Creek Formation to Lake Frome Group in the Flinders Ranges, the succession above the Minlaton Formation on Yorke Peninsula and the upper Kangaroo Island Group of northern Kangaroo Island. The thick turbiditic Kanmantoo Group has no internal biostratigraphic or radiometric age constraints. However, recent U-Pb dating of what is generally considered an early syn-tectonic granitoid (Rathjen Gneiss), which intrudes the Kanmantoo Group, gives an age of around 516 Ma (ie. late Early Cambrian utilising the AGSO Australian timescale). Meanwhile Middle Cambrian marine fossils occur in the upper parts of the 'red bed package'. If the time scale is accurate, this suggests that the 'red bed package' is in part, or entirely, younger than the Kanmantoo Group. A reappraisal of facies, isopach and palaeocurrent data suggests that the 'red bed package' is most likely of syn-tectonic origin. There is evidence that these sediments were largely eroded from the earliest formed part of the Delamerian Orogen which is envisaged as the development and westward encroachment of the sigmoidal fold-thrust belt comprising the Fleurieu and Nackara arcs in the southern part of the Delamerian Orogen. The sharp and often unconformable base to the 'red bed package' may thus pin down the timing of the initiation of uplift. Subsequent phases of the orogeny mildly deformed these early syn-tectonic deposits in some areas such as the Flinders Ranges. In the north, orogenic loading was insufficient to create a true foreland basin. However, further south such a basin may be expected to the west of the region of greatest thrust-generated shortening in the Fleurieu Arc. This is the region occupied by modem Gulf St Vincent. Recent dating of tuffaceous sediments near the base of the supposed syn-tectonic deposits suggest that uplift related to the earliest phase of the Delamerian Orogeny was probably initiated at approximately 522 Ma. If this model is correct, the Kanmantoo Group must have been deposited very rapidly and probably at the same time as the upper Hawker Group and its biostratigraphically established regional correlatives. Although quite different in facies, both groups show significant thickening and deepening into local sub-basins possibly related to strike-slip tectonics near the eastern margin of Gondwana just prior to the onset of the Delamerian Orogeny. In the case of the Kanmantoo Group, the accommodation space was rapidly filled by clastic sediments that entered the basin from the south on palaeocurrent evidence. Recent detrital zircon dating has indicated that the sediment is unrelated to previously proposed local sources. Their source may have been in the region of modem Antarctica, where the contiguous Ross Orogeny had already begun at this time. In contrast, the Hawker Group is largely composed of carbonates and shales. The Narina Greywacke, locally developed at the top of the Hawker Group, may represent a minor incursion of Kanmantoo Group sand into the northem basin.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE ROCKLEA DOME, SOUTHERN HAMERSLEY PROVINCE: A PRE-EXISTING TOPOGRAPHICAL HIGH DURING DEPOSITION OF THE LOWER FORTESCUE GROUP? Charlotte Hall Teaonics Special Research Centre, Dept. of Geology and Geophysics, The University of Western Australia Nedlands, Perth, WA6907,
In the Hamersley Province of Western Australia, exposure of Archaean granite-greenstone rocks of the Pilbara Craton is restricted to small inliers along the southern margin of the province. Late Archaean volcanosedimentary successions of the Fortescue Group rest unconformably on the granite-greenstone terrane. The seven major stratigraphic units of the Fortescue Group, in ascending order, are the Bellary Formation, Mount Roe Basalt, Hardey, Boongal, Pyradie, Bunjinah, and Jerrinah Formations. These formations record a progressive change from pre-rifting volcanism (Mt Roe Basalt) to clastic sedimentation during crustal extension (Hardey Sandstone), followed by a marked increase in basaltic volcanism (Boongal to Jeerinah Formations) with continuing subsidence. Studies of the lower Fortescue Group on the Pilbara craton indicate that active NNE trending, west block-down growth faults controlled the depositional patterns of tholeiitic basalts and clastic sediments on the northern Pilbara Craton, where as in the south, similar formations were dispersed over a relatively stable region with low relief. The Rocklea Dome, a small inlier located 60 km NE of Paraburdoo, has an exposed core of granite-greenstone rocks. Rocks of the Mount Roe Basalt and Hardey Formation unconformably overlie the basement around the edge of the dome. The Mount Roe Basalt consists of basalt flows characterised by a strongly vesicular and amygdaloidal texture, basaltic breccias and a hyaloclastic breccia of pillow basalt fragments. Previous maps of the Rocklea Dome have restricted the Mount Roe Basalt to the southeastern margin of the dome. However, recent mapping has identified two new locations of the basalt on the northwestern margin of the dome. The Hardey Formation is typically a very coarse quartzose sandstone with planar and cross-bedding at the base, gradually fining upwards to very fine sandstones and siltstones. The top of the formation coarsens up to a quartz-rich sandstone. Locally well rounded polymictic conglomerates are present at the base of the formation on the northern and eastern sides of the dome. Palaeocurrent indicators are predominantly found in the medium to coarse sandstone facies of the Hardey Formation and are measured from planar and trough cross beds. Restored palaeocurrent measurements from the Rocklea Dome show a strong pattern of divergence around the dome from east to west, supporting the interpretation that the core of the dome was a topographical high during the deposition of the lower Fortescue Group. Regionally, restored palaeocurrent measurements from along the southern margin of the Hamersley Province indicate a westerly flow for the Hardey Formation The Rocklea topographical high may have been fault controlled. The distribution of the Mount Roe basalt combined with the palaeocurrent information suggest that NNE to NE trending faults could have controlled the distribution of the lower Fortescue Group Formations. Alternatively, the distribution of the Mount Roe Basalt on the Rocklea Dome may have been along palaeovalleys preferentially eroded in metabasalts of the Archaean basement. Palaeovalleys could have existed along the southeastern margin of the dome where the Mount Roe is in contact with metabasalts. However, the new outcrops of Mount Roe along the northwestern margin unconformably overlie cherts, metamorphosed quartz-rich sandstones and granite that are more resistant to erosion.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
WHITHER TERTIARY GEOLOGICAL EDUCATION? Lloyd Hamilton School of Natural Resource Sciences, Queensland University of Technology,Brisbane , Queensland 4001
In these days of economic irrationalism with restructuring, financial cut-backs and downsizing, all Geology Departments are struggling. It could be expected that the old established sandstone universities would weather the storm and the newer and the less financially able departments would be under threat. To a large extent this is true but with some interesting exceptions. Pressure on staff levels has caused amalgamation, retrenchments and staff attrition. Enrollments have been increased or disguised throughout Australia by the following: 1 2 3 4
Integrating geology with biology directly or through environmental science, at the unit level, at the course level, and at Departmental level. The push to environmental science is widespread. Integrating with geography Integrating with civil engineering Going multidisciplinary or interdisciplinary.
Entrance levels have fallen in some places and risen in others. The requirement for a degree, preferably in science, as an entrance requirement into medicine has increased the quality and numbers of students enrolling in biology. This has a flow on effect on geology courses. The ratio of female to male students has also risen dramatically throughout Australia The Minerals Council of Australia is seeking to reform and rationalise geological education with their "Back from the Brink" initiative which proposes to control geological education with a National Tertiary Education Task Force. There is considerable resistance to this and the initiative will probably be largely ineffective. There appears to be a greater demand now for education beyond one degree to gain employment as a geologist. This is not being met so much by PhD programs which are now less cost effective. Honours degrees and second degrees are now popular and double degrees are becoming popular. There also appears to be a new tendancy for advanced geological education to move away from universities to mining companies. On-going post graduate education is a diversifying field and will probably see more diplomas given for aggregate completion of otherwise stand-alone modular units delivered in a flexible manner. Umbrella cooperatives such as VIEPS (Victorian Institute of Earth and Planetary Sciences) will cement institutions together to form relatively strong structures. Industry is likely to become even more involved with post graduate teaching than hitherto. The situation may brighten as an election approaches but there is still an unmet and pressing need for more geological education of secondary school teachers.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
NEW INSIGHTS INTO THE EVOLUTION OF THE NORTHERN AMADEUS BASIN DURING THE EARLY PALAEOZOIC Martin Hand Department of Geology and Geophysics, Adelaide University, Adelaide SA 5005.
Intracratonic basins are commonly considered to be sensitive recorders of the tectonic processes that have affected a continental interior. This is because many intracratonic basins have long histories characterised by shifting patterns and styles of sedimentation that were largely influenced by the intraplate response to forces generated both within the plate and at its margins. However one of the problems with deciphering the record of basin evolution in terms of a tectonic framework is the sedimentological information is often ambiguous, with few obvious pointers to a particular tectonic environment. A good example of a long lived intracratonic basin that contains a complex record of sedimentation is the Amadeus and associated basins in central Australia (e.g. Korsch & Lindsay 1989; Shaw et al., 1991). Although the final stages of the basin evolution are comparatively well known and relate to the Devonian-Carboniferous Alice Springs Orogeny, many of the earlier episodes in the basin history are not well understood. One of these comparatively poorly understood intervals spans the late Cambrian to mid-Ordovician which corresponds to the deposition of the Larapinta Group (510—450 Ma) and Carmichael Sandstone (-450 Ma) and equivalents. During this period, sedimentation was initially localised close to the northern margin of the Amadeus Basin and the southwestern margin of the Georgina Basin. Subsidence rates were initially slow and sedimentation was characterised by comparative uniformity with only minor interuptions, and material was largely derived from reworking of existing sediments. With time the basin expanded southward across subdued topography and sedimentation rates increased. In the mid-Ordovician (c. 450 Ma) a profound change in basin shape with a shift in deposition away from the northern margin of the basin was associated with the deposition of the Carmichael Sandstone which was derived from erosion of the underlying sequences. Previous models for the late Cambrian to mid-Ordovician interval in the Amadeus and associated basins have loosely concluded this period was a time of relative tectonic quiescence, with sedimentation possibly controlled by thermal subsidence from an earlier extensional phase coupled with distal compressional affects associated with the Delarmerian Orogeny (e.g. Korsch & Lindsay, 1989; Shaw et al., 1991). The underlying reasons for the change in basin shape associated with the deposition of the Carmichael Sandstone are unexplained. However the recent recognition that regional high grade metamorphism and deformation occurred in the southeastern Arunta Inlier in the early Ordovician (Mawby et al., this volume. Miller et al., this volume) places the basin evolution over this interval in a new context. Although the full extent of this high grade event is yet to be determined, in the Harts Range region early Ordovician 475 Ma) granulites (8(X)°C, 9 kbar) were overprinted by a system of sub-horizontal shear fabrics (7(X)®C, 6.5 kbar) associated with ~ 10 km of near-isothermal decompression, a tectonic style common in basement terrains undergoing significant extension. This system was reworked at 450 Ma by an upper amphibolite facies compressional event associated with > 30km of south-directed thrusting along a low angle detachment. This data indicates that deposition of at least the upper half of the Larapinta Group occurred in an extensional intraplate setting associated with high grade metamorphism. The change in basin shape at -450 Ma associated with the deposition of the Carmichael Sandstone coincides with significant contraction along a near horizontal deep crustal shear zone, suggesting the Carmichael Sandstone is a partly preserved foreland-style deposit. The fact that previous workers have not recognised that Ordovician sedimentation in the Amadeus Basin was accompanied by major tectonism indicates the sedimentary record associated with significant intraplate deformation may be extremely subtle. Additionally if the general continuity and distribution of sedimentation throughout the deposition of the Larapinta Group is taken as a indicator of the continuity of tectonic processes, it suggests that deformation and metamorphism in the deep crust beneath the basin extends back as far as -510 Ma.
References Korsch, R.J. & Lindsay, J.F. 1989. Relationships between deformation and basin evolution in the intracratonic Amadeus Basin, central Australia. Tectonophysics, 158, 5-55. Shaw, R.D., Etheridge, M. A. & Lambeck, K. 1991. Development of the late Proterozoic to mid-Palaeozoic intracratonic Amadeus Basin in central Australia: a key to understanding tectonic forces in plate interiors. Tectonics, 10, 688-721.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
POSSIBLE CONTROLS ON THE DISTRIBUTION OF INTRAPLATE DEFORMATION ASSOCIATED WITH THE ALICE SPRINGS OROGENY Martin Hand and Mike Sandiford Department of Geology and Geophysics, Adelaide University, Adelaide SA 5005.
The Alice Springs Orogeny was a major intraplate event that affected much of the central Australian region during the Devonian and Carboniferous and was largely responsible for shaping the distinctive geophysical character of the central part of the Australian continent. Deformation resulted in exhumation of the Arunta Inlier and the southern part of the Musgrave Inlier during north and south directed thrusting along crustal scale faults. The fact the Alice Springs Orogeny involved significant shortening well away from active plate margins has received considerable attention which has been largely focussed on aspects of the structural architecture, the thermochronologic record and the long wavelength expression of the deformation. The localisation of compressional intraplate deformation must reflect either: (1) localised reductions in strength in the presence of an in-plane stress, (2) localised stress amplification due to changes in the density structure at depth or (3) some form of intraplate subduction. It is important to realise that prior to the Alice Springs Orogeny, the Arunta Inlier and much of the Musgrave Inlier were covered by sediments belonging two major intracratonic basins, the Officer Basin in the south, and a contiguous Amadeus-Georgina-Ngalia-Wiso system to the north, in which the preserved basins are effectively those regions that suffered relatively little basement involved deformation during the Alice Springs Orogeny. One of the intriguing aspects of the distribution of deformation associated with the Alice Springs Orogeny is a general spatial correlation between the location of major Devonian-aged basement thrusts that bound the exhumed inliers, and the distribution of thick pre-Alice Springs Orogeny sedimentation located in discrete sub-basins within the broader context of the Centralian Superbasin. These variations in sediment thickness reflect the history of long lived depocenters characterised by somewhat contrasting average histories. In the northern Officer Basin, the Munyari Trough has been largely dominated by a flexural evolution that reflects the influence of contractional orogenic belts to the north and east. In contrast in the southwestern Georgina Basin, the accumulation of thick sediment (> 10000 m) is restricted to deep narrow troughs that formed during local extension throughout much of the pre-Alice Springs Orogeny basin history. In the northern Amadeus Basin up to 5000 m of sediment accumulated over -200 Ma in a series superimposed sub-basins that appear to have formed during either compression or minor extension. These differing styles of sub-basin evolution have significant implications for long term regional variations in lithospheric strength. In general, flexural-style basins lead to greater long term lithospheric weakening than extensional basins, which are typically associated with attenuation of crustal heat production and shallowing of the Moho. The mechanical implications of contrasting styles of sub-basin development in the central Australian context are enhanced by the presence of regionally elevated domains of heat production in the Proterozoic basement beneath the basins. These domains of high heat production are best represented in now exhumed the Arunta Inlier, where crustal heat production may have contributed up to 60mWm"^ to the total surface heat flow. The fact that sub-basin formation can lead to localised lithosphere weakening provides a plausible basis for the hypothesis that the distribution of pre-orogenic depocenters exerted a first order control on the localisation of compressional deformation during the Alice Springs Orogeny. The potential influence of sedimentary overburden on the distribution of deformation associated with Alice Springs Orogeny in central Australia is highlighted by the record of reactivation of major faults that formed during the Late Neoproterozoic to early Cambrian Petermann Orogeny, which resulted in exhumation of the Musgrave Inlier. Along the southern margin of the Musgrave Inlier, the Petermann Orogeny was associated with the formation of south vergent thrusts. However, this margin subsequently underwent subsidence during the Cambrian and Ordovician resulting in burial of the southern margin of the Musgrave Block and the Petermann thrusts beneath up to 3 km of sediment. In contrast, the exhumed basement marginal to the southwestern Amadeus Basin did not undergo significant burial subsequent to the Petermann Orogeny. Although the orientation of Petermann-aged structures on the northern and southern margins of the Musgrave Inlier were both conducive to N-S shortening (as evidenced by their original history), reactivation during the Alice Springs Orogeny only occurred in the northern Officer Basin, along the thrusts that had been buried by significant thicknesses of sediment.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
EARLY CRUSTAL EVOLUTION OF THE GEORGETOWN BLOCK, NORTH QUEENSLAND Sarath Hapugoda,^ Kenneth D. Collerson,^ Jo Arnold*, J. Immo Wendt^and Peter W. Gregory^ 1.Department of Earthsciences, The University of Queensland, St. Lucia QLD 4072 2. Formerly BHP brisbane, now 72, Tristania Way, Mt Gravett East QLD 4122
Mid Proterozoic sequences are an economically important, yet tectonically enigmatic element of Precambrian crust in Australia Uncertainty exists concerning their crustal evolution, assembly and stabilization, lateral and vertical extent, and geological correlations (cf., Etheridge et al., 1987; Myers et. al 1996). Debate regarding the nature of crustal evolutionary processes responsible for the assembly of Proterozoic Australia relates to the issue of whether it has long been a single intact continent characterized by intracratonic, tectonic and magmatic activity or a collage of microcontinents that were assembled by plate tectonic processes (e.g., Myers et. al 1996). To establish a framework to better constrain the crustal evolution of Precambrian blocks in North Queensland, a regional field, petrological, geochemical, isotopic and U-Pb zircon geochronological study is being undertaken on the Einasleigh Metamorphics, Georgetown Block (GTB). The Einasleigh Metamorphics are interpreted by Withnall et al.,1988, as an autochthonous sequence of metasediments and metavolcanics >17 km thick. Our field mapping indicates that the Einasleigh Metamorphics comprise a Paleoproterozoic to Archean basement gneiss complex. The gneiss complex comprises orthogneiss derivedfromtonalite, trondhjemite and granodiorite (TTG), with stronglyflattenedpyroxenite and amphibolite dykes, as well as tectonically intercalated supracrustal paragneissic sequences (gamet-sillimanite gneiss, schist, quartzite). The apparent thickness of the ''Group" most likely reflects structural repetition. A relative chronology has been established based on observations in areas of low finite strain. This chronology has formed the basis for subsequent geochemical and geochronological investigations. The earUest package of gneisses (the Black Soil Creek Gneiss Complex - BSCGC) comprises medium to coarse grained compositely layered,migmatitic, trondhjemitic and granodioritic gneisses that are cut by homogeneous sheets of grey, fine to medium, tonahtic gneiss. BSCGC are tectonically intercalated with a varied suite of paragneisses dominated by cordierite-gamet-sillimanite gneiss and quartzite, termed the Stockman Creek Gneiss (SCG). This composite gneiss complex is intruded by a suite of mafic dykes and by subordinate bodies of leucogabbro and pyroxenite. A second supracrustal package (Junction Creek Gneiss - JCG) comprising sillimanite-bearing paragneisses and felsic to mafic metavolcanics (previously interpreted as calc silicate units) which lack mafic dykes is interpreted to have been deposited on, or tectonically intercalated with, the BSCGC and SCG. All of the above units are cut by regionally extensive syntectonic sheets and bodies of granite interpreted as crustal melts. Examples of these include the Upper Stockman Creek Granitic Gneiss and the Mt Surprise Railway Bridge Gneiss. During the final phase of thermotectonism under amphibolite to granulite facies metamorphic conditions, the gneiss complex was intruded by a series of large gabbroic and amphibolite dykes which cut all of the above units. Provisional chronology and Nd isotopic data for the Einasleigh metamorphics are as follows: 1. Black Soil Creek Gneiss Complex => Black Soil Creek TTG ortho gneiss: tonahte: 2.4 to 2.5 Ga (TDM), 8Nd(0): -15.6 to -25.9, granodiorite: 2.5 to 2.7 Ga (TDM), 8Nd(0): -21.8 to -24.4, trondhjemite: 2.5 to 2.6 Ga (TDM), 8Nd(0): -20.2 to -24.1, pyroxenite: 2.5 to 2.8Ga (TDM), 8Nd(0): 2.7 to 4.3, Stockman Creek Gneiss: quartzite, quartzite gneiss, metapelite: 2.5 to 2.6Ga (TDM), 8Nd(0): - 21.4 to -23.0.
Thermotectonism and development of composite layering. M ^ c Dykes (amphibolite): 2.3 to 2.4 Ga (TDM) , 8Nd(0): +2.2 to +4.8. 2. Junction Creek Gneiss => Pelitic gneiss, psammitic protoliths, quartzdte: 2.1 to 2.4 Ga (TDM), 8Nd(0): -21.8 to -22.2, Felsic to mafic metavolcanics: 2.5 Ga (TDM), 8Nd(0): -21.8. 3. Structural intercalation of (1) & (2) or Deposition of (2) on (1), 4. Extensive partial melting produced pegmatite in gneiss complex and S-type melts in JCG: 2.1Ga (TDM) , 8Nd (0):-24.2. 5. Syntectonic Intrusion of Leucocratic sheets: 2.0 to 2.4 Ga (TDM), 8Nd(0): -1.35 to -16.6. 6. Emplacement of Gabbroic bodies: 2.5 to 2.6 Ga (TDM), 8Nd(0): +2.6 to +4.2. 7. Emplacement of Post-tectonic basaltic and rhyolitic Permian to Carboniferous dykes Provisional data indicate that the Einasleigh metamorphics comprise an older basement as well as two supracrustal sequences. They do not represent an autochthonous package of metasedimentary rocks. The presence of this older basement complex indicates that crustal evolution of the Georgetown Block is significantly more complex than previously modeled, resembling the evolution of the Black Angel Gneiss Complex in the Mount Isa Block , (McDonald etal, 1997).
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
ESTIMATING SEDIMENT MOBILITY DUE TO SWELL WAVES AND TIDES ON AUSTRALIA'S CONTINENTAL SHELF P. T. Harris ^ 0 . Andersen and R. Coleman^ 1. Antarctic CRC and Australian Geological Survey Organisation, GPO Box 252-80, Hobart, Tasmania 7001 2. National Survey of Denmark, Rentemestervej 8 DK2400, Copenhagen, Denmark 3. Department of Surveying & Spatial Information Science, University of Tasmania, GPO Box 252-76, Hobart, Tasmania 7001 4. CSIRO, Division of Marine Research, GPO Box 1538, Hobart, Tasmania 7001
Estimates of tidal current speed and significant wave height and period were used to predict the continental shelf area over which sediment is mobilised (threshold exceedance). The ocean tide model is a linear, hydrodynamic model having 8 constituents and a resolution of 4 km. It is forced at the open boundaries using the AG95.1 ocean tide model derived from TOPEX/POSEBDON altimetry. The wave climatology was derived from a global climate model (GCM), produced at the European Centre for Medium-Range Weather Forecasts (ECMWF) which has a resolution of 1.5^. Five assumed grain sizes were used in the model runs: 0.01, 0.1, 0.5, 1 and 2 nun. A new digital bathymetry database for the Australian region, (Cameron Buchanan, AGSO, Pers. Comm.) grided at 1 kilometer resolution, provided the essential framework for the tidal modelling work. Tidal currents are estimated to be competent in initiating motion of fine sand (0.1 mm) over more than 17% of the Australian shelf. Regions of intense tidal sand transport include parts of the Gulf of Carpentaria, Torres Strait, southern Great Barrier Reef, Bass Strait, Spencer Gulf and the Northwest Shelf. The wave climate around Australia is such that 0.1 mm diameter quartz sand had the potential to be mobilised on at least one occasion between July 1992 and July 1995 over 52.7% of the continental shelf. The Australian Bight region has the most energetic wave climate, strong enough to mobilise 0.1 mm diameter quartz sand to water depths of up to 156 m at least once over a three year period. This modelling work used the ETOPO-5 grided bathymetry, which does not resolve complex areas such as the Great Barrier Reef due to its coarse grid spacing (5 nautical miles). Hence the results from these areas are unreliable. In the future we plan to combine tidal and wave-generated currents to assess the relative dominance of each processes regionally. Higher resolution bathymetry and grided shelf sediment properties will also provide input data for the estimate of more realistic shelf sediment threshold exceedance. Applications of this modelling work include predicting pollution dispersal, seafloor stability and benthic habitats. A key goal is to produce a classification scheme of Australia's EEZ using sediment mobility and dominant energy regime as classification criteria.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
COMPARISON OF MARINE SEDIMENT AND ICE-CORE RECORDS FROM THE CASEY REGION OF EAST ANTARCTICA; INITIAL RESULTS AND INTERPRETATIONS P.T. Harris^ T. van Ommen^ V. Mo^gan^ I. Goodwin^ and P.E. O'Erien"^ ^Cooperative Research Centre for the Antarctic and Southern Ocean Environment (Antarctic CRC) & Australian Geological Survey Organisation (AGSO), GPO Box 252-80, Hobart, Tasmania, Australia 7001 ^Antarctic CRC, & Australian Antarctic Division, GPO Box 252-80, Hobart Tasmania 7001 ^Antarctic CRC and SCAR Global Change Progranmie, GPO Box 252-80, Hobart Tasmania 7001 Antarctic CRC & AGSO, GPO Box 378, Canberra A.C.T., Australia 2601
Three sediment cores were collected from an over-deepened glacial trough on the Vincennes Bay inner shelf. Glacial marine sedimentation processes in the bay are characterised by siliciclastic sediments dominating over biogenic sediments in deep, inner shelf troughs. The cores are up to 5.5 m in length and AMS 14C dates at the base of the cores are up to 20,000 years BP. Down-core data includes biogenic silica, total organic carbon, grain size and magnetic susceptibility measured at 2-5 cm intervals . The Dome Summit South (DSS) ice core was drilled near the summit of Law Dome about 150 km to the west of Vincennes Bay and extends through about 1,200 m of ice to bedrock. The site has a high accumulation rate of about 0.7 m/a (ice equivalent) with little seasonal variation. The ice core provides a high resolution record of spanning the past 20,000 years or so, with progressive compression of the record down-core. Down-core variations in sediment properties and magnetic susceptibility are attributed to changes in the relative biogenic/terrigenous sediment supply. Peaks in terrigenous sediment input occur at 5,000, 11,000 and 20,000 years BP. The cause of these peaks and their relation to the ice core record will be discussed.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
NEW CONSTRAINTS ON THE INTRA-DEVONIAN UNCONFORMITY IN THE MOUNT MORGAN AREA Mark A. Havward. Barry G. Fordham, Cec G. Murray, Ross E. Randall, Paul R. Blake and Glenn A. Simpson Geological Survey of Queensland, GPO Box 194, Brisbane, Queensland 4001
The intra-Devonian unconformity in the Mount Morgan area is manifested by the contact relationships of two pairs of units, namely Mount Morgan Tonalite/Mount Hoopbound Formation* and Capella Creek Groups/Mount Hoopbound Formation. The Upper Devonian Mount Hoopbound Formation unconformably overlies the Middle Devonian? Mount Morgan Tonalite in several localities in the Mount Morgan area. This unconformable relationship is typified by the presence of tonalite clasts, up to boulder size, which diminish in abundance upsequence from the contact, and are generally confined to the lowermost 50m. At Bull Creek, a broad zone of brecciation, probably of intrusive origin, may have been mistakenly considered by some to represent the unconformity itself. Although previous workers have suggested that the mtra-Devonian unconformity in the Mount Morgan area was short lived, firm evidence for this view was lacking. Conodont dates from limestones have now constrained this hiatus considerably. The underlying Capella Creek Group reaches as high as the upper Givetian (Late yarcus conodont Zone). Frasnian dates from two horizons in the overlying Mount Hoopbound Formation can, in combination with macrofossil and conodont dates from higher units, be used to extrapolate the base of the formation to a level very close to the Givetian-Frasnian boundary. The hiatus thus appears to be only two million years or so at the very top of the Givetian. Although several hypotheses exist, the tectonic significance of this unconformity is still uncertain. With tighter biostratigraphic control of the unconformity in the Mount Morgan area, the relationship with the intrusion of the Mount Morgan Tonalite can be re-examined. In light of recent and ongoing changes to timescale calibration at this level, it appears that previous radiometric dating of the Tonalite, near Mount Morgan mine, gave results which erred on the younger side of the Givetian-Frasnian boundary. This phase of the Tonalite thus probably mtruded during the hiatus, rather than during deposition of the Capella Creek Group. Importantly however, granitoid clasts have been found in conglomerates at lower levels of the Capella Creek Group pointing to intrusive activity preceding or coexistent with this Middle Devonian unit. New radiometric dates (415 and 417 ± 5) for a granodiorite pluton in the Bajool Sheet area suggest that intrusive activity and volcanism may have persisted in the region from the very late Silurian to the end of the Middle Devonian. Mapping in the Kroombit Creek area, SE of Biloela, has allowed the subdivision of the previously mapped Middle to Upper Devonian Kroombit beds into two distinct units - the Middle Devonian Marble Waterhole beds and the Upper Devonian Lochenbar beds. Conodont determinations from these units have, for the underlying unit, repeated the late-Givetian control obtained near Mount Morgan, and furnished an even tighter, early Frasnian, date for the overlying unit. This suggests that the unconformity may also be represented in this area. Another intra-Devonian unconformity exists in the Bajool 1: 100 000 Sheet area, between the Early Devonian Mount Holly beds and the Upper Devonian to Early Carboniferous Mount Alma Formation. Although no angular unconformity has been observed in this area, its presence is suspected due to the occurrence of Early Devonian limestone blocks at the base of the Mount Ahna Formation, in sedimentary rocks containing Late Devonian brachiopods. The existence of this unconformity can be traced by strongly contrasting responses of the two units on aerial radiometric images (total count RGB). To the north of Mount Morgan, in the Craigilee area, a similar relationship has been established between units previously mapped as 'undivided Siluro-Devonian' and 'Lower Carboniferous' and is regarded as unconformable. Here stratigraphic contacts are probably destroyed by faulting but nonetheless the lower unit, the Craigilee beds, reaches to the upper Givetian and the higher unit, the Mount Ahna Formation, has been dated as early Famermian and very probably extends down into the Frasnian. ^ New name for rocks previously mapped as Dee Volcanics and Boulder Creek Grit ^ New Name for rocks mapped as Capella Creek beds
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
NUMERICAL SIMULATIONS OF FLUID FLOW & METAL TRANSPORT IN THE FRACTURED IRISH Zn-Pb-Ba ORE DISTRICT Timothy J. Hazlett' and Grant Garven^ ^Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, USA 21218 ^Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001
The carbonate-hosted ores of Ireland are associated with to normal faults that penetrate both the sedimentaiy section and Lower Paleozoic (Silurian) metamorphic basement. These faults are thought to have acted as conduits for the transport of metal-bearing brines from deeper basin formations like the Devonian Old Red Sandstone (ORS) and from the crystalline basement. Sphalerite and galena ores are concentrated in the more permeable limestone and dolomite layers immediately above the ORS, commonly in the hanging walls of the normal faults. The ore mineral textures and host rocks are similar in many ways to the large Mississippi Valley-type (MVT) Pb-Zn ores of North America but the Irish ores differ m several attributes. Besides the prominent structural control and association with faults, fluid inclusion data indicate much warmer temperatures for mineralisation, ranging from 150 to 240°C, with salinities of 10 to 25 weight % NaCl equivalent. The Irish ores contain more copper, silver, and iron sulphides than other MVTs, they occur in the stratigraphically lowest, non-argillaceous carbonate bed, and the pre-ore dolomitisation of the host rock is only observed in the southern part of the Carboniferous basin. The ores display both stratabound and stratiform styles of mineralization, with some deposits looking like classic sedimentary exhalative (SEDEX) ores while others feature classic MVT replacement and cavity infilling characteristics of the deeper subsurface.The origin of the Irish deposits is unclear, although at least two different genetic stories have become prominent: 1) densitydriven convection of fluids along NE-SW basement faults which allowed for discharge of brines in a SEDEX environment on the seafloor, and 2) northward migration of basinal brines driven by topography and continental collision of theVariscan orogeny in the mid- to late-Carboniferous. We present numerical calculations of these hydrothermal flow systems to test and evaluate the two competing hypotheses. Finite element codes were deployed to construct mathematical models of both free convection and topography-driven fluid flow in the Irish Carboniferous basin. Some of these model calculations adopt an "equivalent porousmedium" approach to simulate basin-wide flow across the foreland and platform with good success. However, flow modeling at the district and deposit scale require more detailed "dual-porosity" fracture-flow analysis to properly evaluate the role of faults in controlling ore mineralisation. In both cases the models solve the coupled equations for variable-density fluid flow and heat transport in heterogeneous, anisotropic fractured media. We also present calculations for one scenario of ore deposition where lateral flow in the carbonate sequence is fully coupled to a geochemical transport code so as to predict patterns of ore mineralisation and the chemistry of brine evolution in a fractured rock environment. For this model, the fluid phase is assumed to be in chemical equilibrium with the rock at each time step so changes in lithology, pressure-temperature conditions, or mixing with other fluids causes mineral phases to precipitate or dissolve along the flow paths. Density-driven flow to depths of 5 km appears to be a viable mechanism for metal transport in the Irish midlands assuming that fracture plane permeability exceeded about lO'^"^ m^ (30 millidarcys). Two-dimensional simulations predicted large equidimensional convection cells if basement heat flow exceeded 150 mW/m^ about triple the normal continental value. Flow rates from free convection are somewhat small (less than 3 cm/yr) and the temperatures of discharge are not high enough to fit the observed fluid inclusion data. Variscanrelated topography appears to offer a better hydrogeologic model for regional flow across the Irish foreland basin with brines focused by the ORS and deep migration aided by hydraulic connnections with basement faults. Heat flow on the platform was greatly elevated by the regional flow and upward discharge of brines along permeable normal faults. The best hydraulic scenario for ore deposition requires both topography-driven flow and focused discharge controlled by feeder faults. Dual-porosity modeling at the deposit scale showed that NESW conduit faults aligned normal to the basin-scale flow gradient provided optimal hydrologic conditions for "pooling" of metal and brine invasion into juxtaposed carbonate horizons where fluids reacted to precipitate ore at deposits like Lisheen. Geochemical flow modelling of this fracture system shows that a sizeable ore body would form near the feeder fault in less than 500,000 years.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
MODERN SEDIMENTOLOGY AND HYDRODYNAMICS OF NARA INLET, WHITSUNDAY ISLANDS, GBR, AUSTRALIA A.D. Heap, P. Larcombe and K.J. Woolfe School of Earth Sciences, James Cook University, Townsville, Queensland 4811, Australia
Short term (daily) hydrodynamic and sedimentary processes within tropical embayments of continental high islands located in the GBR lagoon are poorly understood. Moreover debate still surrounds the processes that permit high quality fringing reefs to occupy positions juxtaposed to large volumes of unconsolidated material in relatively turbid waters. High resolution hydrodynamic processes and turbidity data were measured for a 12 day period in Nara Inlet, Hook Island and compared with high spatial resolution sedimentary data to predict sediment transport processes within a partially-infilled embayment, containing high quality fnnging reefs and a large volume of unconsolidated sediment. Maximum tidal currents measured by two frame-mounted InterOcean S4 current-meters reach 0.16 m/s within Nara Inlet. Tidal cycles are semi-diurnal with significant high tide diurnal equality and flood dominant. Fairweather significant wave height reached 1 m and wind waves penetrated 500 m into the inlet. No oceanic swell waves were observed during the study period. Near-bed turbidity patterns recorded by three self-logging nephelometers display a regular rise and fall beginning at sunrise and ending at sunset throughout the study period. Sedimentary facies derived from 56 surface grabs exhibit a headward textural fining trend with a modal grain size of 71 jim at the fringing reef crest to 23 jim at the embayment head. Coarser deposits are found on shallow water carbonate platforms (132 fim), and steep carbonate dominated beaches near the embayment mouth (236 jim). Carbonate percentages decrease to headward and also display an inverse relationship with water depth. Sediments from the floor of Nara Inlet consist primarily of large populations of agglutinated foraminifer (Textularina) and carbonate detritus. Modem-day, fair-weather hydrodynamics can not explain the sedimentologic character of Nara Inlet. Tidal currents do not result in sediment transport near the bed, and combined wind-wave and tidal velocities can not explain the patterns in near-bed turbidity. Available evidence suggests that near-bed turbidity is related to the diurnal migration of zooplankton in response to changing light conditions and the probability of death by predators. The pattern of sedimentary facies is in large part relict, reflecting processes that occurred during the early infilling of Nara Inlet. This has been subsequently modified by biogenetic reworking and sedimentation from high-intensity, low-frequency storm events. Fair-weather processes ceased to play a dominant role modem-day sedimentation between 7.4 kybp and 5.4 kybp.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
THE INFLUENCE OF LIMITED ACCOMMODATION SPACE ON THE STRATIGRAPHY OF A CLIFF-BOUND, FAULT-CONTROLLED INCISEDVALLEY SUCCESSION: WEITI RIVER ESTUARY, NEW ZEALAND A.D. Heap' and S.L. Nichol Department of Geography, University of Auckland, Private Bag 92019, Auckland, New Zealand. ' Present address: School of Earth Sciences, James Cook University, Townsville, Queensland 4811, Australia.
A study was undertaken to investigate the influence limited accommodation space has on the stratigraphy of estuarine facies in the Weiti River estuary; a narrow, fault-control estuary of the North Island, New Zealand. The Weiti River estuary occupies a narrow, cliff-bound valley formed by fluvial incision that has exploited a fault trace in the underlying geology. The estuary is at an advanced stage of infilling and sub-surface sediments obtained from 11 vibracores show an overall coarsening-upward facies succession. Eight distinct subsurface lithofacies were identified, based on textural characteristics, sedimentary structures, and contacts. These facies constitute an incised-valley fill that occupies two, non-adjacent depocenters separated by the main estuarine channel, and whose morphology has been determined by limited sediment accommodation space. The seaward depocentre is occupied by a marine-dominated sand accumulation consisting of transgressive estuarine and nearshore deposits overlain by regressive tidal flat, nearshore and shelly beach ridge deposits. The landward depocentre is dominated by a tidal-fluvial transgressive to regressive bay-head delta complex that includes point-bar, channel lag and saltmarsh deposits. Radiocarbon ages for in situ shell and organics are used to establish the chronology of valley filling. Holocene sediment accumulation within the Weiti River incised valley commenced late in the marine transgression (c.8 ka bp), with sedimentation at a rate comparable to eustatic sea-level rise. Transgressive deposition was dominated by estuarine facies in the seaward depocenter and by a tidal point-bar within the landward depocentre that formed by lateral and vertical accretion from the paleo-valley flanks. At the time of maximum transgression (c.6.5 ka bp) most of the valley fill was in place, although the beach ridges had yet to form. Highstand deposition has been characterised by formation of seven shelly beach ridges and vertical accretion of tidal flats in the seaward depocentre, and continued lateral point-bar growth in the landward depocentre. The organization of estuarine facies in the Weiti River estuary has been influenced by limited accommodation space, low fluvial input, and a sheltered setting. In particular, valley confmement has prevented the bayhead delta forming multiple distributary channels and lobes; produced a shallow open bay setting during late Holocene time and, coupled with a restricted marine sediment supply, precluded the formation of a transgressive sandy barrier system; prohibited the development of a continuous transgressive bay-head delta; and negated the accumulation of flood- and ebb-tide delta deposits by producing a narrow, deep main tidal channel that has been confmed to the same position within the paleo-valley for at least the duration of the Holocene transgression. The Weiti River estuary incised-valley sequence documents the influence that limited accommodation space available for sediment deposition may result in the non-preservation and truncation of facies, the absence and amalgamation of certain sequence stratigraphic surfaces (e.g., MFS, WRS, and IFS), and the juxtaposition of facies that should not normally be adjacent. Despite these variations, the Weiti River estuary possesses a transgressive/regressive sediment wedge that is consistent with "idealized" models of incised-valley stratigraphy.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
A NEW TOOL FOR MULTIPLE-IMAGE INTERPRETATION AND GIS DATA INTEGRATION IN A DESKTOP MAPPING SYSTEM A.J. Hender, D.R. Elvey & R.B. Flint World Geoscience Corporation Ltd, 65 Brockway Road, Floreat WA 6014
In mid 1995 World Geoscience Corporation embarked on a study to establish a system for geologists to digitally acquire geological and geophysical interpretations in an efficient and cost-effective manner. One daunting problem confronting geoscientists involves the interpretation of multiple related datasets. Remote-sensing enhancement and various manipulations of airborne magnetic, radiometric and electromagnetic data may easily generate at least 10 discrete images over a particular project area. Traditionally interpretation was undertaken using transparent fihn overlays on large-scale hard-copy imagery and hand drawn using pen or pencil. After comparison with other hard-copy datasets, if available at the appropriate scale, the interpretation map is passed on to the CAD department for digitising. Throughout the digitising process many check-plots are created on which corrections are placed which must then be reincorporated into the digital map by a CAD operator. The capture and revision process is wasteful of time and materials and prone to error introduction. Often only after the geologist is completely satisfied with the CAD map will it be passed onto a GIS department for incorporation into a desk-top mapping system. World Geoscience Corporation's new software provides image interpreters with an innovative and powerful tool for the efficient transformation of multiple satellite, geophysical and analogue map images into digital vector geological maps. Most importantly GIS requirements are at the early stage of data capture rather than after compilation of the geological map. Digital data capture can be achieved either by mouse on-screen or by digitiser pen, or a combination of both. The software was developed within the Extension architecture of ESRI's® Arc View® GIS desktop mapping system and is designed to efficiently run on a Pentium IBMcompatible PC. Attribution of features is achieved by utilising floating palettes for all geological linework and map unit symbols. An important feature of these palettes is their configuration flexibility with simple editing facilities for the user to defme their own geological line types or unit labels. Extra tools include stream digitising, automatic splining and automatic extension to achieve clean and neat linework. Other features include easy reattribution of linework or labels and cutting and flipping of lines if a different interpretation arises - which is frequently! Normal fimctions such as the ability to zoom-in, zoom-out, pan and flip between different images and datasets are provided within ESRI's® Arc View® system. After completion of an interpretation there is a series of logical steps in the polygonisation process leading up to the production of a coloured digital geological map which is fully GIS attributed. These include preparation of linework, identification of overshoots and undershoots, creation of polygons, and identification of and automatic zooming to labelling errors for polygons. Resultant hard-copy maps can be produced by either simply utilising the plot option within WG Tools or by additional software currently being developed by WGC if a higher quality product is desired.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
DEFORMATIONAL RESPONSE TO PALAEOZOIC PLATE CONVERGENCE: THE RECORD FOR THE NORTHERN TASMANTOES Robert A. Henderson and Brett K. Davis School of Earth Sciences, James Cook University, Townsville, Queensland Q4811
The northern Tasmanides encompass the muhiply deformed, low grade Siluro-Devonian metasediments of the Hodgkinson - Broken River Fold Belt in the north and Late Palaeozoic sedimentary and volcanic terranes of the Northem New England Fold Belt, \^ilich have a simpler structural history, to the south. These two tracts are separated by the Charters Towers Province, a block of older crust extensively overprinted by Palaeozoic plutonism and Devonian - Carboniferous basinal development The Hodgkinson-Broken River Fold Belt and Charters Towers Province are coastal tracts which abut Precambrian basement to the west along a major system of fault dislocation. The inboard limits of the New England Fold Belt are not clearfy defined and extensive, Httle deformed basinal tracts correlative with its rock systems extend into the continental interior. That all three of these elements have experienced major deformational episodes is well estabhshed but the crosselement correlation of such episodes has been obscure. Our systematic regional investigations have identified four discrete orogenic episodes for the northem Tasmanides driven by plate convergence. A repeating pattern of crustal response is apparent Compartments of the Tasmanides, separated by major transverse dislocation zones experienced quite different orogenic effects. Penetrative cleavage development for these episodes was concentrated towards the outboard margin with the inboard expression of crustal shortening marked by discrete thrust dislocations and by open, upright folding. The youngest episode, the mid-Permian to mid-Triassic Hunter-Bowen Orogeny, is the best known. For the Hod^dnson-Broken River Fold Belt, penetrative cleavage of this generation is dated by relationships to synorogenic granites as mid-Permian (268 -285 Ma) and occupies a broad northem tract extendingfi"omthe coast inboard for some 100 km. The tract is intermpted to the south by the Desailly Structure across which it narrows to a thin coastal belt. Pervasively cleaved rocks are otherwise restricted to the New England Fold Belt, where their pattem is complex in both space and time. Three discrete belts are represented and their northem limit is generally marked by the Stannage Fault Zone. The age of deformation for the outboard belt comprised of the Shoalwater, Wandilla and Marlborough Terranes, obtained by dating the age of metamorphism and the age of post-tectonic granites as mid- to Late Permian (265-250 Ma). However, stratigraphic evidence indicates that the deformation age of the inboard beh, comprised of the Folded Zone of the Bowen Basin, is Early or mid-Triassic. Major thmst faults were reactivated or initiated by as much as 250 km inboard jfrom the cleaved tracts and appear to extend along the entire Tasmanide margin. The age of movement on these structures, wiiere constrained, is Early or mid-Triassic. The broadscale pattem of strain intensity and age trends imply that shortening strain propogated fi-om a narrow belt of plate coupling located to the east. A Late Carboniferous episode of tectonism, broadly correlative with the Kanimblan Orogeny of the southem Tasmanides was characterised by the modest, but extensive development of fold and thmst belts. Regionally developed cleavage of this age is unknown. Its influence is most marked at the inboard margin of the northem New England Fold Belt, the eastern sector of the Charters Towers Province and the southem perimeter of the Hodgkinson-Broken River Fold Belt A relationship to plate coupling over a broad zone of the Tasmanide margin is implied, and is supported by the distribution of Early Carboniferous igneous assemblages. The age of this episode is poorly constrained but stratigraphic evidence places it in the interval between the Visean and Moscovian (330-310 Ma). Late Devonian tectonism, offset in time but probably related to the Tabberabberan Orogeny of the southem Tasmanides, is marked by deformation of the Hod^dnson - Broken River Fold Belt. Shortening strain shows a strong easterly gradient and terminates to the south at the Clarke River Fauh Zone with the Charters Towers Province unaffected. This episode is represented in the northem New England Fold Belt by a regional unconformity but its effects here were modest Synorogenic granite ages and stratigraphic evidence constrains the episode to within the span of the Frasnian and Famennian Stages (370-357 Ma). Early Silurian tectonism, broadly equivalent to the Benambran Orogeny of the southem Tasmanides, is marked by the emplacement of thrust sheets at the westem margin of the Hodgkinson-Broken River Fold Belt and by the local development of wide shear zones which are generally transverse in orientation to the broadscale orogenic grain. The episode is dated on the basis of synorogenic sedimentary assemblages in the Broken River Province. Regional pattems for it are poorly constrained but it is considered to be largely responsible for the dispersal of Cambro-Ordovician stratotectonic assemblages which are widely scattered in the northem Tasmanides.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
FAULT LINKAGE ACROSS THE YAMPI SHELF-VULCAN GRABEN TRANSITION, TIMOR SEA R. I. Higgins^ and G. W. O'Brien^ ^Tectonics Special Research Centre, Department of Geology & Geophysics, The University of Western Australia, Nedlands WA 6907 ^Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601
Rifting during the Permo-Carboniferous removed a group of continental blocks known as Cimmeria (Sengor, 1987), now preserved in southeast Asia, from the northern margin of Gondwanaland. This event initiated development of two neighbouring and highly prospective hydrocarbon provinces; the Yampi Shelf (Browse Basin) and Vulcan Graben (Bonaparte Basin). These two provinces were further defined by a Jurassic rifting episode which produced the present continental margin of northwestern Australia. This episode involved the rifting of a continental fragment, referred to by some as Argo Land (e.g. Baillee et at. 1994), from the northern margin of Gondwanaland. Much of the Timor Sea is underpinned by the Proterozoic Kimberley Basin, which is transected by numerous faults and fractures. Some of these are intruded by igneous material and have anomalous magnetic signatures. Since a substantial component of this heterogenous Kimberley Block extends out beneath the Timor Sea, such features might be expected to have affecteded the type and location of structures superimposed upon them during the Phanerozoic. Some workers (e.g. O'Brien et al, 1996) propose that both the Yampi Shelf and Vulcan Graben were compartmentalised into areas of differing structural style by N- and NW-trending Proterozoic fractures within the underlying Kimberley Basin. Indeed, potential field anomalies seem to be located in similar positions to rapid changes in syn-rift thickness in this region. This study investigates the nature of basin-fault linkage across the transition zone between the Vulcan Graben and the Yampi Shelf. AGSO's Vulcan Tertiary Tie (VTT) and Yampi Shelf Tie (YST) seismic data have been interpreted to generate fault maps for several key horizons over this region to aid the understanding of how basin-faults link through this major compartment boundary. They are integrated with magnetic, gravity and bathymetry data to to help understand the relationship with, and possible controls of, the underlying basement (Proterozoic Kimberley Basin) features. Analogue models have been designed and run to test ideas of basement control on fault development and to generate models for the kinematics of such systems. These models show that in an extending system, basement fractures may localise the position of relay ramps, changes in syn-rift thickness and deformation due to consequent obhque stresses. Such basement influences will be of interest to petroleum explorationsists in terms of facies distribution, fluid migration and trap development. REFERENCES Baillee, P.W., Powell, C. McA., Li, Z. X. & Ryall, A. M. 1994, The tectonic framework of Western Australia's Neoproterozoic to Recent sedimentary basins, in: Purcell, P. G. and R. R. (Eds), The Sedimentary Basins of Western Australia: Proceedings of Petroleum Exploration Society of Australia Symposium, Perth, 1994,45-62. O'Brien, G. W., Higgins, R., Quaiffe, P., Symonds, P. Blevin, J. & Colwell, J. 1996, Basement control on the development of extensional systems in Australia's Timor Sea: An example of hybrid hard linked/soft linked faulting? APPE4 Journal, 36, 161-201. Sengor, A. M. C. 1987, Tectonics of the Tethyside: orogenic collage development in a collisional setting. Annual Reviews of Earth and Planetary Science, 15, 213-244. Acknowledgements: RH thanks the Australian Geological Survey Organisation for the provision of all geophysical data contained within this presentation, and the Tectonics Special Research Centre (TSRC) at the University of Western Australia for travel assistance. RH is in receipt of a Minerals and Energy Research Institute of Western Australia (MERIWA) top-up scholarship.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
THE STRESS FIELD OF AUSTRALIA Richard R. Hillis. Marian E. Magee, Jeremy J. Meyer and Scott D. Reynolds Department of Geology and Geophysics, University of Adelaide, South Australia 5005
The Australian Stress Map project has increased the number of quality-ranked stress orientation analyses for the Australian Continent from 75, in the 1992 World Stress Map compilation, to 275. These analyses are based on earthquake focal mechanism solutions (55), borehole breakouts (173), engineering tests (hydraulic firacturing and overcoring, 45) and two poor quality 'young' volcanic vent alignments. The majority added since the 1992 compilation are based on borehole breakouts in petroleum wells. The figure below shows the current Australian Stress Map. Analyses yielding stress orientations from A-quality (standard deviation ±12°) to D-quality (>25°) are shown. The poorest, E-quality analyses (18), do not yield reliable stress orientations. Regionally, maximum horizontal stress (SH) is oriented 050°-060°N in the Bonaparte Basin. This is consistent with SH orientation in New Guinea (based on earthquake focal mechanisms) and in the onshore Canning Basin, and thus applies to much of the northern Australian margin. Westward from the onshore Canning Basin, SH rotates approximately 40° to 090°-100°N in the Carnarvon Basin. This rotation is significant and reflects the heterogeneity of plate boundary forces associated with complex and laterally varying convergent plate boundary processes to the north of Australia (eg. subduction in Sunda Arc, continent-island arc collision in Banda Arc and continent-continent collision in New Guinea). In the Amadeus Basin of central Australia SH is oriented approximately north-south. This is consistent with focal mechanism solutions from the Tennant Creek earthquakes of 1988. However, it is inconsistent with focal mechanisms to the south of Amadeus Basin which indicate 070°N oriented SH. Much of the southern part of the Australian Continent exhibits a broad but poorly defined east-west oriented SH. The east-west trend is best constrained by borehole breakouts in the Cooper-Eromanga Basins. It is also loosely supported by scattered earthquake data from the Yilgam Block and the Flinders Ranges. In southeastern Australia (Otway and Gippsland Basins) SH is oriented 120°-130°N. Unlike the SH rotation along the northern margin, the change in orientation from 070°N to northsouth in central Australia cannot be explained by plate boundary forces. Its origm may be linked to the major east-west trending gravity anomalies in the vicinity of the Amadeus Basin. In addition to stress orientations, we are also analysing stress magnitudes in the Australian Continent. The vertical stress (Sv) gradient in the Bonaparte and Cooper-Eromanga Basins increases with depth, as would be expected in sedimentary basins, and increases from 20 MPa/km around 1 000 m, to 23 MPa/km around 3 000 m. The Amadeus Basin displays an overburden gradient of 25 MPa/km that is little affected by depth. In situ measurements in hard rock terranes suggest a higher average overburden stress gradient of 27 MPa/km. Leak-off pressures indicate that the minimum horizontal stress (Sh) is the least principal stress (60-70% of Sv) in the Bonaparte and Cooper-Eromanga Basins. Hence the stress regime in those basins is not, as has been suggested, compressional (SH>Sh>Sv). Consideration of the frictional limits to faulting suggests that, if in a state of incipient faulting, the stress regime is transitional between extensional (Sv>SH>Sh) and strike-slip (SH>Sv>Sh) in the Bonaparte Basin and strike-slip in the Cooper-Eromanga Basins. Applications of the stress data include predicting the orientation of hydraulic fractures induced for geothermal exploitation of hot-dry-rock in the Cooper-Eromanga Basins which would tend to be vertical, and not, as previously suggested horizontal. A revised strategy for hot-dry-rock exploitation might involve drilling deviated wells to mtersect vertical hydraulic fractures. Other applications of the data include mine and wellbore stability, subsurface fluid (groundwater and hydrocarbon) flow directions and seismic hazard/fault reactivation trends. 100"
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
SPECULATIONS ON SYN-DIAGENETIC PROCESSES OF SULPHATE REDUCTION AT THE HYC Pb-Zn DEPOSIT, NT Mark Hinman GeoSOLUTIONS, 43 Gerler Street, Bardon. Queensland 4065
Previously reported structural relationships at HYC have demonstrated that stratiform base metal mineralisation formed belov^ the sediment-water interface w^ithin semi-consolidated Barney Creek sediments prior to the formation of abundant, gravity-driven structures that developed during Barney Creek-time. The refmed syn-diagentic model of ore formation proposed by Hinman (1996) suggests that base-metal mineralisation formed relatively shallowly (-10-20 meters) below the sediment-water interface from a dense, sulphate-bearing brine that flowed within the available permeability of the rhythmically interbedded, organic-rich, silts and shales of the Barney Creek Formation. It has been proposed that redox reactions between very immature organic matter and brine sulphate produced reduced sulphide resulting in the precipitation of base metal sulphides. Base metal mineralisation is texturally associated with styiolamination (Hinman, 1996) which only effects the originally silty portions of the sediments and not the 'muddy tops' of the Barney Creek Formation rhythmites. These 'muddy tops' remain texturally unmodified and unmineralised. Mass balance work suggests that the release of hydrogen ion during base metal sulphide precipitation was neutralised in situ by the dissolution of carbonate to form stylolaminated "mineralised shale" and that this dissolved carbonate plus other hydrothermal and organic oxidation products are redeposited down-flux within the system as secondary "nodular carbonates" and cements. Many organic and carbonate geochemical aspects of the HYC mineralised system support this model and suggest that processes of thermochemical sulphate reduction (TSR) may have played a critical role in the formation of this stratiform base metal deposit. Recent detailed RockEVAL and organic reflectance work (following from Crick, 1989) has indicated that organic supermaturity (Tmax and maxRo) is spatially restricted at HYC to high grade, stylolaminated 'mineralised shale'. In addition, 'mineralised shale' samples exhibit highly depleted TOCs, low Production Indices (PI) and high Hydrogen Indices (HI) compared to background Barney Creek samples. This suggests that the ore sequence's burial potential for hydrocarbon generation was depleted prior to burial. High HI s within the ore sequence (both RockEVAL and limited bitumen DryAshFree analysis (Powell, 1987)) are incompatible with organic burial processes and suggest severe organic modification within 'mineralised shales' before burial. On a fine scale, ongoing detailed reflectance work suggests that bitumen within stylolaminated 'mineralised shale' material has anomalously high reflectances while bitumen within the texturally unmodified, unmineralised, but intimately interbedded, 'muddy tops' has reflectances consistent with simple burial. This suggests an extremely fine-scale permeability control on fluid access and base metal mineralisation process that explains the extensive lateral, fine-scale continuity and correlatability of the mineralised package. Solvent extractions from mineralised samples have extremely low yields of soluble hydrocarbons in comparison with background Barney Creek extracts (consistent with the above), but do yield extremely large amounts of native sulphur. The isotopic composition of this sulphur lies between that of sphalerite-galena and the proposed fluid sulphate composition consistent with two stage processes of sulphate reduction proposed by Machel et al. (1995). Secondary Fe, Mn-carbonate separates from the "nodular carbonate" lithologies are isotopically shifted from the background dolomitic detritus signal: ^C is light-shifted and ^O is variably heavy-shifted for low Fe-Mn, more distal, secondary carbonates and light-shifted for higher Fe-Mn, more proximal, secondary carbonates. Ongoing organic geochemical work of Logan, Hinman and Summons (this volume) aims to constrain these speculations and ultimately resolve details of organic-driven processes of sulphate reduction at HYC. REFERENCES Crick, I. H., 1989. Petrological and maturation characteristics of organic matter from the middle Proterozoic McArthur Basin, Australia. Unpubl PhD Thesis, University of Wollongong Hinman, M. C., 1996. Constraints, timing and processes of stratiform base metal mineralisation at the HYC Ag-Pb-Zn deposit, McArthur River. MIC'96 Abstracts Volume, Townsville Machel, H. G., Krouse, H. R. & Sassen, R., 1995. Products and distinguishing criteria of bacterial and thermochemical sulphate reduction. Applied Geochemistry 10, 373-389 Powell, T. G., 1987. Investigations of organic matter associated with the HYC deposit, McArthur Basin, Northern Territory. Unpubl. BMK report Acknowledgments: This work is presented with the permission of MIM Exploration Pty Ltd.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
CRUSTAL EVOLUTION OF EARLY TO MID-PROTEROZOIC BASEMENT IN THE PEAKE AND DENISON RANGES, NORTHERN SOUTH AUSTRALIA. Derek J. Hopper and Kenneth D. Collerscn Department of Earth Sciences, University of Queensland, Brisbane, Queensland 4072
The Peake and Denison Ranges in northern South Australia are comprised of Early to Mid-Proterozoic metamorphic basement rocks together with Late Proterozoic sediments of the Adelaide Geosyncline. The Early to Mid-Proterozoic rocks metamorphics are located along the northeastern margin of the Gawler Craton and possible northwestern subsurface extension of the Cumamona Craton (Muloorina Gravity Ridge). They are of considerable economic significance as they lie within the G2 corridor (O'Driscoll, 1990) --200 km north of Olympic Dam and contain volcanics and intrusives of similar age to those found in metallogenic provinces such as the Mt Isa block (eg. 1790 Ma, Argylla Volcanics). The basement metamorphics are divided into two subdomains based on lithologic, geochronological and geophysical characteristics. These are the West Lake Eyre Subdomain (WLES) in the south and the Denison Subdomain (DS) in the north. The WLES is characterised by a high magnetic signature and comprises quartzite (local heavy mineral banding), metarhyolite, calc-silicate schist, haematite-calc-silicate gneiss, albite-calc-silicate breccia and subordinate granite and marble. In addition a previously undescribed body of trondhjemitic gneiss is reported. U-Pb shrimp zircon dating of metarhyolite gives an age of 1746 ± 6 Ma for the supercrustal sequence and an age of 1533 ± 6 Ma for granite dikes crosscutting the trondhjemite body. The DS is divided into two parts with similar supracrustal sequences but differing metamorphic grade. In the Kingston and Algebuckina blocks migmatised schist, quartzite, marble and metavolcanics are intruded by diorite, tonalitic, granodiorite, and granite sheets. Metamorphic grade is inferred to have reached upper amphibolite facies. In the Peake block metapelite, metasandstone and metavolcanics with intercalated carbonate (Tidnamurkuna Volcanics) are metamorphosed to greenschist facies. The entire DS is intruded by numerous east-west trending granite pegmatite bodies and the northern and southern parts are divided by the Wirriecurrie Granite. U-Pb shrimp zircon dating gives ages of 1780 ± 12 Ma for the metavolcanics and 1793 ± 8 Ma for the Wirriecurrie granite. Mafic volcanics in the DS are tholeiitic in character and range fi-om basalt to basaltic andesite in composition. Trace element data show that metabasalt from the low grade (Peake Block, Tidnamurkuna Volcanics) and high grade (Kingston) areas within the DS are derived from similar sources, and are transitional between MORB and VAB in composition. Negative Ta-Nb anomalies on N-MORB normalised plots are interpreted as originating from a subduction-enriched source component. Associated metarhyolite and granite (Tidnamurkuna Volcanics, Wirriecurrie Granite) are comagmatic and are marginally peraluminous with I-type affinities (Rb/Nb = 10-13, lOOOxGa/Al = 0.46). Within the WLES felsic gneiss at Lagoon Hill is interpreted as trondhjemitic orthogneiss from outcrop textures and geochemical relationships. The trondhjemites are I0W-AI2O3 type with mildly fractionated REE patterns (La/YbN = 3.4-5), negative Eu anomalies and high HREE contents (Yb^ = 10-40). Crosscutting granites are marginally metaluminous with A-type affinities (Rb/Nb = 2-5, lOOOxGa/Al = 4-6). Sm-Nd TDM ages range from 2565-2424 Ma in the -1780 Ma DS to 2238-2344 Ma in the younger -1746 Ma WLES. The narrow range of T^M (2500-2300) is comparable to data from other Early to Mid-Proterozoic igneous rocks in the eastern Gawler Craton. The DS and WLES are interpreted as distinct lithotectonic packages that were joined together by ~1530Ma. Metavolcanics and granite in the DS and associated quartzite-pelite-carbonate assemblage is considered to have formed in a rifted continental margin setting (initial stage of a back-arc basin?) along the northeastern margin of the Gawler Craton/Mawson Continent. Further accumulation of clastic shelf deposits, volcanic activity and subduction, as indicated by the presence of trondhjemite intrusives, occurred along this margin between 1740 and 1530 Ma (WLES). REFERENCES O'Driscoll, E.S.T. 1990. Lineament tectonics of Australian ore deposits. In: Hughes F.E. ed. Geology of the Mineral Deposits of Australia and Papua New Guinea, pp. 33-41. Australasian Institute of Mining and Metallurgy Monograph series no. 14. Acknowledgments: The authors wish to acknowledge the support of RGC Exploration PTY LTD for this project and for permission to publish U-Pb zircon data.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
MONITORING FLUVIAL SEDIMENT TRANSPORT PROCESSES IN QUEENSLAND Anthony M. Horn Department of Natural Resources, 80 Meiers Road, Indooroopilly, Brisbane, Queensland, 4068
Understanding modem riverine sediment transport processes is essential for approaching sustainable management practices in our water courses. Sediment transport behaviour is an important influence on channel morphology and stability, water and habitat quality and biological speciation. Most studies into sediment transport behaviour have been conducted in Europe and North America. It has been questioned whether the laboratory flume based empirical formulas derived overseas for bed load transport ie. Van Rijn, Ackers and White, Englund and Hansen, Zanke etc. can adequately describe sediment transport processes operating in Queensland. As a result, the Department of Natural Resources has undertaken a major program to assess bed load transport rates, wash loads and nutrient loads in Queensland rivers based on direct flood measurements. The program commenced in 1994 and is expected to run for at least another two years. So far, 25 major flood events have been sampled on the Balonne, Black, Brisbane, Condamine, Comet, Don, Fitzroy, Herbert, Logan, Maranoa and Mary Rivers. Sampling is conducted during flood events from boats, cable ways and on occasions, bridges, using HelleySmith samplers to collect bed load material and P61 samplers to collect suspended load material. Significant development of existing sampling and laboratory methodologies has been undertaken as part of this program. Where possible, samples are taken at several distinct stages of the flood hydrograph, specifically to cover a range of higher flow conditions. Normally, five locations are sampled along a cross-section of the river. These locations are spaced to represent equal flow volumes in the channel. Consequently, the mid channel verticals are generally closer together. Current velocities are measured simultaneously with the collection of suspended sediment samples. Commonly, 5 bed load measurements, 13 suspended load samples and 2 nutrient (Total N and Total P) samples are taken at each cross-section for each particular discharge measured. Bed load sample particle size distributions and size fraction percentages are determined and compared to discharges. Field results are flow weighted and combined for each particular discharge cross-section to give instantaneous; bed load, suspended load, bed material load and wash load quantities. Sediment rating curves (log log plots of the total sediment load components against discharge) are determined for bed load and wash load and sediment transport vs. discharge functions are calculated. These functions are then applied to the historical flow records to provide an indication of the sediment loads transported over a particular time period. Several factors limit the accuracy of the results. The sediment rating curves describe distinct transport rates for each given discharge. Clearly, this is not the actual case. Differing antecedent conditions, armouring histories and variability of material sources (depending on storm activity locations), can result in different transport rates for a given discharge. However, the results commonly represent a significant improvement over the laboratory derived formula. The program has helped to more accurately describe the quantity and variability of sediment being transported by Queensland rivers. The results are currently being used to calculate average material transport rates. This helps provide an indication of sustainable sand and gravel extraction quantities and determine the siltation potential of existing and proposed water storages.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
STAGE 11 IN THE SOUTHERN OCEAN: GEOCHEMICAL AND PALAEOTEMPERATURE ANOMALIES William R. Howard' 'Antarctic Cooperative Research Centre, University of Tasmania, Hobart, Tasmania 7001
The warming of high-latitude oceans that occurred during oxygen isotope Stage 11 (-423 to 363 ka) is one of the most prolonged and extreme events of the late Pleistocene paleoclimatic record. That the thermal and geochemical extremes of this interglacial occurred during a time of relatively weak orbital forcing and a sea-level rise similar to subsequent deglaciations makes it a possible analog for future global warming scenarios, as well an attractive target for paleoclimate studies of "extreme warming" events. The Polar Front and Subtropical Convergence in the Southern Ocean were at their farthest poleward extent of the late Pleistocene. Ice-rafted debris and polar microfossil assemblages were confined to waters south of 50°S, and sea-ice cover was probably less than in the modem ocean. Another manifestation of this event is an episode of high burial fluxes of calcium carbonate throughout the Southern Ocean and in the high-latitude North Atlantic. Southern Ocean calcium carbonate mass accumulation rates were three to four times higher than during glacial stages, and higher than subsequent interglacials. Bulk sedimentation rates are similar to glacial sedimentation rates; thus the carbonate peaks are not likely to be due to reduced dilution by biogenic opal or terrigenous sediments. The pelagic sediments laid down during Stage 11 are nannofossil-rich oozes, representing high seasonal coccolithophorid productivity as well as increased carbonate preservation in deep waters. Piston core carbonate records as well as ODP, and IMAGES long-piston core carbonate and color reflectance records from Indian Ocean and Australasian sectors (South Tasman Rise and Chatham Rise), all record this anomaly as a preiod of prolonged high carbonate deposition and very bright color. This coccolith "bloom" may be an ecological feedback to the ocean's thermal response: as polar waters warm to greater than 3 degrees C, calcite-secreting phytoplankton increase in abund^ce relative to diatoms, with important geochemical consequences. Temperature-driven increases in carbon dioxide partial pressure are enhanced by carbonate precipitation in surface waters. In addition, this large-scale change in the mode of ocean productivity may be partly responsible for a geochemical reorganization of the ocean. The extreme carbonate dissolution that occurred in the Pacific and Indian Oceans during Stage 11 may be partly due to partitioning among pelagic reservoirs of calcite, with sinks lying in the high latitude open-ocean basins as well as in continental margin environments inundated during the glacial termination. The thermal and geochemical extremes of Stage 11 occurred during a time of weak orbital (Milankovitch) forcing and thus may represent the best example in the geological record of a weakly-forced high-amplitude climate event, with possible analogs in the Holocene orbital geometry.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
THE CAPE RIVER AREA - EVIDENCE FOR LATE MESOPROTEROZOIC AND NEOPROTEROZOIC TO CAMBRIAN CRUST IN NORTH QUEENSLAND Ljuneiiutton', C. Mark Fanning^ and Ian W. Withnall' ' Geological Survey of Queensland, GPO Box 194, Brisbane, Queensland 4001 ^ Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200
BASEMENT STRATIGRAPHY - EVIDENCE FOR THE EXISTENCE OF LATE MESOPROTEROZOIC CRUST IN NORTH QLD In the Cape River area, the Cape River Metamorphics of probable Late Neoproterozoic to Early Cambrian age are intruded by the Fat Hen Creek Complex, a series of I and S-type granitoids interlayered with basement gneisses. Quartzo-feldspathic gneiss and biotite-gamet gneiss bands within the Fat Hen Creek Complex may be higher grade equivalents of the Cape River Metamorphics or may be an older basement. Hornblende-bearing granite from one of the gneissic bands contains complex and simple zircons which have yielded 207Pb/206Pb ages of 1105 ± 15 Ma and 1238 ± 38 Ma. Lithic arenite in the Cape River Metamorphics is derived from an almost exclusively Late Mesoproterozoic source with 95% of the 15 zircons analysed yielding an age of 1145 ± 21 Ma. Dating of I and S-type granitoids from the Fat Hen Creek Complex have yielded Late Cambrian to Early Ordovician ages indicating a major magmatic/metamorphic event at that time. The Cape River Metamorphics/Fat Hen Creek Complex are inferred to be overlain by the Late Cambrian to Early Ordovician Seventy Mile Range Group and intruded by Late Silurian to Early Devonian granitoids of the Reedy Springs, Lolworth and Ravenswood Batholiths. GRANITE SOURCE ROCKS - EVIDENCE FOR EXTENSION OF THE PROTEROZOIC BASEMENT A comparison of Late Silurian to Early Devonian granitoids from the Georgetown Region, Reedy Springs Batholith, Lolworth Batholith and Ravenswood Batholith suggest chemically similar source rocks for the Georgetown and Reedy Springs granitoids with different source rocks for the Lolworth and Ravenswood Batholiths. A comparison of inherited zircon populations and €„d model ages from granitoids from all three batholiths suggest a Precambrian component in their source rocks. These data suggest that some Precambrian basement underlies all three batholiths with sources for the Georgetown and Reedy Springs areas being similar but different to the Lolworth and Ravenswood Batholiths. CAPE RIVER METAMORPHICS - AN EXTENSION OF THE DELAMERIAN OROGEN? The Cape River Metamorphics are a sequence of lower to middle amphibolite grade fine grained arenite and argillite with some calc-silicate and mafic volcanics. They are equated with the Charters Towers, Argentine and Running River Metamorphics and possibly with the Anakie Metamorphic Group to the south. The Charters Towers Metamorphics contain mafic bodies (intrusives??) which are dated at 507 ± 12 Ma giving a mmimum age to the sequence. Similarly the Anakie Metamorphic Group have yielded metamorphic ages of ~510Ma with an older age of 560 Ma of uncertain significance. These events are within the range of the Delamerian Orogeny in southern Australia. SIGNIFICANCE OF NEW INTERPRETATIONS IN CONTINENTAL RECONSTRUCTIONS The significance of these data lies in proposed continental reconstructions in eastern Australia during the Late Mesoproterozoic, Neoproterozoic and Early Cambrian. Three conclusions are drawn: • The existence of Grenvillian age (900-1200Ma) crust in or beneath the Cape River area extends the known distribution of rock of this age from the Musgrave area of Central Australia. Continental reconstructions using the SWEAT hypothesis extend Grenvillian belts in America through Antarctica, the Albany-Fraser belt in southern Australia and in the Musgrave region of Central Australia and now into North Queensland. • The distribution of rocks affected by the Delamerian Orogeny has increased northwards to include parts of the Thomson Fold Belt in Queensland. These rocks form the northern end of an orogenic belt which appears to have extended the entire length of eastern Gondwanaland from the Ross Orogen in Antarctica, Delamerian Orogen in southern Australia, to North Queensland. • The presence of Mesoproterozoic and Neoproterozoic to Early Cambrian rocks in the Thomson Fold Belt raises some questions as to the nature of the Diamantina Lineament (Tasman Line) which is previously believed to mark the eastern limit of the Precambrian Australian Craton. It is possible that the Thomson Orogen is floored by thinned Proterozoic crust formed during the breakup of Laurentia from Gondwana in the Neoproterozoic, and separated from the thick craton to the west by the Diamantina Lineament.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
A -1500 Ma PALAEOMAGNETIC AGE FOR THE GOLD-BEARING QUAMBY CONGLOMERATE, EASTERN MOUNT ISA INLIER, QUEENSLAND Mart Idnurm' and Lesley Wvbom' lAustralian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601
The enigmatic Quamby gold deposit lies in the Eastern Fold Belt of the Mount Isa Inlier. It is hosted by the Quamby Conglomerate which comprises mainly conglomerate, feldspathic sandstone, and greywacke: clasts include slate, schist, and quartzite. Hematite is ubiquitous, and is regarded as postdepositional, possibly of hydrothermai origin. The unit crops out in two N and NNE trending graben-like structures, where it is unconformably underlain by metasediments mapped as the Palaeoproterozoic Corella Formation. The northern block, which contains the deposit, is bounded by the Mount Rosebee and Camel Creek Faults; the southern block lies within the Quamby Fault Zone. Although tightly folded at some localities, the unit contains no evidence of metamorphism; its maximum age, therefore, is -1550-1500 Ma — the end of the last period of regional metamorphism in the Mount Isa Inlier. Little is known of the Quamby gold deposit, although several other Cu-Au deposits occur in the Eastern Fold Belt, including the major Ernest Henry deposit -40 km east of Quamby. These other deposits are believed to have been formed by hydrothermai processes related to the intrusion of the nearby ~1500-Ma Williams and Naraku batholiths. A palaeomagneUc sampling program was undertaken to help elucidate the age and origin of hydrothermai hematite in the Quamby Conglomerate. We collected 110 oriented samples from red arenite lenses and the least pebbly beds at twelve spot localities in the northern and southern blocks. Forty-nine other samples were obtained from cobbles of slate within the conglomerate to check that their remanences have random directions (i.e., are not postdepositional overprints). Standard thermal demagnetisation techniques, comprising normally 26 heating steps in the range 100-695°C, were used for analysing the components of remanence. The magnetisation intensity of the typical aremte sample changed litUe up to at least 600°C, and with few exceptions the arenite remanences contained a single component. Corrections for bedding dips halved the directional scatter, indicating that the magnetisation was acquired before deformation. Eleven of the twelve site mean directions form a pair of antipolar groups that represent either identical or closely similar ages, and give a palaeomagnetic pole, QC, at latitude 85.0°S and longitude 7.4°E (A95 = 5.4°) on the Australian apparent polar-wander patli (APWP). Although the remanences of the slate cobbles are poorly defined, most reveal two components. The first demagnetised in the hematite temperature range well above the Curie point of magnetite (580°C), and yielded random directions, as expected for the magnetisation of cobbles deposited in random orientations. The second component unblocked below the Curie point of magneUte. Its directions, although highly scattered, are not random. The mean field-corrected direction for this component is declination 307.8°, inclination 39.6° {o^s = 22.9°), equating to a pole QCC at latitude 21.4°S and longimde 269.2°E (A95 = 22.9°). In view of the poor quality of the results and the large scatter among sample directions, this pole should be regarded as only a rough approximation. The pole QC for the Quamby Conglomerate plots on the APWP near but on the younger side of the pole IM, which represents the final stages of metamorphism in the Mount Isa Inlier. This is consistent with sedimentation of the Quamby Conglomerate postdating regional metamorphism. The closeness of the two poles suggests that the hematite was introduced soon after the metamorphism. Therefore, the age of the umt is inferred to be =1500 Ma, which leaves open two interpretations for the origin of the gold; (I) this age corresponds to the period of widespread circulation of hydrothermai fluids that probably produced the Cu-Au deposits in the Eastern Fold Belt; and (2) the gold may be detrital, having been eroded from older rocks uplifted at -1500 Ma and deposited on the beds of streams that followed the active faults. The pole QCC, related to the remanence component with the lower unblocking temperature in the slate cobbles, appears to be' a partial overprint superposed on either a primary magnetisation of the cobbles or some predepositional secondary magneUsation. Its absence from the arenite samples may be due to the high magneuc stability of the hematite in the arenite to heating. QCC lies roughly on the same part of the APWP as the pole lAR for the postmetamorphic dykes of the Mount Isa Inlier. These dykes include the 1100-Ma Lakeview Dolerite which has been traced to within 35 km of the southernmost sampling site. The pole could thus represent either a thermal or a chemical overprint associated with the lIOO-Ma magmatism which may be part of the Grenvillean orogenesis. Acknowledgments: Our work wasfimdedpartly by the Western Mining Corporation. This abstract is published with the permission of the Executive Director, AGSO. 217
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
CORRELATIONS OF PROTEROZOIC UNITS ACROSS THE MURPHY INLIER Michael J Jackson' Deborah L Scott' and David Rawlings^ 'Australian Geological Survey Organisation, PO Box 378, Canberra, ACT, 2601 ^ CODES SRC, University of Tasmania, GPO Box 252-79, Hobart, 7001
The Murphy Inlier is a 200 x 20 km EW-trending outcrop belt straddling the NT-Qld border near 18°S. The belt is cored by Murphy Metamorphics (>1900Ma), Nicholson Granite Complex and Cliffdale Volcanics (c. 1850Ma). It is overlain and intruded by the Peters Creek Volcanics (PCV). This basement inlier separates the Palaeoproterozoic successions of the Mt Isa Inlier to the south from those in the McArthur Basin to the north. Revised correlations across the Inlier, based on recent field studies in the Seigal-Hedleys Creek area, are combined with existing SHRIMP U/Pb zircon dates to elucidate the relationships between two major parts of a vast Palaeoproterozoic superbasin. The lower part of the McArthur Basin succession along the northern flank of the Murphy Inlier is the Tawallah Group. It comprises a '-2000m thick basal clastic unit, the Westmoreland Conglomerate, overlain by -lOOOm offloodbasalt (Seigal Volcanics), succeeded by a mixed clastic, carbonate and igneous sequence -1500m thick. Studies in 1994/5 by Rogers & Bull (Uni.Tasmania) 300 km to the NW of the Inlier, suggested an inversion event, related to E-W compression, in the middle of the Tawallah Group. This event is recognised as a local unconformity at the base of the Wununmantyala Sandstone (W Sst). Mapping along the northern flank of the Murphy Inlier has identified a subtle, but regionally extensive unconformity at a correlative position in the Tawallah Group, at the base of the "Sly Creek Sandstone"(herein revised to W Sst). Erosion associated with this inversion event appears to be more extensive adjacent to the Murphy Inlier. Sequence stratigraphic studies have also identified a probable important sequence boundary -700m higher up in the succession, within the distinctive carbonate-shale-sandstone Wollogorang Formation. SHRIMP dates (Rod Page, pers. conun.) of 1730±3Ma below the sequence boundary and 1723±4Ma above it, can be interpreted as a hiatus of several million years. These dates also provide a minimum age for the 'mid-Tawallah inversion'. In contrast, stratigraphic equivalents of the Tawallah Group along the southern margin of the Inlier are about half the total thickness. They comprise a -70m thick, coarse-grained, basal clastic unit (Wire Creek Sandstone) and an overlying -600m thick flood basalt unit (Ptpi). These are overlain by a diverse succession dominated by felsic and mafic igneous rocks, with minor carbonates and sandstone (units Ptp2 to Ptpv). Based on earlier interpretations of field relationships and geochronology, most schemes correlate Ptp2-7 with the youngest felsic rocks in the upper Tawallah Group (eg Hobblechain Rhyolite at 1725±2Ma). Detailed investigations offieldrelationships were undertaken in 1997 to better constrain the PCV stratigraphy. New geochronological and geochemical samples were collected to facilitate geodynamic analysis. The identification of key erosion surfaces and thrust duplex repetitions, and the establishment of intrusive-extrusive relationships between various felsic and mafic igneous units indicates a complex history, significantly diflferent from the currently pubhshed extrusive layer-cake stratigraphy. A prominent laterally continuous erosion surface with metres of relief, was recognised in the upper part of Ptpi. It occurs at the base of a conglomeratesandstone unit (Ptpis) previously considered to be concordant within the volcanics. Even though the sandstone unit is relatively thin, we propose correlation with the W Sst on the basis of comparable lithofacies, basal erosion surface and stratigraphic position. Mafic igneous rocks which lie above the clastic unit, and were previously included in Rpi, are clearly sills and are thus not coeval with the underlying extrusive mafic phase. The overlying felsic igneous sheet Ptp2 (1729±4 and 1726±2Ma) is also demonstrably intrusive, indicating sedimentological continuity between the siliciclastics of Ptpu and the lutites of Ptps. The distinctive suite of sedimentary structures within Ptps - carbonate nodules, laminated black shales, wave-rippled microbial mats, 'bladed beds', and chertified domal bioherms of digitate stromatolites - are characteristic of, and favour correlation with, the Wollogorang Formation. As is implied by the non-systematic vertical variation in dates, several other felsic units in the PCV are of intrusive rather than extrusive origin, and provide only minimum depositional ages for the interstratified sedimentary rocks. Comagmatic felsic lavas and volcaniclastics are only recognised from the uppermost part of the PCV. Clustering of dates around 1724-1729 Ma implies the felsic rocks were emplaced during a brief event concurrent with felsic magmatism in the upper Tawallah Group. When refined, the new correlations across the Murphy Inlier should provide better constrained geological models to aid palaeogeographic interpretations, superbasin reconstructions and models of crustal evolution.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
THE LASER ABLATION MICROPROBE (LAM)-ICP-MS: APPLICATIONS TO ORE SYSTEMS AND MINERAL EXPLORATION Simon E. Jackson, Norman J. Pearson and William L. Griffm Key Centre for Geochemical Evolution and Metallogeny of Continents, School of Earth Sciences, Macquarie University, Sydney, NSW
Laser ablation microprobe (LAM)-ICP-MS is firmly established as a fast, sensitive and reliable technique for in situ determination of trace elements in minerals and their inclusions. The technique couples the high resolution sampling capabilities of a pulsed UV laser with the extraordinary detection capabilities of the ICP-MS. Current instrumentation is capable of simultaneously determining 30-40 elements in spots of ca. 30 |im diameter at detection limits down to low ppb level for many elements. Minimal sample preparation is required and a typical analysis takes less than 2 minutes. Using the depth profiling capability of the laser, it is also possible to assess the homogeneity of the ablation volume and so determine whether elements are concentrated homogeneously in the lattice structure, or whether they are chemically zoned or occur in inclusions. The technique already has very useful isotopic applications {e.g., U/Pb dating) and, with the recent advent of multi-collector ICP-MS instrumentation, an age of rapid, in situ, high precision isotope ratio analyses {e.g., U/Pb, Re/Os, Sm/Nd, Lu/Hf, Rb/Sr) is dawning. Applications of LAM-ICP-MS in mineral exploration are numerous and developing rapidly. This paper will briefly describe the technique and detail several case studies of applications of LAM-ICP-MS in the study of mineral deposits. • Indicator minerals: LAM-ICP-MS has tremendous potential for the analysis of indicator minerals for assessing the mineral potential of their source rocks. For example, trace element data from chrome-pyrope garnets in heavy mineral concentrate provide information that can be used to quantify the diamond grade of a kimberlite or lamproitic host rock. The relationships between elements such as Zr, Y and Ti have been used to identify the chemical signatures of different types of mantle processes. Plots involving these elements show that garnet inclusions in diamond and a relatively high proportion of garnets from high-grade diamondiferous kimberlites have depleted trace element concentrations with Zr < 20 ppm, Y < 8 ppm and Ti from 10-2000 ppm. The depleted nature of garnets associated with diamond is also borne out in REE patterns, with many garnets having strongly depleted HREE and enriched MREE, indicative of metasomatic processes which appear to be strongly correlated with the presence of diamonds. Plots involving element ratios indicative of the shape of the REE patterns {e.g., Nd/Y, Sc/Y) provide simple discrimination tests to estimate the diamond potential of a pipe. Case studies of pipes from S. Africa and Siberia will be presented. Other resistate indicator minerals {e.g., tourmaline, chromite, ilmenite, zircon) have tremendous and largely unexplored potential for prospecting for other classes of deposit. • Gangue minerals: Studies of trace element chemistry of hydrothermal gangue minerals {e.g., carbonates, fluorite) in gold and base metal deposits have revealed that complex and extreme zonation of elements, such as REE, is typical and may be indicative of processes that have lead to the economic concentration of the ore minerals. In prospecting for ore deposits formed in submarine environments, systematic variations in REE patterns of gangue carbonate, which are sensitive indicators of hydrothermal fluid chemistry {e.g., sea water magmatic) can be used in tracing the mineralised feeder zone(s). A case study of the Castellanos deposit, Cuba, will be presented. • Ore minerals: The trace element signatures of many ore minerals and their associated sulphides reflect the setting of the mineralisation, allowing quick identification of the geological environment of a showing. The application of LAM-ICP-MS to characterisation of Australian molybdenite occurrences will be given by Blevin et al. (this volume). • Mapping: In zones of very intense weathering, where field mapping can be very difficult, the trace element signatures of certain resistant minerals has been used for correlating volcanic and intrusive units. An example of the application of LAM-ICP-MS to mapping and correlating potentially diamondiferous units using the chemistry of resistant mineral phases will be presented. • Fluid inclusions: LAM-ICP-MS has the capability of sampling and analysing individual fluid inclusions in the ore and associated minerals. This has critical implications for refining our models of ore forming systems. • Age dating: In addition to its many trace element application, LAM-ICP-MS can produce accurate in situ U-Pb age determinations of zircons and other minerals. The speed, precision and accuracy of the technique makes it ideally suited for exploration purposes. The application of LAM-ICP-MS to the study of mineral deposits is in its infancy. However, the wide ranging capabilities, speed and economy of the technique offers a powerful new tool in exploration geology and can also provide data that will give new insights into the genesis of mineral deposits.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
MICROSOFT EXCEL PROVES THE BEST PROTOTYPING TOOL FOR INVESTIGATION SOFTWARE BASED ON THE ELASTIC DATA MODEL. Grant L. Jacquier Computers in Geology, Box 3170, Rundle Mall PO, Adelaide, South Australia, 5000
There are several software programs available for geology field work. These are standard oriented, integrated with a global position system (GPS) and are very categorical. As such they are ideal for exploration and regional surveys. Geological surveys and exploration companies support the development of this software. What is not available is software that is flexible enough to cater for a wide range of data models, with a bias towards geotechnical instrumentation and geological measurement. The flexibility to contain very different data sets has in theory been overcome by generalised data models such as the Enigmatic Long and Skinny Tables Increasing Comprehensiveness (ELASTIC) data model. Software based on this model would be most useful for site investigation and laboratory work. Consultants and auditors will support this development. There are four scenarios for the development of any system: A. Ask for tenders for an off-the-shelf system. B. Analyse and design a proprietary system. C. Form an alliance between the company with the technology and the client who will receive the benefit. D. Prototyping If the problem is well understood but the financial return is low, scenario A gives the best result. If the problem is technically easy but there is a high rate of return then scenario B will maximise the benefit to the client. If the problem is technically difficult and return is high then scenario C is best. Scenario D is used where the problem is technically difficult and also the expected financial return is low. Software utilising a generalised data model is not readily available because: • There are many small client companies who wish to maintain their independence, and do not co-operate. • The increased effectiveness of the client companies using this software will be lower than that achieved by exploration. • The client companies are much more diverse in their activities than geological surveys or exploration companies. Additionally, the ELASTIC data model when implemented in a database exhibits tables of data that cannot be read directly like a book but must be recomposed, that is the tables are Enigmatic and this adds further difficulty to the problem. Therefore prototyping is the best in this situation of technical difficulty and low financial return. Three prototypes have been developed with three alternative software packages. The three development tools tried were the Borland OWL Object library , Borland Visual Basic add-ins and Microsoft Excel. The experience from the previous prototype assisted the selection of the development tool for the next attempt but in particular: Prototype 1. The major weakness of the Object library is that a formal specification is needed to be effective. Prototype 2. The major weakness of the add-ins is it is difficult to create data and manage test sets. Prototype 3. The great strength of MS-Excel is the ease of developing documentation and test data within the application and at the same time maintaining both relational and object models should redevelopment in a more rigid environment be necessary. When taking full advantage of relational algebra, as in the ELASTIC data model, it is not easy to lay out a complete design. Instead, the gradual development of objects, by prototyping, defines a library of independent modules which in total provide more comprehensive and more portable code. This in turn will lead to broader market appeal and greater profitability through economy of scale. The prototyping methodology using Excel is the best way to approach a poorly defined problem with expected low return on investment (scenario D). From the experience with the software development tools in this project the C family of languages would be best for scenario C, object libraries (like OWL) suit scenario B and plug in component libraries scenario A.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
ALTERATION OF A SINGLE KOMATHTE LAVA FLOW FROM KAMBALDA, WA
Mary Jane. Ian Cartwright and David Lambert Victorian Institute of Earth and Planetary Sciences, Department of Earth Sciences, Monash University, Clayton, VIC 3168
The Norseman-Wiluna greenstone belt of the Archaean Yilgam Craton contains the largest magmatic nickel sulphide deposits in Australia. At Kambalda, nickel sulphide mineralisation mostly occurs within structurally modified trough-like embayments at the interface between the host komatiites of the 2.7 Ga Kambalda Komatiite Formation (KKF) and the underlying Lunnon Basalt. A number of post-magmatic processes have affected the KKF, including: (1) seafloor hydrothermal alteration and serpentinisation; (2) deformation; (3) greenschist-amphibolite facies regional metamorphism; and (4) fluid-driven mesothermal gold deposition. It is crucial to the interpretation of trace element and isotopic data that these individual processes be identified. Metamorphic/hydrothermal modification of rock geochemistry can mimic some primary igneous processes such as chemical differentiation and contamination of the lava flows. Alteration can significantly modify the original chemical relationships. The recognition of element mobility is essential to evaluate the chemical history of these lava flows. In order to assess the effect of alteration and metamorphism on the komatiites, we have obtained major element, trace element, radiogenic and stable isotopic data for a complete range of KKF samples from a single deep drill hole above the Hunt ore shoot. All komatiites from this drill hole have undergone metamorphism and metasomatism, with secondary hydrous mineral formation and addition of carbonate. It is most unlikely that these post-depositional events did not result in the exchange of other chemical components. In particular, the careful examination of a single komatiite flow approximately 2 metres thick has lead to a better understanding of chemical mobility of key trace elements that have traditionally been used in the evaluation of the magmatic processes, in particular the REE. This single flow exhibits well preserved and characteristic komatiite textural relationships, including flow top breccia, spinifex textured A-zone and cumulate Bzone. The original mineralogy consisted of olivine, chromite and a small amount of interstitial pyroxene and glass, all of which have been replaced by secondary minerals during seafloor alteration shortly after emplacement, followed by upper greenschist facies metamorphism. Large variations in elements that are incompatible with this original olivine-dominated mineralogy can likely be assumed to be the result of primary magmatic processes, crustal contamination or post-magmatic element mobility. Our data show that, in this single flow, compatible major and minor elements such as Mg and Ni do follow olivine control lines, a primary chemical feature. However, the REE data reveal large variations in the light REE and Eu. (La/Sm)nratios across the single flow range from 0.5 to 1.0, greater than the value of 0.2 to 0.3 expected in a highdegree depleted mantle melt, with Eu unexpectedly exhibiting both positive and negative anomalies. These enriched LREE data have been interpreted to reflect ground melting and incorporation of crustal materials with (La/Sm)n of 3.0-4.0 into the komatiite lava (Lesher & Campbell, 1993). Disturbance of the LREE is also noted in the Sm-Nd isotopic system, with the calculation of a model 1 isochron age of 2608±77 Ma on samples from the single flow. This age, however is in accord with the estimates of the craton-wide gold formation event at 2630 Ma (Kent & McDougall, 1995). Oxygen isotopic data for silicates and carbonates within the same single flow have been homogenised to an average 6^^0(smow) value of 8.1 for silicate whole rock, and a 5^®0(smow) of 11.1 for carbonate, this contrasts with primary 5 O(smow) magmatic values of 5.0-6.0 found elsewhere in the Yilgam Craton. The carbon C(pdb) of -7.5 indicates that a fluid from a deep crustal source was involved, while the isotopic results, an average oxygen isotope results illustrate that the altered silicate oxygen values are in equilibrium with the carbonate oxygen values. In order to cause a shift of this magnitude, the addition of a very large percentage of crustally sourced material is needed, which is not supported by the REE data. These altered values are also found in all the basaltic units, suggesting that large scale C02-bearing fluid flow has occurred throughout the volcanic pile This fluid is presumed to be the gold-bearing fluid as the Sm-Nd isochron age is coincident with the deposition of gold in this area. REFERENCES Lesher C.M. & Campbell I.H. 1993. Geochemical and Fluid Dynamic Modeling of Compositional Variations in Archaean Komatiite-Hosted Nickel Sulfide Ores in Westem Australia., Economic Geology 88, 804-816. Kent A. R. J. & McDougall 1. 1995. "^^Ar-^^Ar and U-Pb Age Constraints on the Timing of Gold Mineralisation in the Kalgoorlie Gold Field, Westem Australia: Economic Geology 90, 845-859.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
A LATE CRETACEOUS AHERMATYPIC CORAL FAUNA FROM WESTERN AUSTRALIA John S. Jeir and Peter A. JelP ^Department of Earth Sciences, The University of Queensland, Queensland 4072 ^Queensland Museum, PO Box 3300, South Brisbane, Queensland 4101
The fauna of the Miria Formation, Carnarvon Basin, Western Australia, exposed in the Giralia Range to the south of Exmouth Gulf is dominated by molluscs but has yielded a moderately diverse assemblage of corals. Ten species of ahermatypic scleractinian distributed among eight genera and one stylasterid have been recognised. The corals are faithfully replicated as sediment casts of the molds within a sparry calcite matrix follow^ing the dissolution of the aragonitic skeleton of the coral. The fauna is the most diverse and abundant Mesozoic coral fauna known from Australia. The only Mesozoic coral previously described from Western Australia is Coelosmilial ginginemis Etheridge, 1913 from the Upper Cretaceous Gingin Chalk in the Perth Basin. A hermatype has recently been found in the Early Cretaceous of the Great Artesian Basin. The stylasterid possibly represents the earliest known occurrence of that group. Other species represent very early occurrences for their genera. The fungiid Fungiacyathus (Bathyactis) is reported from the Cretaceous for the first time. Flahellum is typically Cainozoic with the only other Cretaceous record being in Antarctica. Ballenophyllia is widely distributed in the Tertiary but its only other known Cretaceous occurrence is in southern India. A Maastrichtian age for the fauna is indicated by associated foraminifers and ammonites.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
NARAOIA IN THE MIDDLE CAMBRIAN OF QUEENSLAND Peter A. Jell Queensland Museum, PO Box 3300, South Brisbane, Queensland 4101
Naraoia Walcott, 1912 was originally described from the Burgess Shale of British Columbia and has since been recognised in the Wheeler Shale of Utah and the Chenjiang Shale fauna of southwestern China. Its occurrence only in these well-known shale faunas, each usually considered a lagerstatten, suggests that either it only inhabited muddy environments and/or that its chemical composition required these conditions for its fossilization. Therefore, the discovery during 1997 of two specimens of Naraoia in a thick, relatively clean, limestone sequence with a diverse and abundant trilobite and brachiopod fauna was totally unexpected. The specimens came from the Middle Cambrian (Ptychagnostus nathorsti Zone) V Creek Limestone just off the Camooweal to Burketown Road near Undilla Homestead. The exoskeleton of the specimens is markedly crinkled suggesting that the animal was of some convexity and that when it settled to the sea floor its margin remained in tact so that removal of the vertical dimension has been accommodated by the crinkling. Although appendages are not immediately apparent as in other known occurrences of the genus, close examination of the cephalon reveals at least partial appendages compressed under the exoskeleton. Rudimentary segmentation is also observed beneath the axial region of the pygidium suggesting a multisegmented abdominal region of the animal.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
INTEGRATION OF GEOLOGICAL AND GEOCHEMICAL DATA IN A DATABASE. PRACTICAL SYSTEM DESIGN AND DATA STRUCTURES. David R. Jenkins Terra Search Pty Ltd, P.O.Box 981, Castletown Queensland 4812
The collection of geological observations as part of exploration programs and advanced mining projects are often underutilised due to a lack of planning and inadequate tools for the analysis of the data. The key to having well coded observations is consistency. Lithological information can be described in many ways and how you code the geology should be dependent on how you intend to use the information. Coding is encryption and makes the information data more understandable for computers not geologists. If the data is to be used manually the coding may only reduce the amount of information and the accessibility of the information. However, the amount of data currently available to explorers often dictates that analysis of the data must at least partially be performed using computers. In this case codes save space and avoid confusion for a mechanical analysis. Separating observations into criteria such as lithology, descriptors, alteration, mineralisation, colour, weathering and regolith will afford more flexibility when analysing the data but has the possibility offragmentingthe data. There are several ways of avoiding suchfragmentationsuch as using set coding methods and systems that can group together codes with common traits as single entities. In most exploration and all mining situations a preconceived idea of the likely geology can be formulated using existing knowledge and modeling. This should include ensuring the most critical information for a lithology is coded in the most simplistic way. For example the coding of quartz biotite schist could utilise a single code or altemately use a code for felsic schist with descriptors of quartz and biotite. This would depend on whether it is important to distinguish the type of schist in the analysis of the data or to distinguish between schists and other groups of rocks in the area. Consistent coding can facilitate digital map and drill section creation, statistical analysis and target generation. Any geological database system should have the flexibility to store the data in the way that will facilitate analysis. A system needs to lend itself to the entry of consistent data, simultaneously allowing for the querying of similar inconsistent codes. This later grouping needs to be simple enough for a geologist to use. A mistake commonly made in designing file structures and coding conventions is to give too much flexibility in the coding and too little within the file structures. Geologists require a myriad of lithologies, descriptors and minerals to fully describe their environment. One of the aims of this description is to group together like observations. This is exactly what we should be doing with coding. There should however be a way to distinguish the differences between the observations that we group. Also we need to distinguish certain and uncertain observation. Databases which require high levels of knowledge of computer systems inhibit access to the data. IT departments tend to concentrate on the T and neglect the I. Database structures should be designed by concentrating on the needs of the geologist and the data, not pandering to the most effective methods for a computer. A well designed database, along with a well defined coding system will return the data to the geologist from the computer.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
PRODUCTION OF PORTABLE INTEGRATED GEOSCIENTIFIC CIS DATASETS FROM A CORPORATE DATABASE Greg Jenkins, Mineral Resources Group, Primary Industries & Resources South Australia, GPO Box 2355, Adelaide, South Australia 5001
Geoscientific data at the Mineral Resources Group of PIRSA (formerly MESA) are resident in two main repositories, Oracle and ARC/INFO. At present, point data such as drillholes, geochemistry and field observation points are stored in entirely in Oracle tables, while the spatial components and minimal attributes of line and polygon data including faults and outcrop geology are stored as ARC/INFO arcs/polygons/regions with then- lookup tables stored in Oracle. Raster images such as magnetics and gravity reside in various systems and are usually manipulated through ERMapper. The requirements of mineral exploration companies and other external clients have necessitated the production of a portable version of relevant parts of the database which can be viewed and queried on desktop or laptop personal computers using software provided by the client. Data formats currently supported are ArcView and Maplnfo. A number of additional querying fimctions have been developed in order to replicate some of the most frequently used queries which were previously only available using in-house Departmental facilities. For points, the data are downloaded from Oracle via ODBC, and tables are denormalised where necessary at desktop PC level. For lines and polygons, ARC/INFO coverages are produced by mergmg spatial and attribute data via AML, before translating the coverages to the appropriate format and finally manipulating the data at desktop PC level. The datasets are resident on a Novell file server for in-house use, and are made available to external clients on CD-ROM. At some future time, limited versions of the data will be made available for viewing through an Internet connection.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
CONTRASTING CHARACTER OF CARBONIFEROUS AND EARLY PERMIAN VOLCANISM IN THE SOUTHERN NEW ENGLAND FOLD BELT: TECTONIC IMPLICATIONS S^'^ andWiUiamJ. CoUins^ ^Department of Geology, University of Newcastle, Callaghan, N( New South Wales 2308 ^Present address: P.O. Box 281, Mapleton, Queensland 4560
Three different Late Palaeozoic volcanic rock suites can be identified by age and composition in the Tamworth Belt of the Southern New England Fold Belt, New South Wales. The most volumetric and long-lived suite is of Carboniferous age (350-310 Ma) and consists of regularly spaced volcanic centres that extended some 400 km from Barraba in the north to Port Stephens in the south. Eruption produced widespread calc-alkaline, andesitic to rhyolitic, porphyritic ignimbrite sheets that have remarkably similar trace element characteristics. These include LREE enrichment and moderate negative europium anomalies (LaNAl)N=8.3; Eu/Eu*=0.6), relatively high radiogenic Sr abundance, and distinctive spiked multielement patterns showing large ion lithophile element (LDLE) enrichment and high field strength element (HFSE) depletion relative to mid-ocean ridge basalt (MORB). These features are typical of subduction-zone magmatism and the radiogenic character suggests incorporation of continental crust into a melts derived from the lithospheric mantle wedge. [N.B. all isotopic and age data are from published sources.] Widespread, Early Permian bimodal volcanism occurred around 290-280 Ma, with major centres near Gunnedah, Quirindi and Maitland. High-alumina basaltic andesite lava and rhyolite are dominant. The basaltic andesites have trace element characteristics somewhat similar to the Carboniferous suite, but have a lower radiogenic component and the multielement patterns are smoother with less distinct LILE—HFSE anomalies. The basaltic andesites are LREE enriched compared with MORB, with minor negative Eu anomalies (LaN/YbN=4.0; EuyEu*=0.8). The chemical and isotopic characteristics of the basaltic andesites suggest they were derived from a mantle lithosphere source previously enriched by subduction processes. Later Early Permian volcanism (-280-275 Ma) is volumetrically least significant, with eruptive centres in the Bulahdelah-Gloucester area. Volcanism is dominated by tholeiitic basalt flows and subordinate rhyolite. The basaltic rocks are compositionally distinct from the earlier Permian basalts,, with flat or convex MORBnormalised patterns, MORB-like REE abundances and no Eu anomalies (LaNAn)N=l.l). The basalts are the least radiogenic of the suites. Rhyolite extrusives and sills are distinct from the Carboniferous rhyolites, being less enriched in LREE and with greater negative Eu anomalies (LaNAT3N=2.3; Eu/Eu*=0.4). The chemical and isotopic characteristics of the basalts suggest they were derived from an asthenospheric source without crustal contamination. The chemical and isotopic characteristics of the Carboniferous volcanic rocks, and their close association with adjacent fore-arc and accretionary sediments, suggest an east-facing continental arc existed at this stage. Earliest Permian mafic volcanism is contemporaneous with Sydney-Gunnedah-Bowen Basin formation, which we suggest formed in response to crustal extension during eastward steepening or rollback of the west-dipping subducted slab. The corresponding change in volcanism from intermediate calc-alkaline (crustal contaminated), via a transitional basaltic andesite stage, to primitive tholeiitic basalt compositions reflects progressive intra- to back-arc rifting in the Early Permian, corresponding to progressive replacement of slab-fluxed lithospheric mantle with asthenosphere. Reversal of net plate boundary forces to compressional in the latest Early Permian terminated volcanism and rifting, initiating uplift of tiie accretionary prism and foreland basin formation associated with the Hunter-Bowen Orogeny.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
INDENTER TECTONICS AND THE STRUCTURAL FABRIC OF THE NAMBUCCA BLOCK, SOUTHERN NEW ENGLAND FOLD BELT, NSW. Geology Department, University of Newcastle, Callaghan, NSW 2308
Many tectonic models proposed for the development of the southern New England Fold Belt have attempted to explain the unique position of the Nambucca Block (NB) and the E-W trending structural grain within it. However, there is substantial disagreement amongst authors as to how this E-W trending fabric formed and whether the NB formed in situ or was translated to its current position. Recent structural studies of Early Permian rocks exposed on wavecut platforms on the eastern margin of the NB, combined with a synthesis of the available chronological data, have been carried out to refine previous models. The structural analyses indicate that one to two ductile and two to three brittle-ductile/brittle deformation events are recorded in the lower grade(subgreenschist facies) rocks; in the higher grade(greenschist facies) rocks there is evidence for four to five ductile and two to three brittle-ductile/brittle events. Further, they reveal "a change in fold style from macroscopic open, inclined folds (Fi) at Grassy Head in the south, to near recumbent isoclinal, mesoscopic folds (Fi and F2) at Nambucca Heads and Valla Beach, suggesting that strain increases towards the Coffs Harbour Block to the north. Both fold events were associated with S-SSW-directed tectonic transport and N-S shortening. Subsequently, a change in compressive stress direction occurred, resulting in the formation of meridional trending, open, upright F3 folds and inclined to upright F4 folds, axial plane to which are poor to moderately developed, crenulation cleavages S3 and S4. West-over-east thrusting on the Wongwibinda-Yarrowitch Fault Zone (WYFZ) took place at this time. Folds formed during D3 and D4, developed heterogeneously throughout the higher grade areas (Nambucca Heads to Hungry Head). The absence of similarly oriented structures in lower grade rocks is believed to be due to the low strain associated with these episodes. Change in plate convergence direction to the NE, resulted in en echelon vein arrays and kink bands throughout the eastern margin of the NB. The convergence direction remained north-easterly during later NNE-ENE, strike slip faulting but then rotated to ~SE during the formation of ESE-SE trending, strike slip faults. Termination of faulting occurred prior to 229 Ma, the age of emplacement of the Triassic granitoids. The deformation and associated metamorphism in the NB is believed to be the result of indentation tectonics, whereby south-directed movement of the Coffs Harbour Block (CB) during oroclinal bending produced the eastwest trending structures. The effects of the CB were greatest during Di and D2 on all headlands, with the exception of Grassy Head, where the propagating deformation front resulted in only one ductile phase of deformation. Continued N-S compression post-D2, may have resulted in fold lock-up and the development of a listric thrust fault system exposing deeper crustal levels towards the centre of the block. Shortening of the sedimentary pile in the NB during the southward push of the CB towards the rigid, indurated. Carboniferous, forearc basin sequences in the Hastings Block, resulted in the "escape" of part of itie Early Permian sequence to the E and SE. The deformation associated with this push is believed to have occurred prior to movement on the WYFZ (258-266Ma) and after sedimentation was terminated (267-269Ma).
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
REGOLITH PROFILES ON PALAEOZOIC SEDIMENTS IN THE BALLARATCRESWICK AREA OF CENTRAL VICTORIA: WHAT THEY TELL US ABOUT THE DEVELOPMENT OF DEEP WEATHERING, AND ITS LATER STRIPPING OR PRESERVATION Bemie Jovce.^ Megan Hough^ and David Taylor^ ^ Associate, School of Earth Sciences, The University of Melbourne, Parkville, Victoria 3052 ^ Research Assistant, Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Clayton, Victoria 3168 ^Geological Survey of Victoria, PO Box 500, East Melbourne, Victoria 3002
Centred on the Ballarat-Creswick area is the region now often known as the West Victorian Uplands (WVU) which is the continuation westwards of the Victorian Highlands. The bedrock consists mainly of extensive Cambro-Ordovician deep marine turbidites of the Western Lachlan Fold Belt, extending from the Moyston Fault in the west to the Heathcote Fault Zone in the east. Deposition ended witfi the late Ordovician/early Silurian Benambran Orogeny, which folded the rocks and emplaced numerous Devonian granitic intrusions. The Ballarat area became a landmass, the ancestor of the WVU, although to the east deposition continued during the Silurian in the Melbourne Basin. The regolith and landscape surroxmding Creswick and Ballarat in central Victoria have evolved over millions of years by processes including Permian glacial activity, Mesozoic and later tectonics. Tertiary and Quaternary fluvial activity and Cainozoic volcanism. Remnant higher areas of Ordovician bedrock and Devonian granites are surrounded by early Tertiary quartz gravels (White Hills Gravel) and late-Cainozoic Newer Volcanic flows, which infill valleys of the Tertiary and Quaternary drainage systems. The area includes the historical gold mining townships of Illabarook, Smythesdale, Creswick, Clunes and Maryborough. Radiometric imagery assists with interpreting the field exposures. A red to pink radiometric signature (moderate potassium) indicates preserved deep kaolinitic profiles around Creswick, as well as in parts of the Dereel area to the south of Ballarat, while further north, around Maryborough, the slightly weathered bedrock lies closer to the surface, with bedding and cleavage still evident (Hough, 1996) and has a high composite K, U and Th signature which appears as white on the radiometric image. Kaolinitic bleached profiles tens of metres deep are typically preserved around the margins of the broad domal or plateau areas {in situ Bh "Highly weathered bedrock" of Taylor & Joyce, 1996; Creswick Regolith Landform Unit of Hough, 1996) and may have been exhumed from under White Hills Gravel deposits of early Tertiary age. Higher in the landscape, profiles are shallower, up to 2 or 3 m deep, with relatively fresh rock below and a red cracking clay saprolite extending ahnost to the surface with only a shallow grey soil on top. In some areas, generally at the highest elevations, relatively fresh greyish bedrock reaches almost to the surface, and is capped by a quartz and ferruginous lag, and a shallow soil {in situ Bs "Slightly weathered bedrock" of Taylor & Joyce, 1996; Maryborough Regolith Landform Unit of Hough, 1996). Two contrasting models of landscape evolution from the Mesozoic to the present day can be presented for central Victoria from the profiles described and other evidence which has emerged in recent regolithlandform mapping. Deep pallid weathering profiles developed on an ancient Mesozoic palaeosurface may have been stripped by one or several erosional periods and then subjected to further ferruginous weathering during the Tertiary (Hough, 1996). The altemative model is that much of the deep pallid weathering developed during fte early Tertiary, mainly on the lower parts of the landscape, and especially below a cover of White Hills Gravel, and at the same time shallow ferruginous weathering took place within the upper part of the gravels themselves, as well as higher in the bedrock landscape (Taylor & Joyce, 1996). Basalt lava flows dated by K/Ar are helping provide age controls for Tertiary landscape episodes, as can the sequence of alluvial deposits found at different elevations within the landscape. Such regolith-landscape models have practical applications in the search for gold in Victoria as well as in applications to envh-onmental problems such as groundwater exploration, soil erosion and salinisation in the local landscape. REFERENCES Taylor, D.H. & Joyce, E. B., 1996. Ballarat 1:100,000 regolith-exploration map report. Geological Survey of Victoria, Technical Record 1996/4. Hough, M. 1996. The regolith of the Creswick 1:100 000 mapsheet. Unpublished Honours report. School of Earth Sciences, University ofMelbourne. 228
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
METALLOGENESIS OF THE SILVER SPUR GROUP OF POLYMETALLIC DEPOSITS NEAR TEXAS, SOUTH-EAST QUEENSLAND Karl Jupp and Lloyd Hamilton School of Natural Resource Sciences, Queensland University of Technology, Brisbane, Queensland 4001
The interrelationships and genesis of the deposits in the Silver Spur region of South-East Queensland have been clarified through an integrated metallogenic study. The individual deposits are at Silver Spur (ore commodities in order of economic significance are Zn, Pb, Ag, Cu, Au), Tv^in Hills (Ag, Zn, Pb), Tooliambie (Zn, Pb, Cu), Silver King (Ag, Au), Mount Gunyan (Zn, Pb, Ag), Texas Copper (Cu, Pb, Zn) and Eggleston's Deposit (Pb, Zn). The Silver Spur deposit was one of Australia's richest silver mines and has banded ore in several lenses. All the deposits are confined to a small Permian outlier and are hosted by deformed meta-wackes, meta-tuffs and argillites, with gangue minerals of pyrite (dominant in most deposits), quartz, calcite, sericite and chlorite. The high chlorite content of rocks in the region and especially around the deposits is due to lower greenschist metamorphism. Silicification and/or potassium metasomatism occur as alteration associated with the deposits to various extents. Mineralisation style indicates the stratabound Silver Spur deposit is syngenetic or diagenetic. The other deposits are either in the form of epigenetic veins or occur as fine grained disseminations. Trace element chemistry was conducted using wavelength dispersive spectral analysis in an electron microprobe. S, Se, Co and Ni were determined for indications of depositional environments, and other elements were used for 'fingerprinting' the deposits. This, combined with a study of the ore textures, indicates that Silver Spur is a metamorphosed SEDEX deposit rather than a VHMS deposit as previously suggested. However, it is not a typical SEDEX deposit. Field mapping, mineral assemblages, chemistry and textures indicate the other deposits are genetically related to Silver Spur with respect to source and environment of deposition. The main difference between Silver Spur and the disseminated deposits (Twin Hills and Mount Gunyan) is the thinly banded lenses of concentrated ore unique to Silver Spur. Silver Spur is possibly closer to the source of the ore-bearing solutions or may have been deposited in a clastic sediment starved area. Alternatively the disseminated deposits could have been deposited from more buoyant plumes of ore-bearing solutions. The crosscutting deposits at Egglestone's, Texas Copper, TooHambie and Silver King could have been mobilised into fault zones during structural development of the region, or, more probably, they could simply represent vent facies of the mineralisation.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14'^ Australian Geological Convention, Townsville, July 1998
THE DEPOSITIONAL ENVIRONMENT OF BANDED IRON-FORMATIONS AS RECORDED BY CHEMOSTRATIGRAPHY - AN EXAMPLE FROM THE 2.5 GA KURUMAN IRON-FORMATION, TRANSVAAL SUPERGROUP, SOUTH AFRICA Balz S. Kamber^ ' Department of Earth Sciences, University of Queensland, Brisbane, Qld 4072 Australia
In the Transvaal Supergroup (South Africa) a classical 2.50 Ga old transition from a shallow-water carbonate platform (Campbellrand) into banded iron-formation (=BIF) is well-exposed and many drill-cores through the succession are available. The main aim of this study was to compare geochemical fingerprints across this transition with a view to determine the characteristics of the waters from which these chemical sediments precipitated and further, to use these characteristics to constrain the depositional environment of BIF. By use of a combination of geochemical tracers (trace elements, lanthanide patterns, C-, 0 - , and Sr-isotope signatures) applied to micro-drilled primary and early-diagenetic carbonate cements (calcite, ankerite and siderite) the few samples which were contaminated by detritus and/or have been hydrothermally altered could be identified and rejected from the data-set. Carbon and strontium isotope compositions of the cements change dramatically across the profile and at the same time display relatively well-defined stratigraphic patterns. The stratigraphically lower limestones are characterised by typical marine C- and Sr-isotope values of of ca. -1 and initial ^^Sr/^^Sr of ca. 0.702, supporting sedimentological evidence that these rocks formed in a marine, albeit shallow-water, environment. Across the transition zone and into the BIF, the C-isotopic composition of cements becomes increasingly lighter to reach values of of ca. -8 and the initial Sr isotope composition becomes more and more radiogenic with ^"^Sr/^^Sr values reaching 0.736 to 0.760. These patterns clearly demonstrate that whilst the limestones were deposited in an open-marine environment the waters from which the BIF's precipitated were severely, and apparently progressively, restricted from the open ocean. The C-isotope pattern resembles that of modern restricted basins (e.g. the Black Sea), where the water column is strongly stratified into an oxic surface layer and an anoxic bottom layer. The Sr-isotope composition through the transition zone and into the BIF is so radiogenic that it can only reflect a strong dominance of riverine and/or dust input from the mid-Archaean Kaapvaal craton hinterland over exchange with the open ocean. Rare-earth-element + yttrium patterns (Y inserted into conventional REE patterns between Dy and Ho according to its ionic radius) are compatible with the isotopic constraints, in as much as marine indicators like the shalenormalised La/Nd and Y/Ho enrichment and the positive Eu anomaly are best developed in the low ^"^Sr/^^Sr limestones but show increased dilution with shale-like material in the BIFs. In previous geochemical studies of BIF's several authors found positive Eu anomalies (relative to shale) and claimed that this was proof of a deepwater marine deposition since the excess Eu was suspected to have been derived from hydrothermal alteration of ocean-ridge basalts. However, the chemostratigraphical approach adopted here clearly shows that although weak positive shale-normalised Eu anomalies of 1.25 to 1.67 are found in BIF a distinctively stronger anomaly of 1.68 to 2.28 characterises the true marine limestones. This study thus confirms that the early Proterozoic ocean was indeed characterised by much more prominent ocean-ridge hydrothermal input but also highlights that this signature is diluted in BIF with a continental, shale-like input, most probably in a restricted basin environment. Complete chemostratigraphic data-sets are not available for any other BIF deposit but a comparison of published C-isotope data for North-American and Australian BIFs with the present study indicates that they too may have been deposited in restricted basins. If the existence of vast epicontinental seas was indeed a pre-requisite for the formation and preservation of the major early Proterozoic BIF's, it follows that the evolution of free oxygen in the hydro- and atmosphere was strongly controlled by the size, spatial arrangement and topography of continents. In other words, plate tectonic processes could have had a major influence on the apparently irreversible increase in free atmospheric oxygen by the formation of super-continents as loci for removal and long-term storage of large amounts of Fe and Mn (the major oxygen sinks) from the hydrosphere and by providing ecological niches for the radiation of stromatolite-producing organisms which supplied the required free oxygen.
Acknowledgements: Nic Beukes (RAU, Johannesburg), Michael Bau (GFZ, Potsdam) and Richard Corfield (Oxford) are thanked for supplying samples and analytical data.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
UNUSUAL MINERAL AND MELT INCLUSION COMPOSITIONS IN BASALTS FROM 43®N, MID-ATLANTIC RIDGE: EVIDENCE FOR MELT-PERIDOTITE INTERACTION Vadim Kamenetskv^ Anthony J. Crawford' and Stephen M. Eggins^ 'Department of Geology and Center for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania, 7001 ^Department of Geology, the Australian National University, Canberra, ACT 0200
Basalts AII-Sl D12-7 and D11-177 were dredged during cruise 32 of the RA^ Atlantis 11 in the eastern part of the median valley of the Mid-Atlantic Ridge (MAR) at 42°57'N 29^10'W. This segment of the MAR is adjacent to the 4 3 ^ fracture zone, and has erupted transitional and enriched tholeiites in association with alkali olivine basalts. The location is also on the periphery of the Azores Platform, and a corresponding broad region of the MAR where magmas are anomalously enriched in radiogenic Sr and Pb isotopes, incompatible trace elements, and volatiles. This region is also notable for the occurrence of abyssal peridotites of very refractory character. The samples are Ol-phyric pillow basalts with quenched pillow rim glasses. Clinopyroxene phenocrysts are also present in 012-7. The pillow-rim glasses are primitive (Mg# 63.1 and 69.7, respectively), and are similar to regional MORB in terms of major element composition. The glasses are significantly enriched in the most incompatible trace elements (La/Sm 3.6 and 2.9) and have pronounced positive Nb and Ta anomalies relative to K and La. The isotope composition of glasses is characterized by the most radiogenic Sr (^^Sr/^^Sr 0.703358 and 0.703293) and Pb (e.g., 19.654 and 19.577) and the least radiogenic Nd in the region, and indicate a HIMU component in their genesis. Olivine phenocryst compositions (Fo up to 92) are among the most magnesian known from MORB. D12-7 olivines are unusually enriched in CaO (up to 0.45 wt%). This enrichment correlates with extremely high Ca0/Al203 values in melt inclusions (up to 1.2; c.f., <0.9 in MORB melts), which are a consequence of high CaO (up to 15.2 ) and low AI2O3 (12.8-15.5 wt%) contents in the basaltic melt (MgO 10-12 wt%). This governs an early 01-Cpx cotectic: Cr-diopsidic clinopyroxenes in D12-7 have Mg# (90-92), the highest reported from MORB. Spinel inclusions in olivine demonstrate the most unusual features for MORB spinel: Cr# range up to 70 (c.f., <60 in MORB), and TiOz contents in some D11-177 spinels are as high as 2-4 wt% (c.f., <0.7 in MORB). This high Cr# is in agreement with low AI2O3 content in the melt as evidenced by melt inclusions. High Ti02 abundances in spinel may reflect elevated Ti02 in parental melts recorded in a number of melt inclusions (up to 2.4 wt% vs <1.5 wt% Ti02 in common MORB melts). Other peculiar compositional features of these high-Ti melt inclusions are: (in wt%) very high Si02 (up to 58) and Na20 (up to 3.4) and low CaO (9-11), and that they are trapped in low-Ca (CaO 0.18-0.24) olivines. Such compositions are unusual for MORB and are clearly saturated in orthopyroxene, which has been found as inclusions in high-Si melt inclusions. In summary, the mineral assemblage and melt inclusion chemistry in studied basalts are atypical of MORB and may reflect a variety of parental magma compositions ranging from common MORB to more unusual compositions. The unusual melt compositions enriched in Si, Ti and Na and depleted in Ca {D11-177) could be produced during the shallow upper mantle reaction between percolating melt, which was saturated in Ol+Opx+Cpx (Iherzolitic mantle), and refractory Cpx-poor harzburgite (similar to abyssal peridotites dredged at 43°N). This reaction includes crystallisation of 01+Cpx (in the form of wehrlitic veins) due to expansion of the 01 liquidus field at low pressure at the expense of Opx, and therefore dissolution of Opx from the wallrock peridotite. The resultant melts may still have "mantle" Mg# values, because they are effectively buffered by the mantle, whereas the abundances of Si, Ti and Na increase, and Ca decreases. The origin of the high Ca/Al melts {D12-7) may involve melting of magnesian clinopyroxenite or wehrlite lithologies, which form as described above, though the source of heat for additional melting remains problematic. Melt-peridotite interaction, as it follows from this study, could be an important factor controlling the compositions of MORB primary melts, their evolutionary paths during ascent, and the effects of high-pressure farctionation and re-equilibration on the compositions of residual peridotites and erupted rocks and glasses. Acknowledgements: This work is supported by an Austrahan Research Council Large Grant and funding from RSES, ANU and the ARC Special Research Centers Program.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE GLEN EDEN Mo-Sn-W DEPOSIT, NEW ENGLAND OROGEN, NSW Alirgza Karimzadgh gprnarin and Paul M. Ashley Division of Earth Sciences, University of New England, Armidale, NSW
The Glen Eden Mo-Sn-W deposit in NE NSW is an example of a leucogranite-related, low-grade, large tormage hydrothermal system. It occurs in the southern part of the New England Orogen (SNEO) and is hosted within Permian felsic volcanic rocks, intruded at depth by dykes of porphyritic microleucogranite (Glen Eden Granite). The deposit is hosted within a pipe-like quartz-rich greisen breccia body over 200 m in diameter, surrounded by a greisen zone several himdred metres across, zoning out into altered volcanic rocks containing NW- and NE-striking stockwork veins. A potential resource of 30 Mt of 0.1% M0S2, 0.08% WO3 and 0.04% Sn02 has been outlined above 300m depth. Mo-Sn-W mineralisation is largely hosted in the breccia pipe, with a subordinate amount in stockwork veins. The dominant ore minerals, largely hosted as open space fillings and disseminations in quartz and quartz-rich greisen, are molybdenite, wolframite and cassiterite; they are accompanied by minor to trace amounts of muscovite, fluorite, siderite, pyrrhotite, arsenopyrite, chalcopyrite, sphalerite, bismuth, bismuthinite, joseite A, cosalite, galenobismutite, beryl, anatase and late-stage dickite and kaolinite. Cassiterite is commonly zoned, with considerable variation in Fe, W and Ta contents. Similarly, there are large variations in the composition of wolframite, from Mn-rich to Fe-rich. Accessory anatase is unusually W-rich. Two types of breccia are recognised: (1) greisened volcanic rock fragments (quartz + muscovite), cemented by hydrothermal quartz ± K-feldspar ± ore minerals, and (2) fragments of hydrothermal quartz ± cassiterite ± wolframite enclosed in quartz ± clay. In both types of breccia and in stockwork veins, there is evidence of early precipitation of Mo-Sn-W phases, followed by Bi minerals and base metal sulfides (± fluorite, siderite). Breccia formation and associated hydrothermal alteration are interpreted to be related to devolatilisation of the crystallising, highly fractionated Glen Eden Granite. The process occurred in the early Triassic (240 ± 1 Ma) based on "^^Ar/^^Ar geochronology of greisen muscovite. Major mass transfers accompanied formation of the breccia pipe and surrounding alteration zones which range from quartz-muscovite greisen to K-feldspar- and biotite-bearing, and peripheral sericite, chlorite and clay types. In the greisenised volcanic rocks, there has been enrichment of Mo, Sn, W, Cu, Zn, Mn, Fe, Rb, S and F and depletion of Na, K, Ca, U, Y, Ce, La, Nd, Sr; locally there has been hydrothermal mobilisation of Al, Ti, Zr and REE (possibly due to high F activity). The breccia pipe could have formed in part by rock dissolution and collapse, as well as be explosive degassing of boiling fluids. Fluid inclusion evidence is consistent with boiling, with breccia pipe formation and mineralisation having largely occurred at 3 0 0 - 3 5 0 ° C from fluids with salinity of 0 . 4 to 7 wt% NaCl equivalents in the dilute type and 30 to 47 wt% NaCl equivalents in the hypersaline type. Stable isotopic evidence (O, D, C, S) point to a magmatic contribution for the hydrothermal fluids and metals. The values of various ore minerals vary in the narrow range from 0.6 to 4.4%o indicating a homogeneous sulfur source. The values of siderite lie in the narrow range of -1.56 to - 5 . 2 5 %o suggesting a mantle or deep-seated source for carbon. The calculated 5Dh20 and 8^^0h20 values of water in equilibrium with muscovite show magmatic character. The values of quartz decrease outward the breccia pipe indicating that there has been mixing with isotopically light (high latitude) meteoric fluids, mainly after brecciation and formation of breccia pipe. The main mechanism of mineral precipitation were boiling of the fluid, interaction with wall rock, mixing with meteoric water and cooling. The Glen Eden hydrothermal system has analogies with Climax-type porphyry Mo deposits, although the breccia mass at Glen Eden is a volumetrically major component. There are also links to W-Mo-Bi (-Sn) porphyry-greisen systems (e.g. Mt Pleasant) as well as to local New England Orogen Mo (-Bi-W) pipe and greisen deposits. Glen Eden represents a hydrothermal metal association intermediate between the Sn (-W-As-base metal-Ag) type (e.g. Mole, Gilgai areas) foimd with rather reduced, fractionated I-type granites in the NW part of the SNEO and Mo (-Bi-W-Cu-Au) and Au (-Bi-Mo) types (e.g. Kingsgate, Timbarra) found with more oxidised, fractionated I-type granites further to the east. 232
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Comention, Townsville, July 1998
GEOCHEMISTRY AND PETROGENESIS OF THE GLEN EDEN GRANITE, NEW ENGLAND BATHOLITH, AUSTRALIA A. Karimzadeh Somarin and N. Stephenson Division of Earth Sciences, University of New England, Armidale, New South Wales 2351
The Glen Eden Granite (GEG) is located 17 km northeast of Glen Innes in northeastern New South Wales. It occurs as dikes and it is not exposed at the surface. This granite is spatially associated with the Glen Eden Mo-W-Sn deposit. Based on petrographic studies three types of GEG can be recognised: (1) microgranite porphyry, (2) micrographic granite and (3) aplite. C^artz, K-feldspar and plagioclase are major minerals and biotite, zircon, xenotime, monazite and fluorite are accessory phases. All of the graiute samples analysed are highly felsic, as indicated by Si02 contents between 76 and 78 percent. Aplite samples show potassic alteration and their Si02 contents (%65)are less than other types. The chemical compositions of microgranite porphyry and micrographic granite are similar, however micrographic granite has lower K2O and higher F, Nd and, in some samples, Ce. The characteristics of the Glen Eden Granite are similar to granites associated with Climax-type molybdenimi ore deposits (White et al, 1981). The GEG, like Climax-type rocks, shows enrichment in Si and depletion in Ca, Al, Fe, and Mg with respect to both the average calc alkaline and alkaline granites of Nockolds (1954). Average K20/Na20 in microgranite porphyry (1.53) is close to that of average alkaline granite (Nockolds, 1954) and average normal granite (Le Maitre^ 1976), but this ratio in micrographic granite is less than that of alkaline granite and is more than the average of normal granite. Normative Ab/An ratio is high and reflects the low Ca content of the GEG. The samples of microgranite porphyry and micrographic granite show little chemical variation and no clear trends on Harker-type diagrams using Si02 or MgO as an index of possible fractionation. One of the important geochemical features of the GEG is very low concentrations of P2O5 and CaO. Although low concentrations of CaO along with low concentrations of Sr and Ba may be due to post-magmatic hydrothermal alteration, it seems that they reflect strong fractionation of the GEG magma. The important trace element features of the GEG are low concentrations of Sr, Ba, Zr and Zn and high concentrations of Y, Th, U, and Ga relative to average A-type, felsic I- and S-type granites. Also, GEG contains high concentrations of F and W, similar to other ore-associated granites (Tischendorf, 1977). Microgranite porphyry and micrographic granite samples plot around the minimum melt composition on the Q-Or-Ab ternary diagram for F-poor Q-0r-Ab-H20 systems, whereas aplite samples plot toward the Or apex, reflecting the potassic alteration of these samples. The minimum melt composition of these samples explains the absence of well-defined trends in Harker diagrams. The samples do not have Ab-enriched compositions expected of near-minimum melts in F-rich Q-OrAb-H20 systems. The highly fractionated character of the GEG does not allow unequivocal classification of this granite but it has strong similarities to fractionated I-type and A-type granites. Also, there are many similarities between the GEG and other leucogranites of the New England area and they may derived from similar sources by similar processes.
REFERENCES Le Maitre R.W. 1976. The chemical variability of some common igneous rocks. Journal of Petrology 17, 589-637. Nockolds S.R. 1954. Average chemical compositions of some igneous rocks. Geological Society of. America Bulletin 65,1007-1032. Tischendorf G. 1977. Geochemical and petrographic characteristics of silicic magmatic rocks associated with rare-element mineralisation. In Stemprok M., Bumol L. & Tischendorf G. eds. Metallization associated with acid magmatism 2,41-96. White W.H., Bookstrom A.A., KamilU R.J., Ganster M.W., Smith R.P., Ranta D.E. & Steininger R.C. 1981. Character and origin of Climax-type molybdenum deposits. Economic Geology., 75th Anniversary Volume, 270-316.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
MINERALISED PORPHYRY SYSTEMS WITHIN THE MOUNT LEYSHON CORRIDOR, NORTH QUEENSLAND John R. Kay Normandy Mount Leyshon Ltd, P.O. Box 789, Charters Towers, Queensland 4820
Normandy Mount Leyshon Ltd has carried out a considerable amount of exploration along the Mount Leyshon Corridor, which extends to the northeast and southwest of the Mount Leyshon Mine. This display presents an update of the large data base of geological, geophysical and geochemical information that has been collected. It shows the most recent geological interpretation for the section of the Mount Leyshon Corridor, associated with the Mount Leyshon minerahsation. Extensive exploration work has shown that only a short section of the Mount Leyshon Corridor in this area was active during the period of late Carboniferous to early Permian volcanic activity. This active segment was subjected to structural extension and volcanic activity, while projected extensions of the corridor to the northeast within the Batholith and to the southwest within the Cambro-Ordovician volcanics and sediments were not effected. Aeromagnetic interpretation supports this idea and shows that Permo-Carboniferous structural extension and volcanic activity was confined within two adjoining structural blocks, separated by the cross-cutting Merri Monarch Corridor. Soil geochemistry and gradient array IP have defined two mineralised systems, with features similar to known porphyry systems. One is centred on the Mount Leyshon Volcanic Complex, a large volcanic diatreme hosting the Mount Leyshon gold mineralisation. The other is centred on Matthews Pinnacle; a quartz-diorite plug, with multiple injections. This latter system extends beyond this central plug to encompass the Seventy Mile Mount gold mineralisation. The centre of both systems is characterised by a reversed magnetic core, associated with intense biotite (Potassic) alteration. The reversed magnetics signature is a remnant feature, caused by a reversal of the Earth's magnetic field during the early Permian. Both systems appear to plunge southwards. In both cases there are more intense reverse magnetic anomalies located outside the anomalies to the south. Surface exposures above these southem magnetic features are pre-Carboniferous basement rocks, with moderate to intense biotite alteration. At Moimt Leyshon, an IP-chargeability indicates the pyrite-rich alteration, that occurs throughout the volcanic breccia complex and becomes more intense over the mineralised breccia. At Matthews Pinnacle, an P-Chargeabiity anomaly defines the pyritic halo aroimd the central quartz-diorite intrusion. Soil geochemistry at both Mount Leyshon and Matthews Pinnacle is characterised by circular anomalous features for gold, copper, molybdenum, lead, and zinc. The anomalies at Mount Leyshon are concentrated around the volcanic complex, while at Matthews Pinnacle, the anomalous zone is much larger and extends well beyond the central quartz-diorite intrusive. The gold halo at Matthews Pinnacle tends to form a halo on the outer rim of the anomalies of the other metals. The model being presented shows two porphyry-type systems within the Mount Leyshon Corridor. Each has a deep-seated feeder intrusive, located in the southwest comer of their respective structural blocks. Mineralising fluids travelled northwards from these stocks. At Mount Leyshon, minerahsation was concentrated within a diatreme breccia, located at the contact between meta-sediments and granites. At Matthews Pimiacle, mineralisation occurs as more widely dispersed veins within the Permo-Carboniferous intrusives and adjacent basement granites. The more widespread distribution of the anomalous geochemical halo at Matthews Pinnacle, could also be caused by deeper erosion.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE PALAEOZOIC TECTONIC EVOLUTION OF NORTH QUEENSLAND, AND ASSOCIATED METALLIFEROUS MINERALISATION John R. Kay Normandy Mount Leyshon Ltd, P.O. Box 789, Charters Towers, Queensland 4820
North Queensland contains important gold, copper, lead-zinc, tin and tungsten mineral deposits. Two of Australia's largest modem gold producers are located at Mount Leyshon and Kidston and the Charters Towers goldfield is, historically, the foui^ largest gold producing area in Australia. The dominance of west-north-west structures in this North Queensland mineralised belt is at variance with the north-south structural trends for the rest of the Australian east coast. A tectonic model, which successfully explains this anomaly could lead to the identification of extensions of the mineralisation under sedimentary cover. The model presented is based on recently released aeromagnetic, radiometric and gravity geophysical data from the Australian Geological Survey Organisation. AGSO and the Geological Survey of Queensland have also been advancing the fundamental geological knowledge of the region with geological mapping and geochemical studies of the plutonic suites. The ideas are also based on the work of Normandy Mount Leyshon Ltd, which has carried out detailed geological mapping in the region surroimding the Mount Leyshon Gold Mine and used airborne geophysical data sets to search for gold mineralisation under Tertiary volcanic and sedimentary cover. The tectonic model proposes an early Palaeozoic, west-north-west trending plate rim for the AustraUan Plate, located to the south of Charters Towers and roughly coinciding with the southern margin of what is now the Lolworth-Ravenswood Structural Block. Rock units to the north of this line would have been accreted to the Austrahan Plate during the early to mid-Palaeozoic. Three significant tectonic phases are observed; each with its own characteristic type of mineralisation: • Cambro-Ordovician Volcanic igneous activity formed the east-west oriented calc-alkaline volcanic chain of the Mount Windsor Volcanics, featuring VMS deposits with copper-lead-zinc and gold-barite mineralisation. The early Ordovician syntectonic intrusives, which followed, are highly deformed with east-west mylonitisation and vertical lineations. The model proposes middle Ordovician north-south crustal shortening during collision of the Australian Plate with basement and oceanic crust of a sub-plate to the north. • The northern sub-plate would have been cemented to the Australian Plate by extensive Siluro-Devonian plutonism. Continuing collision activity is indicated by shallow to steeply dipping, east-west oriented, thrusting and brittle fracturing. The important gold mineralisation at Charters Towers was injected at this time. • Igneous activity continued into the Carboniferous-early Permian with development of the Townsville-Momington Is. volcanic belt, with its associated tin-tungsten mineralisation. Further south, the generation of gold bearing volcanic breccia pipes, porphyry systems and some epithermal deposits was controlled by deep seated fracture zones Igneous activity, later in the Permian is north-south and confined to east coast, where it overprints the Palaeozoic trends. East-west trending activity had concluded. The model implies a continuation of mineralisation over a considerable period of time following plate collision. The current Australian Plate margin, to the north in Papua-New Guinea, is examined as a possible modem analogue. Early Palaeozoic stmctural trends of the Lolworth-Ravenswood Block are parallel to those of Papua New Guinea and this block may represent the deeply eroded root zone of similar terrain.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
MULTI-STAGE FLUID INFILTRATION RECORDED BY ZIRCONS IN A COLLISIONAL ENVIRONMENT Sue Keay Research School of Earth Sciences, Australian National University, Canberra, ACT 0200
Infiltration of fluids into the crust can advect heat, cause extensive metasomatism and affect crustal rheology. Determining the temporal and spatial distribution of fluid flow can constrain segments of metamorphic pressuretemperature-time-fluid composition (P-T-t-Xi) paths and help define the thermal and deformational histories of orogenic belts. High precision dating of minerals associated with fluid infiltration and metamorphism is critical in such reconstructions. The tectonic evolution of the Cyclades, Greece, part of the Alpine-Himalayan mountain chain, has involved polyphase metamorphism, deformation, fluid infiltration, anatexis and shearing. Hence it provides a natural laboratory to assess the behaviour of a range of accessory mineral phases, including zircon, developed in response to different metamorphic processes. SHRIMP U-Pb dating of zircon yields ages spanning the Alpine tectonic evolution of the Cyclades from the Cretaceous to the mid-Miocene. As most of these rocks have not experienced temperatures in excess of ~ 600'C, the formation of new zircon must be related to subsolidus processes, an observation that has important implications for the interpretation of zircon ages in medium to low-grade metamorphic terranes. Stable isotope evidence fi-om rock units in the Cyclades supports a history of fluid infiltration during Alpine collisional orogenesis. Zircon development appears to be controlled by the geochemical nature and the degree of deformation of the host lithology. Fluid-related zircon growth can be identified by characteristic zircon morphology, chemistry and textural relations. Such growths are generally unzoned, display low luminescence, occur as distinct overgrowths truncating earher growth structures and have variable but generally low Th/U ratios. Only the youngest layers of new zircon growth can be related to surrounding mineral assemblages and so an understanding of earlier parts of the tectonic history is necessary to interpret older layers of growth. SHRIMP U-Pb dating of these zircon overgrowths can constrain the timing of multiple metamorphic episodes related to fluid infiltration. The reproducibility of morphologies, chemistries and ages from different samples from different areas of the Cyclades and their consistency with other geological evidence suggests that zircon can be a useful tool in unravelling the multi-stage metamorphic histories of complicated orogenic belts. In the Cyclades, clusters of zircon ages occur in the ranges 140-120, 105-95 and 80-70 Ma, suggesting these were periods of active tectonism. 80-70 Ma spans the time of ophiolite formation and high temperature metamorphism in the Cyclades prior to collisional orogenesis. The significance of the older age groupings is unclear, although the ages are too young to be protolith ages and their morphologies and chemistries are consistent with their derivation from hydrothermal precipitation. As new zircon growth can only occur in response to external factors, such as fluid infiltration, this suggests that significant groupings of zircon ages are recording the operation of tectonic processes. Ophiolite formation in the Cyclades was accompanied by sea-floor alteration prior to early Tertiary subduction/collision, producing a crustal sequence containing large quantities of water in the form of chlorite and clay minerals that would devolatilise during high-P metamorphism. The significant period of zircon growth between 50-40 Ma is most likely recording fluid infiltration during the transition from eclogite to blueschist facies metamorphism post-peak high-P metamorphism (Mi). This was followed by retrogression to greenschist facies conditions and further fluid influx recorded by zircon ages of 35-30 Ma. Localised migmatisation produced new zircon during the late stages of partial melting at ca. 18 Ma associated with lower pressure, higher temperature overprinting metamorphism (Mj). Crystallisation of these partial melts led to the release of water post-peak Mj, focussed by shearing during extension, to form the youngest layers of zircon growth at ca. 14 Ma, correlating with ages from monazite and titanite.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
Key Features of Economic Ni-Cu-PGE Sulphide Deposits: Application to Exploration Reid R. Keays, O. Marcus Bumham, and C. Michael Lesher Mineral Exploration Research Centre, Laurentian University, Sudbury, Ontario, P3E 2C6, Canada
Economic Ni-Cu-PGE sulphide deposits are characterized by a number of features, some or all of which may be present in a particular coe deposit (Keays, 1995). The recognition of environments in vMch these features are present can provide powerful exploration guidelines. In^)ortant features include the presence of deep seated lineaments which provided the conduits for the ore-forming magmas, evidence for veiy voluminous magmatic activity which may be the consequence of mantle plume activity, evidence for continental scale rifting, evidence for h i ^ y dynamic environments such asfragment-ladenmagmas and thenaial erosion of the substrate, rocks formed fem high temperature, high MgO magmas that were S-undersaturated and chalcophile metal-undepleted, rocks formedfromco-magmatic magmas that became crustally contaminated and depleted in the chalcophile metals, and, of course, physical traps for the sulphides. In the case of Kambalda-style Ni-Cu-PGE sulphide deposits, the mineralization occurs within or is closely asscociated with thermal erosion channels whereas in the case of flood basalt-related deposits such as Noril'sk, the mineralization occurs within subvolcanic sills which acted as conduits for the magmas vMch formed the flood basalts. An excellent example of a Ni-Cu-PGE sulphide deposit formed by afragment-ladenmelt is the recently discovered Voisey's Bay deposit in Labrador, Canada. Because all economic Ni-Cu sulphide deposits are variably enriched in the PGE, which have extremely large sulphide/silicate melt partition coeflBicients, the primary requirement is that the ore-forming magmas must have been S-undersaturated (and hence PGE-bearing) when intruded to high crustal levels. The only magmas to satisfy this requirement are high MgO, high temperature partial mehs such as komatiites and picrites produced by high degree partial melting. With the exception of boninites, such magmas are probably the products d mantle plumes and are hence associated with very voluminous quantities of tholeiitic magmas. Lower temperature, small degree partial mehs such as MORB leave PGE-rich sulphides in the mantle and hence proAice sulphides with low PGE contents. Another requirement is that the high-MgO magmas become Ssaturatedjust prior to formation of immiscible magmatic sulphides and that these sulphides have high to veiy high Rfector,the ratio of silicate magma to sulphides formed S-saturation may be achieved by either mixing with a S-saturated magma, as in Merensky-style PGE-Ni-Cu sulphide deposits, or by assimilation of S-rich material, as in the case of Kambalda-style Ni-Cu-PGE deposits. Evidence from Kambalda indicates that assimilation of crustal materials which have low S contents not normally lead to S-saturation; for exan^)le, komatiitic basalts at Kambalda that are believed to have formed when komatiitic magmas assimilated up to 30% crustal material on the basis of their REE patterns and isotopic signatures did not become S-saturated. The best mechanism to induce S-saturation and to achieve a high R fector is to thermally erode S-rich rocks, generate sulphide droplets and vigourously mix these with a large volume of silicate magma. To accomplish the latter, the magma must be high temqperature and hence veiy energetic in order to thermally erode and assimilate S-rich material and to guarantee high R &ctors. Favourable magmas which have interacted with Sbearing crustal material can be recognized by features such as low chalcophile metal content, enrichment in the LREE, and high Th/Nb ratios. Other features may include elevated S/Se ratios, more negative c Nd, more positive e Sr, and a shift to either higher or lower 5 34 S values relative to the uncontaminated parental magma. Prospective exploration areas contain rocks formedfromboth primitive chalcophile metal -undepleted magmas as well as their contaminated counterparts. Some of these principal will be illustrated with an example from the Cape Smith Fold Belt, Ungava, C^iebec. Here, although both extrusive and intrusive ultramafic rocks were produced by co-magmatic S-undersaturated komatiitic basalt melts and both are associated with S-rich sediments, mineralization is only associated with the komatiitic basalt flows. The melts which formed the flows were apparently much more dynamic and voluminous than those which formed the sills Reference Keays, R R., 1995, The role of komatiitic and picritic magmatism and S-saturation in the formation of ore deposits: Lithos v. 34, p. 1-18.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
ECONOMIC GEOLOGY AND MINERALOGY OF PROTEROZOIC MAGNESITE DEPOSITS IN THE WILLOURAN AND NORTHERN FLINDERS RANGES, SOUTH AUSTRALIA John L. Keeling Minerals Group, Primary Industries and Resources, SA, GPO Box 2355, Adelaide 5001
Proterozoic magnesites are a major world source of magnesia for the refractories industry, principally from deposits in the Commonwealth of Independent States (former Soviet Union), China, North Korea and Austria. With a theoretical 28.8% Mg, magnesite (MgCO^) is also a potential high-grade source of magnesium metal. Substantial resources of Proterozoic magnesite are known in Australia at Rum Jungle m Northern Territory, Savage and Arthur Rivers in northwestern Tasmania, and from the northern Flinders and Willouran Ranges in South Australia. The South Australian deposits include both coarsely crystalline (sparry) magnesite, at Balcanoona, formed by metasomatic replacement of dolomite, and cryptocrystalline sedimentary magnesite interbedded with dolomite within a 300-500 m thick sedimentary carbonate package. The dolomitic sequence with magnesite interbeds can be traced from Leigh Creek, northwest into the Willouran Ranges, a distance of over 120 km. Investigations during 1997-98 have focused on the potential of these resources to be used as feedstock m magnesium metal production via acid leach and solid sah electrolysis technology. The Balcanoona magnesite deposits comprise four irregular magnesite lenses, up to 580 x 300 m across, in dolomite at the base of Balcanoona Formation (---700 Ma). Investigations by BHP Co Ltd during the 1960s included geological mapping, chip sampling and 3 exploratory adits into orebody "B". These data define an indicated resource of 20 Mt of magnesite ore grading around 44% MgO. Best exposures are along Balcanoona Creek where magnesite forms either massive rounded outcrop with distinctive brown weathering on a rough textured surface or, m the creek bed, a water washed, white "marble-like" pavement. Coarse, bladed magnesite crystals range in length from <1 mm to several centimetres and are commonly arranged as rosettes. The contact between magnesite and dolomite is irregular and in places marked by the presence of very coarse bladed magnesite crystals. Beddmg traces in dolomite continue across the contact with magnesite and equate with subtle pale pink, grey and white colour bandmg in magnesite and linear features formed by alignment of the nuclei of radiating crystal aggregates. The observations suggest that bedding planes in part controlled Mg-rich fluid infiltration leading to metasomatic replacement of dolomite and displacement of Ca ions. Electron microscopy and energy dispersive x-ray analyses of magnesite cleavage fragments show persistent low levels of Fe, substituting for Mg, and minor zones with high calcium content. Non-magnesite phases include Mg-rich chlorite, apatite and dolomite, and traces of euhedral pyrite and rutile. These are typically <0.2 mm across and are concentrated in small aggregates, mainly along the boundaries between large magnesite crystals. Sedimentary magnesite, mainly as thin bands of pebble conglomerate, is a minor but characteristic feature of Skillogalee Dolomite, a lower member of Burra Group sediments of Adelaidean age (-800 Ma). Thickness and abundance of magnesite beds are greatest to the north of Leigh Creek, in the western Flinders Ranges, extending 120 km northwest into the Willouran Ranges. Here Skillogalee Dolomite varies in thickness from 500 to 5000 m with magnesite more common in the upper 300 m. Individual magnesite lenses are relatively thin, ranging in thickness from <0.5 m to a maximum of around 5 m, but are laterally extensive and in some instances have been traced over distances exceeding 10 km. The magnesite is interpreted as a shallow water marine or marginal marine chemical precipitate which typically underwent post deposition processes of consolidation through desiccation, break up to form intraformational conglomerate, and often reworking and redeposition as pebble conglomerate or, less commonly, as coarse magnesite sand. Magnesite lenses are interbedded with carbonaceous dark grey micritic dolomite, dolomitic siltstone and sandstone. Abundant biological activity is evidenced in dolomitic units by the presence of cryptalgal laminations and domed and branching stromatolites. Magnesite is present predominantly as cryptocrystalline particles 1-5 ^im in size. Iron content is low, FejOj varying from 0.08-0.39%, but calcium content is relatively high ranging from CaO 2% to 4.5%. Calcium is present as dolomite or magnesian calcite usually as a slightly coarser crystalline phase, mostly in the matrix around magnesite pebble clasts. Talc is the dominant minor phase. Quartz is present in minor to trace amounts mainly as fine sand grains. Other phases include trace amounts of authigenic dark grey albite, K-feldspar and organic matter. Trace elements show elevated levels of Ba and Sr. The high reactivity of cryptocrystalline magnesite in warm concentrated HCl favours this source for Mg metal extraction over coarse crystalline magnesite from Balcanoona.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
PHYSICAL MODELLING OF DEFORMATION IN THE TASMAN OROGENIC ZONE Mvra Keep and Chris McA. Powell Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia, Nedlands, 6907, WA.
