Geological Society of Australia
ABSTRACTS Number e
Symposium on
Geology and Mineralization in the Lachlan Fold Belt, N.S.W. Held at The N.S.W. Institute of Technology on March 12th, 1982
SYMPOSIUM ON
GEOLOGY AND MINERALIZATION
IN
LACHLAN FOLD B E L T , N . S . W . S Y D N E Y , N . S . W . 1982
A B S T R A C T VOLUME
Edited b y : S . R . Sangameshwar
G e o l o g i c a l Society of A u s t r a l i a
Abstract
No. 6
i CONTENTS* INTRODUCTORY REGIONAL
COMMENTS
1
STUDIES
G.H. Packham What does the Ordovician to Devonian Lachlan Fold Belt stratigraphy tell us about events within the crust and upper mantle?
1
E. SCHEIBNER Some aspects of the geotectonic development of the Lachlan Fold Belt - A critical review
2
S.E. SHAW AND R.H. FLOOD Granitoids of the Lachlan Fold Belt
3
D.W. SUPPEL AND P.R. DEGELING Comments on magmatism and metallogenesis of the Lachlan Fold Belt
4
P.R. EVANS Mid Palaeozoic evolution of the Lachlan Province
6
C. McA. POWELL Late Ordovician to Early Carboniferous Palaeogeography of the Lachlan Fold Belt/ N.S.W. SPECIFIC
6
STUDIES
ASAHEL BUSH The hydrothermal system at Mineral Hill, N.S.W.
8
J. FOLDESSY AND K.L. WILLIAMS Volcanogenic mineralization at Galwadgere, near Wellington
9
E.J. MALONE Base metal mineralization at Currawang *Papers in this volume are presented in groups in the order of their presentation.
H
ii R.P. SINGER Base metal mineralization at the C.S.A. Mine, Cobar, N.S.W.
12
B.L. SCHMIDT Geology of the Elura Ag-Pb-Zn deposit, Cobar, N.S.W,
13
BRIAN MARSHALL AND S.R. SANGAMESHWAR Commonality differences in the ores of the Cobar and Super-Group, N.S.W.
15
CAROL SIMPSON Stratigraphy, structure and geochemistry of Kangaloolah volcanogenic sulphide deposits
16
M.J. VICARY Molybdenum mineralization at Mount Pleasant, N.S.W.
17
I.R. PLIMER A tin silicate Bourke, N.S.W. skarn, Doradilla via
18
R. PATERSON Sn mineralization at Ardlethan
RESERVE
19
PAPER
R.A. BINNS
AUTHOR
Disseminated Ni-Cu-Co mineralization in the ultramafic volcanics at Rockley, N.S.W
20
INDEX
21
INTRODUCTORY
COMMENTS
Abstracts of papers to be delivered at the 19 82 Symposium on the Geology and Mineralization in the Lachlan Fold Belt, N.S.W., at The N.S.W. Institute of Technology, Broadway, N.S.W., are set out below practically in the form in which they were submitted. The symbol Ma has been used to indicate geological time Minor changes in presentation have been made for uniformity, and for clarity. There has, however, been no attempt at stylistic alteration or unification of the abstracts. The papers will be presented in the Turner Hall, New South Wales Institute of Technology, on March 12th 19 82.
REGIONAL
STUDIES
WHAT DOES THE ORDOVICIAN TO DEVONIAN
LACHLAN FOLD B E L T
STRATIGRAPHY T E L L US ABOUT EVENTS WITHIN THE CRUST AND UPPER MANTLE
By G.H. Packham Department of Geology & Geophysics,
University of
Sydney.
In the steady state condition the elevation of the surface of the crust is dependent on the thickness and structure of crust and on the thermal state of the crust and upper mantle. The crustal thickness may be decreased by erosion, stretching or thermal metamorphism. Its structure may be modified by intrusions from mantle sources, crustal shortening, obduction, accretionary processes, metamorphism and partial melting. Changes in thermal state alone can cause some of these modifications to occur and all of them result in modification of the heat distribution in the crust and upper mantle. The sequence of events recorded at the depositional surface can be used to identify some of these processes. Rates of elevation and subsidence are the most important single measurable parameter. The relationship between transgressions, regressions, clastic or biogenic sediment supply and valcanism assist in interpretation. Several examples from the Ordovician to Devonian sequences of South Eastern Australia will be discussed.
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SOME ASPECTS OF THE GEOTECTONIC
DEVELOPMENT
OF THE LACHLAN FOLD B E L T A CRITICAL
-
1
REVIEW *
By E. Scheibner Geological Survey of New South Wales, Sydney. During the decade since the proposal of the first plate tectonic models for the Lachlan Fold Belt, plate tectonic theory has advanced significantly. The following are some examples. *Recognition of A- and B-type subduction. *Two different modes of B-type subduction: and Mariana types.
the Chilean
*The importance of the asthenospheric wedge above the subducted oceanic plate for the formation of volcanic arcs or chains. *Subduction and growth of an accretionary wedge can occur without the formation of a related volcanic arc. Conversely an accretionary wedge does not necessarily develop in association with every subduction related volcanic arc. *The complex relationships which often develop during the interaction of large plates leading to transpression, transtension, leaking transforms, jumps in spreading centres, diffuse sea-floor spreading in plate margin areas, accretion of oceanic plateaux ("suspect allochthonous terranes") and associated orogenic deformation, have not been considered in the regional plate tectonic models for the Lachlan Fold Belt. *The various types of orogenic granites - M,I,A, and S-typeand the implication their genesis has for the tectonic models has not been systematically assessed. It is suggested that by integrating the latest advances in plate tectonics it will be possible to develop and update the earlier regional tectonic models or formulate new ones. These could be tested by future research.
tPublished with the permission of the Secretary, New South Wales Department of Mineral Resources, Sydney.
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GRANITOIDS OF THE LACHLAN FOLD B E L T
By S.E. Shaw and R.H. Flood Macquarie University. Granitoids of the Lachlan Fold Belt of Victoria and New South Wales can be subdivided into "S", "I" and leucocratic types. While some granitoids are Ordovician in age, the bulk are Siluro-Devonian, with the possibility of two intrusive maxima within that time span. Geographically the granitoids fall within three distinct zones:(1)
The eastern zone, containing foliated and massive plutons that are generally aligned in a N to NNE direction parallel to the present coastline. It has been suggested by several workers that the younger plutons lie to the east. A boundary termed the I-S line, running parallel to the coastline and the regional trend, marks the most easterly limit of S-type plutonism. In many instances, plutons or groups of plutons are closely associated with, or intrude, their own volcanic pile. Within the north-western part of the zone is a group of leucocratic adamellites and granites.
(2)
The western zone of granitoids, mainly Siluro-Devonian in age, but possibly containing intrusions of Ordovician age. The eastern limit of this zone lies roughly on a line running from Tumut through Nyngan and Brewarrina. In this zone the granitoids are both I and S-type but the regional trend is NNW. As with the eastern zone, massive and foliated members are present. The Tibooburra granite, a massive I-type and the most westerly member observed in the Lachlan Fold Belt, has a biotite Rb/Sr age of around 410 Ma.
(3)
The north-eastern zone of Bathurst-type granitoids, with their major development around Bathurst. The geographic distribution of these mainly I-type plutons is in a NNWtrending belt that parallels both the Sydney Basin margin and the western edge of the New England Fold Belt. An isolated occurrence of a Bathurst-type pluton, the Mt. Bright granodiorite, in the Hunter Valley suggests that in terms of age and chemistry this zone of plutons has more in common with the Carboniferous development of the New England Fold Belt than with the Lachlan Fold Belt.
87 86 Aspects of the age, Sr/ Sr initial ratios and chemistry of the various granitoids will be discussed.
COMMENTS ON MAGMATISM AND METALLOGENESIS OF THE LACHLAN FOLD B E L T *
By D.W. Suppel and P.R. Degeling Geological Survey of New South Wales.