STRUCTURE OF THE TASMAN OROGEN The Tasman orogenic belt comprises a series of fold belts that have formed since the Neoproterozoic break-up of Rodinia along the Tasman Line. The exposed basement to these fold belts is ocean floor, accreted to the older Precambrian continental edge of Australia in a series of tectonic events starting in the Neoproterozoic and occurring episodically until the Triassic along the east coast of Australia. A remarkable feature of these successive fold belts (Middle to Late Cambrian Delamerian deformation; Early Silurian Benambran deformation; the various mid-Silurian to Devonian deformations culminating in the Middle Devonian Tabberabberan deformation, and the Late Devonian to Early Carboniferous Kanimblan deformation) is that the structure is dominated by upright folds and high-angle reverse faults, most of which verge eastward away from the more stable Australian craton towards the inferred centre of orogenic and magmatic activity. This contrasts with the normal behaviour in orogenic belts where thrusts and nappes, especially in the latter stages of orogenic activity, commonly verge away from the internal parts of an orogen towards the external zones or cratons. Some authors (e.g. Glen, 1992) have interpreted the steep faults exposed at the surface of the Tasman orogenic zone as the listric upturned edges of giant sled-runner thrust faults, hundreds of km, which detach the turbiditic fold-belt sediments from their underlying basement. In the steep high-angle reverse fault slices, normal upright sedimentary contacts occur between the Late Cambrian to earliest Ordovician basal turbidites and the underlying pillow basalts of the oceanfloor basement. These greenstone slices correlate with aeromagnetic and gravity highs, and, in the western part of the Tasman orogen, are curvilinear, mimicking the rectilinear Rodinian breakup margin, with decreasing curvature eastward away from the Tasman Line. We think these aeromagnetic and gravity highs represent the uptumed edges of slices of the oceanic substrate, which have shortened along steep whole-of-lithosphere imbricate fault slices verging away from the thicker, older Precambrian continental lithosphere, and have designed a series of physical models to test this idea.. PHYSICAL MODELLING Physical modelling techniques, scaled for both gravitational body forces and surface forces, have been used to model the lithosphere. These lithospheric-scale models use experimentally-determined flow laws to calculate the strength profile of the lithosphere deforming steadily under simple conditions of uniform strain rate and thermal equilibrium. The lithosphere is modelled by layers representing brittle crust, brittle mantle, ductile crust and ductile mantle. Rheological profiles are recreated in experimental models using analogue materials that are weak enough to flow under their own weight, with the density ratio of the model crust and model mantle mimicking that of the natural system. The models incorporated a thick, strong, cratonic lithosphere (4-layer), with layers representing brittle crust, ductile crust, brittle mantle and ductile mantle, "colliding" with a thinner, 2-layer oceanic lithosphere, representing brittle crust and mantle. Analogue materials include silicon putty, with and without additives to increase density, sand, and castor sugar, incorporated into multilayers of different thicknesses. During deformation a unidirectional compression applied at one end of the model pushes "oceanic" crust towards "cratonic" crust, or vice versa, depending on the model configuration. The cratonic lithosphere incorporates the geometrical outline of the Tasman Line, as interpreted from aeromagnetic and gravity data. Models are recorded using time lapse video and still photography at every increment of deformation. The main results show that during deformation vergence sense in the deforming oceanic lithosphere is commonly away from the craton, reproducible and independent of whether oceanic lithosphere is pushed towards cratonic, or vice versa. Vergence direction appears to be controlled solely by the relative strength and mean density differences between the two types of lithosphere. We note that we did not find large strike-slip or oblique slip faults nucleated from the promontories in the cratonic lithosphere during the modelling. Glen, R.A., 1992. Thrust, extensional and strike-slip tectonics in an evolving Palaeozoic orogen: A structural synthesis of the Lachlan Orogen of southeastern Australia. Tectonophysics 214: 341-380.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
DUCTILE THRUSTING NEAR THE RAYNER COMPLEX-NAPIER COMPLEX BOUNDARY, OYGARDEN ISLANDS, EAST ANTARCTICA. Nigel M. Kellv^ Geoffrey L. Clarke\ Christopher J. Carson', and Richard W. White^ ^Dept. of Geology and Geophysics, University of Sydney, NSW 2006. ^School of Earth Sciences, University of Melbourne, Parkville, Victoria 3052.
The Oygarden Group of islands lie within the Rayner Complex, immediately east of the Archaean Napier Complex, in east Antarctica. The Rayner Complex is interpreted to represent a mobile belt created during continent-continent collision involving the Napier Complex with another crustal fragment at c.lOOOMa. The Oygarden islands record evidence of a high stram, ductile thrusting event related to this collision and involved the reworking of Archaean crust. The Oygarden islands are predominantly layered felsic, intermediate and mafic orthogneisses, with subordinate pelitic, sub-pelitic and calc-silicate gneisses. These gneisses have been multiply deformed and have been metamorphosed at granulite fk^ies conditions. The earliest structural feature, Si, is parallel to compositional layering in layered orthogneiss that has been deformed by F2 isoclinal folds. D2 produced a pervasive S2 gneissosity in post-Di felsic orthogneiss and the transposition of Si into parallelism with S2. Di and D2 are inferred to predate the first episode of Rayner deformation and may be Archaean in age. D2 was followed by the intrusion of a suite of mafic dykes that may be the metamorphosed equivalent of the c. 1200 Ma Amundsen dykes found in the Napier Complex. The first stage of Rayner deformation, Dsa, involved very high strain, subhorizontal ductile thrusting with an E-W transport axis. Strain in this event was strongly partitioned, with highest strain zones showing the complete transposition of earlier structures and lithologies (including mafic dykes) into a subhorizontal gneissic layering. Meso- to macro-scale Fsa isoclinal-recumbent folds and sheath folds have subhorizontal, east trending axes that are colinear with a pervasive Lsa mineral and rodding lineation. Rare nappe structures indicate an east over west thrust movement sense. In areas of lower strain, the transposition of earlier structures was less pervasive, with dykes still transgressive to S1/2 layering and some refolded F2 folds having variably oriented fold axes. In these areas of lower strain a Ssa gneissic foliation is often observed in mafic dykes and the strong L3 mineral and rodding lineation may be present in felsic lithologies. Dsb, the second stage of what is probably a progressive episode of deformation, also shows extensive strain partitioning with the highest strain concentrated in E-W trending, steeply south-dipping shear zones up to 10 metres in width. In zones of lower strain, open to tight, upright-inclined Fsb folds occur, folding S1/2, Ssa and some Fsa isoclinal folds. With increasing strain the intensity of folding increases, with folds ultimately being attenuated into a gneissic layering in high strain zones. Fsb fold axes and Lsb mineral and rodding lineation in the high strain zones are colinear to Fsa and Lsa. The geometry of Dab structures suggests that during D3 deformation there was a rotation of the axis of compression while maintaining a constant stretching direction. An east trending, steeply south-dipping D4 high strain shear zone >lkm across cuts D3 structures in the south of the Oygarden islands. This event has caused the progressive rotation of L3 and F3 fold axes from being subhorizontal and east trending features to steeply southeast-plungmg. The last deformation event, D5, produced mylonites and ultramylonites (with minor pseudotachylite development) that are often marginal to post-D4 pegmatites or developed along other zones of weakness. Ultramylonites may form mutually crosscutting sets but no regular trend of the mylonites can be inferred. Preliminary thermobarometiy implies conditions during D3 involved P=7-10 kbars at T>800°C. Reaction textures in metapelites and mafic gneiss that overprint both S3 and S4 gneissosities indicate that a period of decompression occurred either late in, or after, D4. The development of the large D4 high strain zone may have provided a mechanism to assist in the decompression of the terrain.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
GENERATION OF COMPOSITIONAL DIVERSITY WITHIN GRANITIC ROCKS BY FRACTIONAL MELTING: AN EXAMPLE FROM THE GLENELG RIVER COMPLEX, WESTERN VICTORIA Tony L S. Kemp K D. J. Ellis^ and C. M. Gray^ ^ Department of Geology, Australian National University, Canberra, A.C.T. 0200 ^ Department of Earth Sciences, La Trobe University, Bundoora, Victoria 3083
The Glenelg River Complex (GRC), situated in far western Victoria, comprises a polydeformed metasedimentary sequence intruded by various igneous phases during the Cambro-Ordovician Delamerian Orogeny. Prominent amongst these are leucocratic adamellites and granodiorites containing abundant magmatic muscovite, commonly with garnet and, more rarely, sillimanite ('Harrow type' plutons). Harrow types have tightly constrained Si02 variation (72.84% to 74.66%), with low F e O r , MgO and Ti02 and relatively high Na20 and K2O. Sr contents are >200 ppm and Rb/Sr is less than unity. On geochemical variation diagrams they either plot as a tight cluster of points (AI2O3, Ti02, FeOx) or define almost vertical linear trends (Na20, K2O, P2O5). Such rocks are associated with upper amphibolite facies schist, gneiss and anatectic migmatites. Two groups are recognised, structurally concordant plutons and intrusive plutons; both have unmodified igneous textures and were emplaced late in the deformational history. Structurally concordant phases grade conformably into migmatitic lithologies; all have homogeneous pluton centres, with metasedimentary enclaves concentrated towards the peripheries. Conversely, intrusive plutons crosscut the migmatitic sequence and are commonly gametiferous. Apart from occasional schlieren or micaceous clots, these rocks typically lack metasedimentary enclaves and are associated with voluminous aplite-pegmatite dykes. An exception is the composite Harrow Adamellite, which hosts abundant diverse metasedimentary enclaves, and is the most mafic of the Harrow types. Field and petrographic evidence suggests derivation of both groups by anatexis of the surrounding metasedimentary sequence. This is supported also by their anomalously high Sr signature, which reflects that of the local metasedimentary sequence. Intrusive plutons are thought to be higher level mobilised equivalents of those incompletely segregated concordant plutons transitional to the migmatitic source. To further elucidate petrogenetic processes, detailed studies were conducted of the structurally concordant Nangkita Adamellite, ensconced within stromatic migmatites. The migmatitic layering is commonly contorted and exhibits evidence of extensive mobility of the partially melted portion. Channelling of granitic magma from thick leucosomes, themselves fed by layer-parallel melt veins, into diffuse pools and interconnected networks of irregular crosscutting sheets (0.5-5m wide) is observed. Segregation is quite efficient, with little entrainment of melanosome into magma conduits, and stimulated by compressive deformation. The Nangkita Adamellite appears to have formed almost in situ by coalescence of thick granitic dykes draining magma-permeable stromatic migmatites, and thus contains very litrie unmelted residue. The similar geochemistry of migmatite leucosomes, larger pools of accumulated leucosome and sheets of segregated granitic magma with the adamellite is consistent with this. Quartzofeldspathic schist enclaves at the pluton margins represent refractory material incorporated during incipient ascent of the pluton. Similar deformation-enhanced segregation of clean granitic magma into dilatant structures or dykes is observed within the migmatitic aureoles of other concordant plutons. On this basis, Harrow type plutons are considered to represent amalgamations of efficiently extracted partial melt increments from the surrounding metasedimentary protolith. This proposition is supported by geochemical trends for the most fertile metasedimentary rocks of the GRC, where for many elements (e.g. Ti02, AI2O3, MgO, Na20, K2O) there is no chemical overlap between granitic and metasedimentary rocks. The near vertical geochemical trends defined by plutons that emanate either up (Na20, K2O, Pb) or down (CaO, P2O5, Zr, Y, REE) from the metasedimentary rock cluster are not consistent with restite unmixing or fractional crystallisation, but explicable only by differing degrees of partial melting of the source. Geochemical variation amongst plutons is ascribed to this fractional melting process, with garnetiferous phases representing the lowest partial melt fractions, and Harrow Adamellite the greatest degree of source anatexis. Hence, in terms of major elements, Harrow type plutons are poor overall geochemical images of their protoliths, and regression of chemical trends does not provide a reliable indication of specific source rock composition. This has important implications for high level granitic intrusives of other orogens that cannot be directly contrasted with their inferred sources. Further, it is demonstrated that small degrees of crustally-derived granitic magma may be generated and effectively segregated from migmatitic rocks, leading ultimately to the production of restite-poor peraluminous plutons at or near the source regions; there is no need to invoke total source mobilisation at >30% melt fractions to account for the formation of large granitic bodies.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
SHRIMP AGES OF APATITES FROM PILBARA TIN-BEARING PEGMATITES Align K- Kennedy Dept. Applied Physics, Curtin University of Technology, GPO Box U1987, Perth, WA, 6001.
INTRODUCTION- Tin- and Tantalum-bearing pegmatites occur throughout the central and east Pilbara region of WA. These pegmatites are occasionally adjacent to, peripheral to, or intruded within, post-tectonic adamellite and granite, and it has been assumed that the pegmatites are late-stage melts from these intrusions. To test the hypothesis that the pegmatites are the last vestiges of a single regional event that produced the Pilbara post-tectonic granites, apatites from the Wodinga pegmatite field, and the Moolyella, Spear Hill (Shaw River), Tabba Tabba, and Strelley pegmatites have been dated with the SHRIMP n at Curtin University. GEOLOGIC SETTING- The Wodgina pegmatites intrude the Wodgina Belt, a fragment of the Pilbara Supergroup volcanics sandwiched between the Yule and Carlinda Batholiths. The Kangan and Pincunah Adamellites are the closest post-tectonic intrusions, however they cannot be unequivocally associated with the Wodgina pegmatites. The Spear Hill pegmatites are located approx. 300m south of Spear Hill, a member of the Shaw Batholith post-tectonic intrusions with a Pb-Pb whole rock isochron age of 2847±34Ma. The Moolyella pegmatite dikes invade the Moolyella Adamellite, a small post-tectonic central pluton within the 3.3Ga Mt. Edgar batholith, that has a Rb/Sr whole rock age of 2804±62Ma. The Tabba Tabba and Strelley pegmatites are 20km apart and centrally located within the Carlindi Batholith. SAMPLES- The Wodgina samples were from the South Pit of the mine, the old North Pit workings, and a small lepidolite-spodumene-rich dike ^proximately 1km south of the main pegmatite body. Moolyella and Spear Hill samples are from pegmatite bodies that contain 1cm size crystals of cassiterite. The Strelley sample was from the original mine worldng. The Tabba Tabba sample is from a small pegmatite lens approximately 100m west of the main alluvial workings. Large (1 to 3cm) single apatite crystals from the South Pit and small dike of Wodgina and from Strelley were analysed with the goal of producing internal isochrons and examining disturbance to the U/Th/Pb decay systems. Apatite separates were analysed for the other samples. Cathodoluminescence imaging was used to identify high U regions or grains for analysis. The Spear Hill pegmatite did not contain apatite. RESULTS- Preliminary results show that well defined Pb-Pb 'isochrons' can been obtained for most samples, and that there is usually some disturbance of the U/Pb and Th/Pb systematics. For example, the North pit Wodgina apatite 206pb/204p5 ranges between 18 and 85, and a 14 pt. Pb-Pb model 1 York regression with uncorrelated uncertainties yields an age of 2870±93Ma with a MSWD of 13. If the 206pb/204pb and 207pi3/204pb uncertainties are assumed to have a correlation coefficient of 0.95, the age is 2803±115Ma and the MSWD is 114. A model 2 isochron, which assumes uncorrelated equal magnitude uncertainties, gives 2851±55Ma. Model 1 Pb-Pb results are shown in Table 1 and all uncertainties are 1 sigma. Wodgina Strelley Tabba Tabba Moolyella
206/204 18-85 20-350 340-610 20-200
# of pts 14 15 13 13
Age Ma r=0 2870±90Ma 2831±86Ma 2839±124Ma 3099±125Ma
MSWD 13 5.6 6.0 1.0
Age r=0.95 2803±115Ma 2833±36Ma 2843±840Ma 3068±225Ma
MSWD 114 1.8 1.1 12
A 13 pt. model 1 206p5/204p5 ^^ 238u/204pb 'isochron' for Moolyella gives 3119±568Ma, with an MSWD=12.6; A model 2 York regression gives 3117±445Ma for the same data set. CONCLUSIONS- Our results show that in most apatite samples from pegmatites it is possible to obtain Pb-Pb ages, and that in a small number of samples U/Pb ages can be obtained. In some instances internal 'isochrons' can be derived from single apatite crystals. The principal difficulty with SHRIMP dating of apatite is finding apatite(s) with a sufficient range of ^^^Pb/^^^Pb. With the exception of Moolyella, the analysed pegmatites give ages that are consistent with their being the last vestiges of a single regional event that produced the Pilbara post-tectonic adamellites and granites, and that these intrusions are the source of the Sn and Ta.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
THE RELATION OF LITHOSPHERIC STRUCTURE AND CONTINENTAL ASSEMBLY B.L.N. Kennett, Research School of Earth Sciences, Australian National University, Canberra 0200
Recent studies of the three-dimensional structure of the lithosphere beneath Australia which combine surface wave inversion and body wave tomography have revealed the presence of complex structure in the lithosphere. The main feature is a separation between high seismic wavespeeds extending to depths of 200 km or more in the centre and west of the continent, and much lower seismic velocities along the eastern margin. The transition appears to take place in two stages,. The main boundary lies at about 140-14rE and marks the edge of the thicker lithosphere, major mineral deposits appear to lie close to the edge of this boundary. However, to the east lies a region of slightly elevated seismic wavespeeds which locally extend to 150 km depth or more, and then along the continental margin, associated with Neogene volcanism, are much reduced seismic wavespeeds most likely due to the presence of hot material. The mantle contrasts bear no simple relation to the conventional Tasman line based on the outcrop of Precambrian material with interpolation via magnetic and other lineations. There are many variants of this line, but the seismic structures in the lithosphere are unambiguous and reflected in many different styles of analysis. It may well be that the location of the continental edge left after Rodinian breakup needs to be sought with a different geometry to that conmionly discussed. When a continent is broken or built, the processes do not just affect the crust but will leave an lasting imprint on the mantle. For example, the Kimberley block shows a distinct contrast to the surrounding regions at depths around 80 km and a weaker signature at greater depth. The regions affected by the Alice Springs orogeny can also be recognised by their reduced seismic wavespeeds,. The variations in shear velocity in the lithosphere indicate a distinct separation between the North Australian Craton and the Archaean provinces to the west. The features in the seismic lithosphere most likely represent the scars of past cycles of continental dissipation and amalgamation and represent important constraints on likely processes.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE AUSTRALIAN LITHOSPHERE AND UPPER MANTLE - SURFACE WAVE INVERSION B.L.N. Kennett.^ E. Debayle^ and R.D. Van der Hilst^'^ ^Research School of Earth Sciences, Australian National University, Canberra 0200 ^Dept of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA
The distribution of earthquakes around Australia has been exploited in deployments of portable seismic recorders designed to study the seismic structure of the lithosphere and upper mantle. The whole continent has now been covered in the SKIPPY experiment from 1993-1996 in which a continental scale of broad-band instruments has been synthesised in a sequence of deployments, typically of 10 recorders at a time. The 65 sites occupied during the SKIPPY fieldwork are supplemented by information from the limited number of high-fidelity permanent seismic stations. In 1997 an additional 8 stations at closer spacing were installed in the Kimberley region. By combining all the available data sources, a dense pattern of crossing paths has been achieved from regional earthquakes. Coverage is somewhat better in the east, because of the presence of the the Tonga-Kermadec earthquake belt. For each event for which a reliable source mechanism is available, the large amplitude surface waves and the preceding S body waves have been analysed to determine three-dimensional seismic structure. Two inversion schemes have been employed, but each has the characteristic that the waveforms for a sourcereceiver pair are used in a nonlinear inversion scheme to determine the average structure along the path. Subsequently the path-averaged structures are used as constraints in a linear inversion for three-dimensional structure. In the regions with the densest path coverage, horizontal resolution of about 200 km can be achieved below 60 km depth. By this means we can construct images of the three-dimensional variation of the shear wavespeed, the angular pattern of shear wave anisotropy and, with less reliability, the pattern of attenuation. The results reveal substantial contrasts in structure in the lithosphere and upper mantle. The region beneath the central cratons is characterised by a fast 'lid' extending to at least 200 km and locally to 300 km or more. Whereas on the eastern margin of the continent, the fast seismic 'hd' does not extend to depths larger than about 100 km and there is a pronounced low-velocity zone between 100 and 200 km depth. The transition between these two regimes is complex and does not have a simple relation to the conventional Tasman line marking the separation of Precambrian and Phanerozoic outcrop. The edge of the thick, fast, wavespeeds lies approximately at 140-14rE, and further east there is a second contrast between a moderately fast zone extending to about 150 km depth and the main region of lowered velocities. The pattern of lowered velocities in the east fits in well with the presence of recent volcanism and so it is likely that the cause of the reduction of the seismic wavespeed cames from the presence of hotter material. It is not so easy to explain the fast velocities in the lithosphere beneath the cratons, but there is a distinct correlation with the patterns of azimuthal anisotropy. The fastest velocities lie in the central craton and the Archaean provinces in the west do not seem to have such a distinct signature. Interestingly, the Kimberley block has a distinct contrast with its surroundings with slightly reduced wavespeed. The region affected by the Alice Springs Orogeny also is characterised by lower wavespeeds at 80 km depth than the surrounding cratons. The detailed images of three-dimensional seismic shear structure reveal a complex pattern of seismic wavespeeds and anisotropy in the seismic lithosphere and the mantle beneath. It would appear that structure can persist in the lithosphere over very long intervals of time.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE AUSTRALIAN LITHOSPHERE AND UPPER MANTLE - BODY WAVE STUDIES B.L.N. Kennett.^ Y. Kaiho^'^ and G. Clitheroe^ ^Research School of Earth Sciences, Australian National University, Canberra 0200 ^JAMSTEC 2-15 Natsushima-Cho, Yokosuka-Shi, Kanagawa 237, Japan
Northern Australia lies at a suitable distance from the very active earthquake belt extending from Indonesia, through New Guinea to Vanuatu to examine the seismic wavespeed by the analysis of P and S waves refracted back from the upper mantle. Deployments of short-period instruments provided information on P wave structure in the lithosphere and asthenosphere and have demonstrated the need for complex structure at the base of the lithosphere near 200 km depth. With the advent of three-component broad-band recording, the S wave information can also be analysed and the records from the Warramunga array, the SKIPPY stations (1993-1996) and a deployment in the Kimberley in 1997 enable the construction of a three-dimensional model for mantle structure. Seismic waves trapped in the lithosphere have rather complex seismograms with relatively high frequency arrivals, whereas waves which have been returned from below the asthenosphere have lost their high frequencies. Analysis of the arrival times and frequency content of more than 1500 wavepaths provides a set of valuable constraints on the velocity structure in the lithosphere and the mantle beneath In order to minimise the influence of small-scale heterogeneity we have combined data from many sources and receivers with a conmion direction of propagation. For events at a similar depth the seismograms are stacked together in small distance bins to generate composite record sections which reveal the complex interaction of the phase branches associated with the upper mande discontinuities - particularly the major phase boundaries at 410 and 600 km depth. By matching the time and amphtude behaviour using synthetic seismograms a set of P and S velocity profiles have been constructed for 16 different corridors with different positions and azimuths. The set of velocity information has been mapped into three-dimensions by recognising the sensitivity of refracted waves to the velocity structure in the region when the waves have their turning point. The information obtained from the regional earthquakes is complemented by analysis of more distant events. The structure near the receiver imposes a set of conversions and reverberation which follow the main phase arrivals. This receiver dependent structure can be exploited to provide information on the structure in the crust and uppermost mantle. Three-component seismograms at broad-band stations are rotated so that one horizontal component lies along the great-circle from source to receiver; the influence of the source is then removed by deconvolving this radial component with the onset of the vertical component. The resulting radial receiver functions are stacked over a number of events to improve the stability of the results. Inversion using genetic algorithms yields a stable estimate of the shear wave velocity distribution in the crust and at the top of the mantle. The systematic analysis of seismic receiver functions from over 70 portable and permanent broad-band stations provides information on crustal and uppermost mantle structure across the whole continent. Where earlier seismic refraction results are available the agreement is good, but coverage is now provided in many areas where there was no previous information on crustal structure. Using the definition of the seismic Moho as the base of the transition to mantle velocities, the thickest crust lies in the centre of northern Australia, an area underlain by thick, fast seismic lithosphere. The thickened zone extends from near Mt Isa through Alice Springs towards South Australia. Along the eastern fold belt the crust is quite thick (38-44 km) and mostly transitional in character,. The crust-mantle boundary is somewhat shallower (30-36 km) and sharper near the boundary between Precambrian and Phanerozoic outcrop.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
RAPID GLOBAL CLIMATE AND ECOSYSTEM CHANGES IN THE LATE QUATERNARY: A CALIFORNIA MARGIN PERSPECTIVE J. P. Kennett I. Hendy, and K. Cannariato, Department of Geological Sciences and Marine Science Institute, University of California, Santa Barbara, California, U.S.A.
Marine sedimentary records from the southern California margin clearly reveal major instability of the marine environment and ecosystem during the latest Quatemaiy. We have constructed 50-70 year resolution oxygen isotopic and faunal records spanning 25 to 60 Ka from Santa Barbara Basin, ODP Hole 893A, which reflect the full sequence of D/0 cycles by dramatic sea sur&ce temperature shifts. The ^Oxygen time series of G. bulloides and N. pachyderma define an apparently complete sequence of sub-Milankovitch climate oscillations from 60 to 25 Ka which closely correlates with that of the Greenland Ice Sheet. Interstadials exhibit familiar late Quaternary sav^ooth patterns due to greater magnitude of warmings than coolings. At comparable chronologic resolution to the Greenland ice core records, the two regions show remarkable similarities in speed, magnitude and character of climate changes inferring synchroneity between the north Atlantic and Pacific climatic response. Switches between stadial and interstadial episodes usually occurred within <50-70 years. These rapid global oceanic and atmospheric circulation reorganizations involved brief strong feedback mechanisms that amplified the climatic changes, resulting in inferred sea-surface temperature warmings of up to 5°C within a few decades, and brief warming overshoots of up to 3°C. The large magnitude of the inferred sea-surface temperature and oceanographic changes associated with the D/0 events in Santa Barbara Basin reinforce their global significance. Furthermore, our evidence for tightly coupled atmospheric and oceanic circulation switches during the D/0 events demonstrates strong interdependence between atmosphere and ocean. The rapidity of the changes suggests that the global climatic signals are transmitted via the atmosphere. The evidence of rapid climate variability in surface waters of Santa Barbara Basin and its similarity to the Greenland Ice Sheet record provides further evidence for the global nature of the D/0 events. The California margin records also suggest that the north Pacific Ocean is as sensitive to rapid climate change as the north Atlantic. The oscillatory pattem of climate change during the last ice age suggests that during much of this interval the global climate system was close to a threshold, whereby small perturbations in the climate system were amplified to produce major, rapid climate shifts. Excessive climatic amplification at the beginning cf interstadials, as indicated by the remarkable overshoots in sea-surface temperature increases, can only be explained by involvement of strong greenhouse gas feedback processes of brief duration. We have also documented a history of dramatic, rapid (average -130 yrs) upheavals in the benthic ecosystem that occurred synchronously with the D/0 cycles. These upheavals, as best recorded by benthic foraminifera, reflect the switching between two states of basinal bottom water oxygenation at the initiation and termination cf interstadials during the last 60 kyrs. Benthic foraminifera oscillate between oxic (>1.5 ml L-^ O2) and dysoxic (0.1-0.3 ml L-^ O2) assemblages associated with bottom water oxygenation. Low oxygen assemblages, associated with laminated sediments mark the interstadials (warm episodes); high oxygen assemblages with nonlaminated sediments characterize stadials (cold episodes). Switching between these assemblages and inferred basmal ventilation was rapid (-40 to 400 yrs; mean -130 yr). The D/0 cycles are also marked by large (up to 5 per mil) oscillations in values in benthic foraminifera. The latest Quatemary biotic changes in the basin appear to have been widespread along the California margin in conjunction with ventilation switching. The long-term oxygenation fluctuations aappear to have been controlled by the extent and location of Pacific Intermediate Water production. Well ventilated intervals on the margin, when the oxygen minimum zone (OMZ) was diminished, were associated with young, oxygenated waters from the Subarctic north Pacific. Correlation of laminated intervals in the Santa Barbara Basin to D/0 cycles suggests that fluctuations in upper PIW and the OMZ off central California are tightly linked, and sensitive to global climate change. Millennial climatic oscillations marked by the D/0 cycles during the last ice age continue through the Holocene, although at much lower magnitude. Brief cooling espisodes (average -3°C) punctuate otherwise warm Holocene conditions. Power spectral analysis indicates that sea surface oscillations have a period of 2.2 kyrs, while changes in the strength of the upper mixed layer exhibit periodic oscillations of 2.5 and 1.5 kyrs. The climatic records are similar to those of the north Atlantic, especially the Greenland ice sheet.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
BIOGENIC SEDIMENT ALTERATION BY THE GHOST SHRIMP TRYPAEA AUSTRALIENSIS Geoff Kerr' ^School of Resource Science and Management, Southern Cross University Lismore, New South Wales 2480
Biota have a major effect on sediment alteration, both during and after deposition. Thalassinidean shrimp are major mediators of both structural and chemical alteration of sediment. Trypaea australiensis, a deep burrower of intertidal and shallow sub-tidal sands and muddy sands, occurs along almost all of the Australian east coast, from Melbourne to far north Queensland. This study focussed on the activities of T. australiensis in the lower estuary of the Richmond River, northern New South Wales. Generally regarded as a detrital deposit-feeder, T. australiensis excavates deep into the sediment and winnows fines from bulk sediment during detrital feeding. Sand-sized particles are systematically moved upwards from gallery to gallery and finally discharged, within exhalant currents from the burrow mouth, to settle as small cones on the sediment surface. This burrowing activity results in total sediment turnover to a depth of about 1 m within a few to several months, depending on seasonal and temperature conditions. Observations of animals reared in thin tanks revealed that T. australiensis also gathers suspended material from the water colunm by a filter-feeding strategy. Suspended matter trapped by the animal is used to line the burrow walls, particularly the upper 50 cm of the burrow complex. Periodic burrow irrigation, interspersed with periods of burrow capping, produce alternating redox gradients adjacent to the burrow, resulting in diffusion of dissolved iron and manganese into the burrow lumen and deposition of oxyhydroxides on the burrow walls; the oxyhydroxides on the burrow walls manifest as brown to orange halos around the burrows. The geochemical signature of the thin layer lining the burrow matches closely that of the faecal pellets that the anunal periodically expels from the burrow mouth, suggesting wall grazing as a feeding mode. Animals reared in tanks commence defecation only after three months of burrow construction, indicating the lining of the burrow wall is a long-term gardening strategy; all fines collected during early burrow construction are used to line the wall. Burrow construction and backfilling removes all fine scale bedding structure from the sand, concentrating fines on the burrow walls. Burrow irrigation depresses the redoxcline, preventing substantial sulphate reduction and sulphide formation. Major contemporary geochemical implications of this animal's activities are the concentration of various heavy metals associated with the bulk sediment and the water column into burrow walls, then mobilisation of metals by packaging into faecal pellets that can be transported moderate distances before breaking down under microbial action to form a nepheioid layer at the sediment/water interface. Pellets are rich in organic matter compared to surface sediment. This organic content fuels sulfate reduction within the pellet and during decomposition much of the pellet iron and manganese is reduced to the soluble ferrous and manganous states. Pellet interiors become black, due the presence of iron monosulphides and any subsequent oxidation produces amorphous oxyhydroxides that can scavenge fiuther heavy metals from the water column. Metals concentrated in the pellets, at an order of magnitude greater than the bulk sediment, include lead, zinc, nickel and chromium and the metalloid arsenic. Concentration factors for iron and manganese are somewhat greater and it is thought that these two metals influence the concentration of the other heavy metals.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
PHANEROZOIC DENUDATION OF THE WESTERN SHIELD OF WESTERN AUSTRALIA M.F. Killick CRC LEME do CSIRO Exploration and Mining Private Bag, PO Wembley, Western Australia 6014
The amount of erosion and chemical wasting of the Yilgam and Pilbara Cratons and intervening Proterozoic basins (the Western Shield) that has occurred during the Phanerozoic is critical to exploration for commodities associated with residual and/or shallowly emplaced mineral deposits such as diamonds. The preservation of such deposits on the Western Shield is a function of their age of formation or emplacement, and rates of erosion. Historic denudation rates cannot be measured directly, and therefore must be estimated from analogues. The method used here is by calculation of the volume of clastic sediment deposited in the sedimentary basins marginal to the Western Shield for the period Ordovician-Cretaceous, when the basins were accumulating siliciclastic sediment. Corrections have been made for compaction, the inclusion of an additional (nominal) volume to allow for the abyssal wedge of clastic sediment running offshorefromthe continental margins, and a subtraction of the volume of chemical sediments derived from the ocean budget. The mass of this last volume has been included in the compaction calculation. The corrected volume has been converted using an average crustal density to give and equivalent volume of cratonic rock. When applied to the area of the Western Shield, it is apparent that about 4.09 km of basement has been removed since the onset of basin development and sediment accumulation during Arenigian times, about 500 Ma ago. Isostatic response to unloading means that removal of this thickness of basement would have resulted in loss of about 0.72 km of freeboard from the Western Shield. An average denudation rate of 8.87 m yr"^ has been derived for the period from the Arenig to the end of the Cretaceous, when clastic sedimentation effectively ceased. This rate is comparable with a range of 3-10.6 m yr"^ calculated using coal rank, volume balance, kimberlite erosion and Rb-Sr techniques by other authors. It differs significantly from a single published rate of 0.1 m yr"^ calculated from geomorphic evidence. The oldest Phanerozoic sediments recognised on the Western Shield are Permian channel deposits, found close to the craton margins. Preservation of these deposits under the condition implied by the model presented here requires that these long lived channel systems were either deeply incised into a higher-level shield than exists today, or they were situated seawards of a significant escarpment.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
SIGMOIDAL, STAIRCASE AND SPIRAL INCLUSION TRAILS WITHIN PORPHYROBLASTS; THEIR RELATIONSHIP TO POLYMETAMORPHISM Hyeong S. Kim School of Earth Sciences, James Cook University, Townsville, Queensland 4811
The successive mineral assemblages, mineral equilibria and chemical composition of mineral, especially within garnet and plagioclase porphyroblasts, provide the key for understanding P-T evolution of multiply deformed and metamorphosed rocks. Microstructural relationships between porphyroblast and matrix may also provide the key for resolving and integrating relative timmg between growA of metamorphic minerals and deformation, although it has proved to be a controversial topic. The area that is the focus of this study lies within the Bronson Hill anticlinorium in north-central Massachusetts. Rocks in the Bronson Hill anticlinorium, which were metamorphosed only during the Acadian Orogeny contain kyanite and sillimanite. Metamorphic mineral assemblages in the northern portion of the Pelham Dome, Massachusetts can be divided into 3 ones: (1) kyanite-bearing; (2) gamet-staurolite; and (3) sillimanite-bearing. Both kyanite and sillimanite from assemblages (1) & (3) show two microstructural and petrographic features suggesting they underwent two periods growth. Garnet and staurolite porphyroblasts contain changes in inclusion trail geometries and density and composition of inclusion minerals indicative of more than two phases of growth. In assemblages (1) & (2), some chlorites occur in garnet and staurolite porphyroblasts as an inclusion and around porphyroblasts as a matrix with kyanite, biotite, staurolite and muscovite. Some staurolites were replaced by chlorite before muscovite. It is not final retrograde effect. Also, some staurolites contain plagioclase inclusion which has been sericitized. However, plagioclase in the matrix has not been sericitized. In assemblage (3), staurolite has two distinct habits; (i) staurolite with sillimanite inclusion around muscovite pseudomorph which have been formed by staurolite breakdown; and (ii) new, veryfine-grainedstaurolite without inclusion between grain boundaries in muscovite pseudomorph and around plagioclase porphyroblast. These petrographic observations are very unusual and indicate that there may have been partially experienced disequilibrium condition during prograde metamorphism. The multiple phases of garnet and staurolite porphyroblasts can be distinguished by the trend and relative timing of the/oliation intersection axes (FLA) of foliations preserved within them as inclusion trails. The FIA trends of garnet and staurolite lie in three orientation groupings. The relative timing of each generation of FIA is determined from porphyroblasts with two or three different FIA trends developed from core to rim. The oldest to youngest FIA trends are NW-SE (group A), NE-SW to E-W (group B & B-1) and N-S to NNE-SSW (group C). Furthermore, the FIA distribution patterns and relative timing of garnet and staurolite are similar suggesting the growth of garnet and staurolite were closely associated throughout deformation and metamorphism in this area. The growth of garnet and staurolite porphyroblasts can be separated into 3 stages based on multiple mineral phases and FIA distributions: (1) FIA group A; (2) FIA group B and B-1; and (3) early and late FIA group C. From stage 1 to stage 3, garnet and staurolite porphyroblast growth and dissolution involved multivariant and univariant reactions. There is evidence of local retrograde reactions during pro^ade metamorphism in stages 2 and 3. The characteristics of these retrograde events are: (i) dehydration/rehydration; and (ii) abundance changes of metamorphic minerals. The FIA trends variance in group B probably suggest that porphyroblast has been a relative rotation of FIA to a more E-W orientation with time. It may be resulted from a "fluctuation effect" during retrograde event. Introduction of non-KFMASH components (e.g., Na, Ca, K, Mn, Zn & Ti), disequilibrium textures of chlorite, staurolite and sillimanite, heterogeneous distribution of H2O, and nearorthogonal overprinting of successive foliations, may provide much of evidence for the polymetamorphism history of the rocks, and may be important parameters influencing retrograde events.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
MONAZITE U-Pb AGES FROM EAST ANTARCTIC GRANULITES: COMPARISONS WITH ZIRCON U-Pb AND GARNET Sm-Nd AGES Peter D. Kinny Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, Perth, WA, 6001.
Over the last ten years or so enormous resources and effort have been sunk into bedrock geological investigations of the Prydz Bay region of the East Antarctic Shield by Australian, Chinese and Russian scientific expeditions. New maps have been produced and new models for the tectonothermal evolution of the region have been devised. However, some key aspects of these models remain speculative and unresolved, most notably: 1) the relative intensities and importance of ca. 1000 Ma and 500 Ma tectonometamorphic episodes to the evolution of the various Archaean and Proterozoic basement terranes of the region; and 2) the relationships between the different terranes and timing of their assembly. Large amounts of geochronological data have been accumulated from the region, including zircon U-Pb and gamet/whole-rock Sm-Nd ages. However, since exposures are sporadic and since different isotopic techniques have been applied in different areas, a coherent story has yet to emerge. Therefore, in an attempt to either resolve or further cloud the issues, U-Pb analyses of monazites from different crustal blocks in the region (listed below in order from east to west) have been undertaken using the W.A. SHRIMP. Most samples were dated direct from thin sections. Samples were either collected by the author or else kindly donated by Messrs Black, Fitzsimons, Harley, Hensen, Munksgaard, Snape, Thost and/or Zhou. Monazite dating in high-grade terranes is advantageous in that monazite is stable to high T and P, resistant to Pb loss, rarely inherited, and commonly formed during prograde metamorphism of pelites. Vestfold Hills - Granuhte facies gneisses of the Vestfold Hills preserve concordant zircon ages in the range 2525 to 2475 Ma, with no evidence for significant isotopic disturbance during subsequent events (Black et al., 1991). A monazite sample (VH545) was dated from within a zone of amphibolite facies retrogression that is restricted to the SW comer of the Vestfold Hills, which many workers have associated with younger high-grade events in the adjacent Rauer Islands. Most analyses were concordant at ca. 2460 Ma, a few were slightly discordant, one was concordant at ca. 860 Ma. The 2460 Ma age is interpreted as a cooling age, the significance of the 860 Ma age is unclear, and the timing of retrogression remains unconstrained (except that it clearly postdates tholeiitic dyke swarms of Mesoproterozoic age (Lanyon et al., 1993)). Rauer Islands - The Rauer Group consist of Archaean and Early Neoproterozoic (ca. 1000 Ma) protoliths whose zircons exhibit substantial Pb loss and new growth at ca. 500 Ma and for which gamet/whole-rock isochrons give Sm-Nd ages of 480-600 Ma. The zircon data have been interpreted as indicating that granulite facies metamorphism and major deformation in the Rauer Group occurred ca. 1000 Ma with minor 500 Ma overprint (Kinny et al., 1993) or altematively ca. 500 Ma with major inheritance of 1000 Ma material (Hensen & Zhou, 1995). Monazites analysed from three pelitic paragneisses from Filla Island are predominantly concordant at ca. 1000 Ma, a few show Pb loss consistent with ca. 500 Ma disturbance. This provides strong new evidence for prograde ca. 1000 Ma metamorphism. In contrast, monazites from a late-stage 10cm-wide aplite sheet (FI06) give a mean age of 518±8 Ma. Zircons previously analysed from this sample are essentially bimodal with populations at both 1000 and 550 Ma. Brattstrand Bluffs - Zircons from within two anatectic leucogneisses at this locality have been dated at ca. 535 Ma. Monazites in the same rocks are spectacularly zoned, with high-Th rims yielding slightly younger ages (512-518 Ma) than cores (ca. 527 Ma). These rims are interpreted as having precipitated from fluids associated with hydrous retrogression soon after peak metamorphism. Gamet/whole-rock pairs from the same locality give ages in the range 490-510 Ma (Hensen & Zhou, 1995). Bolingen Islands - U-Pb isotopic data from a metasedimentary banded gneiss at this locality (8820201A) are widely dispersed along concordia. Monazites range in apparent age from 1000 to 800 Ma, zircons from 550 to 750 Ma. Tht garnet/whole-rock Sm-Nd age for this sample is 510 Ma (Hensen & Zhou, 1997). The combined data are consistent with Early Neoproterozoic (? 1000 Ma) metamorphism of a sedimentary protolith followed by a very strong Pan-African overprint. Northern Prince Charles Mountains - Two samples from the NPCMs 80km apart, one a partial melt in metapelite (NM41), the other a gamet leucogneiss (DT009) have given monazite ages in the range 900-950 Ma, similar to or slightly younger than zircons in these and other samples from the region, but again in contrast to gamet/w.r. ages from the same rocks, which are 810 Ma (NM41) and 630-555 Ma (DT009) (Hensen et al, 1997). In summary, most monazite ages from the region are similar to or slightly younger than co-existing zircons (except where the zircons show significant Pb loss), whereas most gamet/whole-rock ages are significantly younger. The new monazite data provide further evidence for Early Neoproterozoic prograde metamorphism and anatexis in the Rauer Group, Bolingen Islands and NPCMs, and for Pan-African metamorphism in the Brattstrand Bluffs. In the Hght of these data, it is questionable whether the Sm-Nd system records the time of initial gamet growth in these polymetamorphic terranes. Isotopic re-equilibration on a regional scale at ca. 500 Ma, similar to that recorded by the Rb-Sr system in biotite/whole-rock pairs, is perhaps more likely.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
fflGH RESOLUTION DIGITAL ELEVATION MODELS IN EXPLORATION: THE MOST COST-EFFECTIVE AND VERSATILE SOLUTION FOR MAPPING, INTEGRATED INTERPRETATION AND SPATIAL ANALYSIS Anne E. Kinsey-Henderson^'^ ^Consultant, 27 Hibiscus Street, Cranbrook, Queensland, 4814 ^Agent for Georeality Pty Ltd
The advent of digital photogrammetry has opened the doors for the production of extremely high detail and accurate digital elevation models (DEMs). Topographic data is such a fundamental dataset for so many applications. Yet, the production of high resolution data is still perceived to be in its infancy due to the historical difficulties in implementing automated routines required for hi^ volume data capture. These difficulties no longer exist and mass production of high resolution DEMs is now possible at competitive prices with traditional photogrammetry. Georeality (a software development and marketing company that also provides combined marketing, services, and product support for Dems Pty Ltd and Terra Sancta Pty Ltd) is one of the first companies in Australia to explore the fiill potential of these extremely valuable datasets. As well, Georeality has developed software solutions such as Virtual Explorer™ to provide geological and structural m ^ i n g capability, as well as 3-D visualisation, on your PC. Vutual Explorer™ also includes access to Terra Sancta's Qn-Line Stress Mapping service. From the Goldfields of Western Australia, to the Gawler Craton and the Drummond Basin, large areas of Australia are being modelled with locational accuracies of under 3m, and in many cases much better. Detection of vertical relief of less than 1 or metre is possible over lO's of kilometres, allowing for identification of extremely subtle structures and drainage patterns, as well as providing the ideal source of information to simplify the brain-twisting tasks of regolith interpretation and surface geochemistry modelling. Whether as hardcopy maps or asrawdigital data or enhancements, high resolution DEMs prove themselves invaluable for a multitude of exploration activities, from regional assessment to prospect evaluation and drill targeting: 1) Typical coverage for a high resolution DEM is 20-30km or more, providing good coverage to help put prospects into their regional context, while the accuracies of the models allow them to be scaled up for prospect-scale analysis - one dataset for all scales. 2) High resolution digital orthophotos and epipolar pairs (digital stereo airphoto pairs for Virtual Explorer™) are by-products of DEM production and are provided at minimal or no extra cost. 3) High resolution DEMs can be used to automatically derive highly detailed drainage and contour data for base maps. 4) Automatic watershed modelling providesflowaccumulation and catchment maps for analysis of stream sediment or other sample data. 5) High resolution DEMs capture the actual (not interpolated) height values for every point, are ideally suited to spatial analysis (eg. in ER Mapper or Arc GRID) as well as to 3-D interpretation and structural measurements (eg. in Vutual Explorer™). Raw digital DEM data can be manipulated in the same way as geophysical datasets - varying sun-angle shading and directional or non-directional filtering for highlighting structures, measuring derivatives (slopes) and orientation. Or they can be used as the basis for very realistic and accurate 3-D images stereographically in Virtual Explorer™ or as perspective views in ER Mapper. 6) As well as providing a stand-alone interpretive datasets, shaded reUef images of high resolution DEMs also make an ideal base upon which to interpret all your maps, sample points, and geophysical data - providing an intuitive and relevant context for interpretation. ER Mapper 5.5 provides tools for doing simple image drapes through its concept of algorithm Surfaces. 3-D stereographic drapes are also possible in Virtual Explorer™ providing fiulher interpretive possibilities - eg for Landsat spectral data or geophysics. Dems Pty Ltd, the production centre for high resolution DEMs, has built up a client base of over 150 customers over the past 3 years, a majority of whom are repeat customers from within the exploration industry. Thus, it is clear that these data are valued by those who use them and are fast becoming the standard for basic mapping and geological interpretive requirements. 251
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
M"" Australian Geological Convention, Townsville, July 1998
DETAILED SEISMIC TRAVEL TIME INVERSION FOR CONSTRAINING EM MODELLING ALONG THE EASTERN GOLDFIELDS GEOPHYSICAL TRANSECT, WESTERN AUSTRALIA Ian J. Kirbv and Karsten Gohl CRC for Australian Mineral Exploration Technologies, School of Earth Sciences, Macquarie University. Sydney, N S W 2 1 0 9
A comparison of seismic refraction models and ground transient electromagnetic (TEM) conductivity-depth images has been conducted along a geophysical transect of the Eastern Goldfields, Western Australia. The principal aim was to determine the concurrence of the models, suggest reasons for any differences, and to investigate the validity of using velocity models as possible constraints on the geometry of geologic structures such as major faults zones for refinement of the TEM images. Seismic first arrival data from a deep crustal reflection survey by AGSO were used to produce 2D refraction velocity-depth models for both P-waves, and where apparent, S-waves, as well as models of Poisson's ratios. This was achieved with a travel time ray-tracing and inversion program allowing for horizontal and vertical velocity gradients. The inversion was performed using the damped least-squares method applied to the residuals of modeled travel times with respect to the observed data. Along much of the transect there is general concurrence between the TEM and seismic models, with the following exceptions. In areas where lateral geologic structure varied rapidly, TEM models tended to be significantly smoothed, an inherent restriction of the resolution of the TEM method as well as a limitation of ID inversion used in TEM modeling. In the presence of a thick conductive overburden, TEM was not able resolve the occurrence of bedrock. In areas with a thin, highly conductive overburden, TEM models differed significantly from the velocity models, probably due to the influence of 2D and 3D effects. Velocity models assist to refine TEM conductivity images where ID inversion of TEM data cannot not closely resolve the actual geology, such as in areas of rapid lateral structural variation. The seismic images presented show that detailed travel time ray-tracing and inversion enhances the interpretation of geophysical data for structural elements of mineralisation zones.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
LATE PALAEOZOIC COOLING OF SOUTHWEST AUSTRALIAN TERRANES INFERRED FROM APATITE FISSION TRACK THERMOCHRONOLGY Barry P. Kohn', Paul B. OOSullivan', Andrew J.W. Gleadow', Garry D. Kamer^ and Jeffrey K. WeisseP 'Australian Geodynamics Cooperative Research Centre, Dept. of Earth Sciences, La Trobe University, Bundoora, Victoria 3083 ^Lamont-Doherty Earth Observatory, Palisades, NY 10964
We report the results of apatite fission track (AFT) analyses and numerical forward modelling to constrain the "low temperature" (^110°C) thermal history of the southwestern Archaean Yilgam Craton, the Proterozoic Albany Mobile Belt and the Neoproterozoic to early Phanerozoic Leeuwin Complex in southwestern Australia. This approach is particularly powerful for reconstructing thermotectonic histories of the upper continental crust of crystalline terranes, especially where the use of traditional stratigraphic and structural parameters is severely limited. The sampling strategy employed principally involved three traverses, which were designed to take into account earlier regional studies. Two traverses extended eastwards for >200 km from the western margin of the Yilgam Craton, at approximately the latitude of Perth, and along the southern coast, covering areas where previous Rb/Sr biotite age trends have been reported. The biotite ages show a distinct zonation eastwards; with ages ranging from 400-600 Ma in a 30-100 km wide western zone, through a 20-40 km wide transition zone, to an eastern biotite domain yielding ages in the range 2000-2500 Ma. One explanation proposed for the reset ages to the west is associated with the denudation of topography created by the Pan African thrusting of Proterozoic sedimentary rocks over the southwest. A third north-south traverse closely followed the western boundary of the Yilgam Craton, marked by the Darling fault system which separates the craton from the Perth basin. The basin contains > 14 km of Silurian to Recent strata, but most of the fill is Upper Triassic to Lower Cretaceous. The Perth basin was formed by a series of rifting events leading to the break-up of Gondwana which culminated in the Early Cretaceous. Kinematic and isostatic modelling of the rift development of the Perth basin suggests a maximum rift flank topography and denudation of 1-1.5 km and 4-5 km, respectively. The AFT data set (58 samples), yields ages with no clear regional trend, rangmg from 185-320 Ma and mean track lengths of ~ 12-14 microns. The youngest samples of this data set (--185-230 Ma) which also include the lowest mean track lengths are located on the Yilgam, within a few kms of the Darling fault, in the Leeuwin Complex and along the south coast. Forward modelling of all the data indicates that samples have generally experienced a significant Late Palaeozoic (Late Carboniferous-Permian) regional cooling by at least -'40-50°C. Assuming the present craton geothermal gradient of -12-14°C/km, this amount of cooling suggests at least -3-4 km of Late Palaeozoic cooling/denudation. Thermal modelling also suggests later cooling possibly during the Late Jurassic-Early Cretaceous and/or Tertiary. These later episode/s are required by the modelling but are poorly constrained temporally since most of the cooling occurred from temperatures <60-70°C, a range over which the thermal effects of fission tracks in apatite are less sensitive. Nevertheless, assuming a similar gradient as above, then the later event/s would in total have involved a further -3-4 km of cooling/denudation. In areas where ages are youngest and track lengths shorter, the post Late Palaeozoic cooling would have occurred from slightly deeper cmstal levels (-15-20°C hotter) so the amount of exhumation would have been even greater and in broad agreement with the estimates above for rift margin topography denudation. Independent evidence for regional Late Palaeozoic regional cooling/denudation is given by the fault-bounded Collie, Wilga and Boyup basins located within the Yilgam Craton -150-180 km SSE of Perth. The largest, the Collie basin, contains -1400 m of Permian tillite and vertically-stacked coal measures. Reported vitrinite reflectance measurements and the stratigraphy indicate that maximum coal burial depth was possibly up to - 100-105°C, and that most of the missing section was removed in the Permian, with a later erosional cycle pre Early (but not earliest) Cretaceous. These observations combined with the AFT data could possibly indicate a substantial thickness of Late Palaeozoic sediment extended across the crystalline rocks of the study area, and that the Collie and adjacent basins are preserved outliers of this accumulation. Altematively, the Late Palaeozoic cooling could be related to the final removal of the Pan African related compressional topography, but the recorded low temperauire cooling appears to extend across all of the Rb/Sr biotite zones previously reported. The existence of significant regional Late Palaeozoic cooling requires rethinking of the long term thermotectonic stability of the Yilgam Craton and highlights the need to further examine the role of tectonic reactivation and denudation patterns in this terrane.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
SEQUENCE STRATIGRAPHY OF THE SHADY BORE QUARTZITE AND RIVERSLEIGH SILTSTONE, UPPER MCNAMARA GROUP, LAWN HILL REGION Andrew Krassay', Barry Bradshaw', Jan Domagala^ Jim Jackson' and Bruce McConachie^ ^ Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 ^Queensland Department of Mines and Energy, GPO Box 194, Brisbane, Queensland 4001 ^Santos Ltd, South East Asia, PO Box 138, Lutwych, Brisbane 4030
In the chronostratigraphic framework constructed as part of the North Austrahan Basins Resource Evaluation (NABRE) project the combined Shady Bore Quartzite (SBQ)-Riversleigh Siltstone (RS) stratigraphic interval of just over 3km thickness is divided into six sequences (probable third-order sequences, -O.S-SMa duration). The six sequences occur within the River Supersequence of the Isa Superbasin (new chronostratigraphic NABRE terminology). The outcrop- and drillhole-based sequence stratigraphy outlined here is based on interpretation and correlation of numerous stratigraphic sections measured during 1995-1997, on drillhole logging, gamma-ray surveys, and on ties to seismic interpretations. At Freemans Creek, Sequence 1 has an erosional contact with underlying mixed carbonate/siliciclastic facies of the Lady Loretta Formation (LLF). The 50m-thick progradational lowstand systems tract (LST) at the base of Sequence 1 comprises coarse-grained, channelised, conglomeratic high-energy fluvial facies. The sequence boundary is interpreted as the base of an incised valley which formed due to significant erosion accompanying a fall in relative sea level and a regional basinward shift in facies. Elsewhere, the contact between the SBQ and the Lady Loretta Formation (LLF) is apparently not erosional, and is interpreted as a correlative conformity. The 200m-thick transgressive systems tract (TST) of Sequence 1 comprises stacked 10-20m thick shoaling cycles of shallow marine to intertidal quartz arenites. Maximum flooding during Sequence 1 is marked by a peak in gamma-ray logs that corresponds to a regionally-correlative, 20m-thick siltstone-dominated interval. A thin progradational, sandstone-dominated highstand systems tract (HST) occurs above the maximum flooding zone. Sequence 2 comprises stacked 50- to 70m-thick shoaling quartz arenite cycles with a broadly retrogradational gamma-ray log trend reflecting progressive drowning. An abrupt facies change to thinly interbedded siltstone and fine-grained sandstone of the late TST and HST of Sequence 2 reflects rapid transgression in early RS time. This facies change also marks the lithostratigraphic contact between the SBQ and RS. An erosional surface at the base of Sequence 3 is overlain by planar cross-bedded and rippled shallow marine shoreface sandstone. This sandbody is interpreted as a lowstand shoreface (LST) created by reduced accommodation rates that lead to a fall in relative sea level and regression. Most likely this rapid fall in relative sea level was caused by tectonic uplift and accompanying tilting of fault blocks. The sedimentary response to the uplift and tilting of fault blocks is most striking in the southern region where Sequence 3 is deeply incised into underlying HST marine siltstones of Sequence 2. Here, the LST comprises conglomeratic fluvial sandbodies. Creation of new acconunodation space resulted in the accumulation of thick TST and HST marine siltstones of Sequence 3. Above this growth-faulted interval is a thick sandbody whose sharp base is interpreted as a sequence boundary marking the base of Sequence 4. Unstructured, lithic sandstones here represent the development of distal LST fan deposits which exhibit marked thickness variations across local growth-faulted sections in the southwest. The TST of Sequence 4 comprises at least 300m of poorly exposed dolomitic siltstones that accumulated under mostly low-energy marine conditions during a major transgression culminating with maximum flooding and development of a condensed section. The HST of Sequence 4 is a thick interval of interbedded hummocky cross-stratified sandstone, thin graded beds, and ripple-laminated siltstone. The sequence stratigraphy of the upper RS {Sequences 5 and 6) is not covered in detail in this presentation. Regionally the SBQ thins markedly to the north. The 50m of LST fluvial facies and most of the 200m TST shallow marine facies of Sequence 1 lap out northwards over 55km between Freemans Creek and Mount Caroline. Seismic images show even greater onlap farther to the north. In contrast, the HST of Sequence 1 and all of Sequence 2 can be correlated over at least 80km north-to-south with a progressive southward-thickening geometry. Unlike the SBQ, the thickness and facies of the RS vary markedly across the Termite Range Fault and also across SW-NE faults (similar in orientation to the Barramundi Fault). Local depocentres are particularly well developed in the central-southern part of the Lawn Hill region where the RS is up to 2700m in thickness. At Mount Caroline 55km to the north the RS thins to only 295m.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
VELOCITY STRUCTURE OF THE ARGO ABYSSAL PLAIN, NORTH WEST SHELF OF AUSTRALIA A.Ye. Kritski^ R. D. Muller\ C.D.C. Collins% H. Stagg% G.l. Christeson^ 'Department of Geology and Geophysics, University of Sydney, NSW, 2006, Australia, ^Australian Geological Survey Organisation, GPO BOX 378, Canberra, ACT 2601, Australia ^Institute for Geophysics, University of Texas at Austin, Austin, TX 78759-8500.
Ocean bottom seismograph data have been analysed to determine and model the crustal structure of the Argo Abyssal Plain in the northeastern Indian Ocean. Two detailed seismic refraction transects were shot to provide constraints on the complex geological and tectonic history of the area. A total of eleven ocean-bottom seismograph (OBS) receivers were deployed at or seaward of the estimated continent-ocean transition (COT) position along the two transects. One transect (96 km long) was oriented orthogonal to the spreading direction of the Argo Abyssal Plain; the other (237 km long) was oriented parallel to the spreading direction and crossed the COT. The AGSO research vessel Rig Seismic was used to deploy the OBS receivers and provide airgun shots every 100 m from a 4800 in^ sleeve gun array. The transects were conducted coincident with previously acquired reflection seismic lines. All receivers recorded a hydrophone, vertical, and two horizontal channels. All OBS were successfully recovered, and arrivals were recorded to offsets of over 250km offshore. Both compressional and shear arrivals are clearly observed in the data. The refraction data have been analysed by iterative forward modelling of travel times and arrival amplitudes. The crustal velocity structure obtained was interpreted in the light of previous seismic reflection results from this area, together with other available geological and geophysical information. The models for compressional and shear velocity structures along the two transects are primarily constrained by variations in two-way-travel time (depth) derived from the reflection profiles. Two-dimensional travel time modelling indicates that the west portion of the study area is associated with unusually high velocities (7.3-7.4 k m/s) for oceanic layer 3. These high velocities are not consistent with a gabbroic composition. They might indicate instead serpentinite alternating with ultramafic material, resulting in a higher average velocity. The data show very good shear wave arrivals which can be used to discriminate between these two compositions as serpentinite has very different shear velocities compared to most other materials. Low angle dipping reflectors are imaged in the upper, mid-, and lower crust near ODP site 765, and in some places penetrate nearly the entire oceanic section. Clear P basement refractions and PmP reflections are observed in the OBS data collected on the strike line at this site. A two-dimensional velocity model provides a good fit to the travel time data along this line; significant features of the model are a 7.4 km thick crust and a velocity of 7.3-7.4 km/s at the base of the crust. The data recorded by the three OBS instruments along the strike line indicates that the unusually high velocities within oceanic layer 3 are not an artifact of two-dimensional features. Velocity modelling and geometrical observations of the low angle structures suggest these features to be a threedimensional dipping surfaces located in the vicinity of fossil ridge-transform intersection. These surfaces may be the low angle detachments between the upper crust and the lower crust/mantle. Geometrical estimates of these structures suggest dips of 30°-35®. Dips as low as those seen here could be explained by the lubricating presence of serpentinized peridonite, fragments of which dredged from both surfaces. Alternatively, these surfaces may instead represent failure surfaces in serpentine-lubricated zones. Two-dimensional models for each seismic line obtained from the forward ray tracing were tested by using the Zelt's and Smith seismic inversion scheme. The testing presents final two-dimensional velocity models as a result of simultaneous inversion for the velocity and interface positions. Final inversion modelling provides an estimation of model parameter resolution, uncertainty and non-uniqueness. Waveform inversion was used to model the Poisson's ration structure of the crust layers for four station with good shear arrivals.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
CALCIMICROBES IN NEOPROTEROZOIC AND CAMBRIAN REEFS Peter D. Kmse Northern Territory Geological Survey, PO Box 2901, Darwin NT 0801
While calcimicrobes are principally a Phanerozoic phenomenon, their known beginnings He in the Neoproterozoic. Spectacular boiindstone massifs in basinal deposits of the Little Dal Group of northwestern Canada represent the earliest occurrences of calcimicrobes in reefs. These are most probably Tonian (early Neoproterozoic) in age. Reef cores consist of digitate, laminar and clotted structures built by 'cellular crusts' and other calcimicrobes comparable with Girvanella and Renalcis, in concert with abundant interstitial marine cement and locally common geopetal structures. Boulder talus surrounds the reef cores. Calcimicrobes were thus frame-builders from their earliest beginnings. Calcimicrobes begin to diversify in the latest Neoproterozoic to Nemakit-Daldynian (early Early Cambrian). Examples of calcimicrobial bioconstructions of this age range are illustrated from the Zavkhan Basin of western Mongolia. These consist mainly of Korilophyton, with or without minor Tarthinia, and with interstices infilled by lime mud. From earliest Tommotian time, archaeocyaths unite with the already established reef calcimicrobes to form a metazoan-calcimicrobe consortium ^\ilich was to dominate the Early Cambrian reef-building phase. During the Tommotian, this consortium was confined to the transitional facies tract of the Siberian Platform. It subsequently spread to nearly all continents during the Atdabanian and attained maximum distribution in the Botomian. Tommotian examples from the Pestrotsvet Formation in southeastern Siberia variously demonstrate nearly all aspects of this Early Cambrian consortium: the full array of archaeocyathan cup shapes, stereoplasm (secondary skeleton constructed by archaeocyaths), mud, fibrous cement, epibionts, cavities, cryptobionts and distinctive peribiohermal sediment. Macroborings appeared m the Atdabanian. Within mounds, calcimicrobes remained the principal bioconstructors in association with marine cement; archaeocyaths contributed to bioconstruction only locally, where stereoplasm was abundant. Both groups could act as cryptobionts. A distinct but less common small skeletal fossil-calcimicrobe bioconstructional consortium existed concurrently. This consortium is exemplified by 'Ladatheca' cylindrica mounds in the West Centre Cove Formation, and Coleoloides mounds in the Fosters Point Formation of Newfoundland. Archaeocyaths declined dramatically in the Toyonian stage, so that archaeocyath-calcimicrobe reefs were confined to a few remnants on some continents. Nevertheless, the 'Tommotian' reef blueprint persisted, at least in argillaceous limestone facies, as illustrated by reefs m the Forteau Formation of Labrador, Canada. With the decline of Early Cambrian-style bioconstruction, stromatolites and thrombolites underwent a resurgence through the Middle and Late Cambrian at the expense of dendrolites. Toyonian reefs with Epiphyton thrombolitic stromatolite framestone cores in the Wirrealpa Limestone of South Australia are illustrative of this trend. They bear a remnant archaeocyathan-radiocyathan biota together with other skeletal invertebrates and macroborers, but calcimicrobes dominate. Middle and Late Cambrian reefs were almost entirely calchnicrobial. Exceptional sponge-calcimicrobe reefs in the late Middle Cambrian Mila Formation of Iran consist of digitate Rankenella encrusted by Girvanella. Interstices are occluded by fibrous and later equant cement, v^th little internal sediment. The ability of calcimicrobes to encrust and bind made them the principal bioconstructors of the Neoproterozoic and Cambrian. Apart from modular forms of the Botomian and Toyonian, archaeocyaths were generally subordinate as bioconstructors, as were other reef dwellers such as small skeletal fossils.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
PORPHYRY COPPER DEPOSITS - THE NEXT 100 YEARS Willard C. Lacy 699 W. Magee Road, #20102 Tuscon, Arizona 85704
THE FIRST 100 YEARS The porphyry copper concept was bom approximately 100 years ago with the economic analysis by D.C. Jackling and R.C. Genmiell of the Utah (Bingham Canyon) deposit. With Utah's success, eight more deposits came on stream in the following 12 years. As potential targets increased, field criteria for capping recognition were introduced by R. Blanchard and A. Locke, and recognition of characteristic alteration patterns by C. Meyer, and later by Lowell and Guilbert. Organized exploration programs began during WWII, organized by E.N. Pennybaker and later by Bear Creek Mining (Kennecott). Various genetic models were developed during the 70's. As a consequence of low per/ton profit margins, the porphyry deposits are sensitive to economic and technological factors. The porphyry operators suffered financial setbacks from 1970 to 1985. Recovery since 1985 to 1997 was the result of expanding production by a few mines and increased labor productivity, as well as development of SX/EW techniques. THE NEXT 100 YEARS Doomsday predictions have all proven to be false, with the continuing production of known porphyry deposits and new discoveries on a world-wide basis. New classes of exploration targets and new porphyry belts are being visualized. In the year 2098 someone will be presenting a paper on "Porphyry Copper Deposits - The Next 100 Years".
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
PETROGRAPHIC EVIDENCE FOR NORTHWARD-DIRECTED BEDLOAD TRANSPORT ON THE INNER GREAT BARRIER REEF SHELF. Alexis Lambeck, Ken Woolfe and Piers Larcombe Marine Geophysical Laboratory, School of Earth Sciences, James Cook University,Townsville 4811 Australia. email Alexis.Lambeck@jcu.edu.au
Investigations into sediment transport on the inner-shelf of the Great Barrier Reef (GBR) Lagoon have inferred northward- directed bedload transport (Belperio,1983; Pye, 1982, 1993; Larcombe & Woolfe, 1995; Orpin & Ridd, 1996). Here we report results from a north-south sediment sampling survey which provides hard evidence confirming that northward-directed bedload transport dominates the inner shelf. Samples were collected at regular intervals along the 10 m isobath between Cape York and Bowen. Each sample was washed through a 125 imi sieve and the remaining coarse fraction was mounted in epoxy resin from which thin sections were prepared. Point coimts (each of 300 points) were made on 153 samples to determine the abundance of common mineral phases. Plotting the results against latitude reveals a marked northward increase in sediment maturity, manifest as an increased total abundance of quartz and decreased relative abundance of feldspars, lithic grains and mafic minerals (Fig. 1). South of the Tully River a smooth overall northward trend is evident, indicating significant mixing on the inner shelf. In contrast, further north, the data is highly variable indicating that sediment inputs from individual rivers may be retained relatively close to source. This may be related to a higher sediment trapping efficiency of the northern inner shelf and or lower net rates of along-shelf transport.
Figure 1. Total concentration of some terrigenous minerals in the > 125 um fraction of samples taken from the 10 m isobath.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
EVIDENCE FOR MANTLE CONTROL ON THE CHARACTER OF I-TYPE GRANITOIDS: EXAMPLES FROM THE NEW ENGLAND BATHOLITH B. Landenberger & W.J. Collins Department of Geology, University of Newcastle University Drive, Callaghan NSW 2308
One of the central controversies in granitoid petrogenesis is whether granitoid melts are generated solely from crustal sources, or whether mantle-derived magmas form an important conq)onent of them. Is the mantle merely a heat source initiating melting within the crust, or does it actively contribute to granitoid magmas? Although petrogenetic studies may convincingly demonstrate that some S-type granites are generated solely from crustal sources (metasediments), models restricting the origin of I-type granites to crustal source rocks are more contentious. For example, while some schools of research maintain that I - t ) ^ granites are derived entirely from pre-existing crustal (meta-igneous) sources, others indicate that mantlederived melts play a vital role in the formation of I-type granitoid magmas. Granitoid suites of the New England Batholith provide a unique opportimity to test whether mantle-derived melts have made major contributions to the character of I-type granitoids within this belt. The three main Itype granitoid suites (Moonbi, Uralla and Clarence River), each have distinct petrographic, geochemical and isotopic characteristics, and are primarily distmguished by their KjO content as: Moonbi Suite (high KjO), Clarence River Suite (low KjO), and the Uralla Suite (intermediate KjO). The Clarence River Suite is the most isotopically primitive while the Uralla Suite is the most isotopically evolved. The suites are also broadly coeval, with most plutons intruding in the latest Permian and early Triassic. Each suite has a distinct spatial distribution, and all but a few plutons are hosted by the Carboniferous New England accretion complex. This suggests that supracrustal sources were unlikely to have caused the observed chemical differences - so what did produce the intersuite variation? Mafic plutonic rocks have long been recognized as part of the batholith, but have been long regarded as unimportant, due to their perceived insignificant volume. Recent investigations reveal that many mafic intrusions (chiefly mafic dykes) have been overlooked, and hence their significance should be re-evaluated. More importantly, geochemical and isotopic similarities provides a clear link between mantle-derived mafic magmas and granitoids within each suite. The mafic rocks associated with each suite largely mirror the element patterns of the associated granitoids on multi-element diagrams and form extensions of the linear trends exhibited by granitoids on Marker plots. Low-K diorites and basaltic to andesitic dykes associated with the Clarence River Suite are chemically akin to the low-KjO granitoids of that suite, while high-KjO calc-alkaline lamprophyres associated with the Moonbi Suite chemically match the patterns of the associated high-K granitoids. Likewise, the granitoids of the Uralla Suite, with their intermediate KjO contents, are also accompanied by diorites, and basaltic to felsic dyke swarms also chemically match their granitoid counterparts. Isotopic evidence also supports a clear link between mantle-derived mafic magmas and granitoids within both the Moonbi and Clarence River suites, with the mafic rocks in each case falling within or near the fields for the granitoids. Although clear explanations for the mechanisms of, and source rocks involved in, magma generation are still sought, mounting chemical and isotopic evidence from the New England Batholith indicates that mantlederived mafic magmas had a controlling influence on the character of granitoids within the I-type suites of the batholith. Problems that require addressing include characterization of the crustal sources involved, and clarification of the mechanism of interaction between the mantle and crustal sources. For example, although the lamprophyres associated with the Moonbi Suite appear partly responsible for the isotopic and geochemical character of the suite, a major problem arises if magma mixing is directly invoked, due to the intrinsically small volumes of lamprophyre magmas produced during mantle melting.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14'^ Australian Geological Convention, Townsville, July 1998
EXPLORATION MODEL FOR COBAR Au-Cu-Ag-Pb-Zn DEPOSITS Kenneth C. Lawrie''^ and Mark Hinman^'^
'James Cook University of North Queensland ^Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 ^Hinman Geosolutions, Bardon, Qld.
The L. Silurian-E. Devonian Cobar Basin, an (inverted) intra-continental ramp basin filled mainly by siliciclastic marine turbidites, is host to a distinctive class of polymetallic Au-Cu-Ag-Pb-Zn massive sulphide deposits. Cobar deposits comprise multiple epigenetic lodes that are highly discordant to stratigraphy and occur as vertically continuous (>400m, often >lkm) pipes, lenses and veins of narrow v^idth (< 100 metres, commonly <30 metres) and short strike length (<300 metres). Au-Cu, Cu-minor Au, and Au-Cu-Pb-Zn deposits occur in the south, with Cu-Zn-Pb-Ag, and Ag-Pb-Zn-(Cu) deposits further north. Metal ratios also vary significantly between adjacent orebodies, with vertical and lateral zonation common within individual lodes. Key elements that form the basis of predictive exploration models for Cobar style deposits are: • Cobar deposits are structurally controlled, localised within steeply dipping, linear, ductile high strain zones that may be components of a thrust complex linked to shallow detachments at depth. Mineralisation was contemporaneous with or shortly post-dated sedimentation and E. Devonian basin inversion. Ore-bearing fluids were focussed withinfracture-enhancedpermeability channelways in brittle-ductile high strain zones which formed primarily at the eastern and northern basin margins. The Peak and Elura deposits are localised within doubly plunging anticlinal fold hinges in high strain volumes. These sites of anomalous vertical extension formed where strain was partitioned heterogeneously at sites of marked competence contrast, and/or where basement heterogeneities (reactivated transverse faults) intersected the high strain zones. • Two fluid components were involved in deposit formation. These are: (1) a relatively reduced (pyrriiotite +/or pyrite-stable) fluid that evolved from n-alkane to C2H4-rich compositions prior to mineralisation, and progressed to hotter, more saline and CHa-rich compositions during mineralisation. This fluid is considered to be of basinal (connate) origin; (2) a more oxidised (pyrite or magnetite-stable) H2O-CO2 fluid that is high temperature but has low salinities. This fluid is considered to have been derived from the basement (metamorphic and /or igneous origin). Ore precipitation resulted from the mixing of these two fluids with contrasting temperatures, redox properties, and compositions including salinities. Pb isotope data supports a mixmg model: the Pb-Zn end member of the mineralisation spectrum appears to represent basinal-source dominance, whereas Cu-Au mineralisation is basement-source dominant, although still mixed. A meteoric fluid component may be present in some deposits. • On a basin scale there is no favourable horizon for Cobar-style deposits. Deposits are localised within relatively reduced siliciclastic turbidite sedimentary units, none of which are distinctive in terms of chemical composition relative to other sequences within the basin. Hence Cobar style deposits may occur anywhere in the basin where the requisite fluids, structural channelways and dilational geometries are present. In contrast, other sedex base metal deposit types form from basinal (connate) brines that are the dominant or sole fluid component. These fluids are thought to utilise bedding-controlled primary or secondary porosity and permeability channelways, and are expelled from basins in response to buoyancy-driven fluid flow along favourable stratigraphic horizons and /or brittle fault structures. Most sedex deposits form where oxidised or reduced host rocks provide the principal redox precipitation mechanism for mineralisation. The metal contents cf Cobar deposits differfromother sedex deposits due to fluid sourcing from regions external to as well as within the host basin. Lower fluid salinities, a greater (and higher) fluid temperature range, and precipitation due to fluid mixing also contribute to the complexity of observed metal ratios in Cobar deposits. More 'typical' sedex PbZn-Ag deposits may occur elsewhere within the Cobar Basin. On the basis of discovery of the McKinnons Au deposit and available isotopic data, exploration for further polymetallic deposits within the Cobar Basin need not be restricted to the basin margins, as long as the requisite fluid channelways and structural trapping geometries are developed. However, fiirther work is required to establish if basement fluids are solely of metamorphic origin, or whether there is a contribution from Early Devonian magmatism that might be expected to be localised by pre-existing basement heterogeneities. Structural mapping and analysis of geophysical data sets is required to identify high strain zones and basement heterogeneities that may have localised blind thrusts and other intra-basinal structures. Exploration for Cobar-style deposits in other terranes should focus on targeting reduced sedimentary basins that were inverted at a relatively early stage in the history of the basin. Thermal mauiration of the basin to conditions consistent with CH4 generation may be necessary to produce the basinal Ag-Pb-Zn component in orebodies. This component may initially be Zn-rich, and evolve to Ag-Pb-Zn compositions. Basin inversion must involve structures that transect the basin and underlying basement to tap contrasting fluids and metal sources. Inversion zones within intra-continental ramp basins, intra-cratonic rift basins, and some parts of foreland basins, are considered favourable exploration targets for Cobar-style orebodies.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
GOLD LOCALISATION IN A LATERAL RAMP WITHIN A REACTIVATED MYLONITE: THE GOLDEN BUTTERFLY DEPOSIT, CROYDON GOLDFIELD, N.Q. Kenneth C. Lawrie'. Subhash Jaireth^ David A-Izzeddin, and Jason Grace
James Cook University of North Queensland 'Current address: Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 ^Current address: Bureau of Resource Sciences, Canberra, ACT 2601
The Croydon Goldfield, in the Georgetown region of Far North Queensland, is a historically significant gold producing district, from which approximately 50 tonnes of gold has been mined. Au occurs within numerous quartz veins which are hosted within the Proterozoic Croydon Volcanic Group and granites of the co-genetic Esmeralda Supersuite. The mineralised veins are typically narrow (<5metres thick), mostly gently-dipping (<30°), and form subparallel lodes that are continuous along strike for several kilometres. Mineralisation is localised within restricted domains along these veins, and generally considered to lie at the intersection of lodes with graphitic zones within granites and volcanics, and/or cross-cutting faults. The gently-dipping veins occur along previously unrecognised mylonites, although earlier workers recognised a relationship between Au lodes and thrust faults. Microstructures within the mylonites include ribbon quartz, stepped feldspar porphyroclasts, quartz- and mica-fish, deformation lamellae, subgrain development and recrystallisation, en-echelon quartz-fUled porphyroclasts, s-c fabrics and extensional crenulation cleavages. Lithology offsets, and sense of shear criteria at both outcrop scale and in microstructure indicate that the mylonites formed as compressional thrusts. Ductile microstructures are developed near their base, in zones up to half a metre thick, with more brittle-ductile fabrics developed towards the top of a zone of shearing which can be up to 5 metres thick. In most thrust zones there is a progression from ductile to more brittle-ductile then brittle stuctures through time, suggesting that a history of progressive unroofing is preserved. A study of the Golden Butterfly Au open pit has revealed that the deposit, which is hosted within graphitic granite, is localised in a sinistral lateral ramp. This ramp appears to be a linking structure between the noriemmost lodes of the Highland Mary-Iguana Ime of lodes (in the east), and the southern-most lodes of the Golden Gate line of lodes (in the west). Low-angle structures, including mylonites and buck quartz veins, are folded on a large (km) scale, and strike at right angles to the regional trend in the mine vicinity. The point of maximum flexure of the lodes is marked by a thickening of the buck quartz vems, localised development of stockwork veins, and a reversal in the dip direction of the lodes. In this zone, buck quartz veins are cross-cut by comb quartz veins, the orientation of which indicates that this was a zone of local vertical transtension. Later E-Wstriking, moderately-dipping faults are associated with drag folds which also indicate sinistral transtensional displacement. These faults are interpreted as break-through structures that link adjacent thrust segments. Visible alteration in the Golden Buterfly deposit forms minor foofwall and more extensive hanging-wall alteration zones (up to 15 metres wide). The alteration assemblage within the graphitic granite host rock at the Golden Butterfly deposit is mainly sericite + silica (+pyrite). Au (as electrum) accompanies base metal sulphides and is intimately associated with galena. The gold is present primarily in comb quartz veins, in fractured earlier veins, and sulphidation of graphitic wall rock adjacent to sheared veins. Au mineralisation is coincident with brittle-ductile shearing of buck quartz, sericitic alteration, and formation of extensional comb quartz texnires. Fluid inclusion data for mineralised comb quartz veins shows that fluids were saline (5-15 % NaCl equiv.), and of moderate temperature (max. Th 259°C). Au was most likely transported as a thio complex, and most likely precipitated as a consequence of redox reactions between graphitic wallrocks and relatively oxidised hydrothermal fluids. However, there is evidence for mixing with a cooler, less saline fluid some in some comb quartz veins. Ar/Ar dating of sericitic alteration alteration associated with Au mineralisation in the Golden Butterfly deposit gives an age of 322± 2 M.a. (Henderson, 1989). In summary, Au-bearing fluids were possibly derived from a Carboniferous (igneous?) source at depth and channelled along a progressively unroofmg thrust complex. Significant Au mineralistion is localised within progressively reactivated mylonites at dilational sites which include lateral ramps and thrust duplexes (eg Federation). Economic Au is linked to formation of more brittle deformation and comb quartz veins which are indicative of formation at sub-volcanic crustal depths. Previous estimates for the amount of stratigraphic section removed from this area (up to 3.5kms) have not taken into account the presence of ductile mylonites which indicate initial development at crustal depths within the brittle-ductile transition zone. REFERENCE Henderson, G.A.M., 1989. Notes on Croydon, NQ, fieldwork July/August 1988, and results of K/Ar dating cf sericitic alteration. B.M.R. Record 1989/46.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
Au-Cu MINERALISATION AT THE BALD HILL PROSPECT, L.F.B, N.S.W. Kenneth C. Lawrie'. Terrance Memagh', Lance Black' & Doone Wybom^ 'Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 ^Australian National University, Canberra
Discovery of significant intrusive-related Au-Cu deposits in the Lachlan Fold Belt has focussed exploration attention on their Ordovician host rocks and associated intrusive complexes. However, younger intrusive-related Au-Cu deposits are known, the largest of these being the Brown's Creek deposit. Exploration in the vicinity of the latter deposit has identified a number of Au and Au-Cu prospects including the Discovery Ridge and Bald Hill deposits. The latter is a Au-Cu prospect, located 4 kms south of the southwards-plunging Carcoar Granodiorite. The deposit is hosted within one of a number of small plutons emplaced within Ordovician metasediments and metavolcanics. The Bald Hill intrusion is elongate N-S and is approximately 1 km in length. It is a hornblende biotite granodiorite, geochemically typical of Carcoar Suite I-type granites. The intrusion is an ihnenite-stable (reduced) I-type. On Harker Diagrams the Bald Hill Granodiorite is ahnost identical to the Garland Granodiorite, and has slightly higher silica levels than the Carcoar Granodiorite. The igneous texture is inequigranular, with scattered larger crystals of plagioclase, hornblende, biotite and quartz in a groundmass of quartz and two feldspars. The texture suggests a relatively high emplacement level, and this is consistent with the small size of the pluton. SHRIMP U-Pb zircon dating yields a crystallisation age of 425 ± 4.7 Ma (earliest Middle Silurian) for the intrusion. Two zones of mineralisation are recognised within the pluton: a northern Au-Cu zone and a southern Audominant zone. Preliminary results from the northern zone show that the main alteration and mineralisation paragenesis is: Pre-ore: Sulphide-poor quartz veins, and silicification and albitisation of groundmass; Syn-ore: Sulphide-bearing quartz veins and disseminated mineralisation containing chalcopyrite + electrum + tellurides + arsenopyrite + pyrrhotite ± sphalerite ± pyrite ± marcasite; the alteration gangue assemblage comprises chlorite + sericite + quartz ± calcite ± epidote ± zircon ± titanite. A reconnaisance fluid inclusion study revealed that pre-ore fluids are H20-CH4-bearing (no COj detected). This suggests that the fluid was reduced. Inclusions have homogenisation temperatures in the range 300-400®C, are liquid-rich, with high potassium in the fluid possibly indicated by small solid mica inclusions. The syn-ore fluid has a similar H2O-CH4 composition (no CO2 detected). In this stage, heterogeneous liquid-vapour ratios within individual quartz crystals suggest phase separation occurred at the time of mineralisation. Vapour ratios up to 50% are preserved in primary and pseudo-secondary inclusions. High salinities are indicated by the presence of halite and other phases within inclusions. In summary, mineralising fluids appear to have been reduced, of moderate temperature and high salinity. Ore deposition may have been coincident with phase separation related to de-pressurisation within active brittleductile faults that intersect the pluton. It is considered likely that the Bald Hill Au-Cu prospect is related to reduced I-type magmas emplaced at relatively shallow crustal levels, although the host pluton itself may not be the phase of magmatism from which mineralising fluids were derived. The age date for the pluton provides a maximum age constraint on the age of mineralisation. Recent studies demonstrate that intrusive-related Au-Cu deposits are not solely associated with oxidised I-type granitoids (Wybom, et al., 1998), and that reduced I-type magmas may be particularly prospective for Au-rich mineralisation (eg Telfer, Tanami and Pine Creek). The association of Au-Cu mineralisation with reduced I-type magmas at Bald Hill, and at the nearby Brown's Creek Au-Cu deposit (Kjolle et al., 1994; Wilkins, 1997), suggests that exploration strategies for Au-Cu mineralisation in the Lachlan Fold Belt should consider targeting Silurian I-type magmas, regardless of whether they are of reduced or oxidised character. REFERENCES Kjolle, I., Walshe, J.L., Surman, J.A. & Whitford, D.J., 1994. Gold recycled from Ordovician shoshonites in the Brown's Creek skam deposit, N.S.W. Geol Soc. Aust, Abs., 37, 221. Wilkins, C., 1997. Structural control of the Brown's Creek Au-Cu skam deposit, Blayney, NSW. Geol. Soc. Aust. Abs. 44, 72. Wybom, L., Budd, A.R., & Bastrakova, I.V., In press. Australian Proterozoic granite-related ore systems. Geol. Soc. Aust. 14th AGC (Abstract). AcknQwledgement? The authors wish to thank Hargraves Resources and the Executive Director of AGSO for permission to publish.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
MODELLING METHANE ADSORPTION ISOTHERMS OF BOWEN BASIN COALS C. Laxminaravana and Peter J. Crosdale Coalseam Gas Research Institute, School of Earth Sciences, James Cook University, Townsville, Queensland, Australia- 4811. Email: sci-cl@jcu.edu.au
Methane adsorption isotherm determination for coal is important in estimating coalbed methane recoverable resources and determining producibility. The Langmuir monolayer adsorption model is usually applied to the adsorption data because coals exhibit a type I isotherm, the model closely describes the data and it is simple to apply. However, coals are known to be microporous solids. Microporous materials probably do not support monolayer adsorption and may be characterised by pore filling processes. Methane adsorption isotherms from a series of coals from the Bowen Basin have been evaluated to compare the relative merits of multilayer and pore filling models. Methane adsorption isotherms were determined on 13 Bowen Basin coals at 23.5®C using a microbalance technique. Samples were hand picked for bright and dull coal pairs and analysed on a dry basis. Coal rank varied from medium volatile bituminous to anthracite. Adsorption isotherm results were modelled using four different techniques viz Langmuir (monolayer theory), Brunauer, Emmett and Teller (BET) (multilayer theory), Dubinin-Radushkevich (D-R) (pore volume filling theory) and Dubinin-Astakhov (D-A) (optimised pore volume filling theory). Characteristic parameters derived for each technique are: Langmuir gives the monolayer adsorption capacity (VJ; BET gives multilayer adsorption capacity (VJ; D-R gives the micropore volume VQ; D-A gives the micropore volume (VO) and a factor relating to the pore size distribution (n). All four models give good fits to the experimental data, with relative errors less than ± 1.5%. Langmuir model estimates higher adsorption capacities, up to 1.5%, than the actual adsorbed volumes in all cases. BET model estimates higher volumes, up to 0.5%, in the medium volatile rank coals and lower estimates, up to 1.1%, in low volatile to anthracite ranks. In the case of the D-R and D-A models, estimates of adsorption capacities are up to 0.005% lower than the actual adsorbed volumes in all cases. Comparison of the model parameters VL, VQ (D-R) and Vo (D-A) shows that VL, and Vo (D-R) are highly correlated with each other, with correlation coefficients exceeding 0.96. These three sets of coefficients are all derived from models which assume a pore distribution. V© (D-A) correlated less strongly with the other parameters, with a correlation coefficients between 0.52 and 0.70. The D-A model adjusts a pore size distribution parameter (n) in the optimisation process. The value of 'n' for Bowen Basin coals varies between 1.1 and 1.8 and shows generally increases with increasing rank. In isorank pairs of bright and dull coals, value of 'n' is higher for bright (vitrinite-rich) coals than dull (inertinite-rich) coals in most cases which could be related to microporosity of coal. For Bowen Basin coals, better models are available than the Langmuir model for characterising the adsorption isotherms, namely, the Dubinin-Astakhov optimised pore volume filling model. Langmuir model for adsorption data is not the best model in estimating the recoverable reserves as it may lead to 1.5% error.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
INTERNET VIRTUAL REALITY TOOLS: A NEW PARADIGM UNFOLDS FOR EXPLORATION AND MINING. G LeBlanc Smith & C Cans CSIRO - Exploration and Mining, 2643 Moggill Road, Pinjarra Hills, Brisbane, Queensland 4069. Email: g.leblanc-smith@dem.csiro.au and c.caris@dem.csiro.au
CSIRO Exploration and Mining has successfully demonstrated a powerful interactive visual (multimedia) communications technology for the integration and display of disparate data in a space-time volume called a "virtual world". This type of virtual world environment is accessible across the Internet and can be experienced and shared between many users. Symbolic representations of mining and exploration data, that are true to scale and time, can be crafted into what are termed "virtual mine" and "virtual exploration" environments using the new ISO-standard Virtual Reality Modelling Language (VRML). These virtual worlds are displayed and shared across the Internet using an intemet browser program called Netscape. User inter-activity with the objects in the virtual world is provided through Java program applets attached to objects in the virtual world. These worlds can include representations for equipment and people. Observing and tracking the position of objects like vehicles and drill-heads (GPS and video) is possible in this multi-threaded process. These virtual mine and exploration worlds run primarily on Silicon Graphics workstations and high-end WinNT PCs, and they can be linked via the intemet to extemal databases. In this way the exploration and mine information can be shared across a network that links people from different disciplines and different locations to a single virtual world. Users can "fly" through the virtual world and input and retrieve information by "touching" objects in a ceertain way. The visualisation software is largely standardised, cheap, and globally accessible. Proof of concept interactive virtual-world demonstrations were completed in 1997 using disparate data from Callide Coalfields Pty Ltd -Trap Gully opencut mine, and BHP Coal - Appin Colliery underground mine. Data types include: various digital terrain models rendered with orthoimages (terrestrial and aerial photogrammetry), drilling, analytical, survey, seismic in 2D, 3D and 4D (micro-), video (blasting), sound, proximity, positioning (GPS), geology, environment, ventilation, stress, mine-workings and infrastructure, and plain text. Data at all scales can be represented in a single virtual world and ranges from the microscopic to the global. This virtual mine technology is particularly effective for both visualising and comparative auditing of disparate spatial data and this includes mine surveying and mapping data. Soft photogrammetry digital mapping (aerial and terrestrial) forms a basis for visualising mining highwall structure and rock geometry, mine development, and environment monitoring. The "virtual world" is an additional tool that will more effectively integrate the output from applications software packages. It does not replace them, but rather enhances them, because it can add to the speed and effectiveness in turning data into information. More effective integration will occur when the principal exploration and mining software packages provide a VRML formatted export for their results. This is a major step for exploration and mine information systems, as for the first time it provides the ability to visually access information from differing software packages and databases and display them in a "virtual world" in full 3D/4D spatial relationship to each other in near real time. Many issues still require attention and include: the handling of streamed data, data volumes and data decimation to manageable sizes, cross-platform inconsistencies and speed of Java, integration of intemet relay chat (ire), methods for optimising 3D and 4D, multi-user avtars, immersive representations, symbolism, simulation, education and training. The need to explore and mine smarter has never been so clear. The trend towards automation continues, not only for mining and exploration equipment, but for rapid data collection, processing, visualisation and interpretation. A new paradigm for exploration and mining is created through using vitmal reality tools and the intemet. A major opportunity exists to refine and implement this new technology that will revolutionise the mining and exploration business. Will people in the minerals industry provide the leadership, support and fimding required to research and implement this powerful new technology?
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
MULTIPLE INVERSIONS, STAIRCASE GEOMETRIES, MILLIPEDE SHAPES AND TRUNCATION OF FABRICS IN PORPHYROBLASTS AND MATRIX: DO ALL THESE FEATURES INDICATE RELICS OF ADDITIONAL DEFORMATION OR ARE SOME CAUSED BY A VARYING STRAIN TRAJECTORY IN 3 DIMENSIONS? Hyun-Woo Lee School of Earth Sciences, James Cook University, Townsville, Queensland 4811
When dealing with thousands of petrographic sections of deformed rocks, we may confront geometries additional to those generally preserved. As microstructural geologists interested in foliation development we try to determine the significance of every single deflection of a foliation preserved in porphyroblasts and the matrix. It is only when we can explain with confidence all microstructures present that we can be sure we have gained an understanding of the tectonic histories preserved. Recent researchers have suggested heterogeneities of a stress and strain in three dimensions can cause traces of foliations projected onto a plane, such as a map, to not represent the true pattern or nature of the deformation. This should also apply in section and as a result, fabrics overgrown synkinematically in strain shadows of porphyroblasts may not reflect real structures. Consequently, if one worked with just 1 or 2 planar sections, there is potential for error although working with multiple sections should eliminate this possibility. Up to 10 radially oriented vertical petrographic thin sections and a horizontal section were made from each rock sample from the Ogcheon Orogenic Belt, in South Korea, which has undergone four ductile deformation events. Most of the garnet porphyroblasts in mica schists along the orogen preserve inclusion trails of the core continuous with the matrix fabrics in three dimensions. This made it easy to follow the three dimensional trajectories of the structural fabrics. However, many garnets contain multiple shear sense inversion, staircase geometries and structural truncations, in certain orientations of thin sections while only one pair of spiral trails is observed in other thin sections from the same rock. These geometries and their larger scale shapes in 3-D are used to interpret the deformation history of the rock and are explained in terms of heterogeneous strain posed upon the existing matrix fabrics around a porphyroblast. The closer the matrix to the porphyroblast, the greater protection from subsequent shear strain provided. Consequently, the foliation, or micro fold axes are progressively rotated away from the porphyroblast margin towards the nearest microshear plane - this region is called a "strain transition zone". Microfold hinges are also inclusive to this category. This can cause more than one pair of shear senses to be revealed in inclusion trail geometries in some cross sections for one deformational event, even though there has only been one shear sense. Similarly, local apparent tmncational microstructures of fabrics can develop where the shear sense reverses. However, these structures, where the shear senses abruptly switch, are the result of a cut effect rather than an additional deformation event. Comparable microstructures can also result from the differential growth of some porphyroblast crystal faces relative to the developing matrix foliation; for example, porphyroblasts tend to grow more into their strain shadows. The fabrics outside the porphyroblast are exposed to subsequent deformation and may be incorporated at a later stage. Consequently, we can expect primary and secondary deflections of foliation in the porphyroblasts. Apparent millipede inlcusion trail geometries in some samples developed as secondary deflections rather than as primary structures. Criteria for recognition of these phenomena are essential for inclusion trail studies and are presented to aid future workers in this field.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
AIRBORNE EM TO FOCUS RECONNAISSANCE DRILLING FOR GOLD EXPLORATION Prue Leeming^ Daniel SatteP, Steve Massey^ ^World Geoscience Corporation Ltd ^ Placer Exploration
Gold exploration relies on extensive RAB and air-core drilling to map and sample regolith cover, depth to bedrock, bedrock lithology, structure, alteration and mineralisation. High bandwidth airborne electromagnetic (AEM) data using the QUESTEM lOOSW system were collected over several knovm gold deposits in the Laverton area, including Sunrise-Cleo, Keringal, Golden Delicious, Beasley Creek and Jupiter. The data were used to map the regolith, estimate depth to bedrock, assist a regional litho-structural interpretation of basement geology based on magnetic data and identify the AEM response associated with known gold mineralisation. Several areas were targeted for future exploration drilling. AEM interpretation by geoscientists is easiest when based on products generated by layered earth inversion techniques (Sattel, 1997). A three-layer model with a conductive layer sandwiched between two resistive layers was used in this study. This model would be applicable to large areas in the Eastern Goldfields. Layer 1, a thin resistive surficial layer represents hardpan, alluvium and colluvium. Layer 2 represents the intervening transported and saprolitic clay and layer 3 is the resistive basement. Inversion models do not perform as well in areas of extreme (high or low) total conductance. However, the interpretation methodology, which uses several other regional data sets can readily identify the limitations of the inversion models. The products used in systematic interpretation of AEM data mclude images of layer 2 conductance (conductivity X thickness), bedrock topography, regolith thickness and digital topography. Sections depicting variation of conductivity with depth clearly show the influence of penetrative basement structures on regolith. Features such as inversion of topography, previous drainage divides, areas of shallow regolith cover, palaeochannels and palaeolacustrine envu-onments were identified in the Laverton study. Bedrock topography permits modelling of a buried landscape, and thereby can predict groundwater movement, palaeodrainage, depth of cover, drilling conditions, and the geochemical sampling environment. Conductance data are useful for mapping hypersaline, saline and brackish groundwater conditions, and bedrock conductors. Known gold deposits are often associated with bedrock topography highs or lows. Sunrise-Cleo lies on a south and westward draining palaeochannel, east of the hypersaline lacustrine sediments of Lake Carey. Supergene gold in transported materials is best developed approximately 100 meters upstream from the main supergene and primary gold zone, and appears to occur at a hypersaline-brackish groundwater interface. Residual supergene and primary gold mineralisation is hosted by complexly sheared intermediate volcanoclastics and BIF. Golden Delicious, Pink L.ady, Voodoo and Black Magic, which are prospects to the northeast of Sunrise form discrete basement highs within thick and highly conductive regolith. In each case, gold mineralisation is associated with silicification, haematite and/or sericite alteration in and around granitoid bodies and porphyry intruding intermediate volcanics. Furthermore, thickenmg of the regolith due to preferential weathering along shear zones, such as Keringal, Beasley Creek and Jupiter where argillic, chloritic, and potassic alteration may accompany gold mineralisation has also been identified. Drilling evidence is presented against conductivity-depth images for comparison and plans of depth to fresh rock are compared with bedrock topography in the Sunrise-Cleo area. Contrasting conductivity of host rock and/or alteration lithologies with country rock is required for successful application of AEM. REFERENCES Sattel D. 1997. Conductivity information in three-dimensions. (In press).
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
THRUST AND LOW ANGLE NORMAL FAULTING IN THE EASTERN TAMWORTH BELT, NORTHEASTERN NEW SOUTH WALES Evan C. Leitch Faculty of Science, University of Technology, Sydney, Broadway, New South Wales 2007
Most faults in the eastern part of the Palaeozoic Tamworth Belt of northeastern New South Wales dip steeply. It is here argued that this was not the original attitude of all these structure but that both pre-orogenic low angle normal faults and early orogenic thrusts can be identified. These were rotated at a late stage during contractional deformation into their present orientation. Major changes in Emsian - Eifelian succession in the eastern part of the Tamworth Beh between Attunga and Nundle occur across major faults that are generally assxmied to have formed relatively late in the defonnational episode that affected this region in the Late Permian-Triassic (Hunter-Bowen Orogeny). Detailed mapping indicates that these faults are now relatively steeply dipping and strike parallel to the overall strike of the Devonian rocks which are likewise steeply dipping. Thus an alternative interpretation of the faults is that they were active in the early stages of orogenic contraction, prior to major rotation of the beds, and that their present attitude is a resuk of subsequent folding that also produced the steep dips in the Devonian rocks. The faults were thus originally low angle structures and the contrasting facies juxtaposed across them is the result of major telescoping of basinal fill. Evidence in support of early thrusting in the eastern Tamworth Belt is present between Attunga and Manilla where stratigraphically lower limestone units structurally overlie younger clastic rocks on a folded contact that is interpreted as a major contractional fault. Here the base of Ae limestones here probably coincides with a major decollement, the presence of which would account for otherwise anomalous structural relationships and would explain why horizons stratigraphically lower than the limestones are not observed. In the upper reaches of the Barnard River south of Nundle evidence for low-angle faulting has also been discovered, although here it is considered to be pre-Hunter-Bowen and of extensional character. In this region the Gleneden Serpentinite occupies the core of a tight macroscopic anticline well defined by reversals in younging and dip in Early Devonian rocks. The anticline is an upright, NNW trending horizontal fold broken by a pair of similarly striking faults between which Early Permian clastic sedimentary rocks have been inserted into the Devonian section. Early Permian siltstones have a well developed anastomosing spaced cleavage parallel to the axial surface of the anticline. To the east the Early Permian strata unconformably overlie Devonian rocks in the fault-disrupted core of a syncline. Further east Early Permian strata yoimg westward and rest unconformably on sheared middle Palaeozoic chert and siliceous siltstone. Restoration of the Early Permian relationship of the rock units indicates that the Gleneden Serpentinite was a slightly transgressive sheet bounded above and below by low dipping faults. The upper fault is now represented by the serpentinite contacts on either limb of the anticline. The inferred lower fault separates the serpentinite from structurally imderlying chert and associates. This fault movement is considered to have occurred just before the onset of Early Permian sedimentation and is viewed as intimately linked to the formation of the Bamard Basin in which Early Permian deposition proceeded. Both exhumation of the serpentinite and basin subsidence were the product of major extension that was relieved by the formation of low angle normal faults.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
CONSTRAINTS ON TECTONIC MODELLING IN THE NORTHERN LACHLAN FOLD BELT FROM GRANITES AND COUNTRY ROCK DEFORMATION. Paul Lennox, Robert Trzebski and Derecke Palmer^ 'Department of Applied Geology, UNSW, Sydney 2052
The relationship between regional deformation, granite-country rock structuring and the three dimensional shape of these granites provides important constraints in developing tectonic models in the northern Lachlan Fold Belt (NLFB). Existing tectonic models for the NLFB rely on either pure extension or oblique extension in a northeastsouthwest direction during the Silurian to form a series of meridional basins and highs followed by late Middle Devonian, east-west compression to develop the dominant meridional structural grain and Early Carboniferous north-south shortening of varying intensity to gently fold or megakink the sequence. The lower Palaeozoic tectonic setting of this region has been suggested to be an single island arc or arcs outboard of a emergent Delamerian Fold Belt to the west, or a back arc or intra-arc setting with active subduction occurring to the east and west in some recent models. This study focusses on the north-south trending, multiply deformed Molong High and the adjacent overall meridional Hill End Trough. In detail the boimdary between these blocks varies from a almost north-south oriented single fault or braided package of faults to one with northeast- and northwest-trending segments. The west-dipping, often braided Copperhannia Thrust separates the Molong High and Hill End Trough south of the Bathurst Granite and an east-dipping duplex separates the blocks to the north. The three granites which have been studied on the Molong High lie across this region of changing basement character. The Carcoar and Barry Granodiorites are weakly deformed, enclave-containing, I-type granites north of the moderately deformed, metasedimentaiy xenolith-containing S-type Sunset Hills Granite. The northern most body the Carcoar Granodiorite is a blocky body bounded by the Carcoar Fault on the west and Ammerdown Fault on the east. The hour-glass shaped, north-south elongated Bany and Sunset Hills granites both lie west and parallel to the concave-shaped extension to the Ammerdown Fault and nearby Copperhannia Thrust. Lennox et al. (in press) suggest these three granites intruded a pull-apart developed between the forerunner of the Carcoar Fault to the northwest and more southeasterly forerunner of the Copperhannia Thrust during the opening of the Silurian Hill End Trough. Northwest- and northeast- trending shear zones identified in these granites may have aided the Siluro-Devonian intrusion and then late Mid Devonian and Early Carboniferous deformation d these granites. The blocky Carcoar Granodiorite appears to be bounded by major faults and have a root zone to the southwest consistent with its emplacement at the leading edge of a south-ward propagating, meridional fault which became the Carcoar Fauh. Significant dextral, strike-slip movement on the extension of this structure further south is reflected in the Browns Creek Mine (Wilkins, this volume). The Barry and Sunset Hills granites appear to have smaller root zones and are bounded on their eastern-sides by west-dipping faults in a similar manner to the Wyangala Batholith. Existing tectonic models assume a meridional shape to the blocks developing in the Early Silurian. If the Early Silurian Hill End Trough opened obliquely and re-activated existing structures it is feasible to envisage a trough developing with a concavo-convex shape consisting of northwest-, northeast- and north-trending steps. The presence of northeast- and northwest-trending shear zones or megakinks in this region may reflect re-activation of existing Ordovician basement structures. The Copperhannia Thrust which is envisaged by recent tectonic models as a major through-going basement feature may be composite and consist of meridional and oblique components perhaps reflecting a earlier history during its growth as a major strike-slip fault. The fault complex north of Bathurst may represent an entirely different strike-slip fault which grew southwards towards the northward-propagating proto-Copperhannia strike-slip fault. REFERENCE Lennox P.G. Fowler T.J. & Foster D. (in press). The Barry Granodiorite and Sunset Hills Granite: Wyangalastyle intrusion at the margin of a regional ductile shear zone. Australian Journal of Earth Sciences 45. Acknowledgements: Hargraves Resources NL are thanked for supportmg our research in the northern Lachlan Fold Belt. Mr R. Cotton and Mr J. Graham have encouraged and assisted our understanding of the Carcoar area.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
A SEISMIC MODEL OF THE CRUST THROUGH THE BROKEN HILL BLOCK AND TASMAN LINE J. H. Leven, D. M. Finlavson. A. Owen, and D. W. Johnstone Australian Geodynamic Cooperative Research Centre (AGCRC) Australian Geological Survey Organisation (AGSO) GPO Box 378 Canberra ACT 2601
INTRODUCTION The Broken Hill Block lies just inboard of the late Neoproterozoic continental rift margin and hosts one of the world's major mineral deposits. In 1997, the Australian Geodynamics Cooperative Research Centre undertook a wide angle refraction survey across the Broken Hill Block and Tasman Line (Darling Lineament) to: • investigate the crustal structure beneath the Broken Hill Block and Tasman Line, • determine if a mid-crustal mafic body had sourced the prominent amphibolite dykes within the Broken Hill Block, and • compare the structure of the Broken Hill Block with the Mt Isa Block. METHOD Eighty six Seismic Group Recorders (SGRs) were deployed along a NW-SE oriented profile extending over 350 km through the Broken Hill Block. The station spacing variedfrom2.5 km over the Broken Hill Block to 10 km near both ends of the profile. The seismic signals were detected by 2 Hz geophones buried beside the SGR recorders. The geophone output was recorded digitally on a tape drive in each instrument during pre-programmed time windows, and shots were fired within these time windows. Six shots were detonated during this program. Two 3000 kg shots (1 and 6) were positioned at the NW and SE ends of the profile to provide wide-angle data to offsets in excess of 300 km, and four intermediate 1000 kg shots (2 to 5) positioned along the profile to provide additional control on the mid-crustal seismic structure. RESULTS • Two layer crust: upper layer 5.9 km s"^ to 6.4 km s'^ with variable thickness ranging between 20 and 30 km. lower layer 6.8 km s'^ to 7.2 km s'^ of relatively constant thickness of around 14 km. • Moho depth ranges from 35 km on either side to 43 km beneath the Broken Hill Block. Sub-Moho Pvelocity of 8.1 km s'\ • No evidence for a mid-crustal body with high P-velocity beneath the Broken Hill Block which might have sourced the mafic dykes. • Thickening of the upper crustal layer beneath the Broken Hill Block by 50%, with the lower crustal layer maintaining a thickness of ca. 14 km. • Low seismic velocities in the upper crust associated with the Menindee and Blantyre Troughs, in the region of the Darling Lineament. • Relatively low crustal P-velocities compared with those of eastern Australia. No evidence of an underplated lower crustal layer with P velocities around 7.7 km s \
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE NATURE OF THE TASMAN LINE SOUTHEAST OF THE BROKEN HILL BLOCK J. H. Leven^ T. Fomin, A. Owen, D. W. Johnstone, B. J. Dnimmond, K. D. Wake-Dyster, and D. M. Finlavson Australian Geodynamics Cooperative Research Centre Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601
The Tasman Line is defined as the boundary between outcrops of Precambrian cmstal elements to the west and Palaeozoic cmstal elements to the east. It separates the older Mt Isa and Broken Hill Blocks from the younger Lachlan and Thomson Fold Belts. In the region southeast of Broken Hill, it is marked by NE-SW trending gravity and magnetic anomalies, and a topographic feature bounded by the course of the Darling River. In 1997 the acquisition of reflection seismic data by the Australian Geodynamics Cooperative Research Centre (AGCRC) and the NSW Department of Mineral Resources was completed across the Tasman Line to the east of the Broken Hill Block, which supplemented the 1996 seismic acquisition of the NSW Government Broken Hill Exploration Initiative across the Broken Hill Block, and the AGCRC across the Tasman Line. Additionally, the AGCRC acquired a regional-scale wide-angle profile trending NW-SE through the Broken Hill Block and the Tasman Line (Leven et al. 1998). Interpretation of the wide-angle seismic data shows the thickening of the upper crust beneath the Broken Hill Block, a lower crust with a relatively constant thickness along the profile, and a region of low velocity in the upper crust beneath the Menindee and Blantyre Troughs of the Darling Basin. The most prominent feature on the reflection seismic data across the Broken Hill Block is the southeast dipping events. These have been interpreted as shear zones; some events can be directly correlated with particular surface-mapped features, such as the Mundi Mundi Fault. Fomin et al. (1998) have interpreted this reflection seismic data to indicate several different levels of sub-horizontal detachments within the crust at -10 and -18-24 km depth. The Moho is not well defined beneath the Broken Hill Block on these reflection data. East of the Broken Hill Block the reflection seismic data shows a series of troughs containing Darling Basin sediments beneath a relatively undeformed blanket of Murray Basin sediments. The Mendinee Trough has a deformed western edge, in which the upper succession has been homoclinally upturned and the lower succession has been truncated, producing a structure analogous to that seen on the northern margin of the Officer Basin (Leven & Lindsay, 1995). This structural analogy and the position of the inflection point of the gravity data suggest the presence of a triangle zone of low-density material to the west of the Menindee Trough in front of a southwest directed thrust sheet, corresponding to the Broken Hill Block. However, unlike the Officer Basin profile, these seismic data do not clearly image the detachment surface at the base of this thrust sheet. The Blantyre Trough has a similarly structured western edge, and a series of west dipping (-17°) reflection events appears to image the associated thrust. REFERENCES Fomin, T., Gibson, G. M., Owen, A., Drummond, B. J., Wake-Dyster, K. D., Maidment, D., 1998. Deep seismic profiling in the Broken Hill region: implications for structural geology in the Willyama Supergroup. Australian Geological Survey Organisation Record 1998/2. Leven,. J. H., & Lindsay, J. F., 1995. A geophysical investigation of the southern margin of the Musgrave Block, South Australia. AGSO Joum. Aust. Geol. & Geophys., 16,155-161. Leven, J. H., Finlayson, D. M., Owen, A., Johnstone D. W., 1998. A seismic model of the crust through the Broken Hill Block and Tasman Line, australian Geological Survey Organisation Record 1998/2.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
MULTIPLE OROGENESIS AND METAMORPHISM OF THE MIDPROTEROZOIC SOLDIERS CAP GROUP, EASTERN FOLD BELT, MOUNT ISA INLIER K^thryn J. Uw^hwaite James Cook University, Townsville, Queensland 4811
It is generally accepted that the mid-Proterozoic rocks of the Mount Isa Inlier underwent a period of orogenesis, termed the Isan Orogeny, between 1590 and 1500 Ma. Previous workers distinguished three regionally significant deformational events that occurred during the Isan Orogeny: Dj, a period of N-S shortening, and two periods of near E-W shortening, Dj and D3. However, integrated study of macroscopic and microscopic structures from the mid-Proterozoic Soldiers Cap Group (1677 Ma), SE of Cloncurry, suggests that the Isan Orogeny constitutes two periods of orogenesis and metamorphism, one dominated by N-S shortening followed by one dominated by E-W shortening. The first orogeny recorded in the Soldiers Cap Group schists SE of Cloncurry, was a period of N-S shortening which comprised four deformational events, D1-D4. These deformations produced four temporally distinct foliations, S1-S4. Stretching lineations on S2, S3 and S4 are N/NE trending and the sense of movement during D3, which produced shallowly-dipping structures, was top-to-the-SW. The ESE-trending folds which dominate the Gilded Rose area SE of Cloncurry, including the Toole Creek Syncline, are structures formed during the fourth deformation of the N-S shortening. The N-S shortening orogeny was accompanied by a period of metamorphism which initiated garnet and chloritoid porphyroblast growth. The second orogeny was dominated by two periods of E-W shortening separated by an enigmatic deformation that produced shallowly to moderately-dipping structures. The first period of E-W shortening can be equated with D2 of previous workers, and produced the NNW-trending Snake Creek Anticline and N/NE trending folds that overprint structures formed during the older N-S shortening orogeny in the Gilded Rose area. Axial planar structures of the Snake Creek Anticline were then reorientated by a deformation marked by the development of crenulations and folds with ESE-striking shallowly to moderately dipping axial planes. This deformation, intermediate to Dj and D3 of previous workers, was characterised by strain thai was hetergeneously distributed and affected the northern-most and southern-most outcrops of the anticline by rotating axial planar structures into E-W orientations. The second period of near E-W shortening, which can be equated with D3 of previous workers, typically reactivated regional Dj structures and produced composite D2-D3 structures. However, D3 also produced NW striking crenulations where Dj structures were reoriented by the deformation intermediate to D2 and D3. The second period of orogenesis recorded in the Soldiers Cap Group was accompanied by the growth of cordierite, garnet, andalusite, staurolite, sillimanite and kyanite porphyroblasts. Peak metamorphism in the Snake Creek area was marked by the growth of sillimanite after the development of D2 structures and before regional D3. In the Gilded Rose area, staurolite and andalusite growth occurred after the development of D2 crenulations. The recognition of two temporally and metamorphically distinct periods of orogenesis during what has previously been known as the Isan Orogeny has significant implications. In particular, multiple periods of deformation during the N-S shortening orogeny, and the recognition of a deformational event intermediate between Dj and D3 of previous workers, provides a new framework in which regional structures should be examined.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
EXPLORATION FOR BROKEN HILL STYLE MINERALIZATION, AN EVOLVING SPECTRUM Wolfgang R. Leyh, Eaglehawk Geological Consulting Pty Ltd, PO Box 965, BROKEN HILL, NSW, 2880.
The Cumamona Craton containing the Broken Hill PbAgZn orebodies is proving to be a highly prospective multicommodity metallogenic terrain. Enormous exploration potential exists for the classical Broken Hill "type" and for a much wider range of Broken Hill "styles", in particular for large v^orld class Cu+/-Au and PbAgZn+/Cu deposits; these are likely to include stratiform, stratabound and replacement variations on the Broken Hill theme. The imminent likelyhood of a major discovery is exemplified by the identification of numerous significant prospects in the region, particularly over the last 20 years since persistent multimillion dollar exploration programmes have moved into the vast tracts of previously under explored soil cover concealing the majority of the craton. Major regional prospects include Polygonum PbAgZn, Grid 4/Ardeetoo CuAu, Thunderdome PbAgZn on the Mundi Mundi Plains in NSW and Kalabity-Telechie CuZn(PbAg), Hunters Dam ZnPbAg, Meningie Well ZnCu, White Dam CuAu, Kalkaroo CuAu, plus the Benagerie Prospects CuAu, PbZnAg in SA. Small economic and subeconomic deposits include Potosi Zn(PbAg), Silver Peak PbAg(Zn), Flying Doctor PbAgZn, White Leeds PbAgZn on the Broken Hill Line of Lode plus Pinnacles ZnAgPb(Au), Copper Blow CuAu, Mutooroo Cu, Dome Rock Cu(Co) and Big Hill / Pyrite Hill Co in the district. More than 150 significantly encouraging but smaller prospects have also been tested with varying degrees of intensity including diamond drilling throughout the outcropping Broken Hill and Olary Blocks. In addition, several thousand smaller occurrences commonly including anomalous PbAgZnCuAuCoWMoSn are known. These prospects are mostly broadly stratigraphically controlled within the Willyama Supergroup and together with Broken Hill constitute a much wider spectrum of Broken Hill Styles. The stratigraphy was originally defmed by the Geological Survey of NSW following high quality lithologically consistent regional mapping in the Broken Hill Block. Mineralization styles have evolved throughout deposition, diagenesis, pro and retrograde metamorphism, complex multistage deformation and accompanying intermittent acid to basic intrusive events. Mineralization is most closely associated with specific iron formations or unusually iron rich metasediments, a range of suspected chemical sediments, calcsilicates, amphibolites and certain quartzofeldspathic gneisses. It is often found in associated metasedimentary sequences. Mineralization is recognised in a wide range of variably magnetic hosts, and more specific lode rock packages associated with a corresponding wide range of alteration features. However, most styles of mineralization appear fundamentally related and at least originally stratigraphically controlled because of links established through the empirically derived iron formation/chemical sedimentary model. In addition there is a demonstrable stratigraphic control on regional zoning ranging from Cu+/-Co+/Mo+/-Au at the base to Pb+/-Ag+/-Zn+/-W+/-Sn towards the top of the Willyama Supergroup. Effective geological exploration for Broken Hill Styles both in the Cumamona Craton and elsewhere in the Proterozoic requires detailed understanding of:- (1) Premetamorphic depositional framework including stratigraphic associations in particular critical facies changes near mineralization (2) Possible overprinting/recycling effects during diagenesis, basinal compaction and tectonism (3) Evolving metasomatic processes during metamorphism and on going intrusion (4) Often extreme metamorphic and structural modification over an extended time frame (5) The role of nearby intrusive heat sources and shears probably generating mixed fluid flow systems leading to variable redistribution, overprinting and replacement of primary stratiform mineralization. Because of the wide variety of Broken Hill styles present in an evolving spectrum, a simplistic comparative model driven exploration approach based on perhaps even a very detailed understanding of only a small part of the spectrum in the Willyama Supergroup or elsewhere in the Proterozoic is doomed to repetitive failure at prospect level, particularly when combined with an excessive drill hole spacing. A purely empirical approach makes no assumptions, but recognises the frequency of occurrence of numerous persistent factual associations in relation to concentration of mineralization. It does this in a wide range of situations and tests these against each newly located target zone modifying the drill target accordingly. It is therefore more likely to succeed but is still subject to excessive drill hole spacing in the majority of prospects explored to date. Hence the lack of exploration success since Broken Hill was first discovered in 1883.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
TESTING THE SWEAT CONNECTION: PRELIMINARY PALAEOMAGNETIC RESULTS FROM THE -«00 MA AMATA DYKE SWARM, MUSGRAVE BLOCK, AND THE WALSH TILLITE, THE KIMBERLEY Zheng-Xiang Li^ Alfredo Camacho^ and David Ellis^ ^Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia Nedlands, WA 6907, Australia ^Research School of Earth Sciences, The Australian National University, ACT 0200, Australia ^Department of Geology, The Australian National University, ACT 0200, Australia
Palaeomagnetism holds the key for testing the validity of Ihe largely geologically-derived Rodinia hypothesis far the late Precambrian. At the core of Rodinia is the so-called SWEAT (South-West Us—East Antarctica) connection (Moores, 1991). A previous attempt (Powell et al., 1993) showed that positions of the ca. 1054 Ma and 720 Ma polesfix>mAustralia are compatible with coeval polesfix>inLaurentia. However, the conclusiveness of such a test was hampered by the lack of data from Australia for the interval between ca. 1000 Ma and ca. 720 Ma. This p^)er report oin* preliminary palaeomagnetic results from both the ca. 800 Ma Amata dyke swarm (or the Amata Suite) in the Musgrave Block of Central Australia, and the cap dolomite above the Neoproterozoic Walsh Tillite in the Kimberley, north-western Australia. We will then examine the SWEAT connection in light of these results. The Amata Suite trends rougjily ENE in central Musgrave Inlier. It has been interpreted as an extension of the NW trending Gairdner Dyke Swarm, and Sm-Nd dating gives ages of 790 ± 40 Ma and 797 ± 49 Ma (Zhao and McCulloch, 1993). Forty-eight block samples were collected from 11 dykes. However, about half of the samples has a tectonically developed magnetic Mfdc which deflected the remanent direction towards the magnetic foliation plane. Twenty remainder samples from six dykes revealed a possibly primary magnetic remanence which was carried by both magnetite and hematite, and with dual-polarities. The mean direction gives a palaeopole at (20®S, 233®E) with dp = 8®, dm = 15®, which is distinct from known palaeomagnetic poles of younger than 800 Ma from Australia. Further sampling is being planned to improve the quality of the data. A primary magnetic remanence, supported by a positive fold test and an unconformity test, is obtained from five sampling sites in the Neoproterozoic Walsh Tillite of the Mt House region, Kimberley, north-western Australia. A palaeomagnetic pole at (21.5®S, 282.4®E) with dp = 12.2® and dm = 15.4® differs significantlyfix)mthat of the ca. 600 Ma Marinoan glacial deposits in the Adelaide Fold Belt, with which the Walsh Tillite was previously correlated based on lithostratigraphy (Coats and Preiss, 1980). Instead, the new pole coincides with the ca. 720 Ma poles from igneous rocks of both Western Australia and India (after being rotated to Australia), and thus supports a Sturtian age fijr the Walsh Tillite (Dow and Gemuts, 1969). The result gives a palaeolatitude of 45® ± 12®N fcr southern Kimberley, and ca. 25® ± 12®N for the Sturtian glaciation in the Adelaide Fold Beh. Our new palaeomagnetic poles from Australia, when combined with existing poles, suggest an early Neoproterozoic loop in the Australian appaimt polar wander path which is broadly compa^le with that of Laurentia, and thus reaffirms the SWEAT connection fortiieearly Neoproterozoic time. REFERENCES Coats R. P. & Preiss W. V. 1980. Stratigraphic and geochronological reinterpretation of Late Proterozoic glaciogenic sequences in the Kimberley region. Western Australia. Precamb. Res. 13,181-208. Dow D. B. & Gemuts 1. 1969. Geology of the Kimberley Region, Western Australia: The East Kimberley. West. Aust. Geol. Sur. Bull 120. Moores E. M., 1991. Southwest U.S. - East Antarctic (SWEAT) connection: A hypothesis. Geolo^ 19, 425-428. Powell C. McA., Li Z. X., McElhinny M. W., Meert J. G., & Park J. K. 1993. Paleomagnetic constraints on timing of the Neoproterozoic breakup of Rodinia and the Cambrian formation of Gondwanaland. Geology 21, 889892. Zhao J-x. & McCulloch M. T. 1993. Sm-Nd mineral isochron ages of Late Proterozoic dyke swarms in Australia: evidence for two distinctive events of mafic magmatism and crustal extension. Chem. Geol 109,341-354.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
STEINWAY . SURFACE EXPRESSION OF DEEPLY BURIED GOLD MINERALISATION OR A "FALSE" ANOMALY? M.J. Lintem Cooperative Research Centre for Landscape Evolution and Mineral Exploration, CSIRO Exploration and Mining, c/o PIRSA, GPO Box 2355, ADELAIDE, SA 5001, Australia.
It is of paramount importance to establish whether relatively inexpensive surficial sampling techniques can be used in lieu of deep drilling for exploration in areas of transported overburden. Many accounts in the open literature and company reports claim that surface sampling can be used successfully to explore in areas with substantial transported overburden. However, independent verification of these case studies by rigorous scientific methods has not been routinely undertaken. This is an unsatisfactory state of affairs since it has left the exploration industry in doubt as to whether they can safely relinquish ground that has no anomalies and is thus supposedly barren. The example of Steinway (Yilgam Craton, Western Australia) is used to show how surface expression to buried Au mineralisation may not be all that it seems. Steinway is a sub-economic Au deposit located 25 km south of Kalgoorlie, WA. The central and northern parts of the area have a variable thickness (>20 m) of transported overburden of presumed Tertiary age consisting of partly consolidated clays, sands and silts. Beneath the transported overburden, saprolite overlies a bedrock of mafic andesites, trachytes, porphyritic tuffs and black shales. There are two types of mineralisation at Steinway: (i) saprolite-hosted supergene mineralisation located at -30 to 40 m and (ii) primary mineralisation associated with quartz stockwork veining within mafic andesites. The Ca- (as calcrete) and Fe-rich soil (0-lm) overlying Steinway is anomalous in Au, with a maximum concentration of 150 ppb compared to a local threshold of 24 ppb. This Au anomaly is one of the strongest in the area that has been drill-tested. Not all the anomalies have proved to have mineralisation beneath them, but a much weaker anomaly to the west. Greenback, does and this has subsequently been mined. A more detailed study of the nature of Au in the surficial material was undertaken to verify whether the Au at the surface was being sourced from the underlying mineralisation or elsewhere. The distribution of gold in the sediments and anomalous soil overlying the mineralization was investigated.. Nearly black, vitreous, sub-rounded ferruginous granules, a few millimetres in size, were found to be abundant in the soil and were separated for detailed study. The principal results are summarised as follows: 1. Gold is associated with both ferruginous granules and calcrete (pedogenic carbonate) in the soil. 2. Some Au in carbonate is highly soluble, and probably present in colloid particles or in a 'chemical' form. 3. Some, but not all, ferruginous granules contain microscopically-visible particulate Au. 4. Gold concentrations of individual, ferruginous granules are extremely variable (<40-15000 ppb). 5. Relict primary fabrics were observed in ferruginous granules. 6. The sediments beneath the soil are essentially barren of gold. These results suggest that the ferruginous granules are the immediate source of Au in the soil, and that both are derived by meciianical dispersion from upslope, rather than Au migrating chemically (into the ferruginous granules) from, the mineralisation below. The relatively-soluble Au in the calcrete is probably derived from either (i) the ferruginous granules, which have weathered and released Au, or (ii) direct chemical dispersion from a similar upslope source as the ferruginous granules themselves. Previous studies have shown that in relict and erosional regimes, sampling of the calcareous horizon may accurately defme drilling targets. However, in depositional regimes, such as Steinway, results mdicate that there may be no causal link with underlying mineralisation and that, in certain depositional areas, sampling of calcareous material, at best, may indicate the potential of the (sub-)catchment. It is suggested, therefore, that for such landscape regimes, wider sampling intervals could be used, with a follow-up requirement that deep samples be collected, including basal sediments and/or ferruginous saprolite. Acknowledgements: This research has been the outcome of productive collaboration between CSIRO and the mineral industry through AMIRA, and the assistance and support of the sponsors of CSIRO-AMIRA Project 409 (1994-1997) and, in particular, Newcrest Ltd, are gratefully acknowledged. CRC LEME is supported by the Australian Cooperative Research Centres Program. C.R.M. Butt and D.J. Gray are thanked for earlier comments on this Abstract.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE APPLICATION OF AIRBORNE HYPERSPECTRAL IMAGERY TO GEOLOGICAL EXPLORATION Paul Linton Geophysical Services Depaitment, Anglo American Corporation of Soutib Afiica, PO Box 61587 Maishalltown 2107 Soufc Afidca
Aiibome Tmaging Spectiometers such as AVIRIS, Geoscan and liie GER have been in operation for several years, althou^ on a limited basis as far as geological e:q)loration is concerned. With advances in technology, these first and second generation instruments are being siq)erseded by new generation systems which provide a means for rapid and accurate geological mapping of large areas. It is envisaged that this technology will become increasingly important for exploration. The new generation aiibome imaging spectrometers typically coflect spectral information in the wavelength range 500 to 2500 nanometres, i.e.fix)mthe visible throu^ to the short wave infra-red. The spectral resolution of such instruments is generally between 10 and 20mn. Some instruments also have the capability to collect data in the thermal infi:a-r^. Spatial resolution varies between three and ten metres. The new generation instruments are more sensitive than their predecessors, and have a high signal to noise ratio. This is particularly important in the short wave infia-red, where energy levels are very low. The increased signal, coiq)led with the high spectral resolution, results in "cleanef spectra which are easy to interpret The iacreased spatial resolution results in smaller pixel area, and therefore the number of "pure" pixels (ie. those that contain only one material) in a flight strip is increased. The reduction of mixed pixels makes classification, and therefore mapping, more accurate. A variety of mineral gmvips display characteristic spectral patterns in the wavelength range covered by these spectrometers. These include a variety of hydrous siHcate minerals, caibonates,su^hates, and iron oxides. Included in this are several minerals and mineral groiq)s that are characteristic of hydrothermal alteration such as pyrophyllite, kaolioite, dickite, micas, chlorites, smectite clays, alunite, jarosite, calcite, dolomite and ankerite. Examples from a number of Australian sites are showrL The data was collected using the HY-MAP (Hyperspectral Mapper) instrument designed and built by Integrated Spectronics Limited of Sydney. These examples illustrate the abihty of airbome imagrng spectrometers for mapping of hthology, alteration, structure andregolith. The experience gained to date has shown t h i the results generated by aiibome spectrometers is of most value when integrated with other geological and geophysical data sets. The ability of these instruments is affected by a number of factors. Caution should be exercised in areas of extensive vegetation cover where the response of rock and soil is muted and diluted by the spectral response of the vegetation. These instruments are passive, Le. they measure reflected solar radiation. Therefore survey flying ^ould be limited to dose to solar noon when the sun's energy is greatest, and shadow is minimised. Qoud cover, and cloud shadows, will degrade flie quality of the data collected. Therefore, a site survey should be conducted prior to flying, and the timing of optimum climatic conditions must be assessed.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
ALLUVIAL FANS IN A MODERN COLLISION ZONE, THE RAMUMARKHAM VALLEY, PAPUA NEW GUINEA AND THE BLAIRMcPHERSON "NATURAL DEPOSITIONAL SLOPE GAP" KeyuLiul and Keith A. W . C r o o k ^School of Earth Sciences, James Cook University. Townsville QLD 4811 %awai'i Undersea Research Laboratory, University of Hawai'i, Honolulu, HI 96822, USA
The Ramu-Markham Valley, Papua New Guinea is situated in the onshore sutured sector of the Australian-South Bismarck plate boundary. Rapid tectonic uplift of the Finisterre arc terrane to the north of the suture coupled with high precipitation have produced extensive coarse-grained sediments fanning south from the mountain frontal thrust into the intramontane valley. The valley is infilled with up to 1000 m thick of gravelly sediments. These coalescing fans have been constantly uplifted and incised forming multiple terraces tens of meters apart at the mountain front. The alluvial fans are terminated at their southern ends by the trunk braided Ramu and Markham Rivers, which follow approximately the contemporary plate boundary. Two different types of alluvial fans have been recognised in the coarse-grained alluvial deposits. These correspond to the Type I and Type U alluvial fans recently proposed by Blair and McPherson (1994). Type I fans are found along the mountain front in the piedmont setting. They are characterised by relatively small in size (<10 km^), steep slope (>2""). These fans are fed by small ephemeral streams with drainage basins less than 10 km^. Debris flow deposits are the major component of the fans. Reworking on the fan surfaces is minimal. Type n fans, which account for over 90% of the alluvial sediments in the valley, are found in association with large streams with drainage basins greater than 40 km^. These fans are large in size with radial lengths ranging from 5 to 15 km and fan areas between 20 and 180 km^. Sheetflood deposits are the predominant facies although channel deposits are sometimes present. Debris flow deposits are extremely rare. Surfacial reworking is confined to the active channels. These fans are fed by relatively large pCTennial streams. The bulk of sediments in Type n fans are believed to have been deposited by catastrophic flood events like those which have been witnessed twice in the region during the last 9 years. The 1988 Kaiapit landslide mobilised an estimated rock volume of about 1.8 billion m^; and the 1993 Gusap landslide mobilised about 1.0 billion m^ of matCTial. Extraordinary floods w^e caused by the bursting of natural dams immediately following the large-scale landslide. Such a process may have been repeated many times in the Quaternary, as is suggested by the presence of multi-level alluvial terraces, and episodic thick sheetflood conglomerate beds up to 1(X) m thick in the uppOTnost part of the deformed Pleistocene Leron Formation. The Type n alluvial fans in the Ramu-Markham Valley commonly have slopes between 0.5® and 2® falling into the "natural depoational slope gap" proposed by Blair and McPherson (1994). The anraialous low fan gradients may be caused by the r^id fan aggradation and valley infill during the large-scale catastrophic floods. Successive floods trigged by the 1988 Kaiapit landslide built up tens of metres of sheetflood sediments on the existing fan surface within a few hours. The average alluvial fan aggradation rate for the Late PleistoceneHolocene period is estimated at about 10 m/ka. The lack of accommodation space in the narrow part of the valley may also contribute to the anomalous low fan gradients. The alluvial fans in the Ramu-Markham Valley may represent an end member (tectonically active, high precipitation, highly deformed lithology and landslide-prone) of a spectrum of Type n alluvial fans defined by Blair and McPhCTSon (1994). Differentiation of such low gradient fansfromtheir braided stream counterparts in the stratigraphic record will be very useful in the reconstruction of the palaeo-tectonic, palaeo-geographic and palaeo-climatic settings, and will have significant implications for mineral and petroleum exploration.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
QUANTITATIVE ANALYSES OF VARIOUS RESERVOIR SEQUENCES IN AUSTRALIA'S MAJOR OIL FIELDS Kevu Liu^ Scott Painter^ and Lincoln Paterson^
1 CSIRO Division of Petroleum Resources, P.O. Box 136, North Ryde, NSW 2113 2 CSIRO Division of Petroleum Resources, P.O. Box 3000, Glen Waverley, Vic. 3150
Well logs and petrophysical data from various oil fields in the Gippsland Basin, Cooper-Eromanga Basin, Barrow-Dampier sub-basins and Timor Sea were analysed using both the conventional variogram analytical method and a new stochastic analytical approach based on the Levy-stable probability distribution recently developed in the CSIRO Division of Petroleum Resources (Painter and Paterson, 1994). The analytical results indicate that the heterogeneity of a reservoir sequence can generally be well characterised by its spatial statistical parameters such as the Correlation Length from the variogram, the Levy Index and Width parameters. Compared with the variogram analytical metliod, the new stochastic approach based on the Levystable probability distribution can better approximate those highly heterogeneous sediment sequences with occasional extreme depositional/erosional events. The variogram and Levy parameters are generally similar for reservoirs in the same basin, where the reservoirs share the same depositional and diagenetic history. Reservoirs with similar depositional environments in different basins also have similar spatial statistical parameters. However, reservoirs deposited in different environments have quite different parameters. Most of the reservoir sequences studied show a statistical self-similarity within a certain range. Within an EXXON depositional sequence or a genetic depositional system, the spatial and temporal variability of a reservoir is similar to that of the whole sediment sequence. This study also reveals that the variability of a sediment sequence is primarily controlled by the depositional processes and sedhnentary environments. The heterogeneity of the reservoirs in the vertical direction can be broadly related to their depositional rates, although their heterogenous nature within the 3D depositional systems is also controlled by the anisotropic ratios. For examples, the relative rapid deposition of the fluvial systems generally produces relatively homogeneous vertical sequences; whereas the slow deposition of the shallow marine depositional systems produces relatively heterogeneous vertical sequences. The submarine fan systems produce semi-heterogeneous reservoir sequences because they comprise both a slow, pelagic deposition and a relative rapid, turbidity deposition. A list of variogram and Levy parameters for various oil fields against their depositional environments is given. This database provides important statistical parameters that can be used for reservoir simulations and evaluation in other oil fields. REFERENCES Painter, S. and Paterson, L., 1994. Fractional Levy motion as a model for spatial variability in sedimentary rocks. Geophysical Research Letters, 21,2857-2860.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
STRUCTURAL FRAMEWORK OF THE NORTHEASTERN YILGARN CRATON FROM RECENT GEOLOGICAL AND GEOPHYSICAL MAPPING Songfa Liu^ & She Fa Chen^ ^ Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, email: sliu@agso.gov.au ^Kalgoorlie Regional Office, Geological Survey of Western Australia, PO Box 1664, Kalgoorlie, WA 6430, email: s.chen@dme.wa.gov.au
Recent geological and geophysical mapping allows a new regional structural synthesis for the northern Goldfields in the northeastern Yilgam Craton, Western Australia. It shows that crustal architecture in the region was largely shaped during regional ENE-WSW compression (D2) and E-W transpression (D3). D2 produced prominent NNW-trending faults and folds, superimposed on locally preserved earlier (Dl) structures. Regional D1 folds occur but no major regional Dl thrusts have been recognised to cause major stratigraphic repetition. Reverse movement is evident along some regional faults during D2. The subsequent E-W transpression (D3) produced Nto NNE-trending macroscopic faults and folds within newly recognised antidilational jogs. D4 resulted in E-Wtrending normal faults that displaced earlier structures and mineralised veins. The exact nature of Dl is not well understood, but it produced faults, folds, and bedding-parallel foliations in greenstones and/or granitoids that were overprinted by later structures. For example, bedding-parallel SI foliations have been folded by the NNW-plunging D2 Lawlers Anticline. Similar relationships have been reported in the Leonora area. The NNW-plunging Dingo Range Antiform is interpreted as a D2 structure that refolded recumbent tight to isoclinal Dl folds. In the Perseverance area Dl faults were reactivated during D2, and Dl isoclinal folds were refolded during D2. Regional D2 folds have wavelengths of 5 to 30 km with fold axes plunging NNW or SSE. They commonly develop penetrative upright axial planar foliations in most rock types although some rocks within these folds and/or between D2 faults/shear zones are only weakly or little deformed due to deformation partitioning. D2 faults and shear zones trend predominantly NNW, some of which define the boundaries between greenstone belts and granitoid terrains. The Waroonga Shear Zone is up to 5 km wide and more than 100 km long, with an arcuate geometry convex to the east. Along its northern arm, geometric relationship between the shear zone and the SI foliations indicates a sinistral movement. Further north, SI foliations in gneissic granitoids have been folded into a NNW-plunging D2 antiform. Along its southern arm, however, dextral movement has been reported. The gravity high extends from the greenstone in the east of the shear zone to the area of gneissic granitoid in the west, which suggests abundant greenstone below the granitoid. This and the convex-to-the-east trace of the shear zone together suggest it dips to the west. The opposite movement senses along its northern and southern arms of the shear zone suggests an eastward thrusting. It probably dips gently to the west at depth, although it appears to be fairly steep at the surface as indicated by the steep foliations west of the Lawlers Anticline. Westward reverse movement occurred along the Eleven Mile Fault between Leinster and Perseverance. Deposition of polymictic conglomerates probably occurred in compressional basins bounded by reverse faults early during D2. The approximately E-W transpression during D3 produced regional N- to NNE-trending faults, shear zones and folds, that developed within antidilational jogs defined by NNW-trending sinistral strike-slip faults or lineaments. Overprinting relationship between D2 and D3 structures is locally observed. Some N- to NNE-trending faults and shear zones might have been reactivated from earlier structures during D3. The NNE-trending Ockerbury Fault in the southern part of the Yandal greenstone belt links the NNW-trending Celia and Keith-Kilkenny lineaments. Several NNE-trending folds wilii wavelengths of 0.5 to 1 km along the Ockerburry Fault were probably developed during D3. The sigmoid shape of the Koonoonooka monzogranite east of the Perseverance Fault was defined by the D3 transpressional deformation. In the Kilkenny-Welcome Well area, several macroscopic S- to SW-plunging D3 folds have wavelengths of 3 to 10 km. Mesoscopic D4 E-W normal faults displace N-trending faults and shear zones in the Bellevue Gold Mine north of Lake Miranda. These normal faults comprise a quartz vein and/or narrow zone of chlorite-rich foliated basaltic rocks. At the SE end of the NW-trending island near the north shore of Lake Miranda, some small E-W trending veins a few centimetres wide in deformed pillow basalts are nearly vertical and filled with alteration materials including amphibole. Regional D4 structures may include the prominent E-W normal faults and fractures. Some regional E-W faults displace NNW- to NNE-trending D2 and D3 structures and greenstone belts. Some are filled widi mafic/ultramafic dykes of probable Proterozoic age. Kink folds and subhorizontal crenulations, observed in many locations, postdated D3. However, their relationship with the D4 faults is not clear.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
M"" Australian Geological Convention, Townsville, July 1998
USING MAGNETIC IMAGES IN REGIONAL GEOLOGICAL STUDIES AND MINERAL EXPLORATION Songfa Liu & Tim Mackey Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, email: sliu@agso.gov.au, tmackey@agso.gov.au
Many images can be produced from the highly useful magnetic data which are now routinely acquired for regional geological studies and mineral exploration. The different images enhance different features. However, they are often not all readily available to the regional/exploration geologist. Therefore, quite a bit of geological information is commonly not extracted from the data. The geologist needs to choose the best images for efficient geological interpretation. Our experience suggests that two or three carefully chosen images provide most of the geological information that can be interpreted from the data. They include a colour image of total magnetic intensity (TMI) reduced to the pole (RTP), a greyscale vertical derivative of TMI (RTP) image, and a greyscale anomaly slope-enhanced image of TMI (RTP) produced from a new method. As interpretation becomes more specific, other images may be produced to help resolve specific geological problems. Geological interpretation of magnetic data involves identification of lithostructural units and structural features, all of which are best shown on a solid geology map. Lithological information can be interpreted from a colour TMI image because different lithologies contain different types and amounts of magnetic minerals, thus having different magnetic intensities. For this purpose, colour images are preferred because they show anomaly magnitudes and long-wavelength features particularly well. However, small and low magnitude anomalies may not be evident in colour images. The positions of colour changes are not only dependent on the position, magnitude, and width of anomalies, but also on how the data are assigned colours in image processing. Using a different colour lookup table presents a different colour distribution in the image. Furthermore, the human eye is easily distracted by the different colours. It is therefore difficult to determine the width of anomalies and to locate their boundaries with colour images. A greyscale image is more useful for showing fine details and locating anomaly boundaries. It should be noted, however, that greyscale images do not give much indication of the magnitudes of anomalies. A composite image using the Hue-Saturation-Intensity (HSI) colour model appears to best show TMI data. It uses a colour image of TMI as hue and a greyscale image of TMI with a sun-angle illumination as saturation and intensity. This type of image combines the advantages of both colour and greyscale images plus the highlighting of features by illumination. However, it should be noted that the interpreter may be biased by the sun angle used for the illumination, which highlights features at high angles to the direction of illumination. Therefore, for illuminated images, several sun angles may be needed to highlight features trending in different directions. A considerable amount of structural information, particularly at macro-scales, can be obtained from the distribution and geometry of magnetic sources. For this purpose, we use a greyscale anomaly slope-enhanced TMI image produced from a new method and a greyscale image of the first vertical derivative of TMI without illumination. The former reflects the slopes of anomalies, the symmetry/asymmetry of which is also preserved, whereas the latter enhances high frequency anomalies by 'sharpening' them. Vertical derivatives of TMI data are useful in resolving composite anomalies and revealing structural details. But they also enhance noise in the data, particularly for the second or higher-order vertical derivatives. In some cases, an automatic gain control image may reveal additional structural information by amplifying and resolving weak signals. In a greyscale anomaly slope-enhanced TMI image, flat areas (eg. peaks and valleys of anomalies) appear as bright (white), while slope areas appear as various shades of grey to black. The darker the shade, the steeper the anomaly slope. Positions of both high and low anomalies can be precisely located. This, therefore, allows close correlation of anomalies with corresponding geological features. It is also possible to determine the dip direction of a planar source from the relative darkness on both sides of the anomaly. If a dyke-like planar source dips to the north, for example, the image will show a lighter grey shade on the northern side of the anomaly whereas its southern side will be darker. Because the symmetry/asymmetry of anomalies is maintained in this type of image, it gives a non-biased presentation of enhanced structural features of an area, unlike illuminated images. Therefore, this type of image assists more objective structural interpretation.