The source rocks of granitoid magmas should strongly influence the metallogenic character of granitoid intrusive rocks and their probable volcanic equivalents. In the Lachlan Fold Belt, source rocks probably were important in the formation of granite related deposits (mostly Sn, W, Mo and base metals), and, possibly, porphyry copper deposits. The role of the source rocks in volcanogenic deposits is less clear. The pre-Carboniferous granitoids of the Lachlan Fold Belt can be subdivided into three belts: a western belt which occupies the Girilambone - Wagga Anticlinorial Zone in N.S.W. and northern Victoria; an eastern belt which occupies the Molong - South Coast Anticlinorial Zone east of the I-S granitoid line of White et al. (19 76); and a central belt occupying the anticlinorial and synclinorial zones between. The three granitoid belts and contained mineral deposits can be summarized as follows: Western Belt Granitoid types: (1) mainly S-type (2) younger leucogranitoids and I-type bodies in southern N.S.W. and northern Victoria, Mineral deposits: (1) Pb - Ag - Zn
Sn - W
(2) Mo - W - Bi;
Eastern Belt Granitoid types:
I-type
Mineral deposits:
Mo - (Bi - W); Cu - Pb - Ag - Zn
Central Belt Granitoid types: (1) S-type (2) I-type and (?) leucogranitoids, commonly younger than S-type Mineral deposits: Cu - (Fe)
(1) minor W, Sn
(2) W - Mo - Bi - (Sn)
Gold deposits in granitoids in all three zones, but one notable line of deposits occurs in or near diorite and (?) I-type granitoids along the boundary between the central and western belts from Adelong to West Wyalong.
^Published with permission of the Secretary, N.S.W. Department of Mineral Resources.
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The source materials of the granites and related mineral deposits are possibly Ordovician quartz-rich flysch sediments in the western belt (Fagan, 19 79); intermediate igneous material underplating crust in the eastern belt (White 1979); and mixed sedimentary-igneous sources including pre-Ordovician sedimentary rocks in the central belt. The "gold line" may represent the western limit of development of oceanic or underplated crust at the depths of granitoid generation. Carboniferous granitoids occur in the northern parts of the central and eastern belts. All appear to be I-type and are accompanied by Mo - Cu - W - (?Sn) and Pb - Ag deposits. Porphyry copper deposits occur in dacitic and dioritic intrusions in Ordovician island arc andesite volcanics in the north of the Lachlan Fold Belt. The age of the inrusions, and hence their source, is uncertain - they could represent magmas generated above a subduction zone or by melting at the base of thick andesitic crust along the Molong Rise. Recent work has shown that some of the felsic volcanic rocks of the Lachlan Fold Belt can be subdivided into S - & I types (Owen and Wyborn 19 79). The felsic volcanics throughout the Lachlan Fold Belt contain deposits of fairly uniform composition: Cu - Pb - Zn - Ag - Au. The larger stratiform deposits in southeast N.S.W. are confined to I - type volcanics close to and east of the I-S line. On a broader scale, however, stratiform deposits may occur in S - type volcanics (e.g. north of Mount Hope) and thus it is possible that rift-type volcanic settings, which provide submarine environments for volcanic exhalations, may be more important than the source of the volcanics in development of stratiform deposits. REFERENCES Fagan, R.K., 1979. S - type granite genesis and emplacement in North-east Victoria and its implications, in Denham, D., (complier) , Crust and Upper Mantle of Southeast Australia, pp.29-30. Australia, Bureau of Mineral Resources - Records 19 79/2 (unpubl.). Owen, M., and Wyborn, D., 19 79. Geology and Geochemistry of the Tantangara and Brindabella 1:100,000 Sheet Areas, New South Wales and Australian Capital Territory. Australia, Bureau of Mineral Resources - Bulletin 204, 52 pp. White, A.J.R., 19 79. Sources of Granite Magmas. Geological Society of America - Abstracts with Programs 11(7), 5 39 . White, A.J.R., Williams, I.S., and Chappell, B.W., 1976. The Jindabyne Thrust and its tectonic, physiographic and petrogenetic significance. Geological Society of Australia - Journal 23(1), 105-112.
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MID-PALAEOZOIC
EVOLUTION OF THE LACHLAN PROVINCE
By P.R. Evans University of New South Wales.
The Lachlan Province is part of a larger domain of interrelated crustal provinces along the eastern thrid of the Australian continent that are of interest to explorers for petroleum as well as for metalliferous ores. The evolution of "transitional" basins (potential bearers of hydrocarbons) in this larger region must be relatable to the contemporaneous evolution of neighbouring more mobile zones that bear minerals. During the Mid-Palaeozoic (Silurian - Early Carboniferous) the Lachlan Province and the Thomson Province to the north changed from "geosynclinal" to largely "transitional" styles of evolution. Variations through time of basin orientations, structures and sedimentary contents in both provinces indicate that a major control on this phase of the cratonization process was the influence of a sinistrally directed stress field that was mainly vectored north - south. The stress vector seems to have rotated anti-clockwise through time and the stress field changed to a dextral form during the Late Carboniferous, when the Lachlan Province achieved sufficient stability to receive only "platform" type deposits thereafter. The strain generated by the stress field varied across the province, apparently in response to the nature of basement as much as to the intensity of shear. West of the Wagga Metamorphic Belt, strain was controlled by pre-existing fracture patterns. Comparable variations in the effects of the stress field are evident in the Thomson Province.