279
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
BENTHIC FORAMINIFERA AS CONTRIBUTORS TO CARBONATE SAND AROUND GREEN ISLAND, CENTRAL GREAT BARRIER REEF PROVINCE, AUSTRALIA Melissa K. Lobescier School of Earth Sciences, James Cook University, Townsville, Queensland 4811
The Great Barrier Reef Province lies on the Queensland Continental Shelf between latitudes 24° and 9° S and occupies 350000 km^. It comprises the largest coral reef associated carbonate system presently known. The major components which contribute to the carbonate sediment in the Reef Province include the skeletal detritus of foraminifera, coral, coralline algae, molluscs, echinoderms, and bryozoans. The relative contributions of these components is still uncertain as is the identity of the main foraminiferal taxa involved. 115 surface sediment samples from Green Island Reef, northeast of Cairns, have been analysed to determine the contribution made by foraminifera. Members of this group contribute an average of 12% of the total weight of reef-top carbonate sediment. Only four foraminiferal genera are responsible for the bulk of carbonate accretion. The genera are in order of decreasing importance: Calcarina, Baculogypsina, Marginopora, and Amphistegina. Only these genera contribute more than 5% by weight of the total carbonate in any sample and together they contribute 90% by weight of the foraminiferal compliment. Calcarina is the dominant genus in the shallow water sediments contributing an average of 45% of the weight of the foraminiferal sediment. Two species are represented: C spengleri (Gmelin), is dominant and favours the reef flat in water depths of less than 5 m; and C. rustica (Todd & Post) is more abundant in deeper water down to 20 m. The monotypic genus Baculogypsina, represented by B. sphaerulata, is subordinate to Calcarina in shallow water sediment contributing an average of 21% by weight of the foraminifera. The third most abundant genus, contributing 12% of the foraminiferal weight to the sediment is Amphistegina. Amphistegina is the dominant contributor to the foraminifera in deeper water between 5 and 20 m. Three species are recognised: A. lessonii, A. lobifera, and A. radiata, in order of decreasing abundance. Amphistegina lessonii is an important contributor to the sediment in deeper water with A. radiata commonly subordinate. Amphistegina lobifera is more common in the shallow reef flat sediment. The fourth most abundant genus at Green Island reef is Marginopora represented by M vertebralis which comprises an average of 12% of the foraminiferal fraction of the sediment. All four main taxa are known to be in symbiotic relationships with diatoms or dinophyceans. A biotope analysis was performed on all the data and produced three distinct assemblages. One assemblage is a shallow water assemblage (0-5m) and is characterised by the four main genera, particularly by abundant Calcarina spengleri. The second assemblage is a deeper water assemblage (5-20 m), the four main genera are not as important and species richness increases. The third assemblage is a deep water assemblage (8-20 m) in \Mc\i Amphistegina lessonii is abundant and dominant, with subordinate A. lobifera and A. radiata and common Calcarina rustica. The distribution and composition of these assemblages can be explained by foraminiferal test shape which is influenced by light and water motion. The most spheroidal species are restricted to shallow, turbulent water whilst flatter species are rarely abundant at depths of less than 15 m. All of the four main taxa are associated with algae and seagrass on Green Island Reef. However there is a mismatch between the floral distribution now mapped and the foraminiferal contribution to the sediment. Therefore we can deduce that the distribution pattern of flora was very different through time and may have followed a cyclical pattern. The changing distribution of flora, with its strong influence on foraminiferal ecology, may be related to crown-of-thoms predation patterns, with two cycles of infestation, each followed by extensive algal colonisation, known over the last 40 years.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14*^ Australian Geological Convention, Townsville, July 1998
TERMINAL PROTEROZOIC MID-SHELF BENTHIC MICROBIAL MATS IN THE CENTRALIAN SUPERBASIN AND THEIR ENVIRONMENTAL SIGNIFICANCE (Graham A.
Clive R. Calver'', Paul Gorjan', Roger E. Summons', John M. Hayes' and Malcolm R. Walter'
'Australian Geological Survey Organisation, PO Box 378, Canberra, ACT 2601, Australia. 'School of Earth Sciences, Macquarrie University, Sydney, NSW 2109, Australia. 'Woods Hole Oceanographic Institute, MS 8, Woods Hole, MA 02543, USA. 'Present Address: Tasmanian Geological Survey, P.O. Box 56, Roshy Park, Tasmania 7018, Australia
A combined sedimentological and biogeochemical study of several Terminal Proterozoic microbial mat facies around the Centralian Super basin has been conducted. The intracratonic setting and vertical and lateral facies variations into shallow-water sediments suggested that depositional environment was probably never very deep (<150 m?). The mats occured in several formations of three distinct ages, and had carbon isotopic compositions differing from those of surrounding sediments, often being depleted in '^C by up to lA%c when compared to organic matter apparently of planktonic origin. The mat facies consisted of beds of dark grey to black shale, sharply delineated from enclosing paler shale or carbonate. The darker shale layers were composed of abundant, weakly anastomosing, thin (< 50 ^m), black seams. Under higher magnification they were seen to contain pyrite framboids tightly clustered around thin, dark brown, organic layers. Pyrite in the horizons containing microbial mats was generally isotopically depleted in ^^S (-8 to -28%o) compared to non-mat facies (0 to -10%o). This suggested the isotopic difference between sulfate and sulfide (A6) in the Pertatataka Fm may have been 30 to 40%o for the non-mat facies but between 40 to 50%o for mat facies. The A5 variation exhibited by sulfides of the non-mat facies was consistent with isotopic fractionations imposed by sulfate reducing bacteria (Habicht and Canfield, 1996). However, the mat facies exhibited A6 variations greater than could be explained by sulfate reduction alone, furthermore of 383 Proterozoic sulfide analyses reported by Canfield and Teske (1996) only 7 showed discriminations greater than 35%o, this implied that sulfide oxidising bacteria must have been involved. ;t-Alkanes <C20 from the microbial mat facies of Wallara-1 had 5 values between -28 to -30%o. In contrast, S^^C compositions for n-alkanes >C20 plotted in a 'saw tooth' pattern in which odd ;t-alkanes had isotopic compositions similar to their <C20 counterparts, and the more abundant even n-alkanes >C20 were depleted in l^C by 4-6%o compared to the shorter compounds (-32 to -34%o). This pattern suggested that the bitumen was derived from two distinct sources. The isotopic composition of associated kerogen was similar to that of the even ;i-alkanes >C20. which suggested that the producers of these compounds formed the bulk of the organic matter in the microbial mat facies. The 'planktonic' source contributed the n-alkanes <C20 and the odd nalkanes >C20. whereas, the 'benthic' source predominantly contributed the even n-alkanes >C20 and the midchain methyl-alkanes and kerogen. Variations in the isotopic compositions of the 'planktonic' organic matter fitted with secular trends observed in widespread stratigraphic sections in the Ediacarian of mainland Australia The combination of sedimentological and isotopic data indicated that the mats may have been at least partly composed of sulfide oxidising bacteria. These bacteria have specific environmental tolerances that set limits on the palaeo-environment. The requirement for oxygen indicated that the water column above the mid-shelf could not have been anoxic. The data in this time interval is extremely limited, however, from the results and age determinations reported here it would appear that the mid-shelf environment was seeing significant levels of oxygen through the Ediacarian. REFERENCES Canfield D.E. and Teske A. (1996) Late Proterozoic rise in atmospheric oxygen concentration inferred from phylogenetic and sulfur-isotope studies. Nature 382, 127-132. Habicht K. S. and Canfield D.E. (1996) Sulfur isotope fractionation in modem microbial mats and the evolution of the sulfur cycle. Nature 382, 342-343.
281
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
ORGANIC MATTER ALTERATION RELATED TO Pb-Zn MINERALIZATION, AT THE HYC DEPOSIT, N.T. Graham A. Logan-. Mark Hinman^ and Roger E. Summons' 'Australian Geological Survey Organisation, PO Box 378, Canberra, ACT 2601, Australia. ^ GeoSOLUTIONS, 43 Gerler Street, Bardon. Queensland 4065.
A Study of organic matter within the Paiaeoproterozoic HYC Pb-Zn deposit, Northern Territory, Australia, has been undertaken to help identify interactions between organic matter and fluids during ore precipitation. Areas of interest to this study were the environment of ore formation within the 'HYC pyritic shales' and unmineralized Barney Creek Fm. We are attempting to define differences in organic matter type between these horizons and seeking evidence for processes of sulfate reduction during ore formation (bacterial or thermochemical). Samples were obtained from core within and outside the deposit and from underground material from 2 Orebody. Analytical techniques common in petroleum geochemistry, such as Gas Chromotography-Mass Spectrometry, have been used to study the biomarkers, which help to define the nature of the organic matter. Organic matter from unmineralized Barney Creek Fm was found to be similar to material from the Clyde Subbasin. Biomarker distributions were similar in all Barney Creek Fm samples and did not differ between ore zones and unmineralized sediments. However, strong even «-alkane carbon number predominances were observed in 2 Orebody and several other of the mineralised horizons. Thermal maturation during burial of Barney Creek Fm did not produce this pattern and does not produce it in other sediments. Even predominances over various ranges <C2o are not uncommon and >C2o have also been reported in various recent sediments and some fossil-fuels. However, the predominance of even A7-alkanes over the C15-C30 range is extremely rare, but has been reported in hydrothermally generated petroleum from the Middle Valley, Juan de Fuca Ridge, ODP Leg 139 (Simoneit, 1994). Marked differences were also observed between 2 Orebody and the zones of less intense ore formation and unmineralized horizons. Some of these differences were also observed in bitumen extracted from Mt Isa sulfide ore (Mycke et al., 1988), in particular the unusual predominance of the 3- and 5methyl even carbon numbered Z?-alkanes. Several important conclusions can be drawn from the HYC data. 1. Primary differences in organic matter can not be responsible for the unusual w-alkane distributions observed in ore zones. 2. Burial maturation of organic matter in unmineralised Barney Creek Fm does not produce this unusual «-alkane pattern. This implies, that the ability of the mineralised zones to generate the normal hydrocarbon pattern had been compromised. 3. Chemical methods used in the laboratory to break apart kerogen which involved oxidation reactions, such as RUO4 or Mn04 oxidation, often produce even carbon predominances in the reaction products. The «-alkane pattern observed in 2 Orebody is similar to that produced by these type of reactions. 4. It would appear that the kerogen in 2 Orebody was attacked in such a way that its hydrocarbon generation potential was removed, and on burial it could not produce the typical hydrocarbon signature of the unmineralised Barney Creek Fm. Without the ability to generate hydrocarbons the even predominance would not be overprinted during normal burial maturation. 5. Removal of the generation capacity of the kerogen before burial catagenesis is indicated by preservation of the unusual even hydrocarbon distribution. This implies that ore fluids reacted with the kerogen before deep burial. 6. The occurrence of similar compound distributions in the Mt Isa ore indicates a similarity of processes involving the organic matter at in both mineral deposits. Acknowledgments: This work is presented with the permission of MIM Exploration Pty Ltd. REFERENCES Mycke B., Michaelis W. and Degens E.T. (1988) Biomarkers in sedimentary sulfides of Precambrian age. Org. Geochem. 13, 619-625. Simoneit B.R.T. (1994) Lipid^itumen maturation by hydrothermal activity in sediments of Middle Valley, Leg 139. In (eds. M.J. Mottl et al.,), Proc.O.D.P., ScL Res, 139,447465.
282
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
ENVIRONMENTAL IMPACTS OF ABANDONED METALLIFEROUS MINE SITES, NEW ENGLAND OROGEN, NEW SOUTH WALES Bemd G. Lottermoser' and Paul M. Ashlev^ ^School of Earth Sciences, James Cook University, Cairns, PO Box 6811, Queensland 4870 ^Division of Earth Sciences, University of New England, Armidale, New South Wales 2351
Metalliferous mine sites can represent important sources of metal pollutants in watercourses and soils, with consequent degradation of local ecosystems. This study investigates the environmental degradation caused by mining and associated smelting operations carried out during the late 19th and early 20th century at six metalliferous mine sites in the New England area of northern NSW. Abandoned mine sites with differing geologic, physiographic, climatic and floral regimes were sampled for waste rock, ore stockpile, mill tailings and slag materials, soils, stream sediments, waters and vegetation. The sites are Halls Peak ZnPbCuAg (VHMS deposit), Gulf Creek Cu(Zn) (VHMS), Webbs Consols AsZnPbAg (vein), Collisons AsZnPbAg (vein), Cangai Cu(ZnAu) (vein) and Mole River As(Cu). Host rocks are mostly quartzofeldspathic (granite, metasediment, felsic volcanic) with relatively limited acid buffering potential. Chemical analysis (Ag, As, Cd, Cu, Fe, Pb, Sb, Mn, Zn) of -270 samples provided information on the environmental behaviour of heavy metals originating from different primary metal and acid-producing sources. Exposure of sulphide-bearing ore and waste dumps to contemporary weathering processes has led to the variable dissolution of sulphide and gangue minerals, formation of post-mine oxidation minerals (e.g. sulphate mineral efflorescences including jarosite, halotrichite, poitevinite, goslarite, antlerite, gunningite, rozenite, bianchite, copiapite, melanterite, as well as Fe oxide species), production of low pH, metal-rich waters and the associated chemical and physical mobilisation of heavy metals into local drainage systems. Seepages from abandoned (xe and waste dumps are acidic and contain elevated metal contents. Where seepage waters enter drainage systems, there is an attendant drop in pH, and aquatic and bankside plant communities disappear. Physical dispersion of secondary metal-bearing minerals into stream sediments is significant. Chemical mobilisation from ore and waste dumps is most pronounced for As, Cd, Cu and Zn whereas Pb remains largely immobile. Stream sediments have metal concentrations exceeding background values by one to four orders of magnitude. Upon entering larger drainage systems the metal concentrations of waters and stream sediments may be diluted to background values, although at some locations, anomalous metal concentrations can be traced for several kilometers downstream. Soils near ore, waste and slag dumps have elevated metal values and contain heavy metal-bearing minerals (eg jarosite, goethite, hematite, clays, scorodite). These sites are commonly devoid of vegetation or support a depauperate assemblage, with consequent marked erosion. At Mole River, regional pollution of topsoils with As has been caused by smelter emissions. At mine and smelter sites, bioaccumulation and biomagnification processes of heavy metals occur in grasses and other plants but patchy recolonistion of contaminated sites by metal-tolerant species is evident. Several species on contaminated sites are common to most locations (eg Aristida sp., Cynodon sp., Chrysocephalum sp., Cassinia sp., Angophora floribunda). Bioaccumulation processes of heavy metals suggest a potential for heavy metal accumulation in the food chain of the surrounding terrestrial ecosystems, including grazing animals. All investigated streams affected by acidic waters contain abundant green filamentous algae {Klebsormidium sp.) with very high As, Cu, Pb and Zn concentrations in dried material. Algae in common with other microbial groups such as bacteria and fungi are capable of accumulating metals from dilute solutions. Thus acid mine drainage environments of New England provide exceptional examples of life adaption to extreme conditions (acidic, metaland sulphate-rich waters), whereby green algae use inorganic chemicals as energy sources similar to the oldest forms of life. Mine seepage waters contain heavy metal values well in excess of WHO recommended maximum acceptable concentrations in drinking waters and ANZECC water guidelines for domestic and stock use. In addition, the heavy metal abundances at all sites exceed contaminated site guidelines proposed by the ANZECC and NHMRC. Rehabilitation of disturbed areas should involve covering and sealing metal-rich mine waste or removal of ore and waste dumps, stabilisation of areas affected by landslides and erosion, construction of wetiand environments and anoxic lime drains, import of topsoil and planting of local, metal-tolerant plant species.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
HISTORY OF ORGANOGENIC BUILDUPS ON THE SIBERIAN PLATFORM AT THE THRESHOLD BETWEEN THE VENDIAN AND CAMBRIAN Veronica Luchinina United Institute of Geology, Geophysics, and Mineralogy, 630090, Novosibirsk, Russia
Extensive organogenic buildups occurred in an epicontinental basin that occupied almost the entire Siberian Platform during the late Vendian. In the north, buildups extend continuously from west to east making a barrier oriented from northwest to southeast. Development of organogenic buildups resuUed from the auspicious geographic position of the platform. At the end of Vendian time, the Siberian Platform was located in the equatorial belt, surrounded by waters of incipient palaeo-oceans. The palaeogeographic regime of the latest Vendian arose owing to the disintegration of Palaeopangea and development of the Palaeoasiatic Ocean. Accompanying processes, which were initiated in the Riphean, include: 1) successive decrease of littoral areas with almost total disappearance of stromatolites as a consequence; 2) shelf reduction; and 3) increase in calcium carbonate removal. Tidal amplitude may have decreased, whereas water temperature rose, occasionally reaching 45°C. In response to the environmental changes, there was abundant calcification of stromatolite-producing algae. As in modem algae and microbes, only certain taxa, mainly cyanobacteria, were calcified. A persistent community of calcareous algae, calcimicrobes, and bacteria were the main bioherm-producers during the Vendian and Cambrian Periods. They formed a concordant system dominated by destructive phototrophic bacteria on the surfaces of calcareous sheaths of calcimicrobes. Calcimicrobes required light, because they had viable cells at the ends of filaments. However, cells inside the sheaths gradually died off, forming organic necromass that was degraded by destructive bacteria, which secreted biogenic elements, deposited carbonates, and lithified the algal communities. As in recent cyanobacterial communities the secretions of Vendian/Cambrian calcimicrobes, which were produced in large quantities with their mass excelling that of the calcimicrobes themselves, were reworked by bacteria. Involvement of archaeocyathans in bioherm production early in the Tommotian, as well as their disappearance late in Early Cambrian, were of little importance for organogenic buildups, thus supporting the contention that non-enzymatic carbonates were more important producers of reef framework in some Phanerozoic reefs than were skeletal organisms. At the beginning of the Cambrian Period (Tommotian Age), the areas occupied by organogenic buildups sharply decreased owing to regression. Rare remaining bioherms were of irregular shape, had small sizes, and were isolated from each other. The situation changed at the end of the Tommotian, owing to the initiation of transgression, the first phases of which were responsible for the formation of the Sakha organogenic buildup belt that extended diagonally across the entire platform. However, at the end of Toyonian time, continuing transgression resulted in the gradual disappearance of the Sakha buildup belt by late in the Cambrian Period.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
EMPLACEMENT AGE OF THE PAPUAN ULTRAMAHC BELT OPfflOLITE : CONSTRAINTS FROM K-AR AND GEOCHRONOLOGY FROM HORNBLENDE GRANULITES AT THE BASE OF THE OPfflOLITE NEAR THE MUSA-KUMUSI DIVIDE Wilfred Y. Lus Ian McDougall' Hugh L. Davies' 'Research School of Earth Sciences, The Australian National University, Canberra ACT 0200 Australia ^Geology Department, University of Papua New Guinea, P O Box 414 University NCD Papua New Guinea.
The Papuan Ultramafic Belt (PUB) ophiolite is exposed over a length of 400 km and a width of 40 km along the northern flanks of the Owen Stanley Range, southeastern Papua New Guinea (PNG). Davies & Smith (1971) considered the complex to be a sheet of oceanic crust and upper mantle which was thrust over the southeastern PNG continental margin (or rifled piece of the Austrahan continental lithosphere) during arc-continent collision. K-Ar and "^Ar^Ar age data suggest that the PUB ophiolite has a minimum crystallisation age of latest Cretaceous to early Cainozoic, consistent also with foraminifei^ data from intercalated sediments. Both K-Ar and "^^Ar-^^Ar ages have been determined for hornblende separates from homblende granulites and amphibolites from the sole of the PUB ophiolite near the Musa-Kumusi divide. The K-Ar ages range from 66 Ma to 56 Ma in the early Cainozoic. These ages are interpreted as broadly reflecting the time of recrystallisation and cooling in the Paleocene cf the aureole rocks during and subsequent to the emplacement of the PUB ophiolite onto the southeastern PNG continental crust. Whether there was an extended interval over which emplacement and recrystallisation of the ophiolite occurred has been investigated by a "^Ar^^Ar study on the same hornblendes. "^^Ar-^^Ar age spectra are relatively flat and most samples yield plateau ages that are in relatively good agreement with one another. Ages mainly in the range of 57 Ma to 59 Ma were obtained, suggesting that the Papuan ophiolite was emplaced and cooled at that time in the Paleocene. The lack of spread in the apparent ages is interpreted as indicating that the emplacement was essentially a single event, perhaps not requiring an extended period of time, and that the larger spread in ages observed in the K-Ar study is due to the presence of variable amounts of excess argon. The results of this study suggest that the arc-continent collision and subsequent emplacement of the PUB ophiolite and possibly the Marum ophiolite occurred during the Paleocene. After its emplacement the ophiolite body and the metamorphic sole rocks were intruded by diorites and tonalites with isotopic ages in the range of 54 Ma to 57 Ma (Davies & Williamson, 1998). REFERENCES Davies H. L. and Smith I. E. 1971. Geology of Eastern Papua, Bulletin of Geological Society of America 82, 3299-3312. Davies H. L. and Williamson N. 1998., Buna, Papua New Guinea, 1:250 000 Geological Series, Geological Survey of Papua Ne^/v Guinea-Explanatory Notes
285
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
GEOCHRONOLOGY OF METASEDIMENTARY ENCLAVES FROM THE S-TYPE DEDDIGK GRANODIORITE, SOUTHERN KOSCIUSKO BATHOLITH, EASTERN VICTORIA Roland Maas^ Ian A Nicholls^. Alan Greig^-^, Alexander A Nemchin"^ VIEPS Dept. of Earth Sciences, La Trobe University, Bundoora, Victoria 3083 VIEPS Dept. of Earth Sciences, Monash University, Clayton, Victoria 3168 Dept. of Earth Sciences, Univ. of Queensland, St. Lucia, Queensland 4027 School of Applied Geology, Curtin University, GPO Box U1987, WA 6001
SHRIMP U-Pb zircon ages were obtained for 3 metasedimentary enclaves and one host granite sample from the Early Silurian Deddick Granodiorite which forms the southern extremity of the Kosciusko Batholith in eastern Victona. The granodiorite is one of the typical cordierite-gamet-bearing mafic S-types common in the Kosciusko Bathohth and contains numerous (5-7 vol.%) enclaves, including microgranitoid enclaves of apparent magmatic origin. Some 90% of the enclaves are of metasedimentary origin and range from unmelted psammitic-pelitic gneisses/schists via migmatites to a volumetrically dominant cordierite-garnet-rich type thought to be residual from extraction of a felsic melt. U-Pb ages of melt-precipitated zircons and zircon rims in the host granite date emplacement at 430±3 (2omean), consistent with ion microprobe zircon ages for other S-types in the area. The 430 Ma age constrains preintrusion low-grade metamorphism and deformation of Late Ordovician-Early Silurian clastic sediments (the Yalmy fold and thrust belt) to the earliest Llandovery. Low Th/U zircon rims in the enclaves indicate similar but less precise ages of 426±5 and 425±15 Ma for metamorphic zircon growth. No older, clearly metamorphic zircon has been found. This indicates that high-grade metamorphism and large scale panial melting at depth (4-5 kb: 15-20 Km), and deformation, low-grade metamorphism and granite emplacement at <2 kb were coeval at 425-430 Ma. Early Silurian metamorphism in the enclave source rocks is supported by a 430±5 Ma step-leach Sm-Nd isochron age for garnet from one of the melt-depleted cordierite-gamet gneiss enclaves. The youngest detrital zircon grains in the enclaves provide upper limits for deposition of sedimentary precursors. The psammitic gneiss enclave yields the most detailed record with two closely spaced age peaks at =475 and 495 Ma, indicating deposition in the Early Ordovician. Corresponding ages in the migmatitic and cordierite-garnetrich enclaves are 490-500 Ma, i.e. near the Cambrian/Ordovician boundary. The youngest detrital grains in the host granite encompass a similar age range, and may be derived in part from abundant disaggregated enclave material. This similarity implies that at least part of the granite source is composed of metamorphosed Ordovician sediments, confirming conclusions drawn from zircon U-Pb studies of metasedimentary enclaves and granitoids elsewhere in the southern LFB. The Early Proterozoic Nd model ages characteristic of S-type granitoids in the Kosciusko Batholith (incl. the Deddick Grd. and its enclaves) reflect the provenance of the clastic source material (with rare 2-3 Ga detrital zircon grains), not the presence of a Proterozoic continental fragment as argued on the basis of granite geochemistry. Further, the inferred Ordovician deposition age for the mid-crustal lithologies does not appear to support the presence of extended Delamerian-type crust at the base of the Lachlan Fold Belt, at least not at the level sampled by the enclaves. The samples studied here have detrital zircon age abundance peaks at 470-550 and 1000-1100 Ma. The host granite sample has an additional cluster at 600 Ma that is not well represented in the enclaves. The ca. 500 and 1100 Ma peaks are typical of detrital zircon suites from Palaeozoic (meta-) sediment and granitoid samples from the Lachlan Fold Belt, New Zealand's Western Province, and the palaeo-Pacific margin of Gondwana in Antarctica. This indicates that mid-crustal lithologies underlying the Kosciusko Batholith during the Silurian were age and paleogeographic equivalents of the Ordovician-Silurian turbiditic sediments at the surface. The relevance of the Deddick metasedimentary enclaves to their host granite is uncertain, although the enclaves' melt-depleted nature and evidence for coeval high-grade metamorphism, migmatite production, and granitic magmatism are compatible with a genetic relationship. However, even if the lithologies represented by the enclaves were part of the source region from which the host magma was generated, Nd-Sr isotopic data for the enclaves (Sri>0.716, eNd -10 to -12.5) require additional source components (rich in Ca, Na, lower Sri, higher eNd) to produce the composition of the host granite (Sq <0.716, eNd =-10). The presence of a detrital zircon age component in the granite that is poorly represented in the enclaves (the 600 Ma cluster) may place an age constraint on this additional component although we can not exclude that further zircon dating may turn up more 600 Ma grains in the enclaves. Potential igneous sources (and/or first cycle sedimentary derivatives) of =600 Ma age are not well known in the Lachlan Fold Belt but may well exist in the form of unexposed sections of the Cambro-Ordovician volcano-sedimentary succession.
286
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
LETPADAUNG, MYANMAR: MINERALISATION CONTROLS AND INDICATOR VARIOGRAPHIC ANALYSIS ON A LARGE PORPHYRY COPPER DEPOSIT Annick Manfrino MINPROC, 140 St Georges Terrace, Perth, Western Australia 6000
Summary- Variographic analysis based on grade indicators and illustrated by variogram maps was used to establish the link between the spatial distribution of copper mineralisation and the structural setting at Letpadaung, a large porphyry copper deposit in Myanmar. The indicator variography demonstrated the superimposed effects of two major sets of structural features on the distribution of the mineralisation m the supergene enrichment zone, thereby allowing adequate modelling of the resource. INTRODUCTION The Letpadaung copper deposit is located in the Monywa District of Central Myanmar. It is a world-class deposit with a Measured and Indicated resource of 1 069 Mt @ 0.40 % Cu and a Proved and Probable reserve of 905 Mt @ 0.40 % Cu, at a 0.1% Cu cut-off grade. Letpadaung is classified as a high sulfidation deposit with hydrothermal alteration and mineralisation occurring in several overlapping stages of hydrothermal activity. Late stage weathering resulted in the oxidation of sulfides and remobilisation of copper, creating a leached zone which overlays a supergene enrichment zone. Copper mineralisation occurs in porphyritic quartz andesite intrusions of late Tertiary age and associated older Tertiary pyroclastics and sediments, and is associated with breccias and fractures zones. The distribution of copper mineralisation is strongly controlled by the structural setting of the project area which consists of two major sets of fault systems. Exhaustive variographic analysis based on grade indicators was completed, coupled with a statistical analysis of the data, leading to the establishment of a convincing model for the spatial distribution of the mineralisation and its link with the structural setting. INDICATOR VARIOGRAPHY Trend analysis with the use of Laplace interpolated maps illustrates the control of the predominant structural features over the mineralisation distribution. A vertical zonation of grades due to the re-distribution of copper mineralisation in the supergene enrichment zone is superimposed on the structural control and is clearly confirmed by a graph of copper average grade vs relative elevation. It was anticipated that, due to its origm, the mineralisation in the supergene zone would present strong horizontal continuity and that the pre-existing direction of primary mineralisation might be partially or totally obliterated by the main direction of the enrichment flow. These assumptions were tested and confirmed in the variographic study using indicator grade variogram maps. Variogram maps visualise the general trends of the mineralisation. Any occurrence of anisotropy of the mineralisation distribution will be represented on the map by elongated features along the direction of preferred continuity. The horizontal variogram maps generated in the supergene zone using increasing grade indicators demonstrate that, at low indicator grades, the mineralisation trend follows the predominant structural direction of the domain. With increasing indicator grade, the mineralisation trends reflect simultaneously two major trends, one bemg the background primary mineralisation trend, overprinted by a trend interpreted to correspond the secondary enrichment water flow. Fmally, at high indicator grades, the primary mineralisation trend is totally obliterated by the secondary one. CONCLUSIONS The use of variogram maps generated with increasing indicator grades explains the complex relationship between mineralisation distribution and structural setting in the case of a two stage mineralisation sequence, where primary mineralisation is overprinted by a phase of secondary enrichment which follows a different structural trend. Understanding the mineralisation controls was paramount when conducting the resource estimation at Letpadaung in order to adequately model the high grade supergene enrichment zone which resource significantly impacts on the economics of the project. The simplicity of use of indicator variogram maps makes this technique a powerful tool when investigating multiple phase mineralising events. Acknowledgments: The author wishes to thank the Myanmar Ivanhoe Copper Company Ltd, a 50/50 joint venture between the Myanmar Government and Indochina Goldfields' subsidiary, Ivanhoe Myanmar Holdings Ltd, for permission to publish this paper.
287
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
D4 IN THE MOUNT ISA INLIER AND ITS IMPLICATIONS FOR MINERALIZATION V. M. Mares
School of Earth Sciences, James Cook University, Townsville, Queensland 4811 and Gnomic Exploration Services Pty Ltd, 9 Stanton St., Belgian Gardens, Queensland 4810 The Isan Orogeny is conventionally subdivided into three main deformation stages, Dj, D2, and D3: N-S shortening during D1 produced E-W-trending folds and shear zones; E-W shortening during D2 and D3 produced approximately N-S-trending folds and shear zones. A fourth event, D4, again with N-S-directed shortening, was previously reported to have produced structures locally. However, schist in outcrop as well as in core from multiple locations within the western part of the Eastern Fold Belt, Mount Isa Inlier, show well developed, approximately E-W-striking crenulations. These may be due to D4 which may therefore have been more widespread than previously thought. It may in fact be a last stage of the Isan Orogeny occurring at a time when the rocks were located higher up in the crust such that deformation produced both ductile and brittle structures during the same event. THE NATURE OFD4: N-S shortening during D4 of a preexisting, very strongly N-S-aligned regional fabric, produced during D2 and D3, resulted in meso- and macroscale kinks as well as tightening and steepening of existing parasitic folds and flexures. Schists display E-W-striking, well developed crenulations orthogonal to S3. Shear along existing fold limbs may have resulted in sheath folds with locally accelerated strain rate and brecciation. Most importantly, however, D4 produced regional-scale dilation in the form of gaping along the contacts between layers of vastly different mechanical strength, such as amphibolite, phyllite, and schist. Gaping may also have occurred along older, layer-parallel, faults and shear zones. In additon, faults and fractures, some many kilometers in length, cutting across preexisting layering, may also have been produced during D4. The dominant orientations of these faults and fractures are E-W, NE-SW, and NW-SE. The apparent limitation to a few locations of evidence for a fourth deformation can be attributed to (1) the overall weaker nature of D4 which allowed only local development of ductile structures, (2) the presence of large granitoid bodies which acted like rigid porphyroblasts in whose strain shadows no D4 structures developed, and (3) the simultaneous development of brittle and ductile structures during the same deformation which makes recognition of this event difficult. IMPLICATIONS FOR MINERALIZATION: Fractures of every size, from kilometer-long lineaments traversing the entire Eastern Fold Belt and gaping parallel to lithological boundaries down to microscopic cracks, provided the regional dilation necessary for fluid flow on a huge scale. The modification during D4 of previously formed folds and flexures created potential structural traps. Evidence exists for the passage of mineralized fluids with formation of pyrite and chalcopyrite along S3 and S4 as well as along later, crosscutting fractures. Examples of mineralized traps are the Mt. Elliott Cu-Au deposit (located within a D4 sheath fold), Starra mineralized ironstones (pods within a N-S shear zone), Fairmile and Partridge (D3 fold, possibly unfolded during D4 with breccia formation), and Osborne (mineralization along dilated D3 flexure). Search for and identification of areas affected by ductile modification of older structures during D4 combined with brittle fracturing and dilation may lead to discovery of further mineral deposits. Acknowledgements: This abstract is based in part on work done during 1997 while the author was under contract to Selwyn Mine. ARIMCO/Selwyn Mine Geology is thanked for permission to publish.
288
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
A MAGMATIC ORIGIN FOR REGIONAL Na-Ca ALTERATION, CLONCURRY DISTRICT, NORTHWEST QUEENSLAND. Geordie Mark and Damien R.W. Foster Economic Geology Research Unit School of Earth Sciences, James Cook University, Townsville, Queensland 4811
Albite-, quartz-, actinolite-, apatite-rich rocks that form a carapace which caps Roxmere quartz diorite, provide direct evidence of the accumulation and subsequent release of magmatic fluids that may have contributed to regionally extensive Na-Ca alteration in the Cloncurry district Actinolite-, apatite-, albite-bearing rocks are common in Proterozoic and Phanerozoic Fe-(Cu-Au) districts worldwide (e.g. Kiruna district; Sweden; Cloncnrry district and Stuart Shelf, Australia; Copiapo district, Chile), and in particular are commonly associated with granite-related alteration/mineralisation systems. Fluids that produced these mineral assemblages are recognised as playing a large role in the tectono-thermal history of many Fe-(Cu-Au) districts, and are commonly considered as important transporters of metals. There are two hypotheses relating to extensive Na-Ca alteration systems which formed synchronously with granitoid emplacement. The significant difference between these systems is whether the granitoids acted as heat engines that drove hydrothermal circulation of the surrounding formation waters through the crust toward the granitoid; or whether the granitoids themselves were the fluid source. Consequently, further definition of the origin of the fluids that produced this type of alteration, and their relationship to mineralisation has considerable implications for effective target generation of auriferous deposits within these systems. The coincidence of Na-Ca alteration, granitoid intrusion and Fe-(Cu-Au) mineraUsation has been well documented in the Cloncurry district (Williams, 1994; Williams et al, 1995), which has been affected by multiple stages of Na and Na-Ca alteration (de Jong & Williams, 1995; Mark, 1998) over a 50 million year («1550-1500 Ma) period. This period is broadly synchronous with the emplacement of the Williams and Naraku Batholiths («1550-1500 Ma), which is highlighted by the spatial and temporal relationship between intrusions of the Mount Angelay igneous complex and stages of regional Na-Ca alteration (Mark & de Jong, 1996). The origin of the fluids that produced Na-Ca aheration is arguable, mainly because of the lack of definitive textural and isotopic data. However, investigation of the albite-, actinohte-, apatite-rich rocks within the carapace of the Roxmere quartz diorite may start to unravel the origin of thefluidsthat caused regional Na-Ca alteration. The carapace rocks comprise two variants; aplite and pegmatite, both of which were affected by aheration and brecciation. The aplitic material is composed of equigranular fine-medium grained albite, quartz, actinolite and apatite. The pegmatitic material typically occurs in layers or pods within aplitic material, and is comprised of mediimi-coarse grained actinolite, albite, quartz and apatite. The aplitic material predominates and the carapace has a brecciated top, and a distinct textural zonation, from an aplite-rich/pegmatite-poor base, to a relatively aplite-poor/pegmatite-rich roof This zonation is interpreted to represent the progressive accumulation of volatile phases toward the roof of the carapace, which culminated in fluid overpressuring and brecciation of granitoid stock. Chemical and isotopic analysis of albite(An<02) and quartz within the carapace suggest a magmatic origin (8^^0(water) 6.0-8.0%o; @450-550®C), and an equilibrium temperature of 540-510°C for these fluids. The contact between the Roxmere quartz diorite and the carapace is covered, hence the parent magma to the carapace is unknown. However, the Mg, Na and Ca-rich and K-poor nature of the quartz diorite is consistent with producing a Na-, Ca-, Mg-rich and K-poor carapace. The chemical and composition of albite, quartz and actinolite compare closely with those produced during regional Na-Ca alteration, and suggests that, at least, some of thefluidthat produced regional Na-Ca alteration may have been magmatically derived. Consequentiy, it is interpreted that if magmas produced the fluids that caused Na-Ca alteration they are likely to have interacted with the surrounding Mg-bearing country rocks. This interpretation is consistent with the higher composition of minerals (e.g. albite and actinolite), and higher Mg content of actinolite in regional Na-Ca alteration assemblages, compared with those in the aplitic carapace. REFERENCES de Jong, G. & Williams, P.J., 1995. Australian Journal ofEarth Sciences 42, 281-290. Mark, G., 1998. Australian Journal ofEarth Sciences 45, (in press). Mark, G. & de Jong, G., 1996. James Cook University EGRU Contribution 55, 81-84. Williams, P.J., 1994. Mineralium Deposita 29, 250-260. WiUiams, P.J. etal, 1995. AUSIMMPublication Series 9/95, 631-636. 289
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
FORMATION AND UPGRADING OF ORE DEPOSITS BY REGIONAL METAMORPHIC REMOBILISATION Brian Marshall^ frank M. Vokes^ and Adrienne C. L. Larocque^ ^Appied Geology, University of Technology, Sydney, PO Box 123 Broadway, NSW 2007, Australia ^Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology, Trondheim, Norway ^Department of Geological Sciences, The University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
Metamorphic remobilisation involves translocation (during defonnation/metamoiphism) of pre-existing, massive, semi-massive or disseminated mineralisation by solid-state, liquid-state, or mixed-state transfer, the latter being dominant in nature. Concepts in remobilisation, such as parent-daughter relationships and upgrading of ore, are discussed. Large-scale selective removal of components from massive ore is difficult to substantiate on both observational and theoretical bases, but such remobilisation cannot entirely be rejected. Textural/mineralogical changes induced by prograde metamorphism impact little on the overall grades of the major, minor and trace elements present. However, effective upgrading by grain-size coarsening and formation of discrete trace-element (e.g., precious-element) minerals enables more efficient recovery. Some prograde changes can be reversed during retrogression. Transfer processes in regional-metamorphic remobilisation comprise cataclasis and granular flow, dislocation flow, and diy- and wet-state diffusion, the latter commonly being grouped with advective transfer. Inter- and intra-granular solid-state transfer are fundamental to dry-state ductile deformation, and of great importance for the degree of internal remobilisation. Dislocation flow has been invoked to explain 'healing' of fractures in high-competence sulfides by low-competence species, but fluid facilitation should not be discounted. Internal solid-state remobilisation can fomi ore-shoots by hinge-zone thickening and elongation processes, though fluid-assisted diffusive and advective mass transfer probably contribute significantly to these cases. External solid-state remobilisation has been invoked to explain piercement cusps and veins, as well as discordant, shearzone-hosted bodies of enriched mineralisation, yet a degree of fluid involvement is usually envisaged. There are few, if any, undoubted examples of solid-state external remobilisation. Internal and external remobilisation mainly involves mixed- or liquid-state transfer. Pertinent mass transfer paths comprise: (1) diffusion around grain boundaries through a static fihn; (2) flow around grain boundaries and through dynamic microfracture networks; and (3) channehsed flow through macrofracture systems, now commonly seen as vein anays related to ductile and brittle shear zones. A substantial extent of remobilisation will only result from the advecting processes (2 and 3). Liquid-state involvement in mixed-state transfer ranges from negligible or subordinate, to totally dominating, before passing into true liquid-state transfer. The spectrum of mixed- and hquid-state processes is evaluated. Magmatic transfer in the remobilisation of sulfide species potentially results from: production of sulfide melts during high-grade metamorphism, dissolution of ore minerals by a migrating silicate melt, and direct incorporation of ore components during production of a silicate melt. Silicate melts perhaps have a role in remobilisation leading to the fomiation of new deposits, whereas the contribution of sulfide melts is at best minor. Nevertheless, both can induce local, as well as effective, upgrading. Magma-related transfer, involving fluids expelled during magmatic cooling, can modify a deposit by replacement and/or addition of phases and result in daughter mineralisation up-flow from the parent body. Transfer by solid-state, aqueous-dominated, or magmatic flow is integral to the remobilisation (modification or formation) of ore deposits. The consequences are recognised at the 'emplacement-site' by the geometry and spatial relationships of one or more of: deformed ore (textural and localised distributional changes); dilational vein-systems; and replacement effects over a range of scales. However, the 'emplacement-site' consequences of remobilisation, syn-tectonic introduction, and even post-tectonic epigenesis involve the same principles. Thus, because 'emplacement-site' features are non-definitive, genetic interpretation can be contentious. Remobilisation reflects a complex interplay between effective upgrading, the remobilisation processes and whether they cause concentration or dispersion, and the selected reference frame. Transport rates and distances involved in external remobilisation are evaluated. External remobilisation seems to be extremely common to a meter, very common to ten meters, moderately common to a few tens of meters, uncommon to a few hundred meters, extremely uncommon to a thousand metres, and unproved beyond that. Despite the formation of daughter orebodies, the relative sizes of parent and daughter imply only low to moderate degrees of external remobilisation. A high degree of selective external remobilisation is unproved and highly improbable; comprehensive external remobilisation (magmatic incorporation excepted) is unproved and even more improbable. 290
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14'^ Australian Geological Convention, Townsville, July 1998
THERMAL HISTORY OF THE HODGKINSON AND LAURA BASINS, FAR NORTH QUEENSLAND Susan J. Marshallsea^. P.F. Green^ and J.A. Webb^ ^ Geotrack International Pty Ltd., 37 Melville Road, Brunswick West, Victoria, 3055 ^ Department of Geology, Latrobe University, Bundoora, Victoria, 3083
The Hodgkinson Basin of Far North Queensland is a Paleozoic basin containing sediments, volcanics and intrusions ranging in age from Lower Silurian to Carboniferous. Unconformably overlying these units are isolated outcrops of coal-bearing Upper Permian to Lower Triassic sediments. The southern margin of the Laura Basin unconformably overlies the Hodgkinson Basin and contains over 1 km of Middle Jurassic (Bathonian) to Early Cretaceous (Albian) fluviatile and shallow marine strata. The occurrence of black coaly horizons in both basins shows that at some time during the Mesozoic and/or Tertiary these basins must have undergone major episodes of paleo-heating. Integration of apatite fission track analysis (AFTA) and vitrinite reflectance (VR) in a reconnaissance study of this region has identified two major episodes of heating and cooling. AFTA data from outcrop samples within the Hodgkinson Basin suggest a major episode of Cretaceous cooling from paleotemperatures varying between 70 to 110®C, or above. During this Cretaceous interval, at least two discrete cooling episodes may have affected the region, a mid-Cretaceous episode in which cooling began between 110 and 100 Ma, and a late Cretaceous cooling episode beginning between 80 and 70 Ma. Furthermore, AFTA data from samples which reached peak Cretaceous paleotemperatures less than ~100®C also show evidence for an earlier cooling episode. These samples cooled from paleotemperatures of least 95°C beginning some time between 250 and 200 Ma (Late Permian-Early Jurassic). VR data from Permian units suggest paleotemperatures during this time may have reached ~ 200 to 220°C. VR and AFTA data from Jurassic and Cretaceous well samples from the Ebagoola-1 well (located in the central part of the Laura Basin) indicate paleotemperatures of around 100 to 130°C prior to cooling beginning some time between 110 and 80 Ma. VR data from two other wells, Marina-1 and Breeza Plains-1, also show paleotemperatures of -95 to 130°C prior to cooling, presumably during the same episode. While a Late Permian to Early Jurassic cooling episode was not recognised from AFTA of the Ebagoola-1 samples, on the basis of VR data from Permian? units intersected in the other two wells, this event is also inferred to have probably affected the pre-Jurassic units of the Laura Basin. Without a constraint on the paleo-geothermal gradients (either from a vertical or well section), the nature of the paleo-heating episodes identified in the Hodgkinson Basin remains uncertain. The regional nature of the episodes suggests the paleotemperatures must primarily be due to greater depth of burial, with subsequent cooling caused by uplift and erosion. Assuming a paleogeothermal gradient of 30°C/km prior to Cretaceous cooling, the total degree of cooling experienced by the samples corresponds to a former depth of burial by between -1.7 and 3.0 km of post Permian-Lower Triassic section, which was subsequently removed by uplift and erosion beginning in the Cretaceous. This seems a reasonable scenario for most of the Hodgkinson Basin. However the geology of Laura Basin suggests the origin of the observed Cretaceous paleo-effects may be more complex in this region. The youngest preserved unit in the Laura Basin is Albian in age (-100 Ma), and thus the time of cooling identified from AFTA along the northern margin the Hodgkinson Basin (110 to 100 Ma) overlaps with the deposition of the overlying units. This conflict in timing is also observed in the Ebagoola-1 well. Thus the Cretaceous cooling identified in the Laura Basin and the underlying Paleozoic sediments is probably unlikely to reflect uplift and erosion, since the region was receiving sediment at the time. The paleotemperature profile derived from AFTA and VR from the Ebagoola-1 well suggests that the Cretaceous paleo-thermal effects may be related to the introduction of heat at a shallow level within the section, possibly as a result of hot fluid circulation. Thus in the Laura Basin, the mechanisms responsible for the observed paleo-thermal effects may be complex and involve more than simple heating due to depth of burial and cooling resulting from uplift and erosion.
291
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
GEOLOGY AND GEOCHEMISTRY OF THE WHITE PEVE SEDIMENT-HOSTED STRATIFORM COPPER DEPOSIT: A REVIEW Jeffrey L. Mauk^ ^Geology Department, The University of Auckland, Private Bag 92019, Auckland, New Zealand
White Pine is a major, well-studied sediment-hosted stratiform copper deposit that is located in the Lake Superior portion of the 1.1 Ga North American Midcontinent Rift System. Mineralisation at White Pine occurs within grey beds in the basal 1 to 3 m of the siltstones and shales of the Nonesuch Fm. The deposit has a classic mineral zonation, with basal native copper and chalcocite passing upward to a chalcocite zone, that is overlain by a cm-scale fringe where cupriferous rocks pass upward to pyritic rocks that characterise the rest of the Nonesuch Fm. At White Piae, there is evidence for three generations of faulting: (1) synsedimentary faulting, (2) subsequent high-angle, dominantiy normal faulting, and (3) thrusting. Two stages of copper mineralisation occur at White Pine. The first, main-stage mineralisation, formed a classic sediment-hosted stratiform copper deposit during early diagenesis. The second stage of mineralisation was synchronous with thrusting and introduced additional copper to the White Pine ore body. This corresponded with a regional-scale compressional event and native copper mineralisation in the Keweenaw district 100 km northeast of White Pine. Main-stage mineralisation provided approximately 90 percent of the copper that was recovered from the mine, though second-stage mineralisation was economically important in some areas of the mine. A 207p|3_206p|3 isochron age of 1081±9 Ma of a synsedimentary limestone at the base of the Nonesuch Fm represents calcite diagenesis during main-stage mineralisation (Ohr, 1993). This is indistinguishable from the depositional age of the underlying Copper Harbor Conglomerate, requiring the precipitation of main-stage mineralisation within 10 m.y. of the deposition of the Nonesuch Fm. Data from second-stage calcite veins indicate that they formed at least 30 m.y. after main-stage mineralisation (Ohr, 1993). Despite relatively low temperatures of mineralisation (<100®C), the rocks at White Pine record a thermal anomaly. Indigenous bitumen from mineralised Nonesuch Fm at White Pine is of significantiy higher thermal maturity than bitumen from unmineralised upper Nonesuch Fm in subjacent portions of the rift. Detritalsh^ed chlorite in the mine area is chemically homogenous, but the Fe/(Fe+Mg) ratios of detrital-shaped chlorite outside the area range from 0.27 to 0.54. These data are consistent with homogenisation of detritalshaped chlorite in the White Pine mine area by hydrothermal fluids during copper mineralisation, producing an "alteration halo" with a radius of 50 to 100 km. Sulfiir isotope studies indicate that that isotopically-enriched diagenetic pyrite in the Nonesuch Formation was replaced during main-stage mineralisation to produce chalcocite with positive ^^^S values. During main- and second-stage mineralisation, modified igneous sulftir was introduced into the mine region to form copper sulfides and hydrothermal pyrite with negative 5^'^S values. Fluid migration during compressional faulting and second-stage mineralisation introduced petroleum into the White Pine region. These petroleums, now trapped as fluid inclusions in second-stage veins, were significantiy water-washed, with water:petroleum ratios ranging from 200:1 to very much greater than 7000:1. White Pine inclusion oils thus represent one of the best end-member examples of water-washing in natural systems. REFERENCES
Ohr, M., 1993, Geochronology of diagenesis and low-grade metamoiphism in pelites: unpub. PhD thesis. The University of Michigan, 147 p.
292
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
THE ORIGIN OF BANDED VEINS AT THE GOLDEN CROSS EPITHERMAL DEPOSIT, NEW ZEALAND Jeffrey L. Mauk^ and Christopher R. J. Simpson^ ^Geology Department, The University of Auckland, Private Bag 92019, Auckland, New Zealand Macraes Exploration, P.O. Box 84, Palmerston, Otago, New Zealand
The Golden Cross mine is a low sulfidation epithemal gold deposit that formed at approximately 200°C, at less than 0.5 km depth. Banded veins consist of alternating black, white and grey bands. By analogy with active geothermal systems, we interpret differences in mineralogy, textures, and chemistry among different bands as reflecting differences in their mode of origin. Black bands contain (1) very fine-grained quartz that presumably recrystallised from amorphous silica, (2) relatively disordered adularia, and (3) more abundant sulfide. In contrast, white bands contain (1) coarsergrained quartz, (2) more ordered adularia, and (3) less abundant sulfide. All three differences can be explained by formation of black bands from vigorously boiling hydrothermal solutions, and white bands from sub-boiling or gently boiling fluids. Vigorously boiling, rapidly cooling fluids formed abundant silica nuclei, leading to very-fine grained amorphous silica precipitation. Gentle boiling conditions allowed formation of fewer, larger silica nuclei, leading to precipitation of coarser-grained silica. With time, amorphous silica recrystallised to form quartz, but our observations suggest that the grain size of the quartz in individual bands in hydrothermal veins is proportional to the grain size of the initial silica deposits. XRD analyses of adularia permit determination of its structural state, or degree of ordering. Relatively disordered adularia in the black bands versus more ordered adularia in the white bands is consistent with precipitation of black bands from vigorously boilmg solutions and white bands from gentle boiling conditions. Vigorous boiling of geothermal fluids causes loss of H^S, which leads to gold precipitation and sulfide formation. Thus, black bands in Golden Cross veins contain more abundant sulfide than the white bands. Gold occurs at the Golden Cross mine as electrum, with variable Ag/Au ratios. The electrum appears to be in equilibrium with acanthite. Electron microprobe analyses reveal that the average electrum composition is 43 wt % Au, with a range from 68 wt % Au to native Ag. Many electrum grains exhibit some zoning, with relatively Ag-rich rims, and relatively Au-rich cores. Some electrum at Golden Cross shows cyclic growth zones, varying from Au60 to AulO. The increase in Ag content from core to rim in electrum grains can be used to help evaluate possible mechanisms for precipitating gold. Published experimental data, coupled with estimates of the temperature of formation of the ore at Golden Cross, can constrain the log fS>^ for the ore-forming fluids. Isothermal precipitation of electrum from a 200°C fluid would limit the log f^^ to approximately -12 to -16. Boiling hydrothermal solutions lose H2S, which can cause precipitation of electrum. Ag-rich electrum, such as that on the rims of grains at Golden Cross, forms at a lower log fSi^ than Au-rich electrum. The observed zoning of electrum grains is therefore consistent with a general boiling model. Several lines of evidence thus support the hypothesis that banded veins at Golden Cross formed from solutions that alternated between vigorously boiling and gentle boiling conditions. As proposed by workers in other systems, changes in boiling conditions most likely result from rapid depressurisation brought about by hydrofi:acturing of host rocks, changes in groundwater levels, orfiracturingfromtectonic activity.
293
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
AN EXPERIMENTAL STUDY OF SULFIDE SOLUBILITY IN MAFIC MAGMAS John A. Mavrogenes' and Hugh St. C. O'Neill Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 ^ also at Department of Geology, Australian National University Canberra, ACT 0200
The solubility of sulfur in magmas affects a wide range of processes, including trace element behaviour, magmatic sulfide deposition, and mantle/core formation. However, the factors controlling sulfur solubility remain poorly understood. For most geological applications, the quantity of interest is the sulfur capacity at sulfide saturation (SCSS), i.e., the amount of S in the magma co-existing with an immiscible sulfide phase. Possible important parameters controlling SCSS in silicate melts include fS2, f02, temperature, pressure and melt composition. In order to investigate quantitatively the relative effects of each of these variables, we have undertaken a series of experiments in which representative basaltic and picritic compositions were studied as a function of pressure and temperature from 5 to 90 kb and 1400 to 1800°C, using piston-cylinder and multi-anvil solid media pressure devices. Three distinct regimes of oxygen fugacity were investigated, imposed by the use of Feioo, Fe4oIr60' ^ ^ Fe2oIr80 capsules.. Runs were rapidly quenched, polished and viewed optically to ensure that an immiscible sulfide phase was present in the charge. The compositions of quenched run products, including the S contents of the silicate glasses, were determined by electron microprobe analysis. Theoretical considerations suggest that SCSS values (in ppm) can be described by an equation of the form: ln[S / ppm Iscss = Y + B +
+
where A and B are functions of the composition of the silicate melt. This equation implies that SCSS is independent of fS2 and f02, except insofar as these factors influence the nature of the sulfide liquid (hence Inafjes). The experiments reported here confirm this. The values of SCSS of both the basaltic and picritic compositions are rather insensitive to temperature, but show a strong exponential decrease with increasing pressure. Consequently, a magma generated in equilibrium with residual sulfide in the mantle becomes undersaturated in sulfide during adiabatic ascent. On emplacement of the magma, sulfide saturation should occur only after substantial crystallization under closed-system conditions, or after significant modification via assimilation (e.g., of S-rich sediments). This simple observation is potentially of extreme importance in understanding the origin of magmatic sulfide deposits. Acknowledgements:: We would like to thanks WMC for generously supporting this work.
294
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
U-Pb, Sm-Nd, AND K-Ar CONSTRAINTS ON THE THERMAL HISTORY OF THE ALICE SPRINGS OROGENY IN THE HARTS RANGE, SOUTHEASTERN ARUNTA INLIER, CENTRAL AUSTRALIA Jo Mawbv^ Martin Hand\ John Foden\ Simon P. Kelly^ Peter Kinny^ and Ian McDougall'^ ^ Department of Geology and Geophysics, Adelaide University, Adelaide, SA 5005, Australia. 2 Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MKT 6AA, UK. ^ Tectonics Special Research Centre, School of Applied Geology, Ciirtin University of Technology, GPO Box U1987, Perth 6001, Australia. ^ Research School of Earth Sciences, Australian National University, Canberra 0200, Australia.
The Devonian-Carboniferous Alice Springs Orogeny (ASO) was a major intraplate event in central Australia that resulted in exhumation of the Arunta Inlier from beneath a once continuous broad intracratonic basin. The affects of the ASO were most intense in the southeastern Arunta Inlier, particularly in the Harts Range region, where up to 50% regional shortening was associated with amphibolite facies metamorphism. In this contribution we present thermochronologic data from the Harts Range region that places some additional constraints on the thermal evolution of the region during the ASO. In the SE Entia Gneiss Complex (EGC) which forms the structurally lowest domain in the Harts Range, monazite U-Pb isotopic data from a kyanite-gamet-biotite-quartz-plagioclase schist gives an age of 343 ± 8 Ma. P-T data from the sample suggests the monazite grew at - 630°C, 6-7 kbar. The monazite is overprinted by sillimanite-bearing shearbands associated with south-directed thrusting and high-T decompression, indicating that exhumation of the EGC began after 343 ± 8 Ma. The monazite U-Pb data are essentially identical to less precise Sm-Nd isotopic data from the EGC. Homblende coronas that formed at - 6 2 0 ^ , 6kbar and overprint the peak assemblages (-TIS^C, 9 kbar) formed at 343 ± 44 Ma, (2-point Sm-Nd hbd-wr). Coarse unfoliated metasomatic gt-hbd-plag±qtz rocks which formed at - 620°C, 6 kbar give 339 ± 39 Ma (Sm-Nd gt-hbd-wri-wrj MSWD = 0.72). In the southeastern EGC pegmatite which intruded at 330 ± 6 Ma (zircon U-Pb) has been isoclinally folded during deformation at ~ 500°C. This age constraint on temperatures > 500°C is supported by homblende K-Ar and IR laser probe single grain "^Ar-^^Ar data which suggests the SE Harts Range cooled through ~500°C at 330-340 Ma. Ar^^-Ar^^ data from muscovite in adjacent samples suggests cooling through '-'300°C occurred at 315 Ma. In comparison, the SW Harts Range cooled much earlier, with K-Ar and "^Ar-^^Ar data from homblende indicating cooling through 500°C occurred at ~ 370 Ma. "^Ar-^^Ar data from muscovite in the same area gives ages around 330-340 Ma. When our data are combined with existing thermochronologic data from the Harts Range a picture emerges of diachronous cooling with the SW Harts Range cooling through '-500°C some 50 Ma before the eastern and northern Harts Range. The combined data shows a step in the -500°C cooling ages of ~ 40 Ma across an approximately NW-SE trending boundary that is oblique to the structural trends in the Harts Range but is roughly parallel to the major shear zones that bound the Harts Range to the north and south. Several implications arise from this expanded dataset: (1) the interpretation that parts of the SE Arunta Inlier were still at > 20 km depth in the mid-Carboniferous means that exhumation rates in the eastern part of the Alice Springs Orogen either increased with time, or began later than in regions to the west, (2) in the EGC, the peak metamorphic assemblages (~725°C, 9 kbar) appear to have formed in the early Ordovician (Mawby et al this volume) and overprinting of these assemblages by lower pressure Devonian-Carboniferous assemblages ('-620®C, 6kbar) associated with the ASO means the EGC records medium to high-T Palaeozoic polymetamorphism, (3) the northward younging in cooling ages in the Harts Range suggests that south-dipping thrusts to the north of the region may have played a major role in the exhumation of the region. The synchronicity of this cooling/exhumation with the development of major shortening structures to the south of the Harts Range emphasises the bi-vergent nature of the ASO in the SE Arunta Inlier, (4) the Devonian and Carboniferous "^Ar^^Ar and K-Ar data should not be directly related to the kinematic evolution of structures in the Harts Range, since most of the map-scale structures formed significantly above 500°C. This is highlighted by the knowledge that much of the structural and metamorphic record in the terrain reflects an Ordovician event (Mawby et al this volume) rather than the Devonian-Carboniferous Alice Springs Orogeny.
295
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
ORDOVICIAN GRANULITES IN THE SOUTHEASTERN ARUNTA INLIER: A NEW TWIST IN THE PALAEOZOIC HISTORY OF CENTRAL AUSTRALIA. Jo Mawbv^ Martin Hand\ John Foden^ and Peter Kinny^ ^ Department of Geology and Geophysics, Adelaide University Adelaide, SA 5005, Australia. Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth 6001, Australia.
The high grade metamorphic history of the Arunta Inlier in central Australia is almost universally considered to record the affects of a complex polymetamorphic Palaeoproterozoic and Mesoproterozoic tectonic history. In the Harts Range in the southeastern Arunta Inlier the peak metamorphic assemblages and structures are thought to have formed during the latter stages of the Strangways Orogeny (-1780 - 1730 Ma). Here we present Sm-Nd isotopic data supported by U-Pb data from monazite which indicates that regional granuhte facies metamorphism in the Harts Range occurred in the early Ordovician. This data represents a radical departure from the existing models of the evolution of central Australia during the early Palaeozoic. In the Irindina Supracrustal Association (ISA) in the Harts Range, the metamorphic peak is defined by coarse grained (3 - 50 nun) gt ± cpx-bearing mafic, and gt-bearing metapelitic migmatites which formed at around SOO^'C and 9 kbar. Sm-Nd isotopic data from 3 peak metamorphic samples give 473 ± 33 Ma (gt-cpx-hbd-sph-wrpwrjwr3, MSWD = 0.98), 485 ± 13 Ma (gt-hbd-wr, MSWD = 1.44) and 462 ± 35 Ma (gt-wr-bi, MSWD = 0.29) respectively. These have been overprinted by a complex system of regionally sub-horizontal mylonitic fabrics that formed at ~700°C and 6.5 kbar and comprise a sheet up to 2 km thick. Sm-Nd isotopic data from the recrystallised mylonitic assemblages and a coarse unfoliated gamet-clinopyroxene-bearing marble interpreted to have recrystallised during the mylonitic deformation gives the following ages: 477 ± 2 Ma (gt-hbd-wr, MSWD = 0.90), 467 ± 4 Ma (gt„ gt2, cpx-wr, MSWD = 0.98), 460 ± 38 Ma (gt-hbd-wr, MSWD = 0.81), 458 ± 63 Ma (gt-sill-wr, MSWD = 0.99). There is a growing awareness that Sm-Nd isotope systems in coarse grained high gra^ rocks can preserve information from temperatures in excess of 750°C, suggesting the data from the Harts Range records a major Palaeozoic high-T thermal event. This is supported by a monazite U-Pb age (467 ± 8 Ma) from a peak metamorphic garnet-bearing metapelitic migmatite and together, the data indicate the Harts Range region contains the youngest known regional granulites in Australia (see also Miller et al this volume). The association of near isothermal decompression and sub-horizontal mylonitic reworking in the ISA strongly suggests the clockwise P-T evolution was associated with extension. In the Entia Gneiss Complex (EGC), peak metamorphic conditions are ~725°C, 8-9 kbar. A two point isochron using the core of a 50 mm garnet and the surrounding whole-rock gives 479 ± 9 Ma suggesting that metamorphism in the EGC and the overlying ISA was coeval. However there is no evidence that temperatures in the EGC were ever as high as in the overiying ISA, implying the terrains were juxtaposed after the peak metamorphism in both, resulting in a pseudo-inverted metamorphic gradient. The two domains are separated by die mylonitic Bruna Granitic Gneiss (BGG) which records south-directed transport at upper amphibolite facies conditions (680°C - 6 kbar), and based on the juxtaposition of peak metamorphic assemblages, we suggest this mylonite zone is a compressional system. Sm-Nd data from the BGG gives an age of 448 ± 7 Ma (gt-hbd-wr, MSWD = 0.96). A less precise date of 445 ± 51 Ma (gt-hbd-wr, MSWD = 0.65) from an amphibolite facies thrust (650°C, 5.5 kbar) at the contact of the ISA and the Strangways Metamorphic Complex in the NW Harts Range also indicates Ordovician compression. We interpret this compression to postdate the extension in the ISA. At present the extent of the early Ordovician granulite event is unknown, however granuhte grade metamorphism and associated intense deformation implies an event of regional significance. In this regard we note that rocks belonging to the distinctive ISA extend > 30 km to the east of the Harts Range. Additionally in the Strangways Metamorphic Complex to the west of the Harts Range, granulite facies reworking is associated with undated E and NE-down extensional mylonites (Norman 1991; Goscombe 1992). The P-T evolution associated with this reworking is quantitatively similar to the Ordovician metamorphism in the ISA in the Harts Range, and although tentative at this stage, we suggest a domain in the Arunta Inlier stretching from the NW Strangways Range to a least 30 km east of the Harts Range has been affected by high grade early Ordovician metamorphism.
References Goscombe, B. 1992. High grade reworking of central Australian granulites, metamorphic evolution of the Arunta Complex. Journal Petrology, 33, 917-962. Norman, A.N., 1991. The structural and metamorphic evolution of the central Arunta Block: evidence from the Strangways Metamorphic Complex and the Harts Range Group, central Australia, unpublished Ph.D. thesis Macquarie University.
296
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998 EXPERIMENTAL STUDIES IN THE SYSTEM PbS-FeS-ZnS±Ag2S: IMPLICATIONS FOR THE BROKEN HILL OREBODY Ian W. Macintosh and John A. Mavro^enes Research School of Earth Sciences and GEMOC, Department of Geology, The Australian National University, Canberra, A.C.T. 0200
Broken Hill N.S.W. hosts one of the world's largest Pb-Zn-Ag deposits. For over 100 years geologists have argued over the genesis of this deposit, yet its origin remains enigmatic because of the intense metamorphism that has obscured/modified the orebody. Various models for the genesis of Broken Hill recognise metamorphic remobilisation of the mineralisation (White et al., 1994). If partial melting of sulfides is possible under peak Broken Hill metamorphic conditions, then many of the unusual characteristics of the Broken Hill ores may be more easily explained. These unusual features include apparent inverted metal (Zn-Pb) zoning (Stanton, 1967) and the presence of Pb-rich sulfide dikes (droppers) extending out from the main lode into shears that cross-cut post ore dikes. The lack of quantitative information in this system limited Lawrence (1967) to speculate that sulfide partial melting during metamorphism may have occurred. An experimental study of melting in the system PbS-FeS-ZnS±Ag2S was undertaken to determine the eutectic temperature in this system as a function of temperature and pressure. Early 1 atm. experiments on the Fe-Pb-S system (Brett and Kullerud, 1967) showed the presence of a melt phase at relatively low temperatures (<850°C) and suggested that the addition of ZnS may produce melts at even lower temperatures. Furthermore, they proposed that sulfide deposits (such as Broken Hill, N.S.W.) may have experienced partial melting during highgra(k metamorphism. Avyetisan and Gnatyshenko (1956) determined the 1 atm. eutectic temperature in the system PbS-FeS-ZnS to be 820°C. Given that peak metamorphism at Broken Hill exceeded 800°C and 5 kbars the affects of pressure on the eutectic temperature must be ascertained before we can determined if partial melting of these ores was possible. In the present study experiments at 1 atm. were performed in sealed silica-glass tubes, while those at elevated pressures were carried out in piston-cylinder apparatus. For the system PbS-FeS-ZnS the eutectic temperature and composition was found to be 800±2°C and 63%PbS, 31%FeS, 7%ZnS respectively, at 1 atm. The eutectic temperature at 27 kbars was found to be 960°±10°C with a composition enriched in ZnS relative to the 1 atm. eutectic. These results allow interpolation of the eutectic temperature at any pressure, with an increase of approximately 6°C/kbar. Thus, at 5 kbars the eutectic temperature for the pure system is 830°C, however the addition of 1 wt.% Ag2S depresses the eutectic temperature to 770±5°C at 1 atm. and 805±5°C at 5 kbars. The conditions of peak metamorphism for the central and southern areas of Broken Hill Block lie above the solidus of the PbS-FeS-ZnS+l%Ag2S system, strongly suggesting that partial melting of the sulfide orebody occurred. If the sulfides of the Broken Hill orebody were partially molten during metamorphism, then information on the current metal zoning of the deposit sheds no light on its genesis. Consequently, mineral exploration programs targeting exhalative deposits in high-grade metamorphic terranes should not adhere too closely to current stratigraphically concordant mineralisation models. REFERENCES Avyetisan, K., and Gnatyshenko, G.I., 1956. Thermal and metalographic study of the lead sulfide-zinc sulfideiron sulfide system. Izvest. Akad. Nauk. Kazak. S.S.R. Ser. Gom. Dela., Stroimaterialov i Met 6, 11-25. Brett, R., and Kullerud, G., 1967. The Fe-Pb-S System. Economic Geology, v. 62, p. 354-367. Lawrence, L.J., 1967. Sulfide neomagmas and highly metamorphosed sulfide deposits. Mineralium Deposita 2, 5-10. Stanton, 1967. Petrochemical studies of the ore environment at Broken Hill, N.S.W.:3-banded iron formations and sulfide ore bodies: Constitutional and genetic ties. Trans. Inst. Min. Metall Sec. B Appl. Earth Sci 85,. 132-141. White, S.H., Rothery, A.L., Lips, A.L., and Barclay, T.J.R., 1994. Broken Hill area, Australia as a Proterozoic fold and thrust belt: Implications for the Broken Hill base metal deposit. Trans. Inst. Min. Metall Sec. B Appl. Earth Sci. 104, 1-17.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
THE USE OF SEAWATER NEUTRALISED RED MUD FROM BAUXITE REFINERIES TO CONTROL ACID MINE DRAINAGE AND HEAVY METAL LEACHATES D. McConchie. M. Clark and C. Hanahan Centre for Coastal Management, P.O. Box 5125, East Lismore, NSW, 2480
Seawater neutralised bauxite refinery residues (red mud) consist of a mixture of minerals dominated by hematite, quartz, boehmite, gibbsite, calcite, sodalite and whewellite, with minor hydrotalcite, hydrocaluminite, halite, lepidocrocite, brucite, ferrihydrite and cancrinite. The red mud is dominated by particles with a high surface area/volume ratio and a high charge/mass ratio, and has a high acid neutralisation capacity due to an abundance of amorphous and finely crystalline mineral phases that form weak bases. Greater than 3.5 moles of acid can be neutralised by 1kg of diy red mud, but despite its high acid neutralisation capacity, and unlike red mud that has not been seawater neutralised, it is not caustic and has a soil reaction pH of 8.5-8.8. The texture and mineralogy of seawater neutralised red mud give it a very high trace metal trapping and binding capacity (>1000meq/kg ^ mud, at pH values >6.5) and the ability to strip metals from water in contact with it (e.g. Table 1), irrespective of the salinity of the contaminated water. The trace metals are largely bound to poorly crystalline Al- and Fe-oxyhydroxides or as constituents of hydroxide, oxide, oxyhydroxide or carbonate minerals. However, the binding is not a simple adsorption process because once the trace metals are bound to the seawater neutralised red mud they can not be readily leached off; evidence from the oldest available seawater neutralised red mud storage ponds indicates that it remains stable for at least 20 years. Table 1: Effect of treating lOL of contaminated water from the Mt Morgan mine near Rockhampton, Queensland, by passing it through lOOg of seawater neutralised red mud.
Species
Before Treatment
After Treatment
pH Alkalinity Chloride Cyanide Sulphate
2.76 0 mg/L 385 mg/L 23.6 mg/L 11750 mgyL
8.53 346 mg/L 530 mg/L 0.5 mg/L 9250 mg/L
Calcium Magnesium Potassium Sodium
677 mg/L 852 mg/L 22.6 mg/L 221 mg/L
884 mg/L 865 mg/L 54.6 mg/L 302 mg/L
Species
Before Treatment
After Treatment
Arsenic Cadmium Chromium Cobalt Copper Iron Lead Manganese Nickel Zinc
334 iig/L 160 |ig/L 1,460 |ig/L 563 \igfL 13,580 iLig/L 59,940 |ag/L 2,340 |ig/L 29,630 ^ig/L 325 iig/L 7,430
<ljiig/L <10 |ig/L <50 iiig/L <20 ^ig/L <20 M-g/L <50 jig/L <20 |ig/L <50 |Lig/L <20 ^ig/L <10 iig/L
Note: Trace metal concentrations were reduced to non-detectable levels for the atomic absorption analytical procedures used and it is highly likely that the actual metal concentrations in the treated water are well below the indicated lower limits of detection. In other tests, using more sensitive analytical procedures, the concentration of arsenic in a 10 mg/L test solution was reduced to < 0.1 ^g/L.
Many different strategies have been used to control the formation and escape of acid mine drainage water and metal-rich tailings leachates, but all are costly and many involve very long term monitoring and management; larger sulphidic waste dumps may also tie up large areas of land indefinitely. Seawater neutralised red mud provides a cost-effective alternative for use in at both current and former mine sites where management of acidic fluids that have a high trace metal load is required. The red mud can be used to treat waste water or solid sulphidic material by direct mixing with the waste, or it can be used to form a permeable subsurface barrier 'filter curtain' between the source of contaminated water and the surrounding environment; the barrier would not impede subsurface water flow but would neutralise any acid and strip any trace metals from the water before it reached natural waterways. In forming a 'filter curtain' a trench would be positioned down the hydraulic gradient from the source of contaminated water, back filled with red mud pellets and covered with clean topsoil; the barrier could be designed with sufficient capacity to treat all the contaminated water that is likely to encounter it, or it could be designed such that the red mud would be periodically removed to a contaminated waste dump and replaced before its acid neutralisation or contaminant trapping capacity was reached. Future work will be directed toward determining exactly how the red mud traps the metals and how it can be converted into a form that is more suited to use in environmental management applications. Acknowledgements: We would like to thank Queensland Alumina Ltd., particularly Richard Fawkes, for instigating and supporting all stages of our geochemical studies of seawater neutralised red mud.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
CADMIUM IN MOLLUSCS FROM NEAR-PRISTINE ENVIRONMENTS IN NORTHERN AUSTRALIA: A NOTE OF CAUTION ON THE USE OF MOLLUSCS TO MONITOR HEAVY METAL POLLUTION p. McConchie and F, Davies-McConchie Centre for Coastal Management, P.O. Box 5125, East Lismore, NSW, 2480
Molluscs, particularly oysters and mussels, have often been analysed as indicators of heavy metal loads in estuarine and near-shore marine environments because they are common in many areas, they do not move between sites, and they are good bioaccumulators of many metals; they are also analysed to determine v^hether they are safe for human consumption. However, recent discoveries of oysters with anomalously high cadmium loads in several near-pristine areas in northern Australia suggest that the use of molluscs as indicators of heavy metal contamination may need to be viewed with more caution and that cadmium concentrations in molluscs may not always provide a direct indication of cadmium loads in their environment. Three mechanisms that can cause a substantial increase in the cadmium content of oysters from unpolluted waters are described. 1. In Shark Bay (W.A.), which is geographically remote from all known industrial or geological sources of heavy metals, the cadmium content of oysters commonly exceeds 5mg/kg (wet weight); concentrations in many Shark Bay oysters are well above the 2mg/kg limit that applies to oysters for human consumption, and above concentrations in oysters from areas with known heavy metal pollution. The cadmium concentration in Shark Bay seawater is similar to that in average oceanic water (0.06^g/L), and in sediment it seldom exceeds 5|Lig/kg. No explanation for the high cadmium load in the molluscs was apparent until it was found that molluscs had their highest cadmium concentrations in areas of the bay where turbulence was high and the sediment contained fme hematite (up to 2%) eroded from nearby Peron Sandstone cliffs. Although the hematite in the Peron Sandstone cliffs has a low cadmium content (< lOp.g/kg), it develops a negative surface charge in seawater (the mean seawater pH of 8.15 is above the pH = 7.8 isoelectric point for the hematite) and a strong tendency to adsorb cadmium from the water column; adsorption is extremely efficient at very low cadmium concentrations. When the hematite particles, with their adsorbed cadmium, are resuspended by turbulence they can be ingested by the filter-feeding organisms and the lower pH in the gut of the oysters releases the cadmium, which c ^ accumulate in the oyster. For areas with a similar turbulence the correlation between the amount of hematite in the sediment and the cadmium in the molluscs is high (r^ = 0.77); the link is also supported by the results of tests carried out in laboratory aquaria. Hence, although the hematite constitutes only a small proportion of the substrate sediment, it efficiently preconcentrates cadmium from the water and enhances its uptake by molluscs. 2. Near Egg Island and Sunday Island and along the southern part of Dirk Hartog Island (Shark Bay, W.A.) oysters have a very high cadmium content but there is no hematite in the sediment. However, these areas host large colonies of seabirds and investigations have shown that the bird droppings have a high cadmium content (average 8.8mg/kg) and that the thin phosphatic (guano-enriched) soil horizon near the colonies has a mean cadmium content 5.6mg/kg. Both the fme soil particles and the bird droppings can be transferred to the sea near the colonies by wind or surface runoff, thereby increasing the availability of cadmium to oysters growing nearby; the cadmium content in the oysters decreases with increasing distance from the bird colonies. On Nauru Island, similarly elevated cadmium loads were found, by Blake and McConchie, in molluscs near facilities where the cadmium-rich Nauruan rock phosphate ore was loaded for export. 3. In Melville Bay (Amhem Land, N.T.) oysters growing near the NABALCO Refmeiy and the Gove Harbour Marina all have a low cadmium content (average l.lmg.kg wet weight) and a zinc content that is typical of that in healthy oysters (300 - 800 mg/kg wet weight); galvanised iron structures and sacrificial anodes on boats are effective sources of zinc in all harbour areas. However, as distance from the refinery and harbour facilities increases, the cadmium content of the oysters increases progressively to over 5mg/kg and the zinc content decreases to less than lOOmg/kg; the increase in the cadmium concentration correlates strongly with the decrease in the zinc concentration (r^ = 0.83). This finding suggests that in remote parts of Melville Bay molluscs are compensating for a deficiency in the availability of zinc by increasing the uptake of the geochemically similar metal cadmium. An increased uptake of cadmium by diatoms and crustaceans in zinc deficient environments has been reported elsewhere and there is a good possibility that high cadmium concentrations in the tissue of sheep and cattle from parts of westem and central Australia where there is no identifiable source of cadmium may reflect a zinc deficiency that could be rectified by adding zinc to salt-licks. Hence, the common assumption that high trace metal loads in molluscs indicate anthropogenic pollution may not always be valid and additional evidence is required to support environmental management decisions. Acknowledgements: This research was supported by grants from Southern Cross University.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
EWEN PLUTONIC ASSOCIATION, NORTHERN MOUNT ISA BLOCK: TECTONIC AND METALLOGENIC SIGNIFICANCE Graeme D. McDonald and Kenneth D. CoIIerson Department of Earth Sciences, University of Queensland, St Lucia, QLD, 4072
The Mount Isa Block (MEB) in NW Queensland is an important segment of the North Australian Craton (NAC) and is host to a number of world class mineral deposits. As a result, a well constrained model detailing the tectonic evolution of the block is critical for the development of further exploration models in the area. The Ewen Block is an area in the northern MIB that provides further evidence in the debate over the regional tectonics. It is a relatively small region, approximately 90 km long and up to 18 km wide, that is dominated by the ca. 1.86 Ga Leichhardt Volcanics and a unit previously mapped as Ewen Granite. This study has shown that outcrops of Ewen Granite can be divided into three geochemically and petrogaphically distinct groups. We therefore propose that these rock types be collectively termed the Ewen Plutonic Association (EPA). Based on major element geochemical analyses the three rock types that constitute the EPA are: 1 - granites (76.3-78.0% SiOj), 2 - monzodiorites (52.2-59.7% SiOi) and 3 - syenites (52.6-57.4% Si02). The granites are characterised by low A l A (12.2-13.1%), MgO (0.22-0.35%) and CaO (0.56-0.86%) and elevated K2O (4.865.49%). The monzodiorites have elevated AI2O3 (15.6-17.8%), FejOs (6.70-11.3%), MgO (2.06-6.30%) and CaO (4.36-8.75%) and depleted NajO (1.84-3.37%) and KjO (1.66-3.89%) concentrations. Finally the syenites show distinctively high AI2O3 (-19.0%) and K2O (7.09-8.89%) with low MgO (1.87-2.04%) and CaO (2.05-3.86%). The granites and monzodiorites display normative hypersthene and the syenites are silica undersaturated exhibiting normative nepheline and olivine. Rare earth element (REE) patterns and trace element abundances for the three EPA groups are also significantly different. The granites have weakly fractionated REE patterns [(LaAn5)N=6.02] and distinctive negative Eu anomalies. The monzodiorites are also weakly to moderately fractionated [(JLa/Yh)^=l 1.8] but do not exhibit a Eu anomaly. The syenite REE patterns are moderately fractionated [(LaA?T5)N=19.4] and display elevated total REE contents and a weak negative Eu anomaly. The granites have trace element compositions typical of highly fractionated granites. They have elevated levels of Nb (12.6-24.3 ppm), Ta (3.31-4.29 ppm), Pb (41.6-54.8 ppm) and Rb (390-477 ppm) and significantly low levels of Ba (100-271 ppm) and Sr (33.5-59.1 ppm). Element ratios include low Ba/Rb (0.26-0.58) and elevated Rb/Sr (6.61-14.2) and Ga/Al (2.74-2.84). They are similar in age to the economically important leucogranite dominated plutons of the Pine Creek Block, Northern Territory, which are associated with U, W, Mo, Bi, Cu, Sn and Au mineralisation. Trace element abundances for the syenites are similar to alkaline igneous rocks. For example they have elevated concentrations of Ba (1244-1694 ppm), Rb (305-406 ppm), Nb (25.3-35.7 ppm), Ta (2.51-2.91 ppm), Y (41.7-57.8 ppm) and Zr (397-527 ppm) and low Sr (209-233 ppm), Ni (3.05-5.28 ppm) and Cr (3.25-5.42 ppm). They also have elevated Rb/Sr (1.46-1.74), Ga/Al (1.99-2.75) and Zr/Y (9.12-9.53) ratios. The syenites also exhibit evidence of potassic metasomatism and alteration by CO2 rich fluids that is similar to fenitisation associated with carbonatite complexes. Sm and Nd isotopic determinations were made on samples from all groups of the EPA. The syenites range in eNd(t) from -3.13 to -3.12 and yield depleted mantle model ages (T DM) between 2.47 and 2.48 Ga. Single samples of the granite and quartz monzoznite gave eNd(t) values of -2.89 and -4.09 respectively. The oldest T (DM), i.e., 2.66 was given by the monzodiorite sample. The granite gave a T (DM) of 2.49 Ga. The generation of these magmas may have involved different source regions, or contamination of mantle derived melts by Archaean crust. SHRIMP U/Pb zircon geochronology provides the most precise age yet obtained for any phase of the EPA. A syenite sample gives an age of 1843±7 Ma. This age is approximately 20 Ma younger than that established for the Kalkadoon/Leichhardt magmatism. McDonald et al. (1997) interpret the early crustal evolution of the MIB, including the formation of the Kalkadoon Granite and Leichhardt Volcanics, as being dominated by subduction related processes. The presence of alkaline igneous rocks (usually associated with extensional settings) combined with this interpretation suggests that the EPA may have been generated in a back-arc environment. Due to the similarities between members of the EPA and lithologies known to be asociated with economic mineralisation elsewhere in the NAC, this suite is considered to have considerable exploration potential. REFERENCES McDonald, G.D., CoIIerson, K.D. & Kinny, P.D., 1997. Late Archean and Early Proterozoic crustal evolution of the Mount Isa block, northwest Queensland, Australia. Geology, V.25, No.l2, 1095-1098.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
LOCAXmG ACTIVE SEAFLOOR HYDROTHERMAL DEPOSITS USING PARTICULATE PLUMES: THE SUSU KNOLLS EXAMPLE Leanne A. McDonald^ Raymond A. Binns^ Anne-Marie Huff® and Steven D. Scott^ ^CSIRO Division of Exploration and Mining, PO Box 136, North Ryde, New South Wales 2113 ^Marine Geology Laboratory, Department of Geology, University of Toronto, Toronto, Canada M5S3B1
Detecting, sampling and analysing the buoyant particulate plumes which rise into the seawater column above active seafloor hydrothermal fields is commonly the initial stage in exploring for present-day massive sulfide deposits, whether these are being sought as potential resources or as natural laboratories in which to study oreforming processes. The method plays the equivalent role to regional geochemical surveying in land-based mineral exploration, and has led to a number of recent discoveries on mid-ocean spreading ridges. Exploration of marginal basins, where massive sulfide deposits tend to be distinctly richer in precious metals, is more difficult because of irregular topography and bottom currents, varied bedrock geology, presence of separate water layers governed by basin sills, and effects related to higher sedimentation rates. However, plume detection remains a valuable tool and we here report an example of its successful application to discovery of the SuSu Knolls hydrothermal field in the eastern Manus Basin, Bismarck Sea, Papua New Guinea. We used a standard oceanographic CTD (conductivity-temperature-depth) rosette fitted with water-sampling bottles and a transmissometer (effectively measuring the cloudiness of water). This was lowered and raised on a conductive cable in "tow-yo" fashion as the ship moved along a predetermined survey track with the CTD and transmissometer signals being relayed to a shipboard monitor. At appropriate points a profile of water samples for subsequent chemical analysis was collected bracketing the level of any apparent plume. SuSu Knolls consists of three porphyritic dacite domes arranged 1 km apart as follows fi-om southeast to northwest: South Su (crest depth 1320 m). North Su (1150 m), and Suzette (1520m). Each dome is now known to possess actively-forming hydrothermal deposits. In 1994 SuSu Knolls was the site of harmonic seismic tremor after the nearby Rabaul eruptions. First indications of hydrothermal activity were obtained in 1993 during a "wildcat" CTD test 5 km northwest of North Su which showed a 0.7% transmissometer anomaly (% reduction in light transmission relative to background) around 1450 m depth. In a short orthogonal traverse the anomaly increased to 1.9%. With time running out a small ridge below these measurements was dredged and photographed but no hydrothermal deposits were found. In 1996 a systematic survey was extended to the southeast, revealing progressively higher anomalies peaking over SuSu Knolls where active massive sulfide chimneys and mound deposits were then discovered. Detailed mapping revealed two distinct levels of intense plume, a subcircular one at 1300-1600 m depth centred (-17% intensity) on the southwestern flank of South Su, and an asymmetric south-southwest trending one at 1050-1150 m depth with maximum intensity (33%) near the crest of North Su. Sporadic broadband sonic emission firom "screaming vents" was recorded over South Su. Bacterialfloewas extensive on the crests of North and South Su, but not at Suzette. When remapped 11 months later in October 1997, the lower plume had vanished apartfiroma small remnant with only 0.8% intensity immediately between North Su and South Su. The upper plume had become more sharply cut off to the north and west, its maximum intensity had increased to 44% over the crest of North Su where black-smoker chimney sources were photographed, and its orientation had shifted to easterly. It remained distinct (2%) and at 1060-1200m depth for the most remote observation point some 18 km to the east. Sonic emission at South Su had ceased, and bacterial activity had retracted to the vicinity of scattered hydrothermal vents. Samples from the peak plume anomalies (40% and 5% respectively) 1 and 10 km downcurrent from North Su showed pH reductions of 0.37 and 0.13 units relative to ambient seawater, and the following levels of total dissolvable metal: Fe, 0.27 and 0.09 ppm; Mn 0.034 and 0.016 ppm; Al, 0.022 and 0.010 ppm (the higher values being 135, 170 and 11 times background seawater). The 1 km sample smelled faintly of H2S, showed dissolved oxygen content 5mM/l below normal, and its temperature was 0.05°C above the ambient level. Other samples across the plume at this site showed correlated variations in Mg, Ca, Na, K and SO4 content suggesting that low chlorinity hydrothermal fluid has been contributed to the plume. The SuSu plume is possibly the most intense ever recorded in association with hydrothermal activity, yel some luck was also involved in this discovery. In 1997 there was no anomaly at the 1993 discovery site, nor at the "eye" of the 1996 combined plume. Plume emission can clearly shut down and current transport directions change rapidly. Neither in 1996 nor 1997 was there a plume component clearly derived from the Suzette chimney field, the richest and largest at SuSu Knolls, despite its activity as indicated by fauna, shimmering fluid and temperature anomalies. The influence of the chemistry and physics of venting on the presence and intensity of plimies is not well understood and must be borne in mind during seafloor mineral exploration.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE OSTEOLOGY AND TAXONOMIC STATUS OF KRONOSAURUS QUEENSLANDICUS LONGMAN 1924 (BRACHAUCHENIIDAE; PLESIOSAURIA). Colin McHenry Department of Zoology, University of Queensland St Lucia QLD, Australia
Although named in 1924 and famous worldwide as the largest pliosaur, Kronosaurus queenslandicus was established on undiagnostic holotype material and subsequent specimens referred to this taxon have never been adequately described. Consequently very little is known about what appears to have been the most massive of all plesiosaurs and one of the largest marine predators of all time. The present study examined the specimens of large pliosauroid that have been collected from the marine Aptian and Albian rocks of the Great Artesian Basin by the Queensland Museum since the 1930's, with a view to reconstructing the anatomy of Kronosaurus and resolving questions about its taxonomy. Although there is a lack of complete specimens the region has produced an number of good skulls, and it was found that Kronosaurus is most similar, not to the giant pliosaurids known from the Jurassic of Europe, but to the smaller pliosaur Brachauchenius lucasi from the Upper Cretaceous of North America, providing further support for Williston's (1925) concept of the Brachaucheniidae as a family of pliosaurs distinct to the better known Pliosauridae. Furthermore, no support was found for Molnar's (1982) suggestion that the famous 'Harvard Specimen', collected from Queensland in 1932, is a different species. There is no evidence of more than one taxon of large pliosaur in the Basin; however, the specimen at Harvard has been heavily restored and is probably inaccurate, and reconstructions of the total size based on this specimen are probably overestimates. The recently named Kronosaurus boyacensis, from the Aptian of Columbia, is probably not congeneric with K queenslandicus, but ftither study is needed to resolve the former's precise taxonomic status. Specimens of Kronosaurus with preserved stomach contents provide direct evidence of predation by Kronosaurus upon other marine reptiles. The giant pliosaurs of the Mesozoic have traditionally been thought of as generalised predators capable of taking large prey, ecologically analogous to the modem killer whale Orcinus orca, and all available data from Kronosaurus supports this interpretation.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
CHARACTERISTICS OF THE MANTLE SOURCE REGION OF THE LADOLAM GOLD DEPOSIT, LIHIR ISLAND, PAPUA NEW GUINEA Brent I. A. Mclnnes^'^. Noreen J. Evans^ ^ Michel G^6goi^e^ Chris Ryan^ ^ and Joanne McCarron' ^CSIRO, Exploration and Mining, P.O. Box 136, North Ryde, NSW 2113 ^GEMOC School of Earth Sciences, Macquarie University, North Ryde, NSW 2109
Are igneous intrusions that host giant Cu-Au ore bodies derived from "enriched" mantle source regions? This hypothesis has been difficult to test by studying porphyry Cu-Au deposits themselves because these deposits are typically hosted within igneous intrusives which are the products of extensivefractionalcrystallization of primary arc magmas. Moreover, there is a dearth of knowledge about Ae composition and structure of subduction-modified mantle at convergent margins because chemically unmodified mantle-derived arc lavas and xenoliths are rare. Dredge and TV-grab sampling of a 1 km submarine cinder cone (Tubaf volcano, 1280 m BSLj 3°15.25'S, 152®32.50'E), located 14km SW of the giant Ladolam gold mine (+40 M oz contained Au) on Lihir Island returned 130 ultramafic, mafic and sedimentary xenoliths (see Herzig et al., 1994, EOS 75(44), 513-515 for details). Petrological studies of these samples has provided an unprecedented view of the source region of an arc magmatic system with a clear propensity to produce giant ore deposits. The xenolith assemblage includes spinel Iherzolite, harzburgite, websterite, orthopyroxenite, serpentinite, gabbro, hornblende gabbro, plagiogranite, diabase, basalt, pelagic deep-sea sediment and shallow-water volcaniclastic sediment, as well as coralline and coralgal limestone. These lithologies represent a jumbled proxy drill hole with a minimum sampling depth of 17 km (depth to seismically determined Moho) and a maximum depth of 70 km (no gamet-bearing Iherzolite). Because these lithologies are distinctly similar to those reported from ophiolite suites, they can be reassembled into an "ophiolite-type" model of oceanic lithosphere. The mineralogy of the anhydrous Iherzolites (olivine: Mg# 0.87-0.91; orthopyroxene: Mg# 0.92 and average Al203=1.7 wt.%; Cr-diopside: En48Wo44Fs3^ with average Cr203=0.75, Na20=0.13 and Al203=2.0 wt.%; and spinel: Cr# 0.35-0.56) is similar to that of the Cr-spinel Iherzolite group (Type I). These mineralogical characteristics and the predominance of harzburgite (87%) over Iherzolite (13%) indicates that Ae mantle source region for the Lihir volcanoes is a depleted peridotite residue from an earlier episode of melting in a mid-ocean ridge environment. The spinel peridotite xenoliths record 2-pyroxene closure temperatures of 850-1050°C and unusually high oxidation states, ranging from +1.5 to +3 logy02 units greater than similar depleted peridotites sampled from abyssal mid-ocean ridge environments (-1 \ogfO2 unit less than the FMQ buffer). Some of the peridotite xenoliths have distinctive vein structures and mineral compositions generated by water-rock interactions within the mantle wedge. The veins are planar, interconnected networks (1 to 6 cm in width) which crosscut earlier ductile deformation. The veins contain fibrous, radiating orthopyroxene withfine-grainedFe-Ni sulfides and minor olivine, clinopyroxene, phlogopite and magnetite. The presence of H20-rich fluid inclusions in the orthopyroxene and the lack of shear structures in these veins indicates that metasomatism of peridotite occurred via hydraulic fracturing, probably as a result of the influx of slab-derived hydrous fluids into the mantle. Oxidation of the peridotite is synchronous with this event, as evidenced by the occurrence of zoned spinels (Fe^'^-rich cores and Fe^^-rich rims), and secondary magnetite in the orthopyroxenite veins. The xenolith suite sampled from the Tubaf submarine volcano allows a reconstruction of the physical and chemical properties of the oceanic lithosphere below Lihir Island. The mineralogical and geochemical characterisitics of the xenoliths indicates that the source region of Lihir magmatism consists of depleted mantie originally generated at a midocean ridge spreading center, and subsequently modified by subduction-related processes. The peridotitic mantle wedge was subjected to hydro-fracturing and hydration metasomatism at T=700-1000°C and 5-20 kbars pressure as a consequence of dewatering of a subducted slab. Peridotite-water interaction has created a hydrofractured network cf oxidized peridotite enriched in H2O, S, Si, Al, Cu, Sr, Pb, U, Th, Na, K and LREE. Work is currently underway to establish to what degree Au and other elements have also been transported. Preferential partial melting of these enriched mantle regions could account for the highly oxidized, sulfur- and metal-rich nature of the high-K calc-alkaline volcanoes of the Tabar-Lihir-Tanga-Feni island arc.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
WEIRD WESTERN WHORLIES E. Donald McKenzie Queensland Museum, P.O. Box 3300, South Brisbane, Queensland, 4101
The ammonites from the Aptian and Albian inundations of the Great Artesian Basin are much more diverse than previously published work would suggest. The last three inundations (conventionally dated as Late Aptian; late Early to early Middle Albian; and Late Albian, respectively) have 'known' characteristic ammonite faunules: The Late Aptian species are relatively cosmopolitan, but those from the two Albian periods are characteristically endemic. These generalisations are derived from the previous studies that concentrated on the more accessible eastern margins of the Basin. Since 1988, when the author started studying the ammonites under Mary Wade's guidance, the Queensland Museum has had a proactive collecting program to enhance its holdings of the Cretaceous ammonites, and much time has also been spent on preparation of the material. Now that the process of sorting and describing the material for publication is well underway, it is becoming clearer that the faunules are much more diverse than previously thought. This is especially true of the material from the western areas of the Basin - the deposits of which can be expected to represent a sampling not only of the fauna living in the epeiric sea but also of new species coming into the Basin from present north (Gulf of Carpentaria) on the anti-clockwise palaeocurrent. If some of the latter entered but did not survive and thrive in the enclosed sea, their remains most likely would be expected to be found along these western margins. And it is in this way that one can rationalise the observation of a few western specimens, including some that are only quite fragmentary, of ammonites not seen in the much larger collections from the east. Such known specimens from the Toolebuc Formation near Boulia include an undoubted Worthoceras - the only known specimen of this genus from the Great Artesian Basin. The interest is not just in the identification but also in the implications for the dating of the events in the Basin: either it is one of the earliest known examples of a genus th at had its greatest expression in the Cenomanian and Turonian; or have we set the starting date of the Toolebuc sea a bit early? Perhaps the Late Albian inundation began well up into this period rather than at 'earliest Late Albian' as is presently most generally accepted.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
RADIOGENIC HEAT PRODUCTION AND HIGH-TEMPERATURE LOWPRESSURE METAMORPHISM IN THE MOUNT ISA INLIER, QUEENSLAND Sandra McLaren. Mike Sandiford and Martin Hand Department of Geology and Geophysics, University of Adelaide, SA 5005
Most recent discussions on the origin of high temperature metamorphism involve either enhanced mantle heat flow or advective heat transfer. Advective heating is supported in many terranes by the abundance of granite plutons, observed geometric and temporal associations between plutons and metamorphic isograds, and evidence for rapid heating and cooling during granite emplacement. There are however many terranes which do not exhibit these characteristics and which call into question the universal applicability of the generally accepted models. We show that the burial of an enriched radioactive sequence beneath a thick sedimentary succession deposited during thermal subsidence following rifting produces two concomitant changes in the thermal structure of the crust. Firstly, the burial of the enriched layer produces high geothermal gradients in the overiying sedimentary succession, with the high gradients propagating down into, but not through, the enriched sequence. Secondly, the deep lithospheric cooling that drives thermal subsidence reduces the heat flowing into the deeper crust from the mantie. Because the process of thermal subsidence promotes burial, it naturally increases the depth extent of the high geothermal gradients in the upper crust, potentially inducing very significant temperature increases in the mid-upper crust during burial. The lowering of the thermal gradients in the deep crust accompanying burial severely limits the temperature changes affecting the Moho; potentially allowing Moho cooling while the midupper crust heats. We show the these combined effects can promote high geothermal gradient (>35°C/km) metamorphism in the mid-upper crust without inducing significant melting in the lower crust, providing the basement heat production contributes >-70mWm*^ to the surface heat flow after burial. These conditions appear to be met in a number of Australian high-temperature, low-pressure Proterozoic terranes where the thermal causes of metamorphism have long been enigmatic. The western Mount Isa Inlier is one such terrane, where peak regional metamorphic conditions are recorded at 600'C and 4kbar (Rubenach, 1992). Despite the concordance of metamorphic isograds about the Sybella Batholith, a discussion of the origin of high temperature metamorphism is motivated by the recognition of an ~130My delay between intrusion of the Sybella Batholith (~1655-1660Ma) and peak regional metamorphism of the sunounding rocks (~1532±7Ma) (Connors & Page, 1995). Both surface heat flow and internal heat production in surface rocks are anomalous in the Mount Isa area, and we use these primary observables in an analysis of the thermal regime. Surface heat flow values are recorded in the range -78-82mWm'^ (Hyndman & Sass, 1966) and surface heat production in the Sybella Batholith averages ~5.5nWm"^ in the main phase and ~8nWm"^ in the microgranite phase (calculated for the time of metamorphism). Heat production is also elevated in other granites (such as the Big Toby Granite underiying the Sybella Batholith) and in the Mount Isa Group sediments, giving an enriched radiogenic upper crustal sequence. In this example a package of sag and rift related (Mt. Isa Group) sediments of at least 4-5km thickness c^jped the orogen at the time onset of metamorphism, and we show that the thermal regime in the middle and upper crust is very sensitive to the thermal conductivity of this uppermost crustal sequence. For example, a decrease in upper crustal thermal conductivity from 3 to 2 Wm'^K'^ can raise temperatures at 15km by 200-250'C. This work has shown that the peak metamorphic conditions in the western Mount Isa Inlier can be achieved simply by burial of the present crustal heat production distribution, without any significant additional crustal or mantle component, and without any advective component. Temperatures at mid-crustal levels are further enhanced by the presence of a low conductivity upper crustal sequence and by a very low mantle heat flux.
REFERENCES Connors K.A. & Page R.W. 1995. Relationships between magmatism and deformation in the western Mount Isa InHer, Australia. Precambrian Research 71, 1-4. p. 131-153. Hyndman R.D. & Sass J.H. 1966. Geothermal measurements at Mount Isa, Queensland. Journal of Geophysical Research 71, 2, p. 587-601. Rubenach M.J. 1992. Proterozoic low-pressure/high-temperature metamorphism and anti-clockwise P-T-t paths for the Hazeldene area. Mount Isa Inlier, Queensland, Australia. Journal of Metamorphic Geology 10, p.333-346.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
APPROACH USED TO DIFFERENTIATE LAYERED GROUNDWATER AQUIFERS WITHIN THE BURDEKIN RIVER DELTA, NORTH QUEENSLAND Gerard A. McMahon^ and Malcolm E. Cox' ^Research Concentration in Natural Resources, School of Natural Resource Sciences, Queensland University of Technology GPO Box 2434, Brisbane, Queensland 4000
The Burdekin River delta is situated about 90 kilometres southeast of Townsville. It is a large (640km^) cuspate delta that overlies a mainly granitic basement to depths in excess of 100 metres near the coast. The area supports extensive irrigation mainly for sugar cane with some rice and fruit (tropical and citrus) crops. Irrigation water is provided by an integrated system of surface water channels and groundwater. The groundwater is extracted mainly from the upper aquifer layers of the deltaic sediments with lesser amounts being derived from lower water-bearing strata by deep bores. The nature of sedimentation of the delta has been quite complex. The sequence of deposits comprise a combination of interfingering fluvial sands, silts, and clays between which are various layers of weathered palaeosols, floodplain silts, and marine muds isolating potential aquifer units. The extent of fluvial palaeochannel connectivity is partly controlled by the rate of sediment deposition over its Quaternary history. Aggradation rates appear to have been rapid (up to 6mm/year) which can cause periodic channel avulsions rather than lateral meandering. Also, the development of distinctive levee banks adjacent to the current Burdekin River causes floodplain sedimentation to occur only during large discharge events. If this is indicative of the history of sedimentation in the delta, then the hydraulic continuity between the various subsurface aquifer units could be more complicated than otherwise anticipated. The depositional history of the Burdekin River delta has been described by various authors in relation to sealevel changes along the Australian coastline, but little attention has been given to the effect this sedimentation style and pattern has had on aquifer formation and extent. Although geological interpretations can be made from existing borehole data, confirmation of aquifer connectivity can only reasonably be established through a study of the character of the groundwater itself. One of the key tools currently being used to assess the aquifer structure of the delta is a hydrochemical assessment of major and minor dissolved ions in the groundwater. By using standard hydrochemical analyses, relationships can be inferred between different water samples. It is therefore possible to determine whether the chemically distinct groups (water types) throughout the delta, and distributed both laterally and vertically, represent different bodies of groundwater. The hydrochemical information can then be compared to stratigraphic interpretations from strata logs as a control as well as to enable an integrated interpretation. The Queensland Department of Natural Resources maintains a groundwater database with detailed records of bores throughout the delta area. Hundreds of strata logs from this database have been used in this study to reconstruct the stratigraphy, and develop a conceptual hydrogeologic model of the aquifer geometry and distribution. In previous investigations, the delta has been categorically divided into two to three vertical aquifer layers. These layers are intermittently separated by estuarine clays and mangrove muds (of the Holocene sea-level maxima) in the coastal zone; and in deeper sections by harder oxidised clayey palaeosols (probably indicative of the Pleistocene lowstand). Evidence from strata logs shows that down-cutting channels often erode these aquifer separators. It would therefore appear likely that there is a significant degree of hydraulic connectivity between layers. However, because of the rapid aggradation rates, the various channel sands would be lens-shaped in cross section, and laterally discontinuous. This indicates that the hydraulic conductivity of each layer could quite possibly vary laterally across the delta as well as with depth. Reconstruction of borehole information shows that the delta is comprised of a complex stratigraphy resulting in equally complex aquifer geometry. This hydrochemical approach demonstrates that the delta is a large multisourced groundwater body and an assessment of the nature and distribution of groundwater types and hydrochemical relationships can complement the stratigraphic variations that are inferred from strata logs.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
CAN A MUSEUM FOR THE COMMUNICATION OF GEOSCIENCES PAY ITS WAY? Greg McNamara Museum Curator, Albert Kersten GeoCentre, P.O. Box 448 Broken Hill NSW 2880
The Albert Kersten GeoCentre is an interactive museum and interpretive centre in Broken Hill dedicated to the public conmiunication of the geosciences. It houses the Broken Hill City Council mineral collection, a significant and valuable collection of Broken Hill minerals, and has displays that feature Broken Hill minerals, Broken Hill geology, geological time, general geological information, local mining history and the development of metaUurgy from the Broken Hill perspective. The GeoCentre was established in 1993 by the Broken Hill City Council in response to the perceived need to explain to the local and visiting public the geological reasons for the existence of Broken Hill. Despite the operation of the GeoCentre being underpinned by local government funding with support from state government grants, there is substantial pressure from within council to reduce costs or to improve income through entrance fee charges and shop sales. Council recognises the value of the GeoCentre to the community as both an education centre and a tourist attraction but also has a duty to reduce the cost burden to ratepayers' where ever possible. The GeoCentre is a relatively popular destination for tourists and locals. 1996-97 and 1997-98 statistics show a seasonal visitation rate. Visitor numbers are highest during autumn, winter and spring school holiday periods and lowest in summer inmiediately after school resumes. In general, income from entrance fees and shop sales reflects this seasonal cycle. However, within the quieter, school-time, periods there are days when the GeoCentre has both high visitation rates and high shop sales. These bursts of activity and income correspond with visiting out-of-town school groups and other organised bus groups. This indicates that even relatively small expenditures by each member of a visiting group can have a big impact on the economic success of organisations such as the GeoCentre. If the GeoCentre is to continue to provide high quality education services while reducing its costs to the local community it must increase both visitation rates and income significantly. National tourism statistics indicate a down turn in inbound tourism numbers and a slackening in the activity of domestic tourists. This clearly indicates conventional, holiday-time, tourism will not provide the increases needed in the short to medium term. In order to increase visitor numbers and income while meeting its goals of geoscience education and community service the GeoCentre must attract new visitorsfromthe education sector. The GeoCentre is in the fortunate position of being able to offer a badly needed service to visiting school groups. Schools are under pressure to reduce costs while providing a greater quality of education through careful attention to syllabus requirements. The GeoCentre can provide an enjoyable activity that is also a syllabus component many science teachers find they are unable to address in the classroom. Anecdotal evidence suggests many teachers welcome geologists intothe class room and geological excursions because geology is the science subject area most teachers feel least able to teach adequately. However, the syllabus has many components and to be an attractive destination Broken Hill must offer school groups a wide range of curriculum oriented experiences, not just geology. Domestic tourism in the form of school and other education interest groups will be the primary target of GeoCentre promotions in the short term to medium term. The GeoCentre can not only meet its educational goals but also improve its income at the same time by attracting a greater number of group visits. The challenge may not be to convince potential group leaders of the merits of the GeoCentre but of the merits of Broken Hill in general. In the foreseeable future the GeoCentre will continue to rely on council flmds to operate, even though it contributes significantly to its operation through entrance fees and shop sales and has the capacity to furdier increase its income. However, the GeoCentre also pays its way through being a popular tourist destination. Through attracting tourists to the city or through adding an extra day and nights stay in the city to a tourist's itinerary the GeoCentre returns extra income to the community from which it is funded.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
A NEW METHOD FOR PRECISE ISOTOPIC DATING OF CLASTIC SEDIMENTARY ROCKS. Neal J. McNaughton. Birger Rasmussen and Ian R. Retcher Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, University of Western Australia, Nedlands WA 6907 AUSTRALIA
Precise dating of coarse- to medium-grained clastic sedimentary rocks such as sandstones and conglomerates, has not readily been achieved. If, however, such a method was available, then several previously intractable geological questions could be addressed, including: (1) providing an absolute time scale for Proterozoic and Archaean sequences of clastic sedimentary rocks; (2) providing an absolute time scale for sequences of clastic sedimentary rocks which have temporal control from fossil assemblages, but lack datable units such as volcanic units; and (3) providing an absolute time scale for unfossiliferous Phanerozoic sequences of clastic sedimentary rocks. With such information, a greatly improved understanding of key aspects of crustal evolution, basin development, sequence stratigraphy, tectonics and ore genesis becomes possible. This abstract is the first presentation of a new method for precise isotope dating of clastic sedimentary rocks. There are essentially two components to this new method: (1) the occurence and understanding of diagenetic xenotime (YPO4) overgrowths on detrital zircon grains in clastic sedimentary rocks of all ages; and (2) the development of an analytical method to determine precise isotopic ages of the xenotime overgrowths at a scale of less than 20 |Lim. Diagenetic xenotime overgrowths on detrital zircons in clastic sedimentary rocks Authigenic xenotime is a common, although volumetrically minor, constituent of Archaean to Cretaceous sandstones (e.g. Warton Sandstone, Kimberley Basin; Millstone Grit, northern England) deposited in fluvial to marine environments. Apart from sandstones, authigenic xenotime is also present in other siliciclastic rocks ranging from quartz-pebble conglomerates to silty shales. Xenotime crystals are mostly present as 1-20 )Lim irregular to pyramidal overgrowths on the surfaces of detrital zircon grains, and appear to have precipitated shortly after burial. Xenotime may contain several percent U substituted for Y in its crystal lattice. Given the relatively high concentration of U in xenotime, as well as the early-diagenetic origin of the mineral, it may be possible for the first time to accurately date the depositional age of a sandstone, or at worst, to establish a minimum depositional age. Dating accompanying detrital zircon provides a maximum depositional age, and together, the two dates constrain the true depositional age of the sedimentary rock. Precise isotopic age determinations of xenotime overgrowths Xenotime has long been recognised as a mineral suitable for high-precision U-Pb geochronology. To date overgrowths at the <20 jim scale, however, requires an in situ method which can only be provided by an ion microprobe with sufficient spatial and mass resolution. We have developed a SHRIMP (Sensitive High Resolution Ion Microprobe) U-Pb method for this purpose, using the SHRIMP II in Perth, Western Australia. An analysis area of about 10 ^im diameter is readily obtained, and methods to analyse smaller xenotime overgrowths without overlapping onto the host zircon grain will be described. The method is dependent on the establishment of a satisfactory xenotime Pb-U standard, against which all unknown xenotimes are measured. We have undertaken the initial characterisation of a potential xenotime SHRIMP standard, which comprises multiple grains from an Archaean pegmatite. Selected grains have a Pb/U ratio which is as reproducible as that for the best available SHRIMP zircon standard, even though the U-content of the xenotime varies by more than a factor of two. The very radiogenic Pb contents permit highly precise Pb ratio determinations. On the basis of analytical experiments on this provisional xenotime standard, protocols for precise age determinations of Precambrian xenotimes (using the 207Pb/206Pb age) and Phanerozoic xenotimes (using 206Pb/238U ages) have been developed. Surprisingly, the degree of metamictisation of the xenotime structure due to radiation damage from U and Th decay is significantly less than in zircons, resulting in appreciably more consistent age data for xenotimes with very high U-contents. These features suggest that xenotime is perhaps the best mineral yet identified for high-precision U-Pb and Pb/Pb geochronology using SHRIMP. Acknowledgements: This development study has benefitted from skills, discussions and advice from the following: Brendan Griffin, Bryan Krapez, Gavin England, Jan Dunphy, Marion Dahl, Matt Godfrey (UWA), Alex Bevan (WA Museum), Ian Williams (RSES-ANU), Allen Kennedy, Pete Kinny (Curtin University), and John Aleinikoff (USGS). In situ isotopic analyses were carried out on a Sensitive High Resolution Ion Micro Probe mass spectrometer (SHRIMP II) operated by a consortium consisting of Curtin University of Technology, the Geological Survey of Western Australia and the University of Western Australia with the support of the Australian Research Council.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
THE HYDROTHERMAL GEOCHEMISTRY OF COPPER D.C. "Bear" McPhail and Matt J. Pomeroy Department of Earth Sciences, VIEPS, Monash University, Clayton, Victoria 3168
The hydrothermal geochemistry of copper is important in controlling leaching, transport and deposition of copper in hydrothermal systems, e.g., ore deposits (particularly porphyry, epithermal, VHMS and SEDEX types). Copper can exist in many forms in hydrothermal waters and brines, e.g., hydroxide, chloride, sulphide, sulphate and other types of aqueous species. There are also two important valence states of aqueous copper, Cu(I) and Cu(n), each with its own family of aqueous species. This means that the hydrothermal geochemistry of copper is complex and predicting its behaviour is difficult to determine and calculate accurately. The aim of the project outlined here is to determine experimentally the important aqueous species in chloride brines and their thermodynamic properties. The least well known and potentially most important aqueous species for Cu in geologic environments are Cu(I)-chloride species. We are using a combination of experimental techniques such as mineral solubility and spectroscopic methods (UVVis-NIR and Raman) to determine directly the important Cu(I) and Cu(n) species. In addition to studying the aqueous species, we are also studying the intriguing possibility of Cu liquid-vapour partitioning. The solubility of cuprite (CujO) has been measured in pH-buffered waters over ranges in NaCl concentrations (0 2m) and temperature (25°C to 80°C, 150°C to 250°C). Experiments at 150°C and 250°C in acetic acid and phosphate pH-buffered solutions (no NaCl) resulted in total dissolved copper concentrations ranging from 0.1 ppm to approximately 7(X)ppm. In general, cuprite solubility increased with increasing temperature and decreasing pH; however, it is likely that there was disproportionation of Cu(I) into Cu(0) and Cu(II) and reaction with the buffer chemicals, especially acetic acid. Lower temperature experiments (25°C to 80°C) for cuprite solubility in pH-buffered solutions and 0.01m to 2m NaCl resulted in total copper concentrations ranging from approximately Ippm to over 3,000ppm. Disproportionation was again evident although measured Cu(I)/Cu(n) ratios show that in most experiments Cu(I) predominated. The high copper concentrations were mainly in experiments using a phthalic add pH buffer where there is evidence for strong complexing between Cu(II) and phthalate. The important copper aqueous species cannot be identified unequivocally yet, but the preliminary results indicate that it will be possible once the formation of Cu(n) is avoided. In addition, it may be necessary to employ alternative pH buffers. The UV-Vis-NIR spectra of Cu(II)-sulphate plus either NaCl or LiCl solutions have also been measured at 25°C, as part of the overall project to understand hydrothermal copper geochemistry. Preliminary results show broad peaks at approximately 750nm to 850nm, with the peak position increasing with increasing chloride concentration. There are other peaks evident at lower wavelengths, e.g., approximately 400nm, that may more useful (i.e., interpretable) in determining copper speciation. It may be possible to study Cu(I) complexing by measuring the charge transfer spectra in the UV part of the spectrum. In addition to UV-Vis-NIR spectroscopy, we are also exploring the use of Raman spectroscopy as another possible technique. We have designed and are in the process of building a titanium cell with sapphire windows that will allow us to measure UV-Vis-NIR spectra in both liquid and vapour phases simultaneously at temperatures up to approximately 350°C. We hope to detect Cu species in both phases and demonstrate, or at least place limits on, the fractionation of Cu between liquid and vapour. The results of our studies should provide reliable thermodynamic properties for aqueous (and vapour?) copper species. With these properties it will be possible to model processes important in ore transport and formation, such as cooling, boiling, fluid-mixing and water-rock interaction. Acknowledgements: Narelle Wolfe ran many of the high-temperature solubility experiments while working as a technical assistant to Dr. McPhail and Dr. Richard Shalders of Monash University designed the new high-temperature spectroscopic cell.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
EARLY PERMIAN PALAEOGEOGRAPHY OF THE DUBBO AREA, NSW - EVTOENCE FROM POLLEN, BRACHIOPODS AND VARVES S. Meakin^ & L. Sherwin^ 'Geological Survey of New South Wales, P.O. Box 536 St Leonards, N.S.W. 1590 ^Geological Survey of New South Wales, PO Box 53, Orange NSW 2800
Recently completed mapping of the Dubbo 1: 250 000 Sheet by the GSNSW and AGSO, under the National Geosdence Mapping Accord, has included investigation of Permian outliers to the Sydn^r and Gunnedah Basins. The discovery of marine and plant fossils helps constrain their age and depositional environments. Palaeontology: Early Permian terrestrial sequences in the area were first recognised when McMinn (1983) identified stage 2 palynomorphs from an outcrop near Mudgee and stage 3 palynomorphs in DMR drillhole Mirrie 1, north of Dunedoo. Limited subsequent sampling by GSNSW has also found stage 2 and 3 palynomorphs, including dominant monosaccate pollen (eg. Caheniasaccites,, Microbaculisporites tentula, Cycadopites cymbatus and Granulatisporites conjluens as identified by Foster, pers comm). These biozones suggest a (?latest Carboniferous-) Asselian to Sakmarian age. Gangamopteris leaf fossils occur. Slightly younger marine brachiopod fossils were recently found in a drill core 2km east of Dubbo, sunk as part of a drilling investigation by Land & Water Conservation. Located 100km west of any other recorded Permian marine shelly fossils in the central west of NSW, the Protoanidanthus and Notospirifer fossils were found at a depth of 200m, extending over a Im interval. The age indicated by this admittedly limited assemblage is the Early Permian (Artinskian) Tomiopsis ovata zone (formerly Fauna II) or E, curtosa/E. warwicki zones (Briggs), approx. equivalent to the Farley Formation of the Hunter Valley sequence or the Pebbly Beach Formation of the far south Sydney Basin. Both species are characteristic of the Early Permian of Queensland. Palaeogeography & environments: It is possible that the Denison Trough and its inferred southern extension into NSW, the Gilgandra Trough (Yoo 1988), had no connection with the Early Permian of the Hunter Valley or South Coast The lack of Permian outcrops north of Dubbo makes it difficult to determine what connections might have existed with other basins. To the east, the NW trending Mudgee Fault, and several subparallel faults, strongly influenced Permian topography, forming valleys in which sediments were deposited Certainly, prominent ridges of Silurian and Devonian rock near Mudgee separated outliersfromthe Sydney and Guimedah Basins. Apart from the isolated marine conditions inferred above, the majority of Early Permian sedimentary deposits in the area arefluvio-lacustrineand represent valley-fill. Plant fossils reflect gymnosperm and pteridophyte vegetation and afreshwaterenvironment The axes of the major Permian valleys were broadly parallel to the present day creeks, with imbrication in Permian conglomerates indicating generally north-flowing currents, with occasional variation to the west or south-west suggesting braiding. Some evidence for glacial activity has been found, with varves and striated cobblesfirstnoted by Dulhunty and Packham (1962) near Mudgee, remote from other glacial sediments in NSW. Varves have also been interseaed by DMR coal drilling north-east of Gulgong (DMR Narragamba DDH3, 4). The laminated sediments, containing dropstones to 2cm, are thought to represent deposition in a glacial melt-water lake. Glacial features are uncommon, suggesting that only seasonal fizzing or very restricted glaciation occurred. REFERENCES Dulhunty J.A. & Packham G.R 1962. Notes on Permian sediments in the Mudgee district, N.S.W. Journal and Proceedings of the Royal Society ofNew South WaleSy 95,161-166. McMinn A. 1983. Permo-Caiboniferous palynology of a sample from Macdonalds Creek, Mudgee district Geological Survey of New South Wales, GS 1983/156 (unpublished). Yoo E.K. 1988. The Rocky Glen Ridge and Gilgandra Trough, beneath the Surat Basin. New South Wales Geological Survey Quarterly Notes 72,17-27. Acknowledgements: Brachiopods were collected by Madhwan Keshwan, L&WC (Dubbo). Thanks to Dr Neil Archbold (Deakin University) and Dr Clinton Foster (AGSO) for palaeontological advice. Published with the permission of the Director General, N. S. W. Dept of Mineral Resources,
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
A FLUID INCLUSION STUDY OF SHEAR-ZONE HOSTED GOLD AT FOSTERVILLE, VICTORIA. Temence P. Meraagh Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia.
Shear-zone hosted gold mineralisation at Fosterville extends for at least 10 km and is hosted within a system of subparallel, NNW-trending reverse faults in a close to tightly-folded sequence of marine sandstones and shales. The refractory gold is very fine grained (typically <1 \xxn) and is closely associated with arsenopyrite and pyrite. In the oxidised zone, gold mineralisation is hosted by lithic fault breccias and fine quartz vein stockworks in silicified and ferruginised sediments. In the lower, sulphide ore zone a complex array of unmineralised, anastomosing quartz/carbonate veinlets and stockworks are present, particularly below shears and faults. Vein textures vary from laminated (crack-seal types), to massive extensional veins often with vuggy zones towards the centre of the veins. Evidence for wallrock alteration in drill core is limited to silicification, minor sericitisation and abundant fine-grained disseminated arsenopyrite and pyrite. The following three types of fluid inclusions occur within the quartz/carbonate veins: • Type la are two-phase, liquid-rich inclusions with less than 10 vol. % vapour and no detectable CO2. Type lb inclusions are also liquid-rich but contain CO2 in the vapour phase. • Type II are vapour-rich (>30 vol. % vapour), C02-bearing inclusions and are the most abundant. Many Type n inclusions have only a thin meniscus of water or appear to consist of only CO2. • Type III are three-phase inclusions containing liquid and vapour CO2 and these are also very common. Many inclusions showed evidence of necking but such inclusions were avoided during further studies. Raman microprobe analysis of the vapour phase of Types II and III inclusions confirms the presence of CO2, but also indicates that N2 and CH4 occur in some inclusions. The N2 content of the vapour phase varies from 0 to 18 mol.% and is believed to reflect interaction of the fluids with organic material within the turbidite sequence. CH4 varies from 0 to 25 mol. % with the highest concentrations occurring proximal to black shales. These results are in accord with microthermometric analyses which showed CO2 final melting temperatures varied between -58.8 and -56.6 ®C with the overwhelming majority being close to -56.6 °C. The majority of C02-bearing inclusions homogenise between 234 and 384 °C with a mode at 270 °C, and similar temperatures are observed for homogenisation into both the liquid and vapour phases with some inclusions exhibiting near critical behaviour. However, a small number of Type II inclusions homogenise between 146 and 198 ®C. Coexisting aqueous inclusions homogenise between 133 and 218 °C with a mode at 162 ®C. Other primary, aqueous inclusions in growth zones in bladed carbonate crystals homogenise between 212 and 338 ®C with a mode at 247 °C. Type lb fluids containing up to 25 mol.% CO2 represent the least reacted, deeply sourced fluids that have migrated through the fault zone at Fosterville. These fluids were initially at near critical conditions and were trapped at about 270 and at pressures from 130 to 200 Mpa. The presence of coexisting aqueous and CO2bearing inclusions which homogenise around 162 ®C suggest that this fluid undergoes phase separation and cooling. Other aqueous inclusions in subhedral to euhedral carbonate indicate the ingress of a lower salinity, meteoric? fluid at temperatures around 247 ®C. Phase separation was enhanced by mixing with CH4 and N2 enriched, reduced fluids in the country rocks and the resulting partitioning of H2S into the vapour phase led to gold and sulphide precipitation. This disseminated style of mineralisation, the brittle fracture nature of the depositional sites, and the lower confining pressures suggest that the Fosterville deposit occurs at a higher structural level than the other major reef gold deposits of the Bendigo - Ballarat zone. Acknowledgments Work reported here was conducted as part of the Australian Geodynamics Cooperative Research Centre and this paper is published with the permission of the Director, AGCRC. I would like to thank Trevor Jackson for his assistance and Perseverance Exploration Ry. Lid. for permission to publish this work.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
MAPPING THE REGOLITH WITH FIELD PORTABLE SPECTROMETERS; THE RELEVANCE TO GEOCHEMICAL SURVEYS. Nick J. Merry and Sasha Pontual AusSpec International Pty Ltd, P.O Box 2235, Kew MDC, Victoria 3101, Australia
One of the main characteristics of the regolith which sets it apart from most other geological environments is the abundance of clay and other fine grained phyllosilicate minerals concentrated in the near surface environment Althou^ previously difficult to study, there is a wealth of information that can be gained from analysing the conqwsition and distribution of these minerals within the regolith. New techniques for analysing and presenting data from portable infrared spectrometers provide an ideal way of mapping these minerals. While not an end in itself the integration of ^ e c t ^ data with geological observations and geochemical data can greatly increase the e3q)lorers understanding of local regolith profiles and can lead to a more accurate interpretation of geochemical survey data. This p^er investigates the use of data from portable infrared spectrometers for moping regolith profiles in a number of geological environments. The results presented show how these data can be used to recognise not only important mineral spedcs but also conqwsitional and crystallinity variations within these species. These have implications not only for logging regolith profiles but also for recognising and moping alteration in weather^ rocks. The integration of these data with other data sets (eg. geochemical results and field logs) can lead to a far greater understanding of the nature of the regolith in a project area in a short space of time. A number of case studies are used to demonstrate the usefiilness of these data and the processing techniques which can be used to analyse them. Case study 1: A summary of data from over 40 profiles though deeply weathered regolith is presented. These data show that in many regions there is a strong correlation between variations in kaolinite crystallinity, smectite proportions and the location of basement contacts beneath transported cover sequences. Case study 2: The integration of 'end of hole' sairq)le ^ectral data with equivalent geochemical data is used to show how the reliability of geochemical results can be assessed using the spectral data. These data can be used to produce maps and sections showing inqx)rtant horizons within the profile as well as the distribution of various clay minerals that make vp these profiles. The same type of data can be used to investigate relationships between mineralisation and alteration in deeply weathered envirormients. Case study 3: Regolith profiles derived fiiom spectral data are presented that show that clay composition and distribution in the regolith can be used as a guide to recognising and mapping the distribution of basement lithologies. For exanq>le weathered mafic and intermediate lithologies are usually associated with nontronite in the lower s^rolite. Ultramafics are frequently characterised by h i ^ smectite contents through the profile with Mg smectites and occasional nontronite dominating the lower s^rolite. The presence of kaolinite and montmorillonite and absence of Fe and Mg smectites are common features of weathered felsics. Case study 4: Results of the analysis of spectral data from a palaeochannel in the NT demonstrate how spectral data can be used as an aid in trying to locate basal gravels in the channel and to correlate between drill holes in an envirormient where visual logging is difficult. One of the important a^ects of !q)ectral analysis is that it can be carried out on site as drilling is being undertaken facilitating more effective geochemical sampling and more consistent logging in deeply weathered terrams. It can also be ^ l i e d to pre-existing samples and the results used to reassess interpretations of geodiemical data and to planfirturedrilling. The exan:q)les presented illustrate how ^ectral data can provide the field geologist with information previously only available using e?q)ensive and time consuming analysis techniques and the geochemist with a method of assessing the reliability of geochemical data sets derivedfromdeeply weathered profiles.
312
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998 THE INTEGRATED ANALYSIS OF GEOCHEMICAL AND SPECTRAL DATASETS IN MINERAL EXPLORATION. Nick J. Merry and Sasha Pontual AusSpec International Pty Ltd, P.O Box 2235, Kew MDC, Victoria 3101, Australia
SWIR (Short Wave Infrared) ^ectroscopy usingfieldportable infrared spectrometers has proved its usefulness in projects involving a range of geologic environments and mineralisation styles, however the practical ^plication of this technology has heca somewhat hmdered by the scarcity of sound analysis tools (ie. analysis methodologies and software) and an understanding of the information contained within SWIR mineral ^ectra. These data have frequently been analysed and interpreted in isolation using conventional analysis techniques removed from the realities of the exploration indus^. In this paper we address the question of how to most effectively analyse this type of data and integrate these analyses with more conventional exploration data eg, geochemical and geological data The methodology presented shows that the integrated analysis of good geological observations, geochemical, spatial and ^pec^ data can quickly lead to a greater understandiog of the nature and possibly distribution of target mineralisation. It relies upon the recognition of inq)ortant spectral charaaeristics, their numerical extraction and integration with more conventional e5q)loration datasets. This methodology represents the synthesis of traditional spectral analysis techniques, derived in part from remote sensing, and techniques frequently employed in the analysis of geochemical data. Various aspects of this integrated approach are illustrated by a number of exanq)les takenfroma variety of e}q)loration targets. In the first exanq)le assay data of percussion drill cuttingsfroman e?q)loration drill hole in the Wiluna region of WA show 3 distina intersections of anomalous Au. Analysis of equivalent ^>ectral data indicate that these are associated with 3 distinct styles of mineralisation: 1) Narrow zones, possibly fault controlled, associated with sericite alteration; 2) Likely siq)ergene enrichment at a felsic/mafic contact and; 3) Fe carbonate alteration in mafic units. It is unlikely that these associations could be recognised by analysing either the geochemistry or spectral data alone. When available, lithological information provides the opportunity to normalise both geochemical and spectral data for the various lithologies within a project area. By removing the influence of lithology anomalous spectral and geochemical r e f u s e s can be more easily recognised. In the second example presented data from the Kalgooriie region of WA show that after normalisation on the basis of lithology, ^ecific ^ectral parameters related to sericite conq)osition identify anomalous (altered) samples more readily than those derived from uncorrected data. Using avail^le geochemistry as a guide, ^ctral data can also be used to search for alteration halo effects surrounding target mineralisation and structures. If present these halos can greatiy inaease the size of the drilling target and may provide veaors to mineralisation. This is particularly useful when target mineralisation is present as narrow zones. Three examples from different projects involving sediment hosted gold mineralisation are presented: 1) In this example drilling samples show a strong association between the presence of Fe carbonate, a broad halo defined by con:q)ositional changes in sericite and a structural feature controlling Au mineralisation; 2) The second example involves drilling samples from a projea where the distance from potentially mineralised anticlinal axes is associated with average sericite con:q)ositions derived from ^ectral data; 3) In the third example Au mineralisation is shown to be associated with alteration defined by visual quartz percentage estimates and sericite conq)ositions derived from spectral data. The association between mineralisation and alteration in these examples was recognised through the integrated analysis of the available data. The final example is of a mineral map derivedfix)m^ectral datafromC horizon soil san5)les over an epithermal system shows how geodiemical pulps can also be analysed ^ectrally thus gr^tiy enhancing the value of these often expensive geochemical datasets. In broader alteration systems including epithermal and porphyry systems the spectral data can be used to describe and m ^ the distribution of various alteration fades. In addition the spectral data can also be used to recognise trends in mineral crystallinity and conqx)sition which can provide vectors towards higher tenq)erature parts of the systera The examples presented show that an analysis approach that integrates spectral data, geochemical data and geologic observations is often essential if the potential benefits of spectral datasets are to be realised. Sudi an approach allows ^ectral data to be used as a practical and valuable new tool in mineral e>q>loration and can lead to important insights that may greatiy improve the chances of ejq)loration success. 313
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
IMPACT OF LANDSLIDES IN AUSTRALIA Marion Michael-Leiba Cities Project, Australian Geological Survey Organisation (AGSO), Canberra, ACT,
On 30 July 1997 a landslide with a head scarp on the north western margin of the Alpine Way at Thredbo destroyed two ski lodges and trapped 19 people. Eighteen of these were eventually confirmed dead and one person was rescued alive. This is Australia's most lethal landslide, but it is not the only one. At least 45 Australian landslides are known to have caused injury or death during the period 1842 to March 1998. At least 80 people have been killed, and 45 injured. The events which caused death varied from the topple or fall of a single rock to a spectacular cliff collapse. Some were the result of human activity, while others were naturally occurring phenomena. There have been two instances of death or injury in 1998 to March. The first was at Aslings Beach near Eden NSW where, on 18 January 1998, a 12 year old boy and his 16 year old brother were digging a 2 m deep hole in the beach. It caved in on the boys and their father, and the 12 year old was buried. He died two days later. The second was on Mt Stuart, Townsville, Queensland on 25 January 1998. A man cHmbed on to a large boulder which dislodged, rolling on to him as he fell, and both he and the rock initially fell a short distance. Eventually, he fell a total of almost 300m and sustained a broken leg, neck injuries and facial cuts. He was rescued by helicopter. Because of the scarcity of readily available data, it is not possible to estimate the total economic loss due to landslides, but most has resulted from damage to infrastructure such as roads and railways. More than 160 landslides are recorded as having caused damage to roads, railways or pipelines, and/or to have required stabilisation. There are too few financial data to estimate the cost in dollar terms. However, a small fill failure destroying half of a lane of the Illawarra Highway at Macquarie Pass, south of Wollongong NSW on 14 February 1997, cost $250 000 for repairs necessitating closure of that section of the highway for about two weeks. On the Wollongong-Sydney-Brisbane railway, costs associated with landslide damage are estimated to average $25 million per year during the period 1989-1996. The costs prior to 1989 are not available. Fifty-three landslides are known to have caused damage to a total of over 200 buildings, many of which were destroyed. There have been two instances to date in 1998. On 10 January, a debris flow at Nelly Bay in Magnetic Island near Townsville hit the Magnetic Island International Resort. Boulders, sand and tree debris in the proximal portion of the debris flow destroyed two resort buildings and a car, and damaged another building, while several other buildings and the driveway of a house were damaged in the distal portion of the debris flow by water and mud. On 21 February, about 18 tonnes of rock fell from a 300-400 m high sandstone escarpment in the Lower Hawkesbury region of New South Wales. A 2.5 - 3 tonne boulder went through the wall of a house about 150 m away, into a walk-in wardrobe. As an adjunct to the Cities Project (National Geohazards Vulnerability of Urban Communities Project), AGSO has established the Australian Landslide Database, from which much of the statistical information in this paper was compiled. AGSO would welcome exchange of information on landslides in Australia to help make the database complete and to keep it current. We need it for community education and as a resource for quantitative landslide hazard and risk assessment. An abbreviated version of the database is on the Worldwide Web at http://www.agso.gov.au/ngis/locator.html. A project is also being developed to capture data at very high resolution in selected local government areas on the east coast by which to develop a better appreciation of magnitude/recurrence rates for landslides and associated causal factors such as rainfall intensity.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
THE P-TUFF BED PROVIDES A SNAP-SHOT OF AN UPPER PERMIAN, CLASTIC SEDIMENT STARVED, PEATLAND SYSTEM Per Michaelsen School of Earth Sciences James Cook University, Townsville, Queensland 4811. Email: Per.Michaelsen@jcu.edu.au
Deposition of the Late Permian Moranbah Coal Measures occurred in a rapidly subsiding retroarc foreland basin, during frequent base-level changes. Compressive deformation and contemporaneous volcanism facilitated the supply of abundant volcanoclastic sediments, resulting in rapid (but punctuated) basinal aggradation. The coal measures represents a dynamic depositional environment with sedimentation dominated by avulsion and crevasse influenced fluvial systems. To the south, frequent transgressive-regressive (?lMa) cycles occurred, resulting in lateral interfingering relationships of the Moranbah Coal Measures with the German Creek and MacMillan Formations. The existence of thick, laterally continuous, low ash coal seams cannot be satisfactorily rationalised in terms of a deltaic depositional model for their development. Facies and stacking patterns of the Moranbah Coal Measures and laterally equivalents have been investigated in extensive opencut coal mine exposures, in drillcores from intensively drilled exploration areas and in spaced regional drillcores. This integrated approach has identified eight fluvio-paralicshallow marine depositional systems: (i) sand dominated, low sinuosity, avulsing trunk river channels; (ii) levee and proximal crevasse splay complex; (iii) distal crevasse splays/overbank; (iv) marsh; (v) peat mire; (vi) shallow floodbasin lake; (vii) intertidal flat and (viii) shallow marine. The sedimentary record shows that alluvial deposition in the northern part of the basin developed as overlapping lobes, generating significant aggradational relief Sequential compaction of thick tracts of peat by clastic overburden appears to have controlled the facies stacking pattern on district scale. Exceptionally well developed, composite, crevasse splay deposits (up to 30m thick by >3km wide) with feeder channels (up to 30m thick by 0.5km wide) indicate that crevassing distributed significant amounts of sediment across the floodplain. Crevassing is considered to be coupled to a flashy fluvial style, aided by ice or log jams, and probably linked to a periglacial climate. In the absence of active clastic sedimentation, peat mire environments expanded, extending across abandoned depositional lobes, and formed the peat precursors of rider seams. The P-Tuff Bed is a basin-wide marker horizon within the Moranbah Coal Measures, traceable for over 200km along strike. The bed is essentially a tephra event unit, the product of a large-scale volcanic eruptive episode involving a pyroclastic volume >10km3. The P-Tuff Bed provides a detailed snap-shot in time and space of the facies mosaic which existed immediately before and immediately after the volcanic eruption. Prior to the emplacement of the P-Tuff Bed, the underlying peat precursor of the regionally developed Pleiades Seam, accumulated over abandoned fluvial tracts on a broad coastal plain close to the palaeoshoreline. The Pleiades Seam is considered to have accumulated rapidly as a function of coastal ponding during the early to middle transgressive phase of the eustatic cycle. The depositional systems immediately proceeding emplacement of the P-Tuff Bed indicate a clastic sediment starved peatland system, with restricted fluvial channels, responsible for little sedimentary aggradation. Facies stacking patterns strongly suggest that this peatland system represents the early part of a base level rise (late LST - early TST). A similar peatland environment developed following deposition of the P-Tuff Bed. The bed is capped by the patchily developed P-Seam. This seam is in turn overlain by paralic-shallow marine deposits in the southern part of the basin, characterised by abundant bioturbation (e.g. Rosselia isp.) and shellbeds, that mark the transgressive system (?late TST - early HST). Acknowledgements: The work presented here was fimded by the Danish Research Academy, Mitsubishi Gas Chemical Resources Australia, Shell Coal, BHP Coal and a Merit Research Grant from James Cook University.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14^'' Australian Geological Convention, Townsville, July 1998
RE-EVALUATING THE METAMORPHIC AND TECTONIC HISTORY OF THE EASTERN ARUNTABLOCK,CENTRALAUSTRALIA J.A. MILLER^ I.S. BUICK^ I.S. WILLIAMS', I. CARTWRIGHT' 1. Department of Earth Sciences, Monash University, Melbourne, Australia 2. Department of Earth Sciences, La Trobe University, Melbourne, Australia. 3. Research School of Earth Sciences, Australian National University, Canberra, Australia.
The Arunta Inlier is a major mobile belt that was multiply metamorphosed and deformed in the Proterozoic and Palaeozoic. Palaeozoic crustal shortening, associated with the --300-400 Ma Alice Springs Orogeny (ASO), may have locally reactivated Proterozoic structures. Consequently, distinguishing between Proterozoic and Palaeozoic tectonic processes is difficult. Until now, granulite-facies metamorphism in the eastem Arunta Inlier was thought to be Proterozoic in age (e.g. Miller et al, 1997). However, new SHRIMP U-Pb ages from partially melted granulites in the Mallee Bore area have challenged this idea and indicate that granulite-facies metamorphism occurred not during the Proterozoic, as previously believed, nor during the ASO, but instead during the midOrdovician 460 Ma), reflecting an older Palaeozoic event not previously recorded in the Arunta Inlier. The Mallee Bore area in the northern Harts Range forms part of the Central Tectonic Province of the Arunta Inlier, which is bounded in the south by the Redbank Thrust Zone and the Illogwa Shear Zone, both of which were probably active during the ASO. The Mallee Bore area is made up of a sequence of interlayered metabasites and metapelites that were correlated with the Harts Range Complex. Both the metapelites and metabasites were metamorphosed to granulite-facies(800-875 "C and >9-10 kbar; Miller et al, 1997) in a narrow, E-W striking zone of intense non-coaxial high-temperature deformation. Both rock types also show evidence for dehydrationpartial melting and contain reaction textures that document a component of high-temperature decompression and cooling that was thought to reflect a segment on a continuous clockwise P-T-t path of Proterozoic age (Miller et al, 1997). Rocks elsewhere in the Harts Range record similar P-T segments that were inferred to represent the effects of superimposed but distinct Proterozoic and Palaeozoic metamorphic events. U-Pb isotope ages were obtained using the SHRIMP II ion microprobe, on zircon and monazite in two granulitefacies migmatitic metapelites and zircon and titanite in two granulite-facies metabasites. Cathodoluminescence imaging showed the zircon crystals in all samples to be composite grains, generally consisting of euhedrallyzoned igneous cores, surrounded by a broadly-zoned or unzoned metamorphic overgrowths. Many of the cores appear to have been corroded before the overgrowth formed, suggesting a period of zircon dissolution before regrowth duringCTanulite-gradeanatexis. The youngest zircon overgrowths from both metabasite samples yielded the same ^^b- ^ U age, --480 Ma, which may record zircon precipitation at an early stage of melt crystallisation. Zircon and monazite from both metapelites also yielded the same age, --460 Ma. The similar ages obtained for new zircon overgrowths and monazite from the metapelites suggest that the terrain initially cooled very quickly from the metamorphic peak or, more probably, that both minerals grew at a very late stage of leucosome crystallisation near the granite solidus and below the monazite blocking temperature (-700 °C). Titanite from the two metabasites yielded ages of -410-420 Ma, which may indicate that the terrain cooled relatively slowly after the metamorphic peak, or alternatively that these ages were partially reset during the ASO. Detrital igneous and metamorphic zircon grains from one metapelite were analysed in detail and yielded a wide range of apparent ages,with major populations at 600-700 Ma and 950-1250 Ma. SHRIMP ages from Mallee Bore have substantially revised the accepted metamorphic history of the Harts Range region and placed new constraints on the Palaeozoic tectonics of the eastem Arunta Inlier. The clockwise P-T-t segment described by Miller et al (1997), thought to be a Proterozoic feature, is instead wholly Palaeozoic in age, and reflects the youngest granulite-facies event (-480-460 Ma) yet documented in Australia. These new data extend the duration of Palaeozoic metamorphism in the central Australia to -180 Ma, and suggests that the Arunta Inlier may have been subject to several periods of reworking over a protracted time period. Several -450-470 Ma Sm-Nd isochrons, thought to reflect cooling below -600-650 'C (Foden et al, 1995), have been obtained in the HRC south of Mallee Bore. However, the geographic extent of -^80-460 Ma high-grade metamorphism is as yet poorly constrained. Sedimentary precursors to the Mallee Bore granulites were inferred to have been deposited prior to -1745 Ma (Cooper et al, 1988). However, the age range ofpre-460 Ma detrital zircons in the Mallee Bore rocks suggests that they were deposited after 600-700 Ma. Therefore, the —480-460 Ma event probably involved deformation and metamorphism of Neoprotoerozoic and Palaeozoic volcanosedimentary sequences. REFERENCES Cooper, J. A., Mortimer, G.E. and James, P.R. 1988. Rate of Arunta Inlier evolution at the eastem margin of the Entia Dome, central Australia. Precambrian Research 40/41, 217-231. Foden, J.D., Mawby, J., Kelley, S., Turner, S. and Bmce, D. 1995. Metamorphic events in the eastem Amnta Inlier: Part 2, Nd-Sr-Ar isotopic constraints. Precambrian Research, 71, 207-227. Miller, J.A., Cartwright, I. and Buick, I.S. 1997. High grade metamorphism in the Harts Range. Petrology and P-T constraints from Mallee Bore, northem Harts Range, central Australia. Journal ofMet. Geology 15, 613-629. 316
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
PROFILES THROUGH HIGH-PRESSURE AND NON HIGH-PRESSURE OPHIOLITE SUITES: IMPLICATIONS FOR FLUID FLOW DURING SUBDUCTION J.A. Miller\ I.Cartwright\ I.S. Buick^ A.C. Bamicoat^
1. Department of Earth Sciences and VIEPS, Monash University, Clayton, VIC, 3168, Australia 2. Department of Earth Sciences and VIEPS, La Trobe University, Bundoora, VIC, 3089, Australia 3. Department of Earth Sciences, The University of Leeds, Leeds, LS2 9JT, United Kingdom
It is widely acknowledged that fluids play a critical role in subduction zone processes. Important among these is the development of subduction related volcanism linked to partial melting in the overlying mantle wedge and the transportation of slab-derived chemical components to sites of magma genesis. Despite the general acknowledgement that fluidflowin subduction zones does occur, our understanding of the scale of fluid flow in subduction zones remains poor. One of the primary reasons for this is the often apparent contradictory nature of differentfluidflowstudies in subduction zones, some of which argue for large-scale widespread fluid flow (e.g. Bebout, 1991) while others suggest that fluid flow is extremely limited and highly focussed (e.g. Bamicoat & Cartwright. 1 9 9 5 ) . Numerous oxygen isotope studies on unmetamorphosed ophiolites have shown that the primary 8 0 value of the ocean crust, considered to be 5.7 ± 0.3 % o (Muehlenbachs, 1 9 8 6 ) , is modified in a predictable manner during seafloor hydrothermal alteration. Elevated 5 ^O values (> 5.7 characterise the upper extrusive sections of the ocean crust while the rest of the crustal section has depleted values (< 5.7 %o) producing a characteristic sigmoidal oxygen isotope profile through the ophiolite. However, few studies have looked at metamorphosed ophiolite sections to examine how this sigmoidal oxygen isotope profile is modified during metamorphism and what this might tell us about fluid flow during high-pressure metamorphism and subduction. Here we compare oxygen isotope values in a variety of rocks types within the high-pressure Corsican and Zermatt-Saas ophiolites with oxygen isotope profiles from these high-pressure ophiolite sequences with ophiolite sequences that have not undergone high-pressure metamorphism. Ophiolites, representing the remains of the Jurassic Tethyan ocean destroyed during the Alpine orogeny, are found throughout the Alpine chain. The high-pressure Corsican and Zermatt-Saas (Switzerland) ophiolites are part of Eoalpine nappes, belonging to the internal Pennine Zone of the Alps. In both examples, all members of the ophiolite sequence (radiolarian and maganiferous cherts, pillow basalts, sheeted dykes, gabbros, serpentinites and ultramafics) are locally preserved but have undergone varying degrees of tectonic dismemberment. The Zermatt-Saas ophiolite was metamorphosed to higher grades (550-600 1700-2000 Mpa) than the Corsican ophiolite (--530 "C, 1200 Mpa) which also underwent a later pervasive blueschist facies metamorphism (-400 "C, 700-900 Mpa). Both ophiolites underwent a later non pervasive greenschist facies event at ~ 45-30Ma. Oxygen isotope ratios have been measured in each of the different units within the two ophiolite sequences. In the Zermatt-Saas ophiolite, serpentinites have S'^O values of 1.5-4%©, gabbros have values of l-7%o, units identifiable as dykes have values of 5-6.5%o, pillow lavas have O values of 5-9.5%o, and undifferentiated metabasalts have values of3-9%o. In the Corsican ophiolite, serpentinites have values of 3.6-6.4%©, gabbros have values of 4.6-12.8%©, dolerites have values of 7.8-8.7%©, pillow basalts have values of 6.4-13.9%©, and undifferentiated metabasalts have values of 5.3-14.6%©. Both ophiolite sequences preserve sigmoidal profiles similar to unmetamorphosed ophiolite sequences (e.g. Gregory & Taylor, 1981). However, within the Corsican ophiolite, values for the differentunits and the transition from "Oenriched to ^®0-depleted rocks tends to be slightly higher than expected for unmetamorphosed ophiolitic rocks. The preservation of sigmoidal profiles, characteristic of ocean floor alteration, suggests that neither ophiolite has undergone large-scale pervasive fluid flow during high pressure metamorphism. Variations in the absolute values of the ophiolitic rocks may be the result of heterogeneous seafloor alteration and heterogeneous dehydration and fluid distribution during high-pressure metamorphism. The results of this study are consistent with previous studies in the Zemiatt-Saas area which suggested that fluid flow during highpressure metamorphism was highly focussed and channelled. If this is the case, it suggests that fluids in subduction zones are more likely to flow parallel to stratigraphic boundaries and the slab-mantle interface, back up the subduction zone and into the accretionary wedge, rather than to circulate within the subduction zone. Therefore the slightly higher values for the Corsican ophiolite may indicate that the Corsican ophiolite does not represent typical subducted ocean crust. REFERENCES Bamicoat, A.C. and Cartwright, I., 1995. Focussed fluid flow during subduction: Oxygen isotope data from high-pressure ophiolites of the western Alps. Earth and Planetary Science Letters, 132, 53-61. Bebout, G.E., 1991a. Field-based evidence for devolatilisation in subduction zones: Implications for arcmagmatism. Science, 251, 413-416. Muehlenbachs, K., 1986. Alteration of the ocean crust and the ^^O history of seawater. Reviews in Mineralogy 6, 425-445. 317
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"'' Australian Geological Convention, Townsville, July 1998
DISCOVERY OF THE GHOST CRAB GOLD DEPOSIT, KALGOORLIE WESTERN AUSTRALIA. Mark Miller'. Ray McLeod^ Steve Devlin^ John Vinar^ 'Newcrest Mining Limited, 89-91 Chaffers Street, Boulder WA 6432, ^ewcrest Mining Limited The Hyatt Centre Level 2 Terrace Road, East Perth WA 6004, 'New Celebration Gold Mine PO Box 223, Boulder WA 6432
The discovery of the Ghost Crab gold deposit, located 34km SSW of Kalgoorlie and 17 km WSW of the New Celebration Gold mine, resulted from a re-evaluation of the major structural features which control gold mineralisation in the Norseman-Wiluna greenstone belt within the Yilgam Craton of Western Australia, and subsequent application of detailed aeromagnetics to target prospective stratigraphy, structure and alteration zones within Newcrest's tenements in this area. This study identified the presence of a NNW-striking sinuous shear, given the name Karramindie Shear Zone, which appeared to form a linking structure between the Zuleika shear to the east and the Kunanalling shear to the west. Along this structure, several underexplored dilational positions adjacent to syn-tectonic granites were recognised, and these were considered to be prospective locations for gold mineralisation. Early in 1995, a program of broadly spaced soil auger sampling was implemented over these dilational zones. Initial sampling at a 400m x 100m spacing, followed by infill sampling at 200m x 50m intervals, defined three parallel anomalies with a maximum value of 552ppb Au. The Ghost Crab deposit is located on the southernmost of these anomalies which has a strike length of 1.1km at the 20ppb contour and a peak value of 134ppb Au. Subsequent drilling showed the bulk of the deposit to be overlain by Tertiary palaeochannel sediments up to 30m thick. Vertical rotary air blast and air core drilling over these anomalies was carried out in September 1995, and this drilling returned a best intersection of 24m @ 13.35g/t Au from highly weathered sediments in hole GCRIO. Infill drilling resulted in the defmition of a saprolite gold anomaly covering a strike length of 480m. Arsenic, while depleted in the mineralised position, defined an anomalous halo in the footwall and hangingwall ultramafics, with values up to 11 OOppm. Multi-element geochemistry, mapping and magnetic interpretation identified a northwesterly control to this mineralisation. Diamond core drilling was then used to test the primary mineralisation and the first hole, drilled below GCRIO, returned an intersection of 15.1m @ 5.22g/t Au from 165.3m down hole. This hole penetrated the centre of what was later named the CIO ore shoot. Newcrest's regional exploration group completed a further four diamond core holes to test the down dip and down plunge potential of the mineralisation during the period to June 1996. Each of these holes intersected the lode gneiss position although gold grades were sub-economic. In July 1996, the project was handed to the New Celebration mine site for drilling to determine the resource potential. During this evaluation phase the down plunge position of the mineralised shoot was recognised. Subsequent drilling would delineate a resource of 570,000oz Au, and the deposit would remain open at depth. The deposit is located in a dilational position on the northeastern boundary of the Depot Granodiorite, within a sequence of komatiites and sediments that have been metamorphosed to amphibolite facies. The mineralisation is conformable and occurs in a sheared and quartz-veined biotite gneiss known locally as the lode gneiss. Some gold mineralisation also occurs in the hangingwall and footwall ultramafics. The lode gneiss averages 1 Im wide and dips at 63° to the northeast. The mineralised envelope appears to plunge approximately 40° to the northwest, although further work needs to be carried out to accurately determine this attitude. Petrological examination of the orebody has identified the lode gneiss as a biotite+cordierite±gamet gneiss after a greywacke protolith, with an alteration assemblage comprising sodic plagioclase + silica + cummingtonite + gedrite/anthophyllite + biotite + chlorite. Disseminated pyrrhotite and pyrite are aligned parallel to the prominent foliation direction, and boudinaged quartz-feldspar veins sub-parallel to this foliation are characteristic of the lode gneiss. Visible coarse gold is occasionally enclosed within these veins while fine gold (2-12^m) occurs along the boundaries of quartz and plagioclase in the groundmass of the gneiss. Discovery of the Ghost Crab deposit can be attributed to a combination of continuous support for exploration by Newcrest's Board and Senior Management, belief in the prospectivity of the tenements in the area and a clearly defmed exploration strategy incorporating a strong commitment to drilling. Permission from Newcrest's Board to present this paper is acknowledged.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
DATING HIGH-GRADE METAMORPHISM: SHRIMP RESULTS ON ZIRCONS FROM MELTS AND TIMS U-PB MONAZITE DATA FOR THE STRANGWAYS METAMORPHIC COMPLEX, ARUNTA INLIER Andreas Moller^*^. Bas J. Hensen^ Richard A. Armstrong^ Dept. of Applied Geology, University of NSW, Sydney, NSW 2052 Max-Planck Institut fiir Chemie, Abteilung Geochemie, Postfach 3060, 55020 Mainz, Germany Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200
Two phases of high grade metamorphism between 1770 and 1730 Ma have previously been recognised in the Strangways Metamorphic Complex (SMC) in the Arunta Inlier (c/. Collins & Shaw, 1995). To resolve the timing and duration of these metamorphic events, SHRIMP U-Pb dating of zircons from orthopyroxene-bearing leucosomes in mafic rocks from two locations were analysed. As the leucosomes crystallised during the peak of metamorphism, the U-Pb ages of the zircons which crystallised from these leucosomes should place age constraints on metamorphism and deformation in the region. From the first location two leucosomes, a layer parallel (Ml, Dl) vein and a second crosscutting (M2, D2) vein were analysed. The results provide unambiguous evidence for a single high grade metamorphic episode followed by cooling at ca. 1715 Ma. The two metamorphic (Ml, M2) events of some earlier workers cannot be temporally resolved with the SHRIMP method. They occurred within 10-15 Ma of each other and the second event apparently disturbed U-Pb systematics of zircons grown during the first event at ca. 1730 Ma. The results confirm our suggestion that M1-M2 and the correlated periods of intense deformation (D1-D2) are part of a single tectonothermal event (Ballevre et al., 1997). The age of the main high-grade metamorphic event in the Strangways range has to be revised to 1715-1730 Ma. Previous higher estimates for granulite metamorphism in the Strangways Range (1730 to 1770 Ma; cf. Collins & Shaw, 1995) were based on U-Pb zircon dating of granitic intrusive rocks (e.g. Lafrance et al., 1995). Those results may represent inherited zircon populations or pre-metamorphic intrusion ages. Euhedral multifaceted zircons from an Opx-bearing leucosome in mafic rocks east of the Woolanga fault in the eastern Strangways Range show a small population of results at 1730 Ma and many older results interpreted as the crystallisation age inherited from the source of the leucosome at 1785 Ma. But these zircons also show zones and rims of bright CL response (low U content) which yield a continuum of ^^'^Pb/^^^Pb ages from 1550-1680 Ma, interpreted to represent incomplete recrystallisation of the zircon during a previously unknown geologic event at about 1570 Ma. This age is identical to the age of granulite facies metamorphism in the Reynolds Range (e.g. Williams et al., 1996) and provides a hitherto unsuspected linkage between the metamorphic history of the two high-grade metamorphic complexes. Conventional U-Pb monazite ages have been determined for samples from six widely-spaced localities in the SMC. Samples from both high grade domains and retrograde shearzones gave ages from 1704-1730 Ma. This age range corresponds to the SHRIMP zircon age in the granulite leucosomes and records the peak of metamorphism. Some younger ages were determined by in situ laser-ablation ICP-MS analysis and further work is needed to properly evaluate their significance. Apparently, monazite in the shearzones was generally not reset during the overprinting amphibolite facies metamorphism which causes extensive retrogression of the granulite facies assemblages. The SMC thus has a quite different metamorphic history from the neighbouring Harts Range where upper amphibolite facies metamorphism during the Palaeozoic (Hand et al., 1998) caused widespread resetting or recrystallisation of monazite. The results also indicate that there are different structural units within the SMC which have to be investigated separately to reconstruct the complex metamorphic and exhumation history of the whole complex. REFERENCES Ballevre, M., Hensen, B. J. & Reynard, B. 1997. Orthopyroxene-andalusite symplectites replacing cordierite in granulites from the Strangways Range (Arunta block, central Australia): A new twist to the pressure-temperature history. Geology 25, 215-218. Collins, W. J. & Shaw, R. D. 1995. Geochronological constraints on orogenic events in the Arunta Inlier: a review. Precambrian Research 71, 315-346. Hand, M., Mawby, J., Kinny, P. & Foden, J. 1998. SHRIMP U-Pb constraints on the timing of Palaeozoic intracratonic deformation in the southeastern Arunta Inlier, central Australia. Tectonics, In Press. Lafrance, B., Clarke, G. L., Collins, W. J. & Williams, I. S. 1995. The emplacement of the Wuluma granite: melt generation and migration along steeply dipping extensional fractures at the close of the late Strangways orogenic event, Arunta Block, central Australia. Precambrian Research 72, 43-67. Williams, I. S., Buick, I. S. & Cartwright, I. 1996. An extended episode of early Mesoproterozoic metamorphic fluid low in the Reynolds Range, central Australia. Journal of metamorphic Geology 14, 29-47. 319
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
IN-SITU DATING OF MONAZITES BY LASER-ABLATION-ICP-MS: 207pb/206pb a g e s Andreas Moller^. Norman J. Pearson^, Bas J. Hensen^ & Simon Jackson^ ^ Dept. of Applied Geology, University of NSW, Sydney, NSW 2052 ^ GEMOC, School of Earth Sciences, Macquarie University, Sydney, NSW 2109
Monazite is a metamorphic mineral abundant in felsic and pelitic high-grade metamorphic rocks and is highly suitable for in-situ geochronology using normal polished thin sections. Unlike zircon, most monazites yield concordant ages in the U-Th-Pb isotopic system and can be recrystallised and isotopically reset by metamorphic events associated with deformation. It has been shown that the correlation of 207p5/206p5 ^ges of zircons determined by laser-ablation (LA) ICP-MS and TIMS U-Pb is acceptable for provenance studies (Machado & Gauthier, 1996). A laser-ablation technique has been optimised on the GEMOC Laser Ablation Microprobe at Macquarie University to obtain 207p5/206pb ages from monazites in 30-40 |im thin-sections or grain mounts. The laser spot size is about 30 jiim. 20'7pb/206p5 ^^tios and ages can be obtained without the need to quantify U or Pb concentrations in the minerals, although these can be determined if the concentration of an internal standard {e.g. LREE, Th, Si) is known. 26 measurements of the NIST 610 standard glass produced an average 2a precision of 0.3 % on the ^^^Pb/^^^Pb ratio. Data acquisition is rapid; a set of 14 unknowns and six standards can be analysed within an hour. The refinement of analytical procedures has improved the precision on the 207p|3/206pb ratio and future developments should enable precise measurements of ^^'^Pb/^^^Pb ages on Phanerozoic monazites grains. The technique has been evaluated by comparison with SHRIMP U-Pb data on Grenvillean monazites from East Antarctica (Post et al., unpublished data) and the Northampton Block, WA (Kriegsman et al., unpublished data). The analysed populations yields an average ^^'^Pb/^^^Pb age of 1074 ±16 Ma which agrees within error with the 207pb/206pb SHRIMP age of 108215 Ma for the Northampton monazites. The age population of monazites from the Windmill Islands sample is more complex. SHRIMP U-Pb dating found older inherited cores of about 1250 Ma and these have been confirmed by a population of LA-ICP-MS ages at about 1280 Ma. Results for a younger statistical population are slightly skewed to higher LA-ICP-MS age results (by 4%). This may be an effect of common Pb, which has not been corrected in age calculationfromthe LA-ICP-MS results or the result of deeper penetration of the laser beam into inherited parts of the grains. Analytical precision of the LA-ICP-MS method for each spot is usually better than 3% standard deviation for ca. 1100 Ma grains. Populations of about 10 analyses yield an external 2a precision of better than 3% or 30 Ma. Monazites in cordierite granulites and shearzones from the Strangways Range in the Arunta Block have been investigated in detail to derive the ages of different stages in the complex metamorphic and exhumation history of the terrane. The study is linked to petrological and structural studies on the same suite of rocks (Ballevre, et al., 1997). The results obtained are consistent with conventional multi-grain TIMS ages of ca. 1705-1730 Ma (Moller et al., this volume); however, the LA-ICP-MS method has identified younger age populations at 1400 Ma and 680 Ma associated with geologic events previously unknown from this area. These ages are found as parts of grains in a metapelite and as new growth of monazite in an amphibolite facies shear zone. The in-situ determination of monazite ^^^Pb/^^^Pb ages in polished thin sections can be used as a relatively inexpensive reconnaissance tool and requires minimal sample preparation in comparison to ion-probe or conventional TIMS analysis. For monazite sufficiently high in ^^'^Pb, the LA-ICP-MS method has the ability to provide precise ages of monazite growth even in texturally complex samples. REFERENCES Ballevre, M., Hensen, B. J. & Reynard, B. 1997. Orthopyroxene-andalusite symplectites replacing cordierite in granulites from the Strangways Range (Arunta block, central Australia): A new twist to the pressure-temperature history. Geology 25, 215-218. Machado, N. & Gauthier, G., 1996. Determination of 207pi5/206p5 ^ges on zircon and monazite by laserablation ICPMS and application to a study of sedimentary provenance and metamorphism in southeastern Brazil. Geochimica et Cosmochimica Acta 60, 5063-5073. Moller, A., Armstrong, R.A., & Hensen, B. J. 1998. Dating high-grade metamorphism: SHRIMP results on zircons from melts and conventional U-Pb monazite data in the Strangways Orogen, Arunta Inlier, central Australia. Geological Society of Australia Abstracts (this volume).
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
DRYLAND SALINITY IN THE NORTH FROGMORE AREA, NSW: GEOLOGICAL CONTROLS ON THE BUFFERING AND MIGRATION OF SALINE WATER. PrC. h, MQQre
Centre for Australian Regolith Studies, University of Canberra, ACT 2601
The physical expression of dryland salinity in the North Frogmore area of NSW includes: the presence of indicator vegetation species, saline scalds, sahne seepage, standing chloride-bearing water and tree die-off. The origin of salt in this region is less commonly from sources within oceanic sedimentary sequences and more commonly as cyclic salt introduced to the area by aeolian accession. Three principal rock types dominate the geology in the North Frogmore area: Ordovician metasediments in the east, in fault contact with felsic volcaniclastics of the Silurian Douro volcanics, which in turn are faulted against mafic Ordovician Kenyu volcanics in the west. Lithologies form belts sub-parallel to the NNW-SSE structural fabric in this region. Regional E-W to ENE-WSW structures offset or truncate the hthological belts. Several small plutons of Devonian granitic rocks, associated with the Wyangala Batholith, have intruded the older lithologies. The highest salinities in the North Frogmore area are associated with modem drainage and with the belt of low grade Ordovician turbiditic metasediments. The metasandstones and slates are composed almost entirely of quartz and kaolinitic clay minerals. Soils formed on this lithology are commonly lithosols and yellow podzolic soils. Although salts may have mobilised in percolating fluids during diagenesis, metamorphism and weathering of the marine sediments, the contribution of salt firom this source to contemporary saline ground-water is considered less significant than the flux of salt introduced from outside the study area. It appears that this lithology is less able to chemically buffer the saline fluid that passes through it than other rock types in the area. Soils developed on the Douro volcanics and Wyangala granitic rocks are dominantly duplex soils ranging from red podzolic to yellow podzolic soils depending on topographic position. Because the Kenyu volcanics do not contain quartz they form more gradational red earth and yellow earth soils. The weathering of these three rock types mobilises a greater range of ions in percolating fluids, so there is greater chemical interaction with the saline fluids than in the Ordovician metasediments. Within the modem drainage there is a range of soil types (soloth, solod and solonetz soils) which reflect salinisation in these zones. These have developed irrespective of the underlying lithology and are particularly well developed in valley floor settings. Fluid migration within lithologies appears to be structurally controlled. Within the Kenyu volcanics a number of the modem drainages flow along shear zones sub-parallel to the local NNW-SSE structural fabric, and may be diverted when they encounter large cross-cutting faults. Sub-surface fluid migration is thought to also follow these major pathways. Compression across the Douro volcanics has caused folding generating an axial plane cleavage sub-parallel to the NNW-SSE bounding faults. Sub-surface fluid percolation is localised along the bounding faults and along cleavage planes. Within the Ordovician metasediments alternation in bedding between metasandstones and cleaved slates focusesfluidpercolation along the slate zones.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
DRILLING A GEOPHYSICAL INTERPRETATION OF THE HORSHAM REGION: RESULTS AND INFERENCES David H. Moore'. C. Mark Fanning", Anthony J. Crawford' and Simon Maher' 'Geological Survey of Victoria, Department of Natural Resources & Environment, PO Box 500 East Melbourne Victoria 3002 'Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 'University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001
This paper presents results and a regional interpretation from the Victorian Initiative for Minerals and Petroleum (VIMP) drilling program carried out in western Victoria. Sixteen holes were drilled beneath thin cover of the Cainozoic Murray Basin, mostly in the Horsham 1:250 000 map sheet area, at targets identified in Moore's (1996) interpretation of airborne magnetic and ground based gravity data. This interpretation divided the region into the Stawell Zone of the Lachlan Fold Belt and the Dimboola, Miga and Ozenkadnook Subzones of the Glenelg Zone, deformed in the Delamerian Orogeny. In the Stawell Zone, two holes intersected highly strained metasediments, probably from the hangingwalls of significant faults. A third hole intersected an altered tholeiitic dolerite, similar to those at the Stawell Gold Mine. It bottomed in a fme grained rhyolitic intrusion that gave a 411±7 Ma SHRIMP zircon age date, the same as felsic dykes at Stawell and typical of igneous rocks straddling the western edge of the Lachlan Fold Belt in western Victoria. The Dimboola Subzone drillholes intersected volcanic rocks, serpentinised ?harzburgite and sandstone. They include a tholeiitic basalt sequence deposited in oxidised shallow water. An andesite to dacite sequence is similar to the Mt Dryden lavas, east of The Grampians, and is part of the allochthon that includes boninitic rocks. The serpentinite is derived from a boninitic ultramafic cumulate protolith, and probably occurs along the boundary between the Stawell Zone and the Glenelg Zone, the northern extension of the Moyston Fault. Drilling in the Miga Subzone intersected andesite and granite. The andesite is similar to the Mt Dryden rocks. A sheared granite has a zircon age of 504±8 Ma, an age commonly seen from Delamerian magmatism and the age of the Stavely Volcanic Complex. An undeformed mafic tonalite gave a zircon age of 404±6 Ma, showing the western limits of the -410 Ma igneous event. The holes in the Ozenkadanook Subzone intersected metasediments, gabbro and migmatite. Two holes targeted magnetic mafic volcanic rocks but intersected altered non-magnetic metasediments, one with a possible volcanic protolith. Low amphibolite facies biotite gave a K/Ar age of 501±4 Ma. The gabbro gave a 524±7 Ma zircon age. It intruded a sequence interpreted to be greenschist facies metamorphosed ocean floor. The intrusion gives a minimum age for the host sequence. The migmatite is a metamorphosed boron-bearing sediment, possibly from a sequence containing evaporites. Metamorphic zircon rims gave an age of 589±14 Ma, whilst the central areas place tha age of the protolith at around 1 040 Ma to 1 130 Ma. The 590 Ma event is the oldest high-grade metamorphic event known east of the Tasman Line on mainland Australia, and is the same age as rift-related mafic volcanism in the Mt Wright area of western NSW. The 1 040 Ma age is unlike any source known from the Gawler Craton or Curnamona Craton, suggesting another provenance for at least some of the sediments. The results further support the correlation of the 500 Ma Miga Subzone with the western Tasmanian Mt Read Volcanics. In both areas 500 Ma calc-alkaline to high-K volcanics occur with Proterozoic basement. REFERENCE Moore, D.H. 1996. A geological interpretation of the geophysical data for the Horsham 1:250 000 map sheet area. Victorian Initiative for Minerals and Petroleum Report 24, Department of Agriculture, Energy and Minerals.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
M"" Australian Geological Convention, Townsville, July 1998
MODERN MICROBIALITES IN WESTERN AUSTRALIA Linda S. Moore Water and Rivers Commission, P O Box 6740, Hay Street, East Perth, Western Australia 6892
Western Australia is not only host to the world's oldest fossil stromatolites, but contains probably the largest and most varied suite of contemporary microbialite formations in the world. Recent studies on living microbialites in Western Australia have led to a greater understanding and appreciation of the complexity of the process of formation. They have also helped to dispel the notion that modem microbialites are restricted to extreme environments that preclude a grazing fauna. Living microbialites occur in fresh, brackish, marine and hypersaline waters that range from sub-tropical to cool temperate environments. They occur in tidal embayments, lakes and groundwater seeps, and are not restricted by extremes in salinity or temperature. However, whilst occurring in a wide variety of envu*onments, presentday microbialites are restricted in global abundance compared to their Precambrian and early Phanerozoic counterparts. Modem and sub-recent microbialites m Western Australia include stromatolites, thrombolites and formations with an ill-defmed internal stmcture such as some tufa deposits in south-western Australia. Apart from the wellknown modem stromatolites of Hamelin Pool, Shark Bay, there is a wide array of other formations in Western Australia. Each microbialite environment and benthic microbial conmiunity (BMC) is distinct from the other and each is significant in its own right. In Lake Clifton, thrombolites are the predominant form of microbialite. They exhibit a range of external morphologies including tabular, domical, discoidal and conical formations which vary considerably in size, as well as more irregular and columnar stmctures up to 1.3 m high. Many of the tabular and domical forms have coalesced to form an extensive reef-like formation over 6 km long. Living thrombolites are also present in Lake Richmond, a fresh water lake surrounded by urban development. Spectacular tufa formations have been described along the south coast near Augusta. The tufa appears in many forms including drapes, curtams, small cylindrical stalactites and larger campanulate masses on the sea cliffs, as well as fans or terraces consisting of a series of rimstone pools and nodular masses in small brackish pools. In Pmk Lake, a hypersaline lake near Esperance, another form of microbialite is present. These fragile formations appear as a carpet of interconnected domes that have an egg-shell-like stmcture. Sub-recent microbialites are present in Lakes Pollard, South Newnham, South Preston, Hayward and Martins Tank Lake. The stmctures are completely lithified and appear to exhibit little sign of an active BMC that is depositing or incorporating carbonate. All occurrences found to date have been confined to the margins of these lakes and those stmctures sectioned so far have proven to be thrombolitic. Tepee stmctures and eroded microbialite mounds are present along the eastern shore of South Preston. In addition to these, some unusual digitate/cmstose formations have been located offshore beyond the tepee zone in water depths of 1-2 m. The nature of the BMC associated with the latter stmctures has not yet been investigated. The Martins Tank lake microbialites, although no longer active, are particularly interesting due to the presence of extemal digitate or columnar protuberances. In section, however, these did not prove to be laminated. Microbial mats are present in the majority of these lakes, but are not associated with the 'relict' microbialites. These mats are dominated by halotolerant and halophilic cyanobacteria. The regional setting of a microbialite locality may provide some clues to the origin of these stmctures, and can provide better insights into the environmental mechanisms involved in their formation. In addition, information on the regional setting of modem microbialites can facilitate interpretations of palaeoenvironments of fossil microbialites, since various factors such as sedimentological features, water chemistry and groundwater discharge are governed by local geology, climate and hydrological regime. In Western Australia, we are fortunate to have a range of environments that support microbialite growth.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE STABILITY OF SUB-CONTINENTAL MANTLE LITHOSPHERE IN A VIGOROUSLY CONVECTING MANTLE Louis Moresi 1, Adrian Lenardic
Slava Solomatov
Hans Miihlhausl.
(1) Australian Geodynamics Cooperative Research Centre, CSIRO Division of Exploration & Mining, PO Box 437, Nedlands, 6009 WA. (2) Department of Geology and Geophysics, University of California, Berkeley, CA 94720,USA. (3) Department of Physics, New Mexico State University, Las Cruces NM 88003-8001, USA.
Some regions of the Earth's surface are rapidly recycled into the interior as part of the general circulation in the mantle (the oceanic plates) whereas other regions have remained almost undisturbed in the midst of this activity for several billions of years (cratons). In addition, cratonic regions have not simply survived for all this time, but have escaped any major tectonic deformation. The major buoyancy source in the mantle which might act to disrupt the integrity of a craton is associated with the subduction of the oceanic lithosphere. Have the cratons simply not been close to major subduction zones in their entire history or are they underlain by a root sufficiently strong (in a broad sense) to resist the stresses associated with nearby subduction ? The former proposition seems less probable both from a statistical point of view, and because there is a record of dynamic topography in cratonic areas which seems to come from nearby slabs. As a result, considerable effort has gone into understanding the properties of the sub-continental mantie lithosphere which give cratonic areas the strength to resist subduction. The requirement that the cratons live long without accumulating significant strain leads to the conclusion that their roots must be both buoyant and mechanically strong. Xenolith data are consistent with inherent buoyancy of continental roots; mechanical strength at depth can be estimated through numerical modeling. In general, the strength has been interpreted as high viscosity and constrained by considering the work available through convection to deform the root. However, such arguments are not entirely consistent as they also imply mechanical stability for the oceanic lithosphere — i.e. they cannot explain the interaction of plate-tectonics with the sub-continental mantle lithosphere. To take the extra step and model continents interacting with subducting lithosphere, we first need a reasonable model of mantle convection with mobile surface plates into which we can introduce model continents. We present models in which we combine mantle convection simulations with a brittle lithosphere, in which platelike surface motions develop, and models in which compositionally distinct materials are included to represent the continents and sub-continental mantle lithosphere of different ages. These models contain many of the desired ingredients: strong, stiff downwellings akin to slabs, buoyant continental crust, and a self consistent parameterization of the brittle behaviour of the lithosphere. The numerical method required to solve this kind of problem is a hybrid finite element, particle-in-cell code which allows the use of fast multigrid solution methods on an underlying grid over which particles swarm carrying strain history and compositional information as they move. The concentration of negative buoyancy in the slab is so extreme that it can often subduct relatively large amount of buoyant crust. Furthermore, in the fully dynamic system, purely viscous roots do not usually give very good protection against strain to the overlying crust — instead in some circumstances they may even act as stress guides between the slabs and the crust. Structures related to the finite yield strength of mantle materials at low temperature, such as localized shear zones, are not impeded by the viscosity of the root. By contrast, we find that there is a simple mechanism for preserving an undeformed region of the continent and this is to surround it with material which is slightly weaker than itself (in the sense of its brittle properties). The sacrificial deformation of regions surrounding cratons prevents the slabs from approaching the cratonic nucleus too closely, and ensures that stresses decay quickly away from the subduction zone. The gradients in material properties between regions which are deformed versus those which are not is quite small and can more readily be accomodated by secular evolution of mantle composition.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
RIDING HIGH THEN DUMPED IN THE TROUGH: THE STRATOTECTONIC DEVELOPMENT OF THE MOLONG HIGH E. J. Mo^gan^ L. M. Ba^•on^ R. G. Came^on^ N. S. Meakin^ D. J. Pogson\ O. L. Raymond^ M. M. Scott', A. Y. E. Warren*, D. Wybom^' 'Geological Survey of New South Wales, P.O. Box 53 Orange N.S. W. 2800; ^Geolgical Survey of New South Wales, P.O. Box 536 St Leonards N.S.W. 2065; ^Australian Geological Survey Organisation, G.P.O. Box 378, Canberra ACT 2601; 'Present address: Department of Geology, Australian National University, Canberra ACT 0200
The Molong High represents a morphotectonic structure in the northeast part of the exposed Lachlan Fold Belt in New South Wales. Stratigraphic development took place during Siluro-Devonian time on a foundation of Ordovician volcanic rocks. The high developed as an elongate platform, flanked by the meridionally trending Cowra Trough to the west and Hill End Trough to the east, the boundary between the platform and basins shifting with time. This work is based on the recently remapped Dubbo and Bathurst 1:250 000 sheets, a project of the National Geoscience Mapping Accord involving the GSNSW and AGSO. Four distinct phases of volcanism and sedimentation took place on the Molong High and adjacent troughs during late Early Silurian to Early Devonian time. Stratigraphic development was associated with major extension of the Hill End Trough, and phases overlap in time and space: Phase One - late Early to Late Silurian felsic volcanism on east side of Molong High; Phase Two- late Early to Late Silurian carbonate and shale development on Molong High and adjacent troughs; Phase Three - early Early Devonian intermediate magmatism on Molong High and in adjacent troughs; distal volcaniclastic deposition in Hill End Trough; Phase Four - late Early Devonian widespread carbonate and shale development. Felsic volcanism dominated Phase One and a chain of largely submarine, rhyolitic to dacitic volcanic centres developed on the eastern margin of the Molong High. The centres were surrounded by volcaniclastic aprons and pyroclastic airfall travelled as far west as the Cowra Trough (Hanover Formation). Volcanism was associated with the inception of rifting on the margin of the Hill End Trough, and is represented initially by the the Mullions Range, Bells Creek and Kangaloola Volcanics and the bimodal Dripstone Formation, and later by the Gleneski Formation and upper Chesleigh Group as the focus of extension moved northwards over time. Widespread, shallow water carbonates of Phase Two developed contemporaneously with felsic volcanism, in areas of the Molong High that were protected from the influx of volcanic detritus. From early Late Silurian time onwards, parts of the Molong High began to rapidly subside and, as well as the adjacent troughs, were inundated by pelagic mud. Limestone on the east margin of the high retreated westwards and by the late Late Silurian, the entire platform was submerged. To the north, parts of the Molong High were later uplifted to sea level and carbonate development locally returned (Camelford Limestone). Phase Three is represented by earliest Devonian basaltic andesite to latite volcanism (Cuga Burga Volcanics) and associated, high level diorite to dolerite intrusions. A series of predominantly submarine, but locally emergent, eruptive centres developed, each surrounded by a thick sequence of volcaniclastic debris that spread out into the adjacent troughs. Volcanic centres have been identified in the Wellington and Yeoval areas and the volcanics are magmatically linked to the source of the coeval Yeoval Batholith and Ordovician volcanics. The presence of minor allochthonous Silurian coral and felsic volcanic clasts indicates volcanism was accompanied by local tectonic uplift. Contemporaneously the subsiding Hill End Trough was flooded with felsic to intermediate volcaniclastic debris (Crudine Group). The material derived from sources to the northeast, east and southeast and was transported as turbidites and debris flows within extensive submarine fans. A syndepositional lystric fault, precursor to the Nindethana Fault formed a fault scarp on the western and northwestern side of the trough, preventing Crudine sediments from travelling further west. The central and western parts of the Molong High, as well as part of the now-filled Cowra Trough were covered by a broad carbonate shelf during Phase Four (Garra Formation), while deposition in the Hill End Trough was dominated by fine grained sediments (Cunningham Formation). The eastern margin of the Molong High consisted of a steep slope and carbonate material regularly slumped basinward as debris flows and turbidites, forming an extensive submarine fan (Nubrigyn Member). The carbonate deposits grade laterally and vertically into shale and siltstone, and evidence does not support the existence of a 'Nubrigyn Submarine Valley'. Acknowledgment: Published with the permission of the Director General, N. S. W. Dept of Mineral Resources.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
GEOCHEMICAL STUDY OF THE MUELHA TIN-SPECIALIZED GRANITE, EASTERN DESERT, EGYPT M. A. Morsv and F.H. Mohamed Geology Department, Faculty of Science, University of Alexandria, Egypt
The Muelha Post-orogenic granite is located at the intersection of major structural weaknesses that dissected the Eastern Desert of Egypt. The common rock type is a muscovitized biotite granite. However, an albitized variety is common, particularly at the S W part of the pluton. This granite is a plumasitic specialized type as reflected from the relatively high levels of Si, Na + K, Sn, Rb, Zn, Pb, Ga and Y, compensated by extremely low contents of Ca, Mg, Ti, Ba and Sr, as well as strongly negative Eu anomalies and HREE enrichment.. The remarkable low values of some elemental ratios like K/Rb and Al/Ga and the high values of Rb/Sr are additional indication for such geochemical specialization. The calculation of mass transfer reveals that albitization is reflected by significant addition of Al, Na, Rb, Sn, Ga, Nb and La, and remarkable loss of Si, K, Fe, Ca, Zn, Zr, Ce and Pb. These chemical alterations of the original rock to a metasomatically albitized granite are concomitant with mineralogical changes. A genetic model illustrates the involvement of both crustal rocks and subcontinental mantle sources in the granite genesis. Fluorine was an important complexing anion during the magmatic crystallization history to yield a volatilerich residual melt enriched in highly charged cations like Sn, Nb and HREE. This resulted in the emplacement of the Sn-specialized Muelha granite along reactivated deep-seated tectonic zone.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
STYLES OF MINERALISATION WITfflN THE ROCKHAMPTON-MONTO REGION, YARROL PROVINCE: WHERE ARE THE FUTURE MINES? David A. Morwood. Paul Blake, Cecil G. Murray and Phil E. Burrows Geological Survey of Queensland, GPO Box 194, Brisbane, Queensland 4001
The world class Mount Morgan gold-copper deposit was by far the most important mine in this region. It has been interpreted by most recent workers as an atypical volcanic hosted massive sulphide (VHMS) deposit, but emplacement of the adjacent Mount Morgan Tonalite has also been considered to have been a key factor in ore formation. A combined model of a Middle Devonian VHMS deposit modified by Late Devonian tonalite intrusion has considerable appeal in view of the imusual size and grade of the Mount Morgan orebody. The relative importance of contemporary volcanism and subsequent granitoid emplacement for mineralisation is the main factor which must be considered in developing exploration strategies for similar deposits. Recent mapping by the Yarrol Project Team of the Geological Survey of Queensland has identified possible correlatives of the Middle Devonian strata which host the Mount Morgan deposit within a separate outcrop area, the Craigilee Block, about 70km to the NW. However, no equivalents of the Mount Morgan Tonalite have been recognised in this area. If emplacement of Late Devonian intrusives played a major role in ore formation, the area of prospective rocks is much reduced. The Early Permian host rocks of the Mount Chahners copper-gold mine, a classic Kuroko-style VHMS, have been intensively prospected for deposits of this style, with limited success. The recent discovery of the Develin Creek VHMS deposit in an essentially coeval volcanic-rich unit re-inforced the prospectivity of these Early Permian sequences for VHMS style deposits of moderate size. Since the recognition of Moonmera as a classic porphyry copper deposit in the early 1960's, several prospects of this type have been identified as a result of extensive exploration programs based mainly on geochemistry. Minor past production has come from the supergene zone of two of these deposits, Moonmera and Mount Cannindah, but the lack of associated gold values has rendered them sub-economic. One aim of the current GSQ Yarrol Project is to compile basic petrological, geochemical and geophysical data on related intrusives, with a view to establishing possible controls on mineralisation. One result from the Yarrol Project is the recognition of major faults which may be related to structurally controlled gold deposits in the Raglan area, about 70km S of Rockhampton. On the Bajool 1:100 000 Sheet, it is believed that several of the historical gold mines are structurally controlled by a series of faults traversing a large portion of the sheet from SE to NW. These faults can be easily identified in both magnetic and radiometric images. Ground truthing of these faults was carried out during field mapping by the Yarrol Project Team in 1996 and 1997. It appears that within this area, there are several fault splays with Mount Holly beds and Mount Alma Formation occurring in repetitive thin wedges. The mines in the Hirons Hill area are particularly noteworthy due to the unusual occurrence of gold not only in quartz and calcite veins, but also developed as free gold along cleavage planes and fractures at the Gully Lease. Other styles of mineralisation of historic importance are intrusive-related gold/calcite/quartz fissure veins, serpentinite hosted gold/quartz/carbonate veins, cupriferous skams, and alluvial and palaeo-placer gold.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
TOWARDS REALISTIC GEODYNAMIC MODELS CONSTRAINED BY GEOLOGY R. Dietmar MQller. * Louis Moresi ^ and Michael Gumis^ ^ Department of Geology and Geophysics, The University of Sydney, NSW 2006 ^Australian Geodynamics Cooperative Research Centre, CSIRO Exploration and Mining, Nedlands, WA 6009 ^Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA
Many parts of the Australian plate have experienced regional uplift and/or subsidence through time. The outer shelf on the northwestern Australian margin from the Browse to the Carnarvon basins shows regional Miocenepresent tectonic subsidence of up to 500-600 m and the Queensland and Marion plateaus exhibit post 9 Ma subsidence of similar magnitude. Neither observation can be explained by collisional processes and forland basin formation alone. In the Cretaceous in the Late Aptian to Early Albian, 120-110 Ma large fractions of eastern Australia experienced marine inundation. After a rapid subsidence event from about 100-90 Ma, intracatonic basins of eastern AustraHa became progressively exposed, with a flooding minimum in the Campanian, 80-70 Ma, when eustatic sea-level was inferred to be near a maximum. Depth anomalies at present are also found south of Australia at the Australia-Antarctic Discordant Zone (AAD), the deepest portion of the global mid ocean ridge system. The depth anomalies are associated with Sr, Pb, and Nd isotopic systematics cf basalts which reveal that there are two distinct isotopic provinces: one to the west of the AAD characteristic cf the Indian Ocean and one to the east characteristic of the Pacific. Understanding such observations requires the development of integrated geodynamic models which are constrained by plate kinematics, geological observations and results from mantle tomography. A threedimensional model of mantle convection in which the known history of plate tectonics is imposed predicts both the anomalous Cretaceous vertical motion of Australia and the present day distinctive geochemistry and geophysics of the AAD. The dynamic models predict that Australia overrode a subducted slab associated with the long-lived Gondawanaland-Pacific converging margin during the Cretaceous when both Australia and Antarctica moved east in a mantle reference frame from 130 to 90 Ma. During this time the sinking slab pulled eastern Australia down dynamically, resulting in marine inundation. We speculate that the post 100 Ma episode of rifting in the Eromanga, Cooper and Surat basins was triggered by an extensional stress field originating from a combination of dynamic topography and extension along the eastern margin of the Australian plate, possibly due to subduction hinge rollback. After 90 Ma, while the slab smking underneath eastern Australia was approaching the upper-lower mantle boundary, its effect on dynamic boundary topography became diminished, and eastern Australia gradually rebounded. Subsequently, the slab partially stagnated in the mantle transition zone, and is presently being drawn up by the South East Indian Ridge, where it separates Pacific from Indian Ocean mantle. The observed post 9 Ma anomalous subsidence of the Queensland and Marion Plateaus may be caused by a similar process, namely the overriding of northeastern Australia of a sinking slab originating from OligoceneEocene subduction north of Papua New Guinea. Shear wave tomography shows a NNW-SSE trending band (f anomalously high velocities in the upper mantle at depths between 300 and 500 km, streching from the Queensland Plateau to Indonesia, which likely reflects slab material that was subducted north of the Caroline back-arc basin in the early Tertiary. Regional anomalous Miocene subsidence of the Northwest shelf as fk* as 1000 km away from the Java trench is more enigmatic, as plate models do not suggest that this area is underlain by subducted slabs in the upper mantle. It may rather reflect a regional elastic response to collisional processes to the north. Geodynamic models constrained by geology, geochemistry and plate tectonics have the potential to become powerful tools for predictmg paleo intraplate stresses, heatflow, the formation and evolution d intracratonic basins and tectonic reactivation.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE CAMBRIAN EVOLUTION OF THE LACHLAN FOLD BELT: NEW STRATIGRAPHICAL, GEOCHEMICAL AND ISOTOPIC CONSTRAINTS FROM NEW ZEALAND Carsten Munker Institut fur Geologic und Dynamik der Lithosphare and Geochemisches Institut, Goldschmidtstr. 3, 37077 Gottingen, Germany Understanding the Cambrian evolution of the Australian-Antarctic Gondwana margin is of major importance for a reconstruction of supercontinent configurations in the Late Proterozoic and Early Paleozoic. In the Late Proterozoic, the Pacific opened due to the breakup of the Rodinia supercontinent [1]. Following the SWEAT hypothesis (Southwest US-East Antarctic connection [2]), Laurentia (present day North America) rifted away from Australia-Antarctica during this supercontinent breakup. Opening of the Pacific led to development of a coherent continental margin in Gondwana from present day eastern Australia through East Antarctica to western South America. Cambrian arc sequences are present in all (present day) SE Gondwana fragments including southeast Australia (Lachlan Fold Belt), Tasmania, Antarcica and New Zealand [3, 4, 5, 6]. These arc sequences are now dispersed by later tectonism and the Late Mesozoic Gondwana breakup so that their sources and stratigraphic relationship are difficult to constrain. This study reports new data from a Cambrian arc sequence in the Early Paleozoic Takaka Terrane of New Zealand, one of the most outboard Pacific Early Paleozoic terranes in Gondwana reconstructions [7]. Cambrian rocks of the Takaka Terrane comprise the Devil River Volcanics, an intra-oceanic arc and back arc suite, and the Haupiri Group sediments which are partially continent derived. Age control by paleontological and radiometric daing (U-Pb SHRIMP, ^Ai/^^Ar) shows that the volcanic evolution lasted for 20-30 Ma and started with an association of Mid Cambrian back-arc tholeiites, low-K type arc rocks and boninites. These rocks are similar in age to ophiolites and intra-ocenaic arc rocks throughout Australia [3,4] and in the Bowers Terrane of Antarctica [5]. Overlying medium- to high-K calc-alkaline arc rocks also correspond in age to arc rocks in SE-Australia and Tasmania, but are limited to mafic compositions, indicating the absence of a mature island arc and large scale crustal contamination. In the Late Cambrian the whole sequence was tectonically overprinted simultaneously to the Ross Delamerian orogeny in Australia and Antarctica. Transition to a passive margin in the Early Ordovician is marked by the emplacement of rift tholeiites (Gendarme Dolerites). All the above tectonomagmatic events are extremely similar to those reconized in Australia and Antarctica, thus closely linking the Cambrian fragments of New Zealand, Antarctica and the Lachlan Fold Belt. Combined Nd-Pb systematics in fresh clinopyroxenes from the volcanic rocks and in sediments [7,8] indicate that the Takaka Terrane sediments are most likely derived from the Australian craton, similar to sediments in the Lachlan Fold Belt of Australia. Extremely radiogenic Pb isotope compositions in the volcanic rocks, however, cannot be explained by a Lachlan Fold Belt source, but require subduction of sediment beneath the arc which was at least partly derived from Archean crust (radiogenic in Pb). This model is consistent with paleogeographic reconstructions [1,9], suggesting that large domains of Archean crust were exposed along the Pacific at both Antarctic Gondwana and Laurentia margins. The application of this New Zealand scenario to rocks of the Lachlan Fold Belt may provide a useful tool to understand stratigraphic and isotopic features of Cambrian sequences in Australia and Antarctica. References [1] Dalziel, I.W.D. 1997. Geological Society of America Bulletin 109, 16-42. [2] Moores, E.M. 1991. Geology 19, 425-428. [3] Crawford, A.J. & Keays, R.R. 1978. Earth and Planetary Science Letters 41, 197-208. [4] Crawford, A.J. 8L Berry, R.F. 1992. Tectonophysics 214, 37-56. [5] Weaver, S.D. et al. 1984. Earth and Planetary Science Letters 68, 128-140. [6] Munker, C. & Cooper, R.A. 1995. Journal of Geology 103, 687-700. [7] Munker, C. 1997. European Journal of Mineralogy 9, 261. [8] Wombacher, F. 1998. Diploma Thesis, Universitat Gottingen, Germany. [9] Borg, S. and DePaolo, D.J. 1994. Geology 22, 307-310.
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
PERMO-TRIASSIC GRANITOIDS OF THE ROCKHAMPTON-MONTO REGION, NORTHERN NEW ENGLAND OROGEN: INTRUSIVE RELATIONS DETERMINED WITH THE AID OF GEOPHYSICAL SURVEYS Cecil G. Murray Geological Survey of Queensland, GPO Box 194, Brisbane, Queensland 4001
Within the Rockhampton-Monto region of the northern New England Orogen (NEO), mapped recently by the Yarrol Project Team of the Geological Survey of Queensland, four main intrusions of Permo-Triassic granitoids occur in a NW trending belt, with an average spacing of about 50km. Several smaller plutons are scattered throughout the belt. The main intrusions, and most of the smaller ones, intrude Devonian-Carboniferous fore-arc basin strata of the NEO. An unusual feature is that all four larger intrusions are elongate in an ENE-WSW or E-W direction, essentially at right angles to the dominant structural grain of the NEO in this region. None of the intrusions was subdivided on first edition 1:250 000 scale maps of the region, although accompanying reports listed a range of rock types for some. The recent mapping program, with the aid of a semi-detailed regional aeromagnetic and radiometric survey, has identified most of Ae intrusions as composite. The smaller plutons are sunply zoned, in some cases with a relatively mafic rim of diorite/tonalite and a core of leucocratic granite. The relative ages of these two phases are easily established, the granite core intruding the mafic rim in all cases where intrusive relationships can be observed. The spatial configuration of rim and core suggests that the mafic phase had not cooled and solidified completely before the granite core was emplaced. The larger intrusions are, as expected, more complex, and consist of a number of coalescing zoned plutons. The western part of the Bouldercombe Complex, W of Mount Morgan, has a rim of foliated granodiorite (characterised by the development of myrmekite) intruded by massive tonalite/granodiorite, in turn intruded by massive granite. The total range of K-Ar cooling ages in this intrusive suite is about 25My, from 258Ma to 234Ma. The eastern part of the Bouldercombe Complex presents a complicated pattern on aeromagnetic images because of strong remanence in some phases. The Glassford Complex, halfway between Gladstone and Monto, can be subdivided into at least 5 plutons, some of which are zoned. A Cretaceous syenite plug intruding the central part of the Complex is associated with a large breccia zone. The Galloway Plains Tonalite, between Biloela and Gladstone, can also be subdivided into several phases, ranging from gabbro to granite. In some cases, the recent mapping program has identified gross errors m earlier descriptions of intrusions. For example, the Mount Gerard Complex was described as diorite and gabbro, and in fact was prospected for platinoids. However, plutonic rocks make up only a relatively small proportion of the Mount Gerard Complex as originally mapped, and are quartz monzodiorite in composition, consistent with their strongly potassic radiometric response. Ages of the plutons range from Late Permian to Late Triassic. A subduction related origin has been suggested for intrusives of Early to Middle Triassic age. This is supported by preliminary geochemistry, and the spacing of the intrusive centres, which is comparable with the distribution of volcanoes in continental arcs.
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
REVISED STRATIGRAPHY OF ORDOVICIAN QUARTZ TURBIDITE AND MAFIC-INTERMEDIATE VOLCANIC SUCCESSIONS, OBERON/TARALGA DISTRICT, NSW Suzanne I. Murray School of Geosciences, University of Wollongong, Wollongong, New South Wales 2522
Ordovician rocks of the Central Tablelands NSW were deposited in a deep marine environment, and now form basement rocks to the Hill End Zone. The succession consists of two contrasting provinces: the Molong Volcanic Province with mafic-intermediate volcaniclastic/volcanic rocks and the Quartzose SedimentaryProvince composed of monotonous quartz-rich turbidites and chert overlain by graptolitic black shale (VandenBerg & Stewart, 1992). A regionally consistent stratigraphy has been developed by VandenBerg and Stewart (1992) for the quartzose sedimentary province however, both the stratigraphy of the Molong Volcanic Province and the relationship between the two provinces in the Taralga/Oberon district is poorly understood and is the main topic of this study. The Sedimentary Province is represented in the study area by rocks of the Adaminaby and Bendoc Groups. The Adaminaby Group comprises mainly thick-bedded quartz rich turbidites of Early Ordovician age. The rhythms display Bouma sequences and each ranges in thickness from 60 to 100 cm, with scouring occurring at the base of some packages. A minimum thickness of 1000 m has been established for this unit although the base is not exposed. The Bendoc Group conformably overlies the Adaminaby Group and is subdivided into three formations: the Numeralla Chert, the Bumballa Formation and the Warbisco Shale. All three of the formations are siliceous and of deep marine origin. The Numeralla Chert is composed of thin bedded (2-10 cm) chert bands often separated by thin mudstone partings; the maximum thickness of this unit in the Oberon/Taralga area is 30 m. Based on conodonts the Numeralla Chert is Darriwilian to Gisbomian in age and contemporaneous with the Sunlight Creek Formation mapped elsewhere (VandenBerg & Stewart 1992). The Bumballa Formation comformably overlies both the Numeralla Chert and the Adaminaby Group and comprises of thin-bedded (5-30 cm) unfossiliferous quartz-rich turbidites. These thin-bedded turbidites are in turn overlain by the siliceous black shales and slates of the Warbisco Shale. Graptolites found in the Warbisco Shale indicate its age ranges from Gisbomian through Eastonian to Bolindian. Coarse-grained, massive sandstone beds of up to 1.3 m thick overlie the Warbisco Shale in the Golspie area. These sandstone beds may be equivalent to the Early Silurian Mundoonan Sandstone of the CanberraA"ass district (Crook et al 1973). In the study area the Molong Volcanic Province is represented by rocks belonging to the Triangle Formation. This formation spans the entire Ordovician, and is subdivided into four members: the Budhang Chert Member, the Gidyen Volcanic Member, the Mozart Chert Member and the Rockley Volcanic Member. The basal Budhang Chert member consists of tightly folded, dark coloured chert and mudstone. Conodont elements from the Paracordylodus gracilis apparatus were identified in this chert and indicate it is early Bendigonian in age. The overlying Gidyen Volcanic Member consists predominantly of basaltic pyroxene porphyritic lava and breccia with minor amounts of plagioclase. Small clasts of chert are present at the base of this member. The Mozart Chert Member overlies this succession of volcanics and volcaniclastics, and consists of thinly to moderately bedded chert bands with interbedded mudstone and sandstone beds. A Darriwilian to Gisbomian age for this member is based on the identification of Pygodus sp. and Peridon sp. elements in the chert. The Rockley Volcanic Member occurs above this chert and is considered Late Ordovician in age. It also comprises basaltic pyroxene porphyritic lava and volcaniclastic sandstone, with dark mudstone. In areas with good exposure, sedimentary features were observed in the volcanic members, indicating parts of these units have been deposited as mass flows of between 1.5 m and 5.0 m in thickness. In thin sections of lavas and breccia clasts, igneous textures are retained although samples have been altered to greenschist facies and contain abundant chlorite, actinolite and some calcite. The stratigraphy of the Quartzose Sedimentary Province for the Oberon/Taralga area is similar to the regional stratigraphy outlined by VandenBerg & Stewart (1992). The stratigraphy of the Molong Volcanic Province in the Oberon area is similar to that describedfromthe Rockley area by Fowler & Iwata (1995) however, the new Bendigonian conodont age indicates that this stratigraphy spans the entire Ordovician and not just the Late Ordovician as previously thought. No evidence for any interdigitation of the two lithological provinces was observed, however, east of Oberon the Rockley Volcanic Member overlies Late Ordovician black shales of the Quartzose Sedimentary Province conformably with widespread fault dismption.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
SUBDUCTION-RELATED MAFIC ORDOVICIAN TRIANGLE FORMATION, NORTHEASTERN LACHLAN FOLD BELT Suzanne I. Murray^. Paul F. Carr', John W. Pemberton', Christopher L. Fergusson' and Tom J. Fowler^ 'School of Geosciences, University of Wollongong, Wollongong, NSW 2522 ^Geology Department, La Trobe University Bendigo, PO Box 199 Bendigo, Vic 3550
The Ordovician Triangle Formation in the Oberon/Rockley area of New South Wales forms part of the Molong Volcanic Province of the Lachlan Fold Beh. The formation is subdivided into four members comprising the Budhang Chert Member (oldest), the Gidyen Volcanic Member, the Mozart Chert Member and the Rockley Volcanic Member (youngest). The basal Budhang Chert Member consists of tightly folded interbedded chert and mudstone. Conodont elements from the Paracordylodus gracilis apparatus identified in chert indicate an early Bendigonian age. The overlying Gidyen Volcanic Member consists of massive pyroxene basalt lava and breccia composed of clasts of pyroxene basalt lava and chert. The Mozart Chert Member consists of thinly to moderately bedded chert horizons with interbedded mudstone and sandstone. A Darriwilian to Gisbomian age is based on the identification of Pygodus sp. elements in the chert beds. The uppermost Rockley Volcanic Member is composed of pyroxene basalt and volcaniclastic sandstone, with minor mudstone. The presence of pelagic sediments and conodont faunas indicate these rocks were deposited in a cold, deep marine environment. Sedimentary features further indicate parts of the volcanic members were deposited as 1.5 m to 5.0 m thick mass flows. Although primary textures and relict minerals are preserved in the volcanic members, the rocks are altered to greenschist facies assemblages with abundant actinolite, chlorite, albite, epidote and calcite. In thin section, basalt lavas are porphyritic with abundant clinopyroxene and minor to rare plagioclase phenocrysts set in a pilotaxitic to cryptociystalline groundmass. Analyses of the least altered lavas and lava clasts indicate that the volcanic units are basalts with SiOj between 45.5 and 53.8% and MgO between 4.0 and 14.9%. The scatter of the more mobile elements (e.g. K, Na, Ba, Rb) on variation diagrams reflects their redistribution during low grade metamorphism and precludes the use of parameters such as the KjO versus SiOj relationship for recognition of magmatic affinity. Samples show slight to moderate enrichment in LREE (LaN = 10.9-60.8) and their REE patterns are fractionated with slight to moderate depletion of HREE (LaJYh^ = 2.4-6.1). Chondrite-normalised elemental plots show negative anomalies for HFSE (Nb, Zr, Hf and Ti) which are compositional signatures characteristic of subduction-related magmas. Initial Nd isotopic ratios are high (e^j >+3.9 at 450 Ma) and are typical of mantle-derived magmas for the Ordovician of the Lachlan Fold Belt. Initial ^^Sr/^^Sr values at 450 Ma are relatively low (<0.70600) but variable and probably reflect redistribution of low, mantle-derived isotopic ratios during interaction with seawater and low grade metamorphism. Based on the above data, basalts of the Triangle Formation have probably been erupted from a submarine volcano in an oceanic island arc setting offshorefromthe continental margin of Gondwanaland. Volcaniclastic material was redeposited into the deep-marine flanks of the island volcanoes. The location of the associated trench is unknown at this time, but subduction-related blueschist blocks in the New England Fold Belt indicate the presence of an Ordovician subduction zone that may have been related to the Ordovician arc. The lack of a crustal signature in these rocks indicates there was no Precambrian or Cambrian continental crust beneath this part of the Lachlan Fold Belt during the Ordovician.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
DISSEMINATED GRANITE HOSTED GOLD DEPOSITS AT TIMBARRA, NORTHERN NEW SOUTH WALES. Roger Mustard Greg Partington, Russel Nielsen and David S. Mitchell Ross Mining NL, 139 Coronation Drive, Milton, Queensland 4064
The deposits are situated at the historical Timbarra Goldfields in the southern part of the New England Fold Belt (NEFB) in New South Wales. The project is currently under development, with the initial gold production expected in last quarter of 1998. 5 deposits with a total Measured and Indicated Reserve of 12.39 Mt at 0.95 g/t gold (377,500 oz of contained gold) will be mined (Ross Mining NL Annual Report, 1997). The deposits represent an economically significant, unusual style of intrusive hosted disseminated gold mineralisation. The Timbarra Goldfield was discovered in 1853 and the subsequent working of alluvial and soft eluvial material (weathered granite) up until 1938 resulted in an estimated total production well in excess of 100,000 oz of gold. Modem exploration commenced in 1969 targeting potential large tonnage low-grade gold deposits. Permo-Triassic (245-238 Ma) high-K Granites of the Tenterfield-Stanthorpe region form a distinct group within the I-type Moonbi Supersuite informally named the Stanthorpe granite group (Blevin and Ch^peU, 1996). Leucogranites of the Moombi Supersuite represent the most significant group of mineralised granites in the NEFB having produced Sn, W, Mo, Ag, As, Bi, Cu, Pb, Au, flourite, beiyl and topaz. (Blevin and Ch^pell, 1993). Three main granite types recognised in the Stanthorpe granite group are the BunguUa, Stanthoipe and Ruby Creek types which form a textuial and compositional continuum considered to represent a diOferentiating magma series (increasing specialisation). Stanthorpe type granite lies directly on top of BunguUa type granite in the Timbarra area forming the present day topographic high known as the Timbarra Tablelands. The Stanthorpe type granite can be divided into a variably porphyritic medium to fine grained homblende-biotite granite called Monty's granite, which is intruded by a later stage more highly fractionated, texturally variable medium to coarse grained equigranular biotite granite termed the Surface Hill granite. The Surface Ifill granite has a chilled contact with the Monty's granite, with locally extensive development of a fine grained carapace facies. Minerahsation is invariably within the Surface Hill granite either immediately beneath the carapace or within 200 metres RL (below internal aphte layers). Structure has localised and enhanced gold minerahsation along fractures, joints and faults. The style of minerahsation at Timbarra, principally disseminated with minor sheeted veins is analogous to that displayed by granite hosted W and Sn deposits. The gold minerahsation has a low total sulphide content (<1%), and is associated with Mo and Bi. Gold (<1 to 100 |im) occurs in interstitial spaces as disseminated grains between primary minerals, within altered feldspars, in dissolution voids within feldspars and primary marioUtic cavities. Minor quartzcarbonate+/-albite-flourite veins contain gold within molybdenite or adjacent pyrite. A weak to moderate pervasive sericite-chlorite (+/- albite-carbonate clay) alteration of feldspars and biotite as well as dark (irradiated) primary quartz accompanies the gold mineralisation REFERENCES Blevin, P L and Chappell, B W, 1993. The influence of fractionation and magma redox on the distribution of mineralisation associated with the New England Batholitk, in New England Orogen Conference, pp 423-429. Blevin, P L and Chappell, B W, 1996. Internal evolution and metallogeny of Permo-Triassic high-K Granites in the Tenterfield-Stanthope region, southern New England Orogen, Australia, in Proceedings of Mesozoic geology of the Eastern Australian Plate Conference, pp 94-100 (Geological Society of Australia; Sydney). Ross Mining NL Annual Report, 1997.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
CADIA GOLD-COPPER DEPOSITS: GEOLOGICAL UPDATE Newcrest Mining Limited Staff (presenter: John Hollidav^ Exploration Department, cA P.O. South Orange, NSW 2800
The Cadia Au-Cu deposits (Cadia Hill, Cadia East, Cadia Ridgeway) are discoveries made since 1992 (Holliday et al, 1998) during exploration of a Late Ordovician porphyry-type alteration-mineralisation system that extends over an area of at least 6 X 2 km within the Ordovician Molong Volcanic Belt of the Lachlan Fold Belt of eastern Australia. The system is spatially related to a relatively small (3 X 1.5 km in outcrop) composite stock of predominantly monzonitic composition (Cadia Hill Monzonite). The stock intruded Forest Reefs Volcanics (volcaniclastics, subvolcanic intrusions and minor limestone) and Weemalla Formation sediments (siltstone mudstone). ' The Cadia Hill deposit is a southwest-tilted (65 deg) tabular body of sheeted quartz veins hosted mostly by monzonite porphyry. The deposit is being developed as a 17Mt per annum open-pit mining operation, based on a resource of 352 Mt grading 0.63 g/t Au and 0.16% Cu. The Cadia East deposit is a much larger body of mineralisation immediately to the east of Cadia Hill, but separated from it by a post-mineral, west-dipping reverse fault and covered by 60-200 m of post-mineral Silurian sediments. The Cadia East deposit has two distinct mineralisation types hosted mostly by volcanics: an upper disseminated type relatively rich in Cu, and a deeper sheeted-vein type known to extend to >1600m depth. The currently published resource for Cadia East, which is in the upper, potentially open-pittable part of the deposit, is 150 Mt of 0.44 g/t Au and 0.43% Cu' Other mineralisation occurences at Cadia are sheeted Cu-Au veins, hosted by monzonite and diorite at Cadia Quarry immediately west of Cadia Hill, and Fe-Cu-Au skam at Big and Little Cadia, hosted by interbedded limestone and volcaniclastics. Newcrest Mining Staff (1996) classified all the then-known mineralisation at Cadia as walbock porphyry type since there v^as no direct evidence linking the observed mineralisation-alteration with cooling and crystallisation of the exposed Cadia Hill Monzonite porphyry. However, new exploration data from the Cadia Ridgeway deposit, discovered in 1996, and from recent deep drilling at Cadia Far East (the deep, eastern extension of Cadia East) strongly suggest that the Cadia Hill Monzonite was the mineralising intrusive. The Cadia Ridgew^ay deposit is located 3 km northwest of Cadia Hill and has a resource of 54 Mt of 2.0 g/t Au and 0.73% Cu at a 1 g/t Au equivalent cut-off. The deposit is an upright bulbous body of stockwork quartz veining zoned about a small (50-100 m diameter) plug of Cadia Hill Monzonite porphyry. The plug has intruded relatively flat-lying, conformable Forest Reefs Volcanics and Weemalla Formation. Spatially-related premineral intrusions include a monzodiorite and pyroxene porphyry dykes. The most intense stockwork veining and alteration, and the highest Au and Cu grades occur immediately adjacent to the monzonite porphyry. The best part of the orebody occurs above the porphyry and the weaker parts along the sides. The intensity of veining and alteration declines both outwards and inwards from the monzonite porphyry margin. This results m a relatively low-grade central portion in the porphyry. Ore minerals are native gold, chalcopyrite and bomite, mostly occurring within vems, but also disseminated. Magnetite is a major accessory mineral in veins Hydrothermal alteration associated with the strongest mineralisation is potassic: orthoclase, albite, actinolite^ magnetite, biotite. This is overprinted by later propylitic assemblages: epidote, chlorite, Fe-carbonkte, calcite,' hematite dusting. Cadia Ridgeway has features characteristic of a small porphyry system subsidiary to a larger system, and is comparable to the Goonumbla porphyries (Heithersay et al, 1990), also in the Lachlan Fold Belt. Characteristics such as the high gold grades, alteration assemblages, and host rock types suggest Cadia Ridgeway is an alkaline-type porphyry. At Cadia Far East drilling to greater than 1 km vertical depth has intersected Cadia Hill Monzonite porphyry dykes and potassic alteration associated with a zone of higher grade Au-Cu mineralisation. The intensity of mineralisation, potassic alteration and dyking appears to increase with depth suggesting that the Cadia East deposit may be the upper part of the main Cadia porphyry system. Under this scenario Cadia Hill and Cadia Quarry are up-faulted, mineralised, peripheral portions of the main porphyry. REFERENCES Holliday et al 1998, Discovery of the Cadia Au-Cu deposits, Lachlan Fold Belt, Australia. Pathways 98 Extended Abstracts Volume, B.C.&Y Chamber ofMines/SEG Heithersay et al 1990, Goonumbla porphyry copper district. AusIMMMonograph 14 Newcrest Mining Staff 1996, Cadia gold-copper deposit. AusIMM Monograph 22 Petrological studies by Mason Geoscience. The strong support of the Newcrest Board is acknowledged.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"'Australian Geological Convention, Townsville, July 1998
NATURE, TIMING AND DURATION OF CONTACT METAMORPfflC FLUID FLOWINSILICEOUSDOLOMITESOFHORSETfflEFCREEKAUREOLE, BRITISHCOLUMBIA,CANADA MARIA C NTFRMANNand G r e g o r y M . DIPPLE,
Dept. Earth and Ocean Sciences, Univ. British Columbia, Vancouver, Canada, V6T 1Z4
Mineral assemblages and reaction textures preserved in 106 samples of potassic siliceous dolomites of the Horsethief Creek contact aureole, British Columbia, Canada record a short-lived episode of heterogeneous fluid infiltration. The distribution of mineral assemblages is comparable to those predicted by forward models of coupled heat flow, fluid flow and mineral reaction in siliceous dolomite. These same models predict reaction histories consistent with preserved reaction textures. The inner aureole expenenced infiltration at or neat its thermal peak while the outer aureole reached its thermal maximum after Ae cessation of fluid flow. This diachronous history of heat and fluid flow requires differentreaction histories for rocksfi-omthroughout the aureole and invalidates the concept of progressive metamorphism. The mineral assemblages observed in the Horsethief Creek aureole are similar to those observed in other contact metamorphic aureoles worldwide, suggesting that our interpretations may be generally applicable to the contact metamorphism of siliceous dolomites. Cretaceous contact metamorphism adjacent to the quartz monzonitic Horsethief Creek batholith produced a -1.5 km wide aureole within Proterozoic siliceous dolomites of the Mount Nelson Formation. The distribution of mineral assemblages defines six metamorphic zones with increasing proximity to the contact: phlogopite, tremolite + K-feldspar, diopside, diopside + phlogopite, diopside + dolomite, and forsterite. The observed sequence records heating to peak temperatures of -400 ooC in the outer aureole and of-^OooC near the contact at a pressure of - 2.2 kbar. Extensive infiltration of H20-rich fluids is recorded in the abundance (up to 65 volume %) of hydrous minerals (phlogopite, tremolite), the prevalence of isobarically divariant mineral assemblages, and a progressive depletion of about 7 a in whole rock carbonate with increasing metamorphic grade. At least three observations support our interpretation of diachronous heat andfluidflowand heterogeneous infiltration for the Horsethief Creek aureole. First, the distribution of index minerals is very irre^lar. Some of the irregularity in index mineral distribution can be accounted for by bulk compositional variations. However, bulk compositional variations alone are not sufficientto explain the heterogeneous distribution of mineral assemblages. The distribution of mineral assemblages can be explained as a result of spatially variable fluid flow, which is also invoked to explain the heterogeneous character of stable isotopic alteration in these samples. Second, diopside cores preserved within large (up to 2 cm long) tremolite porphyroblasts record prograde reaction of diopside to tremolite. Forward models predict that the prograde reaction of diopside to tremolite occurs in all siliceous dolomites that experience heating to 520 ooC or greater prior to fluid infiltration. We interpret the reaction diopside tremolite to record the initiation of fluid flow shortly after the onset of rapid heating of the inner aureole. Third, there is an abrupt change in the petrological character of the aureole at the tremolite + K-feldspar isograd. The outer aureole contains the isobarically univariant mineral assemblage phlogopite (trace) + calcite + dolomite + Kfeldspar which records heating in the absence offluidflow.In contrast, the inner aureole is dominated by isobarically divariant mineral assemblages that experienced extensive fluid infiltration at or near their thermal peak. We interpret the break between univariant and divariant equilibria to be an inevitable result of the timescales of conductive heat transfer. The rocks in the inner aureole were infiltrated early in the contact metamorphic event while near their thermal maximum. At this time, the rocks in the outer aureole experienced infiltration while below their thermal peak. Rocks of the outer aureole attained their thermal peak afterfluidflowhad stopped. The early cessation offluidflowis recorded by the preservation of high temperature divariant assemblages in the inner aureole and by the univariant assemblages in the outer aureole which record post-fluidflowheating. These observations and interpretations are irreconcilable with progressive metamorphism. The sequence of isograds in the Horsethief Creek aureole does not represent the prograde reaction history of rocks fi-om the inner aureole, but rather reflects the complex interaction of heat flow and fluid infiltration over space and time.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
COEXISTING ANDESITIC AND CARBONATE MELTS IN A LHERZOLITE XENOLITH FROM MT. SHADWELL, VICTORIA Marc D. Norman and Norman J. Pearson GEMOC National Key Centre, Macquarie University, North Ryde NSW 2109
Metasomatism is a frequently invoked but poorly understood means for creating compositional diversity within the mantle. Metasomatic agents may include hydrous fluids, silicate melts, and carbonate-rich fluids and melts. We have discovered assemblages of coexisting carbonate melt and andesitic glass in a Iherzolitic xenolith that provides a snapshot of carbonate-related metasomatism in the continental lithospheric mantle. The xenolith is a fertile, protogranular spinel Iherzolite. Silicate glasses and carbonate globules occur in melt pockets and veins. Phenocrysts of olivine, cpx and spinel, and minor sulfide globules are present in the glasses, which are intermediate in bulk composition with 55.5-60.5% Si02, high AI2O3 (19-20.5%), Na20 (4.7-6.2%) and K2O (0.7-2.1%). The carbonate is calcic, ranging from nearly pure CaC03 to -10% MgC03. It occurs as globules with curved boundaries against the glass and as the major phase filling cavities. Internally, most of the carbonate is homogeneous under BSE imaging, with a subtle striated appearance which we interpret as a primary quench texture. The bulk compositions of the glass and the carbonate are distinct from those of equilibrium immiscible liquids, but the form of these globules strongly suggests that they coexisted as poorly miscible melts prior to entrainment. The host Iherzolite was reacting strongly with the melts. Spinels have spongy rims against the glass. Olivine and cpx phenocrysts have magnesian compositions comparable to those of the host Iherzolite, but with distinctive minor element abundances. Olivine phenocrysts have CaO contents more typical of magmatic values (0.15-0.20% CaO vs. 0.05%). New cpx ranges to markedly higher AI2O3 (up to 11%) and Ti02 (up to 2.2% Ti02), and lower Na20 (0.3-1.5%) compared to the host Iherzolite. CaO contents of an olivine grain in the Iherzolite adjacent to a large patch of carbonate show a clear trend of increasing Ca toward the contact, suggesting diffusion of Ca into the olivine. Modelling of the diffusion profile shows that the residence time of the carbonate was extremely short, on the order of days to tens of days, indicating a temporal link to the magmatism that transported the xenolith to the surface. Diffusion profiles were calculated using diffusion data of Jurewicz and Watson (1988) and the thermal evolution model of Lasaga (1983), with values for AT/t of 100-1000 ®C/my and a grain size of 1 mm. The core of the olivine is assumed to represent the initial composition, which is justified by similar compositions in unzoned grains. Trace element compositions of the andesitic glass as determined by laser ablation ICPMS preclude an origin as decompression breakdown of amphibole, and are more consistent with an origin as a small degree melt, perhaps induced by carbonate fluid fluxing of the hthosphere associated with the basaltic magmatism.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
DEFORMATION OF LATE PERMIAN-TRIASSIC GRANITES AND ASSOCIATED THERMAL METAMORPHIC ROCKS OF THE SOUTHERN NEW ENGLAND FOLD BELT, NORTHEAST NEW SOUTH WALES David Och. Pablo Lara, Graziella Caprarelli and Evan C Leitch Dqjartment of Environmental Sciences, University of Technology, Sydney, Broadway, New South Wales 2007
Small-scale deforaiational structures are found within Late Permian and Triassic plutons and associated thermal metamorphic rocks of the New England Batholith. Recrystallization or alteration associated with the structures indicate that movements occurred before the bodies had cooled to ambient temperatures and hence soon after emplacement. Thus although the plutons have many characteristics of postorogenic granites they predate the end of the protracted period of tectonic movements that are collectively grouped as the Hunter-Bowen Orogeny. In the generally massive Carrai Granodiorite, a 150 km^ granodiorite body emplaced in Early Permian strata of the Nambucca Slate Belt, mylonitic rocks are well exposed in a belt at least 20 metres wide beside the Macleay River, The granodiorite is transected by narrow zones ranging from mylonite to ultramylonite, the former with well developed C/S fabrics. The zones mostly strike about NW-SE and dip about 30® NE but N-S striking zones are also present. Individual zones curve, anastomose and bulge irregularly to form mylonitic lenses and knots. Small synthetic and antithetic fractures diverge from the zones. Mineral elongation lineations and shear sense indicators (mica fish, rotated porphyroclasts, C/S fabrics) show that the mylonites are the product of oblique dextral reverse shp involving movement in a direction plunging about 25® N. Offsets of an aplite dyke indicate that individual zones are associated with displacements of 1 m or less. Microstmctures within the mylonites and the presence of newly crystallised biotite and statically recrystallised quartz suggest formation of the mylonites at aroimd 400®C, a temperature significantly greater than that attained by the coimtry rocks beyond the thermal aureole. This temperature is close to that at which the isotope systems used to date tiie Carrai Granodiorite are set. Thus YJAi and Rb/Sr ages of about 225 Ma indicated for the pluton will be close to that of mylonite formation. The Hillview thermal high, an area of about 75 km^ of homfelsic rock associated with a small granite stock and numerous dykes, is located about 40 km south of the Carrai Granodiorite and is believed to be imderlain by a substantial granite mass of similar age to the Carrai body. Along Inlet Creek in the westem part of the high, thinbedded rocks of probable Early Carboniferous age are offset across numerous N-S striking, vertically dipping, mesoscopic faults. In plan view, beds which dip about the vertical, are displaced sinistrally between 0.05 and 0.1 m across individual faults and although the direction of net slip has not been established directly, the consistent sense of displacement of steeply inclined beds on vertical fractures, the presence of steeply plunging breccia lenses associated with small releasing bends and steps along individual fractures, and the orientation of subsidiary fractures, all suggest strike-slip dominates. Hydrothermal alteration has affected the homfels adjacent to the faults, involving widespread destruction of biotite and the crystallisation of chlorite, quartz, calcite and minor sulphides. Faults present prior to thermal metamorphism would have been sealed early in this event and hence we conclude that faulting and alteration were synchronous. Preliminary fluid inclusion studies indicate that alteration involved temperatures higher than those experienced by the country rocks beyond the thermal high, and the alteration is the result of the circulation of fluids along the fractures while Aese rocks were still at elevated temperatures. Hydrothermal alteration has also taken place adjacent to minor shears exposed in extensive road cuttings in the northem part of the 250 Ma Moonbi Adamellite. Quartz, chlorite and calcite occupy narrow veins along the shears adjacent to which the adamellite is a rusty pink colour. The shears belong to a conjugate system, with one set striking NNW and dipping very steeply ENE, and the other striking about E-W and dipping about the vertical. Both the angular relations between the sets and the orientation of uncommon slickenfibres on the shears indicate strikeslip movement with sinistral movement on the NNW striking set. A similar sense of movement occurred late in the history of the nearby NNW striking Peel Fault, and although the latter structure is transgressed by the Moonbi body, it and the shears probably provide a guide to the regional stress field about the time of granite emplacement.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
IMPLICATIONS OF ARGON GEOCHRONOLOGY/THERMOCHRONOLOGY AND METAMORPHIC WHITE MICA STUDIES FOR DELAMERIAN OROGENESIS IN THE DELAMERIAN FOLD BELT,SOUTH AUSTRALIA. Robin Offler^ David A. Foster^ and David R. Gray^ 'Departnient of Geology,Umversity of Newcastle, Callaghan, NSW 2308 ^Australian Geodynamics Cooperative Research Centre, VIEPS School of Earth Sciences, La Trobe University, Melbourne, Victoria 3083 3Australian Geodynamics Cooperative research centre, VEEPS Department of Earth Sciences, Monash University, Melbourne, Victoria 3168
The Delamerian fold belt forms part of the paleo-Pacific margin of Gondwana. It consists of Neoproterozoic to Early Cambrian shallow water sediments which were deposited in intracratonic rifts. Deformation and associated high T-low P metamorphism (HTLP) of these sediments has always been assumed to have taken place during the Delamerian orogeny (523-486 Ma; Turner et a/. 1996). Recent Ar-Ar and K-white mica studies suggest that the tectonic history of this belt is more complicated than previous authors have proposed. Ar-Ar spectra obtained from slates in the fold and thrust belt of the southern Hinders Ranges, north of the HTLP belt in the Mt. Lofty Ranges, indicate that deformation commenced 550-540 Ma ago. Resetting of the system appears to have occurred at 510-505 Ma and subsequently at -453 Ma during movement on detachment zones on the western margin of the fold belt. These relatively younger dates from the deeper-level faults may record a phase of west-directed thrusting caused by major defonnation and accretion of the western Lachlan Fold Belt. The results suggest that deformation is diachronous contrary to previous interpretations that assumed that deformation of the whole belt took place during the one event. Illite crystallinity determined from K-white micas in slates throughout the southern Flinders Ranges, indicate that grade increases from diagenetic (zeolite facies) at the western margin of the belt to epizonal (greenschist facies) adjacent to the eastern margin, reaching biotite grade in some rocks. In addition, biotite is developed sporadically in the the detachment zones and in the hanging wall. These data confirm previous studies by McKirdy et at, (1974). The b cell parameters of these micas (x=9.040; on-l=0.012; n=70) reveal that the rocks in the southern Flinders Ranges have been deformed under much cooler, intermediate pressure type conditions (~16®C/km) than those in the Mt Lofty Ranges (~35®C/km; Dymoke & Sandiford, 1992). Significantly the b cell parameters decrease dramatically in the Sedan area just north of the HTLP belt, suggesting that the cooler conditions prevailing earlier in the fold belt, have been overprinted by the younger HTLP metamorphism which confirms the data obtained obtained from the Ar-Ar analysis. These studies clearly indicate that the tectono-thermal evolution of the Delamerian fold belt is more complex than previously thought and shows the value of a multidisciplinary approach. REFERENCES DYMOKE, P. & SANDIFORD, M. 1992. Phase relations of Buchan facies series pelitic assemblages. Calculations and applications to the Mt Lofty Ranges, South Australia. Contributions to Mineralogy and Petrology 110, 121-132. McKIRDY, D.M., SUMARTOJO, J., TUCKER, D.H. & GOSTIN, V. 1975. Organic, Mineralogic and Magnetic indications of metamorphism in the Tapley Hill Formation, Adelaide Geosyncline. Precambrian Research 2, 345373. TURNER,S.P., KELLEY, S.P., VANDENBERG, A.H.M., FODEN, J.D., SANDIFORD, M. & FLOTTMANN, T. 1996. Source of the Lachlan fold belt flysch linked to convective removal of the lithospheric mantle and rapid exhumation of the Delamerian-Ross fold belt. Geology 24, 941-944.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
DIFFERENTIAL RENEWAL OF U-PB PATTERNS IN ZIRCONS DURING ANATEXIS OF KIMBERLEYS PELITES: IMPLICATIONS FOR GRANITE GENESIS Nichola<; H S Oliver'. Simon Bodo^kos^ Alexander A. Nemchin\ Peter D. Kinny^ & Gordon R. Watt^ ' School of Earth Sciences, James Cook University, Townsville, Queensland 4811 ^ School of Applied Geology, Curtin University, Perth, WA 6001
The analysis of migmatite accessory phases using combined field, micro-analytical and hnaging techniques is a powerful tool for understanding both anatectic processes and thermal evolution of metamorphic belts. In the high grade parts of the Halls creek Orogen in NW Australia, internal zonmg and SHRIMP U-Pb geochronological patterns in zircons from one outcrop of deformed pelitic migmatites of the Proterozoic Halls Creek Orogen reveals a relationship between the type of leucosome and the degree of overprinting ofdetrital U-Pb ages. Peak metamorphism to low pressure granulite faciesis inferred to have occurred at 1850 to 1845 Ma, based on conventional single-grain monazite U-Pb analyses (1845±3 Ma) from stromatic migmatite, and a pooled group of SHRIMP zircon ages from all samples of 1847±7 Ma. Zircons from the oldest, stromatic migmatites with ~ 1 cm wide leucosomes show a relatively h i ^ abundance of zircon analyses with 207/206pb ages corresponding with this peak metamorphic age. Subsequent thick leucosomes (up to 0.5 m wide) developed in response to intrusion of mafic dykes into the stromatic migmatites which had probably not yet frozen. They form as irregular sheaths around the intrusions, cross-cutting tendrils in the host metasediments, and "back-veins" in the intrusions. Zircon analyses within these contact/sheath leucosomes show the greatest proportion of young 207/206p|5 ^ges, with a relatively high abundance in the range 1835±3 Ma, inferred to be the age of intrusion of the mafic dykes. Analyses of zircons from the structurally youngest, cm- to m-scale shear- and vein-hosted leucosomes, show the least approach towards the metamorphic ages, indicating retention of a large detrital population. These leucosomes have the oldest pooled age, with abundant detrital zircon analyses in the range 1860 - 1880 Ma, and extending back to 2600 Ma, comparable to analyses of inferred detrital zircons frompsammitic rocks in the same outcrop which did not undergo anatexis. The correlation between melt-lubricated shear zones and the small degree of zircon overgrowth and U-Pb modification is inferred to reflect rapid deformation-induced melt extraction from the protolith, and/or rapid quenching, as a consequence of shearing. In contrast, the crosscutting contact/sheath leucosomes and the stromatic leucosomes may have had a combination of slow melt segregation, larger interacting melt volume and longer residence times at high temperatures, allowing more complete re-equilibration of U-Pb isotope patterns in zircons during anatexis. Only a small proportion of all the zircons analysed recorded the effects of the granulite facies metamorphism and associated anatexis. This is partly because the Zr content of the host pelites is much higher than the predicted saturation value of Zr in melts at these P-T conditions for the bulk compositions involved. This implies that incorporation of such zircons into a larger granitic magma would potentially preserve a clear signature of the source region, in terms of both trace element signatures and geochronology. In detail, we note that the most readily correlatable aspect of the migmatite types with respect to zircon U-Pb and internal zoning patterns is migmatite melt volume, with the smaller stromatic and shear/vein leucosomes showing little effect of the anatectic events. In this regard, the hypothesis of Watson (1996) concerning interacting melt volumes may be the most applicable. However, we also note that other factors also correlate well, such as the inferred length of time the particular local melt stayed at or above the solidus, the inferred melt segregation rate, and the degree of deformation during segregation, the latter effect appearing to inhibit zircon dissolution and reprecipitation rather than enhancing it. Elucidating which combination of these types of processes is the most important for high volume granitoid batholiths awaits fiuther research. Although much larger volumes of melt are probably required for the accumulation of moderate sized granitoids, our study suggests that different mechanisms of meh extraction may strongly influence the isotopic inheritance of a product magma, and could even dominate over bulk compositional or P-T conditions in some magma types. We aim to test this further on a broader range of melt compositions and anatectic conditions. REFERENCES Watson, E. B., 1996. Dissolution, growth and survival of zircons during crustal fusion: kinetic principles, geological models and implications for isotopic mheritance. Transactions of the Royal Society of Edinburgh, 87: 43-56.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
HAMERSLEY HEMATITE ORE GENESIS BY SYNTECTONIC INFILTRATION OF HEATED METEORIC FLUID Nicholas; H. S. Oliver'. Gerald R. Dickens', Christopher McA. PowelP , P. D. Bons^& Lachlan K. Stewart' ' Ecx)nomic Geology Research Unit, School of Earth Sciences, James Cook University, Townsville, Queensland 4811 ^ Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia, Nedlands, WA, 6907 ' Department of Earth Sciences, Monash University, Clayton, Victoria, 3168
We outline aspects of a new, testable model for genesis of giant hematite ores of the Hamersley Province, which is a modification of a hypogene syntectonic model of Li et al (1993), and has some parallels to models for MVT ore genesis and fluid flow in southwestern New Zealand around the Alpine Fault. The model for ore genesis developed in the 1980s by Morris and coworkers (e.g. Morris, 1985) proposed supergene emchment through protracted post-tectonic surface exposure of BIFs, followed by upgrading of goethite to hematite during subsequent metamorphism. Powell et al (this volume) summarise developments that place considerable doubt on the chronology of mineralising events proposed by Morris and describe a refined model for tectonic evolution of the province during the ca. 2.4 to 2.2 Ga Ophthalmian Orogeny. The model permits a geometry in which topographic and structural uplift of an active foreland fold-and-thrust belt could have provided the necessary perturbation of hydraulic head gradients to allow syntectonic ingress of surface fluids into the belt. A common feature to most previous models, and this current one, is that there are fundamental structural controls on the localization of ores, in particular their association with faults, and at regional scales, their association with the strongly folded southern margin of the Hamersley Province, the Opthahnia fold-and-thrust belt. Around the major ore deposits, many outcrops have been identified in which iron and silica appear to have been dissolved and precipitated over cm- to m-scales, in association with folds. In some samples, the distribution of iron oxides and quartz implies the operation of local (cm-scale) dissolution and reprecipitation, without necessarily requiring external input of mass, and such processes were integral in the accumulation of strain within fold hinge regions. In other locations, however, field and microstructural data, along with the stable isotopes, indicate that mass (particularly oxygen and silica) was transported at greater than 100m scales during the Opthahnian orogeny. These observations and data suggest that fluid transfer was an integral part of the deformation, raising the possibility of an association with genesis of the structurally controlled iron ores. Oxygen isotope data for separated minerals and whole rocks from Mt Whaleback ore and nearby BIFs indicate that the ore fluid was meteoric or modified meteoric in origin. However, magnetite-quartz, hematite-quartz, and hematite-water fractionations (the latter assuming likely fluid compositions for meteoric or basinal fluid) indicate that the most likely temperatures for formation of hematite ores and for metamorphism of BIFs were in excess of200°C and may have reached 400°C. Heating surface fluids is a suitable mechanism for leaching large volumes of silica as its solubility increases substantially over the 20° to 300°C interval. This alleviates some of the problems inherent in a low temperature, supergene model with leaching a billion tonnes of silica to concentrate the 1.6 billion tonnes of Fe residing in Whaleback. Regional observations of fold axial-planar hematite seams with isotopic compositions approaching that of Whaleback ore also suggest that the fluid accessed the rocks during deformation under lower greenschist facies conditions. Although we have not yet fully characterised the links between the giant ore deposits and the evidence for syntectonic fluid flow in the surrounds, the isotope data and broad timing constraints suggest a common link. We propose a major hydrothermal event during the Ophthalmian Orogeny that added oxidised surface water to reduced BIFs over very broad scales, transported this fluid down and along the belt, and culminated in giant iron ore genesis in specific structural locations. REFERENCES Morris R.C. 1985. Genesis of iron ore in banded iron-formation by supergene and supergene-metamorphic processes - a conceptual model. In: Wolf K.H. (ed.) Handbook of strata-bound and stratiform ore deposits Elsevier 13: 73-235. Li Z.X.., Powell C.McA. & Bowman R. 1993. Timing and genesis of Hamersley iron-ore deposits. Explor. Geophys. 24: 631-636. Acknowledgements: We acknowledge support for this aspect of the work by the Australian Research Council, Robe River Mining Associates, and BHP Iron Ore.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
GEORGE V LAND - ADELIE LAND - EYRE PENINSULA: A TENTATIVE CORRELATION R.L.01iver' and C.M. Fanning^ 'Department of Geology and Geophysics, University of Adelaide, South Australia 5005 ^Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia 0200
A number of isolated meta-igneous and meta-sedimentary outcrops are scattered along a 300 km coastal section of Ad^lie Land and King George V Land, East Antarctica. Geochronology indicates that the granitoid at Cape Denison in Commonwealth Bay, and a similar (lithologically) granitoid near Port Martin in Adelie Land, though somewhat retrograded, are the oldest rocks, viz ^2.4 Ga. "Grey layered gneiss", found within the Cape Denison granitoid, however, may represent pre-granitoid terrain. On the coast of the Cape Gray promentory, rock types can be grouped into: i) metasedimentary gamet-cordieritesillimanite-biotite-feldspar-quartz gneiss, ii) igneous hypersthene-K feldspar-quartz-(plagioclase)-(gamet)-(biotite) chamockitic gneiss with variable foliation, intrusive into the metasedimentary gneisses, iii) mafic cpx-opxplagioclase-homblende-(gamet)-dykes and masses (metadolerite), intrusive into both the gneisses and the chamockitic gneiss. Metadolerites and metabasalts, now amphibolites, also abundantly intrude the Cape Denison granodiorite. The chemical composition of these, on average, resembles that of the mafic dyke series east of Commonwealth Bay; the abundance of pyroxene ± garnet in the latter, however, manifests a higher metamorphic grade than that of the Cape Denison dykes, in keeping with a different metamorphic grade of the respective country rocks. Zircons from two chamockitic specimens from Madigan Nunatak, with virtually identical mineralogy, analysed by SHRIMP microprobe, have very different U-Pb ages of 1.709±0.012 Ga and 2.35-2.70 Ga respectively. Generally speaking, the above described rocks from Adelie Land and George V Land appear to manifest events of two ages, ca 1.7 Ga and ca 2.4 Ga. These ages are recorded strongly, also, in southem Eyre Peninsula (viz. late Kimban Orogeny and early Sleaford Orogeny, respectively). Thus the ca 2.4 Ga age of the Camot Gneisses of southem Eyre Peninsula resembles the age of the Port Martin and Cape Denison granitoids. Aluminous, gametcordierite-sillimanite bearing gneisses, similar to those east of Commonwealth Bay, are part of the Camot Gneiss sequence. Also possibly related, here, are phyllitic metasediments at Coffin Bay, southwestem Eyre Peninsula, (with sedimentary and metamorphic ages respectively of ca 1.76 Ga and 1.70 Ga) and rocks of similar lithology and age at Cape Hunter, Antarctica. Mafic dykes, metamorphosed variously to pyroxene gneisses and/or amphibolites in Adelie land and George V Land, may be matched, in southem Eyre Peninsula, by like meta-basalts or meta-dolerites, intmsive into the Hutchison Group during the Kimban Orogeny.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
PALEOCEANOGRAPHY OF THE EASTERN MOST INDIAN OCEAN: A HISTORY OF GLOBAL THERMOHALINE FLOW AND WEATHERING OF THE AUSTRALIAN CONTINENT Bradley N. Opdvke and Anne Muller The Australian National University, Department of Geology, Canberra ACT, 0200 Global Quaternary climate has been characterised by fluctuations in the intensity of global thermohaline circulation. Regional climate changes in Europe and parts of North America are strongly linked to the activity of the northward flowing Gulf Stream. In the southern hemisphere there is some question as to the strength and timing of the climatic "teleconnections" with the northern hemisphere climate changes. Including the question of which areas of the globe "lead" and which areas "lag" with respect to large glacial to interglacial climate shifts. An important component of the global thermohaline circulation is known as the "Indonesian Through Flow", where waters are forced through the Indonesian archipelago when the global thermohaline circulation is strong. In an effort to determine the timing and activity of Indonesian through flow waters a series of cores were taken on the Scott Plateau off north western Australia. This site has also proven a valuable region for monitoring the flux of eolian dust from the north western portion of the Australian continent. One core, taken in 1994 by the RV Rig Seismic on the Scott Plateau, has proven extremely valuable, and shows large shifts in the abundance of proxy indicators for productivity in the area. Opaline silica, one such proxy indicator of nutrient abundance, shows a sixflold increase in accumulation rate during the last glacial maximum, from 25 to 15 ka. This is a time of slower thermohaline circulation globally. Today, productivity is inhibited by the nature of the plume of low salinity water that moves through the Indonesian archipelago and spreads out over the equatorial portions of the eastern Indian ocean. At times of low through flow, we predict that the absence of this low sahnity "cap" would enhance the possibility of stronger up welling in the region. This is due to the increased possibility of wind-driven divergence when the intertropical convergence zone seasonally moves over the area. This would be analogous to the region of relatively high productivity found in the central Pacific ocean. Using this model we have identified two other time periods when the through-flow "shuts down". One approximately 53 ka, at the beginning of Stage 3, and another interval at the beginning of Stage 4, at approximately 70 ka. Given the relatively high sedimentation rates found in this core, from 3 to 15 cm/ka, we have an opportunity to assess the eolian flux of dust to the region from the Australian mainland. There appears to be a direct linkage between the dust flux, productivity, and our perceived change in "through flow" strength. The implication being that there is a direct link between rain/aridity in the north west portion of the Australian continent and the strength of the through flow. This finding correlates well with the data, which show cooler seas at times of through flow "shut down".
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
ARE LITHOSPHERES FOREVER? O'Reillv.S. Y . \ Griffin, W.
and Poudjom Djomani, y /
^GEMOC National Key Centre, School of Earth Sciences, Macquarie University, Sydney, NSW, 2109, Australia ^CSmO Exploration and Mining, PO box 136, North Ryde, NSW. 2113, AustraHa
THE LITHOSPHERIC MANTLE AND 4-0 LITHOSPHERE MAPPING The subcontinental lithospheric mantle (SCLM) carries a geochemical, thermal and chronological record of largescale tectonic events that have shaped die Eardi's crust The SCLM is part of the continental plate, and moves with the plates over the less rigid asthenosphere. It has long been accepted that "old" (cratonic) lithosphere is relatively deep, depleted and cold; more recently it has been recognised that "young" lithosphere is relatively thin, fertile and hot. Development of the 4-D Lithosphere Mapping methodology has provided tools for constructing realistic geological sections of the SCLM. Xenoliths and garnet and chromite xenocrysts from mantle-derived volcanics provide samples of the lithospheric mantle at the time of eruption. These may allow determination of the paleogeotherm, depth to the crust-mantle boundary, detailed distribution of rock types with depth, spatial distribution of fluid-related processes and the depth to the lithosphere-asthenosphere (LAB) boundary. Volcanic episodes of different ages in one region give this information for different time-slices; geophysical data (seismic, gravity, magnetic, thermal) can be used to extend the geologically-derived profiles laterally or to interpret lithospheric domains with geophysical signatures that can be matched with geologically mapped sections. SECULAR VARIATION IN LITHOSPHERE COMPOSITION There is a fundamental distinction between Archean cratonic mantle, represented by xenoliths in African and Siberian kimberlites, and Phanerozoic circumcratonic mantle, represented by xenoliths in intraplate basalts and by orogenic Iherzolite massifs. Archean xenoliths are not only more depleted on average, but have higher Si/Mg (higher opx/olivine); subcalcic harzburgites are well-represented in Archean xenolith and xenocryst suites, but essentially absent in younger ones. Analysis of >13,000 garnet xenocrysts from volcanic rocks worldwide shows a clear correlation of garnet composition with the tectonothermal age of the crust penetrated by the volcanic rocks. The combined xenolith and garnet data indicate that the Archean/Proterozoic boundary represents a major change in the nature of lithosphere-forming processes. In xenoliths, the CrjOj content of garnet correlates well with the AI2O3 content of the host rock and xenolith suites also show good correlations between AI2O3 contents and those of other major and minor elements. These correlations make it feasible to calculate the composition of a mantle section, given the median CriOj content of garnet xenocrysts from that section. The mean compositions of SCLM beneath terrains of Archean, Proterozoic and Phanerozoic tectonothermal age, calculated in this way, show a clear secular evolution in all measures of depletion, such as Al, Ca, mg#, and Fe/Al. SIGNIFICANCE OF SCLM EVOLUTION TO GEOPHYSICAL INTERPRETATION Average mineral compositions for each age group have been used to calculate average modes, densities and seismic velocities. Archean SCLM is 2.5% less dense than the asthenosphere (approximated by PM); for the less-depleted Phanerozoic mantle the difference is <1%. Thermal expansion coefficients are identical within error for all compositions, so that these differences persist to high temperatures. At 25®C, the Vp and Vs of Archean SCLM are higher than that of Phanerozoic SCLM by ca 0.5% and 1.2%, respectively; thus accounting for ca 25% of the range observed by seismic tomography. Typical geotherms for cratonic and Phanerozoic areas were used to calculate the difference in Vp and Vg at 100 km depth; the Archean values are higher by 4-5%, corresponding to the ranges commonly seen by seismic tomography. LITHOSPHERE EVOLUTION AND DESTRUCTION These physical property data are important constraints on the delamination and recycling of the SCLM. Thermal expansion coefficients and bulk modulus of minerals have been used to calculate the temperature-dependent density variation with depth for typical Archean, Proterozoic and Phanerozoic mantles. The results show that the entire section of Archean lithospheric mantle is significantly buoyant relative to the underlying asthenosphere. For Proterozoic and Phanerozoic mandes, a minimum thickness of ca 30 and 60km respectively must be reached before each section becomes buoyant: these are the minimum conditions for lithosphere delamination. This effect explains the thickness and apparent longevity of existing Archean (and thick Proterozoic) lithosphere, but suggests that no deep continental root could be constructed from Phanerozoic lithosphere. CONCLUSIONS Correlations between mantle type and crustal age indicate that continental crust and its underlying SCLM were formed together and remain coupled for geologically long times. Archean (and Proterozoic >30 km thick) lithosphere is forever unless there is physical disruption (eg rifting, thinning and displacement) with associated thermal and chemical erosion (metasomatism). Phanerozoic lithosphere will delaminate unless it remains hot and thin (less than about 50 km thick).
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
DEVELOPMENTS IN GEOLOGICAL MAPPING WITHIN A STATE GEOLOGICAL SURVEY - THE VICTORIAN EXPERIENCE Peter J. O'Shea Geological Survey of Victoria, Dept of Natural Resources & Environment PO Box 500, East Melbourne Victoria 3002
The Geological Survey of Victoria (GSV) was established in 1856 in response to the discovery of gold at Clunes in 1851. Modelled on the British version, systematic geological mapping was, and remains, the primary function of the GSV. Alfred Selwyn was the furst Director of the GSV. Systematic grid mapping at 1:31 680 scale (the Quarter Sheet Series) was carried out between 1856 and 1869, and covered an area from Geelong in the south to Castlemaine in the north. Geological mapping was carried out and published at various scales from the 1870s until the 1950s, but it was not imtil the 1950s that systematic grid mapping was reintroduced with the production of the 1:63 360 series geological maps.
With the change to a metric system in the 1970s, the 1:63 360 scale series was replaced by a 1:50 000 scale series. During the 1970s the first 1:250 000 scale geological map coverage of the State was completed. In the 1980s the 1:50 000 scale series was discontinued in favour of a 1:100 000 scale series in response to an industry needs survey. In the 1990s, in order to meet long term objectives and again in response to industry requests, map production reverted to 1:50 000 scale while retaining 1:100 000 scale areas for coverage of explanatory notes. The 1980s and 1990s has also seen the production of 1:10 000 scale geological maps covering the Bendigo and Chewton goldfields. The program of geological mapping in Victoria between the 1850s and the 1980s was driven at times by gold fever, and at times by urban planning requirements at a State level. A change of focus has taken place during the 1990s as a result of the National Geoscience Mapping Accord (NGMA) and the Victorian Initiative for Minerals and Petroleum (VIMP). Commonwealth NGMA contributions in Victoria have primarily been in the area of airborne geophysical coverage of key 1:250 000 scale sheet areas. Together with airborne geophysical survey coverage fimded at the State level through VIMP, more than 70% of the State has been flown for 400m or better line space magnetics and radiometrics. This new geophysical data is resulting in the production of new generation geological mapping across the full width of the Lachlan Fold Belt in Victoria, which is contributing to an understanding of the crustal architecture at a continental scale. The provision of high quality, detailed airbome magnetic and radiometric data has led to better interpretation and significant time savings in the rate of geological mapping. Map areas are now completed by project teams of up to seven geoscientists together with ancillary support. New map products include 1:100 000 scale and 1:250 000 scale geological interpretation of geophysical features maps on which the geological interpretation is draped over a grey scale image of the magnetics. Regolith maps at 1:100 000 scale are ciurently being produced for key areas in the central Victorian goldfields as aids to mineral exploration geochemical survey planning. Digital data from map areas is now routinely available in a number of formats, and GIS datasets on CD are now available for selected areas of the State. The provision of bigger and better GIS datasets will be a priority in the future as Geological Surveys around Australia, and around the world, catch up with the backlog of database entry of geoscience information. The increasing pressures from Government on Geological Surveys to justify their existence will have an influence on the type of geoscience maps produced in the future. Regolith maps which give an mdication of areas suitable for wine grape production may become as regular an output as the standard geological maps now being produced.
344
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
EPISODIC LATE PALAEOZOIC TO CENOZOIC COOLING/DENUDATION ALONG THE NORTHEASTERN QUEENSLAND MARGIN Paul B. O'SullivaiL and Barrv P. Kohn Australian Geodynamics Cooperative Research Center Department of Earth Sciences, La Trobe University, Bundoora, Victoria 3083, Australia
We report the first results of an apatite fission track (AFT) study designed primarily to constrain the Phanerozoic thermal and tectonic history of the northeastern Queensland margin (north of Townsville). The preliminary results suggest that: 1) major cooling/denudation events have occurred episodically since the Late Palaeozoic, 2) the distribution of these episodes changes from older in the west to yoimger in the east towards the present-day coastline, 3) much of the region has experienced some degree of cooling/denudation during the Cenozoic, and 4) there are strong similarities between the southeastern and northeastern margins of Australia in terms of the timing of major cooling/denudation events. The first recorded episode of cooling/denudation occurred during the Middle to Late Carboniferous, between -320300 Ma. This episode is only recognized in data from rocks located well-inland, at distances >-200 km from the present-day coast, and includes those collected from the Clarke River Basin and the Lolworth-Ravenswood Block. Cooling/denudation during this time is believed to have been the result of widespread mid-Carboniferous tectonism which affected the entire Thomson Orogen, and produced broad, open folds in the Adavale, Drummond, and Burdddn Basins. Following mid-Caiboniferous orogenic activity, cratonization of the region occurred during widespread Late Carboniferous to earliest Permian granitic magmatism. The next major episode of regional cooling/denudation occurred during the Late Permian to Early Triassic between -260-240 Ma. This is recorded in most Permian and older rocks collected throughout the region, from coastal outcrops to those located >300 km inland. This event could reflect a significant decrease in the prevailing geothermal gradients, following their elevation during intrusion of the Late Carboniferous to earliest Permian batholiths. However, since samples of the granites themselves record Late Permian to Early Triassic cooling/denudation, this scenario is unlikely since rocks located well away from the batholiths record identical results. Therefore it is concluded that this cooling event probably occurred in response to the Hunter-Bowen Orogeny which is known to have affected the New England Fold Belt and parts of the Lachlan Fold Belt to the south at about this time. The relationship of denudation to structural style is not clear as cooling/denudation could have occurred in response to compressional reactivation of earlier structures or to more regional rock uplift. Subsequently, two episodes of cooling/denudation are recorded in rocks located near the present-day coast, and have not yet been recognized from the data from any samples collected more than -50 km inland. The first of these occurred during the middle Cretaceous at some time between -90-110 Ma. It is possible that this cooling/denudation occurred due to the onset of continental extension in the Tasman Sea at -96 Ma if extension along the eastern margin of Australia during the middle Cretaceous was not limited to the Tasman Sea. A later cooling/denudation episode, during the Paleocene, is also suggested based on preliminary data from rocks collected near the present-day coastline. We propose that this event occurred in response to a combination of the relaxation of elevated geothermal gradients associated with rifting and rapid erosion at that time. These results suggest that the northeastern margin of Australia has remained tectonically active throughout the Late Palaeozoic and into the Cenozoic. Furthermore, in response to regional tectonics, the northeastern margin has reacted in a similar fashion to the southeast margin of Australia where previousfissiontrack results have already recognized the Late Permian to Early Trias^c, middle Cretaceous, and Palaeocene episodes of cooling/denudation. Fission track dating of apatite samples has provided important constraints on the timing and amount of denudation in response to Mesozoic and Cenozoic uplift and erosion. This information provides previously lacking details on the post-middle Carboniferous cooling history of the region and suggests how that history may be interpreted in terms of specific deformational events.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE PALAEOPLAIN MODEL DOES NOT WORK FOR THE SOUTHEAST AUSTRALIAN PASSIVE MARGIN: EVIDENCE FROM APATITE FISSION TRACK THERMOCHRONOLOGY Paul B. O'Sullivan^-^, Bany P. Kohn^'^, Andrew J.W. Gleadow^'^, and Roderick W. Brown^ ^Australian Geodynamics Cooperative Research Centre, Dept. of Earth Sciences, La Trobe University, Bundoora, Victoria 3083 ^Department of Earth Sciences, La Trobe University, Bundoora, Victoria 3083
The geomorphic evolution of high elevation passive continental margins has been the focus of numerous studies for over a century, and a wide range of landscape evolution models have been formulated in attempting to understand their development. One of these models proposes that a pre-existing land surface, or "palaeoplain", was downwarped towards the axis of the subsiding rift basin to form a continuous, long wavelength flexure or monocline. Present-day topography was then formed by erosion of the monoclinal flexure by a process of escarpment retreat. This concept of a downwarped palaeosurface at the time of Tasman Sea rifting has been suggested to explain the existence of proposed landforms observed along the southeast margin of Australia, including the initial formation of the Great Divide and the presence of coastal facets mapped along the coast. One method which can be used to test this model is Apatite Fission Track Thermochronology (AFTT). This technique is widely recognized as being usefiil in constraining the low-temperature (<~110°C) thermal history of rocks, and identif^ng a thermal stratigraphy through any apatite-bearing sequence. In many cases, AFTT is the only technique which is currently able to provide quantitative estimates of denudation over time scales of the order of 10^-10^ years. Previous AFTT studies of the rifted margins of southeast Australia, have suggested that -1.5-2 km of denudation occurred along the coast and as much as -2-4 km of denudation occurred within localized regions of the eastern highlands. Importantly, most of this denudation occurred in response to either the Hunter-Bowen Orogeny, or the middle Cretaceous rifting event associated with rifting and separation of Australia fi-om Antarctica and the Lord Howe Rise. In general, there is little direct evidence from the apatite fission track data from southeastern Australia for any ongoing large-scale Tertiary denudation. The Palaeoplain Model, as applied to southeast Australia, predicts that: 1) the Great Divide, which runs along the crest of the highlands belt, parallel to the southeast margin, was formed not by uplift, but rather by downwarping of the continental margin during rifting, 2) the Great Divide represents a remnant of the original palaeosurface, and therefore has experienced minimal erosion since the time of downwarping, 3) erosion associated with rifting has been concentrated in regions between the Great Escarpment and the coast, and 4) AFTT ages are prescribed to increase away from the Great Escarpment, particularly in regions where coastal facets have been mapped. A large AFTT database (n=~700 analyses) from southeastern Australia are clearly incompatible with this model: 1) they show clear evidence that localised km-scale denudation occurred within the Southeastern Highlands during the early stages of extension along the southeast passive margin, 2) they indicate that erosion was concentrated along both the eastern flank of the highlands and the coastal regions, but not in intervening regions, and 3) in all cases AFTT ages consistently decrease towards the coast from the crest of the Great Divide, even across proposed coastal facets near Bega and Port Macquarie, therefore clearly demonstrate that landforms along the coast are younger than landforms inland of the escarpment. The AFTT data from southeast Australia are clearly incompatible with the palaeoplain model. In particular the data from the Bega and Port Macquarie districts demonstrate unequivocally that landforms along this sector of the coastal plain are younger than landforms inland of the escarpment. In light of these findings we conclude that the concept of a downwarped palaeoplain at the time of rifting, does not adequately explain the Late Mesozoic and Cenozoic evolution of the southeast Australian passive margin or the landscape features now exposed along that margin.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
ECONOMIC SIGNIFICANCE OF QUARTZ IN ORDOVICIAN SEDIMENTS FROM THE MOLONG HIGH, LACHLAN FOLD BELT G. Packham\ J. Keened L. Barron^ ^Department of Geology and Geophysics, University of Sydney, NSW,2006 ^NSW Department of Mineral Resources, PO Box 536, St Leonards, NSW 2065
Quartz detritus has been found in the Ordovician volcanic arc of the Molong High distributed through rocks ranging in age from Bendigonian to Bolindian. Three volcanic episodes have buih up the High, the oldest represented by the Mitchell Formation south of Wellington, is undated but is overiain by the Bendigonian Hensleigh Siltstone. The siltstone is a predominantly fine-grained clastic unit containing allochthonous shallow water limestone blocks with sparse quartz grains. After a hiatus of about 20 Ma, a second volcanic episode lasted from late Darriwilian to Gisbomian time is recorded by the Fairbridge Volcanics. Two limestone horizons (the Wahringa and Yuranigh Limestone Members) occur within the volcanics. They are Darriwillian to early Gisbomian and Gisbomian respectively. Allochthonous limestone blocks have been found within the volcanic units. Where siliciclastic detritus is present in the carbonate horizons and the blocks, quartz is an important component. The second volcanic cycle can also be identified in the southern part of the Molong High by the Walli and Cargo volcanics. A clastic quartz-bearing succession to the east of Cliefden Caves around Junction Reefs mapped as part of the Weemalla Formation is also of Darriwillian to ?Gisbomian age. A following 14 Ma non-volcanic interval extended through the Eastonian to early Bolindian. Extensive carbonates accumulated, prograding west from Cliefden Caves and Molong over the volcanic core of the Cargo Volcanics. Siliciclastics with common quartz are found in the lower part of the carbonate wedge. Graptolitic shales and spiculites accumulated in deeper waters offshore to the east. The third volcanic phase lasted until the end the Ordovician. Vulcanism, predominantly volcaniclastic and conmionly turbiditic, contains only very minor amounts of quartz. Samples of detrital sands and silts from the Hensleigh Siltstone, Fairbridge Volcanics, Weemala Formation, Transmission Limestone Member, basal Regan's Creek Limestone and Malachis Hill Formation were examined for their quartz types and abundance. A wide variety of quartz types was found including volcanic, vein and strained and aggregates as well as radiolarian chert and possible fine metasediments. The distribution of vein and volcanic quartz are discussed here. The rarity of quartz in the Hensleigh Siltstone is in contrast to its abundance in sediments deposited during the second volcanic episode in the overlying limestone members in the Fairbridge Volcanics. The Wahringa Member is a bioclastic limestone, and basal lithic units contain angular vein quartz and volcanic quartz particles up to Inmi. Both micro and macro quartz is present. Volcanic rock fragments form the majority of the non-carbonate fraction followed in abundance by quartz and then feldspar. The volcanic rock fragments are both silicified feldspar porphyries (containing chaJcedonic quartz veins), trachylatites, and mafic volcanics. The overlying Yuranigh Limestone Member contains a similar abundance of vein quartz, volcanic quartz and similar volcanic rock fragments. The irregular shape of the vein quartz particles indicate their local origin. Samples from the Weemala Formation including one volcanogenic turbidite dated as DarriwiUian (Da3) also contain volcanic and vein quartz. Sediments deposited in the following non-volcanic episode are the richest in quartz detritus. Analysis of the Transmission Limestone Member from the lower part of the overlying carbonate wedge contains the most quartz. This rock is a sub-litharenite with 45% quartz (mostly vein with minor volcanic), and 35% volcanic rock fragments. Particles with vein quartz attached to the volcanic quartz indicate the source as local silicified igneous rocks. The contemporaneous basal unit of the Regans Creek Limestone contains lesser amounts of quartz, but of similar type, it also has clasts that had their origin as amygdales of chalcedony. The volcanic sandstones of the later Ordovician Malachis Hill and Millambri Formations contain rare quartz. Clearly the abundance of detrital igneous vein quartz in the Fairbridge Volcanics and overlying limestones reflects a proximal source and magmatic fractionation throughout the deposition of the Fairbridge Volcanics from Darriwillian to Gisbomian time. The greatest concentration of vein quartz detritus is however in the sediments that surround the Cargo Volcanics, deposited as the extinct volcanic pile was eroded to sea level. Thus palaeogeographic considerations endorse the Cargo Volcanics as the source. There the volcanics are intmded by a dacite porphyry complex and a later adamellite breccia. Quartz-pyrite-sericite reefs radiate out through the intrusive and country rock. Hydrothermal siUcification occurred at the time of intrusion and was followed by rapid erosion. The paucity of quartz in the Millambri and Malachis Hill Formation detrital phases of the third volcanic episode is in contrast to the Fairbridge and Weemalla samples we have studied, suggesting that the major phase of mineralisation occurred on the Molong High associated with the second phase of arc vulcanism in late Middle Ordovician to early Late Ordovician time. The presence of abundant volcanic quartz in the Fairbridge Volcanics 80km north of Cargo suggests there may be a more proximal source in that region.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
GEOLOGY OF THE ESCONDIDA PORPHYRY COPPER DEPOSIT, ANTOFAGASTA REGION, NORTHERN CHILE Ruben A. Padilla-Garza' 'BHP Minerals, 550 California Street, San Francisco, CA 94104
The Escondida porphyry copper deposit, in northern Chile, is currently one of the two largest copper producers in the world. This deposit was discovered m 1981 and the mine operations started in 1989. In 1986 and 1990 two studies used drill hole information to present the first geological description and geochronology of alteration of Escondida (Alpers, 1986; Ojeda, 1990). Since the publication of those works, new drill hole data and mining have further exposed the ore body. This abstract is part of a new study to integrate old and new information to broaden the geological knowledge of Escondida. The porphyry copper system of Escondida is associated with an Oligocene quartz monzonitic intrusion, hosted by Paleocene andesites. The Escondida stock was emplaced in faults of the Precordillera Fault System (PFS), which consists of a >700 km long by 20 to 50 km wide strike-slip fault system that contains the largest porphyry copper deposits of northern Chile. It is interpreted that the PFS was firstly developed during the Mesozoic as a series of north-south faults that broke the western margin of Gondwana. Fragments of the Paleozoic continent were dropped to the west, becoming the basement of the eastern edge of a Mesozoic back-arc basin. This set of faults was reactivated as transcurrent structures during the Cenozoic, forming the present Precordillera Fault System. The geometric patterns observed at regional and district scales indicate the association of intrusive rocks, mining districts, and rhombic structural geometries along the PFS. The Escondida district occurs in one of these rhombic structures and detailed structural mapping suggests that the 34 Ma intrusive stock of Escondida was emplaced in an extensional duplex. The complete evolution of the porphyry system is represented by four alteration-mineralization stages that include propylitic, potassic, sericite-chlorite-clay (SCC), and quartz-sericitic assemblages. Of these four, two alterations stages are mostly pervasive and reveal a consistent zoning propylitic halo surrounding a potassic core. In contrast, the other two alteration styles show strong structural control and are superimposed on the two earlier alteration stages. This overprinting is reflected in the primary copper grades that vary from < 0.2% to 0.4% in the potassic zone and from 0.4% to 0.6 % in areas overprinted by the SCC alteration. Pyrite was the only sulfide deposited with pure quartz-sericitic alteration; thus, the primary copper grade was not affected by this event. The porphyry system of Escondida was exhumed and exposed at the paleosurface relatively rapidly between 34 and 31 Ma. Two rhyolitic domes were intruded at 31 Ma cutting the porphyry system. Therefore, a rate of denudation of 700 m/my is estimated for the period 34 to 31 Ma. The dome shape of these rhyolitic intrusions is well preserved in the present pit of the Escondida mine, which suggests that the rates of erosion after the emplacement of these intrusive domes have been lower. A last hydrothermal event is represented by an advanced argillic system, which postdates the two rhyolitic domes. The distribution of this advanced argillic alteration-mineralization also exhibits strong structural control; where sulfides from this event are present the primary copper grades are typically 0.6% to higher than 1.0 %. Following the emplacement of the rhyolitic domes and the advanced argillic event, the rate of denudation decreased to less than 50 m/my, which was the rate of denudation during supergene enrichment (Alpers, 1986). The early, high rate of erosion and uplift were important factors in the formation of the hypogene part of the porphyry copper system and its characteristic telescoped alteration pattern; however, the continued decrease in the rate of denudation was also important in the formation of the supergene blanket and its preservation. It is observed that the best supergene copper grades overlap the best hypogene copper grades. Another important factor in forming and preserving the supergene blanket was the continual change in climate conditions that resulted in the hyperarid conditions of the Atacama desert (Alpers, 1986). In summary, the variations in climatic conditions, the dramatic changes in rates of denudation associated with uplift, the size of the hypogene porphyry copper system, and its characteristic alteration-mineralization overprinting were allfimdamentalfactors in the formation and preservation of the magnificent Escondida porphyry copper system.
348
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
LINKS BETWEEN EASTERN AND WESTERN FOLD BELTS IN THE MOUNT ISA INLIER, BASED ON SHRIMP U-Pb STUDIES R.W. Page Australian Geodynamics Cooperative Research Centre, AGSO, GPO Box 378, Canberra, A.C.T. 2601
Zircon U-Pb geochronology provides a framework for correlating between the Eastern and Western Fold Belts (EFB and WFB) of the Mount Isa Inlier. Research to constrain the timing and correlation of depositional packages from the EFB and WFB has been motivated in part by discoveries of large stratabound base-metal deposits, many of which are believed to be syngenetic or early diagenetic in origin. Determination of depositional ages contributes to unravelling the evolution of basin phases, and ultimately to the search for other prospective targets. These results have been acquired using SHRIMP U-Pb zircon techniques at RSES, ANU, as part of research programs in both AGCRC and AGSO's NABRE studies. The beginning of rift-related volcanism within both the EFB and WFB appears to young progressively from west to east: from 1790±9 Ma (Bottletree Formation / May Downs Gneiss), to 1781±3 Ma (central part Argylla Formation), to 1774 ±4 Ma (Argylla Formation in the Boomarra Horst), to 1762±3 Ma and 1761±4 Ma (Argylla Formation in the Bulonga and Duck Creek Anticlines, respectively). Major quartzite units above the Argylla Formation to the east (Mitakoodi Quartzite) and west (Ballara Quartzite) of the Pilgrim Fault have been dated utilising sparse, interbedded felsic volcanic units. On the western limb of the Duck Creek Anticline, the U-Pb SHRIMP age for the Mitakoodi Quartzite is 1756±3 Ma, and on the eastern limb is close to 1755 Ma. These ages, in combination with the Bulonga-Duck Creek Argylla ages, also effectively constrain the age of the underlying Manaba Volcanics to 1755-1760 Ma. The same age (1755±3 Ma) has been determined in Ballara Quartzite, 25 km west of the Pilgrim Fault, hence substantiating the stratigraphic equivalence of these two major quartzite packages. The Quilalar Formation, a quartzite-carbonate blanket in the WFB, has a maximum depositional age of 1780 Ma based on the youngest detrital zircon components. Paucity of felsic volcanic rocks in EFB sequences, together with the high grade of metamorphism (especially in the southeast) has meant that correlation of these sequences with WFB has been tentative until now. Using zircon from immature feldspathic sediments and rare felsic tuffs in EFB, the resultant new ages provide some clear stratigraphic links with WFB units. Zircons from Soldiers Cap Group metasediments, at both high and low metamorphic grades, yield maximum depositional ages of ca. 1712 Ma, 1693 Ma, 1677 Ma, and 1676 Ma. A metarhyolite dated at 1654±4 Ma, and a tuffaceous metasediment in the Toole Creek Volcanics dated at 1658±8 Ma, substantiate the relative youthfulness of the group. This supports a possible correlation with the Urquhart Shale (1652±7,1654±5,1656±4 Ma), Lady Loretta (1647±4 Ma), and/or Paradise Creek Formations (1653±7, 1659±3Ma)intheWFB. The SHRIMP geochronology demonstrates that some felsic volcanic rocks in the Tommy Creek Block (TCB) and tuffaceous sediments in the Marimo Slate package (both in the EFB) also belong to a relatively young basin phase. An igneous crystallisation age of 1626±4 Ma is supported by a maximum age of 1618±6 Ma for a brecciated tuffaceous sediment - reinforcing earlier interpretation that this TCB sequence is substantially younger than other known units in the EFB. However, the complexity of the TCB volcanic package is evident from older recrystallised volcanics that have an age of 1650±3 Ma (as well as very much older rocks that formed at 1762±5 Ma and 1758±4 Ma). These results mean that the TCB contains elements of both 1650 and 1620-1630 Ma packages - exactly as found in the Marimo Slate succession (below) - and 'Argylla-type' sequences of the same age as those in the core of the Bulonga Anticline. Two tuffaceous siltstones from the Marimo Slate, having ages of 1655±4 Ma and 161Q±5 Ma, further emphasise the importance of relatively young packages in the EFB. Sediments of equivalent age and possible correlatives in the WFB include the lower Lawn Hill Formation (1611±4, 1615±5 Ma) and lower Doomadgee Formation (1613±5,161915 Ma) in the McNamara and Fickling Groups, respectively. New depositional ages from the Answer Slate and Kuridala Formation provide additional support for the conclusion that major components of the EFB (Soldiers Cap Group, Mary Kathleen Group) are younger, not older, than 1670 Ma, and thus can be chronologically linked with specific packages of the Mount Isa and McNamara Groups in the WFB. Acknowledgement Published with s^proval of the Director, AGCRC and the Executive Director, AGSO.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
SAKMARIAN (TASTUBIAN) SMALLER FORAMINIFERA FAUNAS FROM THE CANNING & CARNARVON BASINS, WESTERN AUSTRALIA. Vincenzo Palmieri 5/63 McLay Street, Cooparoo, Queensland 4151
The marine transgressive Calytrix Formation of the Upper Grant Group (Canning Basin) contains foraminiferids which indicate, by comparison with corresponding faunas of the Boreal Realm, a probable Early Sakmarian stratigraphic age. The first occurrence of Tezaquina clivuli and Nodosinelloides wadeae sp.n. also represents the base of a foraminiferal biozone used in the contest of an Australia wide biostratigraphic scheme. A similar fauna was in fact found also in the Carandibby Formation (Carnarvon Basin). Associated foraminifera include genera and species occurring in Earlier Permian periglacial sediments of Tasmania and South Australia. Correlation with Early Permian foraminiferal faunas from New South Wales and Queensland have not as yet fully accomplished.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
VERA-NANCY GOLD DEPOSIT, PAJINGO Jennifer Paiks and Dave Hall Normandy Pajingo Pty Ltd, PO Box 1271, Chaiteis Towers, Queensland 4820
The Vera-Nancy epithennal vein gold-silver dqx)sit is located I50km south-southwest of Townsville, in the northern portion of the Devono-Caiboniferous Dnimmond Basin. The ore-body is one of a multitude of known gold mineralised veins from within the Pajingo q)ithennal system, whidi covers an area of 150km^. The Vera-Nancy resource at June 1997 was 1.41 million ounces of gold (3.14 mt @ 14g/t Au) The known economic dqx)sits are hosted in andesite-dacite in the lower part of the basin sequence, which at Pajingo con5)rises basal arenite overiain by andesitic to dacitic volcaniclastics, flows and intrusives overlain by arenite and siltstone. The Scott Lode mineralisation has been dated at 342 Ma. Mineralised structures within the q)ithennal system have east-west, northwest, west-northwest and northeast orientations (Porter R G, 1990). The Vera-Nancy gold mineralisation is stmcturaUy controlled within a northwest trending fault The mineralised quartz veins form a series of discrete steq)ly plunging ore shoots along the L4km strike of the ore body, which collectively strike northwest and d^ steeply to the southwest A number of discrete dilational settings have been recognised, whidi mostiy mdicate sinistral strike slip and some corr5)onent of dip slip movement on the northwest structure. However, there is also evidence for reverse movement on subtie northnorthwestflexuresalong this structure. The gold-bearing multiphase quartz veins are 1-lOm thick and are hosted by chalcedonic hydrothermal fault breccia, which ranges iq) to --lOm Mck and locally contains quartz fragments. At least three phases of brecciation have been recognised (Richards et al, 1998). A halo of silica-pyrite-illite-sericite (phyllic) alteration is developed for up to 50m from the host structure, becoming intense adjacent to the veining. The silica is chalcedonic and distinctfromthe later quartz veining. Argillic alteration is characterised by irregulariy developed kaolin alteration Late carbonate (calcite, dolomite-ankerite and siderite), clay (kaolin-dickite and alabaster) and chalcedony are evident as infill in the quartz veins and as cross-cutting veinlets. Adularia in the veins has been con5)letely altered to clay. A 0.10-5.0m wide foliated chloritic fault located between 0-15m below the veining marks a sharp contact to propylitically altered andesite in the footwall. The mineralisation at Vera-Nancy is essentially bhni with most of the ore 200-400m below surface. The possibility of additional mineralisation at some depth below the defined ore body has not yet been tested. Known mineralisation is gold-silver dominated There is sporadic weak anomalism in otiier elements within the alteration package, princ^ally arsenic, molybdenum, antimony, zinc, copper and lead but none of these are can be reliably used as pathfinders to mineralisatiorL However, the silicified fault structure, with minor quartz veins, stringers and quartz fragments evident locally, persists to the surface and detailed structural mapping can be used to target mineralisation. Geophysical methods are the most ^licable e?q)loration tool for areas imder cover. The demagnetised host fault structures are evident in aeromagnetic data, and resistivity methods can be used to detect the silicified structures under up to 50m of cover, and possibly deeper. REFERENCES Porter R. G. 1990. Pajingo Gold Deposits, in Geology of the Mneral Deposits ofAustralia and Papua New Guinea Monograph 14, ed F. E. Hughes, 1483-1487. (The Australian Institute of Mining and Metallurgy, Melbourne). Richards D. R., Elliott G. J. & Jones B. H. 1998. Vera North and Nancy Gold Deposits, Pajingo, in Geology of the Mneral Deposits of Australia and Papua New Guinea, Monograph 22 (The Australian Institute of Mining and Metallurgy, Melbourne) in press.
351
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
MULTIPLE HYBRIDIZATION OF MAFIC AND FELSIC MAGMAS : IMPLICATIONS FOR THE GEOCHEMISTRY AND ORIGIN OF SEQUENTIALLY EMPLACED STANTHORPE GRANITOID PLUTONS Maurice Passmore and Warwick J. Sivell Division of Earth Sciences, The University of New England, Armidale 2351
Field, petrographic and geochemical evidence indicate that compositional diversity among I-type granitoid suites of the Stanthorpe Granite Group (part of the Moonbi Supersuite), including the 245 Ma Undercliffe Falls Adamellite (UCFA), the Stanthorpe Adamellite (SA) and Ruby Creek Granite (RCG) (indistinguishable at 244238 Ma)), is due to repeated mixing and hybridization of mantle-derived mafic magma with various proportions of successively generated discrete crustal anatectic melts, rather than solely tofiractionalcrystallization or restite unmixing. Gabbroid melts intruded both early (pre-UCFA) and late (syn- to post-SA). A major and continuing role for interaction of anatectic melts and derivative rocks with gabbroic magmas throughout the evolution of the Stanthorpe Granite Group also satisfies requirements of volume relations and heat source. A continuous spectrum of mafic-felsic component interactions supporting the magma mixing model is recognized, including: (1) near-complete magma mixing and homogenization indicated by xenocryst-bearing quartz diorite homfels in the UCFA and SA; (2) injection of mafic magma into a still low-viscosity UCFA, which permitted extensive mechanical exchange and thus formation of mafic microgranular enclaves (MME). Quench textures at MME margins indicate injection of hot mafic melt into a cooler felsic magma. Large (12 cm long) K-feldspar megacrysts with numerous overgrowth textures located in MME indicate migration between coeval felsic and mafic melts. Scattering of enclaves throughout the UCFA was achieved by convection, as evidenced by conmion local alignment of both K-feldspar megacrysts and MME. An extensive dyke within the UCFA varies in composition from diorite, through quartz diorite, to hybridized granite along strike, with decreasing proportion of both mafic enclaves and large feldspar megacrysts; (3) injection of mafic magma into partially crystallized SA, represented by a large (1km diameter) gabbro body; The increasing viscosity contrast of the SA prevented formation of MME, however, two small enclaves of S A have been entrained into the gabbro melt. The gabbro body has locally caused melting of the SA to produce aplite. Xenocrysts in small mafic dykes surrounding the gabbro provide evidence for some mixing; (4) late injection of mafic magma into quite solid granitic rock, indicated by numerous xenocryst-rich mafic dykes in both UCFA and SA. Enclaves in dykes in various stages of assimilation, xenocrysts of rounded quartz in disequilibrium with melt, and feldspars with notable changes in growth rims, imply changes in composition of magmas. A high-K diorite dyke cuts the UCFA and also possesses coronal xenocrysts; (5) the evolved RCG has some A-type affinities and contains negligible enclaves, but continued involvement of a mafic magma is indicated by at least one dyke. Discrete chemical trends for the UCFA and evolved, but far more voluminous SA, indicate separate crustal anatectic melt components and imply interaction of these felsic melts with different proportions of mafic magmas. Generation of the UCFA, the most primitive of the Stanthorpe granitoid suites, involved hybridization between predominant mafic magma and a first-stage, relatively Or-rich (disequilibrium) anatectic melt plotting in the vicinity of the 5 kbarfluid-absentmelting minimum. UCFA compositions trend away from the Or apex of the Qz-Ab'Or diagram toward the Kf-Qz cotectic (consistent with sole crystallization of observed abundant Kfeldspar megacrysts). Mafic magmas (gabbro, diorite and quartz diorite) maintain their tholeiitic character (Feenrichment and Y/Nb>3) despite clear chemical evidence for hybridization (due to a relatively small crustal component). SA samples have lower Or components and plot near the 2 kbar granite minimum (along the Kf-Ab cotectic). More extensive crustal melting (at shallower level) is implied, SA compositions approximating to progressive batch crustal melts (i.e. greater crustal component), but closely related to more primitive mixed (dyke) magmas rangingfi-omdiorite to rhyolite with medium- high-K calc-alkaline signatures (low Fe, Y/Nb~2). Higher K/Ba and lower Ba/Rb and K/Rb are consistent with greater degree of partial melting and residual Kfeldspar rather than biotite in the source. Island arc tholeiite chemistry of the mafic magmas and their intrusion to the very shallow level of emplacement of the only just exposed roofs of the granitoid plutons (and remnants of their comagmatic ignimbritic equivalents preserved due to lack of uplift), imply both a subduction-related extensional regime and a heat source other than recovery of normal continental geotherms following Permian uplift and crustal thickening at least 10 Ma earlier. (However, westward thrusting of subduction complex sediments (and emplaced Carboniferous S-types) over deeper parts of the earlier subduction zone could have produced suitable (underplated arc-related K-rich dioritic) source rocks for the Permo-Triassic I-type magmas.) An extensive outboard island-arc probably lay to the east of the southern New England Fold Belt during the Permo-Triassic. Intimate relation between lAT and granitoids may reflect rapid ascent of mantle melts to shallow level in a trans-tensional regime (Demon Fault) in the region behind an accreting island arc. Crustal intraplating and underplating by continued input of hot mafic tholeiitic melts during incipient back-arc spreading could have produced the hybridized granites during terminal stages of subduction. 352
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14'^ Australian Geological Convention, Townsville, July 1998
PETROLEUM PROSPECTIVITY OF THE SOUTHERN LONDONDERRY HIGH REGION REVISITED Virginia L. Passmore' and Jane E. Blevin" ^Bureau of Resource Sciences, PO Box El 1, Kingston, ACT 2604 -Australian Geological Survey Organisation, GPO Box 378, Canberra City, ACT 2601
The discovery of oil in Jurassic sandstone reservoir rocks within the Vulcan Sub basin at Jabini in the mid 1980s, followed by subsequent Jurassic discoveries in the depressions and troughs in the Timor Seas such as the Sahul Syncline and on immediately adjacent highs, concentrated exploration activity within the Timor Sea depressions and their flanks. As a consequence, other areas in the Timor Sea region lacking thick Jiu^sic sediments and/or distant from the depressions and troughs were largely ignored with respect to petroleum exploration. Investigation of the Londonderry High and its adjacent shelves has revealed petroleum potential within the southern Londonderry High region. The Londonderry High which tilts to the north and east is an offshore structural high which lies north of the Kimberly Block on the continental shelf. The southern part of the high, which is in less than 200 m of water, is contiguous with the Yampi Shelf and Berkley Platform of the Browse Basin and Petrel Sub-basin, respectively. These three features form the inboard margins of the Browse Basin, Vulcan Sub-basin and Petrel Sub-basin. They are underlain by shallow basement and flanked the main rift basins during initial periods of extension in the middle Palaeozoic. Sediments ranging in age from Palaeozoic to Cainozoic overlie Proterozoic basement rocks. During the Palaeozoic and Triassic, most of the Londonderry High was a western extension of the Petrel Sub-basin. The Palaeozoic thickness of up to 4 seconds twt is preserved on the eastern side of the high. The overlying sediment thickness is less than 2 seconds twt. All units thin to the south. Several periods of tectonic activity have occurred on the high, creating conduits for migration of hydrocarbons into stratigraphically higher reservoirs and erosion of sediments. The WNW and NE trending basement grain of the Kimberley Block influenced the development of the Palaeozoic and Mesozoic extensional faults and reactivation structures of the high. Uplift and erosion in the Late Jurassic resulted in the removal from west to east of much of the Triassic and most of the Jurassic sediments on the Londonderry High, bringing the Palaeozoic and Triassic reservoir rocks to shallower depths. An overall transgressive cycle which began in the Latest Jurassic/Early Cretaceous produced overlap of eroded surfaces and onlap of basement rocks by progressively younger sediments, creating potential stratigraphic plays in the southern part of the high. Fluctuating sea levels in the Late Cretaceous and Tertiary resulted in repeated incision of the flanking margms and erosion on parts of the inboard areas. The mid-Miocene collision of the Australian and Timor plates caused a downward flexuring that resulted in transgression of Miocene sediments over the eroded flanking margins and extensional reactivation of many of faults in the Londonderry High region. Drilling indicates that the Palaeozoic and Mesozoic sequences in the southern Londonderry High region are marine and non-marine dominantly clastic sequences. Porous Permian limestones and Permian, Triassic, Jurassic and Cretaceous sandstones provide abundant potential reservoir, which are sealed by intraformational shales and regional seals. The overlying Tertiary and younger rocks are clastics and carbonates. Hydrocarbon shows in porous Permian and Mesozoic sediment on the southern Londonderry High and adjacent margins indicate the presence of oil and gas in this region. There are two active petroleum systems in the region: a Cretaceous-Jurassic system and an older Permian-Triassic systems, both of which are prospective. The Cretaceous-Jurassic system has Cretaceous sandstone reservoirs that are sourced by Jurassic and early Cretaceous claystones and sealed by the mudstones and claystones of the Bathurst Island Group, a regional seal for the Timor Sea. This is the petroleum system for most of the oil and gas discoveries in the region. The recent Gwydion and Cornea discoveries demonstrated the potential for long distance from the basins deeps onto the shallower adjacent margin. The recent identification of Permian and Triassic shales with source rock potential on the Londonderry High has shown that there is also a local older petroleum system with potential for oil and gas for this region. Late Permian, Triassic and Early Cretaceous and Jurassic reservoir rocks could be sourced from these older source rocks. Both the Triassic Mount Goodwin Formation and the Cretaceous Bathurst Island Group provide regional seals. Until recently the Permian petroleum system was confined to the present Petrel Sub-basin.
353
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 49 14'^ Australian Geological Convention, Townsville, July 1998
THE LITHOSPHERE BENEATH THE LAC DE GRAS AREA, SLAVE CRATON, CANADA: A XENOLITH STUDY Norman J. Pearson^. William L. Griffm^'^, Buddy J. Doyle^, Suzanne Y. O'Reilly^ Esm6 van Achterbergh^ and Kevin Kivi^ ^ GEMOC National Key Centre, School of Earth Sciences, Macquarie University, NSW 2109, Australia ^ CSIRO Exploration and Mining, PO Box 126, North Ryde, NSW 2113, Australia ^ Kennecott Canada Exploration Inc., 200 Granville St, Vancouver, BC V6C 1S4, Canada ^ Kennecott Canada Exploration Inc., 1300 Walsh St, Thunder Bay, Ontario PTE 4X4, Canada
The composition, structure and thermal state of the lithosphere beneath the Lac de Gras area in the Slave Craton have been determined from a suite of mantle-derived xenoliths. The xenolith studies form the basic element of 4D lithospheric mapping and are an essential complement to interpretation of the garnet xenocryst data (GrifFm et al., 1998). The xenoliths have been brought to the Earth's surface in several generations of kimberlites ranging in age from 47-75 Ma (Davis and Kjarsgaard, 1997). The majority of the xenoliths in this study come from kimberlite pipes DO-18, DO-27 and A154S and have been recovered during the crushing stage of processing of the kimberlite. Several lithological groups have been recognised in the xenolith sample population: Iherzolites (ol+opx+cpx+grt±crt); harzburgites (ol+opx+grt±crt); dunites (ol±grt±crt); wehrlites (ol+cpx+grt±crt); websterites (opx+grt±cpx±ol±crt); garnet clinopyroxenites (grt+cpx); eclogites (cpx+grt±rut±ky); granulites (plag+cpx+grt±opx). Garnet Iherzolites and harzburgites are fine to coarse grained (<lmm to >lcm), with microstructures ranging from equigranular, porhyroclastic to mylonitic. The absence of modal cpx is used to distinguish harzburgite from Iherzolite, but the majority of harzburgite garnet compositions are Iherzolitic (G9) and indicate coexistence with cpx. Subcalcic garnets are abundant in the garnet cocnentrate (Griffin et al., 1998) and Boyd and Canil (1997) have analysed sub-calcic garnets in harzburgite xenoliths from the Grizzly Pipe, north west of A154. The websterites and eclogites can be subdivided on the basis of mineral compositions: the websterites include a high-Cr group (grt Cr203 1.62-8.30 wt%) and a low-Cr group (grt Cr203 < 1.5 wt%); 2 types of eclogites are distinguished using the CaO content of garnet (CaO < 7 wt% and CaO 8.5-13 wt%). Modal variations and mineral compositions indicate that gradations exist between the eclogites with low-Ca garnet and the low-Cr websterites (opx-eclogites). Geothermobarometry on the peridotite and websterite xenoliths produces a P-T array with a non-uniform geothermal gradient. At T < 900°C the P-T estimates fall near a 35 mWm"^ conductive model geotherm, whereas at T between 900 and 1250 °C the locus of P-T points shifts toward a 40 mWm'^ geotherm. This 'stepped' geotherm is unlike the 'kinked' geotherm that characterises a number of cratonic xenolith suites (e.g., Lesotho). The offset in the xenolith paleogeotherm and gross compositional changes in the garnet concentrate (Griffin et al., 1998) defines two compositionally distinct layers, with a boundary at ~900®C. Comparison of the sheared high-T grt Iherzolites with those from the Kaapvaal and Siberian cratons indicates a number of similarities that imply metasomatism by asthenosphere-derived melts. However, the occurrence of undeformed high-T (1200-1250°C) xenoliths requires a minimum lithosphere thickness of > 200 km. Projection of T estimates for the eclogites to the geotherm places this group of xenoliths in the deeper layer. A bimodal distribution of T for the 2 types of eclogites provides evidence for stratification of the deep layer, with the low-CaO grt eclogites concentrated in the upper part and the high-CaO grt eclogites mainly occuring in the lower part. High olivine/orthopyroxene ratios combined with high average olivine Fo contents in the peridotites in the shallow layer confirms the ultra-depleted nature of the shallow layer predicted from the garnet concentrate data (Griffin et al., 1998). These same features distinguish the peridotite xenoliths from other Archean xenoliths and imply that the mantle beneath the Lac de Gras area differs from the lithosphere beneath other Archean cratons. REFERENCES Boyd, F.R. & Canil, D. 1997. Peridotite xenoliths from the Slave Craton, Northwest Territories. Abstracts Goldschmidt Conf., pp. 34-35. Davis, W.J. & Kjarsgaard, B.A. 1997. A Rb-Sr isochron age for a kimberlite from the recently discovered Lac de Gras field. Slave province, northwest Canada. Jour. Geol. 105, pp. 503-509. Griffin, W.L., Doyle, B.J., Ryan, C.J., Pearson, N.J., O'Reilly, S.Y., Davies, R., Kivi, K. and van Achterbergh, E., 1998. Lithosphere Structure and Mantle Terranes: Slave Craton, Canada. Jour. Petrol., subm.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
GRASBERG: 'A VERTICAL MILE OF ORE' Jay Pennington \ Eddy Suwardy^ and Imants Kavalieris^ 1. Grasberg Mine Department, Tembagapura, Irian Jaya 2. Consulting geologist, Jl. Pinang Emas 8/US28, Pondok Indah, Jakarta
The Grasberg ore body (using a 1% Cu equivalent cutoff), mined and insitu, extends vertically for more than 1600m (or one vertical mile), from 4250m elevation (original surface) to presently explored levels below 2650m, and varies in plan diameter from about 150m (pre-mined surface) to a maximum of 950m diameter at 3200m elevation. The current mineable resource (open pit and block cave) is 1.76 billion metric tonnes at 1.30% Cu, 1.20g/t Au and 4.31g/t Ag. The Cu-Au mineralization is centered on multi-stage dykes, of broadly quartz monzodiorite composition, which are divided into 3 intrusive stages. K/Ar dating indicates that these intrusions are 3Ma old. The intrusions are emplaced in the center of a breccia-filled volcanic pipe, about 1km in diameter, that has erupted through a thick sequence of folded carbonate strata. This volcanic structure flares near the surf^ace and has features compatible with a maar-diatreme vent. The Grasberg porphyry system developed below a trachyandesite volcanic dome overlying the breccia. The presently defined ore body forms an irregular column that tapers near surface. Average Au/Cu ratio's (Au in ppm, and Cu in %), increase from 0.9 near surface to 1.2 at 3195m elevation, and then decrease to 0.8 at 2655m elevation. In detail however, Au/Cu ratio's decrease outward at all levels. This zonation corresponds to decreasing chalcopyrite and bomite, and increasing covellite and digenite outward.
355
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
Sedimentology and Sequence Stratigraphy of the Surprise Creek Formation, Lower Mt Isa Group, Mt Isa Basin 1.
Brett Pidgeon^ Simon Lang^ Maitin Neuderdt^ Jan Domagala^ School of Natural Resource Sciences, Queensland University of Technology, Brisbane 2. Department of Mines and Energy, Queensland, Brisbane
The Proterozoic Surprise Creek Formation (SCF) and Warrina Park Quartzite (WPQ) comprise the basal stratigraphic units of ± e Mount Isa Group within the Mount Isa Basin. Sedimentation patterns within the Surprise Creek Formation are indicative of syn-depositional faulting during a rifting event. The Mount Isa Group is the final cycle of four rift-sag cycles that overlie basement rocks. The Surprise Creek Formation unconformably overlies the earlier rift-sag sediments of the Bigie Formation, Myally Subgroup and Eastern Creek Volcanics. The SCF, WPQ and the basal Moondarra Siltstone are subdivided into six units (SCF Unit 1, 2a, 2b, 3, WPQ Unit 4, 5, Moondarra Siltstone Unit 6), each characterised by grain size trends and stacking patterns. Unit 1 fines upward and contains aggradational to retrogradational stacking patterns. Gravelly and sandy lithofacies typical of a fan delta and associated gravelly braided fluvial system dominate this unit. Unit 2a fines upward and has a retrogradational stacking pattern that is evidentfi-omthe gamma-ray logs. Fine grained lithofacies are dominant. Unit 2b is similar to Unit 2a but has a progradational stacking pattern. Lithofacies within Units 2a and 2b are characteristic of lacustrine sedimentation. Sandy lithofacies dominate Unit 3 with gravelly, and fine grained lithofacies subordinate. Progradational stacking patterns are characteristic of this unit. Unit 4 of the WPQ contains sandy and fine grained lithofacies with a progradational stacking pattern, but is laterally restricted occurring only where Unit 5 thins. Unit 5 is more extensive and is dominated by sandy lithofacies in an aggradational stacking pattern characteristic of a braided fluvial system. Unit 6 fines upward with a retrogradational stacking pattern. Facies analysis and palaeocurrent directions suggest that fan delta systems developed along the footwall margins of a lacustrine basin during the deposition of the SCF. The stacking patterns were controlled by the amount of sediment supply relative to the accommodation space created. Proximal to distal variations in lithofacies assemblages within Unit 1 and 3 occur over short distances (1-2 km) reflecting changes in slope and depositional environment associated with the basin margin fan deltas. Stacking patterns show three facies tracts, forced progradational, progradational and retrogradational. Forced progradational facies tracts occurs at the base of the SCF, corresponding to the deposition of braided fluvial systems before the development of a lacustrine environment, resulting fi-om a greater s^iment supply to acconmiodation change ratio. The retrogradational facies tract is a fining upward sequence resuhing fi-om a greater rate of accommodation change relative to sediment supply, whereas the reverse is true for the progradational facies tract. Within the progradational facies tract, back stepping of the southwestern border fauh produced a tectonically derived transgressive-regressive cycle and resuhed in lower gradient fan deltas in the south Stacking patterns and bounding relationships of the WPQ indicate a fall of at least 3 to 4 m in relative base level, producing incision on higher gradient fan deltas in the north whereas no incision occurred on the lower gradient fan deltas in the south. The retrogradational facies tract of Unit 6 overlies the forced progradational facies tract of the WPQ marking the development of more regional subsidence. The basement highs that were the source areas of the fan dehas, partitioned the rift system in the southern Mount Isa Basin into lacustrine basins, whereas further north marine conditions prevailed. The dominant control on stacking patterns has been the rate of movement along the border faults. Superimposed on this has been drainage basin development, seasonal base level fluctuations, and higher frequency tectonic cycles. Acknowledgements: This project was completed under the auspices of the NABRE project, and we grateflilly acknowledge the financial and logistic support of AGSO, QDME, and MIM Exploration.
356
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
STROMATOLITES AND SEDIMENTARY-EXHALATIVE MINERALIZATION IN THE DEVONIAN OF THE CANNING BASIN, WESTERN AUSTRALIA P E Plavford^ and M W Wallace^ 'Geological Survey of Western Australia ^School of Earth Sciences, The University of Melbourne
Massive limestone buildups of columnar stromatolites intergrown with barite occur at several localities in the Devonian reef complexes of the Canning Basin, and are associated with zinc-lead mineralization in at least one of these localities. The stromatolite-barite association occurs in belts from a few hundred metres to three kilometres long and up to 300 metres wide. The stromatolites consist predominantly of inclusion-rich radiaxial-calcite crystals with some interspersed sediment layers. The calcite crystals are commonly arranged in fan-shaped aggregates which display unidirectional grov^. The well-preserved microstructure of the calcite indicates an original calcitic (rather than aragonitic) mineralogy. Barite associated with the stromatolites occurs as two forms; (a) as fibrous crystals filling fractures which crosscut the fibrous calcite, (b) as fan-shaped crystal masses interspersed with the stromatolitic fibrous calcite. Where barite occurs interspersed with fibrous calcite it similarly displays textures indicative of unidirectional grov^. The stromatolites and associated mineralization are believed to have formed during the early Frasnian as sedimentary-exhalative (SEDEX) deposits. They were localized (a) within anoxic shales of the Gogo Formation (basin facies), especially along abrupt contacts of that formation with the Sadler Limestone (marginal-slope facies), (b) beside sandstones and conglomerates that interfinger with the Gogo Formation, (c) beside contemporary faults that intersect the Gogo Formation, and (d) within shales at the base of the Virgin Hills Formation. Outcrops of the stromatolite-barite association are cut by gossans, up to several metres wide and tens of metres long, some of which contain anomalously high zmc values (up to 7,400 ppm). Stromatolite-like structures are developed within some of these gossans. Fragments of gossanous material and large blocks of stromatolitic limestone occur in conglomerate debris flows that extend into basinal deposits of the Gogo Formation. Only one of the stromatolite buildups has been drilled, on the north side of the Emanuel Range, where iron sulfides, galena, and sphalerite were intersected within the buildup, below the weathered zone. This mineralization is thought to have occurred during the early Frasnian, contemporaneous with grov^h of the reef complexes. It has not been proved whether the stromatolites were formed by microbes or inorganic precipitation, but a microbial origin is favoured. The stromatolites were probably generated by chemosynthetic bacteria, growing in mud just below the surface, under water depths of about 200 to 300 metres. A transitional relationship can be seen between limestones of the stromatolite-barite association and cannon-ball concretions ('Gogo nodules'), which are believed to have had a similar bacterial origin. The stromatolite-barite association formed beside or within permeable zones where water, methane, hydrogen sulfide, and mineralizing fluids were being expelled during early compaction of organic-rich shales of the Gogo Formation. Zinc, lead, and iron sulfide mineralization apparently occurred very soon afterwards, and was localized along fractures in the stromatolitic limestones. The gossans found in outcrop are the weathered remnants of this mineralization. Forthcoming research will focus on the detailed petrography and geochemistry of the stromatolite-barite-zinclead association and its relationship to Mississippi-Valley-type mineralization at Cadjebut, Pillara, and elsewhere in the Devonian reef complexes.
357
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
METHANE: ITS PLACE IN ARCHEAN LODE GOLD MINERALISATION? Paul A. Polito^. T. Kurt Kyser^, Yvonne Bone^ Jon D.A Clarke^ ^Department of Geology and Geophysics, University of Adelaide, S.A., 5005 ^Department of Geological Sciences, Queens' University, Kingston, Ont., Canada, K7L 3N6 ^WMC Resources, Exploration division, Melbourne, Vic., 3001
The Junction gold mine is an Archean mesothermal lode-gold deposit. It is hosted in a differentiated dolerite sill. Gold mineralisation formed post-peak metamorphism, and is confined to structures within the Junction Shear Zone (JSZ). Three vein generations are observed Zoning of carbonate species in the alteration halo of the JSZ, together with the abundance of veining associated with mineralisation is indicative of large fluid flow through the JSZ. Pre-gold mineralisation molybdenite-quartz ± pyrrhotite veins are present throughout the JSZ. Laser-Raman microanalysis on fluid inclusions indicates that molybdenite bearing quartz veins host fluid inclusions that contain 25-81% CO2 (ave. 52% CO2) and 19-75% CH4 (ave. 48% CH4). These veins are deformed by shear veins, extension veins and breccias that are spatially associated with the JSZ, hydrothermal alteration and gold mineralisation. Syn-gold mineralisation shear veins are filled with massive quartz and minor albite-chlorite ± dolomite/calcite. Extension veins are filled with quartz-albite-chlorite-pyrrhotite ± calcite/dolomite. Fluid Inclusion microthermometry and laser-Raman microanalysis (FI-LR) indicates that syn-mineralisation, pyrrhotite (±Au) bearing quartz veins host fluid inclusions containing 88-100% CO2 (ave. 98% CO2), 0-12% CH4 (ave. 2% CH4) and formed at approximately 350°C ± 50'C. There are two generations of post-mineralisation, SE dipping, quartz veins. These quartz-calcite-biotite-pyrrhotite veins are observed cutting through all rock units and the JSZ. FI-LR shows that the quartz in the steeper dipping vein-set hosts fluid inclusions containing 100% CO2. In contrast, the shallower-dipping vein-set quartz hosts fluid inclusions containing 37-87% CO2 (ave. 65% CO2) and 13-63% CH4 (ave. 35% CH4). Coupled Elemental Analysis and Mass Spectrometry (EA-MS) of syn-mineralised quartz veins indicates that up to 54% (ave. 30%) of the total gas species trapped in fluid inclusions at the time of quartz ± gold-sulphide mineralisation was CH4. This value is calculated using fractionation curves and equilibrium constraints applied to A^^C values obtained from carbonic fluids liberated from fluid inclusions and pore spaces of the host quartz veins. EA-MS on pre-mineralisation Mo-bearing quartz chips indicate that up to 85% of the total gas species present at the time of gas-fluid entrapment was CH4. EA-MS on post-mineralised Po-Cc-Bt bearing quartz veins indicate that CH4 accounts for up to 33% of the total gas species present at the time of gas-fluid entrapment. Carbon isotopes from syn-mineralised quartz - carbonate alteration have values of -4.8%o to -7.3%o PDB (ave. -6.5%o). Fractionation coefficients for co-existing calcite and CO2 at 350"C indicate that CO2 has a value 2%o heavier than calcite (therefore, CO2 has limits of -2.8%o to -5.3%o, ave. -4.5%o). Temperature estimates from oxygen isotopes obtained from co-existing quartz and calcite, support the fluid inclusion homogenisation temperatures, confirming that the (calcite) values are not the result of a later hydrothermal event. values from CO2 hosted by fluid inclusions in the quartz veins are higher than predicted (+1.6%o to 4.3%o, ave. -2.2%o) suggesting that another carbon species, possibly CH4^ is likely to be present. Although laser-Raman microanalysis indicates low, but variable CH4 contents, initial gas chromatography shows that CH4 volumes indicated by EA-MS are greater than actually present. CO2-CH4 steady state equilibrium is suspected. Mass balance equations incorporating all the known parameters show that the volumes of CH4 indicated by EAMS are possible. Large volumes of CH4 can be produced when a CO2 rich fluid reacts with the wall rock to produce other gangue minerals including sulphides and carbonates. The suggestion that large volumes of CH4 were present in the mineralised system at Junction has major implications for the controls governing gold precipitation in Archean lode systems. Acknowledgement. Special thanks to WMC Resources (Melbourne Exploration division and Kambalda, St. Ives Gold division) for their funding of PAP through a PhD scholarship.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
PRELIMINARY RESULTS OF LITHOSPHERE MAPPING OF THE EASTERN PART OF THE SIBERIAN PLATFORM Yvette H. Poudiom Diomani^ , William L. Griffin^'^, Lev Natapov^ Suzanne Y. O'Reilly^ and Yuriy Erinchek^ ^ GEMOC National Key Centre, School of Earth Sciences, Macquarie University, Sydney, NSW 2109 ^ CSIRO Exploration and Mining, PO Box 136, North Ryde, NSW 2113. ^VSEGEI, Sredny pr. 4, St Petersburg 199026, Russia
The Siberian craton consists of several major Archean terranes that have been mapped from surface geology on the Anabar shield and projected under the platform cover using magnetic data. A 1000-km long kimberlite field with age ranging from Paleozoic to Mesozoic runs in a NNE trend from the centre of the craton to its northern margin. Mantle material from these kimberlites has been analysed to construct mantle sections showing translithospheric discontinuities in the stratigraphy. On this traverse, the boundary between the Archean root and the Proterozoic mantle is marked by the abrupt disappearance of harzburgitic and wehrlitic rocks. In this study, we have first used gravity and topography data on the Siberian platform to determine the flexural rigidity or elastic plate thickness (Te), which is a measure of the mechanical strength of the lithosphere. The density models used in the geophysical modelling are constrained by rock stratigraphy derived from geochemical data. Secondly, we have processed and enhanced the available magnetic data and applied spectral analysis to estimate the depths to the magnetic source bodies and distribution of magnetisation. The gravity and magnetic results can then be integrated with petrological and geochemical data to produce a map of distribution of major lithospheric blocks related to variations in lithospheric mantle composition. The gravity and topo inversion results show that the lithosphere on the Siberian platform is relatively weak compared to values usually observed on cratonic areas. These low values of Te may be explained by the fact that the topographic relief, which is an important parameter in the inversion program, is very low and therefore there isn't enough power in the signal to resolve the range of elastic thickness in this area. Although these values of Te are low, it is evident that the Archean domains within the kimberlite corridor are stronger than the Proterozoic areas, suggesting a cold lithospheric root beneath the southern part of the kimberlite corridor. The depth to magnetic source bodies, which could be approximated to the depth to the Curie isotherm where the rocli lose their magnetisation, are of the same order of magnitude as the elastic thickness values in each lithospheric block. Comparison of Te and the depth to the magnetic bodies suggests that the two parameters may follow the same isotherm. Acknowledgments: This project is supported by a collaborative grant between Western Mining Corporation and Macquarie University.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
ASSEMBLY AND BREAK-UP OF RODINIA Christopher McA. Powell Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia, Nedlands, WA 6907
Final assembly of the Rodinia supercontinent took place during the late Mesoproterozoic along Grenvillian-aged (1300 to 1100 Ma) orogenic belts which can be traced from northeastern Laurentia through west Antarctica and tfience through India and Antarctica into the Albany, Fraser and Musgrave belts in Australia. The eastem margin of Australia in the Rodinia supercontinent lay along the Tasman Line, recently defined by AGSO aeromagnetic and gravity compilations. Published palaeomagnetic data indicate that the Rodinian configuration endured until 750 Ma, but that shortly thereafter Laurentia separatedfromthe Australia-Antarctica margin. In the Adelaidean Geosyncline, the time d the breakup is interpreted to coincide with the end of the Sturtian glaciation, after which a broad sag-phase transgression occurred. Before Rodinian breakup, the interior of Australia was covered by a broad epicontinental basin. Rifting and volcanic activity in northwesterly- and northerly-trending basins along what was to become the eastem margin of Neoproterozoic Australia occurred from --830 Ma until breakup. The NW-trending Gairdner dyke swam was emplaced during this early extension. During the mid-Neoproterozoic breakup the extension direction was oriented NE-SW, implying that the rectilinear margin of Precambrian comprises a series of NE-trending transform and NW-trending rift margins. The preservation of a wide rift margin in South Australia, which formed the base on which younger Adelaidean sediments were deposited, indicates that the South Australian segment of the Rodinian margin had lower-plate geometry. A second breakup, possibly of a smaller continental block, could have occurred in the 600 to 560 Ma interval, as recorded by mafic dykes in western NSW and Tasmania, and a broad zone of dextral transpression across central part of Australia. This continental block was not Laurentia, which by that time had moved to polar latitudes, in contrast with the tropical latitudes for most of Australia at that time. It could have been a small continental block similar to the many continental fragments that broke away from the northwestern margin of Australia over a protracted period from latest Precambrian to Jurassic. The suggestion by Veevers et aL (1997) that Laurentia lay in its Rodinian configuration with respect to Australia until 560 Ma is not supported by palaeomagnetic data, nor is it required by geological information. By 560 Ma, Laurentia attached to the Amazon craton lay on the other side of a collage of continental blocksfromwhich Gondwanaland assembled by the late Early Cambrian by the closure of the Brazilide-Pharusian and Mozambique oceans. Neoproterozoic breakup along the Tasman Line truncated the Mesoproterozoic metallogenic zones, which raises the question of where missing pieces are. An 800-km extension of the Mt Isa Belt is projected to have been lost along the northeastern South Australian rift margin, and a further 600 to 1000 km could have been lost along the rift segment south of Broken Hill. At least four possibilities for where these fragments are can be considered: the first is that the they are located in the conjugate margin of Laurentia, most likely to be in western Canada; the second possibility is that they are located in South China; the third is that they lie in a ribbon of thinned continental crust (c.f the present Lord Howe Rise) now embedded in a Phanerozoic fold belt, and the fourth is that the missing crustal fragment was subducted and recycled in a younger orogenic belt. If the conjugate margin lies in western Canada, the missing pieces could lie buried beneath Phanerozoic deposits of the Rocky Mountains. In South China, the conjugate margin could be concealed under the widespread younger Yangtze carbonate platform. If the metallogenic zones are part of a thinned continental ribbon, they could lie in the basement to the Delamerian Fold Belt, having first separated from Australia in the late Neoproterozoic and later having been brought back during the Late Cambrian contractional phase. If subducted and recycled, the metallogenic beh could be in one of many possible places. REFERENCE Veevers J.J., Walter M.R. & Scheibner E. 1997. Neoproterozoic tectonics of Australia-Antarctica and Laurentia and the 560 Ma birth of the Pacific Ocean reflect the 400 m.y. Pangean supercycle. Journal of Geology 105: 225-242.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
TECTONIC EVOLUTION OF THE PALEOPROTEROZOIC OPHTHALMIA FOLDAND-THRUSTBELT Christopher McA. Powell^ Zhengxiang Li' and David McB. Martin''^ 'Tectonics Special Research Centre, Department of Geology and Geophysics, The University of Western Australia, Nedlands, W.A. 6907 ^Present address: Geological Survey of Western Australia, 100 Plain St, East Perth, W.A. 6004.
The Ophthalmia Fold Belt, along the southern margin of the Hamersley Province in the Pilbara Craton, is part of the broader latitudinally-trending Proterozoic Capricorn Orogen, which separates the Archaean Pilbara and Yilgam cratons and preserves their amalgamation history. Our work over the past eight years has refined the structural and tectonic history of this orogen by recognising sets of superimposed deformations and relating them to the stratigraphic evolution of the basins. The oldest deformation is an extensively-developed NE-trending set of rotational boudins and rarer intraformational recumbent folds, best developed m the late Archaean Marra Mamba Iron Formation. They appear to have formed during the deposition of overlying Hamersley Group units, and are not strictly penecontemporaneous with the strata in which they are found. The implied NW-SE extension occurred during the latest Archaean and earliest Proterozoic (-2600 Ma to 2450 Ma). The second deformation is the E-trendmg set of folds, which are part of a fold-and-thrust belt that involved south-to-north transport of the Hamersley Province cover above the older Archaean (>2900 Ma) granitegreenstone basement. The folds are most intensely developed along the southem margin of the Hamersley Province, where they form overturned to locally recumbent folds verging to the north. In the southem margin of the Hamersley Province slivers of the granite-greenstone basement appear to have been incorporated in the foldand-thrust belt, where the detachment surfaces are interpreted to cut progressively deeper into the Earth's crust. This second deformation, which in places is associated with pervasively developed crenulation-style slaty cleavage, was active during deposition of the Turee Creek and lower Wyloo Groups, which formed in the McGrath Trou^, a foreland basin in front of the advancing fold-and-thrust belt. Folded unconformities, coarse terrestrial clastics derived from the south and truncated early E-trending folds are found along the southem margm of the McGrath Trough. Geological relationships combined with SHRIMP dating of zircons show that the fold-and-thrust belt was active between -2400 Ma and 2,200 Ma, considerably older than previously thought. An interval of extension followed the E-trending fold-and-thrust belt during which ESE- to SE-trending dolerite dykes were emplaced. Tne southem margin of the Pilbara Craton was rifled and the deep-water turbidites of the Ashburton Formation were deposited in a region that previously had been the uplifted source of terrestrial clastics in the McGrath Trough. The June Hill Volcanics, in the lower part of the Ashburton Formation, are related to this extension and have been dated at -<1843 Ma. The second major shortening deformation involves SE-trending folds and high-angle reverse faults that deform the Ashburton Formation but pre-date the Mesoproterozoic Bangemall Basin cover rocks. Best estimates of the age of this contractional deformation are between 1800 and 1650 Ma. Deformation in the westem Hamersley Province after formation of these SE-trending folds include ENE-trending folds and steep ESE-trending faults that formed in a dextral strike-slip regime. The age of these dextral faults and folds is not certain; they could be the late stages of the end-Paleoproterozoic deformation but they could also be considerably younger, possibly even Neoproterozoic. This revised structural and stratigraphic analysis shows that the structure of the Hamersley Provmce involves two major shortening events, one of which is early Paleoproterozoic (2.4 to 2.2 Ga) and the other end-Paleoproterozoic (1.8 to 1.65 Ga). The younger deformation could be related to the collision of the Pilbara and Yilgam Cratons, but the broader tectonic setting of the older fold belt is as yet undetermined. Acknowledgements: Research in past eight years in the Hamersley Province has been funded by BHP (Iron Ore), Hamersley Iron, Robe River Iron Associates and the University of Westem Australia, and has involved the work of 12 UWA BSc(Honours) students.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
GIANT HAMERSLEY IRON ORE BODIES FORMED BY OROGENIC FLUIDS IN AN EARLY PALEOPROTEROZOIC FOLD-AND-THRUST BELT Christopher McA. Powell'. Zhengxiang Li', David McB. Martin' and Nicholas H.S. Oliver^ ' Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia, Nedlands, WA. 6907 ^ Economic Geology Research Unit, School of Earth Sciences, James Cook University, Townsville, Queensland 4811
PALEOPROTEROZOIC OPHTHALMIA FOLD-AND-THRUST BELT Geological mapping and basin analysis over the past eight years has established that the McGrath Trough on the southern margin of the Hamersley Province was a foreland basin in front of the northwards-advancing Ophthalmia fold-and-thrust belt. The McGrath Trough rocks include the Boolgeeda Iron Formation at the base, followed by the Turee Creek Group, Beasley River Quartzite and Cheela Springs Basalt. East-trending folds, inclined to locally overturned to the north, multiple refolded unconformities, and sediments increasing in grain size upwards, comprise the foreland basin, which was yoked to the Ophthalmia fold-and-thrust belt. The age cf deformation is constrained to be younger than --2.45 Ga„ the age of the underlying Woongarra Rhyolite, and mostly older than - 2 2 Ga, the age of the Cheela Springs Basalt inferred from the SHRIMP-dated zircons recovered from an epiclastic unit high in the formation. Some east-trending folding continued after deposition of the Cheela Springs Basalt, which is erosionally truncated by the younger sediments of the Ashburton Basin. The Ashburton Basin has a minimum age of-1.843 Ga, and is deformed by around 1.7 Ga. Clasts bearing microplaty hematite grains are known from the Barrett-Lennard placer deposit at the base of the Ashburton Trough, and have recently been discovered in conglomerate at the base of the Beasley River Quartzite and several horizons above, demonstrating that ore-forming processes were operating during formation of the fold-and-thrust belt. Examination of the stratigraphic, structural and palaeomagnetic constraints on the genesis of the various ore bodies in the Hamersley Province, as well as the characteristics of fluids that were involved in the ore formation suggests that it is possible that both martite-goethite and microplaty hematite ore bodies could have formed during the same time interval as the fold-and-thrust belt. TECTONICALLY-DRIVEN FLUIDS MODEL FOR HAMERSLEY IRON ORES Previous models for the genesis of the Hamersley iron ores have included supergene enrichment followed by burial metamorphism (Morris, 1985) and enrichment by leaching of silica and/or precipitation of iron by tectonically and/or thermally driven fluids (Li et al., 1993). The evidence now available points towards involvement of oxidising fluids at temperatures above 100°C, in places over 200°C, during deformation. Individual ore bodies show evidence of having formed by precipitation either above or below fluid chemical interfaces before and after folding at particular localities (Oliver et al., this volume). The model we propose is that both martite-goethite and microplaty-hematite orebodies could have formed by a combination of orographically-driven groundwater possibly mixed with deeper-seated orogenic fluids being squeezed out of the orogenic pile ahead of the growing mountain belt. If correct, this model for the formation cf the giant Hamersley iron-ore bodies is an example of tectonically-driven lateral fluid flow proposed speculatively by Oliver (1986) as the possible origin of oil and gas fields and base-metal deposits in North America. The principle involved is that hot, variably oxidised fluids moving laterally come in contact with indigenous bodies of formation fluid of different composition, and, if the chemistry is right, form an ore body. Orographic fluids could also have formed orebodies during late-stage extension accompanying orogenic collapse. Whether stripping of silica from the BIF chert layers is the dominant process is a secondary question yet to be addressed. Various palaeomagnetic, isotope and fluid inclusion tests are in progress to refine the model, and we recognise that not all the giant Hamersley iron ore bodies may have formed in this way. The model overcomes some cf the major deficiencies in the existing models, and opens up new horizons for iron-ore prospectivity. REFERENCES Li Z.X., Powell, C.McA. & Bowman, R. 1993. Timing and genesis of Hamersley iron-ore deposits. Exploration Geophysics 24, 631-636. Morris R.C. 1985. Genesis of iron ore in banded iron-formation by supergene and supergene-metamorphic processes - a conceptual model. In: Wolf K.H. (ed.) Handbook of strata-bound and stratiform ore deposits Elsevier 13: 73-235. Oliver J. 1986. Fluids expelled tectonically from orogenic belts: Their role in hydrocarbon migration and other geologic phenomena. Geology 14, 99-102. Acknowledgements: We acknowledge the generous support for this work over many years by MERIWA, BHP Iron Ore, Robe River Mining Associates and Hamersley Iron Pty Ltd.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
LITHOSPHERE MAPPING IN THE SOUTHERN NEW ENGLAND REGION: PRELIMINARY RESULTS William J. Powell GEMOC National Key Centre, School of Earth Sciences, Macquarie University, Sydney, NSW 2109.
Lithosphere mapping is a technique that integrates geochemical, petrological and geophysical information to improve our understanding of the lower crust and upper mantle (O'Reilly & GrifiBn 1996). The geochemical and petrological information is provided by mantle xenoliths - actual samples of the rocks found at depth which are brought to the surfece by basalts. These are also used to provide petrophysical constraints for interpretation of existing and new geophysical data, which can then be used to extend xenolith information laterally. Xenolith geochemistry provides a record of processes in the upper mantle and lower crust such as melting and metasomatism - important parts of the geological history of the area. Geochemical data are also used for thermobarometric calculations, allowing samples to be placed in a depth sequence. Geophysical anomalies in gravity, magnetic and seismic tomography data have been reported for the southern New England region. The tectonic setting has been the subject of much debate over time, and several models for the evolution of the orogen have been proposed. Lithosphere mapping is used to provide new information about the flmdamental structure of the area. Basalts are widespread across the various terranes mapped at the surface of the southern New England region, and many xenolith-bearing localities are known, making it an ideal settmg for a study of this kind. Mantle xenolith suites from Allyn River, Lawler's Creek and Wallabadah Rocks have been investigated. Spinel Iherzolite is the most common xenolith type at each of the localities, with granulite and pyroxenite xenoliths subordinate. Megacrysts are also common at Lawler's Creek. Modal clmopyroxene in the Iherzolites ranges from 1.1-19.6% at Wallabadah, 1.6-14.4% at Allyn River and 2.0-12.0% at Lawler's Creek. The samples lie on the Phanerozoic trend on a Boyd diagram (olivine Mg# against modal olivine %), consistent with their location to the east of the Tasman Line. Preliminary whole-rock data from Wallabadah and Allyn River show simple straight-lie relationships between major elements (eg CaO, AI2O3 and Na20 against MgO), suggesting the Iherzolites are residues after extraction of partial melt. Similar relationships between major elements are shown by calculated wholerock compositions, based on point-counted modes and mineral compositions determined by EMP. Compositions of mantle minerals vary significantly between the localities, indicating chemical heterogeneity in the mantle beneath New England. Clinopyroxene chromium number (lOOxCr/Cr+Al) ranges from 4.6-19.9 at Wallabadah, 6.7-35.1 at Allyn River, and 5.8-17.7 at Lawler's Creek. Cr203 in spinel ranges from 7.0-32.6 wt% at Wallabadah, 11.9-52.9 wt% at Allyn River, and 8.0-30.5 wt% at Lawler's Creek. Clinop>Toxene Cr# and spinel CT2O3 content are commonly accepted indices of depletion of Iherzolites, suggesting higher levels of depletion at Allyn River relative to the other localities. Equilibration temperatures have been calculated for the xenoliths, but no pressures are calculated as garnet is absent from these suites. Garnet-bearing xenoliths from other localities in eastern New South Wales lie on the Southeastern Australian Geotherm of O'Reilly & Griffin (1985), so this is taken to be representative of the ambient geotherm under New England at the time of eruption. Equilibration pressures for the New England samples are then inferred from the geotherm, allowing their depth of origin to be estimated. At Wallabadah Rocks pyroxenite and granulites are dominant to a maximum depth of -30 km, below which spinel Iherzolites are dominant. On this basis the crustmantle boundaiy is placed at 30-35 km beneath Wallabadah Rocks. At Allyn River spinel Iherzolite equilibration temperatures are slightly higher, suggesting a thicker crust at this locality, if the samples represent the full subMOHO mantle section in the spinel Iherzolite zone. Trace element abundances in clinopyroxene, determined in-situ using laser ablation ICP-MS, are used to fingerprint processes in the upper mantle and lower crust. Rare earth element patterns for Wallabadah Rocks clinopyroxenes show flat HREE patterns, and LREE pattems ranging from enriched to depleted (La/SmN-chon <0.8 to 2.8). Allyn River samples have strong LREE enrichments (La/SmN<hon 0.8 to 16.8), with flat to moderately enriched middle and heavy REEs. Lawler's Creek clinopyroxenes have relatively flat pattems, varying from LREE depleted to enriched (La/SmN-chon 0.5 to 1.7), and HREE depletion in some samples. Differences in REE pattems indicate the mantle has undergone different styles and/or degrees of chemical modification at each of the localities. REFERENCES O'Reilly S. Y. & Griffin W. L. 1985. A xenolith-derived geotherm for southeastem Australia and its geophysical implications. Tectonophysics 111, 41 -63. O'Reilly S. Y. & Griffin W. L. 1996. 4-D Lithospheric Mapping: methodology and examples. Tectonophysics 262, 3-18.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
SYN-LATE TECTONIC METASOMATISM IN WESTERN A-LODE, BROKEN HILL Kvlie Prendergast^-^ Scott Stansfield^ and Patrick J. WiUiams^ ^School of Earth Sciences, James Cook University, Townsville, Queensland 4811 ^Now at North Limited, PO Box 1165, Milton, Queensland 4064 ^Pasminco Mining, PO Box 460, Broken HiU, NSW2880
It has commonly been suggested that fluid phase mobilization of ore components during retrograde metamorphism may have occurred at Broken Hill. However recent literature has tended to view this as less significant in orelocalization than mechanical processes influenced by deformation and ductility contrasts. New work has shown that the lode environment was affected by intense post-peak metamorphic metasomatism caused by infiltration of a high salinity aqueous fluid. This metasomatism appears to have had a significant impact on the distribution of ore. The paragenesis of the mineralogically-complex and Zn-rich Western A Lode orebody has been studied in 8 closely-spaced drill holes on mine section 44. A mineralogical record of peak metamorphism is provided by Mnpoor and Mg-rich cores of porphyroblastic garnets in altered quartz-biotite-K feldspar-sillimanite gneisses that envelope the lode rock association. The rims of these garnets along with finely dispersed matrix garnet and secondary garnets in silicified pegmatites have a quite distinct almandine-spessartine composition interpreted to reflect the onset of metasomatism. These altered gneisses locally display biotite depletion and garnet-enrichment associated with quartz veining. Elevated Mn-contents characterize garnets within individual compositional layers towards discordant veins that have selvages of the salmon pink spessartite that distinguishes the quartz and garnetdominated rocks that are the immediate hosts to the orebody. Bulk geochemical comparisons using the isocon approach show this inward transition towards the ore is characterized by enrichment in Fe(t), Mn and Mg, coupled to depletion of Ca, Na and K. Sheeted quartz-hedenbergite veins cut the metasomatic garnet-quartz rocks in the western part of the section and have selvages of comparatively Ca-rich garnet with interstitial hedenbergite ± bustamite. Rhodonite and pyroxmangite occur patchily in the eastern part of the section. The chemistry of coexisting pyroxenes and pyroxenoids and their exsolution textures are consistent with simultaneous crystallization of all these phases. Complex high salinity fluid inclusions occur in quartz, garnet and hedenbergite and are characterized by a distinctive daughter salt assemblage of halite, sylvite, pyrosmalite, a Pb-K chloride and a Mn-Fe phase which is probably a carbonate. A hydrous overprint commenced with cummingtonite followed by actinolite, pyrosmalite and chlorite. The latter are most closely associated with sulphides which constitute the youngest significant paragenetic association in Western A Lode. These minerals commonly display hydrothermal infill textures and metal-contents are generally highest near contacts between quartz-garnet rocks and pyroxene/pyroxenoid bodies. Chalcopyrite-cubanite and sphalerite are relatively concentrated in the core of the lode compared to galena which is enriched around the periphery and accompanied by a minor element association of Ag, Sb, Bi and Au. Pyrrhotite occurs throughout. The metal zoning suggests a temperature influence on the mineralization but is independent of the earlier zoned pyroxene-pyroxenoid assemblages. Marked positive Eu anomalies in the REE patterns of strongly mineralized samples suggest the fluid may have been quite reduced. A later lollingite-arsenopyrite-pyrrhotite association locally depleted the ore by replacement of galena and sphalerite. Abundant partial melting features in the Broken Hill gneisses imply that peak metamorphism was anhydrous and consequently that the fluid responsible for the metasomatism in Western A lode was externally-derived. A further implication is that some of the distinctive components of the Broken Hill lode rocks could have been introduced or enriched by this fluid phase. Several key minerals including various garnets, hedenbergite and pyroxenoids that have previously been attributed to peak metamorphism actually crystallized under sub-peak hydrothermal conditions. The Pb-rich fluid inclusions demonstrate that this metal was highly soluble and imply that extensive metal mobilization could have occurred during the metasomatism. Paragenetic associations are consistent with the possibility that hydrothermal precipitation was the dominant process of mineralization in Western A-lode.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE GEOLOGY OF THE BAJO DE LA ALUMBRERA PORPHYRY COPPER - GOLD DEPOSIT, ARGENTINA J. M. Proffettl D.C.Keough^ and P. J. ForrestaP ^Hunt Ware & Proffett, P. O Box 772066, Eagle River AK, 99577 USA ^Minera Alumbrera Ltd, Av Leandro N.Alem 619, Piso 1 y 2 (1001), Buenos Aires, Argentina ^MIM Holdings Limited, Floor, West Tower, 410 Ann Street, Brisbane QLD 4000, Australia
The Bajo de la Alumbrera porphyry copper gold deposit is situated in the Catamarca Province of Northwest Argentina. To date, a total Mineral Resource (In Situ) of 806 million tonnes at 0.53 percent copper and 0.64 grams per tonne gold has been delineated. The deposit is located within the mid to late Miocene Farallon Negro volcanic complex, situated in the Sierras Pampeanas, a region of basins and ranges controlled by gently to steeply dipping reverse faults on the eastern side of the Andes. The volcanic complex is interpreted as a fossil stratovolcano some 16 kilometres in diameter containing extrusive rocks which evolved from more mafic pyroxene andesites to more felsic hornblende and biotite bearing andesites and dacites. Intrusion of dacite porphyries, which host the copper-gold mineralisation, occurred late in the evolution of the volcanic complex. Small rhyodacite/rhyolite intrusions and domes represent the last phase of igneous activity. The present exposure level at Alumbrera was approximately 2.8 kilometres below the surface at the time of porphyry emplacement and mineralisation, with the copper - gold mineralisation being deposited at a lithostatic pressure range of 0.6 to 0.9 Kilobars. Intrusive rocks related to mineralisation at Alumbrera consist of a series of dacite porphyries intruded into the andesites of the Farallon Negro complex; mineralisation also extends well out into the andesite country rocks. A total of seven distinctive dacite porphyry intrusions have been recognised, earlier phases forming a complex multiphase stock. Later intrusions are in the form of dykes cutting the stock and extending out into the andesites in a broadly radial pattern. Individual porphyry units in some cases vary in texture and phenocryst content but are classified as separate intrusive phases mainly by recognition of igneous contacts and crosscutting relationships. Geochemically, the dacites are typical of subduction-related potassic igneous rocks from mature continental arc settings. Mineralisation at Alumbrera is associated with prominent hydrothermal alteration affecting both the dacite intrusions and the surrounding andesites, with alteration zones arranged broadly in a concentric pattern. A central zone of potassic alteration is surrounded by an outer halo of propylitic alteration. The most intense potassic alteration is closely related in time and space to two early mineralised porphyry phases. Intense quartz veining and pervasive alteration magnetite are characteristic of the strongest potassic alteration, associated with secondary potassic feldspar and secondary biotite. Younger porphyry intrusions show only weak to moderate potassic alteration and un-altered post mineralisation porphyr>' dykes cut the potassic and propylitic alteration zones. Feldspar destructive phyllic and argillic alteration overprints the potassic and propylitic zones and all the porphyry phases and is most intense in a zone between the central potassic core and the propylitic halo. Copper-gold mineralisation occurs in potassically altered dacites and andesites, highest grades being associated mainly with two early porphyry phases and with andesites in direct igneous contact with these intrusions. Younger porphyry phases are less intensely mineralised and the latest phases are essentially barren. The vast bulk of the copper metal is carried as primary chalcopyrite and much of the gold occurs as free metal grains predominantly in the 10-50 micron range. Evidence suggests that the gold was transported by the same fluids as the other metals and that this fluid was oxidised; however a simple, single-stage paragenetic model for coppergold mineralisation will not fit the range of relationships observed. The deposit has been subjected to significant faulting. Steeply dipping, west block down normal faults predominate in the central and eastern parts and a moderately dipping, north-east block down normal fault occurs in the south-west. All of these mine-scale faults are post-ore, displacing all the porphyry phases, the potassic alteration and copper-gold mineralisation. Most faults also displace phyllic alteration but in some cases they seem to have formed conduits for later feldspar destruction. Alumbrera has been described as one of the classic examples of the porphyry copper-gold deposit type. Recent work on the deposit has shown that while it demonstrates several of the features of the general models, it also has many characteristics which are not addressed by these models.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
THOLEIITES AND KOMATIITES AT DIGGERS ROCKS, WA A.C. Purvis' 'Pontifex and Associates, PO Box 91 Kent Town, SA 5071
The Diggers Rocks segment of the Forrestania-Southem Cross greenstone belt in Western Australia has a westdipping and facing sequence. This sequence contains two layers of tholeiitic basalt separated by a layer containing komatiites, pelitic to chemical sediments (including banded h-on formation and chert) as well as tholeiitic basalts. A large layered tholeiitic sill (with bronzitite, gabbro and granophyre) occurs on the western side of this segment of greenstone belt. The tholeiites are derived from a parent with 12-13% MgO, ~ 0.6% Ti02 and a flat mantle-normalised spidergram. This represents abut 30% melting of Archaean mantle with a residual spinel harzburgite. Initial fractionation of olivine was followed by a mixture of bronzitite and gabbro, then by gabbro ± orthopyroxene, as shown by modelling of fractionation assemblages, using outcropping cumulate rocks. The basalts are mostly olivine-orthopyroxene normative but there are some quartz-normative basalts, mostly in the western tholeiites, which seem to show olivine-clinopyroxene-(chromite) fractionation. A few example of siliceous (quartznormative) basalts and magnesian 'andesites' with 4-8% MgO have low CaO and FeO conents and low Ti/Zr ratios, also apparently low contents of Nb, V and Zn, suggesting a lithospheric origin. These may be transitional from normal tholeiite to high magnesium basalt. Most of the tholeiites are amphibolites (locally with clinopyroxene or cummingtonite) but in metasomatic shears may form a wide variety of assemblages, from biotite-plagioclase-gamet-staurolite schist to clinopyroxene-plagioclase-quartz-epidote-(gamet) schist. No contamination is involved in this process as the proportions of Al, Ti, Zr and Nb remain constant throughout. The alteration seems to have been premetamorphic and of several types: silicification, commonly accompanied by siderite ± sulphides ± magnetite, calcite to dolomite alteration and chlorite-clay alteration, also biotite alteration involving a constant K/Rb ratio in the altered rocks. Other altered rocks include metamorphosed granophyres in the western tholeiitic sill, with clinopyroxene-epidote-gamet or homblende-clinopyroxene assemblages and magnesian, partly silica-deficient altered basalts in shear zones (commonly with chlorite, anorthite and/or spinel). K-Ba metasomatism has affected a very few rocks. The komatiites include lens to sheet-like olivine adcumulates to orthocumulates as well as layered units and thin zoned flows. At the Diggers Rocks Mine there are at least three separate olivine-rich units with different Ni/S ratios, commonly separated by high Al-Ti horizons, while the layered unit at Purple Haze has olivine, olivinechromite and clinopyroxene ± olivine accumulative zones as well as upper and lower chilled margins. Peridotitic komatiites have olivine accumulative zones and basaltic komatiites also have clinopyroxeneaccumulative zones. The komatiites are all aluminium depleted types, all of which have high LaAT)^ ratios (also low Ga, V and Sc) due to removal of garnet before or during melting, which also results in positive P (and Zn) anomalies, these elements being generally not concentrated into garnet. Modelling suggests that primitive Archaean mantle can produce 28-40% komatiite with 18-24% MgO (anhydrous) if about 9-11% garnet is removed from the mantle before or during melting. This garnet is pyrope-rich but needs to have minor Ti, Cr and Ca in addition to Fe, Mg and Mn, and a high Al content. Calculations permit significant orthopyroxene [or a 'majorite' component in the garnet] only in the case of the chilled margin of the Purple Haze unit, which also seems to require some chrome-spinel fractionation. Metasomatic alteration to serpentine or chlorite occurred before and after peak metamorphism, with metamorphosed serpentine-rich lithologies having talc, anthophyllite and/or orthopyroxene as metamorphic minerals (no igneous orthopyroxene occurs). Calcium enriched komatiites have metamorphic clinopyroxene. Again the Al/Ti ratios remain uniform throughout as do Ti/Zr ratios where these have been determined with any precision (some of the rocks have < 2 ppm Zr). This suggests that significant contamination has not occurred as potential contaminants (banded iron formation, pelitic schists etc.) have different Al/Ti ratios to those of aluminium depleted komatiites.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
CRUSTAL TRANSECTS IN THE CENTRAL GREAT AUSTRALIAN BIGHT D.C. Ramsay, H.M.J. Staeg. PA. Symonds & L Borissova Australian Geological Survey Organisation, GPO Box 378, Canberra City, ACT, 2601
The southern margin of Australia is a divergent, passive, continental margin extending for some 4000 km from the Perth Basin and NMuraliste Plateau off southwest Australia to the South Tasman Rise in the east. The margin formed during the period of rifting that culminated in the separation of Australia and Antarctica in the Cretaceous. The basins of this rift system (the 'Southern Rift System') include,fromwest to east, the Bremer, Great Australian Bight (Eyre, Ceduna, and Recherche Sub-basins), Duntroon, Otway, and Sorrel Basins. The margin has long been considered a classic example of a rifted margin and has been used to illustrate the applicability of detachment models to continental m^gin development. Over the past few years the Australian Geological Survey Organisation's (AGSO's) Law of the Sea Project has been collecting new geophysical (deep-seismic, gravity, magnetic and bathymetric) data to support Australia's claim to extended seabed jurisdiction beyond the 200 n mile Australian Exclusive Economic Zone (AEEZ) under the United Nations Convention on Law of the Sea (UNCLOS). In late 1997, as part of this activity, AGSO acquired more than 3400 km of deep-seismic data (16 second record length) in the central Great Australian Bight (GAB). The survey had two main elements: eight lines, each 250-350 km in length, on a NNE-SSW azimuth across the lower continental slope and northernmost South Australian Abyssal Plain, straddling the continent-ocean boundary (COB); and two 600+km-long, NNW-SSE oriented transects over the ftill width of the margin from relatively unextended continental crust beneath the shelf in the northern GAB to the Late Cretaceous oceanic crust south of the margin. The transects image two different, but adjacent, provinces of the GAB - the Eyre province in the west and the Ceduna province in the east - each province having quite different extensional styles. These transects are the only data of this type available along the more than 2500 km of continental margin between the western Otway Basin and the central Naturaliste Plateau. This data set provides important new insights into the structural processes, crustal characteristics and extensional styles of the complex and controversial continental margin of the central GAB. In particular, when combined with the extensive refraction velocity data sets acquired during the 1970s, the two margin transects are expected to provide key information for understanding both the tectonic development of the southern Australian margin and the development of rifted continental margins generally. The data should also allow a major upgrading of proposals for drilling in the area by the Ocean Drilling Program. In the central GAB area reflection Moho occurs at about 11.5 s TWT, and generally shallows southwards towards a complex high that separates highly extended continental crust from faulted, slow-spreading oceanic crust. The western transect across the Eyre province illustrates the upper crustal extension of the 'perched' Eyre Sub-basin system with its well defined half graben and ?Late Jurassic-Early Cretaceous synrift section. To the south, much of the deepwater Recherche Sub-basin appears to be underlain by very thick, relatively undeformed section. This area has clearly been subjected to substantialftill-lithospherethinning. In this western province there is good correlation between the various crustal zones imaged on the transect and the satellite gravity data. From south to north these zones represent upper crustal thinning, full-lithosphere thinning, old slow-spreading oceanic crust, and younger 'normal' oceanic aust. The eastern transect across the Ceduna province illustrates a very different extensional style with significant upper crustal and deep thinning occurring beneath the Ceduna Terrace. In this province the direct equivalent of the Recherche Sub-basin zone appears to be missing, and the old slow-spreading oceanic crust zone is much narrower. Beneath the inner Ceduna Terrace about 4-5 s TWT (7-9 km) of largely Cretaceous section overlies the ?Neocomian surface; Tertiary sediments are very thin throughout much of the area. Many of the NNE-SSW oriented deep-seismic lines image a strong band of reflectors at about 8-9 s TWT which has been interpreted as a Neocomian decoUement surface. Faulted NE-dipping ?Aptian section overlies this decollement in the central area, as well as a complex basement high at the outer ends of the lines. Many of these lines cross the NW-SE trending 'Southwest Ceduna Accommodation Zone' (SCAZ) that separates the Eyre and Ceduna provinces. This feature is associated with complex thrust faulting and related deformation. 367
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
CENTRAL VICTORIAN GOLD DEPOSITS AND THE CRUSTAL CONTINUUM CONCEPT WRH Ramsay and M Grover Earth Sciences, School of Ecology and Environment Deakin University, Rusden Campus, Clayton Victoria 3168
Current debate on the mesothermal or orogenic group of gold deposits, recognises that they have occurred discontinuously for the Archaen to the Tertiary. Unifying features associated with this group of deposits include: 1) hosted by a deformed volcano - sedimentary prism, 2) epigenetic structurally controlled, 3) low-salinity fluids, 4) alteration and mineralisation showing minimal vertical zoning, 5) thought to be emplaced late in deforming accietionary prism. In contrast to most other hydrothermal mineral deposits (exhalative volcanogenic, MVT, porphyry, epithermal, volcanic hosted manto) mesothermal gold deposits are characterised by a depositional depth range from near surface to subcrustal depths of 20km, well below the 400°C brittle-ductile thermal transition zone. To account for such a range in depositional depths in the crust various workers have invoked the crustal continuum concept, and applied it, initially to the gold deposits of the Yilgam Craton, where exhumed depositional settings have exposed lode gold deposits ranging in crustal setting from subgreenschist (Wiluna) to granulite grade (Grif&ns Find). Evidence is emerging from younger terranes that mesothermal deposits may likewise exhibit gold emplacement over a considerable vertical extent in the crust. An example is the Palaeozoic gold fields of Central Victoria, which are characterised by a coherent group of epigenetic, structurally controlled, gold-quartz dominated vein deposits. Here a broad gradation in mineralizing style can be recognised with a general decrease in the depth of gold deposition coupled with an upward pseudo stratigraphical younging on progressing from west to east. To the west in the Stawell gold province in the Magdala Mine, mineralisation was emplaced both structurally and stratigraphically in the lowest portion of the exposed Lachlan Fold Belt. Here veining is represented by repeated deformation, shears, multiple foliation with regionally metamorphosed host rocks of greenschist fades. Within the Ballarat province host rocks were broadly subjected to upper prehnite-pumpellyite fades conditions and mineralisation sites were dominated by brittle-ductile features including crack seal, spurs, brecdas, and faultdominated locations. Ore fluids were of low salinity of the C-O-H type, with trapping temperatures in the west of 300± 50^C and in the east 250± 50°C. In the eastern portions of the Ballarat province, where the Ordovidan sediments are charaaerised by diagenic zeolite fades, and in the Melbourne province the Au-Sb-As deposits were deposited from C-O-H-N fluid system with N2 > 15% and homogenisation temperatures and salinities for aqueous and CO2 - bearing fluid inclusions in Sb-bearing veins range from 145-215X and 2.9-5.9 wt% NaCl equivalent and, 180-260°C and 1.9-5.5 wt% NaCl equivalent respectively. Broadly the mineralisation shows a west to east progression from shear-hosted, Sb-poor deposits, which may show a spatial relationship with Cambrian greenstones and sediments in the west to Sb-As-Au mineralisation hosted by Ordovidan and Siluro-Devonian sediments to the east. Initial data suggest that these more easterly deposits are likely to be a little younger and the ore fluids represent an evolved meteoric water dominated source thus suggesting an epithermal style of mineralisation.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14* Australian Geological Convention, Townsville, July 1998
A GEOCHEMICAL ADVENTURE ON EFATE ISLAND, VANUATU ARC, SOUTHWEST PACIFIC Alison M. Raos and Anthony J. Crawford
Centre for Ore Deposit Research, School of Earth Sciences. University of Tasniania, GPO Box 252-79, Hobait, Tasmania 7001 The Pacific plate boundary in the southwest Pacific region is marked by a continuous chain of Tertiary to Recent volcanic island arcs extending from Papua New Guinea through the Solomon Islands, Vanuatu, Fiji, Tonga, and the Kermadec Islands to New Zealand. Volcanism in the Vanuatu Arc is dominantly basaltic, however several highly explosive dacitic caldera collapse centres are known. The Efate Island Group forms part of the volcanically active Central Chain of islands and hosts the record of a large-scale explosive dacitic eruption in the Vanuatu Island Arc. The oldest deposits exposed on the Efate Island Group are those of the Plio-Pleistocene (1.6 Ma) Efate Pumice Formation (EPF). The EPF comprises two members, a lower sequence dominated by coarse pumice lapilli breccias, conformably overlain by an upper foram-bearing, laminated and cross-bedded shard-rich sand and silt package known as the Rentabau Tuffs. Pumice lapilli breccias of the lower unit are clast-supported thick to very thickly, tabular bedded breccias which vary from massive to stratified and cross-stratified. The breccia facies variably show gradational transitions into thinly bedded volcaniclastic sands. The breccias are fines-depleted and composed principally of ragged to subangular fibrous pumice clasts, subordinate perlitic obsidian and dense dacite-rhyodacite coherent clasts, with rare accidental limestone fragments. Interbeds of shard-rich medium to coarse sands, and fine silts commonly containing discontinuous pumice stringers, isolated scour and fill structures and angle of repose cross-bedding occur throughout the succession. Facies characteristics of the EPF pumice breccias and overlying volcaniclastic sands and silts are interpreted to be the product of syn-eruptive, water-supported, volcaniclastic sediment gravity flows. Preliminary glass shard chemistry of these deposits suggest that they resulted from a single, major cataclysmic dacitic eruption in the Vanuatu Arc. Major element compositions of glasses have SiOj in the narrow range of 64-68 wt%, Na20 at 2.06.8 wt% and K2O at 4.0-5.8 wt%, falling in the trachydacite field on a total alkalis versus silica diagram. EPF glasses have unusually high KjO contents in comparison with other Vanuatu Arc ashes and lavas, reflecting more K-rich parental basaltic magmas. Unconformably overlying the EPF and restricted to the north of Efate is the Pleistocene to Recent (~ 0.7 Ma) Basalt Volcanoes Formation (BVF). Deposits include subaqueous to subaerial basaltic lavas, subaerial volcanogenic breccias and airfall ash beds associated with basaltic stratocone development. On mainland Efate small composite cones are now deeply dissected but original constructional morphologies persist in the more recent offshore islands of Nguna, Pele and Emau. Lavas of the BVF are typically plagioclase+olivine+magnetite±clinopyroxene glomeroporphyritic and variably vesicular basalts. The most primitive basalts have 44 wt% SiOj at Mg # = 42, indicating more alkaline affinities for the Efate basalts compared with those in the main southern section of the Vanuatu Arc. In turn, this implies deeper magma generation in the Efate region for which there is some seismic support. The base of the EPF succession is not exposed and much of the original volume has been lost due to erosion. A conservative minimum volume estimate for the EPF volcaniclastics exposed on the Efate Island Group is approximately 200 km^. The initial results of glass geochemistry, coupled with a detailed sedimentological study of the Efate Pumice Formation and the volume of debris produced, suggest that the eruptive episode recorded in these deposits was of significant magnitude and much larger than any other eruptive event in the Vanuatu Arc. This eruptive episode must have been significantly powerful, yet no other record of this eruption has been described from elsewhere in the Vanuatu Arc.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
EARLY EVOLUTION OF THE PROTO-ANDEAN MARGIN OF SOUTH AMERICA r, Robert J. Panlchurst^ C6sar Casquet^ Edgardo G. BaIdo^ Julio Saavedra^ and Carmen Galindo^ ' Centro de Investigaciones Geol6gicas, Universidad Nacional de La Plata, Calle 1 N° 644, 1900 La Plata, Argentina. ^ British Antarctic Survey, NERC Isotope Geosciences Laboratory, Keyworth, U.K. ^ Departamento de Petrologia y Geoqui'mica, Universidad Complutense de Madrid, Spain. ^Departamento de Geologi'a, Universidad Nacional de Cbrdoba, Argentina. ^CSIC, Instituto de Agrobiologi'a y Recursos Naturales, Salamanca, Spain.
The crustal framework of the Southern Andes was formed during Early Paleozoic continental collisions. From detailed study of a 500 km transect in the Sierras Pampeanas, central-west Argentina, two pre-Silurian tectonomagmatic episodes are recognized, each culminating in separate micro-continental collisions against the proto-Andean margin of southwestern Gondwana (Pankhurst et al., in press; Rapela et al, in press). The earliest recognised event in the Eastern Cordillera and northwestern Sierras Pampeanas is deposition of a turbiditic passive margin sequence with trace fossils indicating a Vendian to Tommotian age (Puncoviscana Formation and metamorphic equivalents, Durand, 1996). The Pampean orogeny started in early Cambrian times with a short subduction phase, indicated by ca. 530 ± 3 Ma calc-alkaline granitoids (conventional U-Pb on abraded zircons), partially emplaced along the recently developed supercontinent passive margin. After the Pampean terrane collision, the margin was buried to granulite facies conditions (M2 event, 8.6 ± 0.8 kbar, 810 ± 50 ®C), closely followed by relaxation to pressures ca. 4 kbar (M3 event) and formation of widespread migmatites and highly peraluminous granites in the Eastern Sierras Pampeanas. According to concordant results from SHRIMP U-Pb data on migmatite monazite, Rb-Sr isochron and conventional U-Pb zu-con dating of the anatectic granites and associated cordieritites, these events were essentially synchronous at ca. 525 Ma. After brief quiescence during the late Cambrian, a second major episode, the Famatinian orogeny, started with subduction at ca. 490 Ma, forming a wide continental arc and ensialic back-arc basin to the west of the accreted Pampean terrane. Conventional and SHRIMP U-Pb dating of the main ho + bi granodiorite phase of the batholith yielded a combined age of 490 ± 5 Ma. Younger monzogranites gave Rb-Sr whole-rock ages of 470450 Ma, suggesting that intrusive activity lasted until the late Ordovician. Both the main Early Ordovician metaluminous sequence and the monzogranites have high initial 87Sr/86Sr (0.7075-0.7105) and low 8Ndt (-4.6 to -6.3) inherited from lower crust. Sm-Nd model ages of 1600-1700 Ma indicate that the underlying crust is identical to that beneath the Pampean mobile belt to the east, and therefore strongly suggesting the Famatinian arc was a continental one. The Famatinian subduction heralded the approach of eastern Laurentia to Gondwana, during which the Precordillera terrane separated from the southern Appalachian region, fmally colliding with Gondwana in Silurian-Devonian times. REFERENCES Durand, F.R., 1996. La transicion Precdmbrico-Cdmbrico en el sur de Sudamerica, in Baldis, B. and Acenolaza, F.G., eds.. Early Paleozoic Evolution in NW Gondwana: Universidad Nacional de Tucum^, Serie Correlacion Geologica, No. 12, 195-205. Pankhurst, R.J., Rapela, C.W., Saavedra, J., Baldo, E., Dahlquist, J., Pascua, I. and Fanning, C.M., in press. The Famatinian magmatic arc in the central Sierras Pampeanas, in Pankhurst, R.J and Rapela, C.W., eds.. The Proto-Andean Margin of South America: Special Publication of the Geological Society, London. Rapela, C.W., Pankhurst, R.J., Casquet. C., Baldo, E., Saavedra, J., Galindo, E.,and Fanning, C.M., in press. The Pampean orogeny of the southern proto-Andes: evidence for Cambrian continental collision in the Sierras de Cordoba, in Pankhurst, R.J and Rapela, C.W., eds.. The Proto-Andean Margin of South America: Special Publication of the Geological Society, London..
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Comention, Townsville, July 1998
A PROPOSED GIS PACKAGE FOR TEACHING INTEGRATION OF GEOLOGY WITH IMAGE INTERPRETATION OF REMOTE SENSED DATA - BASED ON AN AREA IN THE ARCHAEAN PILBARA CRATON. M. Ratajkoski, R.S. Blewett & P. Wellman Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601 rblewett@agso.gov.au
The "geo-employment" scene, especially the exploration industry, now demands that geoscientists be computer and digital data literate. The use of digital data in GIS and sophisticated image processing packages are standard tools of the industry. The problem for teaching institutions is the shortage and lack of a variety of good datasets suitable for study. This poster illustrates a proposed teaching GIS package of geophysics and geology of an outcrop area 100 km x 80 km. The data are of classic Archaean geology with greenstones (mafic and felsic volcanics and intrusives) enveloping the Corunna Downs and Mount Edgar Batholiths (granitoid complexes) in the northeast Pilbara of Western Australia. These data are a subset of a craton-scale GIS developed by the North Pilbara National Geoscience Mapping Accord Project (joint AGSO GSWA initiative 1995-2000). The exposures in the northeast Pilbara are excellent and the datasets reveal intriguing aspects of the geology not usually apparent in most Archaean terranes around the world where regolith or cover masks the 'solid' or basement geology. The datasets thus allow for meaningful comparisons between outcrop, the typically shallowlooking Landsat-5-TM and y-ray spectrometry, and the deeper-looking magnetics and gravity. These data will be useful for GIS experimentation and analysis and customised image processing and interpretation purposes. In addition to learning about processing, interpreting and analysing the data, this package is intended to assist the student in the concept of rationalising the "conflicts" between the various datasets and that there are many possible 'solutions' to the geology. As such, these real geological mapping problems revealed by combining the excellent exposure with the range of datasets on offer make it interesting to speculate on the rigour of maps drawn solely from a single dataset (e.g., magnetic data under covered areas!). We believe the package will be especially useful to those working in more poorly exposed Archaean granitegreenstone terranes (e.g., Yilgam) where the geological complexities revealed in the Pilbara are less obvious. The datasets include raster images (band-interleaved or ERMapper files), contour maps. Arc View point and polygon themes. The grids of the magnetic and y-ray datasets are both real and byte data. The package includes: LANDSAT-5-TM image (using the following PC2(4/3;5/7):5/4:l+7 formula) RGB y-ray image map (line spacing 400 m) with the following derivative images Chemical map of principal rock type granite, sediment, mafic rocks Principal Components maps (9 variations) Hue-Saturation-Intensity grid of real data Magnetic Images (line spacing 400 m) with: 1st Vertical Derivative Reduced to Pole Total Magnetic Intensity grid of real data Gravity images and contour maps Digital Elevation Model (9 sec grid) Mineral deposits Whole-rock geochemistry Geochronology Geology (1:250 000 published 2nd edition) Geology shifted (to an accurate base) Solid geology interpretation based mainly on magnetic and gravity data It is planned to release the datasets in late 1998 on a CD accompanied by an AGSO Record with a colour print of each dataset and explanation on how it was generated. The Record will also include GIS exercises.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14'^ Australian Geological Convention, Townsville, July 1998
CRUSTAL STRUCTURE BENEATH THE NORTH AND EAST COASTS OF TASMANIA FROM SEISMIC REFRACTION DATA N. Rawlinson' l T O. Semenova' ^ C.D.N. Collins' " & G.A. Houseman^ 'Australian Geodynamics Cooperative Research Centre ^Department of Earth Sciences, Monash University, Clayton VIC 3168 ^Australian Geological Survey Organisation, Symonston ACT 2609
In 1995, AGSO's research vessel Rig Seismic performed a circumnavigation of Tasmania to acquire seismic data for the TASGO project. Approximately 36,000 shots were fired from its air guns, and the seismic energy from these shots were recorded by a network of 6 digital observatory stations and 38 portable analogue recorders distributed throughout the island. Marine deep reflection profiles, which are not considered in this study, were also recorded as part of the survey. The 3-D refraction and wide-angle reflection dataset provides us with an opportunity to analyse the deep crustal structure of Tasmania much more comprehensively than has previously been possible. Below, we present the results of some 1-D refraction interpretations and 2-D forward modelling along the north and east coasts. The 1-D refraction interpretations assume that the first arrivals are head waves which travel along horizontal planar interfaces separating constant velocity layers. The models that are produced have layer thicknesses and velocities that best satisfy, in a least-squares sense, the observed traveltimes. Consequently, each model represents the vertically averaged seismic structure of the region spanned by the data used to constrain it. The following description of Tasmania's crustal structure is based on these interpretations. Along the north coast, from just south of Three Hummock Island to Forth River, reversed profiles show the crustal thickness to be relatively uniform at 30-32 km with an average Pi velocity of 6.0 km/s and a velocity (the velocity of the mantle beneath the Moho) of about 7.9 km/s. Two crustal layers that are separated by an interface at approximately 8 km depth are discemable from the data; the upper crustal layer has a p-wave velocity of 5.6 km/s while the lower crustal layer p-wave velocity is approximately 6.3 km/s. In addition, a 1-3 km thick layer with a p-wave velocity of about 4.5 km/s overlies the two main crustal layers and probably represents the clastic carbonate sequence known as the Rocky Cape Group. Further east, across the Tamar Fracture System, the crustal thickness decreases to 27 km and remains at this value as far as Banks Strait. Down the east coast, from just north of Binalong Bay to the Freycinet Peninsula, reversed profiles show that the crustal thickness is relatively uniform at 25 km with an average P, velocity of about 6.0 km/s and a Pn velocity of 7.8 km/s. Continuing south to the Tasman Peninsula, the crust thickens slightly to 26 km and the Pn velocity increases to 8.0 km/s. Two distinct crustal layers can be identified beneath the east coast, and are similar in character to the crustal layers of the western north coast, although there is no evidence in the east of a significant overlying low velocity layer. Only a limited amount of 2-D forward modelling can be accomplished with the TASGO refraction data because few shot lines exist that have more than one receiver lying approximately in-plane. A 2-D crustal model of the western north coast of Tasmania has been constructed using both refraction and wide-angle reflection traveltimes from a shot line that runs between Three Hummock Island and Forth River to a receiver located near each end of the line. The model is parameterized in terms of uniform cubic B-spline interfaces in parametric form and layer velocities that are allowed to vary linearly with depth so that ray paths consist of circular arc segments. The advantage of parametric form is that both the x and z coordinates are frinctions of the same independent variable, so complex features such as recumbent folds may be represented. The most prominent lateral feature of the model is an upward deflection of both the Moho and the mid-crustal interface in the vicinity of the Arthur Lineament. The model is supported by free-air gravity data, in which the Arthur Lineament is expressed as a gravity high. Refraction interpretations along the north and east coasts of Tasmania suggest a relatively thin continental crust, with the thickest regions occurring along the western north coast and the thinnest regions occurring down the east coast. The 2-D forward model of the crust beneath the western north coast represents the Arthur Lineament as an upward deflection of the lower crustal layer. These results appear to be in broad agreement with the results of previous seismic and gravity interpretations. Our long term goal with this very large dataset is to invert traveltimes for 3-D crustal structure and velocity to produce a high resolution tomographic image of the whole island to at least Moho depth. Such an image will undoubtedly enhance our current understanding of Tasmania's deep geology and tectonic evolution. Acknowledgements Published with the permission of the Director of the AGCRC and the Executive Director of AGSO.
372
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
FLUID FLOW IN THE MT WELDON SHEAR ZONE, CENTRAL AUSTRALIA: IMPLICATIONS FOR FLUID RECYCLING IN THE MIDDLE CRUST. Caroline M. Read and Ian Cartwright Department of Earth Sciences and VIEPS, Monash University, Clayton, VIC, 3168, Australia.
Shear zones represent sites of intense fluid-rock interaction and can therefore yield important information about fluid flow during exhumation of metamorphic terrains. The Anmatjira and Reynolds Ranges contain high-grade Proterozoic metamorphic rocks that have been substantially rehydrated within discrete shear zones. As in many metamorphic terrains, the source of fluids required to rehydrate large volumes of gneisses is uncertain. The Mt Weldon shear zone in the southeastern Anmatjira Ranges, central Australia, forms the lithological boundary between granulite facies para- and orthogneiss. Shearing within the ca. 250 m wide zone forms heterogenous anastomosing shears with metre scale unsheared blocks. Steeply NE-dipping mineral lineations are present in quartz veins consistent with the NE dipping dominant foliation throughout the orthogneiss and paragneissic layers. The zone of intense schistosity is oriented northwest-southeast, with the Weldon Metamorphic Group rocks thrust up against the Mt Airy Orthogneiss. The bulk of the sheared rocks comprises Mt Airy Orthogneiss containing blocks of the Lander Rock Beds (metapelite). The metapelites of the Weldon Metamorphic Group comprise garnet, cordierite, sillimanite, biotite, K-feldspar, plagioclase and quartz, and are retrogressed to biotite-muscovite schists within the shear zone. The high grade mineral assemblage of the orthogneiss comprises quartz, K-feldspar, plagioclase and biotite, with the sheared orthogneiss containing quartz, epidote, chlorite, biotite and muscovite. The rocks within the shear zone are retrogressed to amphibolite - greenschist facies. Preliminary FT estimates of the metapelites from the Weldon Metamorphics Group indicate granulite facies metamorphism (TOO'C and 5 kbar). This is consistent with estimates from Collins and Vernon (1991) of 750°C and 5.5 kbar within the "Weldon Terrane". The limited number of phases makes estimating FT within the shear zone difficult. An increase in fluid-bearing phases in the sheared equivalents of both granulite hosts requires fluid infiltration during deformation and retrograde metamorphism. For both sheared and unsheared rocks values range from 9.7-11.7%o for the Weldon Metamorphic Group paragneiss and 7.3-8.6%o for the Mt Airy Orthogneiss. Fault rocks within the Orthogneiss at the SW edge of the lithological contact locally have values as low as -1.7%o. Rocks that equilibrated with normal crustal fluids typically have values of more than 5%c (Sheppard, 1986). Closed system isotopic resetting by devolatilisation reactions and partial melting typically lowers the S'^O values by less than 2%c (Kohn, 1993). This resetting therefore indicates an open system where external fluids of surface origin have infiltrated the crustal rocks. The meteoric fluid flow is highly focused within the shear zones. In order to determine the timing of the infiltration of surface fluids into the granulite rocks dating of the shear zones is required. Granites within the Reynolds Range, to the south of the Anmatjira Range also contain evidence of meteoric fluid flow through discrete shear zones (Cartwright et al 1997). "^Ar-^^Ar ages of sheared granites are 332-335 Ma, indicating shearing during the Alice Springs Orogeny (Cartwright et al. 1997). It is likely that these shear zones formed at the same time as the Mt Weldon shear zone. The existence of meteoric fluids within shear zones in central Australia demonstrates surface fluids can penetrate ductile crust. Therefore shear zones can act as conduits for the transportation of meteoric fluids into the ductile crust which can lead to the retrogression of high-grade metamorphic terrains. REFERENCES Cartwright, I., Buick, I. S. and Lambert, D. D. 1997. Infiltration of meteoric water through Alice Springs age shear zones in the Reynolds Range, central Australia: Implications for tectonics. In: Geofluids II (eds) Hendry, J., Carey, F. Farnell, A. Ruffell, A. and Worden, R. Antony Rowe Ltd, Chippenham, UK, 77-80. Collins, W. J. and Vernon, R. H. 1991. Orogeny associated with anticlockwise F-T-t paths: evidence from lowF, high-T metamorphic terranes in the Arunta inlier, central Australia. Geology 19:835-838. Kohn, M. J. 1993. Modelling of prograde mineral 5 O changes in metamorphic systems. Contributions to Mineralogy and Petrology 113:249-261. Sheppard, S. M. F. 1986. Characterisation and isotopic variations in natural waters. In: Valley, J. W., Taylor, H. F. T. and O'Neil, J. R. (eds) Stable isotopes in high temperature geological processes. Mineralogical Society of America, Reviews in Mineralogy. 16:165-184.
373
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
GEOCHRONOLOGY AND PETROGENESIS OF INTRAPLATE LAVAS IN NORTHEASTERN QUEENSLAND Marcel Regelous^. Kenneth D. Collerson^ Jon Stephenson^ ^Department of Earth Sciences, University of Queensland, Brisbane, 4072 ^Department of Earth Sciences, James Cook University, Townsville, 4811
In northeastern Queensland, the intraplate lavas of the Atherton, McBride, Chudleigh, Sturgeon and Nulla volcanic provinces cover an area of -20,000 km^. The oldest lavas are ~7-8 Ma; most of the lavas were erupted after Ma, and the youngest flows in each province are likely to be younger than about 100 ka (Griffin and McDougall, 1975; Stephenson, 1989). In the McBride, Sturgeon, Chudleigh and Nulla provinces, magmas were erupted to form broad lava plains and numerous cinder cones. Magmatism in the Atherton province was more explosive, with maars present in addition to lava shields and cinder cones. Lavas of the McBride province were erupted through metamorphic and granitic rocks of the Proterozoic Georgetown Inlier, whereas lavas from the other provinces overlie Palaeozoic volcanic, plutonic and sedimentary rocks. We have selected a suite of lavas of various ages from each province, for a detailed trace element (ICPMS) and Sr-NdPb-Th isotope study. The objectives of this work are to investigate the petrogenesis of the lavas, and to date some of the younger flows using U-Th disequilibrium. Chemical changes with time, recorded in the lava stratigraphy, may give insights into the relationship between magmatism and tectonics. Pb isotope studies of eastern Australian lavas can potentially reveal the location of the boundary between 'Indian'- and 'Pacific'-type upper mantle beneath the Australian continent. Continental intraplate lavas are also important probes of the structure and composition of the subcontinental mantle lithosphere, and previous trace element and isotope studies of eastern Australian lavas have given insights into the influence of subduction and mantle plume activity on the evolution of the continental lithosphere in this region (O'Reilly & Zhang, 1995; Zhang et al., 1997). The samples we have analysed contain between 5 and 11% MgO; the most common rocktypes are nephelinenormative basalts, hawaiites and basanites. The lavas are enriched in the incompatible elements relative to mid-ocean ridge basalts, and have trace element similarities to many ocean island basalts, with a peak at Nb-Ta on chondritenormalised spidergrams (e.g. Zr/Nb = 3-5). All samples have Ce/Pb and NbAJ within the range of oceanic basalts (e.g. Ce/Pb >11), suggesting that subduction-modified mantle is probably a minor component of these lavas. ^^Sr/S^Sr and 143^(1/144^(1 are 0.7035-0.7050 and 0.51275-0.51300 respectively. The basalts have higher 208p5/204pb and 207pb/204p5 f^^ a given 206p5/204pb than Indian MORB, and lie within the field defined by 'DUPAL' lavas. There is no systematic relationship between lava chemistry and eruption style, suggesting that the geometry of the magma plumbing system at crustal levels does not significantly influence lava chemistry. Within each province, there does not appear to have been systematic changes in lava chemistry with time. There are small but significant differences between the trace element chemistry of lavas from the McBride and Atherton provinces, and lavas from the other areas. The high MgO and Ni contents of most samples, and the presence of mantle xenoliths in many of these lavas, suggests that crustal level contamination has not been significant. These differences were therefore most likely inherited from differences in the chemistry of the subcontinental mantle lithosphere beneath the Proterozoic and Phanerozoic regions. However, the most evolved lavas from the Nulla province tend to have the most radiogenic Sr and the least radiogenic Pb isotope compositions, suggesting that crustal level fractionation and contamination may have influenced the chemistry of these lavas. This is consistent with a general lack of mantle xenoliths in Nulla flows. The large U-Th disequilibria observed in the youngest lavas (^^^hli/^^^U = 1.2-1.5) suggest that several of the flows are younger than about 50 ka. Analysis of mineral fractions will give more precise age constraints. REFERENCES Griffin W. L. & McDougall I. 1975. Geochronology of the Cainozoic McBride volcanic province, northern Queensland. J. Geol Soc. Aust. 22, 387-397. O'Reilly S. Y. & Zhang M. 1995. Geochemical characteristics of lava-field basalts from eastern Australia and inferred sources: connections with the subcontinental lithospheric mantle? Contrib. Mineral Petrol 121, 148-170. Stephenson P. J. 1989. Northern Queensland. In: Intraplate volcanism in eastern Australia and New Zealand Johnson, R. W. (editor) Cambridge University Press 89-97. Zhang M., Stephenson P. J., O'Reilly S. Y., Norman M. & McCulloch M. T. 1997. Geochemistry and petrogenesis of basaltic rocks from North Queensland: has subduction-modified mantle played a role? Geol Soc. Aust. Abstracts 45, 110-113.
374
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
FACIES AND HABITATS IN REEF CAVES, CRYPTIC ENVIRONMENTS, AND DEEP FORE REEF ZONES IN REEFS: A COMPARISON OF CONSERVATIVE BENTHIC COMMUNITIES FROM THE CRETACEOUS OF SPAIN AND MODERN INDO-PACIFIC REEFS Joachim Reitner, Gert WOrheide, Volker Thiel Institut und Museum fUr Geologic und Palaontologie, Univ.GOttingen, Goldschmidtstr.3,37077 GOttingen, Germany; Queensland Museum, Brisbane, Queensland, Australia; and Institut filr Biochemie und Meereschemie, Univ. Hamburg, Bundesstr.55, 20146 Hamburg, Germany
Results on distribution patterns of three coralline sponge taxa and related microbialites from Indo-Pacific coral reef caves and deep fore reef areas are presented. The stromatoporoid taxon Astrosclera, the chaetetid Spirastrella (Acanthochaetetes), and the thalamid Vaceletia are not equally distributed throughout the investigated areas. Distinctive distribution patterns were observed. Thrombolitic micritic microbialites were observed in shallow water caves of fringing reefs on the continental islands of the Lizard Island Section. The microbialites invariably exhibit a light decreasing facies succession. The succession starts with a coralgal community and ends with light independent microbial biofilms and a benthic community of cnistose foraminifera, serpulids, thecidean brachiopods, bryozoans, and coralline sponges. All taxa are living in dark cryptic habitats, but Astrosclera was also found in dim-light zones and shaded overhangs. The related microbialites vary in relation to the sedimentological setting of the environment. Very well developed microbialites were found in environments influenced by the weathering of crystalline basement rocks (e.g. Lizard Island) and deep large caves within a carbonate basement (e.g. Marigondon Cave, Cebu/Philippines). Poorly developed microbialites were observed in open oceanic environments with carbonate basement rocks (e.g. Osprey Reef and reefs of the outer Great Barrier Reef). At depths between 100 and 300 m reef cave microbialites are typically common, forming thick overhangs and mounds. The observed benthic community is very similar to one observed in cryptic habitats of the Aptian and Albian reefs of northern Spain. The facies or habitat zones within the reef caves are extremely condensed ("telescoping effect"). Most of the habitat zones observed in caves also exist in the deep fore reef belts. For long-term studies of microbialite formation and the growth rates of coralline sponges, specimens were stained in vivo, within their natural habitat, with histochemical fluorochromes and nonfluorescent agents. The main results are that microbialite and associated sponges grow very slowly (50-100^m/y). Only a few calcifying microbes participate during microbialite formation. Calcifying acidic organic macromolecules are primarily responsible for microbialite formation because they cement detrital material. Fe/Mn-bacterial biofilms are responsible for strong corrosion of microbialite surfaces. Besides the corrosive activity of the Fe/Mn-bacterial biofilms, boring sponges {Aka, Cliona) are the primary carbonate destroyers. Geochemically, the observed microbialites are composed mostly of high-Mg calcite and exhibit high positive (+3 to +4) values. The observed facies consortia are comparable with facies known from many fossil mud mounds.
375
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
GEOLOGICAL MAPPING USING STATISTICAL ANALYSIS OF GAMMA-RAY DATA IN THE COURTENAY CREEK AREA, EASTERN FOLD BELT, MT ISA INLIER: DEVELOPMENT OF A POTENTIAL EXPLORATION TOOL Julie Richmond School of Earth Sciences, James Cook University, Townsville, Qld. 4811.
The granitoids of the Williams and Naraku Batholiths in the world class Cloncurry minerals province, Mt Isa Inlier, are spatially and temporally associated with Cu-Au mineralisation. Thus using airborne geophysical data to map the distribution and geochemical components of these granitoids would be a highly desirable exploration tool. A project is currently being undertaken to map the components of these prospective granitoids using high resolution airborne gamma-ray data. A pilot study has been completed in the Courtenay Creek area 5 km north of the township of Cloncurry. The area was selected for detailed study as it contains outcropping granite of the Naraku Batholith as well as abundant calcsilicate of the Corella Formation, both of which crop out extensively throughout the Eastern Fold Belt. The aims of the pilot study were firstly, to determine how effectively gammaray spectrometry reflects variations in the geochemistry of rocks and soils, and hence its usefulness as a mapping tool; and secondly to analyse ganmia-ray data to identify statistically independent populations of data and relate these to rock types. The advantage of determining the number of populations is that a limit is placed on the number of lithologies distinguishable using a limited number of elements. To achieve these aims the pilot study was confined to ground based ganmia-ray data to enable direct correlation between gamma-ray and geochemical data for known rock and soil types, and to relate the results of the statistical analysis to a detailed map of the cover types. Potassium (K), equivalent uranium (elJ) and equivalent thorium (eTh) concentrations were measured at random locations within rectangular grids established over representative granitoid and calcsilicate rock units. Rock and soil samples were collected from the same random locations for geochemical analysis. A comparison of the geochemistry and gamma-ray data indicates that K and eTh closely reflect the chemistry of the rock and soil units. A two step statistical analysis procedure was employed to initially determine the number of populations within the gamma-ray dataset, and secondly to use the number of populations as a guide to the maximum number of geological units that could be differentiated using only three elements. Analysis of variance (ANOVA) was used to compare the variance of gamma-ray measurements, which were grouped according to the geological units from which they were collected. The outcome was that the number of geological units identifiable in the field was reduced to the number of units distinguishable using geochemical data. Hierarchical clustering of the ground gamma-ray data was then utilised to successfully map out the distribution of soil and rock units. The number of populations determined using ANOVA, was used as the upper limit of the stopping criteria for the cluster analysis. The clustering discriminated granitoid and calc-silicate from soils. It also highlighted the geochemical variation within the units that have higher K and/or eTh concentrations and those that have a greater variance of those radioelements, i.e. the granitoids. Despite the use of ground based radiometrics and the limited spatial coverage of the data, this study has illustrated the usefulness of statistical analysis, in particular cluster analysis, of gamma-ray data for mapping purposes. It has indicated the potential for a wider application of the methodologies applied here. Similar techniques have been applied elsewhere to differentiate rock types, however the challenge will be in applying these techniques to granitoids with narrow ranges of radioelement concentrations over a larger area using high resolution airborne data. Clusters produced from two grids highlighted geochemical variations within the one outcrop, which implies that mapping variations within a granitoid pluton of varying composition may be possible. Acknowledgments: The author would like to thank MIM Exploration for their support of this project.
376
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
NEW DATA ON SILURIAN GRAPTOLITES FROM YASS, NSW R. B. Rickards^ and A. J. Wright^ 'Department of Earth Sciences, University of Cambridge, Cambridge, England CB2 3EQ ^School of Geosciences, University of Wollongong, Wollongong, New South Wales 2522
Graptolites from the famous Silurian sequence of the Yass district, NSW, have been known since their occurrence was noted by John Mitchell (1886, 1888) and T. S. Hall (1903). Little work has been done on this most important Australian sequence of Silurian graptolites. In particular, the much-publicised ''Monograptus bohemicus"' fauna from the topmost beds of the Black Bog Shale has remained essentially untouched since its study by Brown & Sherrard (1954). New graptolite collections have been made by us in late 1997 from a number of Late Silurian (Ludlow and Pridoli) levels in the Yass sequence, amounting to 24 species-level taxa. A single dendroid specimen has been found low in the Black Bog Shale, from the Yarwood Siltstone Member. More important and varied graptoloid faunas occur as follows: 2 levels high in the Black Bog Shale; 2 levels low in the Rosebank Shale; low in the Cowridge Siltstone; and in the lower part of the Elmside Formation. The first 4 faunas are Ludlow, whereas the latter 3 faunas are Pridoli. The fauna from about 12 m below the top of the Black Bog Shale is dominated by Bohemograptus bohemicus tenuis and B. praecomutus: this fauna is assigned to the praecomutus zone. B. b, bohemicus, which would be expected to occur below this horizon, has not been found in the Yass district. The fauna from the topmost beds of the Black Bog Shale is dominated by a very unusual species, hitherto called ''Bohemograptus bohemicus'\ which we will describe as Bohemograptus sp. nov. This fauna is correlated with the late but not latest Ludlow comutus zone; the latest Ludlow inexpectatus and kozlowskii zones are not yet represented at Yass by graptolites. The next fauna, in the lowest Rosebank Shale, represents part of the ''Monograptus salweyr fauna of Sherrard, and is the Monograptus formosus fauna of Jaeger (1967) and Packham (1968); we agree with Packham that this is earliest Pridoli (parultimus zone). The fauna from the Cowridge Siltstone in Barambogie Creek include forms also assigned by Sherrard (Sherrard & Keble, 1937; Brown & Sherrard, 1954) to M. salweyi; Jaeger studied material from this general area and identified Monograptus bouceki and M. transgediens)\ this fauna is assigned to the mid- Pridoli bouceki zone, with the intervening ultimus and lochkovensis zones being not recognised at Yass. The highest fauna, from the Elmside Formation, is assigned to the latest Pri'doli transgrediens zone as Jenkins (1982) suggested; the pemeri zone has not been identified here. This work has greatly increased the known graptolites from Yass, sharpened the correlations considerably and expanded our knowledge of the biogeography of Late Silurian graptoloids.
REFERENCES Brown, I. A. & Sherrard, K. M., 1952. Graptolite zones in the Silurian of the Yass-Bowning district of New South Wales. Journal and Proceedings of the Royal Society of New South Wales 85, 127-134. Hall, T. S., 1903. On the occurrence of Monograptus in New South Wales. Proceedings of the Linnean Society of New South Wales 27, 654-655. Jenkins, C. J., 1982. Late Pridolian graptolites from the Elmside Formation near Yass, New South Wales. Proceedings of the Linnean Society of New South Wales 106, 167-172. Jaeger, H., 1967. Preliminary stratigraphical results from graptolite studies in the Upper Silurian and Lower Devonian of southeastern Australia. Journal of the Geological Society of Australia 14, 281-286. Mitchell, J., 1886. Notes and Exhibits. Proceedings of the Linnean Society of New South Wales (Second Series) 1, 577. Mitchell, J., 1888. Notes and Exhibits. Proceedings of the Linnean Society of New South Wales (Second Series) 3, 150. Packham, G. H., 1968. Monograptus cf ultimus Perner and Monograptus formosus Boucek from the Hume Series of the Yass district. New South Wales. Proceedings of the Linnean Society of New South Wales 92,217-221. Sherrard, K. M. & Keble, R. A., 1937. The occurrence of graptolites near Yass, New South Wales. Proceedings of the Linnean Society of New South Wales 62, 303-314.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
ROCKS LIKE THE TUCKERS COMPLEX IN THE INGHAM HINTERLAND? Ian P. Rienks^ Mark C. Gunther^ and C. Mark Fanning^ ' Geological Survey of Queensland, GPO Box 194, Brisbane, Queensland 4001 ^Goldfields Exploration Ltd, 14 Williams St, West Kalgoorlie, WA 6430 ^ PRISE Laboratory, Australian National University, Canberra, ACT 0200
A series of Permian plutonic complexes with distinctive rock types ranging from gabbro to granite occur in the Ravenswood Batholith of north Queensland. The Tuckers Igneous Complex is a noteworthy example. The mafic to intermediate members are typically pyroxene-bearing, whereas other intermediate plutons in the batholith tend not to be. K-Ar biotite analyses of the Tuckers and nearby Boon Complexes provide magmatic cooling ages between 288 and 283 Ma. The Ryebum Diorite has recently been recognised in the Mt Fox area southwest of Ingham. It consists predominantly of medium grained diorite but is intemally differentiated. It has 61-67% Si02, and relicts of magmatic pyroxene occur in the cores of homblende. Aeromagnetic images enable the definition of a series of dipoles perhaps delineating separate plutons v^thin this igneous mass. K-Ar biotite ages from a host granite intruded by the diorite are 278 and 283 ± 9 Ma, whereas Rb-Sr total-rock analyses suggest a mid-Carboniferous age. This implies the K-Ar biotite determinations are recording the time of heating during intrusion of the Ryebum Diorite, which could be of a similar age to the Permian Ravenswood Batholith complexes. The major element chemistry of the Ryebum Diorite is broadly comparable to CarboniferousPermian plutons of similar SiOz from the Ingham to Ravenswood region. In detail, the diorite best matches samples from the Tuckers, Boori, First Pocket, and Black Pinnacle Igneous Complexes and from the Percy Douglas Granodiorite in the Drummond Basin (Table 1). All of these plutons can be classed as oxidised from ferric/ferrous ratios. The mineralised Percy Douglas pluton in the Mt Wyatt Goldfield (AGSO Record 94/21), is a correlative that has economic significance. These plutons have very similar absolute and/or relative major and trace element values. Other plutons with a good geochemical match include the Jacobsens Track Granodiorite (280 ± 8 Ma, K-Ar), and an unnamed unit southeast of Kirrama (282 ± 4 Ma, SHRIMP U-Pb zircon), both of which are also oxidised. The Ravenswood Batholith complexes have higher but parallel REE and HFSE but similar major elements, whereas the Percy Douglas is remarkably close in trace elements, but higher in MgO and CaO. Appropriate samples from other plutons are distinct. Slight variations in source, melting, and fractionation conditions could explain chemical differences between the occurrences, and a conmiunity of genesis is possible. The distinctive plutonic suite(s) could have a narrow range of crystallisation ages, and transgress structural blocks. Since sampling, mapping, and geophysical data over the Ravenswood Batholith are more detailed than in most of the Ingham hinteriand, other, unrecognised plutons may yet be found. Geochemical databases are often supposed to be of academic interest. We show here, that a chemical fingerprinting exercise aimed at identifying igneous suites has resulted in correlations which could have implications for the prospectivity of the Ingham region. Table 1. Normalised volatile-free major and trace element data from selected samples from the Ingham-Ravenswood region. Locations in the table are AMG references to nearest 100m. Analyses by QGCL^ and AGSO^.
Units: Sample # Si02 Ti02 A1203 Fe203 FeO MnO MgO CaO Na20 K20 P205 Ba Rb Sr
Ryebum Diorite' RGMP 327
63.83 0.64 16.74 1.52 3.75 0.09 2.33 5.07 3.44 2.45 0.13 375 100 290
Tuckers Complex^ RBRS 161
64.13 0.82 15.74 2.93 2.44 0.10 2.30 4.57 3.63 3.13 0.19 504 99 406
Boori Complex^ RBRS 281
63.07 0.79 16.33 2.33 3.14 0.11 2.43 4.97 3.55 3.07 0.20 520 115 470
Percy Douglas^ 88302 104
63.04 0.53 16.33 1.93 3.76 0.11 3.41 5.73 2.64 2.42 0.09 390 95 255
378
Pb Th U Zr Nb Y La Ce Nd Cu Zn Ga EAST NORTH
Ryebum Diorite' RGMP 327
10 12 -1 105 6 20 25 50 16 10 60 17 3504 79230
Tuckers Complex^ RBRS 161
15 14 4 243 10 25 33 68 31 12 82 19 4503 77799
Boon Complex^ RBRS 281
10 13 3 250 7 22 33 65 27 15 50 18 4716 77773
Percy Douglas^ 88302 104
15 15 3 110 7 21 25 48 19 21 67 18 5234 76891
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
M"" Australian Geological Convention, Townsville, July 1998
EARLY LANDSCAPE EVOLUTION OF THE MONARO VOLCANIC PROVINCE, SOUTHEAST N.S.W. Ian C. Roach Centre for Australian Regolith Studies, University of Canberra, ACT 2601 • •Present address: CRC LEME, Department of Geology, Australian National University, Canberra, ACT 0200
The Monaro Volcanic Province (MVP) is an Eocene-Oligocene intraplate volcanic lava field developed over an undulating basement of Palaeozoic meta-sedimentary rocks and granitoids of the Lachlan Fold Belt. The basement is dissected by major meridional and northwest-northeast conjugate fractures which have had major control over the location of eruption sites (Roach et al 1994). Volcanism appears to have commenced with eruptions from numerous vents located over a major northwestsoutheast fracture and subsidiary northeast-southwest conjugates in the vicinity of what is now the Monaro Range, part of the Great Dividing Range, in the northern MVP. Later, volcanism spread southwards to another northwest-southeast fracture aligned with the Towamba River and other random localities dotted about the west, south and east MVP. The volcanic stratigraphy of the lava pile (Roach 1996) shows that initial volcanism was of tholeiitic to transitional basaltic affinity, taking the form of numerous relatively thin lava flows which dip gently northeast and southwest away from the major eruption axes. Field evidence suggests eruptions created a series of overlapping lava shield volcanoes stretching from Brown Mountain to Lake Eucumbene, the current Monaro Range, much like the overlapping lava shields of Kilauea's rift zones. Early lavas filled the relatively deeply incised Murrumbidgee River valley around Cooma and displaced numerous creeks sideways but were largely contained by a fault scarp on the western side of the Bega Batholith. The lava pile was forced to build upwards because of the relatively high topographic relief, not sidewards as is seen in many other eastern Australian lava fields developed on plains including the Newer Volcanic Province and McBride Volcanic Province. Early lavas may have spilled outwards onto the undulating plains of the Berridale Batholith in the western MVP at this stage. After about 10 Ma of activity subsequent lavas were dominated by alkali olivine basalt, nepheline basanite, olivine nephelinite and ankaramite. At this stage, lava shields were probably becoming replaced by tephra cones and lava flows were extending to the far reaches of the MVP. Some ankaramite and alkali basaltic lava flows entered bodies of water large enough to allow pillow lavas, massive hyaloclastite with quenched lava "rags" and clast-in-matrix hyaloclastite to form. A few alkali basaltic magmas erupted through sodden ground to form maars, at least one of which is now preserved. At the close of volcanism, the MVP consisted of two major NW-striking ridges and a lava pile which filled the southern Murrumbidgee valley and extended eastwards onto the Bega Batholith, westwards onto the Berridale plains and the hills as far as Jindabyne and Adaminaby and southwards into the upper Towamba, Bombala and Snowy River systems. Lavas displaced many rivers and creeks sideways and totally realigned the Murrumbidgee river northwest of Cooma by blocking its southern extent. Post-volcanic erosion has removed all of the tephra, except that buried by lavas, and has dissected the lava pile through to the basement in many places as drainage reclaims some of the pre-volcanic water courses. The large basaltic ridge forming the Monaro Range has provided a means of shifting the Great Divide from its early position east of the MVP to its current position west of the MVP. Scarp retreat and river incision from the east and south of the MVP has lowered the topography in the southeast comer of NSW and northeast Victoria, realigning the Great Divide in a dog-leg from the Tinderry Range, through the Monaro Range (the next highest range) to the Snowy Mountains, west of the MVP, where it again turns south to Victoria. REFERENCES Roach I.e., McQueen K.G. & Brown M.C. 1994. Physical and Petrological Characteristics of Basaltic Eruption Sites in the Monaro Volcanic Province, Southeastern New South Wales, Australia. AGSO Journal of Australian Geology and Geophysics 15(3), 381-394. Roach I.e. 1996. The formation of the Monaro Volcanic Province, Southeastern NSW, Australia. In: Whitehead P.W. (ed). Chapman Conference on Long Lava Flows, Abstracts, James Cook University, 60-61. Acknowledgements: The author gratefully acknowledges the financial support of the University of Canberra.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
STRATIGRAPHIC IMPLICATIONS IN THE MONARO VOLCANIC PROVINCE, SOUTHEASTERN N.S.W. Ian C. Roach Centre for Australian Regolith Studies, University of Canberra, ACT 2601* *Present address: CRC LEME, Department of Geology, Australian National University, ACT 0200
STRATIGRAPHY The stratigraphy of the northern Monaro Volcanic Province (MVP) provides insights into the changing petrology, geochemistry and volcanology of this Eocene-Oligocene lava field. Detailed stratigraphy comes from two profiles on the Monaro Range south of Cooma (part of the Great Dividing Range): a diamond drill hole (Brown et at. 1992); and a road-side profile approximately 3 km north of this (Roach 1996). The drill hole contains up to 22 separate lava flows identified petrographically as alkali olivine basalts and basanites (Brown et al 1992). The road-side profile contains at least 15 separate lava flows identified geochemically as (from the base up) olivine tholeiites, transitional basalts, alkali olivine basalts and a nepheline basanite volcanic plug which intrudes through the whole lava pile. Two ankaramite lava flows near the top of the road-side profile form a prominent marker horizon separating the hypersthene normative sub-alkalic basalts from the nepheline normative alkalic basalts. IMPLIED VOLCANIC STAGES Early stage volcanism yielded dominantly tholeiitic to transitional lavas. Vents from this stage are not visible implying that they are either buried or have been re-occupied. These rocks are topped with ankaramites of alkali basaltic and tholeiitic affinity. Ankaramites occupy a similar stratigraphic level to those of the Haleakala shield volcano of Maui (Macdonald et al 1983), marking a similar major change from sub-alkalic to alkalic volcanism, also implying a similar alkalic resurgence. They do not, however, imply a similar short life span for the MVP. Ankaramite lava from the drill hole is at the same altitude as similar rocks in the road-side profile and has a K/Ar age of 48.9 ± 0.3 Ma (Roach 1996). MVP ankaramites contain large titanian augite phenocrysts and glomerocrysts which display prominent concentric compositional zones and deeply colored rims, unlike those from Haleakala which are not zoned. Ankaramites now occupy ca. 3.3% of the area of the relict MVP lava pile. Late stage alkalic volcanism consisted of numerous basanitic, nephelinitic and alkali basaltic vents which intruded through the underlying sub-alkalic lava pile. Pyroclastics are only rarely preserved within the MVP and most eruption sites now consist of heavily eroded volcanic plugs or dykes consisting of these three rock types. Alkalic lavas now cap much of the lava pile within the MVP and are the most frequently sampled rocks. GEOCHEMICAL CHARACTERISTICS Major, trace and rare earth element studies indicate that no large scale AFC processes occurred within MVP magmas over the life span of activity and most rocks are primary or near-primary. High pressure (olivine + clinopyroxene) and low pressure (clinopyroxene + plagioclase) fractional crystallisation trends are visible within the data however the most fractionated rock has Mg/(Mg + total Fe) of 0.55 and there are no significant Eu anomalies indicating overall relatively low levels of fractional crystallisation. Changing Mg/Fe in amphibole megacrysts in the absence of MREE (Nd - Gd) anomalies and evolved host rock types discounts significant amphibole fractionation within ultramafic magmas. Amphiboles are thus presumed to be derived from disaggregated metasomatic veins. Constant Zr/Nb and Y/Nb indicate parent magmas are from similar sources which have not undergone significant crustal assimilation and that source characteristics varied little during the evolution of the MVP. REFERENCES Brown M.C., McQueen, K.G.& Taylor G. 1992. A core through the Monaro basalt: Bega (BMR) No. 7. Australian Journal of Earth Sciences 39, 555-559. Macdonald G.A., Abbott A.T & Peterson F.L. 1983. Volcanoes in the Sea. 2nd edn.. University of Hawaii Press, Honolulu HI. Roach I.e. 1996. The formation of the Monaro Volcanic Province, Southeastern NSW, Australia. In: Whitehead P.W. (ed). Chapman Conference on Long Lava Flows, Abstracts, James Cook University, 60-61. Acknowledgements: The author gratefully acknowledges the financial support of the University of Canberra.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
SYN- AND POST-DEPOSITIONAL TECTONISM IN THE TAMWORTH BELT, SOUTHERN NEW ENGLAND OROGEN John Roberts and Richard Geeve Department of Applied Geology, University of New South Wales, Sydney 2052
Upper Devonian to Carboniferous successions in the Tamworth Belt, Southern New England Orogen (SNEO), were deposited within a forearc basin. During the Permian the belt was internally imbricated and folded, and converted into a generally west-verging thrust and fold belt between the Peel-Manning System on the east and the Mooki and Hunter thrusts on the west. The thrusts carry the forearc basin over the Permian to Triassic foreland, the Carboniferous volcanic arc and older basement of the Lachlan Orogen. The southern part of the belt has been detached, translated north and rotated to form a separate terrane, the Hastings block, outboard from the accretionary prism of the SNEO. Eight distinct blocks recognised within the Tamworth Belt are separated by faults or major lineaments, a number of which appear to have relatively small displacements. Each of the blocks contains a sedimentary succession that for part or all of its depositional history is different from that of adjacent blocks. The two subdivisions of the Hastings block contain completely different successions. Both parts of the Myall block have major stratigraphic differences prior to the latest Visean but are similar in the Namurian; there is a single possible Namurian link, between the Gresford and Myall blocks; sequences m the Gresford and Rouchel block are different up until the late Visean, after which they are the same; Toumaisian successions in the Rouchel block and eastern part of the Werrie block are similar, but Visean and younger parts require additional resolution; and the Werrie and Rocky Creek Blocks have similarities in the Toumaisian and Visean but are completely different in the remainder of the Carboniferous. Blocks with Imked or common successions are inferred to have been in close proximity to one another during those intervals. However, the presence of different adjacent sequences, either above or below the common sequences, within the same blocks suggests that the latter were in different parts of the forearc basin during deposition of those units. These relationships can be explained by the fore-arc basin being cut by strike-slip faults that were active from time to time during deposition. Such strike-slip fauhs are present in modem environments such as the Aleutians, Cascadia and Sunda forearcs. These types of faults may also have provided the mechanism for dismemberment of the southem part of the Tamworth Belt during the latest part of the Carboniferous. In addition to having different stratigraphies, a number of blocks have been rotated. Published palaeomagnetic data point to rotation of the northem Hastings block (either 130^ clockwise or 230^ anticlockwise), and new data (obtained by Richard Geeve) indicate a 70^ anticlockwise rotation of the Gresford and Rouchel blocks relative to the northem parts of the Tamworth Beh. To date, overprinting obscures any possible rotational history within the southem Hastings and Myall blocks. Whilst the northem Hastings rotation appears to have taken place during the latest Carboniferous it is likely that rotation of the Rouchel and Gresford blocks was associated with late Early Permian NNE-SSW compression which formed the first generation of folds (WNWESE) within the southem Tamworth Beh and established the fore-runner of the Hunter Thmst. Following folding, the Barrington Tops Granodiorite intmded the northem part of the Gresford block. Late Permian to Triassic E-W compression was responsible for the formation of the major N-S stmctures that dominate much of the Tamworth Belt. In the south the Rouchel and Gresford blocks were refolded, and major structures such as the Stroud-Gloucester and Myall Synclines formed in the Myall block; major overthmsting appears to be confmed to the westem margin. In contrast, the northem part of the beh is characterised by as much as 85 km of thinskinned overthmsting, illustrating a major difference in response to compression between the two main parts of the belt. The reasons for these different responses to compression is currently being examined.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
PROSPECnNG BENEATH COLLUVIAL-ALLUVIAL COVER AT QUASAR IN THE BOOGARDIE SYNFORM - MT MAGNET, WA 'Cooperative Research Centre for Landscape Evolution and Mineral Exploration, CSIRO Exploration and Mining, PMB Wembley, 6014. ^armoola Joint Venture Ply Ltd. P.O. Box 710, West Perth, 6872.
The depositional plain overlying the Archaean Boogardie Synfoim at Mt Ma^et is characterized by a heterogeneous, multiphase cover of exotic sediments, unrelated to the underlying rocks, that conceals a complete or truncated, deeply weathered, and leached regolith developed on faulted Archaean felsic and maficultramafic rocks. Palaeochannels, incised into the basement, are infilled with argillaceous sediments and aie overlain by coUuvium-alluvium derived from erosion of a lateriticregolith,in places developed on banded iron formations, that outcrop around the synfomi. Since deposition, precipitation of sihca, to form red-brown hardpan and, locally, carbonates, to form pedogenic calcrete, has occurred. The colluvium has a high background Au content (approximately 50 ppb) due to significant mineraUsation in the source area (e.g., the Hill 50 Gold Mine). This high Au background masks any possible subtle anomalies from post-depositional hydromorphic dispersion from underlying sources. In addition, there are also difficulties in distinguishing transported from residual regoUth, particularly in drill cuttings. The problems confronting exploration at Mt Magnet are typical of many mineralized districts in the Yilgam Craton, and persistent, closely-spaced drilling for bedrock sampling is credited for past exploration success. Orientation drilling on a 50 x 100 m grid at the (Juasar deposit allowed various sampling media, from lag to bedrock, to be compared. The composition of the lag reflects its immediate source, the uppermost layer of the colluvium-alluvium. Although bioturbation has been shown elsewhere to bring a weak geochemical signal to the surface through 0.51.0 m of overburden, the overburden is too thick (4-6 m) at Quasar so there is no correlation between the composition of the lag and the bedrock, or even with deeper units of the colluvium-alluvium. Similarly, there is little correlation between adjacent layers in the colluvium-alluvium or between them and the weathered basement. Geochemical dispersion is minimal in the stripped saprolite so that, although element abundances and anomaly/background contrasts are high, anomalies in the top-of-basement are small in area. Single point anomalies for Au, Pb and Bi indicate the general position of mineralisation at Quasar, although the deposit itself was missed in this orientation program. In comparison, samples across the saprolite-colluvium interface have multipoint anomalies in Au, Pb and Bi that are significantly larger than those in the top-of-basement, although of lower contrast. The interface may incorporate a residual or partly transported palaeosol, as well as targeting a probable seepage zone, hence maximizing the potential for sampling the site of potential mechanical and hydromorphic dispersion, during and after sedimentation. Apart from Au, pathfinders for As-poor Quasar style mineralisation are Umited to Pb, Bi and Zn. However, none of these elements is an unequivocal guide to mineralisation. Use of top-of-basement sampling on a 100 x 50 m drill grid seems barely adequate for detection of a target such as Quasar. Use of interface sampling gave a more certain result and is the best medium in areas of buried, tmncated regolith, although it may be less effective where the basement is covered by a palaeochannel. Improved target sizes and reduction of spurious anomalies were obtained using an additive multi-element index, including Bi, Pb and Zn. Gold concentrations should be used, together with this, to assist in ranking anomalies. Locally, the weathered basement rocks (felsite or maficultramafic) may be distinguished from the covering sediments (colluvium or palaeocharmel clay) by characteristically greater Cr/Fe ratios. Probing for regolith stratigraphy, subsurface facies variations, presenpe of and palaeotopography of unconformities and the characteristics of buried erosional surfaces are important aspects of exploring areas of transported overburden and in deciding which units to sample. Once the regoUth stratigraphy is properly established, routine but accurate logging and sampling of drill cuttings is much more efficient and relevant to data interpretatioa A sample collected a metre too high could be in transported overburden, giving misleading results, whereas sampling too deep could include saprolite devoid of Au and pathfmder elements due to small anomaly sizes and/or leaching. Acknowledgements: This research has been the outcome of productive collaboration between CSIRO and the Australian mineral industry through AMIRA. The assistance and support of the sponsors of CSIRO-AMIRA Project 409 (1994-1997) and, in particular, WMC Resources Ltd., are gratefully acknowledged. CRC LEME is supported by the Australian Cooperative Research Centres Program.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
SEAFLOOR MAPPING SOUTHWEST OF AUSTRALIA: THE FRENCHAUSTRALIAN MARGAU PROJECT J-Y. Royer^ P.J. Hill2, M-O. Beslierl, N.F. Exon2, C. Buchanan2, & H.M.J. Stagg2 1 Geosciences Azur, UMR 6526, BP 48, Villefranche sur Mer, France 2 Petroleum & Marine Division, Australian Geological Survey Organisation, Symonston ACT 2609
The French-Australian cooperative MARGAU (Australian MARGin) survey, scheduled for May 1998 aboard the MS Marion Dufresne II, is designed to investigate the structure and evolution of the southwestern margin of Australia which formed during the breakup and subsequent dispersal of Greater India, Australia and Antarctica in the late Mesozoic. The main scientific techniques to be employed include multibeam swath-mapping (Thomson-Marconi TSM5265 multibeam system) and seabed sampling; hi^-speed seismic, magnetic, gravity and 3.5 KHz echo-sounder data will also be recorded. It is antidpated that approximately 240 000 km2 of seabed will be covered by the swathmapping during approximately 30 days of surveying. The principal objectives of the survey are to study: 1 .the mechanisms of extension, crustal thinning and breakup of the continental lithosphere, with emphasis on the emplacement of materialfi-omthe mantle at the continent-ocean boundary (COB); and 2.the early opening of the eastern Indian Ocean at the junction of the India-Australia and Australia-Antarctica spreading systems. The study area includes the southern margin of the Naturaliste Plateau, the rugged east-west trending and enigmatic Diamantina Zone fi'om south of Albany westwards towards the junction with Broken Ridge, and the intervening abyssal areas. The area lies at the boundary of two orthogonal diverging plate systems. In the Perth Basin to the north, seafloor spreading on a northwest azimuth began at about 129 Ma (chron M9) between Australia and Greater India. While rifting between Australia and Antarctica began in the Late Jurassic, interpretation of the subsequent extensional phase is somewhat controversial, as is the timing of the first seafloor spreading, the location of the COB and the nature of the deep crust between the continent and the Diamantina Zone. Some of the specific problems that are being addressed by the study include: • What is the structure and evolution of the southwest margin around the Naturaliste Plateau? What was the mechanism by which the peridotites dredgedfi-omthe Diamantina Zone were emplaced? • What is the nature and origin of the Naturaliste Plateau - continental or oceanic? • What is the Diamantina Zone - the product of slow seafloor spreading; stretched oceanic lithosphere; or a mixture of the two? • The poorly constrained eariy opening of the Indian Ocean and the early evolution of the Australia-Greater IndiaAntarctica triple junction. • The uncertain age of the initiation of seafloor spreading between Australia and Antarctica. The thin sedhnentary cover and rough basement topography in this region, particularly along the COB and the Diamantina Zone, make it particularly suited to surveying by multibeam swath-mapping and bottom dredging.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
M"" Australian Geological Convention, Townsville, July 1998
DR MARY WADE - COLLECTOR AND FACILITATOR - QUEENSLAND FIELD WORK 1971-1992 Andrew Rozefelds' and Susan Turner^ 'Tasmanian Herbarium, GPO Box 252-04, Hobart, Tasmania 7001 ^Queensland Museum, P.O. Box 3300, S. Brisbane, Queensland 4101
No one person has done more in the recent decades to increase the fossil collections of the Queensland Museum. As Museum Curator of Geology and later of Invertebrate Palaeontology, Mary Wade oversaw the expansion of the geological collections incorporating a vast range of material. The fossil collection quadrupled in size and acquired world class status. Whether it be straight or coiled nautiloids from the far northwest of the state or giant dinosaurs from the centre west, palaeontologist Mary Wade was at the hub of "bringing them back alive". Mary, bom and educated in Adelaide, South Australia, a graduate of the University of Adelaide, moved to live and work in Brisbane in 1971. Her subsequent 22 years of service to the Queensland Museum and research in Queensland Palaeontology was exemplary. Her fieldwork during that time enhanced museum collections and display and facilitated research both her own and of others both from home and abroad. Her unselfish and comprehensive approach built up the collection on almost every level: geographical, stratigraphical; in geological samples, mineralogy, and especially fossils from Queensland. She collaborated in the field with many leading Australian and international palaeontologists and introduced novices (new "chums") to the vagaries of the bush. Her field research style is uncompromising and gutsy. She is the Queensland Museum to many people having achieved almost "legendary" status especially with country people where her background and upbringing gave her the rapport and sympathetic approach needed to facilitate fieldwork in outback areas. For her contributions she was awarded the Queensland Museum Medal in 1994. She collected to extend her own research, mastering several phyla new to her, and becoming a world authority on Palaeozoic nautiloids. Cretaceous dinosaur trackways, and marine reptiles, adding to her earlier specialities of Precambrian fossils and Tertiary foraminifera. Highlights include work in western Queensland on the Cambrian (echinoderms, cephalopods) and Cretaceous invertebrates (e.g., bivalves, teuthids) and vertebrates (e.g., marine reptiles, dinosaur trackways); in north Queensland (e.g.. Tertiary marsupials, Broken River nautiloids); in central Queensland (e.g., Permian plants, fish and ichnofauna, Triassic amphibians).
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
GENESIS OF LARGE-SCALE ALBITIZATION AND ASSOCUTED METASOMATISM IN THE SNAKE CREEK ANTICLINE, EASTERN FOLD BELT, MOUNT ISA INLIER Mike Rubenach^ and Ian Cartwright^ 'Department of Earth Sciences, James Cook University, Townsville, Queensland 4811 ^Department of Earth Sciences, Monash University, Clayton, Victoria 3168
Albitites, mainly resulting from sodic-calcic metasomatism, are quite conmion in the Eastern Fold Belt. Most are post-D2 and many are associated with late-stage granites of the Williams Batholith. However, albitites in the Snake Creek Anticline are the result of sodic alteration that was dominantly syn-D2. They are concentrated in the andalusite/staurolite zone in three belts, each 4-5 km in length and up to several hundred metres wide. The northwest and southeast belts occur adjacent to amphibolite bodies, and it is proposed that ductility contrasts between amphibolite and metasedimentary rocks producedfracturesthat provided access for the metasomatismg fluids. The central belt occurs in the anticlinal hinge, and at least some of the albitites in this belt have replaced metasediments along syn-D2 shear zones. Although dominated by albite, other phases in albitites include staurolite, biotite, andalusite, former cordierite, garnet, anthophyllite, fibrolite, and l^anite. Quartz is rare and muscovite absent. Critical assemblages suggest that formed at around 580^C and 4 kbar. Work in process is investigating to what extent multiple growth stages of porphyroblasts and unusual mineral combinations are the result of disequilibrium or complex P-T-t paths. Albitites have replaced pelitic schists and metapsammitic rocks, and only in the northwestern belt is there a spatial relationship between albitites and quartz veins. Adjacent to some albitites in the northwestern and southeastern belts are metasomatic biotite-staurolite-quartz rocks that have also replaced muscovite-rich pelites. Geochemical studies demonstrate that the formation of albitites required removal of K, Ba and Rb and addition of Na. For some sample pairs, most other elements are close to constant, but for other pairs the isocons are poorly defmed. At 580^C, fluids in equilibrium with the albitites have values of 6.6-9.8 permil, consistent with a largely magmatic derivation. However, schist samples have values of 7.4-11.7 permil, rather low fcr "normal" schists but consistent with with magmatic fluids. It is possible that at least part of the porphyroblast growth in the schists resulted from the destruction of muscovite by infiltrating fluid with a relatively high H^ concentration
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
METAGABBROS AND METATONALITES FROM THE OCEAN-CONTINENT TRANSITION, IBERIA ABYSSAL PLAIN: S YN-RIFT OR HERCYNIAN BASEMENT Mikg Rtib^nagh' and Richard Wysoczanski' • School of Earth Sciences, James Cook University, Townsville, Queensland 4811 ^ Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200
The ocean-continent transition in the Iberia Abyssal Plain region consists of rift blocks of continental crust, followed westward by exposed mantle (the "Peridotite Ridge"). "Normal " oceanic crust, with layer 2 comprised of basaltic rocks, commences at around 300 km west of the Portugese coastline. The nature of the ocean-continent transition and the processes involved in the early Atlantic opening have been investigated in ODP legs 103, 149 and 173 and in sampling by French submersibles. Hole 1067A, ODP Leg 173, penetrated 760 m of Cainozoic sediment followed by 91 m of variably foliated and brecciated amphibolite (metagabbro) with minor metatonalite. Hole 1068 cored 42 m of early Cretaceous talus breccia containing clasts of gabbro, metagabbro, amphibolite, anorthosite, and minor metatonalite. The metagabbro and amphibolite clasts range from amphibolite facies to granulite facies, and some of them show mylonitic microstructures. The breccia, which shows a low-temperature hydrothermal overprint at its base, is in fault contact with serpentinized mantle peridotite, the contact being marked by foliated and hydrothermally altered fault gouge. In accordance with published tectonic models, and shipboard XRF geochemistry which indicated that the mafic rocks have E-type MORB affinities, the shipboard party on Leg 173 concluded that the gabbros and tonalites intruded continental crust during the early stages of the Atlantic rifting. Intrusion into continental crust is fiirthur supported by hornblende geobarometiy on a metatonalite vein and a metatonalite clast, which yielded 4.5 and 6 kbar respectively. Earlier age determinations (conventional U-Pb on zircons and Ar-Ar determinations) appeared to confirm a syn-rift origin for the igneous rocks and their subsequent deformation and metamorphism. However, SHRIMP dating of a metatonalite sample from Hole 1067A yielded an igneous age of 340+/-10 Ma (Hercynian) and a source age of around 595 Ma (Cadomian or Pan-African). It is therefore concluded that the gabbros and tonalites are Hercynian basement and that syn-rift igneous activity was relatively minor during the development of the ocean-continent transition. Likewise, amphibolite to granulite fecies metamorphism and the development of mylonitic metabasic rocks probably mainly took place in the Hercynian, and the detachment faulting producing the continental blocks juxtaposed against mantle was essentially brittle in character.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
A SPECTRUM CONCEPT FOR SAPROLITE Bryan P. Ruxton Applied Science, University of Canberra, Australian Capital Territory 2601
In sedentary soils over crystalline rocks the rate of denudation may be greater than the rate of weathering. The discrepancy defines the degree of weathering of the soil store; fast denudation causing skeletal profiles. Using tolerable erosion in stable landscapes a spectrum of lateral equivalence can be constructed. Equivalence depends on twk where t is thickness, w is percentage weathered and k is the weathering rate constant. Tolerable means where the lowering of the ground surface equals the lowering of the bedrock there is a lateral equivalence of weathering profiles such that: t l w l k l = t2w2k2 = t3w3k3 and 1, 2, 3 are different sites. The series forms a spectrum analogous with the electromagnetic spectrum where frequency and wavelength are comparable with thickness and per cent weathered. The spectrum opens out to duricrusts at one end and to rockslides at the other. The concept of twk equivalence is a special case of the continuity equation. A totally weathered one metre profile is equivalent to a half weathered two metre profile which is equivalent to a quarter weathered four metre profile and so on. When you get up to a thirty second profile you are in the realm of very vigorous mechanical erosion and rockslides as in Taiwan. When you get dovm to twice weathering you are in the duricrust range with cementation. In Papua New Guinea the spectrum can be thought of as tolerable erosion in equilibrium landscapes. The half weathered profile for example is a bulk average value for the whole layer. The base might be only 10 per cent weathered and the top 90 per cent weathered as in many sedentary profiles. There is a sharp break between nearly impermeable rock (grades I, II, III, IV) and previous weathering profiles (grades V, VI). The concept gives simple interpretation of part of the regolith, quantified by measurement of unweathered rock and weathered clay. It applies to many slopes of lower to higher reliefs especially on similar labile rock groups; even shales and mudstone when softening is caused by hydration.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
VOLATILE LEAKAGE FROM ACID DIKE AT 200 TO 500 m DEPTH, HONG KONG, CHINA Bryan Ruxton Faculty of Applied Science, University of Canberra, Australian Capital Territory 2601
A small quartz-porphyry dike 60m wide on an island in western Hong Kong is made up of 24% phenocrysts in a fme grained groundmass containing sanidine. It intrudes porphyritic granite with sharply chilled margins. Thin sections of the selvadge reveal stringers steering the volatiles away into pools of chlorite and soft iron ore. Inwards from the host granite are: 1. Fractured and crushed granite; 2. Fine grained zoisite; 3. Colloidal spherules and collocrysts, and then 2. The margin of the dike material is cryptocrystalline. Greisens and some "boiling" occurred in the region with up to 5% fluorine in a "muscovite" matrix. Upward movement can be stick and slip against rock; rotary dust of collocrysts; or cushioning on gases, in order of greater speed. Viscosity increase due to volatile loss on the walls may have slowed down the dike. Most dikes are in a lenticular envelope; intruded in an active tectonic zone such as a plate margin, and subsequent erosion has removed up to one kilometre from the top of the dike.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
PUTTING SPATIAL DATA INTO AGSO'S GEOSCIENCE DATABASES Rod Rvbum. Ian O'Donnell & Mirek Kucka Spatial Information & Mapping Services, Australian Geological Survey Organisation, GPO Box 378, Canberra City ACT 2601
Until recently, relational databases were for attribute data and spatial data were handled in application-specific data structures tied to CAD, GIS, image processing, and specialist petroleum-exploration and mining software. These systems often maintained parallel relational databases and proprietary spatial structures. Whilst providing good performance, the integrity between attribute and spatial data was often difficult to maintain, and there were generally limits to the amount of data that could be handled by the spatial part. Spatial data could be kept in relational databases, but such systems were slow and lacked facilities for spatial data manipulation and queries. A clear trend has now emerged to integrate spatial data with mainstream corporate data management systems, and the technology to do this is now to hand. GIS is becoming less of a specialist field and more closely tied to general data management. These advances apply not just to 2-dimensional data but to 3 or more dimensions going beyond the bounds of conventional Euclidean space. The technology to handle n-dimensional space that is now widely used in business data-warehousing applications can now be put to use handling the complexities of geochronological, geochemical and geophysical space. This is extremely good news for geoscience, which has never sat very comfortably within the 2-dimensional confines of traditional GIS systems. Now we can expect to see the begirmings of true geoscience information systems that can be applied to space-time chunks of crust in the search for minerals and oil, as well as in the management of natural resources and the environment. For GIS, relational databases and their object-relational successors have some clear advantages over flat files and hierarchical data structures. For example, in geological polygons that are partly fault bounded the fault segment is often digitised twice - once as a geological boundary and once as a fault. This redundancy results in mismatches between the two line segments that leads to lack of closure and consequent area-fill leakage. Redundancy is generally eliminated in relational databases, and one line segment can be shared between many different objects, thus preventing all such problems. With 30 years development and vast resources invested in them, mainstream RDBMS's have flexibility, access control, security and scalability that proprietary GIS systems cannot hope to match. They are also able to maintain performance irrespective of database size. There are currently a number of alternative solutions for integrating spatial and attribute data. In AGSO we plan to experiment with at least two of these - Oracle's 'Spatial Cartridge' and ESRFs 'Spatial Data Engine' (SDE). Oracle's Spatial Cartridge, which is designed to dovetail with the Oracle 8 database engine, allows for very fast indexing of n-dimensional space in a single data type known as an 'HHCODE'. It also adds spatial extensions to the SQL language, thus permitting spatial data manipulations such as polygon overlap. Databases likely to benefit immediately from Spatial Cartridge are the OZROX field database and the OZMAR marine database. OZMAR, which contains ten's of gigabytes of along-ship-track data, is currently AGSO's largest Oracle database. Oracle's Data Cartridge also provides for the automatic partitioning of large tables on a geographic basis when the tables get too big. Performance is thus maintained as the database grows in size. ESRI's SDE is more proprietary, being designed to work with other ESRI products such as Arc/Info and Arc View. SDE uses Oracle for the storage and basic management of it's spatial data, but it does not yet make use of Oracle's Spatial Cartridge solution. AGSO already has a large legacy of Arclnfo-produced datasets, and for the seamless management of the 1:250 000 geology maps of Australia, for example, SDE is likely to provide a more immediate solution than Oracle's Data Cartridge on it's own. Some other GIS products, such as Intergraph's GeoMedia, can also make use of SDE. In the long run, in organisations like AGSO the integration of spatial and attribute data has to be done at the corporate level. Such a solution should be generic and capable of supporting a wide variety of applications in addition to the traditional GIS tools. One can envisage 3-D applications in environmental studies, mining and oil-exploration all sharing the one data-management environment. GIS presentation systems like GeoMedia already work well with either SDE or Data Cartridge, and are also designed for output of spatial information on the World Wide Web. Formida, which supplies tools for the rapid development of multimedia applications on the Web, is already heavily committed to working closely with Oracle's Spatial Cartridge. We look to open, interoperable systems to manage our spatial data.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14'^ Australian Geological Convention, Townsville, July 1998
SEDIMENTOLOGY AND HYDROGEOLOGY OF THE EWINGTON 2 LAKE AND SURROUNDING AREA, COLLIE BASIN, WESTERN AUSTRALIA Krishna K. Sappal, Mohamed EI Tabakh ' and Qadeer Rathur School of Applied Geology, Curtin University of Technology, Perth, Western Australia 6001 ABSTRACT The Ewington 2 Lake is an open cut resulted from coal mining activities of the last fifty years and is located in the Permian Collie Basin of Western Australia. The lake and groundwater are characterised by acidic water which most likely resulted from reactions of groundwaters and sulfur from the coal measures. This study focussed on the geology and hydrology of the Ewington Lake 2 area with respect to enhancement of water quality. The Collie Basin of western Australia is an intracratonic and fault-bounded post-depositional basin containing up to 1200 m of Early to middle Permian sedimentary rocks. The basin covers an area of approximately 230 and extends in the northwest-trending direction and is found within a basement of the Archean gneiss and granite complex of the Yilgarn Craton. The basin has been subdivided into two sub-basins, named as the Cardiff sub-basin to the west and the Premier sub-basin to the east. These sub-basins form two lobe-shaped basins which include early to late Permian coal measures. The sources of water into the lake include rain and surface run-off water, groundwater, and water pumped from the mined area. The color of water is pale blue, with pH values that average 3.5. XRD analysis indicate that sediments are composed of kaolinite, quartz, mica, gibbsite, dolomite, goethite and probably montmorillonite. Four types of sedimentary deposits are defined in the study area including: fluvial, alluvium, colluvium and laterite deposits. Chemistry of the sediments in areas surrounding the lake is given as trace elements of major and minor concern and it is further noticed that the concentration of sulfur increases in areas away from the lake. This is mainly caused by sedimentary and hydrology elements of the study area. Sedimentology analyses based on identification of surface sediments types indicate that the surfacial stratigraphy include basal white and semi-consolidated sand unit with cross bedding and sediments are defined as arenite in composition, and mainly include mature quartz-dominated sand. Middle unite including a laterite layer of up to 1 m thick. This layer is laterally extensive and is found in the southern section of the study area. The upper soil and laterite mix unit which is also up to 1 m thick, composed of loose sand, laterite and organics. Collie basin has a regional groundwater flow system in the Collie Coal Group and is towards the Collie River where groundwater is discharged. There is also a component of vertical flow to deeper aquifers in the basin. These flow systems are defined at the top by the water table and at the bottom by clays of the Stockton Formation or Precambrian rocks. Laterally it is in hydraulic connection with surficial formation and Nakina Formation. Groundwater flow system boundary is at the basin margins and the flow system is unconfined near the surface and confined at depth. These flow systems are defined at the top by the water table and at the bottom by clays of the Stockton Formation or Precambrian rocks. Laterally it is in hydraulic connection with surficial formation and Nakina Formation. Groundwater flow system boundary is at the basin margins and the flow system is unconfined near the surface and confined at depth. Hydrogeology of the Collie Basin reflects the complex nature of stratigraphy and structure of the basin. General groundwater flow system is towards the Collie River to the SW and this flow direction is also reflected in the Ewington 2 Lake area. Groundwater abstraction exceeds the estimated recharge in the basin thus, it appears that the groundwater is being mined. This could result in serious environmental degradation if it is not checked. Groundwater quality in the Collie Basin is variable with low pH and high TDS. This is attributed to the complex hydrogeology which reflects local variations in lithology, topography, land use and depth to watertable. Most groundwaters are of sodium chloride type and are saline in nature.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Comention, Townsville, July 1998
THE CRUSTAL STRUCTURE OF A PASSIVE MARGIN: A TRANSECT ACROSS THE AUSTRALIAN NORTH WEST SHELF Jacques Sayers^ and Clivc Collins^ ^ Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601
Ocean bottom and land seismometers were deployed along a 720 km transect extending from the onshore Kimberley, across the Leveque Shelf, southern Caswell Sub-basin, Scott Plateau and Argo Abyssal Plain. The refraction traverse is coincident with deep reflection seismic lines recorded to 15 seconds two-way-time. This, coupled with gravity data, was modelled to provide a section 950 km long and 70 km in depth. The Kimberley Block onshore has an average crust of 32 km thickness. The upper crust is 14 - 15 km thick with velocities increasing westwards from 5.4 - 6.1 km/s. The onshore Kimberley Basin, with characteristic velocities of 5.4 - 5.7 km/s, thins westwards and is underlain by a layer with higher velocities characteristic of granite. The lower crust has an average thickness of 17 km which thickens to 24 km towards the west. Velocities range from 6.8 - 7.0 km/s, which is characteristic of rocks with a dioritic to gabbroic composition (Goncharov et al., 1997; Sayers, 1998). A major transition from an average crust to a highly thinned crust occurs geographically at the transition from the western edge of the Kimberley Block escarpment to the main Mesozoic Basin. The crust thins rapidly over a 50 km section from 36 - 25 km with the Moho bowing upwards. The area of thinning is coincident with older upper crustal discontinuities; block faulting extends from the Mesozoic Basins into the lower crust and reactivation of older features is demonstrated. The shelf break is also coincident with the western side of the lower crustal thinning. Velocities of 5.8 - 6.0 km/s in the upper crust are probably reflecting granite compositions whilst the lower crustal velocities of 6.6 - 6.9 km/s are indicative of intermediate diorite compositions. A pod at the base of the crust, immediately eastwards of the discontinuities, has a velocity of 7.0 - 7.1 km/s which may be indicative of diorite to gabbro composition. The Scott Plateau is extended continental crust with a thickness of about 22 km. The lower crust is 12 km thick and has velocities of 6.8 - 6.9 km/s which is representative of intermediate diorite composition. The Proterozoic upper crust is highly attenuated, up to 4 km thick. The crust thickens to 23 km under the Wilson Spur and has lower crustal velocities up to 6.5 km/s, implying a less dense continental fragment. The thickness of the oceanic crust of the Argo Abyssal Plain is 12 km, which is anomalously thick and would imply considerable heat flow. Palaeozoic tectonic events clearly shaped the container for Phanerozoic deposition as established from the westward thinning of the Proterozoic across an abrupt 7 km escarpment. Lower crustal extension initiated in the Lower-Middle Jurassic, culminating in the Argo breakup in the Upper Jurassic, was primarily responsible for controlling the development of the margin during the Mesozoic. This is demonstrated by the inverse relation between lower crustal thinning and Mesozoic thickness. The lower crust may have undergone brittle deformation as demonstrated by the presence of possible shear planes interpreted on several deep reflection seismic lines. A deepening of the simple to pure shear transition zone may be a characteristic of passive margins. REFERENCES Goncharov A., Shen-Su Sun & Wybom L. 1997. Balanced petrology of the crust in the Mt Isa region. AGSO Research Newsletter, 26, 13-16. Sayers J. (in press). North West Shelf Ocean Bottom Seismometer Interpretation Report: AGSO Survey 168, lines 301 & 302, Browse Basin. AGSO Record 1998/. Acknowledgments: Published with the permission of the Executive Director, AGSO.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
PALAEOMAGNETIC CONSTRAINTS ON TASMAN TECTONICS P.W. Schmidt', D.A. Clark' and M.A. Lackie^ 'CSIRO Exploration & Mining, PO Box 136, North Ryde, NSW 2121 ^School of Earth Sciences, Macquarie University, NSW 2109
Over the past decade several advances have been made in our understanding of the palaeomagnetism of some of the elements of the Tasman Orogenic System. In 1993 we showed that sedimentary units of the Southern New England Orogen have been remagnetised, and that the remagnetisation occurred before folding in the south but during folding in the north. There is good evidence that only denser, less porous(?), igneous rocks have retained remanences that date from their times of formation. Many of the igneous units are heavily overprinted. Later, in 1994, a study of the Hastings Terrane, now located in an apparently allochthonous position outboard of the subduction complex, showed that it has been rotated 130"" clockwise (or 230 anticlockwise) when compared with coeval Carboniferous magnetizations from regions to the west of the Hasting Terrane. Roberts & Geeve (this volume) build on that study and fiirther show that the Gresford and Rouchel blocks have been rotated 70° anticlockwise. The southern Hastings and Myall blocks have been heavily remagnetised and to date no original magnetisations have been isolated. In 1996 a detailed investigation of the Charters Towers Province provided high precision poles from the Late Silurian to the Early Permian. Regional contact resetting of remanence (a positive contact test) is strong evidence that the Late Silurian magnetisations are primary. A suite of Devonian dykes from the region yields a pole position identical to the Early Devonian Snowy River Volcanics key pole. Early Permian igneous complexes, including the Mt Leyshon complex, yield a high precision well-dated key pole consistent with, though better defmed than, many others of that age from elsewhere in Australia. Furthermore, the Late Silurian pole position agrees with the revised Mereenie Sandstone pole from Central Australia. These results would therefore indicate that these parts of the Tasman Orogenic System have not been grossly rotated with respect to older terranes, unlike parts of the New England Orogen. The results have a bearing on a long-running controversy over the magnetic polarity of Early Palaeozoic rocks from Gondwana. In 1976 McElhinny and Embleton suggested that palaeomagnetic results from the southern Lachlan could be explained by allochthonous terranes. In 1977 Schmidt & Morris showed that the data could be interpreted in terms of autochthonous terranes, if the magnetic polarity of the Early Palaeozoic rocks was reversed. The Late Silurian pole position from the Charters Towers Province, along with the revised Mereenie Sandstone pole, support the autochthonous model. In addition, recent results from other Gondwana fragments are in agreement. Thus there seems to be a first-order difference between the (autochthonous) Lachlan/Thomson Orogen and the (largely allochthonous) New England Orogen. Finally, if palaeomagnetic data are to support the Rodinia concept, then the reverse polarity option implies a similar polarity switch in the Laurentian poles sometime in the Neoproterozoic to Early Palaeozoic.
392
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
LUNULITIFORMBRYOZOANSINLATEEOCENELIMESTONES, ST. VINCENT BASIN, SOUTH AUSTRALIA Rolf Schmidt and Yvonne Bone Department of Geology and Geophysics, The University of Adelaide, South Australia, 5005
A detailed survey of the bryozoan fauna of the Mid to Late Eocene Tortachilla Limestone of the eastern margin of the St. Vincent Basin in South Australia has shown that a common and moderately diverse fauna of lunulitiform (vagrant) bryozoans already existed in southern Australia by this time. This is significantly earlier than the Oligocene age of the first Australian vagrant species recorded in the literature. The discovery of these earlier specimens is significant for several reasons. The Tortachilla Limestone represents the first fully marine Tertiary environment in the St. Vincent Basin (and South Australia in general). It is roughly 2 m thick and can be subdivided into a basal, friable bryozoal limestone which contains a highly diverse fauna, and an upper cemented glauconitic limestone that is less diverse. The bryozoal unit contains very fragmented fossils, whereas the glauconitic unit contains fewerbut more complete specimens. The vagrant bryozoans are predominantly observed in the basal unit, indicating a significant intra-formational change in depositional environments towards the upper unit. Palaeoenvironmentally, vagrants are useful as they comprise several families of the Microporoidea as well as the Cupuladriidae which have convergently developed a distinctive cup-shaped colony growth-form. This is an adaptation for a non-attached, mobile life-style in sandy environments where most other bryozoans either cannot find any appropriate substates for attachment or are burried by the shifting sand. Vagrants, however, are also found in the lower part of the Tortachilla Limestone as just one bryozoan type within a diverse bryozoan assemblage with only minor sand content. Thus, because all bryozoan growth forms are present in significant amounts, it is difficult to relate these communities with modem analogues to determine a likely envu-onment of deposition. Their presence amoung such diversity may indicate that vagrants can also establish themselves in more varied environments. A further explanation for such diversity may be the mixing of sediments from several environments, as the time interval for each bed is unknown. Indeed, modem environments also show such diversity as a result of down-slope slumping. Most of the bryozoan growth forms are preserved in states ranging from excellent to highly abraded and iron stained, indicating some reworking. Specimens of vagrants, however, are mostly observed as moderately well preserved. Considering that their original skeletons are invariably aragonitic, this suggestes that their architecture may partially diminish their susceptibility to early dissolution. Although the underlying South Maslin Sands appear to represent an environment suitable for vagrants, no fossils have yet been found. This may be due to the strong leaching that has occurred towards the top. The upper Mulloowurtie Formation on the westem margin of the St. Vincent Basin on Yorke Peninsula, is the correlative of the upper South Maslin Sands and the Tortachilla Limestone, and may represent a similar environment. In these sandy sediments, the only common bryozoans are nodular pseudovagrants. These live attached by rootlets and thereby can colonise similar environments to the true vagrants. The habitats of vagrants appear to be limited biogeographically to areas where bottom temperatures remain above 10°C, thereby restricting their potential for radiation across abyssal plains. Dispersal via planktonic larvae is a possibility, although life durations of larvae of recent bryozoan species have not been well studied but appear to vary strongly. Australia became fully separated fi-om Antarctica in the latest Eocene, thus initiating the Circum Antarctic Current. Australia and South East Asia did not collide until the Miocene, so it is unlikely that any arrivals occured during this time interval. Research into the bryozoa of the Antarctic Tertiaiy, however, is not extensive and does not appear to give a history of vagrant dispersal.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
THE TECTONOSTRATIGRAPHIC HISTORY OF THE PROTEROZOIC NORTHERN LAWN HILL PLATFORM: AN INTEGRATED INTRACONTIENTAL BASIN ANALYSIS Deborah L. Scott',Barry E. Bradshaw', Chris Z.. Tarlowski' 'Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601
The northern Lawn Hill Platform (NLHP) comprises an area of approximately 16,000 km^ in north central Australia. Thick packages of Proterozoic strata outcrop in the region, although large areas are also covered by younger strata. The packages contain basin sediments and igneous rocks which span '-400My. We present a regional synthesis of approximately 150My of the tectonostratigraphic history of the area. The primary evidence for the tectonostratigraphic history is a sequence stratigraphic re-interpretation of seismic data which links outcrops to the north and south via time lines. Models of subsurface interpretations are tested against observed geopotential data. Newly obtained and existing geochronological, paleomagnetic, geochemical and outcrop and drill core sequence stratigraphic investigations also are integrated into the interpretation. The interpretation and correlations to outcrop made are markedly different than previous proposals. The seismic data image a south-thickening sedimentary megawedge above a sharp acoustic "basement". The internal characteristics of the megawedge are divided into nine supersequences representing distinct basin phases (Scott & Bradshaw, 1997). The packages within the megawedge are correlated to the McNamara Group and the Surprise Creek Formation in the south and the Pickling Group in the north. Characteristic seismic reflection geometries of the basin phases allow predicition of margin geometry changes and subsidence and uplift histories. Distinctive depositional and preserved geometries result in a unified tectonostratigraphic history of the basin. The integration of the seismic and other data allow the separated outcrop belts to be confidently correlated for the first time. The interpretation requires a revision of correlations between the igneous assemblages between the period of ~1760Ma to ITlOMa which comprise the basement underlying the basin phases imaged in the seismic data. The basement interpretation provides new insights into the evolution of the overlying Paleoproterozoic basin phases and models of mineralization within them. Although considered to be deposited on entirely intracontinental lithosphere, the basin stratigraphy records major fluctuations in accommodation space for sedimentation. The geometries of accommodation space vary widely through time. Multiple pulses of deformation are recorded by changing depositional geometries. Investigations in the NLHP have discerned four distinct periods of active deformation. The deformations can be constrained to ~1730Ma, '-1640Ma, ~1595Ma and post-1585Ma. The initial deformation is contained in the basement of the reflective package in the seismic data and is interpreted on the basis of geophysical, geochronological and geochemical data. Interpreted extensional geometries include steep NE-NNE transverse structures segmenting ESE-SE normal bounding faults. Extensional to transtensional 1640Ma geometries are interpreted from the seismic data and include ENE-NE and WNW-NW structures. All fault orientations appear to have syndepositional growth into them, creating a depositional geometry of local deeps which are best developed over NNE 1730Ma structures. The primary trends of the 1595Ma structures are interpreted to result from WNW wrench tectonics. Steep ESE strike slip faults bend or kink over earlier structures into NE splays. The splays create both positive and negative flower structures and rapid changes in thickness in the syn-tectonic depositional geometries. A history of re-activation of a basement template is proposed and styles of reactivation and fault connectivity are presented. Geodynamical mechanisms for accommodation space fluctuations and geometric variability are discussed.
394
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
NORTH AUSTRALIAN PROTEROZOIC INTRACONTINENTAL BASINS: A STORY OF REACTIVATION Deborah L. $cott\ Chris Z. Ta^lo^vski^ Barry E. Braclshaw\ S.E.Martin' 'Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601
The North Australian Basins Resource Evaluation (NABRE) project aims to provide a basin framework and fluid flow model for the Palaeoproterozoic of Northern Australia. Of particular importance in basin analysis is an understanding of the underlying basement architecture, as it can affect the geometry of any subsequent tectonic fluctuation in accommodation space for sediments. The NABRE project area comprises approximately 1/6 of the present Australian continent. Various transformations of geopotential data are presented. Interpretations of lineaments, domains and fabric are provided. A resulting preliminary crustal elements map for die NABRE region is presented in the context of basin systems and resource potential. The central NABRE region can be divided into at least three major crustal elements. The elements are elongate along a NW-SE axis. They are separated by boundaries of tiie same orientation and truncated in tiie east by the N-trending Mt Isa/Kalkadoon-Leichhardt element and in the west by the NE-trending Halls Creek element. The northern element is characterised by a high, relatively random "pitted" gravity signature and a variable magnetic signature which comprises both long and short wavelength, high amplitude anomalies. The boundary (Bl) between tiie northern and central elements is best recorded in tiie gravity data and is herein referred to informally as tiie "Leichhardt Line". Bl is characterised by a steep gravity gradient decreasing from nortii to south, trending NW-SE and stepping en echelon to the NE. The central element is characterised by a generally low gravity signature compartmentalised by slightiy higher elongate gravity signatures that trend NE to ENE. The magnetic signature of the central element is also variable and characterised by long wavelength anomalies. NE trending lineaments dominate, but NW, ENE, E-W and N-S can all be interpreted. The boundary (B2) between tiie central and soutiiem elements also trends NW-SE and steps en echelon to the NE. B2 is less notable in the pseudocolor gravity image but marks a change from a "smooth" character in tiie central element to a "pitted" character in tiie southern element in botii the regional greyscale Bouger gravity and magnetic images. An overall increase in positive magnetic anomalies across B2 into the southern element is clear in tiie psuedocolour Total Magnetic Intensity image. The NE magnetic high lineament trends appear to correspond to tiie jogs in tiie boundaries of the central element. The soutiiem element is characterised by a generally very low gravity signature and widespread positive magnetic anomalies. The southern element is the most complex geophysically. It comprises the eastern part of the Kimberley Block and the Halls Creek belt to the west and the Arunta/Amadeus blocks to tiie south. Limited outcrop restricts geophysical interpretations. At tiie western end of the NW-SE trending elements, a rare opportunity to constrain basement interpretations witii a petroleum reflection seismic data set exists. Interpretation of tiie seismic data is integrated witii tiie regional basement analysis to ascertain the influence of tiie underlying crustal element template. The template is shown to influence the distribution, intensity and geometries of two later deformations. Both later deformations are associated with periods of known mineralization (ie, ~1640Ma and -l595Ma).
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
A MINERAL POTENTIAL STUDY OF PORPHYRY-COPPER-TYPE DEPOSITS IN THE YARROL PROVINCE, CENTRAL QUEENSLAND Margaretha Scott Geological Survey of Queensland, GPO Box 194, Brisbane, Queensland 4001
The increasingly competitive operational environment of exploration organisations has meant that to optimise gains made from investment in geoscientific knowledge (both data and expertise), organisations need to incorporate and manage geological uncertainty within decision processes. Strategies to deal with this issue have been developed and are increasingly being adopted both by industry and government. Advances in computer systems have provided a common mechanism by which major improvements in the handling and interpretation of geoscientific data have and will continue to be made. Notable in recent years has been the release of PC based computer software packages which provide functionality previously only available in 'high-end' computer environments. This factor alone has facilitated the increasing use of a range of geoscientific modelling techniques. Complementing these advances in technology and application has been the development of methodologies that provide: • a formal, systematic approach to the assessment of geoscientific projects • communication of geoscientific knowledge in a way that is useful for decision-makers and incorporates information on risk/uncertainty • linkages for the increasingly important need for effective team-based assessments The ultimate goal of these strategies is to significantly improve the quality of exploration decision-making and mineral resource management. This paper describes the Yarrol Province study, a pilot project with the objective of estimating mineral potential using modem quantitative methods applied to digital data sets of a type provided by Australian State Geological Surveys. Mineral potential of porphyry-copper-type deposits is identified in terms of: • zones favourable for the occurrence of such deposits (favourability modelling). The statistical technique 'weights of evidence' is used in a general PC CIS software environment and programmed in the GIS modelling language 'Avenue'. • estimates of the number of potential undiscovered deposits (probability of occurrence modelling). The approach used involves expert estimation and results of the favourability modelling as well as additional information, such as area spatial limits for porphyry-copper-type deposits and the assignmg of probabilities to anomalies and occurrences. • estimates of the metal endowment within the Yarrol Province (quantitative resource modelling). Monte Carlo simulation is used to produce an estimate of resources in the area consistent with the geological interpretation of the area and the characteristics of known porphyry-copper-type deposits in eastern Australia. The general framework of the study follows the USGS three-part quantitative approach which links the various modelling stages, ensuring internal consistency between the stages and the incorporation of uncertainty within the assessment process.
Acknowledgements: The author would like to acknowledge the contribution made by the Graphical Services Unit of the DME in the preparation of the digital data and of the Airdata Project in the processing of digital geophysical data.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
THERMAL STRUCTURE OF THE CRUST DURING THE 560-520 Ma PETERMANN OROGENY, CENTRAL AUSTRALIA: EVIDENCE FOR A MID CRUSTAL HEAT SOURCE Ian Scrimgeour Northern Territory Geological Survey, P.O. Box 2655, Alice Springs, NT, 0871.
The Peteraiann Orogeny is a major intracratonic obliquely compressional event that affected the Musgrave Block and southwestern Amadeus Basin during the late Neoproterozoic to early Cambrian. The mechanisms by which deformation is localised during intracratonic compression are generally poorly understood, but are likely to be intimately related to the thermal structure of the crust. In the Mann and Petermann Ranges in the northwestern Musgrave Block, a series of south dipping thrusts that were active late in the Petermann Orogeny have juxtaposed domains which underwent peak metamorphism at differing levels within the crust. The ability to examine numerous different crustal levels within the same orogen provides a unique opportunity to gain insights into the thermal structure of the crust during this event. In the Mann Ranges, located along the Northern Territory - South Australian border, Mesoproterozoic granulite facies gneisses and syn- to post-tectonic granites and mafic dykes have been substantially reworked by pervasive mylonitic fabrics that developed during non-coaxial strain at conditions of -12-13 kbars and 7(X)-750®C. Mafic dykes in the Mann Ranges have recrystallised to a sub-eclogite facies garnet - clinopyroxene - hornblende - sodic plagioclase - rutile - quartz assemblage. Peak metamorphism in these deep crustal rocks was followed by nearisothermal decompression to pressures of ~7kbars. In scattered outcrops which extend up to 30 km north of the Mann Ranges, high pressure garnet amphibolite facies assemblages developed at pressures of 9-10 kbars and temperatures of ~700°C. Within this region, anastomosing shear zones, which cut the regional mylonitic fabric, contain abundant leucosomes and garnethomblende-biotite bearing assemblages. A major north-directed structure, the Woodroffe Thrust, juxtaposes these 9-10 kbar rocks against migmatised granites that were metamorphosed at c.630-650'^C at 6-6.5 kbars. This transect from lower crustal to mid crustal levels within the Petermann Orogeny has important implications for the geotherm during this event. The temperatures recorded at different crustal levels suggest that there was little variation in peak temperatures across significant lateral and vertical sections of the mid to lower crust. Although some uncertainties arise due to the fact that the different crustal levels exposed were not necessarily part of identical crustal profiles, it seems clear that the thermal gradient was significantly lower in the deep to mid crust in comparison with the upper crust. The exposure of rocks which were at 630-650°C at 6.5 kbars implies a thermal gradient of ~30°C km"^ in the upper 20 km of the crust, whilst the exposure of rocks at 700750°C at 12-13 kbars suggests a much lower thermal gradient in the order of 5°C km"^ between 20-40 km (the middle of the thickened crust). This suggests that a significant heat source was located within the crust during the Petermann Orogeny, with no evidence for high mantle heat flow. Significantly, in spite of the existence of significant partial melting within the mid crust, there are no granitic or mafic intrusions associated with the Petermann Orogeny as would have been expected if the lower crust had reached elevated temperatures. If the heat source for metamorphism was located in the mid-crust, the absence of any magmatism of this age raises the question of what the nature of this heat source is. In the regions which were the most thermally perturbed during the Petermann Orogeny (20 km depth), there are voluminous Mesoproterozoic granite bodies which had anomalous radiogenic heat production of ~5-7 |LiWm'^ at the time of metamorphism. This is in contrast to the deeper crust, in which the Mesoproterozoic granites and gneisses have relatively low heat production. The correspondence between the most thermally perturbed level in the crust with the layer of anomalously high radiogenic heat production, suggests that this may account for the elevated thermal regime in the mid to upper crust. This is consistent with recent models that suggest that the relatively high heat production within many Proterozoic terrains may be a contributing factor to metamorphism (eg. McLaren et al, 1996). This could also explain the localisation of strain within the Musgrave Block during the Petermann Orogeny, as being the result of thermal weakening of the crust due to an appropriate distribution of crustal heat production. REFERENCES McLaren, S., Sandiford, M., Hand, M., Neumann, N. & Wall, N., 1996. Heat production distributions in Australian Proterozoic terrains: Implications for metamorphic thermal energy budgets and field gradients. In: Evolution of Metamorphic Belts, Geological Society of Australia, Abstracts, 42,69-70.
397
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE REGOLITH AT THE OLIMPIADA GOLD DEPOSIT, ENISEI RANGE, SIBERIA, RUSSIA Nikita B. Sergeev Cooperative Research Centre for Landscape Evolution and Mineral Exploration, CSIRO Exploration and Mining, PMB Wembley, Western Australia 6014
The Northern Enisei district, Central Siberia, has a historical Au production of over 7001 mined, from placers and quartz veins, since 1839. The Olimpiada deposit, discovered as a result of follow-up of a small Sb occurrence in 1979, has reserves of about 6001 Au, at an average grade of 10.8 g/t, within the regolith (Genkin et al, 1994). The deposit is situated in the southeastern segment of the Baikalide geosynclinal belt, in the core and periclinal hinge of the Medvezhinskaya anticline, hosted by Lower Riphean carbonaceous schists. Primary mineralization is associated with quartz-carbonate-muscovite-biotite alteration, with disseminated sulphides and minor quartzcarbonate veinlets. Arsenopyrite and pyrrhotite are the dominant sulphides, with minor stibnite, berthierite, pyrite, chalcopyrite, galena, scheelite and jamesonite. Gold is fme-grained and mainly associated with arsenopyrite and Sb minerals. The deposit straddles a watershed high in the Enisei range, at altitudes of 650 to 700 m. The regolith in the area is commonly only up to 10 m thick, but increases dramatically to 250-280 m thick over the orebody. The weathering profile consists of: brown soil and redeposited weathered products (up to 3 m thick); kaolinitic upper saprolite, coloured yellow to light-brown by Fe oxyhydroxides, (15-50 m); lower saprolite, pale to yellow hydromica with minor kaolinite and goethite (50-120 m) thick; saprock (20-120 m). The most abundant supergene ore-related minerals are Fe, Mn and Sb oxides. Mineralization is outlined by a surface Au anomaly of 200x200m, defined by 100 ppb Au in soil with minor distinct dispersion haloes of As, Sb, W, Cu and Pb. The residual weathered products are enriched in Au, W, Pb, and Cu and depleted in Mn and As relative to primary mineralization. Gold distribution in the regolith is complex, but generally follows that of the subvertical zonation of the primary mineralization. The Au enrichment appears seems to be mainly residual but, nevertheless, there is evidence for some secondary mobilization. Gold in the regolith is fme-grained, with 90% of the Au particles less than 60 ^m in diameter. The majority of Au grains seem to be supergene, formed in situ by oxidation of Au-bearing sulphides. Supergene gold is of high fmeness and occurs as spongy aggregates, and elongate and filiform crystals. Redeposited Au is relatively rare at the deposit, and occurs as globules and platy crystals associated with botryoidal aggregates of tripuhyite (FeSb04). Because of the unusual thickness of the regolith at Olimpiada, some workers have assumed that the clayenrichment is due to low-temperature hydrothermal alteration. However, the chemical and mineral zonation of the profile supports a supergene origin. The location of the deposit within a permeable tectonic structure and relatively high content of easily leached carbonate minerals in the primary rocks caused the development of the specific "pocket" of deep weathering. The data also demonstrate that surface sampling of soil can be effective in exploration in the area. Gold is the most direct indicator of mineralization, with As, Sb, W, Cu and Pb as the most significant pathfinders. REFERENCE Genkin A.D., Lopatin V.A., Savel'ev R.A., Safonov Yu.G., Sergeev N.B., Kerzin A.L., Tsepin A.L, Amshtutz K., Afanas'eva Z.B., Wagner F.& Ivanova G.F. 1994. Gold ores of the Olimpiada deposit (Enisei Range, Siberia). Geology of Ore Deposits, 36, 101 -123.
398
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
PROTEROZOIC ROCK SEQUENCES OF WESTERN TASMANIA David B. Seymour and Clive R. Calver Mineral Resources Tasmania, P.O. Box 56, Rosny Park, Tasmania 7015
Proterozoic rock sequences occupy about one-third of the surface area of Tasmania, dominantly in the western half of the state. They occupy a number of spatially or structurally separate inliers, within and between which are belts of deformed lower Palaeozoic rocks. The largest areas of Proterozoic rocks are the contiguous Tyennan and Adamsfield-Jubilee regions which form a meridional belt in the centre of the state, and the Rocky Cape region which occupies most of the northwestern comer of Tasmania. Several other small fault-bounded inliers occur within the Palaeozoic fold belts. New data on the Tasmanian Proterozoic include detailed airborne geophysical surveys, new geological mapping, and new age controls from U-Pb zircon dating and C and Sr isotope chemostratigraphy. The oldest exposed rocks in Tasmania are interpreted to be Mesoproterozoic in age, based on Rb-Sr model ages of 1100-1150 Ma from metapelite in the Tyennan region, which is dominated by poly deformed and variably metamorphosed (lower greenschist to eclogite facies) quartzarenite, pelite and minor carbonate. The latest interpretation of U-Pb zircon dates from eclogite indicate that peak metamorphism occurred at c. 511 Ma, an event believed to be part of a regional orogenic event (Tyennan Orogeny) at between c. 510-514 Ma, which resulted from a continent-arc collision east of Tasmania and which affected most of the Tasmanian Proterozoic. This event is also believed to have resulted in the structural emplacement of allochthonous Early Cambrian ultramafic and other rock units over much of western Tasmania. In the Tyennan region, faulted juxtaposition of rocks drawn from depths ranging from about 12 km to > 30 km may have resulted from thrusting in the later stages of this event. Weakly metamorphosed, marine shelf-facies sequences of shale, siltstone and mature cross-bedded quartzarenite (Rocky Cape and Clark Groups) form substantial parts of the Rocky Cape and Adamsfield-Jubilee regions, and have been assigned a Mesoproterozoic age based on strong facies similarities with the Tyennan protolith. Independent support for this correlation has come from recent U-Pb age fingerprinting of detrital zircon populations in the quartzarenites. The southeastern part of the Rocky Cape region is transected by the Arthur Lineament, a narrow metamorphic belt formed during the Tyennan Orogeny with grades up to blueschist facies. It contains equivalents of the Rocky Cape Group and of several probable Neoproterozoic units. East of the Arthur Lineament (and partially involved in it), quartzwacke turbidite sequences of the Oonah and Burnie Formations may represent Neoproterozoic precursors to the Cambrian Dundas Trough. The Arthur Lineament also includes the economically important Bo wry Formation, a variably metamorphosed sequence including tholeiitic basalt and mafic intrusives. Recent U-Pb zircon dating indicates that the Bowry Formation predates a 111 ± 7 Ma granitoid. The current radiometric age dataset, including recent U-Pb zircon dates, indicates that the only significant Proterozoic tectonothermal event in Tasmania occurred at c. 760 Ma (Wickham Orogeny). On King Island diis involved polyphase deformation, amphibolite facies metamorphism, and broadly synchronous granitoid emplacement. The Wickham Orogeny appears to have had only a weak effect on the Tasmania mainland, being considered responsible for gentle unconformities below the late Neoproterozoic sequences (see below), and provisionally correlated with the c. Ill Ma granitoid intruding the Bowry Formation. Late Neoproterozoic deposition extended over most of western Tasmania, and comprised two main facies associations, a shallow marine shelf clastic-carbonate-diamictite association and a tholeiitic basalt-volcaniclastic marine rift association. Age constraints on these sequences have been tightened by new C and Sr isotope chemostratigraphic studies, which in the Rocky Cape region date two dolomite sequences, one mid-Cryogenian (c. 700-650 Ma), the other mid-Ediacarian (c. 580 Ma), which sandwich the rift association. Most of the known Proterozoic mineralisation in Tasmania occurs within the Arthur Lineament. The Bowry Formation hosts the Savage River iron ore deposit and large magnesite deposits, silica flour forms residual deposits overlying dolomite, and alluvial gold occurrences are known. The Lineament is also considered to have high potential for Besshi-style massive sulphide deposits. Elsewhere, the extensive Mesoproterozoic and Neoproterozoic marine clastic-carbonate sequences are considered prospective for sediment-hosted Cu, sedex Zn-Pb, and Irish-style carbonate-hosted base-metal deposits. Recent detailed aeromagnetic surveys in the Rocky Cape region have provided further positive indicators for this prospectivity.
399
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
SEGMENTED VARIATIONS IN TECTONIC GEOMORPHOLOGY OF DATONG YANGYUAN FAULT, NW BEIJING, CHINA Cheng Shaoping and Yang Guizhi Institute of Geology, State Seisomological Bureau, Beijing 100029, China
The Datong-Yangyuan fault is the largest normal fault in the late Cenozoic Shanxi rift system. Late Quaternary fault scarps and their along-trending distribution pattern divide the fault zone into two segments, the southwestern segments (SWs) and the northeastern segments (NEs). Between these two segments there are significant variations in mountain front and piedmont slope geomorphology, as well as in the evolutionary history of micro-physiographic stages. Individual segments exhibit different quantitative mountain front geomorphological indices, such as mountain front sinuosity (5), valley width-depth relief ratio {Vj), and stream gradient index {K). The SWs piedmont slope is characterised by the landform assemblage of the youngest alluvial fan A2 (Qs^ + Q4^) which is inset into the older alluvial fan A2 (Qs^), indicating that upwarping occurred on the near-fault side of the older alluvial fan Aj (Qs^) during the late Quaternary. In contrast, the NEs piedmont slope is characterised by the landform assemblage of the youngest alluvial fan Aj (Qs^ + Q4^) onlapped onto the oldest alluvial fan Aj (Qs^), showing that downwarping occurred on the near-fault side of the oldest alluvial fan. Before 36800a B.P., the NEs experienced a history of erosion - aggradation - erosion, while the SWs underwent the stages of aggradation - erosion - aggradation. Rates of fluvial downcutting and faulting as well as fault behaviour and segment boundary geometry seem to be responsible for the geomorphic variability between segments in the fault zone. The SWs has a low fauhing rate relative to the fluvial downcutting rate, which was estimated to be 0.34 mm/a and 0.41 mm/a respectively, with an average displacement per event of 0.9 m and a recurrence interval of 1000 a. The NEs also has a lower faulting rate relative to the fluvial downcutting rate, estimated at 0.99 mm/a and 0.22 mm/a respectively, with an average displacement per event of 3.1 m and a recurrence interval of 1800 a. Segment boundaries are found in areas of overlapping faults between the SWs and the NEs. These areas are characterised by displacement deficits, which have an important influence on the segment geomorphology. Keywords: segmentation, tectonic geomorphology, Datong-Yangyuan fault zone, China
400
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
FLUVIAL TERRACES OF THE JINSHAAN CANYON REACH OF THE YELLOW RIVER AND THE QUATERNARY TECTONIC MOVEMENTS OF THE ORDOS PLATEAU, NORTH CHINA Cheng Shaoping, Den Qidong, Min Wei and Yang Guizhi Institute of Geology, State Seisomological Bureau, Beijing 100029, China
Twenty-one cross-sections have been surveyed and seventeen ESR dates obtained from the fluvial terraces of the Jinshaan Canyon where the Yellow River cuts through the Ordos Plateau. These give a history of regional epeirogenic uplift and local tectonic deformation for the Ordos Plateau during the Quaternary. This paper discusses the relations between the regional epeirogenic uplift, uplift of the Qingzang Plateau, and the effect of base level and climate change. River terraces are landforms that were once constructed and maintained as the active floor of a river but are now abandoned. The tendency of a river to grade its long profile allows reconstruction of ancient river profiles from the remnants of fluvial terraces. Tectonically induced downcutting by streams in an area undergoing general uplift has resulted in a fluvial landscape of high relief Strath terraces are indicative of past tectonic influences and indicate that regional epeirogenic uplift and local deformation has taken place. On the other hand, climatic change will trigger aggradation that forms fill terraces and records brief reversals in long term tectonically induced downcutting. The fill terraces represent time lines in the regional epeirogenic uplift landscape. Six strath terraces have developed along the Jinshaan Canyon of the Yellow River. Their ages are: T^v 1410 ka, Tjy 197 ka, T4t 76 ka, T^^ 44 ka, Tjw 18 ka and T,q 5 ka. The longitudinal profiles of the strath terraces reveal that Quaternary tectonic movements included two components: the regional epeirogenic uplift and the local tectonic deformation. The regional epeirogenic uplift can be divided into five episodes, with an average uplift of 25-46 m each episode. The local tectonic deformation is expressed as either the local differential uplift induced by faulting or the non-uniform uplift, which includes relatively strong uplift and relatively weak uplift and subsidence. The local tectonic deformation is superimposed in the regional epeirogenic uplift. The downcutting induced by the regional by the regional epeirogenic uplift for the Ordos Plateau started after 1410 ka, lagging 250 ka behind the C phase of the Qingzang movement. At 197 ka, uplift rates accelerate for the Qingzang plateau. There are three episodes of aggradation induced by climatic change in the Jinshaan Canyon. They occurred in intervals before 96 ka, between 44-34 ka and 17-9 ka respectively. Regional epeirogenic uplift and periods of aggradation alternated. The aggradation occurred in locations of relatively weak uplift or subsidence and has become time lines in the landscape. The Weihe fault basin acts as the relative base level for the Jinshaan Canyon of the Yellow River. The fault scarps on the Hangcheng fault escarpment zone on the northern margin of the basin record the tectonic fall of base level. The amount of knickpoint retreat upstream from the Hangcheng fault is limited. The geomorphic effect of both the base level fall and the regional epeirogenic uplift is commonly to sculpture a new graded longitudinal profile. Keywords: fluvial terrace. Quaternary tectonic movement, Jinshaan Canyon, Yellow River, Ordos Plateau, North China
401
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Towmville, July 1998
THE CONTRIBUTION OF MIDDLE AND UPPER DEVONIAN ALGAE AND MICROBES TO THE REEF COMPLEXES IN GUILIN, SOUTH CHINA Shen Jian-wei Department of Earth Sciences, The University of Queensland, Brisbane, Queensland 4072
Detailed studies of the volumetric importance of reef framework components are desirable, especially for algae and calcimicrobes. Algae and microbes identified from Devonian to Early Carboniferous carbonate complexes in Guilin include 22 genera, of which 5 or 6 genera are major reef builders. Givetian calcimicrobial frameworks occurring in Guilin are formed mainly by Rothpletzella, Spongiomorpha, Renalcis and synsedimentary cement. Girvanella and cryptalgae occur in back-reef or restricted platform settings. Spongiomorpha generally constitutes biostromes or stromatolites in the platform interior, or joins Rothpletzella in forming stromatolites in the fore-reef slope. Rothpletzella stromatolites, oncoids and Renalcis biocementstones are most common in foreslope facies. Non-skeletal algae-coated bioclasts and spongiostromate oncoids characterize the reef-flat facies of patch reefs. Frasnian algae and microbes in Guilin reef complexes include Parachaetetes, Solenopora, Keega, Diplopora, Renalcis, Rothpletzella, Rivularia, and Ortonella. Reef frameworks contain abundant Parachaetetes, Solenopora, Renalcis and Rothpletzella, which form biocementstones and various oncoids, such as nodular oncoids, oncoid-stromatolites and oncoid-thrombolites. They also contribute to stromatoporoid reefs, receptaculitid-microbial reefs, Smithiphyllum coral bafflestone reefs and microbial mounds. Frasnian algae/microbes generally occur in margin and fore-reef slope or marginal slope facies, as they did during the Givetian. Algae and microbes dominated Famennian reef facies as the major framework contributors. Microbial reef frameworks consist entirely of Epiphyton and Renalcis. These reefs, together with stromatolitic mounds and ooid shoals, constitute massive reef limestones along the Famennian platform margin. Famennian microbes formed rigid reef framework and played roles as binders or were cryptobionts in neptunian dikes. Major extinctions marking Middle and Late Devonian stage boundaries greatly affected Devonian reef types and reef development in Guilin. The end-Givetian biotic crisis affected the major Eifelian and Givetian reef-builders: stromatoporoids and rugose corals. Givetian coral-stromatoporoid reefs were replaced by Frasnian stromatoporoid-coral-calcimicrobial reefs. At the same time, tabular, massive and dome-shaped stromatoporoids were replaced by spherical, branching and irregular forms. Givetian massive rugose corals were mostly absent by the end-Givetian. Algae and microbes migrated to the Frasnian platform margin from the Givetian fore-reef slope and became important in reef construction. The Frasnian/Famennian biotic crisis resulted in extinction of almost all reefal stromatoporoid and rugose coral elements. However, algae and microbes survived the extinction and thrived m the Famennian platform facies soon after the crisis. Excepting the algae/microbes, there were drastic changes in the composition, diversity and guild structure of Famennian reefs as compared to those of the Frasnian. Famennian microbes formed reefs in margin and marginal slope facies, and also occupied the entire carbonate platform, which is characterized by common and well-developed algal/microbial laminites with fenestral fabrics. The algal/microbial species of greatest importance in building Frasnian reefs became the only builders of reefs in the Famennian of South China.
402
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
DIVERSITY AND EXTINCTION PATTERNS OF PERMIAN BRACHIOPODA IN THE ASIAN-WESTERN PACIFIC REGION Shu-Zhong Shen and G. R. Shi School of Ecology & Environment, Deakin University, Rusden Campus, 662 Blackbum Road, Clayton, Victoria 3168, Australia
The end-Permian extinction phenomenon is a widely recognised geological feature. However, details about the possible biogeographical role in regulating the extinction process, intensity and timing remains little known. For instance, we do not know if the global pattern of the end-Permian extinction is consistent with that at realm or provincial level. It is also unclear if the timing of major extinction events or episodes through the Permian is consistent across all the major realms. In this paper, we attempt to answer some of these questions by analysing a databank of 86 families, 411 genera and 1931 species of Brachiopoda from six time intervals of the Permian in the Asian-Western Pacific region. For each taxonomic level (species, genus and family) we calculated two measures of standing diversity, raw diversity, three measures of extinction rates, origination rates and turnover rates. The raw diversity and two measures of standing diversity of the Permian brachiopods in the whole Asian-Western Pacific region show an increase from the Asselian-Tasttibian to the Baigendzhinian-Early Kungurian at generic and specific level, then a minor fall in the Kazanian-Midian, followed by an increase in the Wuchiapingian and a major fall from the Changhsingian to the earliest Triassic Griesbachian. The extinction rates across the Sterlitamakian-Aktastinian, Baigendzhinian-Early Kungurian, Kazanian-Midian and the Changhsingian are more than 50 percent at specific level. However, the extinction rate of the Changhsingian at generic level appears to be the highest among all the stages concerned, seconded by the Kazanian-Midian and Baigendzhinian-Early Kungurian intervals. Our analysis of origination rates demonstrates low origination at specific level for the Asselian-Tastubian, KazanianMidian and Changhsingian intervals and low origination at generic level for the Kazanian-Midian and Changhsingian. The analysis of turnover rates shows a relatively high ttimover rate from the Tastubian to Wuchiapingian at specific level, but a high peak in Changhsingian at generic level. In the Boreal Realm, the raw diversity shows an increase from the Asselian-Tastubian to the BaigendzhinianEarly Kungurian, a major decrease from the Kazanian-Midian to zero in the Wuchiapingian. Similarly, in the Gondwanan Realm the raw diversity exhibits a sharp fall in the Kazanian-Midian and then seemingly a quick recovery in the Wuchiapingian, followed by a deep drop in the Changhsingian and a terminal demise by the earliest Griesbachian. On the other hand, the raw diversity in the Tethyan Realm demonstrates a minor but successive decrease from the Asselian-Tasttibian to Baigendzhinian-Early Kungurian, succeeded by an increase from the Kazanian-Midian to the Changhsingian before a sharp drop towards the end of the Changhsingian.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
LATE PALAEOZOIC BRACHIOPOD FAUNAS OF WESTERN YUNNAN, CHINA: A CASE OF PROVINCIALITY CHANGE INDUCED BY TERRANE RIFTING AND ENHANCED BY CLIMATIC AMELIORATION G R- Shi'. Shu-zhong Shen', Z. Q. Chen', Zong-jie Fang^ and Li-pei Zhan' 'School of Ecology & Environment, Deakin University, Rusden Campus, 662 Blackburn Road, Clayton, Victoria 3168, Australia Nanjing Institute of Geology and Palaeontology, Nanjing 210008, P. R. China 'Chinese Academy of Geological Sciences, Beijing 100037, P. R. China
Wedged between the South China block to the east and the Indian massif to the west, Western Yunnan occupies a critical tectonic position in the long-standing debate over the nature of Palaeo-Tethys during the Late Palaeozoic. Unravelling of the geological fabric of this region has been a focus for many international projects over the past decade. Many have recognised the notion that Western Yunnan was located in northern Gondwana in the early part of the Late Palaeozoic, then drifted northwards and finally docked with Indo-China at about the Permian/Triassic boundary. Accepting this tectonic scenario for Western Yunnan, we attempt to examine if the rift-drift history of Western Yunnan is reflected in the dynamic evolution of the brachiopod provinciality of Western Yunnan through the Late Palaeozoic; if so, how is the tectonic process recorded in the biogeographical signature and would there be any other factors that might have also contributed to the provinciality change. In doing so, we examined brachiopods of Lower Carboniferous, Lower Permian, Middle Permian and Upper Permian fi-om the Baoshan block of Western Yunnan and have also incorporated data fi-om the Tengchong block and previously published literature. Our preliminary results are as follows: (1)
During the Early Carboniferous (Toumaisian and Visean), the brachiopod faunas of Western Yunnan were closest to those of eastern Australia although at the same time strong generic and specific links are also exhibited with West Europe and the Russian Platform;
(2)
No typical Late Carboniferous brachiopods have yet been discovered fi-om Western Yunnan;
(3)
During early Early Permian (?Asselian-Sakmarian), the brachiopods of Western Yunnan from both the Tengchong and Baoshan blocks demonstrate a distinct Gondwanan affinity, evidenced by Callytharrella, Globiella, Punctocyrtella and EHvina;
(4)
The Middle Permian (mostly Early Maokouan) brachiopods are represented by a distinctive mixed Gondwanan-Cathaysian fauna, characterised by such typical Cathaysian genera as Vediproductus, Spinomarginifera, Crenispirfer, and Cryptospirifer intermingled with Gondwanan elements such as Pseudoantiquatonia, ?Callytharrella, and Costatumulus.
(5)
The Late Permian is marked by a typical Cathaysian Wuchiapingian brachiopod fauna, identified especially by Transennatia gratiosa (Waagen; and Spinomarginifera lopingensis (Kayser).
The noted provinciality change of Western Yunnan, namely from a Gondwana-type fauna m the Early Carboniferous and early Early Permian through a mixed Middle Permian Gondwanan/Cathaysian fauna to a Late Permian typical Cathaysian fauna, is comparable with similar changes recognised for the Shan-Thai terrane (Shi & Archbold, 1998). The marked change in marine provinciality can be broadly explained by the 'rift-drift' tectonic scenario recently advanced for the SE Asian region (eg., Metcalfe, 1996), but the effect of climatic amelioration also appears to be required to account for the seemingly rapid biogeographical transition from a characteristic Gondwanan fauna to a mixed (transitional) fauna at the Sakmarian-Artinskian boundary. REFERENCES Metcalfe, I. 1996. Gondwanaland dispersion, Asian accretion and evolution of eastern Tethys. Australian Journal of Earth Sciences 43, 605-623. Shi, G.R. & Archbold, N.W. 1998. Permian marine biogeography of SE Asia. In Biogeography and Geological Evolution of SE Asia, ed. by R. Hall & J.D. Holloway. Backbuys, Amsterdam (In press).
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
LOCALISATION OF MAJOR MAGMATIC SULPHIDE DEPOSITS BY DISCONTINUITIES IN THE LITHOSPHERIC ARCHITECTURE Dr John Simmonds XPLOR Pty Ltd, GPO Box 206, Floreat Forum, Western Australia 6014
As technological minerals explorationists, the fundamental assumption to our business is that mineralisation is an integral aspect of the thermal evolution of the Earth and that therefore, the location and quality of mineralised provinces should be predictable from the broader context of lithospheric structure and evolution. Our vision must therefore be that we should have the ability to confidently target and to rank metallogenetic provinces throughout the globe. This ability will, of course, depend on the availability of appropriate large-scale datasets. The genesis of major magmatic Ni-Cu sulphide deposits is associated with regions of anomalously-high magma flux within the crust. This relationship to intense magma flux reflects, at a more fimdamental level, the thermal input into the system: the second law of thermodynamics dictates that the size of an ore-forming system will be proportional to the energy input into that system. In addition, the more focused is the energy input, the greater is the potential to focus the accumulation of sulphide mineralisation into a smaller volume and hence, to generate a major ore deposit. The key criteria for generation of these magmatic deposits are therefore the amount of heat energy available and the mechanisms for focusing that energy. The magma flux criteria operates at all levels, from global down to camp and discrete deposit scales. At the largest scale, major Ni-Cu sulphide deposits appear to be associated with massive decompression melting generated at the sites of major mantle upwellings and the interaction of the upwelling with the base of the lithosphere. Evidence for these upwellings is found in the characteristic intrusive ± extrusive magmatic provinces generated within the crustal section. At smaller scales, the deposits are associated with discrete sections of the most active magma conduits that connect the regions of partial melting with the near-surface to surface environments. At a detailed level, localisation of sulphide mineralisation is related heavily to factors that affect the physico-chemical parameters and hence sulphide carrying capacity of the host melt. Tectonically, the majority, if not all of the major post-Archaean nickel sulphides are closely associated with the margins of Archaean-age cratonic blocks. This association reflects a more general one, in that much postArchaean magmatism and intracratonic rifting has also been focused along these margins. It is proposed that this localisation reflects variation in the lithospheric stratigraphy and composition between the older cratonic domains and younger, flanking regions. It is the discontinuities between these lithospheric domains that have the potential to focus or generate regions of mantle upwelling. Thus, the secular variations in initial composition and structure of the lithosphere act as a primary constraint on the potential of the discontmuity to focus mantle upwellings and magmatism. However, subsequent tectonic and post-cratonisation modification of the lithosphere have a major potential to significantly change the architecture of the discontinuity. While identification of craton margins becomes a key objective in the targeting for giant nickel sulphide deposits of post-Archaean age, there is considerable debate in both the geological and geophysical literature over what actually constitutes a craton edge. This debate introduces significant uncertainty into the mapping of zones of potential. While we can predict that certain features, such as tectonic intersections, edge inflections and craton boundaries will act to focus magma flow from mantle partial melt zones, there is value in attempting to integrate the most recent data from deep geophysical studies with our empirical knowledge of ore deposit and igneous province distribution. Because of subsequent erosion and terrane fragmentation, the foci of most major magmatic events are not generally obvious. In order to assess a given craton margin for its potential to host major magmatic Ni-Cu sulphide deposits within a melt conduit, it is appropriate to analyse the full spectrum of available geological, geophysical and geochronological evidence to in an attempt to unravel the lithospheric settmg at the time of magmatism. Acknowledgments must be made to WMC Resources Ltd for support of these researches in the USA and Australia during 1996 and 1997.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
LATTICE TEXTURES IN VEIN DEPOSITS AND GEOTHERMAL SYSTEMS IN NEW ZEALAND: CLUES TO UNDERSTANDING BOILING PROCESSES IN EPITHERMAL ENVIRONMENTS Stuart E Simmons and Patrick R. L. Browne Geothermal Institute and Geology Department, University of Auckland, Private Bag 92019, Auckland, New Zealand
Platy calcite occurs as scale deposits in geothermal wells; the oblique orientation of calcite crystals to one another forms an open framework having a lattice texture. Platy calcite, and more commonly its quartz pseudomorph, are also a feature of many ore-bearing epithermal veins. In this case, moulds and casts of platy calcite replaced by quartz form the lattice texture Platy calcite that occur as scales in geothermal wells form crystals that are generally <1 mm long. They form at depths from 500 to 1000 m below the ground surface where ascending hydrothermal solutions begin to flash. The zone of calcite deposition extends upwards for approximately 100 to 300 m above this level. Typically, these calcite scales contain no other minerals. Platy calcite crystals also occur in the openings of back pressure plates, directly upstream from sites where gold and silver deposit in geothermal pipes. Here again, the calcite deposits at the site of first boiling along a cooling flow path. In the Coromandel District, calcite and quartz exhibiting lattice textures line cavities in epithermal veins, such as those at Golden Cross where they occur along the host rock contacts or irregularly in the centers of veins. Crystals range in size from < Inmi to >50 mm across. Lattice textures extend continuously in a single vein for at least 10 m vertical distance. At Martha Hill, lattice textures occur in many of the sub-vertical veins (veins extend 500 m vertical interval); quartz lattice textures dominate in the upper part of veins, whereas calcite lattice textures occur in the deeper parts of veins. In order to assess, the relationship between calcite and gold deposition due to boiling, we modelled several reaction paths using the program CHILLER; the composition of the pre-boiled liquid is based on the deep geothermal fluid at Broadlands-Ohaaki. The results indicate the following overlapping sequence of mineral deposition with decreasing temperature (300-100° C): a) calcite+quartz>K-feldspar>gold and b) calcite+Kfeldspar>gold>amorphous sihca. Except for quartz and K-feldspar, these patterns match the depositional sequence found in geothermal wells. Note that calcite supersaturation only exists in the vicinity of the site of first boiling. Downstream, loss of CO2 makes the residual liquid greatly undersaturated in calcite. We speculate that in a newly opened channel within a mature hydrothermal system, a descending boiling front deposits platy calcite behind it. Once a condition of quasi-steady state flow is established, calcite continues to deposit near the site of first boiling, while earlier deposited calcite, lying above (and downstream of) the boiling front, is dissolved in the cooling residual liquid and replaced by sihca. Thus lattice textures of quartz can be ascribed to single boiling event associated with dilation of a vein structure, with the boiling front descending significantly deeper than the site of precious metal deposition. Once the flow of hydrothermal solutions through the channel subside, most likely in response to sealing, the boiling zone dissipates and ultimately ceases to exist. Repeated cycles of dialation-mineral deposition result in overlapping mineral sequences produced by numerous boiling events. In this way, quartz, adularia, platy calcite, and precious metals that normally deposit along a boiling path extending >500 m occur together in a hand-sized sample.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
EARLY TO MIDDLE DEVONIAN SHEAR ZONES IN THE CANN VALLEY REGION, EASTERN VICTORIA Carol J. Simpson^ and Christopher L. Fergusson^ ^Department of Earth Sciences, Monash University, Clayton, Victoria 3168 ^School of Geosciences, University of WoUongong, Wollongong, NSW 2522
Recent mapping of the Craigie 1:100 000 Geological Sheet for the Geological Survey of Victoria has reevaluated the significance of a branching system of shear zones developed within Early Devonian granitic rocks. The oldest rocks in the Cann Valley region of east Gippsland are a monotonous succession of quartz-rich turbidites (Early Ordovidan Pinnak Sandstone) interspersed with fault-bounded wedges of black siliceous shale and mudstone (late Gisbomian to Eastonian Warbisco Shale). The Ordovidan succession has been intruded by biotite granite and hornblende granodiorite of the I-type Early Devonian Bega Batholith. The Ordovidan strata and Devonian granites are un(nQn)conformably overlain by the Late Devonian Combyingbar Formation which is a thick sequence of mainly fluvial sandstone, conglomerate and red mudstone. Two major deformational events have affected the Palaeozoic succession in the Cann Valley region. The first event produced tight upright folds and cleavage in the Ordovician rocks southeast of the Combienbar Fault, two steeply dipping schistosities in the Ordovician Kuark Metamorphics northwest of the Combienbar Fault and major ductile/brittle shear zones in some of the Bega BathoUth plutons. The occurrence of foliated plutons adjacent to non-foliated plutons within one of the shear zones suggests that the ductile phase of the deformation was synchronous with early granite emplacement. Age data to support this is lacking and the first deformational event can therefore only be broadly constrained as post-Late Ordovician and pre-emplacement of some of the Bega Batholith plutons. The second deformational event post-dated deposition of the Late Devonian Combyingbar Formation. Regional east-west compression produced a northeast-trending zone of en-echelon north-trending synclines indicative of dextral movement on a basement fault. Gentie folds developed in the overlying Combyingbar Formation and reactivation of faults on the syncline margins contributed to the preservation of the down-faulted Late Devonian rocks. The ductile/brittle shear zones developed during the first deformational event are assodated with a system of branching north- and northeast-trending faults. Deformation of some of the granites as they were intruded produced zones up to 1.5 km wide of steeply dipping, north-trending foliation and variable, but mainly vertical, down-dip lineations and the development of more intense northeast-trending and some north-trending shear zones containing mylonitic rocks. In both the northeast- and north-trending shear zones the strike of the mylonitic foliation is northeast. Zones of mylonites range from a few metres to >200 m wide and are characterised by elongate mafic enclaves, ribbons of recrystallised quartz, shear bands, S/C fabrics and zones of very fine-grained ultramylonitic rocks usually 10-20 cm wide. The major northeasterly Fiddlers Green Shear Zone has undergone variable movement; predominantly vertical stretching involving southeast-side-up and northwest-side-down at the southwest end near Buldah, to oblique dextral strike-slip movement near the NSW border. Based on the presence of gendy plunging lineations in the mylonitic rocks and shear sense criteria, the major north-trending Caim Valley Shear Zone has undergone mainly dextral strike-sHp movement. The minimal amoimt of observed offset on granite boundaries within the shear zones in the Caim Valley region is in marked contrast with the reported 24 km of dextral strike-slip offset on the Burragate Fault immediately north of the NSW border. This disparity is consistent with faulting synchronous with plutonism such that some early granites are offset while other granites may have been emplaced into voids created during strike-slip movement along faults and hence show no displacement. The model proposed here involves intrusion of some of the Bega Batholith plutons into a regional compressional stress regime which produced a series of zones of north-trending foliation and mainly vertical stretching lineations in the granites during cooling. The compression direction is broadly east-west and was resolved into east-west shortening involving west-over-east thrusting. On a regional scale tiiis produced a system of northwest-trending (eg Berridale Fault) and northeasttrending (eg Burragate Fault) strike-slip faults. In the Cann Valley region, northwest-trending sinistral faults are poorly developed and most of the deformation has occurred on the northeast-trending portion of the Fiddlers Green-Caim Valley Shear Zone. The regional strike-slip faulting formed northeast-trending mylonitic fabrics (akin to C planes) by deformation of the north-trending fohation (akin to S planes). Continued deformation during the final stages of cooling of the granites produced brittie deformation structures including gouge zones andbrecdas. Acknowledgment: The Geological Survey of Victoria is thanked for their permission to publish this abstract.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
EVIDENCE FOR MID-PALEOZOIC EXOTIC TERRANES IN THE YARROL PROVINCE, CENTRAL QUEENSLAND Glenn A. Simpson. Paul R. Blake, Cecil G. Murray, Mark A. Hayward and Barry G. Fordham Geological Survey of Queensland, GPO Box 194, Brisbane, Queensland 4001
Recent studies by the Yarrol Project Team of the Geological Survey of Queensland have revealed new evidence which suggests that there are three separate mid-Paleozoic terranes in the Yarrol Province which have different compositions, age ranges, and stratigraphic successions. All of these assemblages were formed in close proximity to volcanic centres and all are entirely composed of marine sedimentary and volcanic rocks. The Yarrol Province forms part of the strato-tectonic unit known as the New England Orogen. Previous workers have considered the oldest component of the Yarrol Province to be a single tectonic unit named the Calliope Volcanic Assemblage. Regional geological mapping by the Yarrol Team has led to the subdivision of the Calliope Volcanic Assemblage into three stratigraphic assemblages. The Upper Silurian to Middle Devonian Mount Morgan Stratigraphic Assemblage includes the Capella Creek Group, Craigilee beds, and Marble Waterhole beds (part of the former Kroombit beds). The Mount Holly beds comprise a stratigraphic assemblage interpreted to have formed during the Lower to Middle Devonian. The Calliope beds constitute a stratigraphic assemblage which includes an autochthonous Ordovician limestone bed and a limestone bearing conglomerate which contains clasts bearing Ordovician and Middle Devonian conodonts. Airborne radiometric and magnetic signatures and field mapping observations reveal that rocks of these stratigraphic assemblages differ in overall composition and contain sequences which are difficult to correlate. There is no stratigraphic contact between the three assemblages. The differences between the three stratigraphic assemblages are interpreted to be a consequence of the assemblages being derived from different sources. Indeed, it would seem that the lack of any stratigraphic contact between these assemblages and their differing compositions supports the interpretation that each stratigraphic assemblage represents an exotic terrane emplaced either by accretion at a subduction zone or through transform slip movement. Field mapping also revealed evidence for a marine setting for volcanism and sedimentation in the Mount Morgan Assemblage and these rocks are intruded by the Mount Morgan Tonalite, previously interpreted as an arc related intrusion with similar chemistry to plutonic rocks on New Britain. None of the stratigraphic assemblages contain any evidence of subaerial sedimentation. Furthermore, conglomerates in the Calliope beds have allochthonous limestone clasts which have produced Ordovician and Middle Devonian conodonts prompting comparisons with the Gamilaroi Terrane in New South Wales. The Gamilaroi Terrane contains extrusive volcanics and volcaniclastic sediments which, based on provenance studies and geochemistry, have been previously interpreted as being formed in an exotic intra-oceanic island arc setting. However, not all evidence points towards an exotic terrane model. There is a lack of any major change in style of volcanism across the Middle Devonian Mount Morgan unconformity in contradiction to the change expected between different terranes. There is also local quartz bearmg sediments in the Mount Morgan Stratigraphic Assemblage and the Mount Holly beds, which could be interpreted to be more in keeping with a continental derivation than that of an oceanic island arc. Re-evaluation of the new evidence at hand has led the Yarrol Team to consider new models for the development of these mid-Paleozoic stratigraphic assemblages.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
CHARACTERISTICS OF SUBAQUEOUS BASALTIC ANDESITE FIRE FOUNTAIN DEPOSITS: AN EXAMPLE FROM THE MOUNT WINDSOR VOLCANICS, NORTHERN QUEENSLAND, AUSTRALIA KirstieA. Simpson^ and Jocelyn McPhie^ ^Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001
The Cambro-Ordovician Mount Windsor Volcanics (MWV) are located south of Charters Towers in northern Queensland, Australia. They extend approximately 160 km east-west, and comprise four regionally mappable formations, two of which (the Mt. Windsor and Trooper Creek Formations) are submarine volcanic successions that host several economically significant massive sulphide deposits and prospects. Recent detailed facies mapping in parts of the belt has resulted in the identification of extremely well preserved volcanic textures in a wide variety of facies types and compositions. Of particular interest is a distinctive coarse basaltic andesite breccia interpreted to be a subaqueous fire fountain deposit. Such deposits are poorly documented and understood, although they have been identified in ancient subaqueous volcanic successions in Japan, Sweden, Canada and elsewhere in Australia and on the modem seafloor. The subaqueous fire fountain deposit identified in the MWV is characterised by fiuidally shaped, highly vesicular clasts ranging in size from 2 cm to 10 cm. Vesicles in the fluidal clasts are largest in the cores and become smaller towards the margins, with non-vesicular, formerly glassy, quenched rims. The fluidal clasts strongly resemble volcanic bombs. They are set in a matrix composed of <2 cm non-vesicular, formerly glassy clasts that are dominantly angular or splintery in shape and identical in composition to the bombs. The texture of the matrix suggests it comprises finely quench fragmented debris generated synchronously with the bombs. Internally the unit is massive, -lOOm thick, varying only in the ratio of hyaloclastite matrix to bombs. The bomb-rich breccia facies was generated by weakly explosive fountaining of low viscosity magma in a subaqueous setting. The considerable thickness of the unit probably accumulated within hours to days, reflecting a high magma discharge rate as is typical of fountaining in subaerial settings. The bombs resulted from tearing apart of magma ribbons jetted upward from the vent. Contact with cold seawater quenched the outer margins of the bombs while the interiors were insulated and able to retain sufficient heat to allow for continued vesiculation prior to deposition on the seafloor. The hyaloclastite matrix formed as a by-product of fountaining, through a combination of two main processes: 1. Initial quenching of the hot bombs by cold seawater resulted in partial or complete quench fragmentation, depending on the size of the bombs. The glassy margins of large bombs were completely fragmented while the interiors remained intact. Small bombs were completely quenched prior to vesiculation and disintegrated. 2. Fragmentation due to bomb to bomb impacts or impact with the seafloor. Fragile quenched glassy rims were preferentially spalled off on impact, resulting in accumulation of fine, angular glassy fragments. Both fragmentation modes can produce abundant fine, angular, glassy particles which would settle out onto the sea floor infilling spaces between the larger bombs. The poorly sorted, massive nature of the unit indicates that the hyaloclastite matrix accumulated simultaneously with the bombs, and that there were no significant breaks nor fluctuations in the discharge rate. Recognition of bomb-rich breccias attributable to submarine fountaining is important in volcanic facies analysis because such deposits on the modern seafloor only occur very close (<10's m) to source vents. The distinctive association of vesicular, fluidal, large clasts with non-vesicular, angular, formerly glassy finer clasts is diagnostic of submarine fountain deposits and serves to distinguish them from the subaerial counterpart. Presently there is considerable debate on the range of water depths in which fire fountain eruptions may occur. Consequently these deposits tell us little about the water depth at which they formed.
Acknowledgements: Support for this research was provided by RGC Exploration.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
THE GAP AN UNUSUAL BORNITE-RICH VHMS ORE BODY, MYRA FALLS DISTRICT, BC CANADA B.J. Sinclair & J.B. Gemmell CODES University of Tasmania, PO Box 252-79C, Sandy Bay, Tasmania, Australia 7004
The Gap ore body forms part of the Myra Falls VHMS deposit on Vancouver Island, Canada. The Myra Falls camp is operated by Westmin Resources Limited and currently consists of two active underground mines, H-W and Battle. The current proven and probable ore reserve for the district is 12.3 million tonnes grading 7.8% zinc, 1.8% copper, 1.9g/t gold and 40.4g/t silver. The Myra Falls deposits are hosted by the Devonian andesitic Price formation and the overlying Myra formation of the Sicker Group. The Myra formation is composed of rhyolitic to basaltic rocks with lesser sedimentary units. Massive sulphide deposits occur within felsic volcanic units of the Myra formation. The Gap ore body is situated near the top of the H-W Rhyolite, the lowest unit of the Myra Formation. The ore is hosted within a quartz-feldspar porphyritic rhyolite body and surrounded by sericite-quartz shear zones with minor sphalerite l± pyrite, chalcopyrite, tennantite, galena) stringers. The ore is overlain by hanging wall H-W Andesite and underlain by a pyrite-sericite stringer zone hosted within altered volcaniclastics of the H-W Rhyolite. The Gap ore body has a pipe shaped cross section 20-30m wide by 40-50m high, and is approximately 250m long. It is located 2(X)m north-west of the currently mined Battle main massive sulphide lens. The Gap has a current proven and probable reserve of 783,300 tonnes grading 12.5% zinc, 1.7% copper, 3.3g/t gold, and 118.2g/t silver. It is distinguished from other ore lenses within the Myra Falls district by the minerals bomite, digenite and colusite (CuaCAs, Sn, V)S4) and by its high precious metal content. The Gap ore body also contains sphalerite, pyrite, chalcopyrite, tennantite and galena. A classical VHMS mineral zonation from footwall pyrite-sericite stringer zone through pyritic massive sulphide, chalcopyrite-rich ore, sphalerite-chalcopyrite-rich ore to a sphalerite-galena-barite-rich top is observed in the Gap ore body. This zonation has been overprinted by later copper-rich solutions through the core of the ore body forming a bomite-digenite-rich spine. Evidence for overprinting includes reaction rims of bomite replacing chalcopyrite, bomite replacing sphalerite along fractures and grain boundaries, and bomite surrounding and invading pyrite grains along fractures and as embayments. The digenite occurs as grain boundary inclusions and fracture fill of the bomite, and as perthitic textured intergrowths between digenite and galena surrounded by bomite. The colusite is concentrated at the top of the bomite-digenite-rich spine as inclusions in sphalerite and bomite. The Gap ore has been interpreted as facies and permeability control replacement style mineralisation by Allen (1993). Abundant relict pumiceous mass flow textures within the mineralization and sulphides replacing pumice is evidence for the replacenmt style origin (Allen, 1993). At this stage it is not clear if the mineralisation is replacing volcaniclastic sediments within the quartz-feldspar porphyritic rhyolite or the porphyritic rhyolite unit itself. Bomite in VHMS deposits is not uncommon, however it is usually only an accessory phase. When observed bomite forms as a late stage replacement of chalcopyrite (± pyrite). Bomite-rich ores have been documented at Kidd Creek , Canada; Neves Corvo, Portugal and Mt Lyell, Australia. The Gap ore body places the Myra Falls VHMS district into a select class of VHMS deposits which have significant bomite-rich ores.
REFERENCES Allen R.L., 1993. Volcanic Facies analysis of massive sulphide deposits in British Columbia: Preliminary results from field work August-September 1993. Report to MDRU, University of British Columbia. Unpublished.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
CONVERGENT- / TRANSFORM-MARGIN TECTONIC TRANSITIONS ALONG THE PALAEO-PACIFIC GONDWANA RIM: IMPLICATIONS OF MAGMA GEOCHEMISTRY FOR TECTONIC PROCESS Warwick J. Sivell Division of Earth Sciences, The University of New England, Armidale 2351
As well as the broad magmatic cycle related to Late Carboniferous subduction transitional to Early Permian extension in the Connors-Camboon Province, models for the Early Permian tectonic evolution of Gympie Province in the northern New England Orogen (NEO) must explain the development of the (in part coeval) Gympie Island Arc, and an hiatus in magmatism within the arc which also marks a transition in chemical compositions of erupted magmas from predominantly island arc tholeiites (Highbury Volcanics) to high-K andesites (Rammutt Formation) which cannot be related by fractionation alone. In addition, the formation of a proximal Early Permian back-arc basin in an extensional (trans-tensional) regime behind the arc (and a brief phase of effectively bimodal volcanism within the arc), and in particular, the distinctive OIB-like isotopic and geochemical signatures of some of the back-arc basin basalts, require explanation. Accretion of the arc to the continent with the resumption of (reversed polarity) subduction, syn- and post-collisional magmatism and goldmineralisation are other considerations. Geochemical features of mantle-derived magmas generated during these events have implications for mantle geodynamics and the nature of sequential tectonomagmatic processes accompanying the development, translation and docking of the resulting terranes of Gympie Province. The following tectonomagmatic sequence of events is recognised: initiation of the Gympie Island Arc with eruption of island arc tholeiites (lAT) on oceanic basement, either nearby or distal to the Gondwana rim with its well-established tripartite morphology of continental margin Camboon Volcanic Arc, Yarrol Forearc and subduction complex above a west-dipping Benioff zone; rafting of the island arc toward the continent upon closure of a marginal basin to the rear of the arc (probably involving west-east subduction of Carboniferous seafloor (Amamoor Beds)); eruption of chrome diopside megacryst-bearing ankaramites and bimodal basaltdacite volcanics in the immature island arc in an extensional regime which ultimately led to a (short-lived) cessation of subduction-related magmatism; synchronous development of a nascent Early Permian back-arc basin (Cedarton Volcanics-Cambroon Beds) in which both incompatible element enriched OIB-like basalts and lAT, and later MORB-like magmas were emplaced. [In the supra-subduction zone mantle wedge beneath the evolving back-arc basin proximal to the Gympie Island Arc, sequential mixing between an isotopically enriched mantle end-member (similar to enriched (E-) MORB or ocean island basalt-source mantle), subduction-modified mantle and depleted N-MORB-source mantle is inferred on the basis of basalt geochemistry, Nd-isotope systematics and field relations (Sivell & McCulloch 1998)]; resumption of subduction, probably from east to west, the Benioff zone having stepped-out oceanward of the Gympie arc, with eruption of mainly high-K andesites requiring different source components to the earlier LAT (particularly subducted sediment or partial melts of accretionary complex); accretion of the arc to the continental rim and intrusion of the more alkaline (Langton) dolerite sill sourced in sub-continental lithospheric mantle; post-collisional intrusion of dolerite and shoshonite dykes derived by different degrees of melting of refractory asthenospheric mantle intensely metasomatised by fluids and diorite partial melts of stalled lithospheric slabs or subducting oceanic lithosphere. What relationships do these events show to regional crustal evolution and convergent margin tectonics at the palaeo-Pacific Gondwana rim? One of the consequences of seafloor spreading is the subduction of ridgetransform fault (RTF) triple junctions. A short-lived Early Permian transition from convergent to transform tectonism along the northern NEO portion of the Gondwana margin could have resulted from ridge-trench interaction in the north, with subduction of a dextral transform fault leading to southward propagation of an FFT-triple junction. Cessation of volcanism in the Gympie Island Arc and the emplacement of OIB-like lavas within an extensional basin that developed behind the arc may reflect incremental expansion of a slab-free window beneath the region. Slab window magmatism in this distinctive tectonomagmatic setting would have been triggered by decompression melting of upwelling mantle akin to that proposed for Cenozoic volcanism in coastal California. Other trans-tensional basins oceanward of the Camboon Volcanic Arc (e.g. Grantleigh Trough) may have had a similar origin. REFERENCES Sivell W.J. & McCulloch M.T. 1998. Geochemistry and Sm-Nd isotope systematics of Early Permian basalts from Gympie Province and fault basins in southeast Queensland: implications for mantle sources in a back-arc setting at the Gondwana rim. Geological Society of Australia Inc. Special Publication 19, 148-160.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
PETROGENETIC AND GEOCHEMICAL EVOLUTION OF THE GYMPDE ISLAND ARC AND ITS ACCRETION TO THE GONDWANA RIM Warwick J. Sivell Division of Earth Sciences, The University of New England, Armidale 2351
Geochemical analyses for 60 stratigraphically well-constrained mafic-felsic igneous rock samples from extensive (>13 km) drillcore that penetrates the entire (2km thick) Late Palaeozoic volcanic-intrusive sequence of the Gympie Group (including important gold-mineralised units and at least 7 sequentially emplaced, chemically distinct magma suites) permit the detailed petrogenetic evolution and tectonomagmatic history of part of the Gympie Island Arc to be determined, as well as its relation to proximal Early Permian back-arc basin eruptives to the west and the palaeo-Pacific Gondwana rim. The stratigraphically lowermost volcanics of the Gympie Group comprise the Highbury Volcanics. MORB-like geochemical affinities (e.g. Zr/Nb=30, TiA^=30) of the earliest-erupted Alma Member basalts - island arc tholeiites (lAT) interlayered with hyaloclastite and chert - imply an asthenospheric mantle wedge source not yet strongly depleted by extraction of arc-magmas, but significantly modified by addition of soluble LILE from slab-derived fluids. Overlying Tozer Member basalts show more pronounced depletion in HFSE (Zr/Nb~45; TiA^=20) and greater slab additions (Pb/Ce>0.2, La/Nb=8-15, Ba/Zr up to 9). These magmas have low Nb (<2 ppm), Zr (40-70 ppm) and Si02, with high Mg# and Ca0/A1203 indicating rapid ascent with httle fractionation. Systematic variation in trace element ratios within overlying Mary Member lAT (e.g. Zr/Y ratios increasing from 2.5-3.5) heralds imminent fundamental change in magma generating processes. The Mary basalts are more fractionated than the earlier emplaced magmas (higher Zr (70-90 ppm), Nb (up to 4 ppm), and A1203; lower Ti/Zr, Mg# and Ca0/A1203). Mary ankaramites possess clinopyroxene megacrysts with broad unzoned cores that require total equilibrium crystallisation leading to minor dacitic residual liquids (effectively bimodal volcanism). These lost most of their mafic phenocrysts and underwent significant plagioclase crystallisation in near-surface magma chambers. Ankaramite production, limited involvement of more enriched mantle source components, greater proportion of highly differentiated eruptives (diminished rate of magma generation) and ultimately an hiatus in volcanic activity, reflect a period of rapid extension with ensuing sedimentation. This is probably linked to coeval development of a nascent back-arc basin (Cambroon Beds-Cedarton Volcanics) with both OIB and lAT type basalts) in a trans-tensional regime behind the Gympie arc, with mixing between enriched OIB-source mantle and subduction-modified mantle. Volcanism resumed with the eruption of (non-bimodal) high-K andesites of the Rammutt Formation. These felsic lavas and pyroclastics show fundamental geochemical differences to, and are incompatible with fractionation from the underlying lAT. They represent evolved magmas with a significant sediment component (subducted sediment or partial melts of accretionary complex) (increasing NbA!', ZrA"(>5), ThA^b, LaAf, K/P, Nb, Rb, Zr, A1203 and Si02; decreasing FeO/MgO, La/Nb, Zr/Nb, P/Nd (despite high P) and Sc). The felsic nature of the bulk of the eruptives may in part be due to thicker crust of the rejuvenated arc acting as a density filter for more basic magmas. High Ti in (parental) basaltic andesite dykes suggests the presence of some enriched mantle components, possibly from sub-continental lithospheric mantle (SCLM) due to imminent accretion of the arc to the Gondwana rim, following closure of Carboniferous and Early Permian back-arc basins. Distinctly continental affinities of the multiphase Langton Dolerite sill owe their origin to the involvement of an enriched mantle source in their genesis, probably SCLM beneath the now accreted arc. These rocks are compositionally quite distinct from OIB-like (asthenospheric) mantle melts in tiie proximal back-arc basin, and are displaced from OIB-MORB mixing trends, with higher Y/Nb, Zr/Nb and La/Nb than OIB. Postcollisional partial melting of the SCLM may be due to heat from asthenospheric convection in the mantle wedge, or triggered by subduction fluids lowering the solidus. Lowest Ca0/A1203 in Langton samples that represent the smallest degrees of partial melting suggests substantial CPX in the mantle residue after partial melting. Suites of hornblende-bearing shoshonites (with Mg#=70-75 and Cr>10(X) ppm indicating near-primary melts) and coeval or earlier-formed dolerite dykes appear to be related by partial melting of very depleted (asthenospheric) mantle that was metasomatised and oxidised by addition of (solidus-lowering) slab-related fluids as well as dioritic partial melts (increased Zr/Nb, Ba/Zr, constant Y/Nb; Zr/Y=4.5-9) sourced either in stalled lithospheric slabs from back-arc basin closure or at depth in the subducting slab itself. Extensive partial melting which gave rise to these basic dykes may postdate the Langton, and bears an intimate relation to gold mineralisation. The shoshonites were generated by smaller degrees of melting in the waning (gold-mineralising) hydrothermal regime. High Ca0/A1203 (0.7) and Cr/Ni in the basic dykes may indicate an absence of CPX in the refractory asthenospheric mantle residue of extensive melting.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE HOLOCENE-LATE PLEISTOCENE SEDIMENTARY RECORD OF MYALL LAKES, NSW. Greg Skilbeck'. Ed Frankel', Adrian Cramp^ and Jane Tribble^ ' Department of Environmental Sciences, University of Technology, Sydney, Australia. ^ Marine Geosciences Research Group, Department of Earth Sciences, University of Wales (Cardiff)» UK. ^ School of Ocean and Earth Sciences and Technology (SOEST), University of Hawaii, Hawaii, USA.
The Myall Lakes system of the central NSW coast of Australia (latitude 32®30' S) contains a potentially continuous epiclastic and biogenic sequence that may be as old as 50,000 years. The main lake lies within 2.5 km of the Tasman Sea from which it is separated by a sandy Pleistocene dune system (Melville, 1984) overlying irregular Carboniferous basement. A direct connection with the sea is located approximately 30 km to the southwest of the study area. Despite this connection, and its proximity to the sea. Myall Lake contains virtually fresh water (2-3 ppt) and has no current tidal or wave current influences. The maximum water depth approaches 5m. This situation is unique along the NSW coast where all other lake systems are either directly or periodically open to the sea and contain widespread reworked sand deposits. We have collected 26 cores ranging from 0.5 to 6.0 m in length across Myall Lake in mid 1997. Although detailed analysis has not yet commenced, preliminary investigation suggests that in many, the last Pleistocene sea level low-stand (-12,000 y.a.), recognised by red and orange oxidation staining, has been intersected. Seven major lithologies are present in the cores. Along the southern shores of the lake quartz-rich sand representing the distal edges of the dune system dominates. Around the northern shores of the lake irregularly distributed lithic sand, clearly derived from the Carboniferous basement, predominates. In the central parts of the lake organic facies ("gyttja" - olive-yellow/green amorphous organic matter (AOM); sapropel - dark brown to black silty clay, AOM and disseminated plant material; peat - brown to dark brownfriableplant debris), silty clay with disseminated sand, and a possible palaeosol unit, have been identified. Lithostratigraphic units were established mainly on colour and the layers occur in a sequence that can be correlated across large areas of the lake. Variation in grain size (determined by laser particle size analysis), geochemical, and other data all correspond closely with the colour variation and confirm the initial stratigraphic subdividion of the sampled sequence. Thin quartz sand layers and shell beds are irregularly present within the finer grained facies, but are not sufficiently widespread to permit finer-scale correlation within the lithostratigraphic units. Apart from root growth, there is little evidence for in situ reworking of the sediment, and so we believe that there is potential for an undisturbed sedimentation record to be present. High-resolution (10cm spacing) magnetic susceptibility profiles have been determined on a selection of cores to date and correlation of these profiles suggests that a more detailed resolution of the stratigraphy is possible. This in turn supports the hypothesis that the sequence, at least in the central parts of the lake, is essentially undisturbed. AMS '"^C dating is currently being undertaken on the designated type section in order to date the profile. Twenty-two carbon analyses have been carried out at a down-hole spacing of 10 cm on Core ML-19. Total carbon ranges between 1.16 and 36.45% and variation corresponds well with the visible lithostratigraphy. Apart from the few visible shells present, the majority of the carbon is organic (range 1.12-31.12% TOC). We have recently commenced ^^C of these sediments and ^^O analyses will commence once a suitable diagenetic silicate phase is identified. The total thickness of the unconsolidated sediment column is as yet unknown. A single-channel seismic survey carried out concurrently with coring was unable to resolve basement because of the dampening effect of abundant vegetation within the lake. We intend to undertake a GPR survey later this year in order to establish basement level. If the lake sediments are undisturbed as we suspect, they offer a unique opportunity to reconstruct a continuous sea-level and palaeoclimate history for temperate coastal eastern Australia that may stretch back into the Late Pleistocene. REFERENCES Melville, G., 1984. Headlands and Offshore Islands as dominant controlling factors during Late Quaternary Barrier Formation in the Foster-Tuncurry Area, New South Wales, Australia. Sedimentary Geology 39, 243-271
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE FUTURE FOR GEOSCIENCE Brian Skinner Department of Geological Sciences, Yale University
The 20th Century has seen geological discoveries of such extraordinary magnitude that we might well ask whether issues of similar magnitude remam to be addressed in the 21st Century. I will argue that there are many problems that offer challenges that equal or exceed those of the past, and that as a result the earth sciences will be a growth industry in the century ahead. Two obvious scientific challenges facing us concern space and time. The space challenge arises from our inability to directly observe the largest bulk portion of the earth, the mantle. The dynamics of the mantle in response to thermal inhomogeneities and mass flow directly determine what happens to the crust, yet most aspects of the mantle remain conjectural. The time challenge involves the first half of the earth's history. Our understanding of the interactive evolution of the four major parts of the earth system, the biosphere, atmosphere, hydrosphere and lithosphere, is still at a very rudimentary stage. We will not fully understand the scene today until we understand how the present pattern came to be. Understanding the earth's evolution must proceed hand in hand with the continuous monitoring of the present-day earth system. Challenges arising from the still-growing human population become continually more numerous and more difficult to address. These are problems such as the inhomogeneous distribution of potable water, the eventual loss of soil fertility as a result of increasingly intensive agricultural practises, and the changing chemical makeup of the earth's surface as a result of the redistribution of materials mined, processed and eventually discarded. Many of these human induced problems are as much social as they are scientific, but geological issues underlie them all and as a result it seems inevitable that people with geological training will be involved in their eventual resolution. Our profession already faces an internal split in which the producers of raw materials sometimes fmd themselves at odds with those whose job it is to monitor. It is essential that we not allow this split to become any larger and that we present a united front. If we fail in our efforts we may well finish as scapegoats.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
INTER-ARRAY AND INTRA-ARRAY KINEMATICS OF EN ECHELON VEIN SYSTEMS John V. Smith School of Resource Science and Management, Southern Cross University, PO Box 157, Lismore , NSW 2480
En echelon vein array systems are crucial to understanding the ways in which brittle and ductile deformation of rocks develops dilatant pathways through which fluid can flow. The development of both interconnected spaces within individual vein arrays and the linking of numerous arrays must be considered. The bulk deformation defined by the combination of orientation, sense of shear and displacement magnitude of each array constitutes the inter-array kinematics of the vein system. The fracturing and rotation of rock elements within a vein array constitutes the intra-array kinematics. To illustrate kinematic interpretations of vein array systems two examples from eastern Australia are discussed. MERRIMBULA GROUP, SOUTHEAST NEW SOUTH WALES En echelon arrays of quartz veins are well developed in the Devonian Merrimbula Group near Merimbula, south coast New South Wales. The orientation, sense of shear and displacement magnitude of approximately 30 vein arrays at Short Point Merimbula indicate an inter-array kinematic pattern of flats, ramps and backthrusts within an incipient thrust fault system comprising three parallel west-dipping thrusts. The distribution of arrays and array types has been strongly influenced by the complex anisotropy of bedding and cross-bedding of the fluvial sandstones. The geometry of veins within individual arrays indicates intra-array kinematics of extension fracture and buckling of wall-rock bridges between fractures. Measurements of the amount of vein material and wall-rock bridges in the centre of arrays is compared to the predictions of the 'bridge buckling model' for sigmoidal veins. The comparison shows that some bridges have been thinned by the development of local pressure-solution cleavage. The morphology of veins has been strongly influenced by the angular relationship between fractures, arrays and bedding/cross-bedding anisotropy. BROKEN RIVER PROVINCE, NORTH QUEENSLAND En echelon arrays of calcite veins are well developed in the Silurian Jack Limestone of the Broken River Province, north Queensland (Smith, 1997). Important geometric parameters used to characterise these vein systems are (1) the angle between veins and their host array and (2) the angle between conjugate pairs of arrays. The vein-array angle has been found to typically average 35^, a value consistent with dilatancy. The angle between conjugate pairs of arrays has been found to typically average 40^, a value also consistent with dilatancy. Combining these two values the typical conjugate vein array system would have a conjugate angle of 40° and the veins of each array would be within a few degrees of being parallel with the trend of the opposing array. Such a configuration has been called 'convergent' since the trends of veins from each array converge toward the acute bisector of the arrays. Most analyses of the formation of en echelon vein systems have been founded on the supposition that the individual veins, are parallel to the acute bisector of the arrays and therefore parallel to the principal compressive stress. Convergent vein arrays pose a conceptual problem since the individual veins are oblique to the inferred bulk principal compressive stress of the array system yet conmionly the veins have morphological characteristics of extension fractures. The geometry of veins in individual arrays indicates brittle fracture without bending of wall-rock bridges. Many of the veins contain elements which appear to be faults sub-parallel to arrays. It is inferred that these faults initiated first and the orientation of opened extension fractures reflects the incremental strain pattern resulting from slip on the faults. This kinematic model explains the origin of convergent extensional veins in the conjugate arrays. The inter-array kinematics comprises conjugate systems of arrays produced by layer-normal compression. The degree of dilatancy, as indicated by conjugate angles, varies in a manner consistent with the predictions of the distribution of strain in neutral surface folding. The orientation of the principal extensional strain is horizontal at the top and base of the limestone but sub-vertical in the centre of the limestone adjacent to the central clastic lagoonal facies. This is interpreted to indicate extrusion of the clastic facies from between the limestone members in a direction controlled by the three-dimensional distribution of facies rather than by the regional strain pattern. REFERENCE Smith, J.V. 1997. Initiation of convergent extension fracture vein arrays by displacement of discontinuous fault segments. Journal of Structural Geology, 19, 1369-1373.
415
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998 KINEMATIC AND RHEOLOGICAL INTERPRETATIONS OF MAGMATIC TEXTURES John V. Smith School of Resource Science and Management, Southern Cross University, PO Box 157, Lismore , NSW 2480
The presence of flow features such as banding and aligned crystals in magmatic rock textures is well known. Other more complex textural assemblages are also commonly recorded and their interpretation has implications for magma flow kinematics and rheology. The development of banding by the stretching of magma is corroborated by analysis of associated microstructures. Analysis of textures also records evidence of timedependent rheological behaviours, principally shear thickening in the crystal mush prior to solidification. These interpretations are substantiated by examples of dyke and lava textures from eastern Australia. Application of the techniques to plutonic textures is discussed together with description of a newly discovered strain marker in plutonic rocks - folded magma interfaces in composite magmas. TRACHYTE DYKES, ERASER ISLAND, QUEENSLAND Multiple trachyte dykes exposed near Waddy Point on Eraser Island contain a flow texture comprising dykeparallel banding and crystal alignment. The flow texture also includes groundmass shear zones which are oblique to banding and overprint the crystal alignment fabric. The shear zones formed during the last stages of solidification as deformation changed from being distributed throughout the material to being localised in shear zones. The presence of large phenocrysts (up to 6mm) of feldspar disrupts the shear zone fabric by causing shear zones to anastomose and preserves relict textures in 'strain shadows'. Detailed three-dimensional analysis of the texture indicates that the shear zones formed as conjugate pairs of structures with opposite shear senses. The dihedral angle between the conjugate shear zones is 63^ which is consistent with the interpretation that crystalrich lavas experience dilatancy during flow. Dilatancy is commonly associated with shear thickening rheology indicating that time-dependent effects would have inhibited magma flow rates during the last stages of magma flow (Smith, 1997). BASALT LAVAS, NORTHEAST NEW SOUTH WALES A survey of textures in basalt of the Tertiary Lamington Volcanics, northeast New South Wales reveals the common occurrence of textures comprising distinct structural domains. Two domainal types are described 1) groundmass shear zones and 2) sub-spherical clusters of groundmass crystals (n=~100). Groundmass shear zones record the localisation of deformation within a homogeneously distributed crystal alignment texture and are mainly associated with intense shear flows near the base of lavas. Crystals within clusters are commonly aligned suggesting that the clustered configuration overprints a pre-existing homogenous crystal alignment texture. The groundmass crystal clusters are interpreted as resulting from either 1) melt segregation, 2) degassing or 3) clustering during flow as predicted by recent published numerical models of flowing suspensions. Published numerical models record a relationship between particle cluster formation and the onset of shear thickening rheology. COMPOSITE PLUTONIC ROCKS, STEWART ISLAND, NEW ZEALAND Composite mafic-intermediate-felsic plutonic rocks of the Anglem Complex Stewart Island New Zealand have been interpreted as the result of magma mingling. The interfaces between contrasting magma types have foldlike morphology. Interfaces perpendicular to magmatic layering and foliation have fold forms with high amplitude/low wavelength whereas interfaces parallel to magmatic layering are smooth or have low amplitude/high wavelength fold forms. These fold forms record the layer-perpendicular strain (approximately 50%) which occurred at the end of the magma mingling process. Image processing analysis of the magmatic fabrics shows alignment which is not as intense as expected from the strain recorded by interface folds, mechanical interactions of crystals and the growth of space-filling grains impair the application of fabric analysis. Comparison with strain recorded by features such as interface folds promises the possibility of calibrating fabric analyses. REFERENCE Smith, J.V. 1997. Shear thickening dilatancy in crystal-rich flows. Journal of Volcanology and Geothermal Research, 79, 1-8.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
PALAEOZOIC MAFIC MAGMATISM AND MANTLE-DERIVED MELTS IN THE EVOLUTION OF THE SOUTHERN LACHLAN FOLD BELT Alvar Soesoo & Ian NichQllg Department of Earth Sciences, Monash University,Clayton, VIC. 3168, Australia
There are a number of geological and geochemical observations and theoretical constraints which strongly suggest involvement of mafic, mantle-related melts in continental felsic magma generation and emplacement. Many granitic rocks contain dark fine-grained enclaves, typically more mafic in composition than their hosts. These enclaves are widely believed to reflect at least mechanical interaction (mingling) and probably also chemical hybridisation (mixing) of basaltic to andesitic magmas with host granitic magmas, and they provide supporting evidence that mafic magma influx fi-om the mantle provides heat needed for crustal melting. Many granitic suites within the Lachlan Fold Beh (LFB) show variation in isotopic signatures compatible with an origin involving contrasted crustal and mantle source components. Experimental studies have shown that granitic magmas with compositions similar to LFB S-type granites can be generated by interaction between high-alumina olivine tholeiitic magma and alimiinous pelitic sediment. Studied Devonian basaltic to andesitic dykes (sometimes occurring as swarms) and compositionally equivalent plutons of the southern Lachlan Fold Beh (in the Mt. Stirling-Mt. Buller, Snowy Bluff, Tabberabbera and Tambo Valley areas of eastern Victoria) are often temporally and spatially closely associated with large granitic complexes. Geological and geochemical evidence indicates that mingling/mixing between mafic-intermediate magmas and dominant felsic magmas has produced abundant microgranitoid enclaves, partially homogenised hybrid zones and/or composite dykes. Most basaltic-andesitic rocks of the Early to Middle Devonian occurrences (Mt. Stirling-Mt. Buller dioritic-gabbroic stocks and Tambo Valley high-Mg andesitic dykes) show low Ti levels and trace-element abundance pattems with significant negative Nb anomalies, similar to those of rocks of continental margins related to current or recent subduction. However, some Tambo dykes demonstrate MORBlike trace-element signatures. By contrast, the Late Devonian Snowy Bluff basalts, emplaced as part of a bimodal basalt-rhyolite suite associated with coarse terrestrial sediments indicative of crustal extension, have relatively high Ti contents and lack significant Nb anomalies, show "intra-plate" characteristics and define the other end d the observed spectrum of major and trace element characteristics. A group of Tambo Valley dykes, and the Middle Devonian dioritic dykes and stocks of the Tabberabbera area, show intermediate characteristics. Geochemical distinctions between rocks of the studied complexes are most evident when they are plotted on a VTi diagram. The Mt. Stirlmg-Mt Buller rocks and Tambo Valley high-Mg dykes plot in an "arc-tholeiite" field while other Tambo Valley rocks occupy the MORB and back-arc basm basaltic fields. Tabberabbera rocks lie between the fields of alkali and MORB/back-arc basin rocks with one exception plotting in the MORB/back-arc basin rock field. This sample also shows other geochemical similarities to MORB. The Snowy Bluff basalts demonstrate chemical characteristics which are typical of ocean-island and alkali basalts. Recent models of magma generation for LFB granitic complexes, involving contributions fi-om mantle-derived magmas, are supported by Sr, Nd and Pb isotopic studies. Fractional crystallisation of a mantle magma to produce an I-type granitic end-member will be exemplified by the Mt Buller igneous complex. This includes plutonic rock types rangingfi*omgabbros and norites through quartz-diorites and granodiorites to I-type granites and leucogranites. The complex defines linear geochemical trends on Harker diagrams. Gabbros, diorites, granites and mafic enclaves within the granites show consistent initial ^^Sr/^^Sr ratios, 0.7037-0.7062 and ENd values +5.6 - +2.6, which suggest a close genetic linkage. This interpretation is supported by the results of thermodynamic modelling (using COMAGMAT & MELTS software), which suggest that there were at least two stages of fi-actionation, one at intermediate pressure, probably in the deep crust, and a second at shallow emplacement depths. We argue that fi^ctionation of mantle-derived melts is primarily responsible for geochemical variation within the Mt Buller suite and that this mechanism may play a more important role in the evolution of Lachlan Fold Beh I-type granitic suites than previously recognised. Negative Nb and Ti anomalies are consistent with emplacement of the Mt Buller complex within an Early to Middle Devonian tectonic environment influenced by subduction, possibly related to a double subduction system in the western Lachlan Fold Belt. Following closure of an oceanic basin in the Middle Devonian, the sinking of an "inverted U" shaped slab caused it to detach from the overlying crust. This detachment would have triggered inflow of sub-continental asthenospheric mantle which then underwent decompressional melting. Some pieces of the sinking slab may have remained attached to the overlying crust and undergone restricted partial melting. The progressive replacement of "mantle wedge" magma sources by "asthenospheric" sources may explam the observed transition fi-om magmatism with "subduction-related" geochemical signatures to magmatism with "intraplate" characteristics.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
MINERAL SYSTEM PREDICTION USING A CHRONOSTRATIGRAPHIC BASIN FRAMEWORK: AN ESSENTIAL DATASET FOR REDUCING EXPLORATION RISK IN NORTHERN AUSTRALIA. Peter N Southgate^ Barry E Bradshaw\ Jan Domagala^, Mart Idnurm^ M. Jim Jackson^ Andrew A. Krassay\ John F. Lindsay\ Rod Page^ Terry Sami^ Deborah Scott' ' Australian Geological Survey Organisation, PO Box 378, Canberra, ACT 2601. ^ Queensland Department of Mines and Energy, GPO Box 194, Brisbane 4001 ^Department of Geological Sciences, Queen's University, Kingston, Ontario K7L 3N6 Canada
Current base metal exploration models are based on interpretations of near-surface strata made from geological maps, geochemical databases and potential field datasets. A growing interest now exists in predicting exploration targets beneath shallow cover. Traditional exploration strategies are unable to accurately predict such targets. Unlike the petroleum industry where seismic data and well logs provide insight into basin shape, stratigraphic architecture, regional fluid migration pathways and fluid flow events, the mineral exploration industry rarely has chronostratigraphic information available in a form suitable for its inclusion in regional exploration programs. In consequence, identifying prospective areas under shallow cover and predicting where metal bearing fluids were focussed during migration is difficult. One of the objectives of the North Australian Basins Resource Evaluation project (NABRE) is to provide industry with the tools, techniques and methodolgies necessary to reduce exploration risk when exploring beneath shallow cover. The project is integrating geophysical (seismic and potential field) datasets with outcrop-derived information (measured sections, drill core and gamma ray logs), SHRIMP zircon and Apparent Polar Wander Path data to provide a time series structural and sequence stratigraphic framework for the Palaeoproterozoic of northern Australia. Based on chronostratigraphic methods of correlation, spanning the interval 1780Ma -1580Ma rocks of the Mt Isa - McArthur region can be repackaged into two Superbasins: the Leichhardt and Isa Superbasins. Leichhardt Superbasin rocks include the Eastern Creek Volcanics, Haslingden Group sediments and their correlatives. Isa Superbasin rocks, the focus of this talk, include the Bigie Formation, Fier\^ Creek Volcanics, Mt Isa and McNamara Group rocks and their correlatives across northern Australia. Rocks of the Isa Superbasin span a period of -150 My. and are grouped into 9 supersequences. Each supersequence represents a major phase of basin evolution. Unconformity surfaces, or their correlative conformities, bound each supersequence. With the exception of the lowermost supersequence, which remains poorly defined, each supersequence has a shortened name reflecting the lithostratigraphic unit that hosts its maximum flooding surface. Sediment accumulation in the Isa Superbasin conmienced with the Big Supersequence and comprises the fluvial (conglomeratic) Bigie Formation, possibly part of the Quilalar Formation and the bimodal Fiery Creek Volcanics (FCV). A local unconformity forms the base of the overlying, shallow marine, clastic-dominated Prize Supersequence. A major igneous event between -1680 Ma and 1671 Ma (Carters Bore Rhyolite and Sybella Granite) led to the formation of a 25 My depositional hiatus, incision and an unconformity above the Prize Supersequence. Marine carbonates of the Lower McNamara Group form a -1500 m thick southward thickening ramp within the Gun and Loretta Supersequences. The Upper McNamara Group contains up to 7000 m of marginal to deep marine clastics deposited in the final 50-60 My of the Isa Superbasin. Five Supersequences are present: the River, Term, Lawn, Wide, and Doom Supersequences. The sequence stratigraphic correlations and associated tectonic events synthesised in the Isa Superbasin permit construction of a first pass conceptual regional model depicting stratigraphic and structural architectural styles along a NW-SE transect from the Murphy Inlier to the Kalkadoon Leichhardt Block for the period 1580 Ma1725 Ma. This architectural model can be used as a template for addressing the following questions: 1. Which parts of the stratigraphy host the source rocks for base metals? 2. When and at what depth of burial or temperature did the basinal brine(s) become enriched in base metals? 3. When did brine expulsion occur and where in the stratigraphy did the fertile brines reside prior to expulsion? 4. Which pathways did the metal-bearing brines migrate along? 5. What are the relationships between metal bearing brine and organic matter at the site(s) of metal precipitation? These questions and possible mineral exploration implications related to the architectural stratigraphic model will be addressed during the talk.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
COPPER AND GOLD: THE ECONOMIC BACKGROUND Malcolm Southwood J.B. Were & Son Umited, Aldermary House, 10/15 Queen Street, London, EC4N ITX
Ultimately, many of the delegates at this conference depend on the prices of copper and gold for their livelihoods. Historically, copper and gold prices have moved in response to geological, technological, political, economic and speculative influences. Such factors can shape both the supply and demand sides of the market equation, but this paper concentrates mainly on supply-side issues. It attempts to demonstrate how geological, mining and metallurgical developments in particular have influenced the cost of producing metal, and the extent to which the changes in production costs are in turn reflected in long term prices. At present, the mining and metals industry is enduring a cyclical downturn. Lx)w metal prices threaten not only the activity of exploration companies, but also the operations of many higher cost producers. Prices will recover, but are we seeing a longer term structural change in the cost/price relationships?
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
TECTONIC SIGNIFICANCE OF OCEANIC CRUSTAL SLICES AND INTERMEDIATE F METAMORPfflSM IN THE WESTERN LACHLAN FOLD BELT, VICTORL\. Catherine V. Spaggiari'. David R. Gray' & David A. Foster^ Australian Geodynamics Cooperative Research Centre ^Department. Earth Sciences, Monash University, Melbourne VIC. 3168, Australia, ST-cath@earth.monash.edu.au ^Department of Earth Sciences, La Trobe University, Melbourne VIC. 3083, Australia.
Within the turbidite-dominated western Lachlan Fold Belt of Victoria, ?Cambrian meta-igneous "greenstone" belts of largely mafic to ultramafic composition occur at the boundaries of major structural zones. Determining the significance and nature of these belts has important implications for the tectonic history of the westem Lachlan Fold Belt. In both the Heathcote Belt and the Howqua River area of the Mount Wellington Belt, underplated and accreted slices of the upper stratigraphy of an oceanic crust occur in association with either black mud-matrix or serpentinite matrix melange. At Howqua, the oceanic succession is a basal layer of strongly deformed and serpentinised greenstone melange, followed by less strongly foliated, MORB pillowed metabasalts which includes fault slivers of pelagic chert, which pass up into a section of bedded chert and siliceous black shale, overlain by turbidites. Strongly sheared, veined and boudinaged black mud matrix melange with chaotic blocks of greenstone and turbidites are likely to be decollement related. These are similar to melanges of subduction accretionary complexes, such as the Franciscan of California, where exotic blocks of high or intermediate pressure mineral assemblages occur within melange. Isolated blocks juxtaposed with rocks showing different metamorphic assemblages have been found at both Heathcote and Howqua. Pods of coherent, "bluecoloured" rocks occur surrounded by strongly foliated and fragmented greenstones with minor chert in fault contact with strongly deformed Ordovician turbidites at Red Hill, Heathcote. The coherent rocks consist of the assemblage Ca-Na amphibole, albite, stilpnomelane, Mg-Cr spinel, chlorite and epidote. Electron microprobe analyses show the amphibole to be winchite, which is transitional between glaucophane and actinolite. Optically, the amphibole is blue-green to lavender blue. Geobarometry using this assemblage indicates pressures up to 6-7 kb, which translates to a depth of around 18 km. This is the approximate depth of an inferred detachment zone (Gray et al, 1991). Intermediate pressures (up to 4 kb) are also indicated by bo cell parameter and illite crystallinity data of phengitic micas in the turbidites (Ofiler et al. in press). As the turbidites would occur at a structurally higher level these data supports the 6-7 kb of the underlying meta-igneous rocks. At Heathcote, Ar-Ar geochronology has shown that low grade metamorphism and deformation occurred at the basal level at '«458Ma. Reactivation related to thrusting and shallow level emplacement occurred at --420Ma. In the Mount Wellington Fault Zone deeper level deformation occurred at -410Ma and later reactivation related to shallow level emplacement was at ~390Ma. Intermediate pressure metamorphism, the timing of deformation, ongoing sedimentation during deformation, and structural relationships (melange formation) can be explained in terms of subduction-accretion processes. Initial deformation occurs as oceanic crust is subducted and the top layer is effectively "peeled off" and accreted to the base of the accretionaiy complex, (e.g., Kimura & Ludden, 1995). Intermediate pressure metamorphism suggests accretion at the base of the accretionaiy complex, rather than the toe. The accreted layer is subsequently duplexed, thrusted and stacked, in parts preserving the uppermost oceanic stratigraphy and allowing exposure of basal level rocks at the siuface. This model, and the data which supports it gives emphasis to the tectonic significance of these two "greenstone" belts. They are interpreted as remnants cf oceanic crust preserved in major fault boundaries, due to the closure of a small ocean basin. REFERENCES Gray D.R., Wilson C.J.L., Barton T.J. 1991. Intracrustal detachments and implications for crustal evolution within the Lachlan Fold Belt, southeastern Australia. Geology, 19, 574-577. Kimura G. & Ludden J. 1995. Peeling oceanic crust in subduction zones. Geology, 23, 217-220. Offler R., McKnight S., Morand V. 1998. Tectono-thermal history of the westem Lachlan fold belt, Australia insights from white mica studies. Journal of Metamorphic Geology (In press).
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
THE PETROLOGY AND PGE MINERALISATION OF THE GREENHILLS COMPLEX, SOUTHLAND, NEW ZEALAND Carl Spandler, John Mavrogenes\ Steve Eggins and Richard Arculus GEMOC, Department of Geology, Australian National University Canberra ACT 0200 ' also at Research School of Earth Sciences, ANU Canberra ACT 0200
The Greenhills Complex is a relatively small (14km^) ultramafic to mafic layered intrusion, located on the Bluff Peninsula, 30km south of Invercargill in the South Island of New Zealand. It was emplaced into the Greenhills Group sediments during the Early Permian, causing localised homblende-homfels metamorphism. Most of the complex is made up of a series of concentrically arranged cumulate rocks (the layered series), ranging from dunite and olivine clinopyroxenite in the outer shells, to gabbroic rocks in the core. This sequence is enclosed by a thin gabbroic ring dyke and is cut by numerous dykes and small intrusive bodies. On the basis of whole-rock and mineral chemistry, together with calculated high oxygen fiigacities (FMQ 0 to FMQ +3), we suggest the layered series was formed by fractional crystallisation of a wet, primitive magma, in an island-arc setting. Mineral chemistry trends were constructed through the layered series and were used to infer the injection of several magma pulses into the chamber during development of the cumulate pile. The whole-rock and phenocryst chemistry of ankaramite and porphyry plagioclase dykes suggest they are directly related to the layered series cumulate rocks. Therefore, the composition of these dykes are thought to represent the compositions of the magmas that formed the layered series. Several of these porphyritic dykes were analysed for trace elements by Laser Ablation ICP Mass Spectrometry (LA ICP-MS). Melt inclusions trapped within cumulus chrome spinel grains were also analysed for trace elements by LA ICP-MS. Multi-element trends constructed from both the dyke and melt inclusion data closely compare to the trend of typical low-K island arc tholeiites and it is suggest^ that the parent magmas to the Greenhills Complex were also low-K island arc tholeiites, derived from an incompatible element-depleted mantle source. Both primary and secondary platinum-group element (PGE) minerals are preserved in chrome spinel-rich pods in the basal dunite sequence. Isoferroplatinum (PtsFe) is the most conmion phase, occurring as euhedral inclusions enclosed within unaltered chrome spinel grains. Sperrylite (PtAs2) is the only other phase identified to date, and occurs as anhedral forms around the margins of spinel grains. The characteristics of the isoferroplatinum grains leave little doubt that they were formed as primary magmatic phases, prior to or during the crystallisation of chrome spinel. Recent experimental work (Borisov and Palme, 1997; O'Neill et. al., 1995) has shown that the solubility of PGEs in silicate melts increases with increasing oxygen fiigacity. Therefore, the highly-oxidised parent magmas of the Greenhills Complex may have contained very high PGE contents. A decrease in the solubility of PGEs in magmas occurs with, 1) an increase in the Si02 content of the melt; 2) an f02 decrease in the melt or; 3) a drop in the temperature of the melt (Borisov and Palme, 1997). Any, or all of these factors may have caused the precipitation of isoferroplatinum at Greenhills. In contrast, the Sperrylite grains are believed to have formed as secondary phases, precipitating from As-rich fluids that had previously dissolved isoferroplatinum. Alluvial PGE mineralisation in southern New Zealand is well documented (Mitchell, 1996), though primary sources for these deposits had not been found until the present discovery within the Greenhills Complex. The geochemistry and modal mineralogy of the PGE minerals at Greenhills supports the suggestion that they are the source for the alluvial deposits located along the beaches further to the east of the complex. REFERENCES Borisov A. & Palme H. 1997. Experimental determination of the solubility of platinum in sihcate melts. Geochimica et Cosmochimica Acta 61, 4349-4357. Mitchell M. J. 1996. Alluvial platinum-group minerals from southern New Zealand. Unpublished PhD thesis, University ofOtago, New Zealand. O'Neill H. StC., Dingwell D. B., Borisov A. Spettel B. & Palme H. 1995. Experimental petrochemistry of some highly siderophile elements at high temperatures, and some imphcations for core formation and the mantle's early history. Chemical Geology 120, 255-273.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
GENESIS OF PALAEOPROTEROZOIC VOISEY'S BAY STYLE NI-CU-CO MINERALISATION IN THE EAST KIMBERLEY, WESTERN AUSTRALIA R.A. Sproule^ D.D. Lambert^ and D.M. Hoatson^ Victorian Institute of Earth and Planetary Clayton, VIC 3168, Australia. 7 Sciences, Department of Earth Sciences, Monash University, I ^Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601, Australia.
The discovery of the Voisey's Bay deposit in Labrador, Canada has renewed interest in exploration for magmatic NiCu-Co deposits. In this study, we have obtained Re-Os isotopic data from the Voisey's Bay-type Ni-Cu-Co-bearing Sally Malay intrusion in the East Kimberley, Western Australia. This will allow us to determine the source(s) of the Ni-Cu-Co mineralisation, and to understand better how these metals were concentrated to produce the mineral deposits. The Re-Os isotopic system was employed as it provides excellent insights into magma source reservoirs and as a tracer of processes which may promote mineralisation, e.g., crustal assimilation. The Sally Malay intrusion consists of four small layered ultramafic-mafic bodies with a combined surface area of km^. The southern-most body hosts a basal Ni-Cu-Co bearing massive accumulation of pyrrhotite, pentlandite and chalcopyrite with an indicated and inferred resource of 5.5 Mt @ 1.75% Ni, 0.66% Cu and 0.1% Co. The Sally Malay intrusion shows many similarities to the Voisey's Bay Ni-Cu-Co deposit. Both mineralised intrusions occupy similar tectonic settings (Proterozoic 1.85 Ga collisional suture zone) and both contain troctolites. The most mineralised section of Voisey's Bay is interpreted to be a feeder conduit to other sections of the intrusion, which has also been inferred for the southern body of the Sally Malay intrusion. Massive sulphides have low common Os concentrations (2-4 ppb) and high Re/Os ratios (-20), similar to data for massive sulphide ores from the Babbitt deposit (Duluth Complex) and Voisey's Bay. Re-Os isotopic data from the Sally Malay intrusion do not form an isochron, possibly the result of variations in R-factor (the effective mass of silicate melt with which a given mass of sulphide melt has equilibrated) within the ore system. However, samples with only disseminated sulphides yield a Model 3 isochron of 1893 ± 57 Ma, within error of the magmatic 1845 Ma U-Pb zircon age, suggesting that these samples have remained isotopically closed. Massive sulphide ores yield exceptionally radiogenic initial Os isotopic compositions (y^^ = +1100 to +1300), whereas disseminated sulphide mineralisation yield lower y^ values (+450 to +470). Unmineralised troctolite and peridotite range from y^^ = +60 to +370. The Os isotopic data suggest that the Sally Malay intrusion and associated Ni-Cu-Co mineralisation were derived from crustally-contaminated mantle melts. The R-factor can be crucial fc«- radiogenic isotopes with a strong affinity for magmatic sulphide melts, in particular the Re-Os isotopic system. If the R-factor is high (i.e., dynamic magmatic systems), extensive equilibration can occur between immiscible sulphide melt and chondritic mantle melt lowering the apparent y^^. Thus, crustal contamination processes which may promote sulphide-saturation may be underestimated using the Re-Os isotopic system in dynamic magmatic systems. Incorporating the R-factor into Osisotopic modelling demonstrates that the variation in the initial Os isotopic compositions in different parts of the mineralised system may be due to variable R-factor: massive sulphides appear to have experienced a lower R-factor (-200) than disseminated sulphides (-500 to 800). We prefer a two-stage model for mineralisation at the Sally Malay intrusion which may be appropriate for other magmatic sulphide ore systems (Duluth, Noril'sk-Talnakh, Sudbury, Voisey's Bay). In this model, sulphideundersaturated basaltic melts assimilated old crustal materials en-route to their mid-upper crustal magma chambers, which promoted sulphide-saturation in conduits or staging chambers. The immiscible sulphide "proto-ore" was then entrained in the silicate magma and transported to the crustal magma chamber where the sulphide melt was passively deposited as a result of fluid dynamic contrasts. As this process resulted in a low R-factor, the sulphide melt retained an old crustal y^^ value inherited from the contaminant. In contrast, disseminated sulphide ores from the Sally Malay intrusion preserve a record of higher R-factors in some portions of the magmatic system, potentially the result of more extensive/effective mixing in a replenished magma chamber. Thus, mineral exploration programs should focus on magma feeder conduits or the bases of intrusions for Voisey's Bay Ni-Cu-Co mineralisation. Acknowledgments: Financial assistance and permission to publish were kindly granted by Helix Resources NL. Access to diamond drill core was kindly provided by Normandy Exploration.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
ASPECTS OF REGOLITH-LANDFORM EVOLUTION IN COBAR, CENTRAL WESTERN NEW SOUTH WALES. Melissa J. Spry^ 'Co-operative Research Centre for Landscape Evolution and Mineral ExplorationUniversity of Canberra, Belconnen, A.C.T. 2616.
The Cobar region in central New South Wales has been the subject of many geological studies related to the base metal and gold mineralisation for which the region is renowned. Less attention has been paid to the region's regolith. Recent studies have begun to develop an understanding of regolith materials in the region, however these have not been formulated within the context of a regional landscape evolution model. Many exploration programs at Cobar are based on geomorphological models formulated earlier this century by workers such as Dury, Langford-Smith and Stannard. The widespread acceptance of a pedeplain model for the Cobar area needs to be re-examined as the search for new mineral deposits continues. Models of pediplanantion in this area are based on the generally low relief of the area and the presence of duricrusted residuals, such as silcretes, believed to have formed on a regionally extensive pedeplanated surface which has since been uplifted to its present position. New models of landscape evolution are required to accommodate recent developments in the geomorphological and regolith sciences. Palaeochannels of differing morphologies are present in the Cobar area and suggest that a number of different climate conditions have existed in the Cobar area. The modem drainage consists of dendritic drainage with ephemeral flow. An earlier drainage system within the Cobar area is preserved in a series of maghemite filled palaeo-channels which may have formed in the Tertiary in response to lowering of regional base levels in the neighbouring Murray Basin, or in response to possible local tectonic activity within the Cobar area. The deposits were formed in discrete channels which incised deep narrow valleys. These channels have been reported in the literature by Ford (1996) for an area north of Cobar but are also identifiable from aeromagnetic imagery over the whole Cobar Basin. Further palaeodrainage deposits are preserved to the northwest and west of the Cobar area at localities such as Belah Trig (35km west of Cobar), and in the Tyncin Trig area (northwest of Elura Mine). These gravels are dominantly composed of quartz pebbles and boulders, and contain no maghemite indicating that they formed before maghemite development in this region. Duricrusts in the Cobar area include silcretes, calcretes and ferricretes. These materials form localised occurrences and may represent different stages of landscape formation. Calcrete sampling for gold exploration is being conducted by Hill et al (1998) for the Cobar area and initial results suggest that low magnesium nodular pedogenic carbonates reflect the best sampling medium. Studies of regolith materials and their relation to landform provide a more comprehensive understanding of the origin of sampling media and its previous and present landscape position (eg. Alipour et al 1997). The development of a regional landscape model incorporating the origin of sampling media will ensure companies sample the most suitable regolith materials within exploration programs. REFERENCES Alipour, S., Cohen, D.R. and Dunlop, A.C., 1997. Characteristics of magnetic and non-magnetic lag in the Cobar region, N.S.W. Journal of geochemical exploration, 58, 15-28. Ford, A.J., 1996. Re-interpreting the North Eastern margin of the Cobar Basin, using drainage channel morphology, in The Cobar mineral field- A 1996 perspective, (Ed Cook et al.), 113-123 (Australasian Institute of Mining and Metallurgy: Melbourne). Hill, S.M., McQueen, K. and Foster, K,A., 1998. Regolith carbonates in western and central NSW: characteristics and potential as an exploration sampling medium. Regolith 98. Third Australian Conference on Landscape Evolution and Mineral Exploration, Conference Proceedings. CRCLEME, Canberra/Perth.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14'^ Australian Geological Convention, Townsville, July 1998
THE USE OF SULPHIDATION-OXIDATION HALOES AS GUIDES IN THE EXPLORATION FOR METAMORPHOSED MASSIVE SULPHIDE DEPOSITS: CONSTRAINTS FROM THE BLEIKVASSLI Zii-Pb-(Cu) DEPOSIT, NORWAY ^ Paul G. Sprv^. Jill L. Rosenberg', Carl E. Jacobson' and Frank M. Vokes^ ^ 'Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa, U.S.A. 50011-3212 Institutt for geologi og bergteknikk, Norges teknisk-naturvitenskapelige universitet, 7034 Trondheim, Norway
The effects of sulphidation and oxidation on country rocks enclosing metamorphosed massive sulphide deposits constitute an exploration guide to ore. Sulphur and oxygen from sulphide orebodies react with the Fe component of ferromagnesian silicates or oxides during metamorphism to produce more Mg- or Zn-rich silicates (e.g. garnet, staurolite, chlorite, biotite, amphibole) and Zn-rich oxides (e.g. gahnite, hogbomite) with proximity to ore. Haloes of Mg- or Zn-rich minerals, tens of meters wide, can be superimposed onto zones of pre-metamorphic, syngenetic hydrothermal alteration associated with ore formation or can envelop ore deposits where syngenetic hydrothermal alteration is apparently absent. A corresponding halo is also observed for minerals in the system Ca-Ti-Fe-S-0 (e.g. titanite, ilmenite, rutile). The compositions of ferromagnesian silicates and Zn-rich minerals, as well as the distribution of members of the system Ca-Ti-FeS-0, are dictated by bulk-rock composition and by a variety of physicochemical conditions including T, P,yS2, j02, and The presence of pyrrhotite, pyrite, and magnetite (high JS2-f02 conditions) in a metamorphosed ore body that is enveloped by graphite (low f02 conditions)-bearing country rocks ensures such a gradient. The width of the sulphidation-oxidation halo is, in part, dictated by the proximity of graphite to the orebody, the amount of graphite present, the size of the orebody, and the magnitude of the jS2'f02 gradient. Where graphite occurs adjacent to an orebody, the halo will be narrow but if graphite occurs tens to hundreds of meters from sulphide mineralization the likelihood of a wider halo is increased. Maiga (1983) proposed an unusually wide (159 m) sulphidation-oxidation halo surrounding the 7 Mt Bleikvassli Zn-Pb-(Cu) deposit, Norway. However, more detailed mineralogical studies by us show that compositional variations of biotite, garnet, and staurolite are restricted to within 50 m of the Bleikvassli orebody (Xpe (biotite) = 0.06 to 0.45, Xpe (gamet) = 0.28 to 0.79, and Xpe (staurolite) = 0.34 to 0.78), and that the distribution and composition of these ferromagnesian silicates is dependent upon the competing effects ofj02'J^2 and bulk-rock composition. The enrichment in Mg-content of biotite, garnet, and staurolite in and up to 5 m away from the orebody is due to the/)2 and conditions imposed on the silicates by various oxide-sulfide-graphite assemblages, whereas at distances >5 m from the ore, the bulk-rock chemistry controls silicate composition. Gamet-zincian staurolitebiotite assemblages commonly occur in and immediately adjacent to ore, but there is no obvious spatial zoning of mineral assemblages around the Bleikvassli deposit resulting from fluctuations in JO2 and JS2 conditions. Complexities in evaluating the limits of the sulfidation-oxidation halo are compounded by the presence of up to 2.4 wt. % F in biotite (which results in F being preferentially bonded to Mg due to the Fe^^-F avoidance principle) and up to 9 wt. % ZnO in staurolite. The sulphidation-oxidation halo surrounding Bleikvassli is approximately 5 m wide rather than 159 m as previously proposed by Maiga (1983). This narrow width is due to the small size of the orebody and the fact that abundant graphite occurs in the wall rocks immediately adjacent to ore. It is evident that the delineation of a sulphidation-oxidation halo spatially associated with a metamorphosed massive sulphide deposit cannot be done exclusively on the basis of the distribution and composition of ferromagnesian silicates. In metamorphic terranes that contain heterogeneous rock types, bulk-rock analyses of host rocks in the vicinity of massive sulphide deposits must be obtained before attributing mineralogical and compositional characteristics entirely to sulphidation-oxidation reactions. REFERENCE Maiga, B.S. 1982. Halo effects around the metamorphosed Bleikvassli, Norway, polysulfide ore deposit. Unpublished M.A. thesis. State University University of New York, Buffalo, U.S.A.'
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
DEEP CRUSTAL STRUCTURES AND THEIR INFLUENCE ON THE OFFSHORE OTWAY BASIN, SOUTHEAST AUSTRALIA H.MJ. Stagg & A.M.G. Moore Petroleum & Marine Division, Australian Geological Survey Organisation, Symonston ACT 2609
The northwest-trending Otway Basin extends for some 500 km along Australia's southeast margin from southwest of Cape Jaffa in South Australia to the northwestern tip of Tasmania, and covers an area of more than 100 000 km^. The basin is an element of the 'Southern Rift System', which extends for some 4000 km from Broken Ridge in the far west to the South Tasman Rise, and formed during the Jurassic-Cretaceous rifting episode associated with the breakup of Australia and Antarctica in the Late Cretaceous. The basin contains at least 6-8 km of Late Jurassic to Tertiary sediments underlain by basement rocks of the highly dissected Palaeozoic Lachlan Fold Belt. The main Late Cretaceous depocentre lies beneath the continental slope. In 1994-95, as part of its studies of the southeast Australian continental margin, the Australian Geological Survey Organisation acquired a regional grid of deep-seismic data (16 second record length) that was designed both to tie the principal exploration wells and to provide a first look at the hitherto largely unidentified basinforming structures underlying the margin. In addition to the reflection seismic data, recording of wide-angle reflections at land stations has allowed an interpretation of the crustal velocity structure (Finlayson et al., 1997). In brief, the crustal velocity structure indicated that the Otway Basin section is underlain by basement and lower crustal rocks (velocities of 6.15-6.8 km.s'^) that thin from a combined thickness of approximately 30 km onshore to about 6 km thickness beneath the deep-water part of the basin. The reflection character of the deep crust / upper mantle varies widely beneath the basin and there is a strong correlation between that character and the gross basin configuration. Beneath the relatively thick crust underlying the continental shelf, the crust/mantle boundary (Moho) is interpreted to lie at the base of highly reflective lower crust that is approximately 10 km thick. Beneath the strongly-thinned crust underlying the Cretaceous depocentre, the boundary comprises a thin band of high-amplitude reflectors immediately below a zone of low reflectivity. Beneath the rugged oceanic crust southwest of the basin, possible Moho reflections are occasionally visible as a series of discontinuous, sub-horizontal reflections. The wide variations in crust/mantle reflection character across the Otway Basin are probably a function of the style of crustal thinning and the resulting velocity contrast at the boundary. On at least one transect, it appears that the strongly laminated deep crust that is pervasive beneath the landward flank of the basin thins over a distance of some 50 km and is entirely absent beneath the Cretaceous depocentre. This suggests that accommodation space was generated by removal of extended lower crust such that mid-crustal rocks may now directly overlie mantle. This scenario could account for the high reflectivity (and velocity contrast) at the interpreted reflection Moho beneath the main part of the basin. However, it should be noted that this interpretation is in some contradiction with the velocity information referred to above. A prominent feature of the deep crust of the Otway Basin is the strong relief in the deep crust / upper mantle beneath the outer part of the Otway Basin, and the influence that this has had on the overlying crust at least until the latest Cretaceous. This relief largely appears to be controlled by shearing and/or magmatism It is as much as 4 km over a horizontal distance of less than 20 km, and is reflected by variations in thickness of more than 1 km in the directly overiying Late Cretaceous section. The wide line spacing precludes confident interpolation of these Moho structures between lines. However, it is likely that they either reflect the original oceanward basin boundary, which was probably a major strike-slip zone, or they are the expression of deep-seated NNW- or Ntrending crustal lineaments which later evolved into the major fracture zones that dominate the CretaceousTertiary oceanic crust west of Tasmania. In either case, it is evident that major dislocations in the deep crust and upper mantle have influenced both the gross configuration of the Otway Basin and more localised and shallow structural and sedimentary features. REFERENCE Finlayson, D.M., Lukaszyk, I., Chudyk, E.C. & Collins, C.D.N, 1997. The Otway continental margin transect: crustal architecture from wide-angle seismic profiling. Exploration Geophysics, 28, 58-62.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
EPISODIC PORPHYROBLAST GROWTH IN THE FLEUR DE LYS SUPERGROUP, NEWFOUNDLAND: TIMING RELATIVE TO THE SEQUENTIAL DEVELOPMENT OF MULTIPLE CRENULATION CLEAVAGES Aaron R. Stallard School of Earth Sciences, James Cook University, Townsville , Queensland 4811 email: aaron,stallard@jcu,edu,au
Inclusion trails in garnet and albite porphyroblasts in the Fleur de Lys Supergroup preserve successive generations of microstructures, some of which correlate with equivalent microstructures in the matrix. Microstructure-porphyroblast relationships provide timing constraints on a succession of seven crenulation cleavages (S1-S7) and five phases of porphyroblast growth. Significant destruction and alteration of early fabrics has occurred during the microstructural development of the rock mass. Garnet porphyroblasts grew episodically through four growth phases (G1-G4) and preserve a succession of 5 fabrics (S1-S5) as inclusion trails. Garnet growth during each of the four growth phases did not occur on all pre-existing porphyroblasts, resulting in contrasting growth histories between individual garnet porphyroblasts from the same outcrop. Albite porphyroblasts grew during a single stage of growth and have overgrown microstructures continuous with the matrix. The garnet and albite porphyroblast inclusion trails record a succession of crenulation cleavages without any rotation of the porphyroblasts relative to other porphyroblasts in the population. The contrasting growth histories of different porphyroblast species determined in this study has significant implications for investigations of the geothermobarometry of orogens. Geothermobarometry techniques, such as the Gibbs Method, depend upon all mineral phases required for a particular continuous equilibrium to be coexisting and in local equilibrium at all time during mineral growth. This study has identified considerable variation in growth history within single mineral species (garnet) and contrasting growth histories between different phases (garnet and albite). A geothermobarometry study of the Fleur de Lys Supergroup using garnet, albite and matrix phases would prove meaningless given the contrasting growth histories and hence disequilibrium of the different mineral phases. Geothermobarometry studies in other orogens with poorly constrained microstructural histories may produce inaccurate or even meaningless P-T estimates, especially if the timing of mineral phases is not understood. Complex microstructural histories are best resolved by preparing multiple oriented thin sections from a large number of samples of different rock types within the area of study. The succession of matrix foliations must be understood, as it provides the most useful time frame against which to measure the relative timing of phases of porphyroblast growth, and comparable microstructures must be identified in different porphyroblasts and in the rock matrix.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
PERMO-TRIASSIC EVOLUTION OF THE NEOTETHYAN DOMAIN
Gerard M. Stampfli^. Gilles D. Borel^. Jon Mosar^ hnstitut de G6ologie et Pal€ontologie, UNIL-BFSH2, CH-1015 Uusanne, Switzerland ^Tectonics Special Research Centre, Dept. of Geology and Geophysics, UWA, Nedlands, WA 6907, email: gborel@cyllene.uwa.edu.au
Analyses of subsidence patterns along the Neotethyan southern margin from Iran, Oman, Himalaya and Australia allows the definition of the thermal history of this margin. Palinspastic profiles have been made, based on tectonic and sedimentological observations, and when combined with the geohistory diagrams, allow the definition of the major geodynamic events of the evolution within the Neotethyan domain. The Neotethys rifting occurred in two phases. On the Gondwanan margin, rifting initiated in the Early Carboniferous possibly as a result of the partial collision of this supercontinent with Laurussia, reorganising the intraplate stress pattern at that time. But it was only when Paleotethys engaged in an advanced stage of subduction and slab roll-back eastward (since early Permian) and slab detachment westward (in the Variscan orogenic area) that the Cimmerian blocks were removed fi-om the Gondwanan margin. The Gondwanan margin then split to give birth to Neotethys. Oceanic sea floor spreading took place in the late Early Permian to Late Permian. Thermal subsidence of the newly formed margin started in the Late Permian to Early Triassic including the flooding of the rift shoulder reliefs and is characterised by the onset of a large carbonate platform development lasting until at least to the Late Cretaceous. In the southern Zagros, Upper Permian sediments are unconformably transgressive on the Ordovician series in the western part of the margin (Guniz) marking the rift shoulder zone. To the east, an almost complete Palaeozoic sequence faulted by normal and strike-slip faults exists, corresponding to a closer position to the transform rift. There, the subsidence evolution is characterised by a strong syn-rift tectonic subsidence starting during the Early Permian. In Oman, a Late Cretaceous accretionary complex is situated between the autochthonous sequences from the Arabian margin and the Semail ophiolite. Within the complex, exotic blocks represent major elements of Neotethyan margin. Early Carboniferous marine sediments in the Jebel Qamar Exotic have been recognised to rest with an angular unconformable contact above Cambro-ordovician series, and are covered by pillow lava asociated with late Early Permian platform limestones. In Jebel Akhdar, the transgressive formations (Late Permian) overlie Lower Paleozoic or Proterozoic rocks, and together with the subsidence curve patterns demonstrate an uplift of the rift shoulder. Syn-rift sequences usually start with Sakmarian deposits, slope facies are found since the Early Triassic. The sedimentary series of the Tethyan zone in the Himalayas recorded the evolution of the northern border of Gondwanaland since the Late Precambrian. In the SW of the Zanskar area the Upper Permian post-rift sediments are uncomformably overlying Precambrian to Lower Cambrian strata. The Panjal Traps, basaltic extrusions which occurred during the Late Early Permian are followed by the onset of thermal subsidence, recorded by the development of a carbonate platform since the Early Triassic. This platform and the subsequent stratigraphic record up until the Paleogene reflects the evolution of a continental plateau near a passive margin. Consequently the Neotethyan border in NW India corresponds to the shoulder of a flexural margin. The thickness of the eroded series gives an Early Permian age for the maximum thermal uplift as in Oman. In the NW margin of Australia, a complete synthetic stratigraphic record from the Palaeozoic to the Cainozoic shows mainly uplift during the Carboniferous. The Devonian rocks are often unconformably overlain by the Permian sandstones. Triassic continental to shallow marine sediments are recognised everywhere on the continental margin. The subsidence curve patterns for that time demonstrate a thermal subsidence following an Early Permian uplift prooved by a decrease in sediment thickness and a change in sediment type. This situation is locally overprinted by episodic tectonic subsidence affecting the margin due to a right lateral wrenching during the Triassic. However, most of the faulting affecting the Triassic sequence apparently occurred in the Jurassic.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
PRELIMINARY STRUCTURAL ANALYSIS OF THE WANDOO AREA USING 3D SEISMIC (DAMPIER SUB-BASIN, AUSTRALIAN NORTH WEST SHELF) Christian Steiner^ Robin Marchant^ Satyavan B. Reymond^ Claude Signer^, Lars S0nneland^ and G6rard Stampfli^ 'Institute of Geology, University of Lausanne, BFSH-2, CH-1015 Lausanne, Switzerland ^Schlumberger Geco-Prakla, P.O. Box 330, 4001 Stavanger, Norway
The Wandoo 3D seismic survey is located in the off-shore Dampier sub-basin, which belongs to the Northern Carnarvon basin on Australia's North West Shelf. The Dampier sub-basin, with a stratigraphic column starting at the least in Early Triassic times (probably already in Permian), has recorded a long and polyphased tectonic history. Four major geodynamic events have affected the sedimentation and structures of this area: the Permian opening of the Neotethys, the Jurassic opening of the Argo ocean, the Cretaceous opening of the Cuvier ocean and the Neogene collision with the Band arc (Baillie et al. 1994). The Mesozoic structures of the North West Shelf reveal both extensional and compressional features which have been interpreted either as: - a succession of distinct extensional and compressional events, or as - the result of transpressional stresses. Thanks to very detailed seismic and well information provided by Ampolex (now MEPA), which include a high density 3D seismic survey, a grid of over 3'500 km of 2D regional seismic lines and 11 wells, this project is focused mainly on the structural evolution of the Wandoo area. For this purpose, we have benefited from state-of-the-art technology for the interpretation of this 3D survey: - different types of structurally relevant attribute maps: dip, azimuth, curvature, correlation, edge enhanced grids, etc., - different types of new edge enhanced proprety cubes, - seismic facies classification and reservoir analysis tools, which also helped to map subtle faults. The first phase consists of stratigraphical and structural mapping of the 3D survey; in a second phase the larger 2D grid will be guided by the analysis of the high resolution of the 3D data. An extremely detailed tectonic history through space and time should come out for the Wandoo area and help to clarify the structural evolution of a much wider part of the North West Shelf. Furthermore detailed fault analysis through displacement maps, stereographic projections and stress-strain analysis could also lead to a better understanding of wrench tectonics.
REFERENCES Baillie P.W., Powell C. McA., Li Z.X. & Ryall A.M. 1994: The tectonic framework of Western Australia's Neoproterozoic to Recent sedimentary basins. In: Purcell P.G. & Purcell R.R. (Eds), The sedimentary basins of Western Australia: Proceedings of Petroleum Exploration Society of Australia Symposium, Perth, 1994.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
PETROLEUM PROSPECTIVITY OF AUSTRALIA'S CONTINENTAL MARGINS A. (Tony) E. Stephenson Bureaa of Resource Sciences, PO Box E l l Kingston ACT 26C4
This poster paper presents an overview of the petroleum prospectivity of the Australian continental margin, with an emphasis on 28 major sedimentaiy basins and 8 subsea plateaux contiguous with the continent - several known petrolexmi-bearing basins were also assessed at sub-basin or regional scale. The petroleum exploration status of each area has been evaluated, including whether hydrocarbons have been discovered, and whether new or untested oil and/or gas plays are known or likely. New quantitative del&nitions of petroleum exploration status for Australian basins are proposed, based on density of exploration activity (for this purpose, exploration wells include appraisal wells but exclude development wells): Mature basins: >50% probability that >50% of the basin's hydrocarbons have been discovered. The average seismic grid spacing is likely to be <2 km, with >10 exploration wells drilled/1,000 km^. Submature basins: >50% probability that between 20-50% of the basin's hydrocaibons have been discovered. The average seismic grid spacing is likely to be between 2-5 km, with 1-10 exploration wells drilled/1,000 km^. Immature basins: >50% probability that <20% of the basin's hydrocaibons have been discovered. The average seismic grid spacing is likely to be >5 km, with <1 exploration well drilled/1,000 km^. Frontier basins: No hydrocarbons have been discovered. The level of exploration activity is variable in these basins, but sparse seismic coverage and very few wells is the norm. Other than the likelihood of hydrocarbons being present, five other factors are identified as relevant to perceptions of the prospectivity of Australia's continental margins. These are: Likely maximumfieldsize is a fimction of basin geology and exploration history. Water depth is linked to the economic factor of development (and to a lesser degree exploration) costs, with different production mechanisms currently being required at water depths greater than around 400 m. The cost of acquiring an exploration permit will only be an important factor in areas perceived as highly prospective by industry, and therefore hotly contested in the permit bidding process. The likely cost of development of any hydrocarbons discovered is an important factor in perceptions of prospectivity. This is related to the factors of likely field size and water depth, but also incorporates proximity (and access) to existing infirastructure and markets, and prevailing climatic or oceanic conditions. Political risks vary greatly from coimtry to country, and over time. In Australia at the present time, political risks are perceived to be fairly minor, a major positive factor when we are compared to some other countries by the international petroleum industry. Those risks which do exist can be characterised into three risk types: highly sensitive conservation areas, areas which may be claimed under native title legislation, and areas facing potential competitionfi-omother industries. Maps incorporating the author's assessment of exploration status and all of the above variables for the Australian continental margin are presented. Current geological knowledge suggests that many of the sedimentary basins and some of the subsea plateaux may have generated significant quantities of hydrocarbons. Commercial risk factors vary in each area, and perceptions of prospectivity will depend upon the goals and exploration strategies of players in the petroleum industry. It is concluded that most of Australia's continental margin has not yet been thoroughly explored for petroleum.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
MAGMATIC AND ALTERATION TRENDS IDENTIFIED FROM THE NSWDMR, BROKEN HILL WHOLE ROCK GEOCHEMISTRY DATABASE by B.P.J. Stevens^ and K.R.D. Capnerhurst' ' NSW Department of Mineral Resources, 32 Sulphide St Broken Hill 2880
The BHchem98 database contains 2300 validated whole rock analyses, obtained from many sources. Reliability and completeness of data vary, but the size of the database overcomes some of these deficiencies. Each sample has been allocated a systematic rock type symbol, manually allocated to a stratigraphic unit, and tagged as near to, or away from the Broken Hill mines. Those close to the mines are tagged as on the ore-bearing NW limb of the Broken Hill Antiform, or on the non-ore SE limb. The database can be used to provide clues about the origin of various rock types and identify hydrothermal alteration which may have accompanied mineralization. A twostage process is recommended; the first stage is to identify primary chemical trends related to igneous and sedimentary processes; the second is to identify and evaluate deviations from these trends. The deviations may reflect hydrothermal alteration related to mineralising systems. Basic gneisses (amphibolite and hornblende granulite) occur throu^out the sequence up to the top of the Broken Hill Group. The bulk of the basic gneisses in the Thackaringa and Broken Hill Groups conform chemically to a single tholeiitic ferrobasalt fractionation trend, similar to trends in some ocean floor and Icelandic tholeiites . The least fractionated rocks contain about 48% SiOj, 10% MgO, 12% FejOj* (total Fe), and 1% Ti02. As MgO decreases to about 5%, Fe203* and Ti02 increase markedly. With further decrease in MgO there is a drop in Fe203* and Ti02. Fe/Mg and P2O5 increase throughout the process, and Ni and CaO decrease. Si02 appears to drop to 45% or less, at 5% MgO, and then increases strongly. The pattern is interpreted to result from initial crystallization of Mg-rich pyroxenes with some plagioclase, then at about 5% MgO, strong crystallisation of ilmenite. Basic gneisses in the Thackaringa Group show less extreme fractionation, while those in the Broken Hill Group occupy the whole range. y
Basic gneisses in the Pamell Formation near the Broken Hill mines are some of the most extreme in composition, largely as the result of extreme fractionation. On the SE limb of the Broken Hill Antiform the trend is to very high Fe, Ti, Ti/Fe, Y and Na, while on the NW limb where the Pamell Formation originally underlay the orebody, there is a trend to lower Mg, Fe, Ti and Ti/Fe values and higher Si and P, and there is much greater variation. The scatter of Na and Ca in particular is difficult to reconcile with fractionation, and probably indicates hydrothermal alteration. Compositions of basic gneisses such as those on the SE limb, containing more than 20% FejOg* and only 41-46% Si02, are very unusual for basalt, suggesting perhaps large-scale hydrothermal removal of silica, or incorporation of cumulate phases. However the relative consistency of composition favours neither of these models. It is difficult to suggest any genetic connection between the Pb-Zn orebody and extreme fractionation of a tholeiitic magma, but the variability of chemistry of basic gneisses on the NW limb of the Broken Hill Antiform, may point to hydrothermal alteration beneath the orebody. Elsewhere in this volume, Stevens, Wybom and Jagodzinski have interpreted chemical trends and textural data from "Potosi-type" gneiss, to suggest an origin for most of it as volcaniclastic mass-flow sediment. Ratios of K2O to NajO are quite variable, but there appear to be two populations (Main et al. 1983), one with very approximately equal K2O and Na20, as in normal acid volcanic rocks, and the other with 2.5-4.5% K2O and less than 1% NajO. Most samples from "Potosi-type" gneiss on the NW limb of the Broken Hill Antiform are in this low-Na group, here probably representing part of an alteration zone around the Broken Hill orebody. Oxidation state, particularly in metasediments, may be another indicator of proximity to ore. The data suggest that the metasediments close to the orebody have lower Fe^VFe^"" ratios than any other metasediments in the sequence. However the database includes many surface samples in which oxidation may have increased during weathering. REFERENCES Main J.V., Mason D.O. & Tuckwell K.D. 1983. The characteristics and interpretation of whole rock geochemical data, Willyama Supergroup, New South Wales - trends towards ore. In Broken Hill Conference, 1983, Australasian Institute of Mining and Metallurgy. Acknowledgements This abstract is published with permission of the Director, NSW Department of Mineral Resources.
430
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE ORIGIN OF HORES GNEISS AT BROKEN HILL, N.S.W. AND ITS ROLE IN MINERALIZATION by B.P.J. Stevens^. L.A.I. Wybom^ and E.AJagodzinski^ Geological Survey of NSW, 32 Sulphide St Broken Hill 2880 ^Australian Geological Survey Organisation, GPO Box 378 Canberra 2601
Most of Hores Gneiss comprises a gamet-biotite-rich quartzofeldspathic gneiss locally known as 'Potosi-type' gneiss. The immediate host to the Broken Hill Pb-Zn-Ag deposit is siliciclastic metasediment in Hores Gneiss, but "Potosi-type" gneiss occurs very close to the orebody, probably stratigraphically equivalent, and stratigraphically higher and lower. There has been much speculation over the origin of "Potosi-type" gneiss, ranging from ashflow to pure sediment, to granitic intrusive. Regional mapping indicates that Hores Gneiss extends over a current (folded) area of 600-900 km^ and ranges in thickness from about 20 m to 150 m. Estimated volume of Hores Gneiss is in the order of 90-200 km^, most of which is "Potosi-type" gneiss. Most of Hores Gneiss is in upper amphibolite to granulite grade zones, where metamorphic grain growth and deformation obscure primary textures. However, north of Yanco Glen, near the andalusite/sillimanite transition, some original textures are preserved. The current study confirms the identification by Page and Laing (1992) and Laing (1996) of volcanic quartz and feldspar phenocrysts, and rip-up clasts of sediment. The quartz and feldspar phenocrysts show no gramsize sorting. Biotite clots may in part represent deformed biotite phenocrysts. Zircon shapes are typical of volcanic rocks (Page and Laing 1992). Spherical to ellipsoidal blobs 5-10mm across, consisting of cloudy albite, commonly radially arranged, with a central cuspate filling of biotite, muscovite, quartz and/or opaque minerals, are probably amygdules. Volcanic quartz phenocrysts have been tentatively identified near the Southern Cross mine, in upper amphibolite grade, preserved in the outer zones of metamorphosed carbonate-bearing concretions. Chemically, "Potosi-type" gneiss does not conform to a normal igneous rock. In contrast the time-equivalent (ca. 1690 Ma) Rasp Ridge and Alma "granitic" gneisses show normal granitic chemical trends. Potosi gneiss chemistry could have been produced by entry of massive ashflows into the sea, where steam explosions tend to winnow off glassy ash in preference to crystals; water and sediment are incorporated into the flow, and the ashflows transformed into sedimentary mass flows. Mass flows of the size required to produce Hores Gneiss tend to rip up and incorporate sediment. The resultant chemistry is a mixture of the ashflow minus some of the glassy fraction, plus entrained sediment. In possible analogies, volcaniclastic turbidites are interbedded with coeval lava and intruded by comagmatic hypabyssal sills. It is possible that parts (particularly those interpreted as amygdaloidal) of Hores Gneiss were lavas and/or comagmatic sills, but the geochemistry suggests that the bulk was volcaniclastic sediment. A sandy sediment mass-flow deposit tens of metres thick could act as an aquifer and control the flow of basinal fluids, including any pre-metamorphic mineralismg fluids. A study has commenced, to identify the effects of post-depositional fluid alteration in Hores Gneiss. Tentative conclusions are that a diagenetic fluid resulted in patchy carbonate alteration, including the development of concretions, while a later fluid produced uregular oxidation. Chemical studies will be undertaken to determme whether either of these fluids changed the concentrations of base metals. REFERENCES Laing W.P. 1996. Volcanic-related origin of the Broken Hill Pb+Zn+Ag deposit, Australia. New Developments in Broken Hill Type Deposits, CODES Special Publication I, 53-66. Page R.W. & Laing W.P. 1992. Felsic metavolcanic rocks related to the Broken Hill Pb-Zn-Ag orebody, Australia: geology, depositional age, and timing of high-grade metamorphism. Economic Geology, 87, 21382168. Acknowledgements This abstract is published with the permission of the Director-General, NSW Department of Mineral Resources and the Executive Director of the Australian Geological Survey Organisation. Some of the conclusions are from the Metallogeny of Australian Proterozoic Granites Project, sponsored by 20 companies.
431
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
PARENT ROCKS FROM WEATHERED EQUIVALENTS, NORTHERN GOLDFIELDS, YILGARN BLOCK, WESTERN AUSTRALIA Alastair Stewart and Julienne Kamprad Minerals Division, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601; e-mail astewart@agso.gov.au, jkamprad@agso.gov.au.
XRD and portable infrared mineral analyser (PIMA) analyses of drill cuttings of weathered assemblages can identify felsic, mafic, and ultramafic parent igneous rocks where drilling did not reach bedrock, and so yield a solid geology map. The PIMA is particularly useful in distinguishing: smectitic clays from chlorite, as XRD peaks for these overlap; montmorillonite from nontronite, Fe-chlorite and Mg-chlorite; and kaolinite, dickite and halloysite from each other. Quartz is present in 85 percent of samples from the Lake Violet 1:100 000 Sheet area, and kaolinite in 75 percent. Much of the quartz is cryptocrystalline silica set free by weathering of feldspar, pyroxene, amphibole, and mica, as these minerals are richer in silica than kaolinite (46.5 percent Si02). Hence, quartz is of no use in assigning an initial felsic or mafic parentage. Partly weathered felsic rocks contain illite [K(Fe, Mg, Al)2(Si, A1)40io (0H)2], and so the presence or absence of illite was used as the initial criterion for respectively assigning a felsic or mafic parent The procedure for assigning a parent rock is: Illite present? Yes - felsic parent. No - plagioclase present? Yes - quartz present? Yes - felsic parent. No - mafic parent. No - kaolinite present? No - ultramafic parent. Yes - quartz present? No - intermediate or mafic parent (secondary silica migrated away to form silcrete). Yes - quartz as discrete grains? Yes - felsic parent. No - intermediate or mafic parent (quartz secondary cryptocrystalline). A felsic rock can be so intensely weathered that illite has gone to kaolinite, but in this case a silicic rock should retain discrete quartz grains (phenocrysts or groundmass) detectable in the field. An intensely weathered intermediate rock, containing no primary quartz, could be misassigned a mafic parent. Samples of recognisably sedimentary origin (eg, from bedding lamination) are identical witii weathered felsic igneous assemblages. Texture and structure are needed to distinguish a felsic igneous from a sedimentary parent. ARE THE RESULTS REASONABLE? Yes, because the weathered assemblages contain other clays and minerals that were not used for diagnosis, but which can be expected to occur in the weathered equivalent of the assigned parent. Thus: chlorite is present in most ultramafic assemblages; talc occurs almost exclusively in ultramafic assemblages; Fe-oxides are noticeably more common in mafic assemblages compared to felsic ones; muscovite is absent from mafic and ultramafic assemblages; serpentine is virtually absent from felsic assemblages; and montmorillonite and quartz are virtually absent from ultramafic assemblages (secondary silica having migrated away), although nontronite (iron-rich smectite) is commonly present. ARE THE CLAY ASSEMBLAGES IN SITU? Yes, for two reasons: the weathered assemblages of the three igneous groups are markedly and consistentiy different, and so are unlikely to have been mixed together; the assemblages form well defined groups of one or a few closely related types, which change suddenly to another type. This suggests that a lithological boundary has been crossed, consistent with a corresponding change in parent rock type below. Acknowledgements: Colin Pain, David Tilley, Ivor Roberts, Chris Cuff, Tony Eggleton, and Bob Gilkes gave much appreciated help with this study.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
SULPHUR AND CARBON ISOTOPIC STUDY OF THE ELURA Pb-Zn DEPOSIT, COBAR, NSW Yanyan Sun and Philip K. Seccombe Department of Geology, The University of Newcastle
Systematic sulphur and carbon isotopic analyses for sulphides and carbonates have been carried out at the Elura Pb-Zn deposit, Cobar. These results complement a comprehensive study on the origin and depositional conditions of sulphide mineralisation at Elura. The results support an ore-forming model involving a mixture of sulphur and carbon from a deeper crustal reservoir, together with a contribution from the Early-Devonian metasedimentary hostrocks. The sulphur isotopic data exhibit a distinct grouping of higher 5^S values in the country rocks and lower 6 "^S values in the ore. Disseminated and fr^boidal pyrite samples from country rocks have a range of b^^S values between 9.5 and 20.1%o with a mean value of 13.9%o. These S^^S values are similar to those of Early Devonian seawater sulphate and the sulphides are likely to have been derived from contemporaneous seawater sulphate by reduction in a closed system. Sulphide samples from ore have a range of values between 5.9%o and 10.2%o with a mean value of 8.1%o. Exceptions are two pyrite samples in pyrite-ore which are a mixture of syngenetic pyrite and late hydrothermal pyrite and have higher values of 11.2 and 11.8%o. A general 6'^S order of S^^Spy > 5^Spe & > S^S^p > 8^Sgn occurs among the coexisting sulphide minerals in the same ore specimens. It suggests that chemical and isotopic equilibrium was reached during ore deposition. Ore-forming temperatures calculated from values of the coexisting mineral-pairs range from 220® to 449®C. H2S(aq) is the dominant aqueous sulphur species (ZH2S(aq)>90%). 5^S composition of H2S(,q) isotopically equilibrated with sulphide minerals crystallised at high temperature would approximate to an initial b^^S^s of ~6.5%o for the ore-forming fluid and subtantially lower than the values of syngenetic pyrite in hostrocks. This lower initial value indicates a deeper sulphur source, derived from basin or basement rocks at a higher metamorphic grade than the hostrocks to the deposit. An overall increase of 1.5 to 2 per mil for the value of H2S(aq) was observed in the ore-forming fluid from high temperature to low temperature mineralisation. This increase was considered a mixture of deeply sourced sulphur with a lower value and isotopically heavier sulphur obtained by replacement of early formed sulphide in hostrocks by the late hydrothermal sulphides. This mterpretation is also supported by structural and textural evidence. Calcite from two fossil fragments and one limestone have values ranging from 1.1 to -0.2%o with a mean of 0.3%o relative to PDB. Calcite from weakly altered hostrocks have obviously low values (ranging from -3.7 to -4.9%o, mean -4.2%o). data for calcites from calcite-quartz-sulphides veins vary from -3.5 to -9.2%o with a mean of -6.2%o. Ankerite and siderite samples from altered walh-ock and hostrocks possess consistent values of-4.5 to -4.8%o (mean -4.6%o) and -2.9 to values 4.6%o (mean -3.6%o), respectively. To compare values of calcite, ankerite and siderite, of ankerite and siderite have been recalculated to equivalent calcite values, ranging from -5.5 to -5.8%o (mean -5.6%o) and -3.9 to -5.7%o (mean -4.6%o), respectively. It is clear that values increase from calcite in mineralised veins, through ankerite, siderite and calcite from altered hostrocks to calcite from limestone and fossil fragments. The explanation for this increase may result from mixing of isotopiclly light carbon in hydrothermal fluids with heavier carbon from local sedimentary rocks. The low carbon source may contain more reduced carbon derived from underlying sedimentary sequences.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
SERIAL SEA FLOOR - MANTLE PLUME SYSTEMS AND THEIR VOLCANIC ODYSSEYS ACROSS AUSTRALASIAN - ANTARCTIC REGIONS F. Lin Sutherland Geodiversity Research Centre, Australian Museum, Sydney, New South Wales, 2000
Basaltic intraplate volcanic rocks characterise continental margins along eastern Australia, Ross Sea, Marie Byrd Land and southwestern New Zealand and form migratory chains along Tasman and Southern Ocean floors. An estimated 50,000 km^ of eruptives probably conceal magmatic underplates a magnitude greater in volume. Migratory lines within the volcanic fields back track to old sea floor rifts, suggesting the underlying plumes originated with short term spreading events (10-30 Myr). The spreading rift-plume events differ in extent and age of initiation and were part and parcel of continued eastern Gondwanan breakup since the Late Cretaceous. The volcanic migrations reflect past plate movements, but because of relative motion between Indian and Pacific hotspots are best matched using an averaged hotspot motion. After sea floor formation, plume traces cross ocean floors, continental margins and even plate boundaries under lithospheric movement controlled by long term Southern Ocean and Pacific spreading. Proposed sea floor-plume systems and their serial initiation are summarised in Table 1. PLUME SYSTEMS The plume systems seem to develop on old triple points and include up to 5-10 plume sites in the larger spreading systems (1650-2750 km long Tasman and Coral Sea systems). Individual systems may exhibit different isotopic plume characteristics, the Tasman-Iselin? systems being dominated by HIMU plumes and the Coral Sea-Rennell system by OIB and EMI plumes. Passage of Australia over Tasman plumes generated early Cainozoic volcanic fields in southeastern Australia. Coral Sea plumes generated early-mid Cainozoic fields in north-central eastern Australia and mid-late Cainozoic fields in central-southeastern Australia. Migratory trails over Coral Sea plumes show westward deviations from absolute motions during episodic plume surges ('boomerang' volcanism). This may reflect westward mantle flow bending the rising plumes, as western flow of Pacific mantle mto Indian Ocean mantle is observed on the Australian-Antarctic spreading ridge near present plume positions. VOLCANIC TRENDS The serial sea floor-plume model accounts for prolonged volcanism along extended belts and can predict future trends of activity. It suggests northern Australia is over riding a young plume series and will be active over another 10 Myr. Victoria will move off the Coral Sea system, which will encroach under Tasmania, while the older Tasman system will maintain Ross Sea marginal volcanism under the slow moving Antarctic plate. Full blown episodic activity at any of the 30 or so serial plume sites in the Australasian-Antarctic region appears to depend on tensional or compressive states in the overlying lithosphere. However volumes of eruptives and their petrogenetic range within plume series gradually decrease with time. Table 1 Progressive spreading rift-plume series, SW Pacific margin System
Size
Age(Ma)
Main Trace
Present Position
Tasman Capricorn Coral Rennell Iselin Solomon D'Entrecasteau South Fiji New Guinea?
Large Small Large Small Medium? Medium Medium Medium Medium
95-55 95-85 65-55 65-55 55? 45-42 36-24 36-24 24-?
S. Tasman-Balleny Rockhampton Hillsborough-W Victoria Tasmantid chain Marie Byrd Land Bundaberg-Barrington Lord Howe chain Dunedin-Banks North Queensland
Balleny-McMurdo volcanoes ? Bass Strait West Tasman seismic zone Marie Byrd Land volcanoes S E Sydney Basin S. Lord Howe S E Campbell Plateau NE-SW Queensland
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
TANTALUM-TIN MINERALIZED PEGMATITES AT WODGINA AND MT. CASSITERITE, PILBARA CRATON, WESTERN AUSTRALIA Marcus T. Sweetapple and Peter L. F. Collins Mineral Deposits and Exploration Research Group, School of Applied Geology, Curtin University of Technology, Perth, W.A., 6001
There are two suites of tantalum-tin mineralized pegmatites in the Wodgina Pegmatite District of the Wodgina Greenstone Belt. The Wodgina pegmatite suite is equivalent to the lepidolite subtype of the Complex Type, and the Mt. Cassiterite pegmatite suite is equivalent to the Albite-Spodumene Type (classification system of Cerny 1993). Both fields of pegmatites have a strongly peraluminous chemistry, typical of tantalum mineralized pegmatites of the LCT class of Cemy (1993). These pegmatites were probably derived from the post tectonic Numbana Granite of around 2950-2850 Ma. The Wodgina pegmatite field is hosted mostly in undifferentiated mafic-ultramafic volcanics of the Gorge Creek Group, whereas the Mt. Cassiterite field is within psammopelitic metasediments of the Corboy Formation. These host rocks are separated by a major shear zone which predates pegmatite emplacement, and which was reactivated at the time of emplacement. Pegmatites of both groupings show evidence of syn-tectonic emplacement into low angle shear zones, typically with a reverse sense of movement. The principal pegmatite of the Wodgina group is the Wodgina main lode pegmatite, which has a strike length of approximately one kilometre and is between 5 and 40 metres in thickness. The principal structure of the body is a sheet dipping 20° to 50° east, with adjoining lenses of pegmatite, sub-parallel to the local foliation. The north end of the body is shown to be intruded into the limbs and around the hinge zone of asymmetric parasitic folds. Zoning typically takes the form of layering developed sub-parallel to contacts. These layers typically comprise an almost pure coarse grained albite-cleavelandite and an aplitic to granitic textured medium to coarse grained muscovite-albite-quartz±microcline unit. Classical concentric zoning around a quartz core, typical of highly fractionated pegmatites is rarely present. Late metasomatic activity is represented by fine grained lepidolite and muscovite alteration of microclinelspodumene bearing aplitic/granitic-type units. Tin and tantalum minerals are mostly associated with coarse grained cleavelandite units, and are believed to have formed during primary magmatic crystallization. Coarse grained tantalum minerals are associated with the base of these units, suggesting gravitational settling. Gravitational effects are also observed in the development of lode cast and orbicular structures between cleavelandite and aplitic units. Late redistribution of tantalum appears to have accompanied metasomatism, involving transport of tin, tantalum and alkali elements in fluoride complexes. The Mt. Cassiterite pegmatite is the principal pegmatite within a field of structurally complex and interlinked pegmatites and associated greisen veins. The Mt. Cassiterite pegmatite consists of 5-12m thick sheets dipping 15-20° west covering an area of approximately 400 x 250m. Adjoining pegmatite dykes appear to be controlled by S2 parasitic refolded folds in a similar structural setting as described for the northern part of the Wodgina main l ^ e pegmatite. This pegmatite is mostly unzoned, being composed entirely of very coarse spodumene and microcline crystals in a matrix of fine-medium grained quartz, albite and muscovite. Secondary quartz and albite crystallization overprints the primary fabric. This alteration is most concentrated in the upper parts of the pegmatite sheets, possibly due to an up-dip fractionation of fluids related to later greisen veins. Both of pegmatite fields have distinctly different tantalum-tin mineralogy. The Wodgina main lode pegmatite contains mostly manganotantalite, with subordinate tapiolite and wodginite, all of which have undergone microlite alteration. The Mt. Cassiterite pegmatite has mostly cassiterite and wodginite (with stanniferous microlite alteration). These differences are reflected in a higher niobium : tantalum ratio in the Wodgina pegmatite, and a higher tin : tantalum ratio in the Mt. Cassiterite pegmatite. Reference Cerny, P. (1993) Rare Element Granitic Pegmatites. Part 1: Anatomy and Internal Evolution of Pegmatite Deposits. Pp 29-47, Ore Deposit Models Volume II, Editors Sheahan, P.A. and Cherry, M.E., Geoscience Canada Reprint Series 6.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
GEOCHEMICAL ZONATION AND PARAGENESIS OF A PORPHYRY RELATED GOLD DEPOSIT AT RAVENSWOOD, NORTH QUEENSLAND. Cameron K Switzer, J Steve Hinde, Chris J Green, Derek L Webb, Barry A James, MIM Exploration, Level 2, 55 Little Edward Street, Spring Hill, Queensland, 4000.
The gold deposits of Ravenswood constitute a significant goldfieldwith combined historical production and identified resources in excess of 3.2 million ounces. Gold has and currently is being extracted via various forms of seleaive miningfi"oma series of low angle quartz-sericite-pyrite veins and structured stockworks as well as chlorite-carbonatebase metal rich breccia veins hosted within tonalites and diorites of the Ravenswood Granodiorite Complex. Detailed observations, structural mapping, niche geochemical sampling of the complex vein system and an extensive soil sampling database has confirmed many previous workers' interpretations and identified a geochemical zonation and paragenesis that suggests that the gold mineralisation is both spatially and genetically related to a porphyry style CuAu system. Mineralisation and associated alteration are preferentially localised along fault zones of numerous orientations, the intensity of which is dependent upon the scale, geometry, association with other faults and the relative components of movement. Gold depoation occurred via the process offiactureinfill whereas alteration is developed as both veinlet infill and as replacement selvage proximal to the structures. Field mapping of the area has demonstrated that the main gold minerali^ng event associated with sericite and varying proportions of chlorite, crosscut and dislocate alteration assemblages consisting of K-Feldspar-biotite, biotite, and cWorite-albite-epidote-carbonate with minor actinolite. Gold mineralisation is associated with a phyllic overprint that postdates the earlier developed potassic and propylitic alteration. Recent ^Ar/^^Ar age dating by Perkins, C. and Kennedy, A.K, 1998., suggest that the biotite has an age of ca 330 Ma whilst there are two distinct sericite ages of ca 330 Ma and ca 310 Ma. Exploration must therefore attempt to discriminate between the early alteration and the subsequent overprinting alteration and mineralisation. The early alteration geochemical zonation is as follows. The K-Feldspar-biotite zone is dominated by Cu-Au with elevated Zn"-Mo. The biotite zone, which is evident in the Nolan's operation, exhibits the same association as above except at reduced levels. The propylitic zone is characterised by strongly elevated Pb-Zn at much higher levels than the potassic zones, along with accessory Cu. The overprinting zonation is complex with zones most likely representing a geochemical continuum fi-om early proximal veins to later lateral manifestations of the imderlying system. The proposed sequence consists of early high Te-Bi-Mo-Cu^-Au veins associated with mtense sericite-illite{?)-silica alteration. Crosscutting these vein sets is the main mineralising gold event which has the characteristic geochemical character of Au-Cu-Bi^-Zn v^th the dominant sulphide phase consisting of pyrrhotite and/or pyrite. Overprinting this, are veins vdth the association of Au-Cu-PbZn-As-Bi which are characterised by the alteration assemblage of sericite-chlorite-carbonate. Laterally outward fi-om this, veins more typically associated with sericite-clay-pyrite have a signature defined by high Sb-As-Pb-Zn-Ag-Hg"^Au. Minor weakly colloform banded quartz veins have been identified distal to these zones and have the elemental association of Sb-Hg-As. It is unsure if these veins are related to the Ravenswood ore system. Soil anomalism is coincident with the abovementioned zonation but does not accurately define the paragenesis. Detailed observational data comprising niche sampling, structural and alteration mapping have defined a geochemical zonation and vein paragenetic sequence that suggests a close spatial and genetic relationship to a porphyry Cu-Au of Carboniferous age. Further work is required to confirm the details of this ore system. REFERENCES Perkins, C., and Kennedy, AK., 1998. Permo-Carboniferous gold epoch of northeast Queensland. Australian Journal of Earth Sciences, 45, 185-200. Acknowledgements This work was published with the permission of MIM Exploration Ltd. Many geologists have contributed to the understanding of this system over the years. Their reports and dedication to this project are acknowledged.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
GENESIS OF GOLD MINERALISATION AT THE HENTY DEPOSIT: AN UNUSUAL STYLE OF IHGH GRADE MINERALISATION IN THE MOUNT READ VOLCANICS OF WESTERN TASMANIA Jafar Taheri and Geoffrey R. Green Mineral Resources Tasmania, PO Box 56, Rosny Park, Tasmania 7018
The Henty gold deposit, with a pre-mining resource of 506 000 tonnes of ore with a grade of 26.9 g/t Au, is associated with zones of intensely silicified, cherty, felsic lavas and volcaniclastics in the Cambrian Mount Read Volcanics (MRV) of western Tasmania. The deposit represents an unusual style of mineralisation for a district noted for its major polymetallic VHMS deposits and disseminated Cu-Au mineralisation. It lies within the lower part of the Tyndall Group, the uppermost unit of the MRV, adjacent to the footwall of a major structure, the Henty Fault, which bisects the volcanic belt (Halley and Roberts, 1997). The silicified unit (locally termed the MQ unit) is enveloped by strongly sericitised rocks, with disseminated sulphides (MZ), generally with an intervening zone with characteristics intermediate between the former (MV). Strain partitioning has resulted in intense cleavage development in the MV and MZ with repeated fracture development and formation of quartz veinlets in the MQ. Thin zones of massive pyrite and polymetallic massive sulphides occur in places towards the top of the alteration zone. Higher in the sequence is a unit of pervasively albitised felsic lava, again with repeated fracturing and quartz veinlet development, but little mineralisation. The MQ, MV, and MZ alteration interfmger downdip with carbonate-rich assemblages. Most of the gold mineralisation in the MQ occurs within the quartz veinlets in association with chalcopyrite, galena, and tellurides. This feature, together with the proximity to the Henty Fault, suggests that the deposit may be of mesothermal origin, formed during Devonian deformation. However, a number of geological and geochemical factors favour a mode of formation in a shallow, sub-seafloor, environment beneath a relatively low temperature VHMS-forming system: • Randomly orientated relict flakes of sericite in the MQ suggest that silicification was pre-cleavage. • The lack of mineralisation in the stratigraphically higher albitised lava, despite its similar brittle response during deformation to the MQ, is inconsistent with a mesothermal origin. • The downdip passage from silicified and sericitised rocks to carbonate-rich alteration is difficult to reconcile with upward flow of hydrothermal fluids during deformation. • Etched samples of pyrite from the massive sulphide mineralisation show similar brittle behaviour to the MQ. Early microcolloform and framboidal pyrite is overgrown by zoned euhedral rims. These early phases of pyrite are strongly brecciated, infilled and partly replaced by a later pyrite generation. S isotope data, obtained by both conventional and ion microprobe techniques, show a similar range of values of 4 to \5%c for the early pyrite, late pyrite and sulphides in the MQ, MV and MZ. These values indicate that it is unnecessary to invoke an additional source for the sulphur in the late veinlets in the MQ and MV. 13
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• Isotopic values for carbonates show ranges of -2.4 to 0.7%o for 5 C and 9.3 to 13.1%o for 5 O and quartz from the MQ alteration has a 5 O range of 11.9 to 13.7%o. These ranges are consistent with mineralisation at a temperature of about 200°C and carbonate alteration at temperatures of between 170 and 200®C from fluids with the isotopic composition of seawater. • Lead isotope data are similar to other Tasmanian VHMS deposits and identical to sulphide clasts in a nearby Cambrian mass flow deposit. • Co and Ni contents of pyrite are similar to those from massive sulphide deposits in the MRV. It is concluded that there is no need to invoke any source for the mineralisation other than those associated with Cambrian hydrothermal activity and that the gold mineralisation was initially formed just below the sea floor in a VHMS forming system. The associated intense silicification was related to the extreme temperature gradients in this regime, with hydrothermal fluid-seawater mixing facilitated by the high permeability of the volcaniclastic host rocks. Inferred formation temperatures of about 200°C were insufficient to form a large polymetallic deposit and fluid boiling was not necessarily responsible for ore formation. The spectacular gold grades may be partly due to local scale (i.e. within the alteration system) mass transfer during Devonian deformation. REFERENCE Halley S.W. & Roberts, R.H. 1997. Henty: A shallow-water gold-rich volcanogenic massive sulfide deposit in western Tasmania. Economic Geology 92, 438-447.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
THE GRAMPIANS ALLOCHTHON, VICTORIA: EXTENSIONALLY EMPLACED REMNANTS OF A THRUST DUPLEX STACK David H. Taylor & Ross A. Cayley Geological Survey of Victoria, PO Box 500, East Melbourne, Victoria 3002
The Grampians lie just to the west of the Lachlan Fold Belt (LFB) and Adelaide Fold Belt (AFB) terrane boundary (the Moyston Fault) in western Victoria. Until recently the Grampians was interpreted as a relatively gently deformed, fluvial to shallow marine sequence approximately 7000 m thick that was deposited in a passive basin setting. Detailed structural and lithological mapping (Cayley & Taylor, 1997) now recognises numerous bedding-parallel thrust faults that stack large parts of the stratigraphy, effectively thickening the original sequence by around 100%. The thrusts are folded about large scale folds, and the whole complex is truncated at depth by a low angle decollement which gives the Grampians an allochthonous relationship to its basement of AFB rocks. Early Devonian post-tectonic granites intrude the complex. On structural, sedimentological and temporal grounds, the fold-and-thrust belt of the Grampians is regarded by us as a structural outlier of the LFB. The restored stratigraphy is approximately 3700 m thick, consisting of a lower quartzo-felspathic to micaceous sandstone package (Red Man Bluff Subgroup; 1900 m thick), an intervening micaceous mudstone dominated package (Silverband Formation; 750 m thick) and an upper, quartzose sandstone package (Mount Difficult Subgroup; 1050 m thick). The sediments comprise fluvial, shallow marine, and aeolian deposits which probably accumulated in passive basins developed along the AFB margin, transitional to the Victorian LFB turbidite sequence accumulating in an oceanic setting to the east. A Late Ordovician to Early Silurian age for the Grampians Group is suggested by their regional geological setting and history, contrary to an Upper Silurian/Lower Devonian age suggested by a poorly defined fossil assemblage. A marked absence of internal sediment reworking, lateral facies changes or high grade metamorphic detritus argues against the development of the Grampians Group as a foreland basin to the accreting the LFB to the east. The thrusts are difficult to observe where bedding-parallel but become obvious where they bifurcate or ramp through the sequence, truncating bedding and juxtaposing beds of different dip. Where exposed, the faults are narrow, metres-wide zones of intense fracturing and brecciation, with strain strongly partitioned into cataclastic gouge and slickensided faults. These zones are often intruded by sill-like felsic dykes related to Early Devonian post-tectonic granites. The systematic repetition of various units suggests three main thrust detachment horizons: (1) low in the Redman Bluff Subgroup (2) high in the Redman Bluff Subgroup, and most significantly (3) v^thin the Silverband Formation. There are approximately 12 major thrust faults of significant extent splaying from these detachment horizons, but not all of these faults repeat stratigraphy. The thrust ramps variously show footwall and hangingwall control and this suggests a duplex thrust stack rather than an imbricate thrust stack geometry, a deduction further supported by the systematic repetition of adjacent stratigraphic units without the lowest stratigraphy ever being emplaced over the highest stratigraphy. The internal duplexing of stratigraphy rather than wholesale imbrication may have been controlled by the structurally incompetent Silverband Formation lying between the two thick sandstone packages—in effect a roof thrust for the lower package and a sole thrust for the upper package. The absence of any in-faulted 'exotic' rocks into the Grampians Group suggests internal thrusting that did not involve the AFB basement. There are several large-scale open folds, notably the Wartook Syncline, which affect the thrust-stacked stratigraphy. Crustal shortening at the time of the Grampians deformation is not expressed in the underlying AFB basement, and is therefore inferred to be related to the accreting LFB to the east. The Grampians Group was probably deformed in a thin-skinned fashion as the LFB was emplaced against the AFB along the Moyston Fault during the Early Silurian. The Marathon Fault is the low angle decollement which truncates the thrust faults and separates the Grampians from the underlying AFB. The fault is poorly exposed but Grampians outcrop distribution, company drilling and gravity data support a thin-skinned allochthonous setting—^more than 3000 m of thrust-stacked and folded sequence may be missing from beneath the Wartook Syncline. The Marathon Fault omits stratigraphy in contrast to the earlier thrust faults, and geometrical relationships suggest that it was initiated as a listric extensional structure, segmenting the thrust-and-fold belt. This fault may be one expression of postorogenic extensional relaxation of the LFB/AFB suture. REFERENCES Cayley, R.A. & Taylor, D.H., 1997 Grampians special map area geological report. Geological Survey of Victoria Report 107. 438
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE LANDSCAPE EVOLUTION AND REGOLITH OF THE WESTERN VICTORLVN GOLDFIELDS: IMPLICATIONS FOR EXPLORATION David H. Taylor & Ross A. Cayley Geological Survey of Victoria, PO Box 500, East Melbourne, Victoria 3002
Western Victoria has been a prolific producer of gold from mesothermal mineralisation hosted by quartz veins in Palaeozoic bedrock. Lying scattered across the landscape are a series of Tertiary fluvial placer deposits derived from this source. In historical times these were also extensively mined but today they mainly complicate geochemical exploration. The landscape evolution which produced the placers also generated a variety of bedrock regolith profiles whose distribution also affects the geochemical sampling regime. Large areas of prospectivity are masked from direct exploration by relatively young basalt flows whose total thickness may exceed 100 m. The Geological Survey of Victoria is producing regolith maps to aid exploration of the western Victorian goldfields (eg. Taylor & Joyce, 1996). Understanding the landscape evolution and the resulting regolith of the goldfields is important for geochemical exploration because the regolith controls the sampling strategy. Although the regolith is often viewed as an impediment to exploration, dispersed geochemical haloes within it, and geochemical plumes in the groundwater may enhance the expression of the small parent ore bodies and allow inexpensive exploration beneath the cover. Large amounts of transported gold lie scattered across the landscape, reworked by several cycles of Tertiary erosion. The erosion was driven by stream rejuvenation when Australia and Antarctica rifted apart in the late Mesozoic. The fu-st erosion cycle extensively dissected a low relief Mesozoic landscape—of which some remnants remain—^by several hundred metres. This produced a new landscape of emergent meridional bedrock strike-ridges and broad shallow valleys filled with an auriferous, sheet-like, gravel lag composed almost entirely of vein quartz from the reefs; the White Hills Gravel (WHG). The scarcity of bedrock clasts in even the most proximal WHG deposits suggests that the Mesozoic landscape had been deeply weathered to great depths. Where the WHG lies close to bedrock mineralisation it usually carries economic gold grades and was extensively mined in the past. This new landscape—an early Tertiary palaeosurface—has remained largely intact to the present day and strongly controlled the distribution of younger units. Weathering of this landscape slightly ferruginised the elevated bedrock interfluves, strongly silicified/ferruginised the low-lying WHG into a duricrust and deeply weathered the bedrock that underlay it. The concentration of this weathering to the low-lying areas suggests that surface run-off and groundwater flow were responsible. In the middle Tertiary a new drainage system formed, possibly in response to increasing aridity when the circumpolar current developed as Australia and Antarctica drifted apart. Less energetic streams inherited the akeadyformed broad shallow valleys, and in a second erosion cycle, carved steeper, narrower valleys through the WHG duricrust back into the underiying deeply weathered bedrock. The WHG remnants occur about 30-80 m above these valleys, with this depth of incision probably reflecting the exhumation of fresher bedrock from below the deep weathered material. The gravel lag of this new drainage system, restricted to the incised channels, is the highly auriferous deep leads that are now a major groundwater aquifer system. The deep leads are mainly comprised of recycled material from the early Tertiary palaeosurface and include ironstone and pallid clay. Towards the end of the Tertiary the deep lead drainage system was effectively shutdown by voluminous outpourings of basalt. Flows filled the valleys and in places overtopped the bedrock interfluves and completely buried the landscape to form basaltic plains which mask large areas from simple exploration. Around Ballarat there was large-scale northward divide migration caused by stream diversion. Impounded streams deposited poorly sorted outwash in their headwaters and where these deposits are close to nearby mineralisation they may be phenomenally rich in nuggetty gold—much of which is probably of supergene rather than detrital origin. Since that time the present drainage has reestablished largely as lateral streams which have started to deeply incise. High standing areas of bedrock in the early Tertiary palaeosurface have remained relatively unweathered and are uncontaminated by transported gold liberated from the Tertiary deposits. Low standing areas of bedrock exhumed from below the early Tertiary palaeosurface range from highly weathered to almost fresh and are contaminated by transported gold. Major deposits completely hidden beneath the basalt plains may express themselves as geochemical plumes in the major aquifer system of the deep leads. REFERENCES Taylor D.H. & Joyce E.B. 1996. Ballarat 1:100 000 regolith-exploration map report. Geological Survey of Victoria technical record 1996/4.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
HYDROTHERMAL ALTERATION AND ASSOCIATED GOLD MINERALISATION FROM THE MOUNT TERRIBLE VOLCANIC COMPLEX, NSW GRAHAM TEALE, Werrie Gold Ltd, PO Box 740, North Adelaide South Australia, 5006
The Mount Terrible volcanic complex (MTVC) is located approximately 45 km south west of Tamworth, NSW. It is approximately 5 km in diameter and is bounded by ringfeults,dykes, and occasional breccia zones. It forms a strong positive magnetic anomaly complex which provides a stark contrast to the extensive negative anomaly associated with the surrounding Werrie Basalt. The MTVC is located within a poorly understood belt of Permo-Carboniferous volcanics and intrusives situated between Boggabri and Murrurundi and it has an Early Permian age. The MTVC is dominated intermediate volcanics (50%-65% SiOj) which, on a total alkali-silica diagram, lie in the basaltic trachyandesite and trachyandesite fields. The rocks have been described petrographically as high-K andesites, shoshonites and latites. Flows, coarse and fine grained pyroclastics and epiclastics of the above volcanic types have been mapped. Minor trachyte, high alumina basalt, trachybasalt and trachydacite are also present. The volcanic rocks tend to be trachytic textured with phenocrysts of clinopyroxene, plagioclase (generally labradorite), titanomagnetite and lesser apatite and rare hornblende. Orthopyroxene phenocrysts are virtually absent and Fe^'^rich ilmenite is rare. Phenocryst phases sit in a groundmass dominated by plagioclase miCTolites and microphenocrysts of magnetite and clinc^yroxene and interstitial K-feldspar. Fine grained biotite is present in the matrix of shoshonitic variants. A diverse grouping of intrusive rock-types is present with gabbroic through dioritic to granophyric syenodiorite (banatite) in one Imeage. In addition, h i ^ level dacitic domes and numerous other dioritic porphyry stocks and dykes are also present. Vent, diatreme and milled breccias have been identified and sheeted vein systems occur in some areas. Rare tourmaline-quartzpyrite breccias range in widthfi-om1 cm to 2-3m and are subhorizontal to shallow south dipping. They appear to crosscut potassic and phyllic altered zones, but may be affected by argillic alteration. Gold mmeralisation has been delineated in two areas of the complex and occurs within low temperature (150® - 200®C) carbonate-base metal-gold veins and higher temperature (-300^-350°) copper dominated veins andfeultfillsassociated with a specific, highly altered trachyandesite dyke suite. These veins commonly trend 120® with steep north-east dips. Some north and north-east trending structures are also mineralised, often containing lower gold grades but elevated zinc concentrations. (Juartz-magnetite veins v^iiich sometimes contain molybdenite ± chalcopyrite ± sphene ± epidote ± actinolite are early and higher temperature. Low temperature base metal-gold veins contain predominantly arsenian marcasite, galena, matildite, low Fesphalerite, pyrite and electrum in a gangue of Mn-calcite, smectite, interlayered smectite-chlorite, heulandite, quartz and laumontite. Gold in these veins can contain up to 65% silver, 1.5% mercury and minor tellurium. Native silver is also present. The more copper-rich and higher temperature vein occurrences contain chalcopyrite, Ag-Bi-rich galena, gold, numerous Bi-Pb sulphosalts, pyrite and sphalerite (exsolving chalcopyrite) in a gangue dominated by quartz (sometimes amethystme), chlorite, sericite, carbonate, k-feld^ar, albite and rare tourmaline. Moderate to intense hydrothermal alteration occurs throughout large areas of the MTVC. Early propylitic alteratioQ is widespread and potassic, phyllic, argillic alteration and variations of these alteration types have been noted Propylitic alteration causes replacement of clinopyroxene by epidote and chlorite aggregates, the patdiy development of epidote-calcite-chlorite ± actinolite, the introduction of pyrite and the albitisation of plagioclase phenocrysts. Tourmaline developed in association with the development of biotite ± k-feldspar (potassic) alteration. It often occurs replacing plagioclase phenocrysts (with adjacent hornblende totally pseudomorphed by biotite). This tourmaline development is not related to the tourmaline-quartz-pyrite breccias. An inferred resource of 130,000 tonnes at 8 g/t gold has been identified at one of the prospects in the MTVC.
440
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
POST MID-MIOCENE EPISODES OF ACTIVE UPLIFT OF INTRPALTE HIGHLANDS: THE MACQUARIE RIVER VALLEY, AUSTRALIA. Kerrie M. Tomkins and Paul P. Hesse Department of Physical Geography, Macquarie University, Sydney, NSW 2109
The denudational history of the Macquarie River valley in the Transition Zone was investigated to test Bishop and Brown's (1992) model of episodic isostatic uplift in response to denudational unloading of the highlands and sediment loading of the basin. Longprofiles of the basalt filled Mid-Miocene valley, the inset Pliocene bedrock valley and highest terrace were reconstructed to look for evidence of incision and deformation due to highland uplift. The timing and nature of sedimentation in the valley was determined to identify periods of base level stability and instability. The history of erosion and sedimentation in WeUington Caves, located in the Bell River (a major tributary) was also included to provide additional evidence for incision between basalt extrusion and erosion of the Pliocene valley. It was found that the Mid-Miocene valley and Pliocene valley showed evidence for two periods of major incision, the first during tiie Mid-Miocene, the second during the Late Miocene. The longprofiles of tiie valleys showed clear evidence for deformation and oversteepening in the Transition Zone which triggered knickpoint retreat during each incisional period. The evidence of extensive subaerial erosion from Wellington Caves during the Early Pliocene is consistent with knickpoint retreat. The periods of incision and oversteepening in the longprofiles thus provide clear evidence for uplift events of the highlands during the Mid-Miocene and the Late-Miocene. It was found that sedimentation in the valley, between the Late Miocene and Late Pliocene was conformable which indicated a period of base level stability. Minor incision resulting in terrace formation and lateral river migration forming a ridge and swale floodplain, began in the Early Pleistocene. The terrace longprofile does not show evidence of significant divergence. It does however, show convergence with the floodplain towards the Darhng Basin. The implications of this convergence for highland uplift in the Pleistocene is unclear. The evidence for uplift in tiie Transition Zone in the Macquarie River valley shows that uplift is episodic, occurring over a return period of millions of years which implies a tectonic (active) force. The valley length over which inflexion occurs (>50km) provides evidence for longwave flexure of the lithosphere. The timing and response of tiie Macquarie River valley to uplift shows that tiie model of Bishop and Brown (1992) does not apply to the entire highlands. The suggestion of similarity between otiier valleys in the Darling Basin which drain from the highlands may indicate that uplift of the highlands operates on a regional scale.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
PORPHYRY COPPER AND GOLD MINERALISATION AT CARGO, NSW. Christopher E. Torrey' and Peter D White^ 'Golden Cross Resources NL. 22 Edgeworth David Avenue, Homsby, NSW 2077. ^Geomin Exploration Ltd. 18 Giles Way, Tanners Point, RDl, Katikati, New Zealand.
The Cargo porphyry Cu-Au project is located 35km southwest of Orange, NSW, adjacent to the old goldmining town of Cargo. Total recorded production from the goldfield was 318kg of gold, predominantly from quartzcarbonate veins. Modem exploration of the Cargo Intrusive Complex for both copper and gold began in the mid 1960s when porphyry-style mineralisation was first recognised. Recent consolidation of the previously fragmented mineral tenure has led to a re-evaluation of the project. Detailed geological mapping, petrography, airborne magnetic and radiometric surveys and metal distribution studies have outlined a large concentrically zoned, Cu-Au-(Mo) mineralised and altered igneous complex which, prior to faulting, would have been in excess of three kilometres in diameter. The protolith comprises lavas, pyroclastics and epiclastic sediments of the Ordovician Cargo Andesite which include rocks ranging in composition from andesites to trachyandesites. These are intruded by a coeval calcalkaiine suite which includes diorites, quartz diorites and dacite porphyries informally referred to as the Cargo Intrusive Complex. Dacite porphyries predominate as both quartz- and feldspar-rich varieties and locally grade compositionally into rhyodacite. In the centre of the intrusive complex coalescing zones of intense brecciation occur. Breccias contain fragments of all of the above rock types in a matrix of quartz monzonite. This late intrusion is alkalic, and ranges in composition from monzodiorite to syenite. The limits of the intrusive complex are well defined by a distinctive aeromagnetic anomaly which, in plan, is semicircular in shape, comprising a central core of relatively subdued, and a rim of high, magnetic susceptibility. A sharp, north-trendmg, faulted contact marks the western boundary of the anomaly and a broad, concavewestward arc of low magnetic susceptibility marks the southern, eastern and northern boundaries. Prior to faulting the intrusive complex was probably circular in plan, and with diameter in excess of 3 kilometers. Intrusive rocks, including breccia bodies, occupy the central core of the magnetic anomaly, whereas homfelsed biotite-magnetite-bearing andesites occur on the magnetic rim. Alteration is concentrically zoned around the breccia with some elongation along a northwest-trending structural corridor which cuts through the central part of the complex. East-, northwest- and north-trending subvertical and flat lying sheeted and stockwork quartz veins occur in this corridor. The core of the complex is characterised by biotite and K-feldspar alteration with extensive and strong overprint by retrogressive chlorite and sericite. Beyond this potassic zone, an outer rim of sericite-chlorite-pyrite-silica (phyllic) alteration occurs within andesites and dacite porphyries, and overprints early biotite and magnetite in homfelsed andesites. This zone gives way to propylitic alteration characterised by chlorite-epidote-carbonate-quartz-pyrite assemblages on the periphery of the complex. Gold- and basemetal-bearing quartz-carbonate veins appear to form a radial vein pattern on the margin of the magnetic anomaly at the transition between phyllic and propylitic alteration assemblages. These locally cut sheeted and stockwock quartz veins. The paragenesis is outlined as follows. Stage 1 was a magmatic event characterised by extensive albitisation of feldspars and formation of biotite and magnetite especially within the protolith. Stage 2 was a magmatic-hydrothermal episode resulting in the deposition of sheeted and stockwork quartz and K-feldspar veins, magnetite and biotite. Stage 3 was a hydrothermal event caused by cooling and dilution of fluids and was characterised by formation of retrogressive chlorite and sericite, and the deposition of sulphide minerals, abundant carbonate and gold. The intmsive complex has a distinctive metal zonation which consists of a central core containing elevated copper and molybdenum with anomalous gold, a peripheral gold-rich halo which straddles the edge of the magnetic anomaly and an outer zinc-rich zone. The core contains chalcopyrite, molybdenite and lesser bomite in interstices between breccia fragments and within stockwork veins and is coincident with potassic alteration. The gold halo corresponds to the zone of radial quartz-carbonate veins, is locally anomalous in Pb, Zn and Cu, and straddles the outer potassic, phyllic and inner propylitic zones. Geochemically elevated zinc overlaps the outer part of the gold halo within the propylitic zone. The outer limit of propylitic alteration is unknown.
442
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
MORPHOGENESIS AND TECTONIC SETTING OF THE SILURO-DEVONIAN GRANITOIDS IN THE NORTHERN LACHLAN FOLD BELT Robert Trzebski. Paul Lennox and Derecke Palmer' 'Department of Applied Geology, UNSW, Sydney 2052
Granitoids constitue nearly 30% of the area of the Lachlan Fold Belt and potentially ofSsr important clues as to the basement, tectonic setting and crustal architecture. Large data sets have been collected on their geochemistry and mineralogy but structural analysis has only been attemped on a few bodies. Substantive evidence as to their 3D geometry and subsurface geometry has not been available. This study was designed to address this short coming in the northern Lachlan Fold Belt (NLFB). The NLFB granites intruded already deformed Ordovician mainly volcanic and minor metasedimentary rocks during Late Silurian-Early Devonian extensional deformation. The three granites in this study outcrop near the margin of the meridional Molong High with the Hill End Trough. The Carcoar and Barry Granodiorites are weakly deformed I-type granites north of the moderately deformed S-type Sunset Hills Granite. The northern most Carcoar Granodiorite is a blocky body bounded by the Carcoar Fault on the west and Ammerdown Fault on the east. The hour-glass shaped, north-south elongated Barry and Sunset Hills granites both lie west and parallel to the concave-shaped extension to the Ammerdown Fault and nearby Copperhannia Thrust. Gravity and structural studies potentially reveal the relationship at depth between the intrusive bodies and adjacent faults. They may help us to reconstruct the morphogenesis of the granites and the tectonic setting during magma genesis, ascent and emplacement. In this study we provide details of these three granites which have contrasting emplacement modes, intrusion geometry and deformation style. Using high-resolution gravity and detailed structural data we demonstrate that the nearly coeval magma emplacement occurred during both transtensional and shear deformation. Further work is proceeding to better control the timing of granite intrusion, fault kinematics and the relationship between magmatism and regional deformation. Gravity is a most effective geophysical method to model granitoid intrusions at depth. Granitoids are generally characterized by negative gravity anomalies due to their relative low density. Two methods were used for the granite modeling: the Linsser filtering and the 2.5D density modeling. The Linsser filtering generates a tomography of the gravity field in selective depth levels and indicates the depth, orientation and position cf geological source bodies, including intrusives and tectonic faults which are causing high density contrasts. By stacking the individual Linsser displays a 3D model is obtained which in turn provides starting parameters for the 2.5D density modeling along cross-sections. The gravity models and structural data suggest that the Carcoar Granodiorite intruded transtensional faults which were formed due to pull-apart movements related to the oblique opening of the adjacent Early Silurian Hill End Trough. In contrast, the Barry Granodiorite and the Sunset Hills Granite show some evidence for emplacement via oblique shear zones between the precursors of the Ammerdown Fault and Copperhannia Thrust. The gravity models will be compared with tectonic models akeady developed for this area to characterise the time-space geological history of this part of the northem Lachlan Fold Belt. This is important because current tectonic models for this part of the Tasmanides are poorly constrained in the third dimension. We are working towards a better understanding of crustal architecture of the Tasmanides through dating the key events, characterising of fault movement succession and modelling of the granites. This has important implications for the style, timing and structural setting of mineralisation m the northem Lachlan Fold Belt. Acknowledgements: Hargraves Resources NL are thanked for supporting our research in the northem Lachlan Fold Belt. Mr R. Cotton and Mr J. Graham have encouraged and assisted our understanding of the Carcoar area. Dr C. Rizos, School of Geomatic Engineering, UNSW is thanked for his guidance regardmg the DGPS.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
STRUCTURAL AND GEOCHEMICAL CONTROLS ON GOLD DEPOSITION AT THE TANAMI GOLD MINE, NORTHERN TERRITORY Andrew J. Tunks* and David R. Cooke Centre for Ore Deposit Research (CODES), University of Tasmania, GPO Box 252-79, Hobart, 7001 ^Current Address - Paladin Resources NL, 245 Churchill Ave, Subiaco, W.A., 6008.
INTRODUCTION The Tanami Gold Mine (TGM), located 650 km northwest of Alice Springs, is one of several significant Au deposits in the Palaeoproterozoic Granites-Tanami Inlier of the Northern Territory. Gold was discovered in 1904, and the mine has been worked intermittently since then. Large scale, open-cut mining commenced in 1987 and finished in 1994. During this time, approximately 7 Mt of ore were mined at an average grade of 2.1 g/t Au. New deposits in the area south of the original mine site are currently being evaluated (mining re-commenced at Dogbolter, 10 km south of the TGM in December 1995) and are expected to increase the total resource to over 1,000,000 ounces of gold. These new deposits occur along strikefromthe TGM GEOLOGICAL SETTING The sedimentary and volcanic rocks of the Mt Charles Beds, a subdivision of the Black Peak Formation, host the TGM. Deposition of the Black Peak Formation occurred in a continental rift setting that developed on the multiply- deformed high-grade metamorphics of the Ditjiedoonkuna Suite during the Palaeoproterozoic Leichardt rifting event. In the Tanami area, the Mt Charles Beds can be subdivided into three separate informal units; the Footwall, Mine and Hangingwall sequences, based on aeromagnetic interpretations and limited surface exposure. The Mine sequence is composed of up to five <150m thick pillow-basalt units separated by sedimentary units of similar thickness. The thick basalt-dominated units are intercalated with minor fine-grained sedimentary lithologies. Individual beds within the Mine sequence are laterally continuous, and it is possible to trace centimetre-thick chert beds for several hundred metres. Thicker units, such as the carbonate-mudstone facies and basalt breccias, can be correlated over several kilometres. STRUCTURE Gold mineralisation at the TGM is hosted in brittle shear zones and veins that transect all Mine sequence lithologies. Mineralised shear zones are characterised by a progressive increase in fracture and vein density towards the centre of the zone. Vein textures include open-space fill, crack-seal veins, and internally deformed veins that contain discrete slip surfaces which have truncated earlier-formed crystals. These textures indicate periodic reactivation of the mineralised structures related to cyclical variations in fluid pressure and shear stress. Within the array of mineralised shear zones and veins, three main orientations occur at 350-010°, 030-040° (sinistral strike-slip), and 070-080° (dominantly dextral strike-slip). Stress-inversion calculations using faultstriation data indicate a subhorizontal, NW-SE-directed Oi stress for the mineralising event, with 02 subvertical. GENESIS Auriferous quartz ± sericite ± chlorite ± pyrite ± ankerite ± chalcopyrite veins and breccias were deposited from high-temperature (300°C), low-salinity (5 m.% NaCl), I0W-CO2 fluids (0.5 m) with a^^S values around \2%c and calculated S^^O^ater values between 9.0 and 11.4%o. Mineralogical, isotopic and fluid inclusion data are consistent with a weakly acidic (pH = 5), reduced (mH2s > niso42-)5 H2S-rich (~ 0.006 m) hybrid magmatic/metamorphic fluid generated during granite emplacement and associated contact metamorphic devolatilisation. When mineralising fluids reached the base of the mine sequence, a build-up of fluid pressure combined with the existing regional stress field to create the appropriate conditions for brittle failure in the basalts. The competent basalt units were spaced close enough to allow faults to propagate through the intervening low-competence sedimentary units. Cyclic rupturing and sealing of the fault zones due to variations in thefluidpressure and shear stress coupled with subsequent hydrothermal self-sealing created conditions favourable for fluid-wall rock interaction and the formation of sericite-pyrite-carbonate alteration halos. Pyritisation of magnetite and chlorite in the basalts, and hematite in the sedimentary rocks destabilised AuHS and Au(HS)2', causing gold deposition in the veins and country rocks. Localised decreases in P(f) associated with opening of dilational fault jogs may have also been important in causing phase separation, H2S loss and gold deposition. Shear zones did not penetrate far into the hangingwall sequence, causing dispersion of the mineralising fluids and preventing the deposition of additional ore-grade gold in the overlying sedimentary rocks. 444
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
UNESCO/I.U.G.S. IGCP 328: AN UNQUALIFIED SUCCESS? Susan Turner Queensland Museum, P.O. Box 3300 S. Brisbane, Queensland 4101
International Geological Correlation Programme Project 328 on Palaeozoic Microvertebrate Biochronology and Global Marine/Non-Marine Correlation (1991-1996) was part of the Unesco-IUGS collaboration programme which brings together scientists with common goals to solve key geological problems. The project was an Australian initiative with Drs Gavin C. Young (then A.G.S.O.) and S. Turner, joined in mid 1993 by Dr Alain Blieck of France, acting as co-leaders (Turner & Blieck 1997). Major international meetings took place in Canada, China, Germany, Russia, Belgium and France, each with resultant published abstracts and/or symposium volumes. Regarded as one of the most successful IGCP projects, with over 1000 publications to date, it involved geologists and palaeontologists working in around 50 countries. The main aim of IGCP 328 was to coordinate international research on, and to promote the geological use of micro vertebrates (scales, teeth, denticles, fin spines, bones, otherwise known as " ichthyoliths") and to integrate the data initially on the main groups of Palaeozoic fishes. By using fish microremains and the individual databases set up, biostratigraphy was investigated, leading to refined biozonation especially in the mid Palaeozoic. Correlation of marine and non-marine sequences where vertebrates come into their own (complementing palynology) has been enhanced. The data have also been applied to gain insights into palaeoenvironments, palaeogeography, and distribution of the main taxa. During the project younger Mesozoic faunas including those in the North West Shelf of Australia, East Timor and Irian Jaya were described for the first time. In addition, by analysing the microvertebrate data, controlling factors and geological processes such as climatic changes, geographic changes such as sea level rise and fall, catastrophic events, and palaeoenvironment adaptation were assessed. Major outcomes include a greater understanding of the number, and temporal and geographic ranges of the early vertebrates which gave rise to higher tetrapods, including ourselves. Of the seven major groups of fishes, both jawless agnaths which dominate from late Cambrian?-()rdovician to late Devonian (Frasnian), and jawed cartilaginous and bony fishes, many new taxa have been described and discovered. Detailed search of later Devonian and early Carboniferous rocks worldwide has led to increased knowledge of the transition from fish to tetrapod and work in train on related microfaunas has recognised co-occuring taxa to pinpoint possible tetrapodbearing deposits. We now have a greater understanding of the microfossils, which the often disparate fish groups provide. These data have now entered the university courses on micropalaeontology in many participant countries and several graduate students have defended theses on microvertebrate topics both in Australia and elsewhere. From the basic systematic work has come a series of detailed schemes of vertebrate zone fossils, particularly in the Ordovician of Australia, Silurian of the northern hemisphere, Devonian and Carboniferous worldwide, utilising a range of taxa, particularly thelodonts, acanthodians, and sharks (e.g., Blieck et al. in press). Comparison of faunas in eastern Australia, Tasmania and New Zealand with those of e.g., Iran and China is aiding assessment of Gondwanan terranes (e.g., Turner & Young in press). Studies on Triassic oil and gas-bearing deposits use shark remains to aid geochronological assessment. These standardised schemes offer dating constraints for geologists working particularly in non-marine rocks, such as the central and western parts and the northwest margins of Australia. At least one successful IGCP project, 406 on Circum-Arctic Palaeozoic faunas has resulted and another on Late Palaeozoic/Mesozoic faunas in major economic basins is planned. REFERENCES Blieck, A., Turner, S. & Young, G.C. with contributions from Luksevics, E., Mark-Kurik, E., Talimaa, V.N. & Valiukevicius, J.J. in press Devonian vertebrate biochronology and global marine/non-marine correlation. In P. Bultynck & R. Feist (eds) Subcommission on the Devonian System Review, Courier Forschungsinstitut Senckenberg. Turner, S. & Blieck, A. 1997.The final flings of IGCP 328: Palaeozoic microvertebrate biochronology and global marine/non-marine correlation. Episodes 20, no. 1, March, 48-52. Turner, S. & Young, G.C. (in press) Devonian marine/non-marine correlation in East Gondwana. In A. Blieck & S. Turner (eds.) IGCP: 328, Final Report. Acknowledgements: ST grateftilly acknowledges the support of the Australian IGCP Committee throughout the years of IGCP 328 and to attend the 14'^ AGC meeting.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
COUPLED MECHANICAL/FLUID FLOW MODELLING OF PAPUAN FOLD BELT MINERALISATION Phaedra Upton and Paul Gow CSIRa, Exploration and Mining, PO Box 437, Nedlands, Western Australia 6009 and the Australian Geodynamics Cooperative Research Centre
New Guinea is situated within a broadly oblique-convergent collisional setting, representing the margin between the Pacific and Australian tectonic plates. Associated with the convergence, initiated in the Oligocene, was formation of several major late-Tertiary Cu-Au deposits of the porphyry- or related styles. The major deposits include Porgera, Ok Tedi, and Frieda River in Papua New Guinea, and the Grasberg deposit in Irian Jaya. As a result of variable subduction rates at the plate margin, the regional stress regime in New Guinea has fluctuated from extensional to compressional during convergence (Hill et al., 1993). However, from approximately 8 Ma to the present, the stress regime in central-northern Papua New Guinea has been compressional, resulting in formation of the Papuan Fold & Thrust Belt. This was accompanied by synchronous intrusion of felsic-intermediate magmas, and possible reactivation of transfer structures formed during earlier extension. The Porgera and Ok Tedi deposits were formed within the fold belt during this period (6 Ma and 1 Ma, respectively). The genetic relationship between the compression, intrusion, fault reactivation, and mineralisation is currently unclear, although Corbett (1994) has suggested a direct link between the transfer structures and mineralisation. This paper presents results from two- and three-dimensional mechanical and coupled mechanical/fluid flow modelling. These models are based on current geometries from the Papuan Fold Belt as well as pre-fold belt development geometries determined from restored cross-sections (Hill 1991). The models consider the mechanical effects of collision of the belt of Miocene granites located to the north of the Papuan Fold Belt and help constrain possible fluid-flow environments that existed in the Papuan Fold Belt during the main period of mineralisation (6 - 1 Ma). Mechanical modelling of the Papuan and Andean convergent margins (Mason and Ord 1996) has shown the change in the deformation, volume change and orientations of the stress tensor that occurs as the amount of partitioning between strike-slip movement and internal deformation of the material is varied by changing the relative strengths of the faults and the material. If highly partitioned, that is the fault takes up most of the strikeslip movement, then a fold belt develops in the material adjacent to the fault. This is the situation we see in Papua New Guinea. If the system is not highly partitioned, the we see both strike slip movement and the development of a less intense fold belt. Combining techniques in structural geology, geophysics and mechanical modelling provides a powerful methodology which we can use to unravel the dynamic, thermal and fluid flow histories of complex regions such as the Papuan Fold Belt. By studying the prospective terranes in New Guinea, we hope to understand more clearly the dynamic controls on ore body location within both ancient and active convergent margins. REFERENCES Corbett G. J. 1994. Regional structural control of selected Cu/Au occurrences in Papua New Guinea. Rogerson L. ed. Proceedings of the PNG Geology, Mining and Exploration Conference 1994.
In:
Hill, K.C., Grey, A., Foster, D. & Barrett, R. 1993: An alternative model for the Oligo-Miocene evolution of northern PNG and the Sepik-Ramu Basins, In: Carman, G.J. and Z., eds. Petroleum Exploration and Development in Papua New Guinea: Proceedings of the Second PNG Petroleum Convention, Port Moresby, 31 St May - 2nd June, J993, Hill K.C. 1991. Structure of the Papuan Fold Belt, Papua New Guinea, AAPG Bulletin, 75, 857-872. Mason R. & Ord A. 1996 Modelling the effects of crustal structure during convergence Abstract Third International Symposium on Andean Geodynamics, St Malo, France, 17-19 September 1996
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
COUPLED MECHANICAL/FLUID FLOW/THERMAL MODELLING: APPLICATION TO THE PETROLEUM ACCUMULATIONS OF THE NORTH WEST SHELF OF AUSTRALIA Phaedra Upton and Chongbin Zhao CSIRO, Exploration and Mining, PO Box 437, Nedlands, Western Australia 6009 and the Australian Geodynamics Cooperative Research Centre
In the Timor Sea, northwestern Australia, convergence of the Australian and Eurasian plates in the latest Miocene has induced reactivation of Mesozoic rift faults. Associated with this faulting is rupture of seals, reduced trap integrity and a fluid flow event. This event involved the flow of hot, saline brines up major faults and through the Mesozoic sequences with the associated breaking of seals/traps and the loss of hydrocarbons(0'Brien et al 1996). The biodegradation of the leaked hydrocarbons has been associated with the development of the Hydrocarbon-Related Diagenetic Zones (HRDZ) within the Eocene aquifer sands. We have created mechanical/fluid flow, fluid flow/thermal and mechanical/fluid flow/thermal models based on the Swan Graben within the Vulcan Sub-basin to consider the effects of the mechanics and the thermal structure upon the fluid flow regime within the basin MECHANICAL/FLUID FLOW MODELLING Under a hydrostatic pore pressure regime, extension drives fluids up the reactivated Mesozoic faults. If brines are present at depth within the basin, these move up the faults with the faults failing first at depth. A brine source at depth is not vital to the flow regime, however it provides excess fluid which leads to pore pressures greater than hydrostatic in the basin and increases the chance of seal rupture. Seal permeability plays an important part in the development of the fluid flow system within the basin. If the seal does not rupture, a steady state deformation and flow regime develops. If the seal ruptures, intermittent failure and flow occur along the faults with periods of through going failure and flow occurring. FLUID FLOW/THERMAL MODELLING As a first step towards a fully-coupled three-fold system of mechanics, fluid flow and thermal transport, we have run coupled fluid flow/thermal models. These produce a three cell convective system within the basin for a nondeforming material under a thermal gradient of 10°C per kilometre. This flow system has fluid flow up one fault and down the other. This produces an asymmetric temperature pattern with the region around one fault warmer than the other. MECHANICAL/FLUID FLOW/THERMAL MODELLING Initially we consider only the mechanical response of the material to the fluid flow and thermal transport that is occurring. In subsequent models we will consider the effect of an extensional boundary condition on the models. The three cell convective system observed in the fluid flow/thermal model becomes a two cell system and fluid is driven up both Mesozoic rift faults. The result of this fluid flow system is to produce a thermal pulse up both faults. SUMMARY These models investigate various driving forces for fluid flow and thermal transport in basins such as those of the North West Shelf of Australia. Deformation and a thermal gradient can both drive fluids up the Mesozoic rift faults in the region. Numerical modelling of coupled mechanics, fluid flow and thermal transport offers an inexpensive method of testing geometric models, which are often based on observations alone. Consideration of the f^undamental mechanics which have affected buried accumulations will increase our understanding of the preservation and migration of hydrocarbon pathways. REFERENCES O'Brien G.W. 1996. Late tertiary fluid migration in the Timor Sea: A key control on thermal and diagenetic histories? APPEA Journal 36 399-427.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
CHARACTERISATION OF NOVEL NANO-ORGANISMS ON TRIASSIC AND JURASSIC SANDSTONES PhilippaJ.R. Uwins^ Richard I. Webb ^ and Anthony P. Taylor^'^ 1 Centre for Microscopy and Microanalysis, and 2Department of Microbiology The University of Queensland, St Lucia, Queensland, Australia 4072
The identification and existence of nannobacteria has been proposed but remains a controversial issue and the subject of intense scientific debate. Until now the evidence for the existence of sub-bacteria sized organisms has been based solely on the morphological similarity of mineralised structures to living bacteria. Bacteria range in size from ISOnm-SOjimi in diameter, while the proposed nannobacteria are an order of magnitude smaller with diameters reported to range from 20-150nm in diameter. Nannobacteria are therefore considered by many to be too small to contain the enzymatic and genetic material essential for life. It remains unresolved whether nannobacteria are the fossilised remnants of autonomous life-forms, artefacts of sample preparation, or unusual mineral deposits. In this presentation we report active and spontaneous growth of organic features on Triassic and Jurassic sandstones which correspond in size to those referred to by Folk and others as nannobacteria (eg. Folk, 1993) We do not follow Folk's convention of spelling, nor do we relate the organisms we describe to bacteria until their phylogeny has been established. Instead we refer to these features as nano-organisms or nanobes to indicate their significant difference in size to proteobacteria and archaeons, loosely following SI convention. We were able to establish enrichment cultures of these organisms and document their morphology, elemental composition, and structural detail by a variety of electron optical, spectroscopic and molecular analysis techniques. Nanobes have cellular structures similar to Actinomycete^ and fungi (spores, hyphae and fhiiting bodies) with the exception that tiiey are up to 10 times smaller in diameter (20nm-1.0|Lim). Nanobes have hollow, membrane bound structures that are composed of C, O and N and stain positive for DNA using DAPI stain, a common intercalating agent for DNA. Whilst morphologically distinct, nanobes are in the same size range as the controversial nannobacteria described by others in variety of different rock types and in the Martian meteorite ALH84001. Based on the evidence that we present, it is our thesis that nanobes are biological structures. For example, 1. Nanobes are communicable and grow spontaneously at atmospheric pressure at 22°C on a variety of substrates; 2. Nanobes closely resemble tiie morphology of actinomycetes and fungi, but are much smaller. The characteristic radial and axial symmetries governing the morphology of the spores, filaments and branched hyphae are typical of membrane bound structures; 3. Nanobes consist of C, O, and N as shown by energy dispersive spectroscopy. 4. Nanobes are not solid objects, but contain a central cavity, which extends the length of the hyphae. Nanohyphae and microhyphae have closed ends, 5. Nanobes have smooth to finely granular non-crystalline hydrophobic wall structures; 6. Nanobes contain DNA as indicated by DAPI staining. We did not find evidence to suggest that the observed features could be explained as inorganic compounds. In particular, a crystalhne mineralic origin is excluded by the absence of electron diffraction patterns, demonstrating the amorphous nature of nanobe walls. If the nano-organisms which we describe are amorphous mineral deposits, it is difficult to propose a mechanism for growth given the environment of culture (22°C and atmospheric pressure), and tiie highly restricted elemental composition lacking significant quantities of silicon, sulphur, and metals. Consequentiy any inorganic compound mimicking nanobes could not be a silicate, sulphide or metal oxide in either crystalline or amorphous form. In addition, carbonates can also be excluded since no element was detected in sufficient abundance to represent the cation in any carbonate material. Future DNA analyses will determine whether these organisms are related to bacteria or fungi or belong to a different phylogenetic tree altogether.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE BENAMBRAN DEFORMATION, EASTERN LACHLAN FOLD BELT: SOME OBSERVATIONS ON A COMPLEX EVENT A.H.M. VandenBerg Geological Survey of Victoria. PO Box 500, East Melbourne, Victoria 3002
The Benambran Deformation was the first strong deformation to affect the entire eastern Lachlan Fold Belt. Recent mapping by the Geological Survey of Victoria has helped in elucidating the complexity of this event, and provides a good framework for understanding the tectonic history of the region. The first event that may be linked to deformation is a regional facies change, at about the Ordovician-Silurian boundary, from black shale and siltstone of the Bendoc Group, to quartzitic turbidites of the (entirely Llandovery age) Yalmy Group. This transition is concordant and apparently conformable in most places, but evidence that some deformation has occurred in the region occurs in the form of a chert pebble dominated conglomerate wedge (Seldom Seen Formation) apparently derived from Bendoc Group rocks, and mass flow deposits comprised largely of carbonate. Sporadic, widely scattered carbonate lenses have been shown to be condensed sequences that, together, record continuous or near-continuous carbonate deposition through much of the Silurian (Simpson & Talent 1992)—the oldest sediments are contemporaneous with the Yalmy Group. They regard the lenses as in situ, and thus require radical age revision of much of the Silurian rock record, but all the sedimentological evidence points to them being olistoliths. The single exception is the small, late Pridoli Pyle's limestone that lies unconformably on Pinnak Sandstone. The presence of these limestones adds a considerable complexity, implying shallow shelf conditions through much of the Silurian in parts of the region. Large rift-like basins (Wombat and Limestone Creek grabens) preserve very thick, dominantly dominantly coherent marine rhyolitic lava and porphyry—the Mitta Mitta and Thorkidaan volcanics. Pyroclastics in one of the limestones suggest the volcanism occurred close to the Wenlock-Ludlow boundary. The volcanics are overlain by thick clastics (Enano and Wombat Creek Groups) in which the first granite clasts occur. It is these that carry most of the limestone lenses, both small and very large, suggesting that much of the limestone shelf was destroyed at this time. The youngest limestone olistoliths in these sediments are of Pridoli age. Which of these events is the Benambran Deformation? The type area, in the Wombat Creek Graben, is of little help because the type unconformity is between Upper Ordovician Bendoc Group and Upper Silurian Wombat Creek Group. The various lines of evidence indicate that deformation occurred at different times in different areas. Where the limestones accumulated, the deformation must have been completed by early Llandovery times whereas farther east, in the Yalmy Fold and Thrust Belt, it occurred either in the late Llandovery or early in the Wenlock, prior to the eruption of the Thorkidaan and Mitta Mitta volcanics. The oldest granites in the region show late Llandovery cooling ages and include granites intimately associated with the amphibolite-grade Omeo Metamorphic Complex (430 ± 1 3 Ma, Nunniong) and some well outside the metamorphics, that intrude folded Yalmy Group (427 ± 3 Ma, Amboyne Granite). The Benambran Deformation is therefore a complex sequence of events that included uplift and denudation, folding and faulting, granite intrusion and regional metamorphism to amphibolite grade, whose effects were different in different areas. The events are part of a proposed major southward migration of part of the Omeo Zone (see Willman et al., this volume). REFERENCES Simpson, A.J. & Talent, J.A., 1995. Silurian conodonts from the headwaters of the Indi (upper Murray) and Buchan Rivers, southeastern Australia, and their implications. Courier Forschungsinstitut Senckenberg 182, pp. 79-215.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
VICTORIAN PALAEOZOIC TERRANES A.H.M. VandenBerg Geological Survey of Victoria, PO Box 500, East Melbourne, Vic. 3002
Victoria provides the only well exposed section across the southern part of the Lachlan Fold Belt (LFB) and the easternmost Delamerides. Although the plate tectonic setting of this region in the Palaeozoic is still uncertain, the exposure provides important insights into its depositional, magmatic and structural evolution from the Cambrian to the end of the Devonian. In Western and Central Victoria, three prominent geological trends all show eastward migration with time. The oldest of these is depositional, marked by an eastward migrating turbidite sheet, which ranges from Cambrian or perhaps older in the Glenelg Province, to Cambrian in the Stawell Zone, Ordovician in the Bendigo Zone, Silurian in the western part of the Melbourne Zone and Lower Devonian in the eastern part. The second trend is the timing of the earliest fold-forming deformation—Delamerian (Cambro-Ordovician) in the Glenelg Zone, Late Ordovician or Early Silurian in the Stawell and Bendigo Zones, and Early to Middle Devonian in the Melbourne Zone. The third trend is plutonism, with the earliest granites in the Glenelg Zone being Cambrian and Early Ordovician, those in the Stawell and westernmost Bendigo Zone being Early Devonian, and those in the eastern Bendigo Zone and Melbourne Zone being Late Devonian—although there are localized Middle Devonian intrusives in the eastern Melbourne Zone. This pattern stops at the Governor Fault, at the boundary between the Melbourne and Omeo structural zones. East of here, each of these three stratotectonic events occur earlier: the main turbidite deposition occurred in the Early Ordovician, the first deformation in the Silurian, and the first plutonism also occurred in the Silurian. This was followed by several cycles of silicic volcanism and sedimentation in the eastern LFB and this causes particular problems in understanding the Melbourne Zone, where there is no trace of these events. Palaeocurrents show westerly and southerly sources until very high in the sequence, in the Lower Devonian Walhalla Group, where the first easterly-derived currents occur. The Governor Fault (VandenBerg et al., 1995) is a poorly documented structure, largely because it is so poorly exposed. The fault strikes northwesterly and thus cuts across the generally north-south structural trends of the region to the west. It marks a major change in the direction of tectonic transport, from eastwards, in the Melbourne Zone of the footwall, to westwards in the hangingwall Omeo Zone rocks. Virtually nothing is known about its other outcrops. The change in major geological trends across the Governor Fault suggests it is a true terrane boundary. The position of the Eastern Lachlan Terrane (ELT) is conjectural—^Fergusson (1987) suggested large-scale dextral displacement along the fault. Although such displacement has not yet been demonstrated by structural observations, moving the Eastern Lachlan Terrane 350 km to the northwest removes some of the anomalies. In such a reconstruction, the Adaminaby Group of the ELT is brought along strike with the Castlemaine Supergroup of the Bendigo Zone—both are the main turbidite pulse in their respective region, and both are Lower Ordovician. Both of these rock units suffered their first major deformation at about the same time. It is suggested that the docking of the two terranes occurred during the Middle Devonian Tabberabberan Deformation and was, indeed, responsible for this deformation. REFERENCES Fergusson, C.L., 1987. Early Palaeozoic back-arc deformation in the Lachlan Fold Belt, southeastern Australia: implications for terrane translations in eastern Gondwanaland. In E.C. Leitch and E. Scheibner (Editors), Terrane Accretion and Orogenic Belts, American Geophysical Union, Geodynamics Series 19, pp. 39-56. VandenBerg, A.H.M., Willman, C.E., Hendrickx, M.A., Bush, M.D. & Sands, B., 1995. The geology and prospectivity of the 1993 Mount Wellington Airborne Survey area. VIMP Report 2, 165 p.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE GRETA COAL MEASURES CLASTIC SEDIMENTATION IN A HIGH ACCOMMODATION SETTING A. Van Heeswijck and R. Boyd Department of Geology, University of Newcastle NSW 2308
The Early Permian Greta Coal Measures are a regressive wedge of coarse clastic sediments with generally discontinuous coal seams of variable thickness. They formed in a high accommodation setting developed by isostatic adjustment of a thrust beh in a retro-arc foreland basin. The Coal Measures are boimded by the lower marine Dalwood Group and the upper marine Maitland Group. In outcrop an erosional unconformity was located at the base of the Greta Coal Measures in the proximal Cranky Comer Basin and the distal Maitland-Cessnock-Greta Coalfield. Close study of borehole and electric log data showed that these unconformities were unrelated. Two sequence boundaries are therefore recognised in the Greta Coal Measures. The Mirannie Sequence comprises coarse clastic sedimentation with very minor, thin peat production. Regressive parasequences are topped with fme grained floodplain sediments. Electric logs show aggradational and retrogradational parasequence stacking patterns. This is interpreted to represent a lowstand systems tract with a remnant transgressive systems tract formed in a high accommodation setting. Clastic sediment supply in the early-middle lowstand tract equals or exceeds the increase in accommodation. This sediment supply competes and overwhelms localised peat swamps. Unloading of the thrust zone caused rebound of the Mirannie Sequence and the removal of the remainder of the sequence. This relative fall in base level formed the erosional unconformity represented by the Gilliston sequence boundary. The Gilliston Sequence has a similar depositional style to the Mirannie Sequence. The lowstand systems tract is represented by progradational to aggradational coarse clastic parasequences. The parasequences terminate in sandy bartops with occassional floodplain sediments or very thin peat swamps. Localised, thicker coal seams show that the rate of increase in accommodation or sediment supply was more variable in the Gilliston Sequence. Generally the high increase in accommodation and large sediment supply precluded the development of thick, extensive peat mires. The very thick Greta and Tangorin Coals developed at the end of the lowstand tract when the increase in accommodation was less than the sediment supply and a bypass surface occurred. The top of the Greta Coal represents a transgressive surface. The Greta and Tangorin Coals are a high volatile, low ash bituminous coal with a moderate sulphur content. Proximate analysis results show an upward increase in volatile matter and ash in both seams. Cannel coals are found in the top portion of the Greta Coal. These show an increase in the rate of groundwater rise and subsequent drowning of the mire as the accommodation rate increases. An upward increase in the sulphur content of both seams suggests that the drowning of the mire was caused by brackish water. The high vitrinite content of both seams shows that groundwater conditions were stable both proximally and distally. Low vitrinite values near the base of both the Greta and Tangorin Seams are reflected by higher inertinite and liptinite values. This shows a sharp fall in the water table at some point in time probably due to tectonic adjustment in the hinterland. The two distinct inertinte populations are due to a greater allocthonous content of the proximal swamp. The nearby hmterland shed organic matter which had undergone oxidation before incorporation into the peat. Liptinite content also shows two distinct populations. Gammidge (pers.comm.1997) reports that the majority of the liptinite in the Greta Seam is composed of sporinite. In the Tangorin Seam, Marshall and Draycott (1954) note that cuticular material is common and spores form a very minor proportion. The significance of this is that the amount of leaf matter reduces significantly from proximal peats to distal peats. The upper portion of the Greta Coal Measures in the study area is contained within the transgressive systems tract of the Gilliston Sequence. As the rate of base level rise increases backstepping fluvial to estuarine parasequences contain thin, discontinuous, high sulphur peat mires. Peat growth is unable to keep up with the increase in base level and the mires rapidly drown, often by the incursion of brackish water. The shoreline eventually transgressed over the fluvial sediments ending Greta Coal Measure deposition.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
DETRITAL ZIRCON GEOCHRONOLOGY AND THE PROVENANCE OF THE OMUNG METAMORPHICS, BENA BENA AND GOROKA FORMATIONS, CENTRAL HIGHLANDS, PAPUA NEW GUINEA Nicholas Van Wvck^ and Ian S. Williams^ 1 Geology Department, Box 414, University of Papua New Guinea, Port Moresby, PNG (e-mail :nvw@usa.net) 2 Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia (e-mail: ian.williams@anu.edu.au)
Detrital zircons from two basement blocks in the Highlands of PNG possess a strong Australian provenance signature. Two samples of the Omung Metamorphics, from the south-eastern end of the Kubor block near the junction of the Meril and Wahgi Rivers, yield zircon with concordant ages dominantly -1.8 Ga (8%), -1.50 Ga (11%), 435-480 Ma (16%), -350 Ma (9%), and -270 Ma (41%). Sedimentary petrography and detrital zircon ages indicate deposition of the Omung concomitant with active volcanism. Recumbent isoclinal folding occurred, and an F1 axial planar fabric developed, prior to intrusion and contact metamorphism by the Kubor igneous complex. Two stocks of the Kubor Complex have zircon ages of 245 ± 6 Ma and 238 ± 6 Ma. The deposition and subsequent deformation of the Omung Metamorphics is therefore Late Permian. The two basement units within the Bena Bena block are the Goroka and Bena Bena Formations. The Bena Bena is mostly quartzose and volcanic-rich, while the Goroka contains mostly schist and phyllite. The stratigraphic relationships between the two units and the adjacent Owen Stanley Metamorphics have never been established, however the higher metamorphic grade of the Bena Bena Formation has led most to consider it the oldest unit. The Goroka Formation and an interbedded Bena Bena tuff were sampled near Rintebe. A second Bena Bena sample was collected 5 miles east of Henganofi. The sample of Goroka Formation contained detrital zircons with ages dominantly -1.55 Ga (47%) and 270-310 Ma (37%), with minor mid Paleozoic and mid Proterozoic components. This resembles the detrital signature of the Omung Metamorphics, suggesting the Goroka Formation has a similar provenance and might be correlative. In contrast, a meta-psammite from the Bena Bena Formation yielded only ages of 270-300 Ma (85%) and 235250 Ma (15%). An interbedded tuff yielded only igneous zircon, 222 ± 5 Ma, Late Triassic. The lack of a -240 Ma component in the Goroka Formation suggests that, contrary to earlier interpretations, it is older than the Bena Bena Formation. The age of metamorphism in both units remains unresolved. Detrital zircon ages in metasediments from both the Kubor and Bena Bena blocks show a strong correlation to basement ages m the adjacent northeastern part of the Australian craton. With the exception of a small component of Ordovician ages, all of the detrital zircons in both the Bena Bena and Kubor blocks could have been derived directly from the adjacent Georgetown-Coen Inlier (GCI) of northern Queensland. The Ordovician component, although not present as basement ages, could have been present in the cover sequence on the GCI derived from the adjacent New England Orogen. The likely provenance of all detrital zircons is adjacent northeastern Australia. Furthermore, the Bena Bena block was located close enough to the Kubor block to receive a -240 Ma detrital component during the deposition of the Bena Bena Formation. We favor a simple tectonic model where the Kubor and Bena Bena blocks are not suspect terranes as previously has been asserted, but rather part of the Australian craton. The cratonic margin was modified by rifting during the Mesozoic and has since been reinverted during Cenozoic compression. The Australian craton, in the eastern Highlands of PNG, probably extends at least as far north as the northern edge of the Bena Bena terrane.
452
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
SMALL-SCALE ISOTOPIC SAMPLING OF SINGLE IGNEOUS FELDSPAR CRYSTALS IN THE WILSON'S PROMONTORY BATHOLITH, VICTORIA Tod E. Waight^'^, Roland Maas^ and Ian Nicholls^ 1 VIEPS Dept. of Earth Sciences, La Trobe University, Bundoora, Victoria 3083 2 VIEPS Dept. of Earth Sciences, Monash University, Clayton, Victoria 3168 3 Danish Lithosphere Centre, 0ster Voldgade 10,1350 Copenhagen K, Denmark Small-scale sampling of individual feldspar crystals from a microgranular enclave and from enclosing enclave-rich granite within the Wilson's Promontory Batholith (SHRIMP zircon age = 395 Ma) has revealed internal Sr-Nd isotopic heterogeneity or zonation - 'isotopic stratigraphy' believed to reflect temporal change in the isotopic composition of the host magma during feldspar crystallization. Pov^der samples weighing 1-2 mg were collected from small (-1 mm) shallow drill pits along traverses across several large (10-20 mm) plagioclase crystals believed on textural grounds to have been transferred from the host magma to the enclave during mingling of enclave and host magmas. One of the conspicuous alkali feldspar megacrysts in the adjacent host adamellite was also sampled in this way. In some cases, the presence of minute biotite inclusions in the feldspars sampled was indicated by elevated Rb/Sr ratios of the powder samples; these samples were discarded. Mineral separates of fine-grained alkali-feldspar, apatite, biotite and zircon from the enclave were also analysed. The enclave chosen for study was found to have relatively evolved isotopic signatures (Sri -0.7088, eNd i^ot typical of the bulk of the enclave population (Sri 0.707 to 0.710, eNd 0 to -3). All studied feldspar crystals show roughly similar isotopic trends from core to near rim of decreasing Srj and increasing eNd- Observed mirror-image trends of Sr- and Nd isotopic variation indicate that any effects of post-crystallization diffusion and disturbance of the Rb-Sr and Sm-Nd isotopic systems were imimportant. Sr-Nd isotopic variation is believed to reflect changes in the isotopic compositions of the host magma over the period of feldspar crystallization, related to either intrinsic heterogeneities within the magma body inherited from the source, or due to open system behaviour (e.g. magma mixing/mingling). Magma mingling is supported by the presence of abimdant microgranular enclaves. All feldspar crystals also display a pronoimced shift to higher Sri (0.710) and lower e^d (-6) near their rims. These rim compositions are not mirrored by whole rock data for either the host granite (Sri = 0.709, eNd= -3) or the enclave (except for eNd ^ the latter). By contrast, feldspar rim eNd is similar to that of fine-grained biotite, feldspar and apatite separated from the enclave, and also the bulk enclave (eNd / feldspar and apatite have Sri -0.710, similar to the rimts of the large feldspar crystals. This suggests that feldspar rim compositions reflect growth in the evolving enclave magma, after magma mingling and transfer of feldspar crystals from the felsic host to the enclave. The following preliminary model is proposed: Feldspar crystals formed in a felsic magma imdergoing isotopic exchange with admixed more primitive magma (represented by the bulk of the mafic enclave population). Some of this enclave magma was then contaminated by a third, previously imrecognized component with evolved Sr-Nd isotope ratios (possibly sedimentary wall rocks of the intrusion), producing the low eNd and some internal isotopic disequilibrium within the studied enclave. Some plagioclase crystals from the host magma were later transferred into the enclave, e.g. by convective shearing. The large alkali feldspar crystal analysed was not found in direct contact with the enclave but the isotopic data for the rim of this crystal, and the presence of several other similar alkali feldspar crystals which straddle the enclave-host boimdary, suggest i t may have resided in the enclave at least temporarily. Our results indicate i) that effects of crystal transfer between isotopically distinct magmas may be detected from small-scale internal isotopic stratigraphies in single large crystals; ii) that large feldspar crystals in the WPB adamellite preserve a record of changing magma isotopic ratios not readily appreciated from whole rock analyses; iii) the Sr-Nd isotopic variations in the feldspars suggest that both magma mixing/mingling and crustal assimilation processes influenced the evolving composition of the felsic magma system. 453
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
TRACE ELEMENTS IN SEDIMENTARY PHOSPHORITES Stephen Walsh Centre for Coastal Management, Southern Cross University, PO Box 5125, East Lismore NSW, 2480
Trace metal contamination of soil, water and biota as a result of phosphate fertiliser use on farms has recently been recognised as a problem and produce from some farms no longer complies with food quality standards for cadmium. However, few studies of trace metals in potential phosphate ores have been published and most published studies involve few samples or present data for a limited range of trace elements. This paper reviews published trace metal concentration data for a wide range of potential phosphate ores and provides new data for trace metals in phosphorite nodules from the Chatham Rise and other sites in the southwest Pacific Region. Past studies have estimated that around 100 million tonnes of phosphorite nodules occur along 400 km of the Chatham Rise, a submerged platform east of New Zealand. Most of the Chatham Rise phosphorites have similar mineralogies and texture and very similar major and minor element contents except for phosphorus, cadmium and zinc; P2O5 concentrations range from 12.6-24.4 % and cadmium concentrations from 0.5-3.3 ppm (Table 1). Compared to Nauruan phosphate ore and world averages for other phosphate ores, the Chatham Rise phosphorites have a notably low cadmium content but a high uranium content; this finding needs to be considered in future resource assessment. The reason for the generally low trace metal concentrations in marine phosphate nodules remains unclear, but it probably reflects microbially mediated redox conditions in the genetic environment, the chemistry of upwelling bottom water and exsolution during diagenetic recrystallisation. Table 1. Phosphate and trace metal contents of phosphorite nodules from Chatham Rise and the southwest Location
%
Cd ppm
Co ppm
Cu ppm
Mn ppm
Ni ppm
Pb ppm
Zn ppm
U ppm
24 23 22 13 18 20 18 14 22
2.4 2.2 2.4 0.7 3.7 2.4 3.9 1.9 3.7
21 21 21
6 13 7 n.d. 9 7 5 3 36
92 91 103 394 76 62 58 259 2467
10 20 10 n.d. 37 12 19 49 24
55 55 60 18 61 56 65 69 124
11 97 19 9 79 15 114 128 142
230
P2O5
Reserve Bank, CR i612, Chatham Rise 1613, Chatham Rise h622, Chatham Rise h676, Chatham Rise Kaikoura, NZ Stoneyhurst, NZ p847 Smokey Cape t278, Fiji
8
54 61 24 61 125
* * *
* * *
* *
Wonarah
16
*
*
58
140
30
500
200
22
Nauru
39
71
3
25
*
*
4
300
90
18
21
35
1352
200
74
195
120
World Average
* indicates no data available
n.d. signifies not detected
Further evaluation of possible controls on trace metal concentrations in different phosphorites will require the collection of data for other phosphorites that can be compared to the Chatham Rise material and to existing published data. Detailed investigation of trace elements in bedded, nodular and guano-derived phosphorites using laser ablation ICP-MS will provide data that is expected to reveal a more clear relationship between particular phosphate ore types and geochemically distinct groups of trace elements. Preliminary data suggest that the trace element content of different phosphate ore types is sufficiently distinct that trace element composition may be useful in classifying phosphates with different genetic and diagenetic histories. At present the recovery of marine phosphorite nodules from the seabed is more costiy than mining existing terrestrial deposits but this limitation is being reduced by: improved seabed mining techniques, depletion of easily mined terrestrial resources and the environmental and economic benefits of the lower contaminant content in marine phosphorite nodules.
454
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
PERMIAN CORAL REEFS IN HUNAN CHINA Yang Wanrong Nanjing Institute of Geology and Palaeontology. Academia Sinica, Nanjing 210008, China. Email mpIab@Publiclpyy.js.cn
This reef outcrops in Permian Changhsingian strata at Cili, northwest Hunan, consisting of massive limestones more than 50m in thickness. Reef-building corals are monotonously the tetracoral genus Waagenophyllum which appears as bushy masses, each of which are fan-shaped, preserved in growth position. The bushy coral masses are 0.3 to 1.0m high, densely distributed in the limestone, comprising over 85 percent total rock, and forming a skeletal framework. Forming the reef core are lime mud-filled-framestones. Due to strong dolomitization, the internal structures of the coral branches are destroyed. The dolomitized branching corals are honeycomb-shaped, considered a favourable petroleum reservoir. The ecological coral reef displays a complete lateral facies belt of reef composition. Distributed from north to south are basin marginal slope facies (thin bedded spicular siliceous lime mudstone), fore-reef slop facies (reefbreccia containing bioclastic packstone), reef core facies (dolomitized reef framestone), backreef facies (greenalgae bioclastic packstone). The vertical sequence consists of echinodermal bioclastic packstone (reef base), dolomitized coral framestone (reef core), and dolomitic striped wackestone (reef cap) Overlying strata are Triassic shaly mudstone, with oolitic and oncoid limestone. The microfacies sedimentary sequence of coral reef indicates a shallowing upward environment and the progressive reef-forming model. This type of ecological coral reef is the first reported from the Permian of China. It is characterised by its great size, well-preserved original growth characteristics, and the dominance of Waagenophyllum\ such a coral reef has not been reported from Permian strata elsewhere in the world. Flugel and Kahler ( 1992 ) listed types of Permian reefs of the world, but they cite no examples of Waagenophyllum coral reefs. Compared with modem coral reefs (Carbone et al. 1992) the Permian Changhsingian bushy coral reef at Cili, Hunan, as indicated by its paleoecology and sedimentary features, is from a restricted, calm-water, low energy environment.
455
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
GEOMETRY OF THE TAMWORTH BELT (NEW ENGLAND OROGEN) AND ITS RELATIONSHIP TO THE BOWEN BASIN IN THE SUBSURFACE BENEATH THE SURAT BASIN IN SOUTHERN QUEENSLAND Wolfram Waitenherg'-. Russell J. Korsch' and Andreas Schiifer ' Australian Geodynamics Cooperative Research Centre, Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601 -Geologisches Institut, Rheinische Friedrich-Wilhelms Universitat Bonn, NuBallee 8, D-53115 Bonn, Germany
The New England Orogen consists of arc-related rocks produced during Devonian-Triassic plate convergence at the interface of eastern Gondwana and Panthalassa. The Tamworth Belt is exposed in northern New South Wales but continues northwards beneath the Early Jurassic-Early Cretaceous Surat Basin into southern Queensland. The belt is generally interpreted as a Devonian-Carboniferous forearc basin within a continental margin convergent plate setting. Cropping out immediately to the west of the Tamworth Belt is the Gunnedah Basin, considered to be initiated by back-arc extensional processes in the Early Permian. The Gunnedah Basin forms part of the Early Permian-Middle Triassic Bowen-Gunnedah-Sydney basin system, which also continues to the north, beneath the Surat Basin. The western margin of the Tamworth Belt is defined by the Mooki Fault, which is part of a major near-longitudinal Middle Triassic system of thrust faults and displacement transfer zones defining the approximate eastern limit of the Bowen-Gunnedah-Sydney basin system (Korsch & Totterdell 1995). In northern New South Wales, the Tamworth Belt has been thrust over the eastern margin of the Gunnedah Basin. Here, we examine the geometry of the Tamworth Belt and its relationship to the Bowen Basin in the subsurface in southern Queensland, using a variety of geophysical techniques. We have used aeromagnetic and gravity data to define the limits of the Tamworth Belt in the subsurface. The belt extends northwards into southern Queensland and then curves to the northeast, forming part of the Texas Orocline. We have also interpreted a regional network of shallow (4 seconds two-way travel time) seismic reflection profiles collected by the petroleum exploration industry. In the subsurface, the Tamworth Belt consists of a very thick sedimentary package (at least 2.5 sec TWT, approximately 5 km thick) containing several pronounced sequence boundaries. It is difficult to date these sequences because palynological control is provided by only seven samples from drill core. The oldest dating is considered to be of Visean age; the other samples providing microflora were dated as Late Carboniferous to earliest Permian. The Tamworth Belt has been deformed by a series of thrust faults, most of which appear to be west-directed and forming part of a foreland thrust-fold belt. Nevertheless, within the westward-propagating thrust system there are some east-directed thrusts which we interpret as backthrusts. The eastern edge of the present limit of the Bowen Basin is related to the western limit of the thrust system, which is formed by the Leichhardt-Moonie-Tingan fault system, separated by displacement transfer zones. Thus, in the area of interest, the Bowen Basin has been deformed and has been incorporated into the Late Permian to Middle Triassic retro-foreland thrust and fold belt of the New England Orogen. The Moonie Fault, in particular, shows a classic fault-bend fold geometry in the hanging wall, with the Tamworth Belt in the footwall possibly extending to the west beneath the sedimentary rocks of the Bowen Basin in the Taroom Trough for an unknown distance. The base of the Bowen Basin is defined by a sequence boundary which, in places, shows an angular relationship with the underlying volcanic-volcanoclastic pile. To the east within the thrust sheets, the orientation of the sequence boundary essentially mimics that of the underlying thrust fault. REFERENCE Korsch R. J. & Totterdell J. M. 1995. Eastern margin of the Bowen-Gunnedah-Sydney Basin: Geometry of the Burunga-Leichhardt-Moonie-Goondiwindi-Mooki-Hunter fault system. In: Geological Society of Australia, Specialist Group in Tectonics and Structural Geology, Field Conference, Clare Valley. Geological Society of Australia, Abstracts 40, 85-86. Acknowledgements: We wish to thank AGSO, Geological Survey of Queensland, Geological Survey of New South Wales and Werrie Gold Ltd for providing access to new aeromagnetic data. Published with permission of the Executive Director, AGSO and the Director, AGCRC.
456
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"" Australian Geological Convention, Townsville, July 1998
GEODYNAMIC MODELLING OF ASPECTS OF THE BOWEN AND SURAT BASINS IN EASTERN AUSTRALIA Paula Waschbusch'. Christopher Beaumont' and Russell J. Korsch" ' Canadian Institute for Advanced Research, Department of Oceanography, Daihousie University, Halifax, Nova Scotia, Canada B3H 4Ji - Australian Geodynamics Cooperative Research Centre, Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601
The easternmost tectonic unit in eastern Australia is the New England Orogen, consisting of arc and arc-related rocks produced during Devonian to Triassic plate convergence at the interface of eastern Gondwana and Panthalassa. Several sedimentary basins developed inboard of this margin during this time span and are initmately related to convergent plate processes. The Early Permian-Middle Triassic Bowen and Gunnedah basins and the Early JurassicEarly Cretaceous Surat and Eromanga basins in eastern Australia developed due to intraplate deformational events in a backarc tectonic setting behind the active convergent plate margin. These events v^ere a response to interplate tectonic events occurring in, and outboard of, the present position of the New England Orogen. We have undertaken geodynamic modelling of selected aspects of the basins in an attempt to constrain the mechanisms that were operating during the formation of the basins. In the Bowen and Gunnedah basins, the initial event was extensional and stretched the continental crust to form a significant Early Permian East Australian Rift System. Several contractional events from the mid-Permian to the Middle Triassic are associated with plate convergence and the development of a retro-foreland thrust belt in the New England Orogen. The subsidence during the foreland basin phase in the Bowen and Gunnedah basins may be a flexural response to loading of the retro-lithosphere by supralithospheric loads (such as the accretionary wedge thickened orogenic crust and retro-thrust belt). Alternatively, it may be a dynamical platform tilting caused by viscous comer flow in the asthenospheric wedge above the subducting plate, or a combination of the two processes. Here we focus on the Late Permian and Triassic 'foreland basin' phase of the basins. An elastic, finite element plate model is used to calculate the flexural response to orogenic and sedimentary loads, and can incorporate spatial variations in flexural rigidity, broken plate conditions and variations in these properties with time. A comparison of the observed and model isopachs indicates that the primary large-scale features of the observations are reproduced by the models if the effective elastic thickness of the lithosphere was approximately 10 km at the beginning of the loading phase in the Early Permian and increased with time to approximately 20 km at the beginning of the Triassic. This model requires that the loads were placed adjacent to the relatively-undeformed current eastern edge of the Bowen Basin throughout the Late Permian to Middle Triassic period of retro-thrusting in the northern New England Orogen. This requirement is difficult to reconcile with geological constraints such as the original distribution of stratigraphic units now correlated with the Bowen Basin (e.g. Fielding et al., 1997), the fine-grained turbiditic nature of early sediments suggesting a depositional setting somewhat remote from the thrust front at that time, and the notion that foreland thrust belts usually evolve progressively from the internal (eastern New England) to external (western New England) positions as the width of the orogen increases. The dichotomy is removed if the early foreland phase of subsidence, in particular, was primarily a dynamical platform tilt, which removes the need for proximal supralithospheric loads. Mantle corner flow can create both the necessary near-field large amplitude-short wavelength subsidence and a far-field small tilt observed of the Springsure Shelf of the Galilee Basin. In later phases of the Bowen Basin the two mechanisms may have acted together, or supralithospheric loads may have dominated. Preliminary modelling of the Surat and Eromanga basins suggests that subsidence was controlled by dynamical platform tilting, or subsidence owing to viscous comer flow which provides a mechanism for both the near- and f^ar-field effects (see Mitrovica et al., 1989, for a description of the mechanism). REFERENCES Fielding C. R., Stephens C. J. & Holcombe R. J. 1997. Permian stratigraphy and palaeogeography of the eastern Bowen Basin, Gogango Overfolded Zone and Strathmuir Synclinorium in the Rockhampton-mackay region, central Queensland. Geological Society of Australia, Special Publication, 19, 80-95. Mitrovica J. X., Beaumont C. & Jarvis G. T. 1989. Tilting of continental interiors by the dynamical effects of subduction. Tectonics, 8, 1079-1094. Acknowledgements: PW and CB thank the AGCRC for financial support to work in Australia during part of this study. RJK publishes with permission of the Executive Director, AGSO and the Director, AGCRC.
457
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14'^ Australian Geological Convention, Townsville, July 1998
NEOPROTEROZOIC ATTUNGA ECLOGITE IN THE NEW ENGLAND FOLD BELT Teruo Watanabe^ C. Mark Fanning^ and Evan Leitch^ Dept. Earth and Planetary Sciences, Graduate School of Science, Hokkaido Univerisity, Sapporo, Japan, 060 ^Research School of Earth Sciences, The Australian National University, ACT 0200 ^Dept. of Applied Geology, University of Technology, Sydney, NSW 2007
In the southern New England Fold Belt, the Woodsreef melange is a serpentinite zone formed along the Peel Fault dividing the middle Palaeozoic subduction complex from an associated fore-arc basin. In the serpentinite melange zone, Cambrian ophiohte and middle Ordovician high P metamorphic rocks occur as small blocks. Eclogites, age previously unknown, have been reported from Attunga, Gleneden and Port Macquarie. As the eclogite always occurs as isolate small blocks, their occurrence was uncertain other than they are found adjacent to meta-gabbro/serpentinite. In this study we recognise that eclogite from Port Macquarie is retrogressively metamorphosed lawsonite eclogite whereas the eclogites from Attunga and Gleneden are petrologically similar mafic eclogite. At Attunga, the eclogite outcrop is only of a few metres in length and is surrounded by meta-gabbro with a narrow (-10 cm) transitional zone. Both the eclogite and meta-gabbro have suffered retrogressive greenschist facies metamorphism producing actinolite, chlorite, epidote and very rare green biotite. No relict pyroxene has been identified from the rock fragments of the meta-gabbro part, but the eclogite often preserves nearly equigranular garnet and clinopyroxene. For the Attunga eclogite, an unusually wide range of garnet compositions has been previously described. Our studies of the garnet zoning pattern demonstrate that at least a two-stage garnet growth has occurred. High Mg garnet rims are considered to be evidence for a rapid increase in the thermal gradient during a later, second stage of garnet formation. From the jadeite mole content in clinopyroxene, together with Mg/Fe partitioning between garnet and clinopyroxene we estimate that these rims formed during the second metamorphic event at -SOO^C and -15 kb pressure. The early stage P-T conditions for the eclogite are difficult to determine as there is no distinct zoning pattern in that clinopyroxene. However, high Ca and relatively high Mn contents of inner garnet suggest a lower thermal gradient. The Gleneden eclogite has experienced blueschist metamorphism, with Na-amphibole and Na-pyroxene present as inclusions within garnet cores. In the -10 cm transitional zone between the eclogite and meta-gabbro, garnet becomes gradually more sporadic and finally chlorite pseudomorphs after garnet can be seen near the meta-gabbro. Thus it is evident that the eclogite has been carried into its present association by the gabbroic intrusion, resulting in a complex history for the eclogite. SHRIMP U-Pb analyses have been made on zircons separated from both an eclogite enclave and host gabbro collected at the Attunga locality. The zircons from the eclogite are clear elongate grains with round terminations. Cathodoluminescence (CL) images reveal an internal structure that is consistent with a metamorphic origin. The areas analysed by SHRIMP are very low in U (<5 ppm) and extremely low in Th (<0.1 ppm); though one anomalous area has -240 ppm U and -20 ppm Th. The U-Pb analyses are relatively enriched in common Pb yet define for the most part a simple mixing hne between a -570 Ma radiogenic end member and common Pb. A weighted mean of the Pb/ U ages gives 571 ± 22 Ma which we interpret to reflect the time of eclogite formation. There is one significantly older area at -650 Ma which is interpreted as being inherited. Zircons from the gabbro host are similarly clear elongate grains with subround terminations. These zircons do not have simple magmatic morphologies and the CL images of the sectioned grains record evidence for a previous metamorphic history. The areas analysed are more enriched in U (1-15 ppm) than those from the eclogite, though are still very low in Th (<0.1 ppm). The U-Pb analyses are similarly enriched in common Pb but define a simple mixing line between a -460 Ma radiogenic end member and common Pb. The crystallisation age of zircon in the gabbro is 460 ± 15 Ma. The Neoproterozoic to Early Palaeozoic elements recognized in the serpentinite melange provide us with evidence for tectonic evolution in the eastern Gondwana margin following the break-up of Rodinia. This evolution results in the formation of the Ordovician arc-trench system. Above all, the chronological study for the Attunga eclogite and surrounding gabbro leads us to a speculation of the tectonic development along the Peel Fault with -570 Ma eclogite facies metamorphism and Ordovician uplift of eclogite together with gabbro intrusion and blueschist exhumation. At this stage we are not able to resolve the process and mechanism for disruption of the supposed ^c-trench system and on the formation of the serpentinite melange. Nevertheless, a major conclusion is that the first subduction zone had been produced in the Palaeo-Pacific ocean in Neoproterozoic times, approximately 200 my after the break-up of Rodinia.
458
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
CONTRASTING STYLES OF FELSIC SUBMARINE VOLCANISM, EASTERN MANUS BASIN, PAPUA NEW GUINEA. John C. Waters and Raymond A. Binns CSIRO Division of Exploration and Mining, P.O. Box 136, North Ryde, NSW, 2113
The Manus Basin is an extensional back-arc basin behind the New Britain trench and volcanic arc, to the north-east of mainland Papua New Guinea. The eastern part of this basin includes a series of en-echelon neovolcanic mafic to felsic ridges, which developed on older arc crust and parallel the extension direction. Detailed sampling and bottom photography from five research cruises in the eastern Manus Basin has uncovered two areas of submarine felsic volcanism, Pual Ridge and SuSu Knolls, which are dominated by dacites exhibiting contrasting styles of volcanism. These provide excellent examples of varying flow morphologies within felsic submarine volcanic complexes, some of which are normally treated as typical of intermediate to mafic compositions. Pual Ridge is a narrow, linear, steep sided ridge 15 km long which rises to 1655 metres, about 500 m above the surrounding basalt to basaltic andesite seafioor. Volcanics along the ridge range in composition from andesitic through to rhyodacitic. Dacitic volcanism predominates and is dominated by aphyric, poorly to moderately vesicular glassy lavas which show a wide range of flow morphologies. Sheet, lobate and more commonly chaotic autoclastically fragmented flows, all dacitic in composition have been photographed along the crest of the ridge. Fragmented flows composed of jumbled chaotic blocks to 5 m in size are conmion. These blocks are characteristically glassy with jagged, hackly upper surfaces. Some of these jumbled flows appear to extend for several hundred metres and have subcircular collapse pits to 5 metres in diameter. They produce a highly irregular topography and resemble subaerial block lavas. Rarer smooth to ropy surfaced sheet flows occur at several localities along the ridge crest. These flows have surface textures more commonly found in basalts and andesites. Lobate flows have individual lobes up to 5 m across and have flow fronts with well developed ropy wrinkles and fractures. Locally developed gradations between chemically similar flow types suggests that cooling-related increases in viscosity rather than variations in magma chemistry are responsible for the observedflowmorphologies. Associated with most dacitic lava types at Pual Ridge is poorly sorted hyaloclastite debris. Most of this debris is cobble to sand-sized angular dacitic hyaloclastite spalled from the glassy margins of theflows.Elongate platyfragmentschemically similar to the lavas are also common along the central part of the ridge. These 1 to 15 cm long fragments have an outer pale coloured glassy margin characterised by veiy fine, highly attenuated tube vesicles. Similar pumiceous glassy margins have been found on some larger blocks dredged from Pual Ridge suggesting that this debris represents the fragmented outer pumiceous margin of flows. At SuSu Knolls porphyritic dacitic volcanics form three prominent, moderately sedimented, steep sided conical peaks informally known as North Su (1150m), South Su (1320m) and Suzette (1520m). These are developed above a surrounding platform of gentiy deformed Pliocene aged sediments. These knolls are comprised of moderately to strongly plagioclase-clinopyroxene-orthopyroxene-magnetite-phyric dacitic volcanics. The groundmass of the volcanics is strongly microlitic and shows a preferential alignment of microlites around glomerociysts and larger phenociysts. Samples dredged from near the crest of the knolls only vary rarely have preserved millimetre-thick glassy rinds. Dredging often returns blocky debris to several tens of centimetres in diameter suggesting the outer carapace to the lava domes may be partlyfragmented.Cobble-sized volcaniclastic breccias composed of variably altered volcanic and minor cherty fragments in a sulfidic mud matrix recovered from the summit of South Su may possibly be autoclastic in origin. The main volcaniclastic deposit associated with the dacites at SuSu Knolls however, is a fine mixture of muds and sandy hyaloclastite composed of glass and crystal fragments. Depth records during camera tows across the tops of both North and South Su suggests that there may be lava spines of up to 100m high preserved on their summits. The linear nature of Pual Ridge parallel to the extension direction within the eastern Manus Basin suggests that emplacement of the dacites was likely to be via fault controlled fissure eruptions. These eruptions involved relatively low viscosity dacite melts which were erupted at, or above, their liquidus temperatures. These flowed away from their vents, cooling and increasing in viscosity while interacting with the seawater to produce the observed glassy aphyric lavas. In contrast to this, SuSu Knolls appears to have formed as a result of 3 discrete point source eruptions of crystal rich high viscosity lava which formed domes. While the morphologies of the dacites at SuSu Knolls are typical of felsic volcanics in ancient sequences, those exhibited along Pual Ridge are more typical in old mafic-intermediate volcanics but can no longer be considered diagnostic of such.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
MINESITE POLLUTANTS: IN SEARCH OF THOSE THAT GOT AWAY RT. Watkins Environmental Inorganic Geochemistry Group, School of Applied Geology Curtin University of Technology
Mining and processing of ores leads, directly or indirectly, to the breakdown of their relatively refractory and resistant minerals and the release of metals into the environment in a more mobile form. New metal compounds enter the hydrosphere and pedosphere as particularly active chemical species, which are readily incorporated into the biogeochemical cycle. Even metal compounds that become associated with so called "chemical sinks", such as sediments, retain the potential to be remobilised as a result of bioturbation and resuspension, or through a change in the geochemical environment, and consequently their toxicological risk remains intact. Their toxicity, however, largely depends upon the degree to which they are assimilated by biota and this, in turn, is dependent upon the chemical form in which they exist. To date, ecotoxicology, public awareness, and environmental legislation have all been based upon the total amounts of contaminant metals present. The study of environmental geochemistry, and more particularly questions of minesite pollution control and rehabilitation, necessitate that we know not only "how much" of a metal is present, but also in which chemical and physical form it exists. Such metal speciation will directly determine the degree to which a pollutant metal will emanatefromthe mine source, its mobility and mechanisms of transport and dispersion, and how and where it will eventually be incorporated into a pollutant sink. Precise determination of metal species is a difficult and complex task. The analytical difficulties and the time requirements for "hands-on" experimentation have thus far precluded the development of techniques by which species analysis might be routinely applied to minesite monitoring and investigation. Nevertheless, if our understanding of mining pollution is to advance, and with it our abilities to control pollution and predict the longterm envu-onmental impacts, it is essential that methods for routine measurement of individual metal species be found. The development of robotics in chemical analysis, in conjunction with novel approaches to the separation and analysis of chemical species, provides avenues along which the necessary developments may take place. In this presentation, the importance of metals speciation in the control and remediation of minesite pollution will be outlined. Present methods for the determination of metal speciation will be reviewed. Techniques for the routine analysis of metal species being employed by EIGG will be discussed, and results will be presented of on-line ore leaching analysis and differential thermal release ICP-MS analysis. Future directions to expand the availability of metal speciation analysis will be discussed. The potential value of such developments to the understanding and control of minesite pollution is great. Together with advances in empirical study of the behaviour of individual metal species in the environment, they can usefully advance the present simplistic approach of total metal determination.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
OOIDS, REEFS, AND BEACHROCK: WHAT CONTROLS THE DISTRIBUTION OF MODERN MARINE "ABIOTIC" CARBONATES? Gregory E. Webb Department of Earth Sciences, The University of Queensland, Brisbane, Queensland 4072
One of the fundamental enigmas of modem marine geology is the rarity of "spontaneous" carbonate precipitates, despite the supersaturation of shallow tropical waters with respect to aragonite and calcite. Among the better known presumed abiotic precipitates are ooids, reef cements, and beachrock cements. The shallow, high-energy settings of these precipitates have led to a preponderance of abiotic, physico-chemical models for carbonate induction. In particular, degassing of carbon dioxide owing to water agitation and solar warming, evaporation, and provision of abundant pore volumes of water by continuous tidal pumping widely have been considered sufficient cause for spontaneous carbonate precipitation in tropical shoal, reefal and littoral settings. The failure of most sediments in these environments to undergo cementation has been blamed on a variety of poorly defmed organic and inorganic inhibitors. Although shallow, warm, high-energy marine settings clearly should promote carbonate precipitation, particular physico-chemical parameters do not coincide well with the distribution of known marine precipitates. (1) Ooids clearly form in waters that promote carbonate precipitation, yet they typically remain uncemented despite spending the majority of their time buried below the sediment water interface. Encrustation of ooid/pisoid surfaces by interstitial skeletal biota in the Early Carboniferous Gudman Seas of Queensland, followed by continued cortex accretion, demonstrates that the ooids were immobile in normal circulating marine waters for relatively extended periods of time, but remained unlithified. (2) The windward margins of many reefs are preferentially cemented, but cementation is patchy and high energy areas may be less well cemented, whereas low-energy areas have well cemented patches. Additionally, major accumulations of synsedimentary cement occur in the fore-reef slope environments of some Devonian reefs, well below the zone of surficial heating and wave agitation. (3) Beachrocks contain a variety of different marine cements. Both aragonite and Mg-calcite cements have been described and acicular fringes and micritic morphologies occur. Both mineralogies and both morphologies commonly occur in close spatial and temporal proximity. How can ambient water chemistry induce such a range of mineralogies and fabrics in beachrocks, while proximal sands remain unlithified? Clearly the physico-chemical parameters of ambient sea water do not control the distribution of carbonate precipitates. Organic matter has commonly been considered an inhibitor to marine carbonate precipitation, either through chemical means or physical obstruction, however, other organic matter, both living and dead, has been demonstrated to induce carbonate precipitation. A variety of microbial and metazoan metabolic and degradational processes locally increase pH and/or carbonate alkalinity, thereby favouring precipitation. However, the primary control on the distribution of particular morphological types of biologically induced carbonates may relate to the occurrence and distribution within organic matter of specific macromolecules that function as nucleation sites. Stereochemically active organic matter plays an important role in biomineralisation in skeletal organisms, and macromolecules with similar calcium binding activity occur in degraded organic matter. The abundance and geometric relationships of such reactive organic matter within biofilms may control the distribution and orientation of favourable nucleation sites for non-skeletal carbonates on marine substrates. Biologically induced carbonates can be related to organic matter in at least three ways. 1) Crystals may nucleate directly on organic surfaces and then grow away from the organic matter into the ambient water. At least some aragonite fringe cements result from this "external" nucleation. 2) Crystals may also nucleate on randomly oriented sites confined within a three-dimensional biofilm. Resulting felts of crystals produced by this "confined" nucleation form some micritic cements. 3) Carbonate minerals may also replace organic matter in the sheaths of filaments or in the basal mucilage-rich layer of thicker biofilms. Hence, thin films of specific organic matter may promote cementation of grains where exterior nucleation occurs. Thicker biofilms may promote precipitation within, or beneath them, but not upon their surfaces (e.g., enclosed and replacive nucleation). Hence, some types of organic matter may promote the lithification of sediments, whereas other types of organic matter may promote the accretion of ooid corteces, while inhibiting cementation of the ooids to each other.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
ORIGIN OF THE ESCARPMENTS OF SOUTHEASTERN AUSTRALIA - EVIDENCE FROM DETAILED GRAVITY TRAVERSES John A. Webb. Robert J. Musgrave and Ben Kovac Department of Earth Sciences, La Trobe University, Bundoora, Victoria 3083
The Great Dividing Range of southeastern New South Wales terminates along its eastern side as an abrupt north-south scarp, separated from the Tasman Sea by a narrow coastal plain. To the south, in eastern Victoria, the southern margin of the Dividing Range is a more dissected, less well marked escarpment that runs east-west. The origin and history of these scarps, particularly the one in NSW, are controversial. Most authors beUeve that the NSW scarp has retreated by erosion from a position at or seaward of the present coast, but other theories involve dissection of a large-scale easterly downwarp or an origin as a fault scarp. The Great Dividing Range is located over a prominent negative Bouguer anomaly, which is flanked by a short wavelength positive anomaly running parallel to the escarpments in both NSW and Victoria. To see if the scarps are related to the gravity anomalies, two detailed gravity traverses were measured across the scarps, an east-west one in southern NSW and a north-south one in eastern Victoria. The traverses showed that the location of the NSW scarp closely matches the maximum gradient of the gravity anomaly, which is narrower and has a higher amplitude than the anomaly in Victoria. Modelling of the results shows that the anomaly cannot be due to the offshore edge effect, as previously postulated, and is best explained by a substantial upward offset of the middle crust, seaward (eastern) side up. This implies that the upper crust is thinner to the east of the offset, as also indicated in NSW by the lower elevation topography of the coastal plain and continental shelf. In NSW the scarp lies directly above the modelled ofifeet; in Victoria the scarp is located inland of the offset. The mid-crustal step-up is probably a major fault; the detachment model for eastern Australia proposes a steeply west-dipping master detachment along the eastem continental margin. To the east of the offset the average crustal density suddenly increases, reflecting the thinner upper crust; to remain in isostatic equilibrium, this requires a topographic offset of 1-1.5 km, with the seaward side lower than the landward side of the mid-crustal offset. The localized isostatic imbalance was probably adjusted by upper crustal faulting. The scarp in Victoria represents the extreme northern margin of the Gippsland Basin, and, from the gravity interpretation, is most likely an eroded surface expression of an upper crustal fault above a mid-crustal detachment. Seismic sections within this basin clearly show numerous, east-west, down-to-the-south normal faults. Rifting in the Gippsland Basin began at -160 Ma, so the scarp could have originated then, and has been dissected smce that time. The scarp in NSW has a similar gravity signature to the Victorian scarp, and is therefore also likely to represent the surface expression of a fault. Rifting along the southern NSW coast began 80-90 Ma ago, so this scarp is younger than the Victorian escarpment, accounting for its less dissected appearance. There are also fewer large rivers in this region that could erode into the scarp. The original scarp may have been close to its present location, as it coincides with the maximum gravity gradient and therefore the mid-crustal detachment, implying that this scarp has retreated little since it formed.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
MAPPING OF GRANITES USING REMOTELY SENSED DATA Peter Wellman and Richard S. Blcwett Minerals Division, Australian Geological Survey Organisation, P.O. Box 378, Canberra ACT 2601
This poster compares the results of traditional geology, and traditional and new remote sensing products, with respect to mapping the composition and structures in felsic batholiths. The project area is the Shaw, Corunna Downs and Mount Edgar Granitoid Complexes of the north Pilbara Craton, Western Australia. Weathering is minimal, and transported sediments are largely restricted to river channels. Traditional geological mapping resulted in the separation of seven mapping units. Importantly, it was recognised that the granites can be subdivided into three series: early, more mafic granites of mainly biotite granodiorite composition, intermediate-age porphyritic granites of intermediate composition, and later more felsic granites of biotite granite and biotite adamellite composition. Landsat5 thematic mapping data has been optimised for greenstone geology by A.Y. Glikson and L. Macias using directed principal components (pc) and band ratios, with hydrosilicate clay (pc2[4/3:5/7]) shown red, iron oxide (bands 5/4) shown green and quartz (bands 1+7) shown blue. This display did not show any internal subdivisions in the granite, but did clearly show the small areas of mafic and sedimentary rock within the granite and less clearly the distribution of granite tors. Residual gravity anomalies clearly showed the relative depth extent of the various parts of the granites. Magnetic anomalies gave disappointing results. Maps of the first vertical derivative allowed one to map positively and negatively magnetised fractures, mafic dykes and the extent of material of mixed granite/greenstone. The magnetic susceptibility of the granites was estimated from the square-root of the amplitude of short-wavelength magnetic anomalies, and from the medium-wavelength variation in total magnetic intensity. These estimates of the susceptibility, do not correlate well with other measures of either composition, or the extent of individual intrusions. Gamma-ray spectrometric data in the traditional display (potassium as red, thorium as green and uranium as blue), or by display of supervised or un-supervised classification, shows reasonably well the amount of differentiation, but shows poorly the relative proportions of the elements. The K-Th-U data for all granites correlate with each other, forming a elongate cloud in K-Th-U space - the mean evolutionary path. The preferred method of presenting the spectrometric information is for the K, Th, U data for each pixel to be transformed to distance along the mean evolutionary path (differentiation distance) (most of the variance), and deviations of the elements from the mean path (a minor part of the variance). Displays of differentiation distance show that in some batholiths it changes systematically across the batholith, while for parts of other batholiths it is near constant. Displays of the deviations show that some granites are composed of numerous discrete intrusions, and some show no sharp intrusion margins possibly because of deformation. Batholiths largely composed of well-defined equidimensional intrusions, are relatively felsic, with no deformation structures, may be presently exposed near the top of the original batholith. Batholiths with few well-defined equidimensional intrusions, are relatively mafic, and have prominent deformation structures, may be presently exposed part way down the original batholith. The three granite series recognised by field geology can be separated using spectrometric data, and each has a separate evolutionary path within the mean path for all granites. The result of this study is that gravity, magnetics and spectrometry all have a contribution to the mapping of batholiths in the north Pilbara Craton, but that the major contribution will be from modelling the spectrometric data. This dataset should give useful information into the large-scale structure, discrete intrusion and chemistry of the batholiths. The mapping granites on magnetic characteristics alone may give misleading results.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
THE GRENVBLLEAN fflSTORY OF THE WESTERN MUSGRAVE BLOCK, CENTRAL AUSTRALLi. ^Richard W. White', Nigel M. Kelly', Geoffrey L. Clarke' and Ron H. Vemon' ^ ' School of Earth Sciences, Macquarie University, Sydney, NSW 2109, Australia Department of Geology & Geophysics, University of Sydney, Sydney, NSW 2006, Australia
The western Musgrave Block of central Australia is dominated by massive felsic orthogneiss, megacrystic granitoids and layered mafic intrusions, with smaller amounts of metasedimentary gneiss. The oldest rocks are metapelitic gneiss, calc-silicate, layered pyroxene gneiss and orthopyroxene-gamet-bearing gneiss, which all contain a high-grade foliation (Si) cut by areally extensive c. 1300 Ma felsic orthogneiss. Metasedimentaiy rocks and orthogneiss preserve c. 1200 Ma S2 assemblages formed under peak conditions of P=5-7 kbar and 7>800°C. Corona reaction textures in metapelitic gneiss are consistent with a period of near-isothermal decompression, having occurred soon after c. 1200 Ma ago, most probably associated with the uplift and erosion of the high-grade Musgrave basement. A c.l070 Ma magmatic pulse resulted in large, layered, mafic to ultramafic sills of the Giles Complex, granitoids, and comagmatic volcanics that unconformably overlie gneiss containing S2. Magmatic thickening of the crust, due to intrusion of Giles Complex gabbros, preceded D3, which resulted in variable recrystallization of earlier assemblages at P=5-7 kbar and r~750®C. Differences in P-r estimates for mineral assemblages that define S3 throughout the western Musgrave Block are inferred to be the resuh of crustal-scale faulting during the c. 550Ma Petermann Orogeny, when the Musgrave Block was thrust northwards onto a juvenile Amadeus Basin. The Grenvillean history of the western Musgrave Block involved orogenic events of duration similar to those inferred to be shaping Earth now.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
SYNGENETIC KARST IN PLEISTOCENE DUNES IN SOUTH WESTERN VICTORIA: DIAGENESIS AND SPELEOGENESIS Susan White Department of Earth Sciences, La Trobe University, Bundoora, Victoria, 3083
Karstification is a complex process controlled by lithology, tectonic structure and climatic conditions. In particular, lithological variability, especially porosity, chemical composition and strength, can be very important for the degree of karstificatiort Whereas massive, well jointed and relatively chemically pure limestones are traditionally perceived as having the best karst development, the extensive but poorly consolidated Pleistocene carbonate dunes in southern Australia have developed extensive karst systems. Earlier studies have assumed that diagenesis of the limestone occurs prior to speleogenesis and that caves develop mainly in massive limestones of sufficient age for this to be the case. In the mid Pleistocene calcarenite dunes cf the Otway Basin, speleogenesis occurs in limestones which have been only partly undergone diagenesis. The karst features and processes at Bats Ridge and Codrington are integral parts of the landscape of such a mid Pleistocene calcarenite dune system and can be used to show the processes involved. The resolution of problems of the rapid subaerial speleogenesis is achieved by the synthesis of the known karst features of such ridges and an appreciation of diagenesis ard lithification in calcareous dunes. Karst development on these extensive mid Pleistocene aeolianite ridges (Bridgewater Formation) in south eastern Australia is dependent on several interrelated conditions: lithological conditions such as the purity of the limestone, its porosity and its ability to support a cavity as well as the availability of aggressive water capable of solution. The dmie ridge must have a sufficiently high proportion of soluble material, pure enough for the solution processes not to be impeded by insoluble residues left after solution has occurred. The rock is a wellsorted, fine-to medium-grained bioclastic carbonate sand which is commonly laminated and variably cemented by calcite. Cross-bedding is a prominent feature and fossil soil horizons are an integral part of the formation but at intensive karst sites such as Bats Ridge and Codrington, the fossil soil horizons are absent. An important lithological aspect is the development of an indurated layer in the original calcareous dunes. It is not clear from the nature of the feature whether this can be classed as a calcrete as there is no clear evidence of either biological activity or pedogenic processes. This indurated layer (kankar or cap rock) is necessary for the development of any major karst features such as caves, as the original sand dunes have insufficient strength to resist collapse. Insufficient tensional and compressive strength in the limestone will result in solutional cavities collapsing before they are very large. The cap rock is present near the surface of the dune and can be seen in cave entrances where a cross-section of part of the dune is obtainable but its full extent and exact position in the dune is unknowa The thickness of the caprock as obtained from these exposures at cave entrances is about 1 metre. Generally, where the cap rock exists, caves form beneath it. In this case it is the tensile strength of the caprock which enables cavities to remain rather than completely collapsing. However, the fiesh collapses which are a characteristic modifying feature in the caves, are usually composed of non indurated limestone. These collapses often show domed roof structures which do not expose the caprock, and the caves retain their linear pattern despite modification by collapse. The cap rock then should be seen as a layer of dune limestone, formed during diagenesis which has sufficient strength to withstand complete collapse when caves and hollows are dissolved beneath it. The Pleistocene calcareous ridge systems of the Otway Basin were deposited as the sea retreated from the c o ^ during the mid Pleistocene. Karst landforms are found in ridges at Bats Ridge, Mumbannar, Strathdownie, Puralka, Ardno and Codrington but some dunes show more intensive karst development than others. As the dunes were deposited during the mid Pleistocene (between about 235 ka and 300 ka), it is difficult to envisage the evolution of this karst landscape from the standpoint of the usual speleogenetic schemes which depend on the presence of compact, well jointed limestones with sufficient time for diagenesis and subsequent karst development. The study of dune karst at Bats Ridge and Codrington develops further the thesis that diagenesis and karstification can and do occur simultaneously in less consolidated limestones. The diagenesis of the calcarenite is occurring now and must have been occurring by the mid Pleistocene. This simultaneous lithification of the carbonate dunes into aeolian calcarenite rock and the development of solutional karst features in the dunes is the characteristic feature of the speleogenesis in this area.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
GEOLOGY OF THE KUCING LIAR DEPOSIT S. Widodo, D. MacKenzie, L. Johnson, C. Brannon, and A. Edwards P T . Freeport Indonesia, Tembagapura, Irian Jaya, Indonesia
The Kucing Liar replacement deposit is located in the Ertsburg Mining District, in the highlands of Irian Jaya, in the western half of the island of New Guinea. The deposit was discovered in 1992 and is being drill delineated at present. The deposit is one of numerous deposits in the district with total mineable reserves exceeding 1.9 billion tonnes, dominated by the Grasberg copper-gold porphyry deposit. Kucing Liar is a complex, multistage copper-gold skam/replacement deposit. An indicated possible resource has been announced of at least 250 million tonnes at a copper-equivalent grade of greater than 2%. The deposit lies on the south limb of the Yellow Valley Syncline, whose axis trends 110°, The deposit ranges from 50 to 200-1meters thick, and is a moderately dipping tabular body with dimensions of at least 1700 meters east-west and 600 meters north-south.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
STRUCTURAL CONTROL OF THE BROWNS CREEK GOLD-COPPER SKARN DEPOSIT, BLAYNEY, NSW. Colin Wilkins' ^Department of Geology & Geophysics, Sydney University, NSW 2006. Gold-copper mineralisation at the Browns Creek underground mine, worked by Hargraves Resources NL occurs within skamed marble and mafic volcanics at their western contact with a major granodiorite intrusion. Massive garnet, pyroxene and wollastonite dominated skams contain disseminated copper sulphides (chalcopyrite, bomite) and gold. The Browns Creek total mineable resource (measured + indicated + inferred) at 30 June 1996 comprised L36 million tonnes at 6.10 g/t gold and 0.44% copper. The mine resource continues to the south and at depth and current mine development is progressing into the deeper southern ore zones with a present published resources of 1.84 million tonnes grading at 6.25 g/t gold. The Cowriga Lode, in the Hargraves Resources underground operation, consists of structurally controlled coppergold bearing skams and gold-rich retrograde skams located between multiple strands of the 4000E Fault ^ n e which propagated at the contact between granodiorite and the Blayney Volcanics / Cowriga Limestone country rock sequence. The structural complexity of the ore body became apparent when continuous exposures were available for mapping prior to stoping. Banding within marbles, in the hangingwall to mineralised skams, was not rehct bedding but an intense foliation in calcite mylonites developed within the 4000E Fault Zone. Dextral transcurrent motion on the 4000E Fault Zone (equivalent to the Mt. David Fault Zone at surface) produced vertical anastomosing fault duplex systems that controlled the location and formation of metasomatic prograde wollastonite-gamet-pyroxene-plagioclase exoskam in both marble and homfelsed basaltic volcanics. Fault controlled high temperature marble-derived and basalt-derived skams were the locus for three subsequent stages of retrograde mineralisation as temperatures began to fall: Stage 1 - Pyrite-pyrrhotite-arsenopyrite-biotite-homblende (barren). Stage 2 - Copper-gold mineralisation consists of chalcopyrite-bomite-(chalcocite)-gold-andradite gamet-epidotechlorite-quartz-calcite. Movement on the 4000E Fault Zone, a dextral transcurrent fault system (with gently plunging shckenside lineations on fault surfaces) constrained the prograde skams and mineralisation stages 1 and 2, to an area of extensional duplexes within the overall fault zone. Fault movement then switched to dextral transtensional dipslip motion (with steep slickenside lineations on fault surfaces). Stage 3 - Transtensional movement along duplex fault slices caused brittle failure and the formation of large volumes of extensional vein arrays preferrentially hosted in pyroxene-gamet basalt-derived skam units. These stmcturally controlled sheeted quartz vein arrays host lower temperature retrograde ore skam assemblages (quartzcalcite-epidote-prehnite-chlorite-gold-chalcopyrite-bomite) characterised by high, and locally bonanza, gold grades (up to 40 g/t). Post-mineralization retrogression, and the intmsion of monzonite / aplite sills and dykes are ubiquitous. Prominent east and west dipping low angle conjugate fault sets are the final phase of deformation recognised in the underground mine and substantially increase the stmctural complexity of the deposit. Inspection of a geological level plans illustrates the main stmctural features of the ore body which is developed in Blayney Volcanics and Cowriga Limestone, bounded to the west by the Carcoar Granodiorite and to the east by the 4000E Fault Zone. Hydrothermal fluid flow was guided by the permeability of fault networks and caused heterogeneous skarn formation and mineralisation in alteration zones up to 40 m in width. Skam reaction fronts, for example those affecting marble, are clearly fault controlled. The disposition of Blayney Volcanics, Cowriga Limestone and their skarn derivatives are controlled by dextral strike-slip duplexes at a major bend in the irregular granodiorite contact that imposed a fundamental boundary constraint during the propagation of fault strands. The extensional form of the fault duplex also controlled the intrusion of post-ore monzonite bodies that cut and invade sheeted quartz vein ore and contain rotated blocks of ore. A genetic model for the Browns Creek orebody involves a primary late stage intmsive-related hydrothermal fluid focussed into permeable fault zones that propagated adjacent to pluton margins. Orebody mapping has so far delineated a major dextral transtensional fault system controlling mineralisation to +450m below surface, and within mine exploration drilling has traced this stmcture down to 1.2 km below surface. Gravity and stmctural studies on the emplacement of the Carcoar Granodiorite (Trzebski et al., 1998 and Lennox et al. 1998, this volume) have shown independently that the orebody-controlling fault system is prominent down to 4 km below surface and the intmsion itself is likely emplaced into a transtensional stmctural setting.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
INHERITANCE IN GRANITES: THE RESPONSE OF ZIRCON, MONAZITE, AND THEIR PB-U ISOTOPIC SYSTEMS TO REGIONAL METAMORPHISM LEADING TO HOST METASEDIMENT PARTIAL MELTING M S. wuiiams
Research School of Earth Sciences, The Australian National University, Canberra, ACT, 0200
The nature of the sources of the early Palaeozoic granites of the southern Lachlan Fold Belt (LFB), eastern Austraha, remains a subject of lively debate. While it is generally acknowledged, on the basis of the granites' chemical and isotopic compositions, that most of the magmas are unlikely to have been derived directly from the mantle, the relative contributions of components from the crust and mantle in producing the range of granite compositions observed, and what those components might be, remain contentious. A feature of the majority of the Lachlan granites is the presence of inherited zircon as cores within zircon grains precipitated from the melt phase of the magma. The abundance of inheritance varies widely as a function of granite composition. As a group, the generally metaluminous I-type granites are relatively poor in inheritance, particularly the relatively high temperature granites which are chemically and isotopically the most primitive. In contrast, inherited zircon is very conmion in the peraluminous S-type granites, the proportion of total zircon which is inherited tending to be highest in the most mafic host rocks. The presence of the most inheritance in the relatively cool S-type magmas argues in favour of the inherited zircon being an intrinsic component of the magmas, not remnant from assimilated country rock. If it is accepted that the LFB granite magmas were not total melts, but rather partial melts containing some unmelted source material, then it is highly likely that the inherited zircon is remnant from that source. That being so, the ages of the inherited grains potentially provide a characteristicfingerprintby which the source rocks might be identified. A point of caution however; although the inherited zircon appears to have remained chemically suble in the granite magmas, and the measured blocking temperature of the Pb-U isotopic system in zircon (^OOT) is above the magmatic temperatures, it is not necessarily correct to assume that the Pb-U ages measured on the inherited zircon cores accurately reflect the ages they would have given prior to incorporation in the magma. In other words, the measured ages of the inherited cores might be biased. To test this possibility, a study has been made of one case in which a granite and its probable source rock can both be sampled. At Cooma, in southeastern NSW, turbidites of the Ordovician Binjura Beds locally have been subject to low pressure regional metamorphism. Over an exposure distance of about 8 km the grade rises progressively from chlorite, through biotite, cordierite, andalusite and silhmanite grades to migmatite. At the centre of the complex is the small, S-type Cooma Granodiorite. Previous studies suggest that at least the western side of the metamorphic complex is probably intact and virtually in situ. There is some debate about possible relative movement between the higher and lower grade rocks, but very close similarities between the chemical and isotopic compositions of the metasediments, migmatites and granodiorite suggest that the granodiorite is anatectic— a consequence, not the cause, of the metamorphism. If so, the low grade Binjura beds are in effect the granodiorite's source rock. Zircon and monazite extractedfrommetasediments of different grades, the migmatite and granodiorite, reveal how these minerals responded to the metamorphism and host rock partial melting. In the biotite grade metasediments both minerals have abraded detrital grain shapes and preserve a range of dominantly Proterozoic ages reflecting theu- vanous protosources. The zircons, dated by SHRIMP U-Pb, are dominantly 460 to 600 Ma old, with a second major group between 1000 and 1200 Ma, and lesser groups at -1.8 Ga, -2.2 Ga and -2.1 Ga. With rising grade the monazite becomes increasingly embayed and ultimately very rare, only to reappear as well-formed crystals in the migmatites and granodiorite. The partly-dissolved monazite preserves Proterozoic ages, but that which is newly formed records only the age of metamorphism/magmatism. No isotopic evidence for the survival of mhented monazite was found. In contrast, the zircon preserves its detrital crystal form without apparent modification up to the sillimanite zone, where overgrowths develop which eventually enclose the detrital grains, giving them stubby euhedral crystal forms. Larger overgrowths in the leucosomes generate prismatic grains! Overgrowth ages match the ages of the new-grown monazite, and even in the granite, the overgrowths have the low ThOJ common in high grade metamorphic zircon. Ages of the inherited cores in the granodiorite zircons match the ages of the detrital zircons in the low grade metasediment very closely; they are dominantly 460 to 600 Ma old, there is a major group between 1000 and 1200 Ma, and lesser groups at -1.8 Ga, -2.2 Ga and -2.7 Ga. Incorporation of older zircon as inheritance in a granite magma appears to have a negligible effect on its Pb-U isotopic system.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
PUBLIC SECTOR GEOSCIENCE IN AUSTRALIA IN THE NEW MILLENNIUM Neil Williams' 'Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601
To forecast what might be in store for public sector geoscience in the new millennium the starting point must be the present. Today, agencies involved in public sector geoscience in Australia include the Australian Geological Survey Organisation (AGSO), the State/Territory Geological Surveys, the Commonwealth Scientific and Industrial Research Organisation (CSIRO), and several Cooperative Research Centres, as well as various State/Territory bodies with responsibility for soil conservation, agriculture, and water resources. The nation's geological surveys account for some 70% of public sector geoscience expenditure, and for much of this century their role has been basic geoscientific mapping and research to encourage economic growth through mineral and petroleum exploration investment. The pre-competitive geoscientific data, information, and knowledge generated by the geological surveys increases perceptions of the nation's mineral and petroleum prospectivity and reduces exploration risk. In recent years there has been growing concern within society about the impact of development on the environment, and about the sustainability of development. In response to these concerns, the traditional role of geological surveys has expanded to include environmental geology and the geoscientific aspects of sustainable development, including resource management, access and security. For example, AGSO is presently increasing its activities in land and groundwater degradation and management. It is also collecting, interpreting and preparing the data necessary to support Australia's United Nations Convention on the Law of the Sea (UNCLOS) claim for extending its offshore jurisdiction beyond the 200 nautical mile exclusive economic zone. Will Australia's public sector geoscience still be focussed on economic growth and sustainability in say the year 2025? The answer to this question will in all probability be yes, but through time the character of the geoscience seems destined to change dramatically. Global trends towards increasing affluence and population growth will continue to fuel demand for earth resources. New mineral deposits remaining to be found in Australia are likely to be buried or concealed by the regolith, and future public sector geoscience research to attract mineral exploration investment will shift from its traditional surface mapping approach to the unravelling of Australia's geology at depth. This will necessitate a growing emphasis on geophysical mapping and on new techniques for interpreting geophysical data. In the case of new petroleum fields, future public sector geoscience research will be increasingly focussed on frontier basins in Australia's marine jurisdiction, as well as on unconventional resources, such as gas hydrates, that might be abundant in the jurisdiction. Public sector geoscience research on sustainability issues seems destined to grow as efforts are made to minimise, and where possible reverse, society's growing impact on the Australian environment. The maintenance of environmental quality through the minimisation of land and water degradation will be a growing priority because of the economic and social importance of land and water resources to the nation. The goal of research should be a full understanding of our natural resource systems and their interaction with the increasing demands of society in order to develop good sustainable management strategies. Remote sensing and geophysical mapping will again be important, not only for elucidating natural systems, but also for monitoring their responses to management initiatives. A particular resource management challenge will follow the acceptance of Australia's Marine Jurisdiction by the United Nations, as the acceptance will give Australia one of the largest maritime regions in the world, and one that embraces substantial parts of the Pacific, Indian and Southern Oceans. Our present knowledge of vast tracts of the jurisdiction is embryonic at best and if Australia is benefit from the jurisdiction and satisfy its obligations under UNCLOS, it must begin to systematically gather baseline information to support the effective management of the jurisdiction. Another challenge for geoscientists working on sustainability issues will be the integration of their work with the activities of other disciplines relevant to sustainability, such as the life sciences, economics, sociology and politics. To be successful the integration will need to take place at all levels of Government, as well as internationally, because many of the sustainability issues facing society are global in scale, yet will have to be managed at either a local or regional level.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
TWO-STAGE, DUAL SOURCE ORIGIN OF BROKEN HILL-TYPE PB-ZN-AG DEPOSITS Patrick J. Williams^ Guoyi Dong Peter J. Pollard ^ Christopher G. Ryan^, Lucy H. Chapman^ and Kylie Prendergast^'"^ ^School of Earth Sciences, James Cook University, Townsville, Queensland 4811 ^CSERO Exploration and Mining, PO. Box 136, North Ryde, New South Wales 2113 ^Now at Great Central Mines, 580 St Kilda Road, Melbourne, Victoria 3004 %ow at North Limited, PO Box 1165, Milton, Queensland 4064
Broken Hill-type Pb-Zn-Ag deposits are rare but significant insomuch as some of them, such as Broken Hill (discovered 1883) and Cannington (discovered 1990), have exceptional metal-content and grade. A long history of genetic controversy partly reflects the complex structure and highly metamorphosed settings, but also relates to the distinctive low S and Mn-Fe-Ca±P±F-rich chemistry which does not equate well with other known types of base metal deposits. New data demonstrate that the ore environments at Broken Hill and Cannington were infiltrated by hot saline aqueous fluids after the local metamorphic peak and are consistent with the possibility that Broken Hill-type deposits evolved with a two stage history involving dissolution, transport and redeposition of metals. The enigmatic geochemistry of the deposits can be interpreted as a product of mixing of primary and fluid-introduced components. Recent work at Cannington and Western A-lode at Broken Hill has revealed similar histories of postmetamorphic metasomatism in both cases though Cannington is distinguished from Broken Hill by its higher F content and Fe:Mn and Ag:Pb ratios. Higher temperature skam assemblages with garnet, hedenbergite and pyroxenoids are overprinted by retrograde assemblages containing amphiboles, pyrosmalite and various low temperature phyllosilicates. Sulphides are paragenetically associated with the younger parts of these parageneses. Mineral and metal zoning patterns are present that mimic those of magmatic hydrothermal skam deposits. Fluid inclusion data from medium to high temperature (300-500''C) postmetamorphic veins in large Proterozoic Pb-Zn-Ag ore systems at Cannington and Broken Hill show that high concentrations of distinctive ore and gangue components (Ca, Mn, Fe, Zn, Pb) were present in a mobile aqueous fluid phase. Very similar multiphase inclusions are present in both deposits and are hosted by quartz, garnet and hedenbergite. The characteristic daughter mineral assemblage consists of halite, sylvite, pyrosmalite and a Pb-K chloride. The veins commonly lack Zn and Pb minerals, and any sulphides that are present are paragenetically younger than the minerals that host the inclusions. Coupled with the presence of Pb-K chloride daughter salt (as opposed to galena) this all implies that the fluids were strongly undersaturated with metals at the time of entrapment despite the fact that proton microprobe analyses commonly show extremely high Pb concentrations in the range of 5-10 wt. %. Abundant postmetamorphic S-poor, Fe-rich Cu-Au deposits occur in the Cloncurry district near the Cannington deposit. These formed from Ca-Mn-Fe-rich saline fluids of likely magmatic derivation that had variable low to high concentrations of Zn and Pb. This demonstrates that such high salinity fluids were widespread and also suggests that highly soluble metals such as Pb and Zn were abstracted from the host sequences. The distinctive chemistry (high metal grades, low S-content, Ca-Mn-Fe-P-halogens), zoning similar to that in skarn deposits, down temperature paragenetic evolution, homogeneous S isotope geochemistry, Pb-isotope geochemistry related to the host sequences, and structural controls of Broken Hill-type deposits can all be explained by a two stage process involving interaction of preexisting base metal concentrations with hot, S-poor, high salinity fluids. Transport of metals along with additional components contributed by the fluid, followed by sulfide precipitation during cooling, fluid phase separation and/or by wall rock or fluid mixing reactions would have led to ore emplacement in appropriate structural sites. The (hot) crustal environments that characterize these deposits and very high measured Pb and Zn concentrations imply that the scale of metal transport could have been large in terms of both mass and distance. Some bulk compositional differences between individual Broken Hill-type deposits (e.g. variable Fe:Mn ratio) may be a reflection of varying {O2 in the postmetamorphic fluid which is likely to have been buffered by the host sequences. Other differences may be inherited from the fluid source and yet others could reflect differing degrees of fluid access to the mineralized sites at various stages in the paragenetic evolution of the deposits.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THRUST SHEETS BOUNDED BY STRIKE-SLIP FAULTS IN THE SOUTH-EASTERN LACHLAN FOLD BELT Clive E. Willman, Marc A. Hendrickx, Vincent J. Morand, Alfons H.M. VandenBerg and Suzanne.J. Haydon Geological Survey of Victoria, P.O. Box 500 East Melbourne, Victoria 3002
New mapping by the Geological Survey of Victoria using an integrated approach between traditional mapping and geophysics has identified a set of thrust sheets bounded by strike-slip faults in the complex Omeo structural zone of eastern Victoria. The Omeo region lies near the junction of four major structural zones: the Omeo, Tabberabbera, Buchan and Mallacoota zones. The structural history is complicated because three major periods of deformation occurred over a relatively short interval of about 45 Ma (Early Silurian to Middle Devonian). The major effect was the wholesale southeastern transport of the Omeo and Tabberabbera zones which led to collision with the Buchan and Mallacoota zones in the Late Silurian. The first deformation in the Early Silurian (Benambran Deformation) produced major northwest- to northtrending folds and faults in the Omeo and Tabberabbera zones which were then parts of a single major zone subjected to low-pressure metamorphism and southwest directed thrusting and folding. After a period of shelf carbonate sedimentation during the Silurian (early Llandovery to ?Pridoli) the newly formed crustal blocks were subjected to localised extension and major southeast tectonic transport. The Tabberabbera-Omeo zones probably started to move in the Mid Silurian culminating in the Bindian Deformation at about the Silurian-Devonian boundary. This movement was concentrated along a number of major marginal strike-slip faults which link with dip-slip thrust faults at the leading edges of thrust sheets. The Tallangatta Creek Fault Zone (TCFZ) accommodated major internal dislocation within the Omeo Zone causing differential movement between two subzones, the High Plains and Corryong subzones. Initial sinistral strike-slip movement along the TCFZ led to the formation of the Wombat Creek Graben in a dilational jog. After deposition ceased in the Wombat Creek Graben and the related Limestone Creek Graben their contents were deformed by the continued southeastern movement of the Corryong Subzone which was thrust over the Limestone Creek Graben in the Buchan Zone along the Indi Fault (reverse dip-slip). The TCFZ links with the Indi Fault which probably underiies the zone at depth as the sole thrust. Further northeast the Indi Fault links with the sinistral strike-slip Gilmore Fault. The Corryong Subzone therefore forms a sheet bounded to the west by the dextral strike-slip TCFZ and to the east by the sinistral Gilmore Fault. At its leading edge, the Indi Fault forms a major southeast-directed thrust. As the Corryong Subzone thrust over the Buchan Zone it became tilted, exposing progressively lower stratigraphic units (and higher metamorphic grade) towards the Indi Fault. Another Late Silurian (Bindian Deformation) linked fault system occurs along the boundary of the Tabberabbera and Omeo Zones. During southeastward transport, the zones were separated by dextral strike-slip movement along the Kiewa Fault Zone which has now been shown to link with the Ensay Shear Zone, also a dextral strikeslip fault. This caused collision of die Tabberabbera Zone and High Plains Subzone with the Buchan and Mallacoota zones. Regional aeromagnetic data and geological mapping indicate the Ensay Shear Zone and the Yalmy Fault Zone may be linked under the Buchan Rift. The Yalmy Fault Zone is inferred to be a major northwest-dipping thrust fault which forms part of the leading edge of this thrust sheet, cutting earlier east-west Benambran thrusts. Although the Snowy River Volcanics and Cainozoic rocks cover the boundary between the Tabberabbera and Mallacoota zones, interpretation of aeromagnetics suggests this boundary runs north-south and is cut by the Ensay-Yalmy fault system. This in turn indicates the amalgamation of the Tabberabbera and Mallacoota zones occurred either early in the Bindian Deformation or during the Benambran Deformation. An episode of brittle faulting associated with mineralisation occurred after the Bindian Deformation but probably before the Tabberabberan Deformation. This was characterized by brittle faulting which initiated the Haunted Stream Fault and reactivated the Ensay Fault. The third major deformation was the Tabberabberan event which had variable effects in the area and occurred in two phases. In the first phase Lower Devonian graben sequences were folded, the Tabberabbera Zone was mildly cleaved and folded but the Omeo Zone was largely unaffected because east-west compression caused sinistral reactivation of the Ensay Shear Zone which partitioned much of the shortening into the Tabberabbera Zone. The second main phase involved northwest movement of the Nunniong Domain (Buchan Zone) which was thrust over the southern margin of the High Plains subzone (Omeo Zone) along the Jam Tin Fault, juxtaposing Early Silurian granite against the Early Devonian Buchan Group.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
PALAEOMAGNETIC AND GEOCHRONOLOGICAL CONSTRAINTS ON THE NEOPROTEROZOIC BREAK-UP OF THE EASTERN MARGIN OF RODINIA Michael Wingate, Christopher McA. Powell. Zhengxiang Li and Robert Bird Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia, Nedlands, WA 6907
The primary juxtaposition of Laurentia near the eastern margin of Australia in Rodinia is permissible by the overlap in the Rodinia fit of palaeomagnetic poles from the '-1110 ± 10 Ma (U-Pb) Upper North Shore Volcanics and the 1109 +4/-2 Ma (U-Pb) Logan Dykes in Laurentia, and the 1116 ± 12 Ma (Rb-Sr) Lakeview dolerite and dykes, the 1076 ± 33 Ma (Sm-Nd) Stuart dykes and the 1090 ± 32 Ma (Sm-Nd) Kulgera dykes from Australia. Between -1100 Ma and 900 Ma, the Laurentia Apparent Polar Wander Path (APWP) describes a large loop (Grenville Loop) for which there are no coeval palaeomagnetic data in Australia. Newly-determined palaeomagnetic directions from the Mundine Well Dyke Swarm, dated by U-Pb SHRIMP on zircon as 755 ± 3 Ma, appears to pass a contact test, and, in combination with previously-determined palaeomagnetic directions from the Northampton dyke swarm, gives a palaeomagnetic pole (Wingate & Giddings 1998) that fits on the previously defined Australian APWP, but in a position that implies that Laurentia and Australia could have begun to separate by this time. Australia is in low northerly latitudes at 750 Ma, and remains in low latitudes for the rest of the Neoproterozoic and the early Palaeozoic. Laurentia, in contrast, moves to high southerly latitudes between 723 +4/-2 Ma, when it was near equatorial and 597 ± 18 Ma (U-Pb) when the palaeo-South pole lay near northeastern Laurentia. Breakup of Laurentia from Australia can be inferred from these data to have occurred by 750 Ma. If the stratigraphic evidence for the rift-drift transition at the end of the Sturtian glacial epoch is accepted as the time of the breakup, the age of the Sturtian glacial rocks could be older than previously postulated. The age of the proposed correlative glacial unit in Laurentia, the Rapitan Group, can be no older than 755 ± 18 Ma (G.M. Ross in Klein & Beukes 1993). An older limit to the age of the breakup is 802 ± 10 Ma, given by the U-Pb age on zircons from the Rook Tuff, which is stratigraphically well below the Sturtian glacial interval. The age of breakup of Laurentia from Australia is thus possibly between 800 Ma and 750 Ma, possibly close to the younger limit. New palaeomagnetic data from South China (Zhang & Piper 1997), combined with unpublished data from one of us (ZL) show that South China could have lain near the eastern margin of Australia in Laurentia, possibly more northeasterly than previously thought (Li et al 1996). The Neoproterozoic stratigraphy of South China is comparable with that of southeastern Australia and western Laurentia up to, and including the first glacial unit (Liantuo = Sturtian = Rapitan), but differs thereafter in that there is no equivalent of the younger Marinoan (= Ice Brook) glacial unit in the succeeding carbonate succession. We infer that South Chma broke away from the Australia during the <-750 Rodinia breakup. Palaeomagnetic evidence does not support the suggestion of Veevers et al. (1997) that Laurentia did not break away from Australia until 560 Ma. However, a second breakup of a smaller continental fragment from eastem Australia near the end of the Precambrian, is possible, though this was not Laurentia. REFERENCES Klein, C. & Beukes, N.J. 1993. Sedimentology and geochemistry of the glaciogene Late Proterozoic Rapitan Iron-Formation in Canada. Economic Geology 88, 542-565. Li, Z.X.., Zhang, L. & Powell, C.McA. 1996. Positions of east Asian cratonic blocks in the Neoproterozoic supercontinent Rodinia. Australian Journal of Earth Sciences 43, 593-604. Veevers J.J., Walter M.R. & Scheibner E. 1997. Neoproterozoic tectonics of Australia-Antarctica and Laurentia and the 560 Ma birth of the Pacific Ocean reflect the 400 m.y. Pangean supercycle. Journal of Geology 105, 225-242. Wingate, M.T.D. & Giddings, J.W. 1998. Paleomagnetic test of the Australia-Laurentia connection at 755 Ma. (in review). Zhang, Q.R. & Piper, J.D.A. 1997. Palaeomagnetic study of Neoproterozoic glacial rocks of the Yangzi Block: palaeolatitude and configuration of South China in the late Proterozoic Supercontinent. Precambrian Research 85, 173-199.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
KNOWLEDGE BASED SYSTEM TO UNDERTAKE PATTERN RECOGNITION OF THE STRUCTURAL CONTROLS TO GOLD MINERALISATION
Consulting Structural Geologist, 28 Wildwood Drive, Salisbury Park, South Australia, 5109
Metamorphic gold mineralisation often exhibits a structural control, related to the deformation process effecting the rock sequence before or during the movement of mineralised solutions. Significant deformation types include wrench fault systems (e.g. Western Australian Goldfields, Mueller & Harris 1988), folding and saddle reefs (e.g. Victorian Goldfields, Cox et. al 1985) or a thrust fault duplex (e.g. Otago Schist, New Zealand, Teagle 1991). In each case there is 'rough' geometrical relationship evident and a control established. In the past these relationships have been manually established and extrapolated, which allows for human expertise and intuition, however a degree of bias is inherent and differences of opinion are evident. Future exploration beyond the turn of the century could be aided by the development of a fuzzy logic pattern recognition system, in association with methods currently used. Fuzzy logic is widely used in many areas, where there are a number of possibilities that cannot be assigned exact numbers, providing an inference morphology that enables approximate human reasoning capabilities to be applied to intelligent systems, as a tool for modelling uncertainties associated with human thinking and perception. Geology is an excellent area in which fuzzy logic systems could be applied as information about the third dimension is often uncertainty and little meaningful data can be gathered in terms of probability. Recently this approach has been proposed as an add to mining engineers for the purpose of interpreting and extrapolating geology (Winsor in press). Developing a 'fuzzy' knowledge based system involves establishing a number of crisp and fuzzy rules, based on local conditions, local relations, regional associations and intuition. For example a rule could be if the major shear direction is sinistral, secondary structures will exhibit the following relation.... Another might be if dilation is maximised at a bend in a fault other bends should be examined. The rules used can have different degrees of complexity. By establishing a membership function for the situation being considered and identifying the related variables, extrapolation is possible using this function. Examples are illustrated, indicating that second order structures display distinct geometries (Hancock 1985). Crisp and fuzzy rules that have been identified will be reviewed. Whether or not there will be extensive use of fuzzy logic knowledge based systems in the new millennium is currently uncertain, however this approach should be examined as a mineral exploration tool and could possibly be extended to other areas of geoscientific investigations. REFERENCES Cox, S.F., Sun, S.S., Etheridge, M.A., Wall, V.J., Potter, T.F., 1995. Structural and geochemical controls on the development of turbidite - hosted gold deposits. Wattle Gully. Economic geology, 90, 1722-1746. Hancock, P.L. 1985. Brittle microtectonics : principles and practise. Journal of structural geology, 7, 437458. Mueller, A.G., & Harris, L.B., 1988. An application of wrench tectonic models to mineralised structures in the Golden Mile district, Kalgooriie, Western Australia. In: Ho, S.E., Groves, D.I., (eds) Recent advances in understanding Precambrian gold deposits. Vol. 2, Geology Department, University Western Australia, 11, 67107. Teagle, D.A.H., Norris, R.J., & Craw, D., 1991, Structural controls on gold - bearing quartz mineralisation in a duplex thrust system, Hyde Macraes shear zone. New Zealand. Economic geology, 8 5, 1711-1718. Winsor, C.N. in press. An expert system to perform : discontinuity analysis and prediction in singularly deformed areas. International Conference on Geomechanics/Ground Control in mining and underground construction, Wollongong, July 1998
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
N"" Australian Geological Comention, Townsville, July 1998
ANTARCTIC BRYOZOANS: COMPARISONS WITH SOUTHERN AUSTRALIAN CONTINENTAL SHELF BRYOZOA Geraldine Winzar and Yvonne Bone Department Geology and Geophysics, University of Adelaide, South Australia, Australia 5005
The margins of East Antarctica and southern Australia were produced by rifting during Gondwana times. The resulting ocean filling the rift precludes modem biyozoans on either side migrating to the opposite side. Environmentally-controlled assemblages, when compared, show differences,particularly in nature of substrate, water depth and associated biota. Bryozoa from three Australian Antarctic Territory research stations (sampled 67 years ago by Sir Douglas Mawson) have recently been studied. Their ecology can now be compared to the widely distributed, diverse species and dense populations ofbryozoans living on the modem southern Australian continental margin. Here, on the largest area of cool-water carbonate deposition in the world, biyozoan bioclasts are one of the dominant contributors to the sediments, along with molluscs and foraminifers. The adjacent Tertiary basins, which contain biyozoan-rich limestones, can be better understood by information gained from these temperate and polar modem analogues. ENERGY LEVEL In Australian waters, living biyozoans are particularly common at depths below wave-base and in the region of the shelf break, i.e. depths of 80-250 m. There is a major differencein bottom current velocities and strengths in this depsth range, i.e. energy levels, from the high energy open-shelf area resulting in subaqueous dune formation to the quiet conditions of the upper continental slope. In Antarctica, similar genera occur at greater depths, i.e. 200-600 m, where their remains and associated sponge spicules form >80% of the sediment, but current velocities and strenghts are similar to those at the shallower depths in the Australian region SUBSTRATE Antarctic bryozoans colonise the highly irregular sea-floor that is either rocky or coated with muddy substrates. Glacial processes have produced, and indeed, are still being modified by such processes. Yet still we see the same genera on the Australian margin where the substrate is predominantly shiftin<^ calcareous sand. ^ TEMPERATURE Bottom and surface water temperatures are relatively constant in Antarctica (average of - 1.7°C), but show marked thermoclines in Australia, often varying more than 10°C with additional seasonal variability. The bottom temperature does not drop below 5°C on the Australian margin. NUTRIENT AND PHOTIC ZONE LEVELS These two are linked inasmuch as the major dietary component for bryozoans is the descending remains of phytoplankton. In areas of upwelling, the increasing nutrient level encourages increased bioproductivity in the surface waters, therby counterbalancing the reduced light input, e.g.roof of translucent sea-ice. Thus, the year-round circumpolar upwelling in Antarctica may well be an indirect major contributor to the high productivity of bryozoans in this environment. On the other hand, the Australian margin has limited areas of upwelling, e.g. Bonney Shelf, and even these are seasonally controlled.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
TECTONIC PREPARATION FOR MINERALISATION IN THE GYMPIE GOLDFIELD. Bronwvn Witham and Ron Cunneen Gympie Eldorado Gold Mine Pty Ltd, P.O. Box 784, GYMPIE, 4570
The Gympie Goldfield, located in south-east Queensland, is hosted within a sequence of Permian island arc volcanics and sediments, the Gympie Group. The Goldfield extends over an area of 10 kilometres by 4 kilometres, elongate north-south and centred on the City of Gympie. With continuous mining between 1867 and 1927 responsible for a hard rock production of 116 tonnes of gold bullion, Gympie was the sixth largest historical goldfield in Australia. STRATIGRAPHY The stratigraphic succession of the Gympie Group reflects the progressive evolution and drowning of an island arc system. Initial basic volcanics are cyclically replaced by volcaniclastics as volcanism wanes, and are finally covered by a shallow marine limestone and deep water turbidites. The Gympie Goldfield itself is situated on the eastern limb of a domal feature disrupted by thrusting and block faulting. The stratigraphy strikes north-south, dips shallowly east and is essentially conformable. Some of the bounding faults have been identified as growth faults; stratigraphic thicknesses and compositions have been influenced by the rate and degree of subsidence of each of the fault block basins. Gold mineralisation is a late event in the tectonic history of the Gympie Group. Dating indicates that it is probably no older than the Mid Triassic. Dykes and vems that crosscut the Permian stratigraphy are likely to have started forming in the Late Permian to Early Triassic. VEINING/MINERALISATION All the veins are of a similar style: quartz-calcite, mesothermal, massive and laminated, overprinted by goldgalena-calcite. These veins occur as two different types: Crosscourses and Gympie Veins. Crosscourses strike northwest, crosscut the stratigraphy and are subvertical. Mineralisation occurs over large areas within the Crosscourses, which are themselves feeder structures to the Gympie Veins. Gympie Veins occur as an array of parallel fissure reefs striking parallel to the stratigraphy and dipping normal to bedding. A striking example of the influence of wallrock composition on ore deposition occurs where the Gympie Veins cut a sequence of carbonaceous shales (the Productive Horizon), creating bonanza grades. DYKING Mafic dykes occupy the same or parallel structures to the reefs. Dolerites are intimately associated with the quartz veining events while diorites are syn- to post-mineralisation. One of the most important intrusives identified to date is the Langton Sill (dolerite). This intrudes between the Productive Horizon and the overlying limestone cap. This is a brittle unit that has enhanced vein density and size. It seems probable that this dyke aided vein (and therefore fluid) penetration into the Productive Horizon that produced many pockets of dense visible gold. The Langton Sill is known to exist over at least a third of the Gympie Goldfield. ONGOING WORK Gympie Eldorado Gold Mine Pty Ltd is in the process of undertaking geochemical studies of the veins, dykes, and stratigraphy to identify patterns surrounding the mineralisation. This work will also help to clarify relationships between the dykes, and to the mineralisation. Geological remapping of the Gympie Goldfield is expected to identify the most prospective areas for strong Productive Horizon development, and the focussing of vein and dyke development. In addition, an aeromagnetic survey is also planned to obtain greater detail on the structural framework of the Gympie Goldfield. The combined detailed, high quality geology, geochemistry and geophysics will lead to a better understanding of the source, pathways and traps for gold mineralisation in the Gympie Goldfield.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
THE DINKIDIAU-CU PORPHYRY - AN ALKALINE PORPHYRY DEPOSIT FROM NORTH LUZON, PHILIPPINES. Rohan C. Wolfe'. David R. Cooke' & Paul Joyce' 'CODES SRC, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001 'Climax Arimco Mining Corporation, Level 13, Gold Fields House, 1 Alfred Street, Sydney, NSW 2000
The gold-rich Dinkidi Cu-Au porphyry deposit is located within the Early Miocene Didipio Igneous Complex of northern Luzon, Philippines. Drilling by Climax-Arimco, a Philippine subsidiary of the Australian company Climax Mining, has delineated a 110 Mt orebody at 1.2 g/T Au and 0.5% Cu. The alkaline Didipio Intrusive Complex has intruded a sequence of Late Oligocene calc-alkaline andesitic lavas and volcaniclastics (Mamparang Formation) and overlying potassic latitic to trachytic lavas. This igneous complex is comprised of a series of early gabbros (pyroxene-plagioclase cumulates), clinopyroxenites (pyroxene cummulates), diorites and monzodiorites that have been intruded by a large, weakly-mineralised monzonite pluton. The Dinkidi deposit is located at the southern margin of the monzonite pluton. Mineralisation is hosted within a composite monzonite stock. Within the stock a large, 800 meters long by -200 meters wide, elongate monzonite stock (Tunja Monzonite) predates and hosts the bulk of the Au-Cu mineralisation. This has been intruded by a thin, highly-mineralized variably-textured magnetite- and clinopyroxene- phyric monzonitic pegmatite (Balut Dyke). The youngest phases of the composite stock are a leuco-monzonite porphyry (Quan Monzonite) that grades texturally, in its core, into a post-mineralisation crystal-crowded lueco-quartz monzonite to lueco-granite (Bufii). This late porphyry phase is inferred to be the source of much of the Cu-Au mineralisation. The intrusions of the composite stock are interpreted to be late-stage felsic differentiates that were released from the neighbouring monzonite pluton. Unmineralised late-stage andesite porphyry dykes have intruded throughout the igneous complex and surrounding units. Based on whole rock geochemical analyses, we conclude that the intrusions are co-magmatic with each other and with the potassic latite lavas in the surrounding region. The intrusive complex is related to, but chemically distinct from, the silica-undersaturated potassic intrusives of the Cordon Syenite Complex and the nearby Palali Batholith. Five main stages of hydrothermal activity are recorded at Dinkidi: 1) INITIAL K-SILICATE STAGE An unmineralised K-silicate alteration assemblage formed during the intrusion of the Tunja Monzonite stock. The diorites surrounding the stock have intense selectively pervasive biotite alteration, patchy pervasive orthoclase alteration, intense selectively pervasive magnetite alteration and are cut by an extensive network cf orthoclase veins and monzonite dykes. 2) CALC-SILICATE STAGE The silica-undersaturated assemblages of Stage Two carry substantial high-grade Au mineralisation and are associated with the intrusion of the Balut Dyke. The emplacement of this dyke is associated with formation of an extensive array of orthoclase-actinolitelsulphide-fluorite veins, orthoclase-bomite±chalcopyrite-native gold veins, emplacement of pegmatoidial clinopyroxene-orthoclase-magnetite-sulphide dykes, and development of selectively pervasive actinolite alteration and pervasive orthoclase alteration on the margins of the dyke. 3) MAIN QUARTZ-SERICITE-CARBONATE STAGE The main stage of Au-Cu mineralisation at Dinkidi is intimately associated with the intrusion and initial crystallisation of the lueco-monzonite porphyry (Quan) and deposition of quartz±chalcopyrite-carbonate veins, a later carbonate-sulphide vein set, and vein-related sericite-c^onate alteration. Stage 3 was terminated by crystallisation of the Bufii phase in the core of the intrusive, with associated antiperthite veins, deposition cf massive silica on the walls of the crystallismg intrusive and the subsequent explosive brecciation of the roof cf the intrusion, forming a quartz-fragment breccia. 4) CLAY STAGE Retrograde metasomatism during Stage Four was intimately associated with brecciation of the flanks of the monzonite porphyry, and resulted in the formation of selectively pervasive clay alteration. 5) CARBONATE STAGE Extensive, late-stage selectively pervasive carbonate alteration is associated with widespread carbonate±sulphide and zeolite veins.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
SEDIMENT THREATS TO THE GREAT BARRIER REEF: A SYNOPSIS OF CURRENT UNDERSTANDING KenJ.Woolfe School of Earth Sciences, James Cook University, Townsville, QLD 4811
Historically sediment entering the Great Barrier Reef lagoon has been considered a threat to reefal and other ecosystems. This perception of risk is widespread, and in part justifiable in that neither the corals or tourists on the outer reef would likely take kindly to a rapid inundation of mud. However, an examination of the last 10,000 yrs of reef evolution together with an increased understanding of the processes that control sediment transport, accumulation and turbidity suggests that riverine sediment entering the GBR lagoon is not in itself a threat. In reaching this conclusion it is noted that: • • • • • •
•
Many reefs have muddy cores, suggesting that reef initiation probably occurred in turbid water. Turbid water reefs are common along the present-day GBR coasdine. Sediment supply to the ocean from rivers is small. Most of the sediment reaching the coast trapped within a near-shore sediment wedge and little if any escapes to the midshelf or beyond (figure 1). Resuspension by waves is the principle mechanism for generating turbidity, and turbid water thus generated is generally trapped with in a turbid coastal boundary layer. North facing bays are the sediment vacuum cleaners for the lagoon, without these (or should these become full) the muddy coastal wedge common in much of the central and southern GBR would extend northwards to Torres Strait. The limiting factor controlling turbidity and sediment transport along the coast is wave energy. Consequently, increasing the supply of sediment to the coast will not increase turbidity, nor is it likely to increase sediment supply to areas other than those directly proximal to river mouths.
With the exception of sites directly adjacent to riverine sources or within the outer portions of north-facing bays, it appears that increased sediment supply to the GBR does not pose a threat.
"Coastal turbid-zone gravel pat^s xene substrate sedifnent wedge Patch reef
Depth >20 m
Prevailing SE trade winds
Terrigenous source Sediment wedge Coastline
2(M0km
Figure 1: Schematic representation of terrigenous sediment distribution within the Great Barrier Reef Lagoon, modified from Woolfe and Larcombe (1998).
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
AIRBORNE ELECTROMAGNETICS - MAPPING GEOLOGY THROUGH COVER Lisa Worrall, Tim Munday & Andy Green Co-operative Research Centre for Australian Mineral Exploration Technologies, do CSIRO, Private Bag PO Wembley, Perth, W.A 6014 Australia
As the new millennium approaches, mineral exploration in Australia is increasingly focussed on regolith dominated terrains. Airbome geophysics, particularly aeromagnetics and radiometrics, has become a routine component of exploration programs as a result. However some of the most prospective terrains in Australia are magnetically quiet, and a thin cover of transported materials will preclude the use of radiometrics for geological mapping purposes. In these circumstances airbome electromagnetics (AEM) may be the most cost-effective mapping tool. The promise of AEM is this regard is examined principally with reference to AEM surveys flown in the Yilgam Craton, Western Australia. The geological mapping capability of AEM has been enhanced in recent times by the development of systems such as GEOTEMDEEP, QUESTEM450. S A L T M A P and most recently, G E O M A P . These are time domain systems descended from the I N P U T system, however improvements in bandwidth, instrument sensitivity and signal to noise ratios have brought greater resolution from surface to depths of over 100m. In addition, data processing strategies and presentation styles developed over the last decade have made AEM data more amenable to analysis and interpretation by geologists. AEM data is now routinely grided to produce images of physical property variations, and inverted to produce conductivity depth sections. We have found that AEM data are also suited to analysis using statistically based methods developed and refined for the analysis of multispectral remote sensing data. These techniques enable rapid tumaround times for the display and analysis of AEM data for geological mapping purposes.
In general terms however, the success of AEM as a geological mapping tool in regolith dominated terrains is dependant on an adequate contrast in the conductance, or conductivity thickness product, of saprolite developed over different rock types. This contrast may be obscured by saline groundwater. Similarly saline groundwater may obscure the conductivity contrast between in-situ and transported regolith materials. Perversely, information on the groundwater and the movement of solutes may be critically important to the development of effective geochemical sampling strategies for mineral exploration.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
BLOWING THE LID OFF OPERCULATE TETRACORALS: THE KEY TO THE PHYLOGENY OF THE GONIOPHYLLIDAE A J. Wright School of Geosciences, University of Wollongong, Wollongong NSW 2522
Goniophyllidae, the small Silurian-Devonian tetracoral family with opercula, has largely been studied in thin section as is customary for ancient corals. My studies suggest strongly that the morphology of the operculum is characteristic at the generic level; however, it can only be studied satisfactorily in silicified material, isolated calcareous specimens or moulds; calcareous material of some genera will thus be difficult to identify. The 2 common genera on the world scene, Rhizophyllum and Calceola, have very different internal and external features on their opercula, as does Pararhizophyllum Pedder, 1997 and a new genus from NSW, Victoria and Queensland. Other external features such as rootlets, and internal features including dissepiments and tabellae, are also important characters at the generic level. MORPHOLOGICAL CHARACTERS. Goniophyllum is distinctive in its square cross-section, abundant rootlets, 4 opercula and dissepiments and tabulae; the rare, cylindrical Llandovery Araeopoma also has several opercula, dissepiments and tabulae. Rhizophyllum is characterised by dissepiments, tabulae, rootlets and, in particular, its relatively isometric (concentric) opercular growth rings and subdued opercular septa. Pararhizophyllum has rootlets but lacks dissepiments and has, according to Pedder (1997), distinctive aspects of the septa on the inner opercular surface. The new East Australian genus lacks rootlets, dissepiments and tabulae, and has weakly anisometric growth lines on the operculum and very strongly developed septa on the inner surface of the operculum. Calceola has strongly anisometric opercular growth lines and almost vestigial septa on mature opercula. STRATIGRAPHIC RANGES. Goniophyllum and Rhizophyllum appear in the Llandovery. Of these 2 genera, Rhizophyllum ranges from the Llandovery to probably the Emsian, whereas Goniophyllum became extinct in the Ludlow. The new genus ranges from the Lochkovian (Manildra, NSW: eurekaensis zone Garra Formation), through the Pragian (Garra Formation, Wellington, NSW) to the Emsian (Tabberabbera, VIC). Calceola ranges from mid-Emsian (the Eifel, Germany; Cabrieres, France; Buchan, VIC; Brogans Creek, Mount Frome, NSW) but is mostly Eifelian to late in the Middle Devonian (Burma, the Eifel, North Africa). PHYLOGENY. The phylogeny of the group is becoming more clear. Both Goniophyllum and Rhizophyllum appeared in the Llandovery (Early Silurian), but their phylogenetic relationship is not apparent. It is tempting to infer from the stratigraphic and geographic distributions of the genera that Rhizophyllum gave rise first to the new genus in Australia in the Lochkovian, and then to Pararhizophyllum in the Pragian delta zone of Nevada. It seems reasonable to derive Calceola from the new genus, rather Rhizophyllum, in the mid-Emsian. BIOGEOGRAPHY. Goniophyllum, the name-giving genus for the family, was restricted to Baltica, Avalonia and the eastern part of Laurentia. The rare Araeopoma was even more restricted, and has not been reported outside the Llandovery of Godand. The only cosmopolitan genus in the family, Rhizophyllum, occurs widely in the Silurian and Early Devonian of Europe, Asia, Australia and North America; it appears to be the stock which evolved into the new genus in Australia. Calceola may have migrated from SE Asia to eastern Australia, but did not reach the Americas. FUNCTIONAL MORPHOLOGY. The ways in which these corallites articulated with the operculum will be discussed.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
USING GIS IN A DATA-DRIVEN, MINERALS SYSTEMS APPROACH TO ASSESS THE MINERAL POTENTIAL OF AUSTRALLVN PROTEROZOIC GRANITES Lesley A.I. Wvbo^l^ Irina V. Bastrakova^and Anthony R. Budd\ 'Australian Geological Survey Organisation (AGSO), GPO Box 378, Canberra, ACT, 2601
Increasingly new strategies for exploration involve manipulation and integration of large digital data sets within GIS. Many new methodologies have been developed and are utilised by government and the exploration industry, primarily for information management, but more recently for assessing mineral potential. Recently AGSO, in collaboration with Commonwealth, State and Territory Geoscience agencies, utilised GIS in a 'datadriven' approach to assess the mineral potential of granites in all Australian Proterozoic Provinces. At the start our understanding of the Australian Proterozoic granite-related mineral system (based mainly on the Pine Creek and Mt Isa Inliers) was that specific granite types and distinctive host rocks tend to be associated with certain types of Au, Cu, Zn, Pb, Sn and W mineralisation. Rarely is mineralisation hosted by the granites: it is more conmionly hosted in the country rock several kms from the granite contact. Using whatever data were available the aim of the project was to determine 1) which Proterozoic granites have metallogenic potential, 2) what commodities they may be associated with and 3) where the better host rocks are located for potential ore deposits. To assess the data, instead of treating ore bodies as anomalies equivalent to their actual diameter, a mineral systems approach was used in which ore bodies are considered to result from the coincidence of many associated geological factors which can cover tens of kms (district scale) or hundreds of kms (regional scale). To investigate potential relationships Australia-wide, 3 data sets were built. The granite data set comprised data for 648 plutons for 60 attributes including field criteria (size, shape, mineralogy, breccication, etc.) and chemical criteria (-7500 analyses were assessed). Most plutons were grouped into suites and supersuites: these were then divided quantitatively into 10 types, based on similarities to known mineralised granite systems (eg, Olympic Dam, Cullen types, etc). The second data set focussed on the host rocks and for 380 units compiled information on 40 attributes including lithology, the abundance of reactive minerals (eg, carbonate, graphite, magnetite) and the commodities that occur in these units. Botli data sets recorded the age of the unit: if not available a relative age was calculated. The registered number for each unit from the Stratigraphic Names database was included to give a unique identifier to all units Australia-wide. The third data set comprised digital maps of all provinces, at 1:250 000 scale or smaller, highlighting the granite polygons and host rocks within a 5 km buffer from the granite contact. Using GIS techniques, the granite and host rock data sets were joined to the maps and host units that were of an equivalent age to the granite or older were selected and integrated with the MINLOC data base (Bureau of Resource Science/AGSO) to determine which commodities occurred within the 5 km buffers and whether the mineral occurrences were in preferred host rock types. In essence, this project was a data-driven, 'bottom-up' exercise in which simple proximity analysis was undertaken to determine which commodities were related to specific granite types. Advantages of this nonmodel driven approach were that interesting and unexpected relationships were uncovered for subsequent 'forensic' analysed to determine a cause. For example gold was found unequivocally (but rather surprisingly) to be spatially related to granites that were reduced: the data set clearly showed reasons for this association. Another advantage of the mineral systems approach was that serendipity could uncover other aspects of other Proterozoic systems (eg, the regional oxidised alteration of the Pb-Zn mineral system of the McArthur-Mount Isa region versus the highly reduced alteration related to the Pb-Zn Broken Hill mineral system). Although the final model of the Proterozoic granite-related mineral system can be portrayed as a simple cartoon, in reality it is an empirical model built on a scale that has never been attempted before in Australia, if not globally. The bottom-up methodology adopted in the project contrasts with current popular methods which either statistically assess known ore deposit localities in specific areas and then digitally look for analogues, or else utilise digital empirical models of known deposits and search for them within a purpose-built data set. These 'top down' approaches essentially result in only finding what you put in, and the result is heavily biased by the quality of the input data. In contrast, the advantage of the data driven approach is that it is the actual analysis of the data that develops the model. A major limitation of the bottom up approach is a lack of standardisation of Australian digital data formats: much time was spent getting various data sets into compatible formats. The project also highlighted the current high loss of digital data and geoscientists need to seriously consider the permanent preservation of their digital data: hard disks on personal PC's are woefully inadequate.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14'^ Australian Geological Convention, Townsville, July 1998
AUSTRALIAN PROTEROZOIC GRANITE RELATED ORE SYSTEMS Lesley A.I. Wvbom\ Anthony R. Budd', and Irina V. Baslrakova\ 'Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601
AGSO, in collaboration with Commonwealth, State and Territory Geoscience organisations, has completed a project on the "Metallogeny of Australian Proterozoic Granites" (which was sponsored by 20 mineral companies). Using GIS techniques, the project investigated spatial relationships between specific granite types, host rock compositions and hydrothermal Au, Cu, Zn, Pb, Sn, W and Mo mineralisation. Data were collated on the mineralogy, geochemistry (--7500 analyses), and age of Proterozoic granites and felsic volcanics, as well as the age and mineralogical composition of sediments within 5 kms of granite boundaries for 20 provinces. KEY RESULTS BY GRANITE TYPE Overall although there is a strong spatial relationship with specific granite types for many commodities, much of the mineralisation is hosted in the country rocks and particularly for Au, up to 5 kms from the nearest granite contact. I-(granodioritic) compositions dominate and these are inevitably Sr-depleted and Y-undepleted which contrasts with igneous rocks normally found associated with porphyry style mineralisation. Some significant differences were found between Proterozoic granites and their Archaean and Phanerozoic counterparts (e.g., Proterozoic granites have elevated K, Th U, and High Field Strength Elements contents). Hence classifications for A- and S-types, and some metallogenic indicators developed from Palaeozoic granites were not meaningfiil. The mineralogy of the country rock was found to play a crucial role not only in determining the site of mineralisation, but also in affecting the redox state of the granite. More specific relationships are as follows: • I or S-type granites designated as unfi-actionated were restite-rich and consistentiy unmineralised. • Fractionated I-type granites can be divided into 2 groups: those that were either F-poor or F-rich throughout most of their fi-actionation history. The F-rich granites are often the true rapakivi types and have littie mineralisation presumably because CI had been partitioned into the granite melt early. • Fractionated F-poor I-type granites can be further divided into 2 classes: Oxidised and Reduced. The Oxidised granites are most commonly associated witii Cu-Au deposits and crystallised at higher temperatures than the Reduced class. Altiiough it is commonly stated that Au mineralisation is only associated with oxidised granites, an unequivocal spatial association occurs between the Reduced class and Au-dominant mineralisation which also has variable amounts of Cu, Sn, W, and Bi. Reduction is due to several causes including increasing ASI, interaction with country rock and perhaps magma cooling. • Rare fractionated S-types are conmionly associated with Sn mineralisation. KEY RESULTS BY REGION Mount Isa: The most prospective areas for Cu-Au are those surrounding the Williams Supersuite. The Burstall Suite in the eastern Wonga Belt has some Cu-Au potential including potential epitiiermal style deposits. Georgetown : The Esmeralda Supersuite has strong similarities to tiie Hiltaba Supersuite of the Gawler Craton. Pine Creek: The Allia Suite and CuUen Supersuite have proven potential: other suites are not highly rated. Kimberleys: Major suites are essentially unfractionated (restite-rich?) and are not considered prospective. The small Koongie Park Suite (related to the Koongie Park VHMS mineralisation) shows evidence of fractionation. GraniteS'Tanami: Altiiough data are limited, many granites are reduced, but some become oxidised at shallower level of emplacement: tiie two contrasting types may require different conditions for precipitation. Tennant Creek/Davenport: Au is most likely to be related to tiie 1820 Ma Treasure Suite. Gawler Craton: The Hiltaba Supersuite comprises tiie oxidised Roxby Downs Suite which has Cu-Au-U potential and tiie more widespread, reduced Kokatiia Suite which is associated witii Au ± Sn mineralisation. Paterson: Granites closest to tiie Telfer deposit are reduced, in contrast to the oxidised Mount Crofton Granite. Gascoyne: 1800 Ma granite suites are fractionated, although tiie metal potential may be limited by the high metamorphic grade of many hosts. Prospective targets are likely to be small and focussed as at Tennant Creek. Arunta: Many suites present: tiie most prospective are tiie 1710 Ma Alarinjela and Barrow Creek Suites, the 1640 Ma Mount Webb Suite, and tiie 1567 Ma Soutiiwark Suite. High metamorphic grade may be a limitation. Broken Hill: Dominated by restite-S-types tiiat are unlikely to be related to tiie Broken Hill Pb-Zn-Ag deposit. Olary: If the 1590 Ma suites are true analogues of tiie Hiltaba Supersuite, tiien tiie potential for Cu/Au is high, although by analogy with the Cloncurry district the presence of calc-silicates may cause limitations. Northhampton, Rocky Cape, Albany Fraser, Leewin, Musgraves Insufficient data available for confident predictions of metallogenic potential.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
INTERACTIONS BETWEEN CRUST AND MANTLE THROUGH TIME AND THE RELATIONSHIP TO THE EVOLUTION OF AUSTRALIAN ORE DEPOSIT TYPES Lesley A.I. Wybom', Murray S. Hazell^ Laigee M. Bell'and Suzanne M. Edgecombe'. ' Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601
Compilations of the distribution of ore deposits types in time clearly show them to form three broad groups that are coincident with the Archaean, Proterozoic and Phanerozoic eons {eg Meyer, 1981). As many deposits are related to the intrusion/extrusion of igneous rocks, the changes in dominant ore deposit types appear to parallel temporal changes in the dominant type of igneous rocks in response to mantle cooling. Some examples are: • The Archaean mantle was at least 100°C hotter than the present mantle as is reflected in the abundance of komatiites derived from deeper parts of the mantle. Hence the Archaean dominance of komatiite-hosted Ni. • The Archaean lacks the significant volumes of the high-T, oxidised l-(granodioritic) types that are usually associated with Cu/Au mineralisation in the Proterozoic. Thus ironstone hosted Cu-Au deposits are absent. • Some early Palaeoproterozoic mafic igneous rocks are Mg and Si enriched, suggesting that mantle melts remained hotter than at present and were derived fi-om shallower levels than in the Archaean. These compositions would promote the formation of layered intrusions within the crust (with associated deposits of Ni, Cr, Pt) and would also result in underplating of the crust which is so prevalent in the Proterozoic. • Late Archaean/Palaeoproterozoic granites have higher K, Th and U values than granites of other ages. (However, an oxygenated atmosphere is required to free U allowing unconformity-style U deposits to form.) • Widespread underplating in the Proterozoic could result in more effective conductive heating of the lower crust, whilst the high K, Th and U values would increase radiogenic heat outputs. Both factors would cause higher crustal geothermal gradients, and therefore high temperatures at shallow crustal levels, which in turn would facilitate the generation of the large size of some Proterozoic hydrothermal deposits. • The physical size of igneous suites is generally larger in the Precambrian than in the Phanerozoic, suggesting also hat the related thermal anomalies that influence basin formation were vast. In the Palaeoproterozoic some basin formation resulted in extensive sag phases characterised by evaporitic sequences that were an important source of ligands (such as CI ) for transport of U and base metals. • Australian Proterozoic VMS are rare asfi-actionatedvolcanics rarely coincide with subaqueous sediments. • I-(granodioritic) types dominate Australian Proterozoic/early Palaeozoic felsic melts and the majority are Sr-depleted, Y-undepleted, possibly signifying high crustal geothermal gradients. Related deposits are mostly hosted in countiy rock, possibly because the granites are too felsic and/or intrusions are too deep. • S-types granites or I-types that fractionate to more peraluminous compositions are more prominent in the Palaeozoic, this may explain the greater abundance of Sn mineralisation during this era in Australia. • Magma types change notably in the Phanerzoic and more are clearly related to activity at plate margins. • Genuine shoshonites with associated Cu/Au mineralisation made their first appearance in the Ordovician • Porphyiy-style mineralisation is more conunon in the Phanerozoic, where it is hosted within I-(tonalitic) or M-types above subduction zones. Most melts are Sr-undepleted signifying lower geothermal gradients. • Ophiolite-related deposits (Cu-pyrite deposits, podiform Cr deposits) are more common in the Phanerozoic. Thus the changing thermal regime of the crust and mantle through time exerts a significant control on the dominance of different magma and ore deposit types through time. Although the Archaean was characterised by highest mantle temperatures, crustal geothermal gradients were probably lower possibly due to a thicker lithosphere. Mantle temperatures were intermediate in the Proterozoic, whereas crustal geothermal gradients may have been higher perhaps due to lithospheric thinning, thus allowing mantle melting at shallower levels than in the Archaean. Because of the hotter thermal structure of the continental lithosphere in the Precambrian, deformation was substantially less partitioned into narrowly defined plate boundaries than at present (Etheridge & Wall, 1994). Hence in the Precambrian intraplate deformation was more widespread resulting in more opporttmities for fluid migration (and hence some larger ore deposits!). The Phanerozoic represents a transition to a dominance of igneous activity and deformation at plate margins driven by a cooler mantle, explaining the increase of ore deposits that characterise the plate margins, viz. porphyry Au/Cu and ophiolite-related deposits. REFERECENCES: Etheridge, M.A., and Wall, V.J, 1994. Tectonic and structural evolution of the Australian Proterozoic. Geological Society of Australia, Abstracts, 37, 102-103. Meyer, C, 1981. Ore-forming processes in geologic history. Economic Geology, 75th Anniversary Vol, 6-41.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
A NEW PROTEROZOIC GRANITE-RELATED ALTERATION SYSTEM, MOUNT WEBB REGION, WA/NT: IMPLICATIONS FOR Cu-Au MINERALISATION Lesley Wybom', Murray Ha2ell\ Rod Page', Mart Idnurm\ and Shen-Su Sun' ' Australian Geological Survey Organisation, PO Box 378 Canberra City, ACT, 2601.
The Mount Webb Granite and its comagmatic volcanics of the Pollock Hills Formation form part of a newly discovered significant hydrothermal alteration system that crops out in the remote Mount Webb region in the western Anmta Inlier on the WA/NT border. New petrological, geochemical and geochronological data confirm that this magmatic system has many similarities to granite systems in other Australian Proterozoic regions where hydrothermal Cu and/or Au deposits have been linked to magmatic sources (e.g.. Eastern Mount Isa Inlier, Gawler Craton). The Mount Webb Granite intrudes rocks shown as 'unnamed Archaean?' which consist mainly of interbedded quartzite and mica schist with some amphibolite. Late-stage quartz and quartztourmaline veins are abundant and some brecciation is recorded. At one locality close to the granite contact quartz veining and minor ironstones are prominent and the rocks show a strong metasomatic overprint characterised by quartz, biotite and magnetite. Both the granite and the country rock are intruded by prolific NNW trending dolerite dykes which do not intrude Neoproterozoic and younger sequences. The Mount Webb Granite is heterogeneous, ranging from more mafic diorites/tonalites through to granodiorites, monzogranites, syenogranites and aplites, with monzogranites dominating. The granite is I-type containing hornblende, biotite, magnetite, plagioclase, K-feldspar and quartz. Two alteration types are present, firstly a Sodic-calcic alteration characterised by the assemblage albite + diopside + epidote ± tremolite which is prominent in a major linear shear zone trending 310° to the NW and SE of Mount Webb. This alteration contains few quartz veins and there are no open space fillings, sulphides or anomalous concentrations of elements. The second alteration type is Sericitic. It is more regionally restricted and is usually associated with brecciation and fracturing, quartz veining, open space filling and a higher modal abundance of sulphides (mainly pyrite, although chalcopyrite was observed in one sample), fiuorite and tourmaline. The felsic volcanics of the Pollock Hills Formation consist predominantly of black porphyritic dacites, rhyodacites and lavas, which are overiain by tuffaceous and non-tuffaceous sediments. The volcanics are predominantly porphyritic ignimbrites with phenocrysts of plagioclase, magnetite and quartz in decreasing order of abundance. Alteration is less pervasive in the volcanics than in the Mount Webb Granite and consists of two types: hematite and epidote. Three granite samples gave U-Pb zircon ages of 1643 ± 4 Ma, 1639 ± 5 Ma and 1639 ± 5 Ma. The sNd, values ranged from -1.5 to -2.1, giving single-stage TDM model ages of --2320 Ma. In an Australian context this 1640 Ma represents a new granitic age which is coincident with a major inflection point on the Australian Apparent Polar Wander Path. Both magmatic zircon and baddelyite were dated from an unmetamoiphosed dolerite dyke and gave Neoproterozoic ages of 976 ± 3 Ma and 972 ± 8 Ma respectively, removing any possible connection between the these dykes and the Mount Webb Granite. Sixty samples of granites and volcanics were analysed: Rb and Rb/Sr increase exponentially with increasing Si02. and most samples are oxidised. The ASI values are mostly <1.1, indicating that the granites are metaiuminous to weakly peraluminous. These are just some of the characteristic features of granites associated with Cu-Au mineralisation in the Australian Proterozoic. Some samples from the sericite alteration type also had anomalous F, Cu (up to 348 ppm) and S whilst some samples of quartz veins had elevated Mo (145 ppm) and Pb (1800 ppm) values. Recent exploration suggest that this truly 'greenfields' area may have some economic significance. Semi-continuous rock chip samphng returned results of 9.1% Cu, 3 g/t Ag and 0.38 g/t Au over a true width of 4m and 0.3% Cu and 8 g/t Ag over 10m true width. An aircore drilling programme has located three Cu-Au-Ag anomalous areas, the largest had peak values of 0.21 ppm Au and 896 ppm Cu on three adjacent 800m spaced grid lines (Anonymous, 1997). REFERCENCES Anonymous, 1997. Aurora Gold Limited. Quarterly report, December 1997. Acknowledgments: Lesley Wybom and Murray Hazell thank Aurora Gold for access to this remote region. Assistance of Loma Fitzgerald & Dean Butler of Aurora Gold, and Bobby West & the Kiwirrkurra community during sampling is gratefully acknowledged. Published with permission of the Executive Director, AGSO.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
WHAT DO -10 000 WHOLE ROCK GEOCHEMICAL ANALYSES TELL US ABOUT AUSTRALLVN PROTEROZOIC INTRAPLATE IGNEOUS ACTIVITY? Lesley A.I. Wybom', Mart Idnurm\ Anthony R. Budd\ Irina Bast^akova^ Murray S. HazelI^ and Suzanne M. EdgecombeV ^Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT, 2601
A major compilation of Australian Proterozoic Igneous rocks reveals broad patterns which provide major constraints on geodynamic reconstructions in the Proterozoic. The key results show that the Australian Proterozoic can be subdivided into major magmatic provinces each of which is coincident with one or several major recognised fold belts/orogenic domains and contains magmatism that is clearly episodic. The majority of Australian Proterozoic granites are I-(granodioritic) type derived by melting of pre-existing crust. Proterozoic granites are also dominantly Sr-depleted and Y-undepleted, signifying high geothermal gradients and sources dominated by plagioclase. Genuine S-type granites with visible cordierite and garnet in the more mafic compositions are rare, as are intermediate or felsic types that are distinctly Sr-undepleted, Y-depleted: a signature which is most commonly found in granites associated with subduction in island arc or continental margin settings, and infers the presence of garnet in the source and lower geothermal gradients. Wybom et al. (1997) have shown that in most major Proterozoic magmatic provinces the dominant Srdepleted, Y-undepleted I-(granodioritic) types can be divided into three groups which show a time progression in geochemistry. The oldest group (Group 1) at 1870-1850 Ma comprises restite-rich suites. Group 2, emplaced at 1840-1800 Ma is a low Ca type that shows evidence of magmatic fractionation. The youngest group (Group 3) is enriched in incompatible elements and comprises three subgroups: Subgroup 3u dated at around 18001780 Ma, has very high values of Zr, Nb and Y; Subgroup 32, usually emplaced between 1760 and 1650 Ma, is enriched in F and has variable amounts of Y, Zr, and Nb; and Subgroup 33, emplaced from 1640 to 1500 Ma, is more oxidised with a wide range in Si02 values. A simple explanation for the geochemical evolution from Groups 1 to 3 is that as the temperature in the source region increases, the magma is dominated first by minium melt, then by biotite breakdown and finally by amphibole breakdown, with evidence of source temperatures of up to 1000°C during the latter phase. The temperature increase of granite melts contrasts with a general decrease in the temperature of the mafic melts with time, with high Mg-tholeiites dominating before -1850 Ma and continental tholeiites after -1850 Ma (with the exception of high Fe-tholeiites at Broken Hill and Mt. Isa). It is difficult from the geochemical viewpoint to relate these consistent, continent-wide changes in composition to magmatism at the plate boundaries. Further, given the abundance of continent-wide magmatic activity, this would require an unlikely large number of small plates. Likewise, an explanation for melt emplacement in terms of tectonism, related to coeval mantle heating events is difficult to reconcile with time constants for heat transfer from the mantle to the crust. For crustal thicknesses of 25, 30, 40 and 50 kms the time lags have been calculated as 19, 27, 48, and 76 Ma respectively (Upton et al., 1997). Given the likely Proterozoic crustal thicknesses of at least 30 kms, the time lag between mantle magmatic activity and crustal melting is at least 25 Ma and may exceed 70 Ma, suggesting that melting of the lower crust and the emplacement of granitic magmas were due to different tectonic events. The clue to solving these problems may be in the shape of the apparent polar wander path (APWP) which, for those parts of the Australian Proterozoic where it is defined, confirms plate mobility and also shows that magma emplacement was coincident in time with inflection points on the APWP. The latter are recognised as significant interplate tectonic events with associated intraplate effects that cause major episodic migration of basinal fluids. Similar intraplate tectonic responses to later plate boundary tectonic events may have also allowed granitic melts to migrate into the upper crust, with the composition of the melt being simply dependant on the temperatures in the lower crust at the time of 'escape'. This hypothesis is consistent with (1) the increase in temperature of the felsic melts with decreasing age, (2) the progressive change in chemical compositions with time in each province, and (3) the related decrease in the temperature of the mantle melts. Finally, the hypothesis implies that major magma emplacement in the Proterozoic of Australia may have occurred in the interior of the plate rather than along numerous plate boundaries. REFERECENCES: Upton, P, Hobbs, B, Ord, A, Zhang, Y, Zhao, C, Drummond, B, and Archibald, N, 1997. Thermal and deformation modelling of the Yilgam Deep Seismic transect. Abstracts, Geodynamics and Ore Deposits Conference, Australian Geodynamics Cooperative Research Centre, Ballarat, 22-25. Wybom, L, Ord, A, Hobbs, B, & Idnurm, M, 1997. Episodic crustal magmatism in the Proterozoic of Northern Australia - a continuum crustal heating model for magma generation. AGSO Record, 1997/44, 131-134.
484
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
PALYNOLOGICAL AND STABLE ISOTOPIC STUDY OF PALAEOENVIRONMENTAL CHANGES ON NORTH-EASTERN TIBET PLATEAU IN THE LAST 30ka G. Yan'. RB. Wang% G.R. Shi' ^School of Ecology and Environment, Deakin University, Rusden Campus. 662 Blackburn Road, Clayton, Vic 3168. Australia ^Department of Geography, Nanjing University. Nanjing 210094, P.R. China
Palynological and stable isotopic study of two lacustrine sequences (the No.2 peat section and the Wasong section) revealed regional palaeovegetation and palaeoclimate evolutionary history of the Zoige Plateau, northeastern Tibet Plateau in the last 30 ka. Both palynological and stable isotopic analyses v^ere done to the No.2 peat section, while only palynological analysis was performed to the Wasong section. Pollen diagrams of both sections, coupled with carbon stable isotope curve from the No.2 peat section, show that the territorial palaeoenvironment has experienced a series of dramatic changes: 2 periods of Glacial climate during 30.0 - 26 ka B.P. and around 18.0 ka B.P., the Late-Glacial series of Boiling, Alleroid and Younger Dryas events, the Holocene Megathermal event during 9.4 - 4.05 ka B.P., and the change towards coohng and drying ever since. The changes in the palaeoenvironment record the history of the South-western Monsoon since 30 ka B.P.. 1. The climate during 30 - 19.8 ka B.P. is divided into 2 sub-stages. In the first sub-stage (30 - 26 ka B.P.), there existed a Zoige palaeolake, but the water level dropped significantly due to the cold-and-dry glacial climate condition. The water level rose in the second sub-stage (26 - 19.8 ka B.P.). The palaeoenvironment is reconstructed as being semi-humid grassland inferred from the pollen assemblage composition and the discovery of big mammal fossils. 2. The Glacial Maximum climate at 18 ka B.P. and the rapid climatic change series during the Late Glacial: The Glacial Maximum occurred during 19.8 - 17.5 ka B.P., when it was characterised by cold and dry climate. The drop in temperature during the Glacial Maximum is about 4.8-7.9° C. There is a "blank" stage of vegetation during this period. The Late glacial period started from 17 ka B.P. Temperature rose gradually during 17 - 14.2 ka B.P. Then from 14.2 to 10.0 ka B.P. the climate suffered a cycle of cooling and warming, which is almost synchronous with that of Boiling, Allerod, Old Dryas and Younger Dryas as recorded in Greenland Ice Core. 3. The Holocene Megathermal during 9.4 - 4.05 ka B.P. Climate began to ameliorate since 10 ka B.P., when the palaeolake started to dry up gradually and peat deposits started to develop widespread. The Holocene Megathermal emerged at 9.4 ka B.P., thereafter the pollen assemblage features show frequent fluctuations between humid and dry, with similar variation records in The optimum of the Megathermal took place during 9.4 -9.05, 7.2-6.75 and 5.25-4.05 ka B.P.. The temperature during 9.05-8.8 ka B.P. and 6.75-5.25 ka B.P. did not change significantly but the humidity decreased greatly.. 4. Climatic fluctuation during 6.75-3.8 ka B.P. Picea /Abies forest shrank to isolated islets since 4.05 ka B.P.. Climate changed towards colder and drier. The value of the No.2 pit section rose from -28.5%o up to -27.8%o, also suggesting a drop of temperature during this period. 5. Modem climate since 3.8 ka B.P. Picea forest started to recede and even disappeared out of southern slopes of the flat hills at southern margins of the Plateau. Almost all Abies disappeared out of the Plateau since 2.6 ka B.P.. Meanwhile, marsh stopped developing in the upper wide valley, and gradually migrated to the lowlands. Such a phenomenon continued till 1.8 ka B.P., thereafter climate ameliorated again with increased precipitation. Then from 0.65 ka B.P. onwards the precipitation decreased again.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
TRACE ELEMENTS IN CHROMITES FROM KIMBERLITES AND RELATED ROCKS: RELATION TO TEMPERATURE AND MANTLE COMPOSITION ShixinYao^ William L. Griffin^^^ and Suzanne Y. O'Reilly^ ^GEMOC National Key Centre, School of Earth Sciences, Macquarie University, NSW 2109, Australia ^CSIRO Exploration and Mining, P.O. Box 136, N. Ryde, NSW 2113, Australia
About 1500 chromites from 47 kimberlite, lamproite and lamprophyre localities world-wide have been analysed to determine their major and trace element contents by using electron microprobe and laser ablation microprobe ICPMS, respectively, in the GEMOC National Key Centre at Macquarie University. Chromites analysed are heavy mineral concentrates embedded in epoxy resin and polished and their sizes are typically 0.5-1 mm in diameter, with a few larger than 1 mm. Chemical compositions reported here represent analyses of the cores of grains. Chromites from xenoliths have been analysed in-situ using thick polished sections. There are some good inter-element correlations observed in chromites from kimberlites and related rocks. All chromites show a positive correlation between Co and Zn and a negative correlation between Co and Ni. A negative correlation also exists between Zn and Ni. Some analyses of Cr-spinel (low Cr) in xenoliths from Nushan alkaline basalt, southeastern China and chromites in xenoliths from Wesselton kimberlite, South Africa show the same correlations, within single suites. This implies that most chromite macrocrysts in kimberlites and related rocks are xenocrysts derived by disaggregation of mantle peridotites and captured by magmas ascending through the mantle sections. Those scattered off the main trends are considered to be magmatic or modified (metasomatised or metamorphosed) chromites. Chromites along the trends are defined as a "Mantle Array", representing chromites equilibrated with mantle olivine. Zn contents are temperature-dependent (Griffm et al., 1994; Ryan et al., 1996), and Co shows a good correlation with Tzn ("C) derived from Zn composition, therefore suggesting the locus of Mantle Array is controlled by temperature. The overall negative correlation of Ni with Zn indicates that Ni contents in chromites are also controlled by temperature. The behaviour of Mantle Array chromites is attributed to the partitioning of these elements between chromite and mantle olivine, which serves as a reservoir of these elements. Generally, the Mantle Array chromites can be defined on 3 axes: Zn, Co and Ni. In order to test whether these Mantle Array chromites record differences in the chemical composition of the lithosphere related to age or tectonic position, we have classified the localities according to the age of the last tectonothermal modification of the crust through which these kimberlites and related rocks penetrated, using a version of the scheme proposed by Janse (1984). "Archon'" represents a craton stabilised in Archean time with latest crustal modification >2.5 Ga. "Protons" are cratons with latest crustal modification before 1000 Ma. "Tectons" are basically younger tectonic units, mainly Phanerozoic in age. The trace elements of mantle-array chromites show differences among different tectonic environments. Chromites from Archons contain significantly higher V than those from Protons and Tectons, and chromites from Tectons have relatively higher Ga than those from Protons although they have similar V contents. Ga is positively correlated with Al^, and the higher Al and Ga in Tecton chromites reflects the enrichment of Phanerozoic mantle in Al relative to Proterozoic and Archean mantles. The higher V in Archon chromites correlates positively with Cr^, which is an mdicator of the degree of depletion of the mantle. Chromites from Archons tend to have high Nb and Nb/Zr, while those from Protons show a much greater spread in Zr contents and Nb/Zr. Chromites from Tecton environments, and all chromites from lamprophyres, tend to have low Nb and Zr. These differences may be related to time-integrated differences in metasomatic style, related to tectonic setting; chromites from xenoliths in the Wesselton kimberlite (S. Africa) have high Nb contents which can be related to observed phlogopite-related metasomatism in the mantle. References Griffin, W.L., Ryan, C.G., Gumey, J.J., Sobolev, N.V. and Win, T.T., 1994. Chromite macrociysts in kimberlites and lamproites: Geochemistry and origin, in H.O.A. Meyer and O.H. Leonardos (Eds) Kimberlites, Related Rocks and Mantle Xenoliths. CPRM Special Publication 1/A, pp.366-377, Companhia de Pesquisa de Recursos Minerals, Brasil. Janse, A.J.A., 1994. Is Clifford's Rule still valid? Affirmative examples from around the world. In H.O.A. Meyer and O. Leonardos (eds) Diamonds: characterization, genesis and exploration, CPRM Spec. Publ. lA/93, Dept. Nacional da Prod. Mineral., Brazilia, 215-235. Ryan, C.G., Griffin, W.L. and Pearson, N.J., 1996. Garnet geotherms: Pressure-temperature data from Cr-pyrope garnet xenocrysts in volcanic rocks. Journal of Geophysical Research, Vol.101, No.B3, pp.5611-5625.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
MAFIC TO FELSIC VOLCANIC HOSTS TO HYDROTHERMAL ACTIVITY IN THE EASTERN MANUS BASIN, PAPUA NEW GUINEA Christopher J. Yeats and Raymond A. Binns CSIRO Division of E3q)loration and Mining, PO Box 136, North Ryde, New South Wales 2113
The Manus back-arc extensional basin is located to the northeast of the New Britain subduction trench and volcanic arc. In the eastern Manus basin, an en echelon series of mostly northeast-trending neovolcanic edifices, ranging in compositionfi-ompicritic basalt to dacite-rhyodacite and referred to as the Eastern Manus Volcanic Zone (EMVZ), are develop^ as constructional features overlying older arc crust equivalent to exposures on the islands of New Britain and New Ireland. The edifices of the EMVZ trend at 90° to the inferred extension direction and appear to occupy a tensional jog which is related to a complex triple-junction between the New Britain trench and a series of major transform structures (the Wide Bay, Djaul and Weitin fault systems) which extend from the trench into the eastern Manus basin (Binns, 1998). The EMVZ contains four known sites of hydrothermal activity. These are, from west to east, Marmin Knolls, PACMANUS, DESMOS and Susu Knolls. DESMOS is a mafic volcano which was discovered by Japanese scientists in 1990 and contains a 1.5 to 2.0km diameter, 250m deep caldera (Gamo et al., 1997). Onsen, a small, veiy active hydrothermal field hosted in basaltic ^desites, was discovered in the wall of the caldera at a depth of 1930m. Disseminated pyrite and extensive silicic and advanced argillic alteration are associated with hydrothermal venting. To date, no massive sulfide mineialisation has been detected. The PACMANUS group of massive sulfide deposits were discovered by the CSIRO-University of Toronto led PACMANUS I cruise in 1991. Subsequent cruises in 1993, 1996 and 1997 have successfully photographed and sampled 4 separate fields which extend discontinuously along the crest of the northeast trending dacitic to rhyodacitic Pual Ridge over a distance of approximately 3km. Sulfide mineralisation at PACMANUS occurs at a depth of approximately 1750m and includes both Cu-Zn (dominantly chalcopyrite and sphalerite) and Zn-Pb (sphalerite and galena) rich massive sulfide chinmeys and possible subhalative massive sulfide mineralisation. Susu Knolls is a series of three dacitic volcanic cones, each 1.0 to 1.5km in diameter which form a northnorthwesterly trending edifice. The central and largest cone. North Su, rises to a depth of approximately 1200m. Sulfide mineralisation was discovered at Susu Knolls during the PACMANUS HI cruise in 1996. Curich massive sulfide chimneys and pyritic subhalative mineralisation has been dredged, along with porphyritic dacite, from the crests of the three volcanoes. Marmin Knolls is an area of mafic volcanism in the western part of the EMVZ. During the 1997 PACMANUS cruise, a single sample of porous barite was recovered from approximately 1650m depth in a dredge of picritic vesicidar pillow basalt The sample contains approximately 3% disseminated sulfide: dominantly pyrite with traces of sphalerite, chalcopyrite and galena. The sample has not been analysed as yet, but appears similar to gold-rich sulfate chimneys dredged from FrankHn Seamount in the Woodlark basin (Binns et al., 1993). The EMVZ occupies an area of approximately 1500 km^. Hydrothermal activity and associated massive and disseminated sulfide mineralisation occur at depths ranging from 1930 to 1200m and are hosted by mafic (Marmin KnoUs), intermediate (DESMOS) and felsic (Susu Knolls and PACMANUS) volcanic rocks. The variety and extent of mineralisation arguably makes the eastern Manus basin the most appropriate modem analogue for volcanic-hosted mineral provinces in ancient back-arc environments. REFERENCES ^ , Binns, R.A., 1998. Tectonic framework of the eastern Manus basin, Papua New Gmnea. This volume. Binns, R.A. & 10 others, 1993. Hydrothermal oxide and gold-rich sulfate deposits of the Franklin Seamount, western Woodlark basin. Papua New Guinea. Economic Geology, 91, 2122-2153. Gamo, T. & 18 others, 1997. Acidic and sulfate-rich hydrothermal fluids from the Manus back-arc basin, Papua New Guinea. Geology, 25, 139-142.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
SHRIMP n U-PB EVIDENCE FOR MORE THAN ONE LODE-GOLD MINERALISING EVENT IN THE ARCHAEAN YILGARN CRATON, WESTERN AUSTRALIA Christopher J. Yeats , Neal J. McNaughton and David I. Groves Ceito for Strate^c Mineral Dq)osits, The University of Western Australia, Nedlands, Western Australia 6907 current address: CSIRO Dmsion of Exploration and Mining, PO Box 136, North Ryde, New South Wales 2113
The lode-gold deposits of the Yilgam Craton are hosted by a variety of rocks and have widely varying structural jyles, associated alteration and ore mineralogy. However, most recent research correlates the variations in deposit parameters to the metamorphic grade and geochemistry of the host successions and interprets the lodegold deposits to r^resent a coherent group of epigenetic deposits, the majority of which formed during a Craton-scale, broadly synchronous hydrothermal event late in the tectonothermal evolution of the granitepeenstone terrains at 2640-2630 Ma. The majority of published geochronological data for the Yilgam is consistent with ca 2640-2630 Ma gold imneralisation. Several studies, however, suggest that gold may have been introduced at other times MueUer et (1996) dated the mam gold mineralising event at Big Bell at 2662±5 Ma, using U-Pb in garnet thought to have formed durmg gold mineralisation, and a later, secondary event (±gold) is dated at 2614±2 Ma using titoute. However, the structural timing of mineralisation at the Big Bell gold deposit remains controvert and the texti^ relationship between the minerals used for the geochronology and the gold mineralisation is e^ocal. At Lawlers, however, a SHRIMP U-Pb titanite study by Hetcher et al. (submitted) has dated the later of two gold mineralising events at 2592±9 Ma, distinctly younger than any other mineralisation rehably dated elsewhere m the Yilgam. Together with the younger (minor) mineraHsation at Big Bell, there now appears to befirmevidence for post-2630 Ma gold mineralisation. A number of ^V-^OAr plateau ages for hydrothermal muscovite from the eastern part of the Craton also give younger ages for gold mineralisation. However the validity of this method given the uncertain cooling histoiy of the Yilgam remains a matter of debate. Recent SHRIMP work at UWA has dated pre-mineralisation felsic porphyries at Mount Charlotte (2673±3 Ma) Mount Percy (2672±12 Ma), Racetrack (2663±3 Ma), Porphyry (2657±8 Ma) and Lawlers (2663±3 Ma) at ca 2670-2660 Ma, providing a maximum age for gold mineralisation at these deposits. Cross-cutting felsic dykes are present in the Jundee and Mount McClure gold deposits in the Yandal greenstone belt. Two of these post-ore dykes from Jundee (2656±7 and 2669±7 Ma) and three from Mount McClure (2656±4, 2663±4 and 2668±10 Ma) have been dated using SHRIMP U-Pb zircon geochronology at ca 26702660 Ma, within error of the ages of the pre-mineralisation porphyries at Mount Charlotte, Mount Percy Racetrack, Porphyry and Lawlers. It is probable that these dykes are all part of the same ca 2670-2660 Ma intrusive event. However, gold mineralisation at Jundee and Mount McClure clearly occurred prior to the mtrusion of the dykes, providing evidence in this area of a gold mineralising event at least 20 m.y. prior to the mam ca 2640-2630 Ma event which has affected most of the Craton. A further constraint on the timing of gold nuneralisation at Jundee is provided by the age of 2678±5 Ma for a mineraUsed porphyryfromthe Deakin South pit. This unit is cross-cut by the 2669±7 Ma post-ore porphyry, effectively bracketing the age of mineralisation between ca 2680 and 2670 Ma. In conclusion, recent SHRIMP studies have confirmed the presence of a craton-scale lode-gold mineralising event at ca 2640-2630 Ma which appears to have affected the majority of the Yilgam Craton. However, evidence has emerged for the presence of earlier and later gold mineralisation. At this early stage of research! pre- and post- ca 2640-2630 Ma mineralisation appears to be much less widespread than the main event! However,fiirtherwork is needed to confirm this. REFERENCES Hetcher, I.R., Mikucki, J.E. & McNaughton, N.J., submitted. SHRIMP dating of felsic magmatism and lodegold mineralisation in the Lawlers area, Yilgam Craton, Westem Australia. Economic Geology. Mueller, A.G., Campbell, I.H., Schiotte, L., Sevigny, J.H. & Layer, P.W., 1996. Constraints on the age of gramtoid emplacement, metamorphism, gold mineralisation, and subsequent cooling of the Archean greenstone terrane at Big Bell, Westem Australia. Economic Geology, 91, 896-915.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
INTERNATIONAL GEOLOGICAL CORRELATION PROGRAMME PROJECT NO. 396 ^CONTINENTAL SHELVES IN THE QUATERNARY' W.W.-S. Yim Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China
Sediments on continental shelves provide a direct geological record of Quaternary sea-level changes but have played only a minor role to date in land-sea correlation. This is attributed mainly to the lack of a major international initiative like the Ocean Drilling Programme on continental shelves. The main objectives of this project are to study and interpret the Quaternary sequences on continental shelves to permit global correlation of sea-level and climatic changes while at the same time identify beneficial uses for humankind. Major topics being studied include: (1) Sea-level and climatic changes. (2) Marine flora and fauna including corals, molluscs and microfossils. (3) Terrestrial flora and fauna including migration routes. (4) Soil development during low sea-level stands. (5) Continental shelf hydrology. (6) Correlation with the deep sea sedimentary record. (7) Correlation with the continental sedimentary record. (8) Engineering properties of offshore sediments and their relationship with environmental change. (9) Dating methods including the reliability of pre-Holocene radiocarbon dates and using a wide range of dating methods to facilitate comparison. (10) The contribution of continental shelves to the global carbon cycle. (11) Living and non-living resources and their management. In order to fulfill the above, six working groups have been set up: (1) Dating and sequence stratigraphy - Leader Prof Allan Chivas (Fax: 61-42-214250 E-mail: a. chivas@uow. edu.au). (2) Siliciclastic shelves - Leader Dr Yoshiki Saito (Fax: 81-298-543533 E-mail: yoshi@gsj.go.jp) (3) Carbonates - Prof Peter Davies (Fax: 61-2-93510184 E-mail: pjd@es.su.oz.au) (4) Palaeo-oceanography - Dr Bill Austin (Fax: 44-191-3742456 E-mail: bill.austin@durham.ac.uk) (5) Marine processes and marine geotechnics - Dr Wyss Yim (Fax: 852-25176912 E-mail: wwsy im@hkucc. hku. hk) (6) Living and non-living resources - Dr Heiner Josenhans (1-902-4264104 E-mail: josenhan@agc.bio.ns.ca) The project is currently in its third year out of five. In 1998, the annual conference will be held in Goa, India from October (contact Dr M. Veerayya Fax: 91-832-223340 E-mail: veerayya@csnio.ren.nic.in for details). Examples of important achievements over the past two years will be presented.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
PHANEROZOIC DRAINAGE EVOLUTION IN NORTHEASTERN TASMANIA, AUSTRALIA W.W.-S. Yim Department of Earth Sciences, The University of Hong Kong, Pokflilam Road, Hong Kong, China
A heavy mineral provenance study of tin placers in northeastern Tasmania has provided valuable information on the Phanerozoic drainage evolution with the earliest recognizable event dating back to at least the Permian. Stratigraphical control of the stanniferous deep leads is based on the K-Ar dating of Tertiary basalts; fissiontrack dating of alluvial zircons; palynoflora within deep leads, and, post-Middle Miocene duricrusts. The sequence of events are: (1) Pre-Permo-Triassic erosion and unroofing of mineralised Devonian to Carboniferous granitic batholiths. (2) Intrusion of Jurassic dolerite sheets followed by extensive erosion. (3) Middle Eocene basaltic volcanic activity. (4) Post-Middle Eocene period of 'rapid' erosion. (5) Post-Oligocene lateritisation. (6) Middle Miocene basahic volcanic activity. (7) The capture of northwesterly flowing streams draining from the Blue Tier Massif formerly connected to the Boobyalla River system by the present day Ringarooma River. (8) The capture of the tributary of the Great Mussel Roe River by the Ringarooma River. (9) Reworking of alluvial deposits along the course of the Ringarooma River associated with periods of high discharge in the Quaternary. Along parts of the present day river course of the Ringarooma River between Branxholm and Pioneer, the direction of river flow may have been reversed in the past. The evidence to support this include the barbed pattern of present day tributaries, the misfit pattern of stream channels in the present day Boobyalla River system and the provenance of the 'zircospilic' (zircon, corundum, spinel and magnesium ihnenite) suite of heavy minerals known to have been derived from the Blue Tier Basalts. The Cainozoic events identified are in general agreement with sea-surface temperatures obtained from oxygenisotope studies of the deep sea cores from waters adjacent to Tasmania by Shackleton & Kennett (1975). This study highlighted the dramatic changes in erosion rates over the Phanerozoic. REFERENCE Shackleton, N.J. & Kennett, J.P. 1975. Palaeotemperature history of the Cenozoic and the initiation of Antarctic glaciation: oxygen and carbon isotope analyses in DSDP sites 277, 279 and 281. In Kennett, J.P. et al eds. Initial Reports of the Deep Sea Drilling Project 29, 743-755.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
PROSPECTIVITY ANALYSIS OF GREENSTONE BELTS FOR LODE-GOLD DEPOSITS WITHIN A GIS ENVIRONMENT Grace Yaiuan Yun\ David Groves', Carl Knox-Robmson\ Stephen Gardoll' 'Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, The University of Western Australia 6907
A major problem in area selection for exploration is assessing the relative merits of incompletely explored areas, which show similar indications of mineralization but have somewhat different geological features. To solve this problem, it is necessary to define the relationships between geological characteristics of mineral deposits, and their environments, and their economic characteristics. To solve the problem of area selection in gold exploration, conventional geological map data and known golddeposit information for major Archaean greenstone belts, in the Yilgarn, Pilbara, and Zimbabwe Cratons and Abitibi Belts, have been integrated into a Geographic Information System (GIS). The database allows the identification of regionally important spatial parameters, which control the siting of gold deposits. With the aid of GIS, spatial relationships can be quantified and integrated, and the results can give a guideline for target selection in gold exploration. In the early stage of an exploration period, it is important to reduce the size of the search area based on the presence or absence of specific geologic features, which show a spatial and temporal relationship to mineralisation. Several reviews of gold deposit genesis and greenstone evolution have led to the recognition of numerous important craton-scale parameters, which potentially control the distribution of gold deposits. The most widely accepted geological parameters that influence the occurrence and siting of gold deposits are linearity of greenstone belts, presence or absence of crustal-scale faults and shear zones, the geometry of these and lower order structures, the siting of anticlinal crests and domes, lithological contacts, the competency and composition of potential host rocks, and the proximity to granitoid intrusion. Geographic Information Systems (GIS) have grovm dramatically over the last decade, and have been adopted in many scientific fields to perform spatial analysis. The methodology used in this study comprises three main steps: identification, quantification, and integration and comparison. Once suitable data have been collected and assimilated into the GIS environment, the spatial relationships between known deposits and mineralized terranes to certain geological features can be identified and quantified. If a significant relationship is discovered, it is then quantified into a map, which expresses how the observed relationship behaves over the entire area of interest. Finally, two or more quantified spatial relationships are integrated, and the prospectivity of areas of the major greenstone belts can be determined on a province to craton scale, and comparison can be made between the studied terranes. The following spatial relations between known gold deposits and geological features are quantified in this study: 1) linearity of greenstone belts, 2) orientation in terms of regional stress fields, 3) shape of granitoid intrusions, 4) portion and orientation of regional-scale fault zones, 5) anticlinal zones/uplift zones, 6) competency contrasts across lithological contacts or faults, and 7) lithological units. Once several spatial relationships have been identified and suitably quantified, the final step is to integrate these data and define the areas of highest prospectivity on various scales. Spatial relationships, conmion to all or most provinces or cratons, can be used in exploration on a global scale. GIS-based prospectivity analysis has the potential to identify and summarise subtle spatial relationships that exist between known gold deposits and the surrounding geology. The results not only have significance in exploration, but may have implications for the origin of Archaean lode-gold deposits, in the sense that they indicate which geologic features are an essential part of the ore-forming environment and which are merely incidentally associated with mineralization. Acknowledgment: This research has been funded by AMIRA/MERIWA and is now supported by Great Central Mines. We are particularly grateful to Dr G.Neil Phillips for support and discussion.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
LATE PALAEOPROTEROZOIC AND EARLY MESOPROTEROZOIC TECTONICS AND MINERAL DEPOSITS IN THE SOUTHEASTERN GAWLER CRATON, YORKE PENINSULA, SA Wen-long Zang. Conor, C. H. H., Cowley W. M. Mineral Resources, Department of Primary Industry and Resources, South Australia
Late Palaeoproterozoic rocks on Yorke Peninsula form the southeastern margin of the Gawler Craton, including magmatic Lincoln Complex (1850-1700 Ma, c£ Parker, 1993) in the south and the Wallaroo Group (Conor, 1995, -1770-1730 Ma) to the north. The Wallaroo Group consists mainly of low grade metasediments and metavolcanics and was intruded by the Tickera and Arthurton Granites (1600-1575 Ma). Emplacement of these granites is considered to have generated hydrothermal systems to form Cu-Au deposits in a fault-controlled mineralisation zone, eg. the Moonta and Wallaroo Mines. The Lincoln Complex intruded the Archaean Sleaford Complex (>2450Ma) and Hutchison Group (2000-1850 Ma) on southern Eyre Peninsula. On Yorke Peninsula it is interpreted to form the basement of the Wallaroo Group. At Corny Point, a suite of migmatitic paragneisses, which is concordant with the Lincoln Complex, contains calcsilicate boudins of probable Hutchison Group. Emplacement of mafic and felsic plutons and migmatisation marked the end of middle Palaeoproterozoic sedimentation. Of these, widely distributed Gleesons Landing Granite (Donington Granitoid Suite equivalent), an I-type granitoid, intruded the paragneiss and was deformed. Four deformation events can be recognised in the Lincoln Complex on Yorke Peninsula. On northern Yorke Peninsula a late Palaeoproterozoic 1770-1730 Ma) rift event resulted in deposition of the Wallaroo Group. This sedimentary basin might have an Archaean (suggested by Nd-Sm isotopic analyses, Huffadine, 1993) provenance and east to northeast deepening sedimentary regime, with shallow-water arenaceous and arkosic deposits in the south and southwest and relatively deep-water, laminated argillites, carbonates and chemical sediments in the northeast. The metasediments are intercalated with mafic and felsic volcanics, locally forming hyaloclastic deposits at the contact margins. Bi-modality of volcanism suggests potential to form SEDEX or VMS deposits but those were probably remobilised during subsequent deformations to form those low grade Cu, minor Pb and Zn deposits in the Doora and Weetulta regions, in which the minerals were re-distributed along foliation or fold axial planes and compositional layers. Mineralisation of fracture systems probably coincided with intrusion of the Arthurton Granite during the early Mesoproterozoic (-1590 Ma) on Yorke Peninsula. Magmatic fluids were probably generated by ascending A-type granites during the event and the fluids forced into fault zones and dilatant fractures. The important Moonta and Wallaroo Cu-Au lodes formed during this period and are hosted by the Moonta Porphyry and Doora Formation of the Wallaroo Group. Contemporaneous deformation progressed as a two-stage event. The first was NE-SW compression and reverse-faulting which created NW300-340°-oriented brittle-ductile shear zones and was accompanied by NE-oriented en echelon tension gash arrays and E-W shear zones. The subsequent deformation was a conjugate shearing event and occurred during the cooling stage of magmas; the associated shearing and faulting displaced the Cu-Au lodes. Probable equivalents of the Wallaroo Group metasediments occur near the world-class Olympic Dam deposit which is hosted by granites of the Hiltaba Suite (-1590 Ma). The magmatism and mineralisation both on Yorke Peninsula and at Olympic Dam are considered to have occurred because of extensive continental crust that slowed mantle cooling during the early Mesoproterozoic. Subsequent intracontinental extension may have focussed mineralisation (Hitzman et al., 1992). REFERENCES Conor, C. H. H., 1995. Moonta - Wallaroo region, an interpretation of the geology of the Maitland and Wallaroo 1:100 000 sheet areas. Department of Mines and Energy, South Australia. Open File Envelope 8886. Hitzman, M. W., Oreskes, N. and Einaudi, M. T., 1992. Geological characteristics and tectonic setting of Proterozoic iron oxide (Cu-U-Au-REE) deposits. Precambrian Research, 58, 241-287. Huffadine, S. J., 1993. Environment, timing and petrogenesis of a Middle Proterozoic volcanic suite: Port Victoria, South Australia. University of Adelaide, honours thesis (unpublished). Parker, A. J., 1993. Palaeoproterozoic. In: Drexel, J. F., Preiss, W. V. and Parker, A. J. (editors), The geology of South Australia. Vol. 1. The Precambrian. South Australia. Geological Survey, Bulletin 54, 1:51-105.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
EARLY CAMBRIAN ACRITARCHS, TRILOBITES AND BIOSTRATIGRAPHY IN SOUTH AUSTRALIA Wen-long Zang, Jim Jago and Tian-nii Lin Mineral Resources, Department of Primary Industry and Resources, South Australia, PO Box 151, Eastwood, SA 5063 Department of Applied Geology, University of South Australia, The Levels, SA 5095 Department of Geology, Nanjing University, People's Republic of China
The purpose of this paper is to integrate newly recognized acritarch assemblages from the Early Cambrian of South Australia with previously reported archaeocyathid and particularly trilobite biostratigraphy. Lower Cambrian sediments in the Arrowie and Stansbury Basins were deposited on a rifting continental platform and contain thick carbonates and siliciclastics. Five unconformity-bounded depositional sequences or subsequences have been recognized; these contain abundant trace fossils, small skeletal fossils, archaeocyaths, trilobites and organic-walled microfossils. Generally faunas and acritarchs are abundant in transgressive and highstand, but less common in lowstand deposits, responding to both sedimentary and preservational environments. Early Cambrian faunas have been known from the Stansbury and Arrowie Basins for over a hundred years. Jell in Bengtson et al (1990) erected four trilobite zones (Abadiella huoi (base), Pararaia tatex, P .bunyerooensis and P Janeae Zones) which equate approximately with the Atdabanian and Botoman of Siberia. Gravestock (1984) and Zhuravlev & Gravestock (1994) described three archaeocyathid zones from the Wilkawillina Limestone which they correlate with the Atdabanian. The highest of these zones, the Jugulicyathus tardus Zone overlaps with the Abadiella huoi Zone. The informal Syringocnema favus beds of Zhuravlev & Gravestock correspond to the P. janeae Zone. A higher informal archaeocyathid assemblage, the Archaeocyathus abacus beds occurs in the Wirrealpa Limestone above the P. janeae Zone. Acritarch biostratigraphy is relatively new in Australia but well studied on the European Platform. Recent studies have revealed the presence of seven acritarch assemblages in the Early Cambrian of the Stansbury and Arrowie Basins, both in outcrop and drillholes. Study of acritarchs from outcrop is difficult due to weathering effects, although the three bottom acritarch assemblages described here are known only from the Uratanna and Parachilna Formations in the Flinders Ranges. Recent work on both acritarchs and trilobites from the fully cored Yalkalpo-2 hole in the eastern Arrowie Basin has allowed a linkage between acritarch and trilobite zones. Acritarch Assemblage 1, which contains mainly long ranging spheroids crosses the Precambrian-Cambrian boundary. Assemblage 2 represents a dramatic increase in spinose acritarchs about Im above the first appearance of the trace fossil Rusophycus\ it contains fragmentary spinose specimens of Comasphaeridium, Filisphaeridium and relatively well-preserved Fimbriagloerella minuta, Vendomyces sp. and Corollasphaeridium. Future studies relating to acritarchs, trace fossils and small shelly fossils should help locate the the Precambrian-Cambrian boundary in South Australia and assist in global correlation. Assemblage 3 occurs in transgressive siltstones in the upper Parachilna Formation and is marked by the appearance of several new forms which are widespread and well correlated in the Arrowie and Stansbury Basins. Assemblages 4,5,6 and 7 are best knovm from Yalkalpo-2. Assemblages 4,5 and 6 are dominated by Skiagia; the appearance of S. ornata followed by S. ciliosa in Assemblages 4 and 5 indicates correlation with the Talsy and Vergale Horizons respectively on the East European Platform. Assemblage 7 indicates a major acritarch decrease during the late Botoman. Continuous occurrence of acritarch groups of Skiagia, Ceratophyton and Corallosphaeridium provide valuable evidence of evolutionary development and therefore for biostratigraphic zonation. Yalkalpo-2 is frilly cored from the Parachihia Formation to the Billy Creek Formation. It contains abundant acritarchs, scattered trilobites and inarticulate brachiopods plus some trace fossils. The trilobite Abadiella huoi Zone and the archaeocyathid J.tardus Zone partly correspond to acritarch Assemblage 5; the trilobite P.tatei Zone may be equivalent to the bottom part of acritarch assemblage 6; the P. bunyerooensis Zone may be equivalent to the mid part of Assemblage 6 and the P. janeae Zone may be equivalent to upper Assemblage 6 plus Assemblage 7. REFERENCES Bengtson, S., Conway Morris, S., Cooper, B.J., Jell, P.A., & Runnegar, B., 1990. Early Cambrian fossils from South Australia. Memoirs of the Association of Australasian Palaeontologists, 9, 1-364. Gravestock, D.I., 1984. Archaeocyatha from lower parts of the Lower Cambrian carbonate sequence in South Australia. Memoirs of the Association of Australasian Palaeontologists, 2, 1-139. Zhuravlev, A.Yu & Gravestock, D.L, 1994. Archaeocyaths from Yorke Peninsula, South Australia and archeocyathan Early Cambrian zonation. Alcheringa, 18, 1-54.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
PETROGENESIS OF LATE CENOZOIC BASALTS IN NORTH QUEENSLAND AND GEODYNAMIC IMPLICATIONS Ming Zhang\ Jon Stephenson^, Suzanne Y. O'Reilly^ M.T. McCulloch^, M. Norman^ ^GEMCX:, School of earth Sciences, Macquarie University, NSW 2109, Australia ^Dept of Earth Sciences, James Cook University, Townsville, Qld 4811, Australia Research School of Earth Sciences, Australian National Univ., ACT 0200, Australia
Late Cenozoic basalts (6 Ma-13 Ka) of six volcanic provinces (Atherton, McBride, Chudleigh, Nulla, McLean, and Piebald) in North Queensland (NQld) consist of a chemical spectrum from nephelinite, through basanite,' alkaline olivine basalt, and hawaiite, to olivine tholeiite. '^Srr^Sr ratios (n=31) range from 0.70340 to 0.70472 and 'Nd/'^^d ratios (n=29) range from 0.51302 to 0.51279 (ewd =+7.5 - +3.0) (Zhang et al., 1998a). Although these Sr and Nd isotopic ratios overlap those for oceanic island basalts and the NSW lavafield basalts, they clearly differfromthe latter m their high ^'Sr/^Sr at a given ENd, thus forming a high ^'Sr/^^Sr isotopic ratios (n=12) range in ' ' W P b of 17.93-18.62, in of 15.52-15.62, and in Pb/ Pb of 37.75-38.55, ubiquitously displaying the Dupal-type Pb isotopic signatures (A8/4Pb = 37-63, A7/4Pb = 3.3-10.9). The Sr-Nd-Pb isotopic data fit v^ith tvy^o-component mixing between an isotopically depleted Indian-MORB type component and an enriched one with EM2 signatures. The basalts with the enriched isotopic signatures are generally high in K/Nb, K/U, Rb/Sr, and Zr/Nb, but low in U/Pb and Ce/Pb, consistent with derivation from a subduction-modified source and possible participation of phlogopite in the lithospheric mantle melting domain. We propose that the enriched mantle source is a subcontinental lithospheric mantle modified by subduction-related processes during the late Paleozoic orogeny in the region. On the other hand, the depleted mantle component represents an asthenospheric mantle chemically identical to the mantle source for the present-day Indian MORE. The basalts with the depleted isotopic signatures show fractionated incompatible element pattems with high Ce/Pb, U/Pb, LREE/HREE, but low K/Nb, K/U, Rb/Sr, and Zr/Nb, similar to or even more fractionated than some HIMU oceanic island basalts. We attribute these distinct incompatible element signatures to melting of amphibole- and apatite-bearing assemblages of metasomatic origin resided in the lithospheric mantle. The high |li (>70) and low Sm/Nd (-0.15) of the nephelinites are obviously decoupled from their low ^^Pb/^^Pb (< 18.0) and high eNd (> 6.7). This implies that the metasomatism must be a close precursor of the nephelinitic magmatism, probably connected with the early Tertiary subduction at the NE margin of the Australian Plate during the rapid northward motion of the Australian continent after 50 Ma. Our data also show that some lava-field basalts (55-30 Ma) in NSW may require a Pacific-MORB upper mantle as one of their major sources. This discovery is consistent with the proposed secular distribution of the Pacific and Indian MORE reservoirs surrounding the Australian continent in the eastem Gondwana, as deduced from studies of back-arc basin basalts in the SW Pacific and ocean floor basalts in the Southern Ocean (eg Hergt and Hawkesworth, 1994; Lanyon et al, 1995). We further suggest that the Indian MORE mantle is a long-term mantle reservoir beneath most part of the East Gondwana and the westward migration of the P-MORB mantle may have partly associated with the Tasman Sea opening (ca 82-60 Ma) along a broad front southeast of the Australian continent (Zhang et al., 1998b). An isotopic framewoiic of mantle reservoirs, based on mixing relationships between various theoretical endmembers, can now be established in eastem Australia. The delineated source end-members include the Pacific and Indian Ocean asthenosphere, two mantle plumes (one presently located near the Bass Strait and another at Balleny Islands), and SCLM domains with both EM2 and EMI signatures. The secular distribution of these mantle reservoirs are consistent with the evolutionary history of the Tasman foldbelts in eastem Australia. REFERENCES Hergt, J.M., and Hawkesworth, C.J., 1994. The Pb, Sr, and Nd isotopic evolution of the Lau Basin: implications for mantle dynamics during the back-arc opening. Proceedings of the Oceanic Drilling Program, Scientific Results, 135, 505-518. Lanyon, R., Crawford, A.J., and Eggins, S.M., 1995. Westem migration of Pacific Ocean upper mantle into the Southem Ocean region between Australia and Antarctica. Geology, 23, 511 -514. Zhang, M., Stephenson, J., O'Reilly, S.Y., McCulloch, M.T. and Norman, M., 1998a. Petrogenesis of late Cenozoic basalts in North Queensland and its geodynamic implications: trace element and Sr-Nd-Pb isotope evidence. Submitted to J. Pet. Zhang, M., O'Reilly, S.Y. and Chen, D., 1998b. Pacific- and Indian-MORB mantle as source reservoirs for the Cenozoic basalts in eastem Australia: Pb-Sr-Nd isotope evidence. Submitted to Geology. 494
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
NUMERICAL MODELLING OF PORE-FLUID, THERMAL AND MASS FLOW IN HYDROTHERMAL BASINS: CASE STUDY OF NW SHELF BASIN Chongbin Zhao^ B. E. Hobbs", K. Baxter'^ H. B. Muhlhaus' and A. Ord' 'CSIRO Division of Exploration and Mining, P. O. Box 437, Nedlands, WA 6009, Australia 'Australian Geodynamics Cooperative Research Centre ^now at Badley Earth Sciences Ltd.
We are using numerical methods to develop a holistic and predictive tool for orebody exploration in hydrothermal basins. Towards this end, we have to deal, mathematically, with a fully coupled problem between material deformation, pore-fluid flow, thermal transfer and species transport/chemical reactions in porous rock masses. The fmal product of our development is to produce a reliable and practical means to answer many whatif scenarios in relation to orebody formation in any given hydrothermal systems. As the first step of our development, we have focussed on the numerical modelling of convective flow of porefluid in the earth's crust consisting of fluid-saturated porous rocks. To achieve this purpose, we have developed the progressive asymptotic approach procedure, which is linked with the finite element method to successfully solve the natural convection problems of pore-fluid in hydrothermal basins (Zhao et al. 1997). According to the relative time scale of the convection cell roll over time to the species chemical reaction relaxation, we have divided the species transport problems in the earth's crust into the following three categories: the transportdominated (non-reactive) transport, the quasi-instantaneous reaction dominated transport and the chemical reaction dominated transport. Since the orebody formation is usually relevant to the quasi-instantaneous reaction dominated transport problem (Phillips 1991), we have developed the related numerical technique to qualitatively predict the most probable dissolution/precipitation regions of minerals in hydrothermal basins. We select the Australian North West Shelf Basin to illustrate the performance of the developed numerical and predictive tool. In particular, the following what-if scenarios are discussed in this presentation: (1) W h a t . . . if the basin is comprised of homogeneous porous rock masses? (2) W h a t . . . if the basin has two major relatively permeable faults? (3) W h a t . . . if the basin has two major relatively impermeable faults? (4) W h a t . . . if the basin is stratified and has two major relatively permeable faults? (5) W h a t . . . if the basin is stratified and has two major relatively impermeable faults? (6) W h a t . . . if there is a hydrocarbon source in the centre of the basin? (7) What ... if the porous rocks of the basin is deformable during the convective flow of the pore-fluid? We can, of course, investigate many other what-if scenarios in any hydrothermal basins if necessary. The list above is just a few illustrative examples. It is noted that in terms of the occurrence regimes of pore-fluid flow in permeable rock basins, the flow can be generated by the following main processes: mechanical process, thermal process, chemical process, or a combination of some/all of them. This presentation mainly shows the pore-fluid flow generated by the thermal process in hydrothermal basins. Our final goal is to produce a powerful and robust numerical tool so that the pore-fluid flow generated by all the possible processes mentioned above can be effectively and efficiently predicted and modelled in any geological basins. REFERENCES Phillips O. M. 1991. Flow and Reactions in Permeable Rocks. Cambridge University press, Cambridge. Zhao C., Muhlhaus H. B. & Hobbs B. E. 1997. Finite element analysis of steady-state natural convection problems in fluid-saturated porous media heated from below. InternationalJoumal for Numerical and Analytical Methods in Geomechanics, 21, 863-881.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
MAJOR GOLD PROVINCES AND TECTONICS OF CHINA Taihe Zhou Quantum Resource Limited,
Floor, 580 St Kilda Road, Melbourne, Victoria 3004, Australia
China has been producing gold for over 4,000 years. Its gold production has increased substantially since 1980s, about 25 tonnes in 1980 to some 120 tonnes in 1996. The gold reserve was about 2,800 tonnes by 1992, now the accumulated reserve is some 4,000 tonnes, and the estimated potential resource is about 10,000 tonnes. The major new fmdings are from NW China. Gold distribution appears to be patchy in China in terms of geography. More than a quarter of gold production and reserve in China is from eastern Shandong (ie the Jiaodong gold province), and some twelve-percent form western Hanan (ie the Xiao Qinling gold province). The others are mainly from the Shaanxi, Hebei, Liaoning, Jilin, Inner Mongolia, Guizhou, Yunnan and Hunan provinces, and some recent new findings mainly from Gansu, Xinjiang, Shaanxi and Inner Mongolia provinces, northwestern China. However, when the data are put into the tectonic frameworks, it appears that most of the gold deposits are located along margins of the major continent blocks and the convergent zone related fold belts. Accordingly, the following gold provinces may be recognised: •
• •
• • •
Jiaodong (ie eastern Shandong) gold province tectonically on the southeastern margin of the China-Korea craton, along the ultra-high-pressure suture zone between the North China and South blocks, mainly comprising greenstone-granitoid belt gold (> 25% reserve and production); North China gold province along the north margin of the Sino-Korea block, mainly comprising greenstonegranitoid belt gold (20-25% reserve); Qinling gold province (including the Xiao Qinling) along the fold belts between, and margins of the SinoKorea blocks and the Yangzi block, including greenstone belt, slate belt and epithermal golds (20-25% reserve); Southeastern gold province along the South China Fold Belt, including slate belt and epithermal gold (2-3% reserve); Dian-Gui-Qian gold province along the south margin of the Yangzi block, mainly Carlin type and epitermal gold (5% reserve); and North Xinjiang gold province along the north margin of the Tarim and Yili blocks and the fold belts including slate belt and epithermal gold (main new fmdings).
Formation of the major gold province appears to be controlled by the major tectonic movement in china. The major Precambrian cratons in China are the Sino-China, Tarim and Yangzi blocks, and the tectonic history of China is dominated by the interaction between the three blocks and with the Siberia, Kazakhstan and Indian blocks, and deformation of accreted sequences between the Precambrian massifs. Apart from strong tectonic movements during the Late Archaean and Proterozoic, the major Archaean blocks have had a complex tectonic history since the Palaeozoic in China. Most of the Archaean blocks elsewhere (eg, the Yilgam block in Australia and the Canada Shield) underwent tectonic movement and gold mineralization by end of the Archaean, and have been isolated from ftirther significant orogeny since then. The major Archaean blocks of China, however, has undergone multiple orogenic tectonics and magmatism along its margins since the Palaeozoic. The most important tectonic events are the Hercynian orogeny, which led to the begining of merging of the Siberia and North China (ie Tarim + Sino-Korea) blocks forming the Altay-Xinganling and Tian Shan-Beishan-Ynshan fold belts, and the Indosinian orogeny (Triassic) which led to merging of the North and South China blocks forming the Kunlun-Qinling-Dabie-Sulu fold belts. Much of the Chinese gold deposits are distributed along the two major tectonic belts. During the Yanshanian (208-90 Ma) and Himalayan (<90 Ma), the tectonic patterns and gold distribution were largely influenced by the subduction of the Izanagi-Pacific plates under, and indentation of the India continent into the Eurasia continent.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14'^ Australian Geological Convention, Townsville, July 1998
LAOWANCHANG: AN UNUSUAL REGOLITH GOLD DEPOSIT IN KARST DOMINATED TERRAINS, GUIZHOU PROVINCE, CHINA Taihe Zhou'', Ken McQueen' and Jianquan Mao^ 'CRC LEME, Faculty of Applied Science, University of Canberra, PO Box 1, Belconnen, ACT 2616, Australia ^Guizhou University of Technology, Caijiaguan, Guiyan City, Guizhou Province 550003, China
An unusual "red soil" goldfield, i. e. the Fuyuan-Xinren goldfield, has been found in karst dominated terrains in southwestern Guizhou Province, China approximately 180km southwest of Guiyang. The goldfield is located in the Carlin-like gold province of southwestern Guizhou, within the western part of the Yangzi tectonic block. Many epithermal "Carlin-type" gold/antimony deposits occur in the tuffaceous breccia and mudstones of the Late Permian Dachang Formation in this area. The goldfield occupies a belt about 80km long and 20km wide. More than thirty locations of Au-Hg-Sb geochemical anomalies have been found in the goldfield, and further exploration has confirmed more than ten "red soil" gold deposits. The Laowanchang gold deposit is the largest one in the goldfield, occupying an area 2km long and 1km wide, within the eastern part of the Fuyuan-Xinren goldfield. It is located in a moderately hilly karst erosion area with a relative relief of some 300m, where the Early Permian Maokou limestone is exposed. Karst features include depressions, caves, sinkholes and funnels, which are partly buried by regolith and transported Quaternary sediments. The sediments are mainly residual and slope wash silty clays and clayey silts, with some clluvial and transported overburdens of clays, pebbles and fragments of locally derived tuffaceous mudstones, brecciaed and silicified tuff, sandstones, and limestone brecciated. The Quaternary sediments infill karst collapse depressions and valleys, and cover different karst topographies with a thickness of 5 to 20m, locally up to more than 50m. Gold is hosted in the Quaternary red soils and clays, which occur in the karst depressions, and on the north slopes of the karst hills. There are five ore bodies, which are currently being mined in the deposit. The ore bodies are about 200 to 300m long, 20 to 100m wide and 5 to 20m (up to >50m) deep. The size and shapes of the ore bodies vary depending on the characters of the underlying walls, which are controlled by the karst topography in the Maokuo limestone. The indicated resource of the deposit is some four million-tonnes with an average grade of approximately 5 grams per tonne (ie ca 20 tonnes gold). The potential gold reserve should be larger since the bottom of the main ore body is still open, and lower grade gold ore has been found in the surrounding area. The Quaternary sediments hosting gold mainly consist of red, yellow, brown with minor grey and pallid silty clays, clayey silts and soils with fragments of tuffaceous mudstones, tuff breccia, sandstones, limestone, and some heavily oxidised ferruginous pebbles. Most of the fi-agments are angular and unsorted, and appear to be derived from the Late Permian Dachang Formation, since remnant tuffaceous and breccia textures, commonly observed in the Dachang Formation, can still be recognised. The fragments have been heavily oxidised and weathered, and commonly display pseudomorph breccia textures where previous breccia fragments have been replaced by quartz aggregates of radiated elongated large crystals normally 0.1 x 0.7cm in size. Detailed XRD analysis of the soils and clays from the ore bodies indicates that the soils and clays are mainly composed of quartz, kaolinite, illite, with lesser amounts of goethite, anatase and ± chlorite. Geochemical studies indicate that the Late Permian Dachang Formation contains background gold levels more than ten times higher than those from other rock units do. Analysis of 2,249 rock samples of the Dachang Formation collected in the surrounding area by the provincial Geological Survey Brigade gave gold values from 35 to 157ppb. High gold anomalies of 0.1 to 0.3 grams per tonne, with rare locations of up to a few grams per tonne, are also reported in the nearby Dachang epithermal antimony deposit. At Laowanchang gold appears to have been relatively concentrated in the Dachang Formation during epithermal/Carlin type Sb-Au mineralising processes. It was leached, transported and further concentrated during weathering and erosion, and finally accommodated by the karst topography. The high grade of the ores, the simple mining processes and the extremely low cost for gold recovery make the Laowanchang type regolith "red soil" gold deposits economically significant. Deposits of this type and scale have not been reported previously in the international literature.
Current address: Quantum Resources Limited, 8*^ Floor, 580 St Kilda Road, Melbourne, Victoria 3004, Australia
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
Australian Geological Convention, Townsville, July 1998
AGE CONSTRAINTS ON DEFORMATION OF THE EASTERN HODGKINSON PROVINCE, NORTH QUEENSLAND: NEW PERSPECTIVES ON THE EVOLUTION OF THE NORTHERN TASMAN OROGENIC ZONE. R.G. ZUCCHETTO^ R.A. HENDERSON^ B.K. DAVIS' and R. WYSOCZANSKf ^Department of Earth Sciences, James Cook University of North Queensland, Townsville, Qld 4811, Australia. ^Research School of Earth Sciences, GPO Box 4, Australia National University, Canberra, ACT 2601, Australia. Granitic plutons of the Wangetti and Mount Formartine Supersuites intrude the Hodgkinson Formation of the Macalister Range district of the eastern Hodgkinson Province, north Queensland. The Hodgkinson Province represents the northern extremity of the Tasman Orogenic Zone, the dominant crustal element of eastern Australia. Along with the subjacent Broken River Province to the south, it comprises a discrete tract within the fabric of the Tasmanides characterised by distinct rock assemblages and a common history. With the exception of a belt of diverse rocks at its western margin and a narrow suspect terrane in the southeast that comprise the Barnard Metamorphics, the Hodgkinson Province consists dominantly of Quartz intermediate greywacke sandstone and interlayed pelite attributed to deep marine, turbidity current deposition and minor conglomerate, chert, metabasalt and rare limestone collectively referred to as the Hodgkinson Formation. Four discrete populations of structures have been recognised, and are assigned to separate deformation events (D1-D4) and placed in relative age sequence on the basis of overprinting criteria. The earliest event (DO was characterised by melange formation in which the rhythmic interlayering of arenite and pelite was disrupted to form what are now meta-arenite phacoids in a matrix of slaty meta-pelite. D2 is characterised by a pervasive cleavage (S2) which is commonly steeply dipping and strikes approximately NNW-SSE. Field observations combined with microstructural data from spatially oriented thin sections from a traverse linking the Macalister Range district with the Hodgkinson Formation to the west, suggest that this fabric correlates with S2 in the central and eastern Hodgkinson Province. A weak deformation event (D3), which involved sub-horizontally oriented shortening is apparent from microstructural studies. The dominant structures represented were produced by the re-use and/or reactivation of S2 during D4. S4 is generally a slaty cleavage striking NNW-SSE. The similar shortening direction for D2 and D4 has resulted in the production of a composite S2-S4 fabric. Structural relationships show that plutons of the two supersuites were emplaced at different times. The Wangetti Granite lacks fabric development, but the deflection of country-rock cleavage trends around it and the microscale crenulation of S? on the rims of D4 porphyroblasts within its aureole indicate syn-D4 emplacement, consistent with the Early Permian crystallisation age attributed to this pluton from isotopic evidence. For the Mount Formartine Granite, microstructural observations indicate the presence of structures which pre-date D4. Overprinting criteria and fabric mineralogies indicate that within the Mount Formartine Granite, sub-vertical S2 was succeeded by sub-horizontal S3; these correlate with D2 and D3 fabrics, respectively, within the Hodgkinson Formation. The pervasive S2 geometries indicate that emplacement of the granite pluton occurred during or prior to imposition of the regional S2 cleavage. An emplacement age of 357 ± 6 Ma (latest Devonian) was obtained for this granite by SHRIMP U-Pb analyses of zircon. The U-Pb zircon age for the Mount Formartine Granite is particularly significant because it identifies the oldest episode of plutonism known for the Hodgkinson Province. The isotopic age is consistent with its inferred time of emplacement and, by association, that of a substantial tract of the Hodgkinson Province. The 357 ± 6 Ma age probably falls within the Famennian Stage of the Late Devonian and clearly predates the major D4 tectonic episode. Field relationships indicate that emplacement post-dated Di, whereas microstructural evidence indicate that D2 structures were imposed on the granite subsequent to its solid state. This indicates that the Mount Formartine Granite was emplaced pre- to syn-D2. Our study has identified a discrete early episode of plutonism for the Hodgkinson Province, most granites from which are of Permian age. The new date constrains the age of D2 for the Macalister Range district and the age of the protolith of the Hodgkinson Formation as pre-Carboniferous. It matches the age previously determined for one of several small related granitic stocks in the southeastern Camel Creek Subprovince of the Broken River Province. Structural relationships for granites of this age suggest that their emplacement was broadly associated with the first episode of regional-scale orogenesis and the development of penetrative fabrics in the Hodgkinson Broken River Fold Belt.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
AUTHOR INDEX ABBOTT, Steve ADABI, Mohammad H. ADAM, John ADSHEAD-BELL, Nicole S. AHMAD, Masood A-IZZEDDIN, David AL AROURI, Khaled AL BAKRI, Dhia ALARD, Olivier ALEXANDER, Jan ALEXANDER, Kevin ALLEN, Charlotte M. ALLEY, Neville F. ALLIBONE, Andrew ANDERSEN, O. ANDERSON, Bruce ANDREW, Anita APAK,N.S. APPOLD, Martin S. AQUINO, Joel S. ARBOLEYA, Maria- Luisa ARCHIBALD, Nick ARCULUS, Richard ARMSTRONG, Richard A. AKNE, Dennis C. ARNOLD, Jo ASHLEY, Paul M. BAIN, John H.C. BAKER, E. Max BAKER, Simon F. BALDO, Edgardo G. BALLANTYNE, Geof&ey H. BALLEVRE, Michel BALLHAUS, Chris BARLEY, Mark E. BARNES, R.G. BARNETT, Elizabedi J. BARNICOAT, A.C. BAROVICH, Karin M. BARR, David BARRON, L. BARRON, L.M. BARTON, Tim J. BASrRAKOVA,IrinaV. BATEMAN, Roger BATIZA, Rodey BAXTER, K. BEAUMONT, Christopher BECKER, Thomas BEGG, Graham BELL, Laigee M. BELOUSOVA, E.A. BELPERIO, Antonio .P BENDALL, Betina
1 2 183 3 4 261 5 6 7 142 96 8 9 96 202 10 95 68 11 12 129 96 421 319 13,32 201 14, 130,232,283 15, 39,123 16 17 370 18 19 20,21 162 22 65 317 23 34 347 325 24,126 480,481,484 25 106 495 457 90 34 482 26 50,66 27
BERNECKER, Tomas 28 BERRY, Andrevir J. 29 BESLIER, M-0. 383 BETTS, Peter 30 BEYER, Eloise E. 31 BIERLEIN, Frank P. 13,32,33 BILLS, Robert 34 161,301,459,487 BINNS, Raymond A. BIRCH, Gavin F. 35, 36 BIRD, Robert T. 37,38,472 BLACK, Lance.P 15,39,123,262 BLAKE, D.H. 40,41,192 BLAKE, Paul R. 42, 151, 204, 327,408 BLEVIN, Jane E. 353 BLEVIN, Phillip 43,44 BLEWETT, Richard S. 123,371,463 BLODGETT, Robert B. 90 BOBIS, Renato 45 BODORKOS, Simon 46,339 BONE, Yvonne 47,55,63,109,115,358, 393,474 BONS, P.D. 340 BOREL, Gilles D. 427 BORISSVA, Irina 48,49,367 BOURMAN, Robert P. 50,65 BOYD, R. 451 51,254,394,395,418 BRADSHAW, Barry E. BRAKEL, Albert T. 52 BRANAGAN, D.F. 53 BRANNON, C. 466 BRAUN, Jean 97 BROWN, A.V. 126 BROWN, Catherine E. 54 BROWN, Kirety M. 55 BROWN, M.C. 56 BROWN, R.E. 22 BROWN, Roderick W. 346 BROWN, Warwick M. 57 BROWNE, Patrick R.L. 406 BROWNLOW, J.W. 22 BRUCE, Michael 58 BUCHANAN, C. 383 BUCHANAN, Cameron 59 BUCKERIDGE, John. StJ.S 60 BUDD, Anthony R. 480,481,484 BUI, Elisabeth N. 61 BUICK, I.S 62,71,316,317 BULL, Stuart W. 91 BULTITUDE, Robert J. 15,116,123,157 237 BURNHAM, 0 . Marcus 327 BURROWS, Phil E. CALVER, Clive R. 281, 399 273 CAMACHO, Alfredo CAMERON, R.G 325
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14"" Australian Geological Convention, Townsville, July 1998
CAMPBELL, Liz CAMPBELL, Lorraine M. CANN, John H. CANNARIATO, K. CAPNERHURST, K.R.D. CAPRARELLI, Graziella CARIS, C. CARLSEN, G.M. CARR, Paul F. CARSON, Christopher J. CARTER, L. CARTER, R.M CARTWRIGHT, Ian CASQUET, C6sar CASSIDY, Kevin F. CAWOOD.PeterA. CAYLEY, Ross A. CHAMPION, David C. CHAPMAN, Lucy H. CHAPPELL, B.W. CHEN, Aigen CHEN, She Fa CHEN, Z.Q. CHEN, Zhongqiang CHENHALL, B.E. CHILD, David CHOW, Nancy CHRISTESON, G.I. CLARK, D.A. CLARK, M. CLARK, M.W. CLARKE, Geoffrey L. CLARKE, Jon D.A. CLIFFORD, H.T. CLITHEROE, G. CLOSE, Dorothy COLEMAN, R. COLLERSON, Kenneth D. COLLINS, C.D.C. COLLINS, C.D.N. COLLINS, Clive COLLINS, Peter L.F. COLLINS, William J. CONOR, Colin H.H. COOK, Alex COOKE, David R. COOPER, Roger W. CORKERON, Maree CO'I'I ER, Stephen J. COUSINS, Sonia COWLEY, W.M COX, Malcolm E. COX, Simon COX, Stephen F.
63 64 65, 66 246 430 67, 175, 337 264 68 141,332 84,240 69 69 62, 70,71,221,316, 317,373, 385 370 72 46, 195 73, 74,438,439 72 75,470 76, 77 78 79,278 404 80 81 143 82 255 392 298 83 84, 240, 464 358 85 245 86 202 87,153,201,213, 300, 374 255 372 171,391 435 95,226,259 88,492 89,90 91,444,476 139 92 93,94 95 492 306 96 97
CRAMP, Adrian CRANFIELD, Len C. CRAWFORD, Anthony J.
413 98 99, 100, 114,231, 322,369 CROOK, Keith A.W. 101,276 CROSDALE,PeterJ. 102, 103,263 CROUCH, Simon B.S. 141 CUNNEEN, Ron 475 DAHLHAUS, Peter 138 DALY, Susan J. 104 DAMASKE, Detlef 145 DANIEL, Richard 105 DANYUSHEVSKY, Leonid V. 99, 106, 136 DAVIDSON, Garry 107 DAVIES-McCONCHIE, F. 299 DAVIES, Hugh L. 108,285 DAVIES, Miles 109 DAVIES, Rondi M. 110 DAVIS, Brett K. 111,209,498 DEBAYLE, E. 244 DEFANT, M.J. 153 DELLA-PASQUA, Fernando 112 DENTITH, Michael C. 193 DEVLIN, Steve 318 DICKENS, Gerald R. 113,340 DIPPLE, Gregory M. 335 DIREEN, Nick G. 114 DITCHBURN, R.G. 174 DOGRAMACI, Shawan S. 115 DOMAGALA,Jan 116, 117,254,356,418 DONAGHY, Tony 118 DONCHAK, Paul J.T. 116 DONG, Guoyi 470 DONG, Yu 119 DONNELLAN, Nigel 120 DOYLE, Buddy J. 354 DOYLE, Mark G. 121 DRAPER, John J. 15, 122, 123 DRUMMOND, Barry 25, 150,165,171 DRUMMOND, Barry J. 124,125, 126, 195,270 DUNPHY, Janet M. 127 DURNEY, David W. 128,129 EDGECOMBE, Suzanne M. 482,484 EDGOOSE, Christine 86 EDRAKI, Mansour 130 EDWARDS, A. 466 EGGINS, Stephen M. 106,231,421 EL TABAKH, Mohammed 390 ELLIOT, Greg 143 ELLIS, David J. 21,241,273 ELVEY, D.R. 208 ENGLAND, R. 131 ERINCHEK, Yuriy 359 ESTERLE, Joan 132 EVANS, Noreen J. 133,303 EVANS, W. Ray 134
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153 135, 383 104 136 404 104, 116, 141,216, 322, 341,378,458 137 FARDON, Ross 136 FAUL, Ulrich H. 138 FAWCBTT, Jonathon FERGUSON, Kenneth M. 139 FERGUSSON, Christopher L. 140,141,332,407 64,142 FIELDING, Christopher R. 143 FINK, David 144, 269,270 FINLAYSON, D.M. 145 FINN, Carol 146,308 FLETCHER, Ian.R 208 FLINT, Richard .B 147,148 FLOOD, P.G. 86, 149,196 FLOTTMAN, Thomas 23,27,295,296 FODEN, John 162 FOLKERT, S. 150,165,270 FOMIN, Tanya 42,116, 151,204,408 FORDHAM, Barry G. 365 FORRESTAL, P.J. 289 FOSTER, Damien R.W. FOSTER, David A. 13,32,71,152, 180, 338,420 128 FOUNTAIN, Andrew J. 332 FOWLER, Tom J. 413 FRANKEL, Ed 87, 153 FRANKLAND, Robyn A. 175,176,177 FRANKLIN, Brenda J. 154, 155 FREEMAN, Michael J. 156 GAINA, Carmen 370 GALINDO, Carmen GALLAGHER, Robin 48 GARDOLL, Stephen 491 GARRAD, Paul D. 157 GARTRELL, A .P. 158 GARVEN, Grant 11,159,205 GAUL, Oliver F. 160 GEEVE, Richard 381 GEMMELL, J. Bruce 10, 161,410 GEOLOGY DEPT, HAMERSLEY IRON 162 GEORGE, Annette D. 163 GHORI, A. 68 GIBSON, David L. 164 GIBSON, George M. 118,150, 165 GILES, Alan D. 166 GLEADOW, Andrew J.W. 167,253,346 GLEN, R.A. 144 GOHL, Karsten 168, 169,170,252 GOLEBY, Bruce R. 125, 195 GONCHAROV, Alexey 171 GOODFELLOW, A. 81
EWART, A. EXON.N.F. FAIRCLOUGH, M.C. FALLOON, Trevor, J. FANG, Zong-jie FANNING, C. Mark
203 GOODWIN, I. 281 GORJAN, Paul 172 GOSTIN, Victor A. 173,446 GOW, Paul 261 GRACE, David 174 GRAHAM, I.J. GRAHAM, Ian T. 175, 176,177 GRANGER, Ken 178 GRAY D.J. 179 GRAY, C.M. 241 GRAY, David R. 152, 180,185,186,338,420 GREEN, Andy 478 GREEN, Chris J. 436 GREEN, David H. 136 GREEN, Deborah K. 181 GREEN, Geof&ey R. 182,437 GREEN, P.F. 291 GREEN, Tim J. 141 GREEN, Trevor H. 183 GREGOIRE, Michel 184, 303 GREGORY, Peter W. 39,201 GREGORY, Robert T. 185, 186 GREIG, Alan 286 GREIG, John 98 GREY, Kathleen 68, 187 GRIFFIN, W.L. 188 GRIFFIN, William L. 7,26, 110, 160,219, 343, 354, 359,486 189 GROENEWALD, P.B. 368 GROVER, M. 57,146,488,491 GROVES, David I. 190 GRUNDY, Mike J. 400,401 GUIZHI, Yang 191,192 GUNN, P.J. 378 GUNTHER, Mark C. 5 GUNTHER C.O. Bischoff GUO, Wanwu 193 GURNIS, Michael 328 HACK, Alistair C. 194 HACKNEY, Ron I. 195 HAGEMAN, S. 162 HAINES, Peter W. 149, 196 HALL, Charlotte 197 HALL, Dave 351 HALL, Mike 118 HAMILTON, Lloyd 198,229 HANAHAN, C. 298 HAND, Martin 27, 86,199 , 200,295,296, 305 HAPUGODA, Sarath 201 HARBORT, Terrence 58 HARRIS, Peter T. 202,203 HASSAN, Lee Y. 139 HAYDON, Su22nne J. 471 HAYES, John M. 281 HAYWARD, Mark A. 42, 151,204,408 HAZELL, Murray 54,482,483,484
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 Australian Geological Convention, Townsville, July 1998
HAZLETT, Timothy J. 205 HEAP, A.D. 206,207 HENDER, A.J. 208 HENDERSON, Robert A. 111,209,498 HENDRICKX, Marc A. 471 HENDY, I. 246 HENLEY, H.F. 22 HENSEN,BasJ. 19,319,320 HERCZEG, A.L. 115 HESSE, Paul P. 441 HIGGINS, R.L 210 HILL, P.J. 383 HILL, Peter 135 HILLIS, Richard R. 211 HINDE, J. Steve 436 HINMAN, Mark 212,260,282 HINZ, Karl 169 HOATSON, D.M. 41,422 HOBBS, B.E. 495 HOLCOMBE, Rodney 58,64 HOLLIDAY, John 334 HOLLINGSWORTH, David A. 195 HOPPER, Derek J. 213 HORN, Anthony M. 214 HOSKIN, Paul 0. 194 HOUGH, Megan 228 HOUSE, Michael R. 90 HOUSEMAN, G.A. 372 HOWARD, William R. 215 HUFF, Anne-Marie 301 HUSSEY, Kelvin J. 120 HUSTON, David L. 121 HUTTON, Laurie 15,216 IDNURM, Mart 217,418,483,484 JACKSON, Jim 254 JACKSON, M. Jim 418 JACKSON, Michael J. 218 JACKSON, Simon 44, 133,219,320 JACOBSON, Carl E. 424 JACQUIER, Grant L. 220 JAGO, Jim 493 JAGODZINSKI, E. 431 JAIRETH, Subhash 261 JAMES, Barry A. 436 JAMES, Noel P. 55,105 JANE, Mary 221 JELL, John S. 116,222 JELL, Peter A. 222,223 JENKINS, David R. 224 JENKINS, Greg 225 JENKINS, Ross B. 226 JOHNSON, L. 466 JOHNSTON, Anthony J. 227 JOHNSTONE, D.W. 144,269,270 JONES, B.G. 81 JOYCE, Bemie 228
JOYCE, Paul 476 JUPP, Karl 229 JUSMARDY 57 KAIHO, Y. 245 KAMBER, Balz S. 230 KAMENETSKY, Vadim 99, 231 KAMPRAD, Julienne 432 KARIMZADEH SOMARIN, A. 232, 233 KARNER, Garry D. 253 KAVALIERIS, Imants 355 KAY, John R. 234,235 KEAY, Sue 13,236 KEAYS, Reid R. 237 KEELING, John L. 238 KEENE, Jock 135, 347 KEEP, Myra 239 KELLY, Nigel M. 240,464 KELLY, Simon P. 295 KEMP, Tony I.S. 241 KENNEDY, Allen K. 242 KENNETT, B.L.N. 243,244,245 KENNETT, J. P. 246 KEOUGH, D.C. 365 KEPERT, Doug 195 KEPEZHINSKAS, P. 153 KERR, Geoff 247 KILLICK, M.F. 248 KIM, Hyeong Soo 249 KING, J.D. 382 KINNY, Peter D. 250,295,296, 339 KINSEY-HENDERSON, Anne E. 251 KIRBY, Ian J. 252 KNACKSTEDT, Mark A. 97 KNOX- ROBINSON, Carl M. 57, 491 KOHN, Barry P. 253,345, 346 KOJAN, M.J. 155 KORSCH, R.J. 126, 144,456,457 KOVAC, Ben 462 KRASSAY, Andrew 51,254,418 KRITSKI, A. Ye. 255 KRUSE, Peter D. 256 KUCKA, Mirek 389 KYSER, T. Kurt 358 LACKIE, M.A. 392 LACY, Willard C. 257 LAMBECK, Alexis 258 LAMBERT, David D. 71,221,422 LANDENBERGER, B. 259 LANG, Simon 356 LARA, Pablo 337 LARCOMBE, P. 206,258 LARGE, Ross R. 91 LAROCQUE, Adrienne C.L. 290 LAWRIE, Kenneth C. 260,261,262 LAXMINARAYANA, C. 263 132,264 LEBLANC SMITH, Guy
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LEE, Hyun-Woo 265 LEEMING, Prae 266 LEITCH, Evan C. 177,267,458,337 LENARDIC, Adrian 324 LENNOX, Paul 268,443 LESHER, C. Michael 237 LEVEN, J.H. 144,269,270 LEWTHWAITE, Kathryn J. 271 LEYH, Wolfgang R. 272 LI, Zhengxiang 361,362,472 LI, Zheng-Xiang 193,273 LIN, Tian-rui 493 LINDSAY, John F. 418 LINTERN, M.J. 274 LINTON, Paul 275 LIU, Keyu 101,276 LIU, Keyu 277 LIU, Songfa 79,278,279 LOBEGEIER, Melissa K. 280 LOGAN, Graham A. 281,282 LORAND, Jean-Pierre 7 LOriERMOSER, Bemd G. 283 LU, Xiuqin 119 LUCHININA, Veronica 284 LUS, Wilfred Y. 285 MAAS, Roland 100,286,453 MacINTOSH, Ian W. 297 MacKENZIE, D. 466 MacKENZIE, Douglas E. 15,123 MACKEY, Tim 145,192,279 MAGEE, Marian E. 211 MAKER, Simon 322 MAIDMENT, David 150, 165 MANFRINO, Annick 287 MAO, Jianquan 497 MARCHANT, Robin 428 MARES, V. M. 288 MARK, Geordie 289 MARSHALL, Brian 166,175,176, 177,290 MARSHALLSEA, Susan J. 291 MARTIN S.E. 395 MARTIN, David McB. 361,362 MARTINAITIS, P. 50 MASON, David 195 MASSEY, Steve 266 MATTHAI, Carsten 36 MAUK, Jeffrey L. 292,293 MAVROGENES, John A. 194, 294,297,421 MAWBY, Jo 295,296 MCABE, M. 189 McCARRON, Joanne 303 McCAVE, N. 69 McCONACHIE, Bruce 117, 254 McCONCHIE, D. 298,299 McCUAIG, T. Campbell 25,72 McCULLOCH, M.T 95,494
McDONALD, Graeme D. McDONALD, Leanne A. McDONALD, Robert McDOUGALL, Ian McGOLDRICK,PeterJ. MCHENRY, Ben McHENRY, Colin McINNES, Brent LA. McKENZIE, E. Donald MCKNIGHT, Stafford McLAREN, Sandra McLEOD, Ray McMAHON, Gerard A. McNAMARA, Greg McNAUGHTON, Neal J. McPHAIL, D.C (Bear) McPHIE, Jocelyn McQUEEN, Ken MEAKIN, N.S. MEAKIN, S. MEIXNER, A. MERNAGH, Terrance MERRY, Nick J. MESSENGER, P. MEYER, Heinrich MEYER, Jeremy J. MICHAEL-LEIBA, Marion MICHAELSEN, Per MIKUCKI, E.J. MIKUCKI, J.A. MILLER, J.A. MILLER, John Mc. MILLER, Mark MITCHELL, David S. MOHAMED, F.H. MOLLER, Andreas MOORE, A.M.G. MOORE, C.L. MOORE, David MOORE, David H. MOORE, Linda S. MORAN, Christopher J. MORAND, Vincent J. MORESI, Louis MORGAN, E.J. MORGAN, V. MORRISON, Gregg W. MORRISON, Robert S. MORSY, M.A. MORWOOD, David A. MOSAR, Jon MUHLHAUS, Hans B. MULLER, Anne MOLLER, R. Dietmar MUNDAY, Tim
503
300 301 142 285,295 91 109 302 133,303 304 32 305 318 306 307 13, 127, 146,308, 33,488 17, 181,309 409 497 325 310 191,192 112,262,311 312,313 131 169 211 314 315 146 146 62,316,317 186 318 333 326 19,319, 320 425 321 145 322 323 61 471 324,328 325 203 12, 45 120 326 327 427 324,495 342 37,156,255,328 478
GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"" Australian Geological Convention, Townsville, July 1998
MUNKER, Carsten 329 MURRAY, Cecil G. 151,204,327, 330,408 MURRAY, Suzanne I. 331,332 MURRAY-WALLACE, Colin V. 66 462 MUSGRAVE, Robert J. MUSTARD, Roger 333 359 NATAPOV, Lev 169 NEBEN, Sonke NELSON, David 10 286,339 NEMCHIN, Alexander A. NEUDERDT, Martin 356 334 NEWCREST MINING LIMITED STAFF 207 NICHOL, S.L. NICHOLLS, Ian 286, 417,453 183 NICHOLS, Geoffrey T. 333 NIELSEN, Russel 335 NIERMANN, Mma C. 153 NIU, Y. 58 NIU, Yaoling 494 NORMAN, M. 336 NORMAN, Marc D. NUIDERT, Martin 117 118 NUTMAN, Allen 210 O'BRIEN, G.W. 203 O'BRIEN, P.E. 337 OCH, David O'DONNELL, Ian 389 227,338 OFFLER, Robin OLIVER, Nicholas H.S. 46,339, 340,362 341 OLIVER, R.L. 29,294 O'NEIL, Hugh St. C 342 OPDYKE, Bradley N. 495 ORD, A. O'REILLY, S.Y 7,26,110, 160,184, 188,343, 354,359, 486,494 344 O'SHEA, P.J. O'SULLIVAN, Paul B. 253, 345,346 OWEN, A. 25,150,165, 269,270 347 PACKHAM, G. 348 PADILLA-GARZA, Ruben PAGE,R.W. 117,349, 418,483 189 PAINTER, M. 277 PAINTER, Scott 268,443 PALMER, Derecke 350 PALMIERI, Vincenzo 370 PANKHURST, Robert J. 351 PARKS, Jennifer 161 PARR, Joanna M. 333 PARTINGTON, Greg 28 PARTRIDGE, Alan D. 98 PASCOE, Garry 352 PASSMORE, Maurice 353 PASSMORE, Virginia L. 277 PATERSON, Lincoln PEARSON, Norman J. 219,320, 336,354 332 PEMBERTON, John W.
PEMBERTON, M.R. 85 PENNINGTON, Jay 355 PICKARP,A.L 162 PIDGEON, Brett 356 PILKINGTON, Mark 37 PLAYFORD, P.E. 357 PLIMER, Ian R. 14 POGSON, D.J. 325 POLITO, Paul A. 358 POLLARD, Peter 470 POMEROY,MattJ. 309 PONTUAL, Sasha 312,313 POUDJOM DJOMANI, Yvette H. 343, 359 POWELL, Christopher McA. 193,195,239, 340,360,361,362,472 POWELL, Roger 84 POWELL, William J. 7,363 POWER, Bill 96 PRENDERGAST, Kylie 364,470 PRESCOTT, J.R. 50 PRICE, Richard C. 100 PROFFETT, J.M. 365 PURVIS, A.C. 366 QIDONG,Den 401 RAMSAY, D.C. 367 RAMSAY, W.R.H. 368 RANDALL, Ross E. 204 RAO,PrasadaC. 2 RAOS, Alison M. 369 RAPELA, Carlos W. 370 RASMUSSEN, Birger 308 RATAJKOSKI, M. 371 RATHUR, Qadeer 390 RAWLINGS, David 218 RAWLINSON,N. 372 RAYMOND, O.L 325 READ, Caroline M. 373 REGELOUS, Marcel 87,374 REICHERT, Christian 169 REITNER, Joachim 375 REYMOND, Satavan B. 428 REYNOLDS, Scott D. 211 RICHARDS, Ian J. 185 RICHMOND, Julie 376 RICKARDS, R.B. 377 RIDSDILL-SMITH, Thomas A. 37 RIENKS,IanP. 15,378 ROACH, Ian C. 379,380 ROBERTS, F.I. 189 ROBERTS, John 381 ROBERTSON, Evette 35 ROBERTSON, I.D.M. 382 ROEST, Walter R. 38 ROSENBERG, Jill L. 424 ROWE, Edward G. 45 ROYER, Jean-Yves 156,383
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49 14"' Australian Geological Convention, Townsville, July 1998
ROZEFELDS, Andrew 384 RUBENACH, Mike 385, 386 RUXTON, Bryan P. 387, 388 RYAN, C.G. 188 RYAN, Chris 112, 303 RYAN, Christopher G. 470 RYBURN, Rod 59 RYBURN, Roderick 389 SAAVEDRA, Julio 370 SAMI, Terry 418 SANDIFORD, Mike 200, 305 SAPPAL, Krishna K. 390 SATTEL, Daniel 266 SAYERS, Jacques 391 SCHAFER, Andreas 456 SCHMIDT, P.W. 392 SCHMIDT, Rolf 393 SCHOLZ, Joachim 47 SCOTT, Deborah 51,218,394, 395,418 SCOTT, M.M. 325 SCOTT, Margaretha 396 SCO 11, Robert J. 194 SCOTT, Steven D. 301 SCRIMGEOUR, Ian 86, 397 SEARGENT, M.J. 170 SEARLE, Ross D. 190 SECCOMBE, Phillip K. 433 SEMENOVA, T.O. 372 SERGEEV, Nikita B. 398 SETYONO, Didik 96 SEYMOUR, David B. 399 400,401 SHAOPING, Cheng SHARP, David 10 SHEN, Jian-Wei 402 SHEN, Shu-Zong 403,404 SHERWIN, L. 310 SHI, G.R. 80,403,404,485 SIGNER, Claude 428 SIMMONDS, John 405 406 SIMMONS, Stuart F. SIMPSON, Carol J. 407 SIMPSON, Christopher R.J. 293 SIMPSON, Glenn A. 42, 151,204,408 SIMPSON, Kirsty A. 409 SINCLAIR, B.J. 410 SIVELL, Warwick J. 352,411,412 SKILBECK, Greg 413 414 SKINNER, Brian SMILLIE, Robert 96 SMITH, John V. 93,94,415,416 SMITH, Stuart 135 SMURTHWAITE, A.J. 155 417 SOESOO, Alvar SOLOMATOV, Slava 324 S0NNELAND, Lars 428 SOUTHGATE, Peter N. 51,418
SOUTHWOOD, Malcohn 419 SPAGGIARI, Catherine V. 420 SPANDLER, Carl 421 SPROULE, R.A. 422 SPRY, Melissa J. 423 424 SPRY, Paul G. 255 STAGG, H. 367,383,425 STAGG, H.M.J. STALLARD, Aaron R. 426 STAMPFLI, G6rard M. 427,428 STANSFIELD, Scott 364 82 STASIUK, Lavem D. STEINER, Christian 428 STEPHENSON, A. E. 429 STEPHENSON, Jon 374,494 STEPHENSON, N. 233 STEVENS, B.P.J. 430,431 STEVENS, M.K 68 STEWART, Alistair 432 STEWART, Lachlan K. 340 22 STROUD, W. Jim SUMMONS, Roger E. 281,282 SUN, Shen-su 483 SUN, Yanyan 433 434 SUTHERLAND, F. Lin SUWARDY, Eddy 355 SWEETAPPLE, Marcus T. 435 SWITZER, Cameron K. 436 21 SYLVESTER, Paul 48, 59, 367 SYMONDS, P.A. 156 SYMONDS, Phil SYMONDS, Philip A. 49 TAHERI, Jafar 437 394,395 TARLOWSKl, Chris Z. 131 TAUBE, Alex 448 TAYLOR, Anthony P. TAYLOR, David 228 73, 74,438,439 TAYLOR, David H. 57 TAYLOR, Geof&ey R.T. 35, 36 TAYLOR, Stuart E. 440 TEALE, Graham TEIXELL, Antonio 129 375 THIEL, Volker 441 TOMKINS, Kerrie M. 442 TORREY, Christopher E. 413 TRIBBLE, Jane TRZEBSKI, Robert 268,443 444 TUNKS, Andrew J. 384,445 TURNER, Susan 98,157 TUTTLE, John S. 170 UENZELMANN-NEBEN, Gabriel 446,447 UPTON, Phaedra 448 UWINS,PhilippaJ.R. 244 VAN DER HILST, R.D 451 VAN HEESWIJCK, A. 203 VAN OMMEN, T.
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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 49
14"' Australian Geological Convention, Townsville, July 1998
VAN WYCK, Nicholas VANDENBERG, A.H.M. VARNE, Rick VERNON, Ron H. VINAR, John VOKES, Frank M. VON DERBORCH, Chris WAIGHT, Tod E. WAKE-DYKSTER, K. WALLACE, M.W. WALSH, Stephen WALTER, Malcohn R. WANG, F.B. WANRONG, Yang WARREN, A.Y.E. WARTENBERG, Wolfram WASCHBUSCH, Paula WATANABE, Teruo WATERS, John C. WATKINS, Ron T. WATT, Gordon R. WEBB, Derek L. WEBB, Gregory E. WEBB, J.A. WEBB, John A. WEBB, Richard I. WEI, Min WEISSEL, Jefifrey K. WELLMAN, Peter WENDT, J. Immo WHELLER, Graeme WHITE, A.J.R. WHITE, Peter D. WHITE, Richard W. WHITE, Susan
452 449,450,471 112 464 318 290,424 105 453 150, 165,270 357 454 5,281 485 455 325 456 457 458 459 460 339 436 461 291 28,462 448 401 253 15, 123,371,463 87,201 87 77 442 240,464 465
WHITEHEAD, N.E. WIDODO, S. WILKINS, Colin WILLIAMS, I.S. WILLIAMS, Ian S. WILLIAMS, Neil WILLIAMS, Patrick J. WILLMAN, Clive E. WINGATE, Michael WINSOR, Colin N. WINZAR, Geraldine WITHAM, Bronwyn WITHNALL, Ian W. WITT, Walter K. WOLFE, Rohan WOOLFE, Ken J. WORHEIDE, Gert WORRALL, Lisa WRIGHT A.J. WYBORN, Doone WYBORN, Lesley A.I.
174 466 467 316 8,452,468 469 364,470 471 472 473 474 475 15, 39,123,141,216, 79 99,476 79, 103,258,477 375 478 377,479 262,325 41,54,171,217,431, 480,481,482,483,484 WYSOCZANSKI, Richard 386,498 YAN, Ge 485 YAO, Shixin 486 YEATS, Christopher J. 487,488 YIM, W. W-S. 489,490 YUN, Grace Yajuan 491 ZANG, Wen-long 492,493 ZAW, Khin 112 ZHAN, Li-pei 404 ZHANG, Ming 494 ZHAO, Chongbin 447, 495 ZHOU, Taihe 496,497 ZUCCHETTO, Ricardo G. 111,498
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14th AGC Organising Committee Bob Henderson (Co-convenor) Simon Beams (Co-convenor) Kaylene Camuti Peter Crosdale Tony Hespe
School of Earth Sciences, JCU Terra Search Pty Ltd Lantana Exploration Pty Ltd Coalseam Gas Research Institute, JCU Aberfoyle Resources Limited
Stuart Jeffrey
BHP Cannington
David Johnson
School of Earth Sciences, JCU
David Lemcke
Outer-Rim Exploration Services
David Mason Greg McNamara Jim Morrison Nick Oliver Peter Pollard Mick Roche Michelle Turner
Homestake Gold of Australia Albert Kersten GeoCentre, Broken Hill Jim Morrison & Associates/Resolute Limited EGRU, School of Earth Sciences, JCU Pollard & Taylor Geological Services Pty Ltd BHP Cannington Queensland Manufacturing Institute