LATE ORDOVICIAN TO EARLY
CARBONIFEROUS
PALAEOGEOGRAPHY OF THE LACHLAN FOLD B E L T ,
N.S.W.
By C. McA. Powell School of Earth Sciences, Macquarie University. The palaeogeography of the Lachlan Fold Belt from Late Ordovician to Early Carboniferous can be considered in terms of three palaeogeographic configurations. An island-arc/marginal sea system, similar to the present-day Andaman Sea region, was developed by Mid-Ordovician (4 80 Ma) and lasted until Early Silurian (440 Ma). A NNW-trending andesitic volcanic arc in eastern New South Wales and Victoria (=Andaman/Nicobar island chain and submerged volcanic seamounts) was separated from land mass to the west (=Malaysian Peninsula) by a youthful marginal sea (=Andaman Basin), and faced an oceanic realm to the east (=Bay of Bengal). Basaltic andesites and associated volcaniclastics, together with calcareous deposits, were derived from wtihin this fold belt, but the main sediment source was quartzose
- 7 -
from land masses to the south (cf. present-day Irrawaddy and Ganges sediment inputs to the Andaman System). In the Mid-Silurian (430 ± 10 Ma) there was widespread deformation with development of the NNW-trending Wagga Metamorphic Belt over the site of the former marginal sea, and by Late Silurian (420 Ma) the second palaeogeographic configuration was established. A horst and graben system similar to the Basin and Range terrain of western U.S.A., and Mexico developed east of a line from Cobar to Sale, with a foreland basin onlapping the exposed older Australian continent to the west. The Cowra, Tumut and Hill End Troughs and Molong and Capertee Highs were part of the eastern horst and graben system, in which individual meridional grabens trended 20^ oblique to the NNW-trending western bounding fault from Cobar to Sale, suggesting sinistral shear during their formation. Silicic volcanics and associated clasti cs of both terrestrial and marine facies and minor but important, high-K mafic extrusions were widespread throughout the eastern horst and graben provinces, and extensive limestones reflected the low palaeolatitude. In the western foreland basin, a quartzose clastic wedge prograded from the southwest so that Early Devonian deep-marine conditions near Cobar were replaced by fluvial environments by the Middle Devonian. Local deformation occurred in the eastern provinces from the Mid-Silurian to Early Devonian, probably associated with transcurrent movement on the various graben faults, but in MidDevonian widespread deformation, most intense in southern New South Wales and Victoria uplifted the southern Lachlan Fold Belt to form the Tabberabberan Highlands. The effects of this deformation died out to the north and west. By latest Devonian (370 Ma) the third palaeogeographic configuration was established, consisting of an andesitic continental arc along the eastern edge of the Lachlan Fold Belt with a foreland basin and pericratonic basins to the west, similar to the present-day Sumatran-Malaysian system. The continental arc (=volcanic arc in Sumatra) extended north from just east of Mudgee to Anaki in Queensland, with the foreland basin (Drummond and Lambie Basins) parallel to it on the west, and extensions further west into the pericratonic Darling and Adavale Basins (=Sunda shelf). Most of the sediment in the foreland basin is quartzose being derived from the remains of the Mid-Devonian Tabberabberan Highlands and the Australian continent, but along the eastern part of the Lachlan Fold Belt, there is a significant input of volcanolithic detritus from the bounding eastern continental arc. The Tamworth Trough was a prearc basin in the latest Devonian, with the Woolomin Beds in the New England Fold Belt representing trench slope and/or abyssal plain deposits. This third palaeogeogrpahic configuration lasted until Mid-Carboniferous (330 - 10 Ma) when widespread deformation throughout eastern and central Australia uplifted the entire region west of the former continental arc.
SPECIFIC STUDIES
THE HYDROTHERMAL SYSTEM AT MINERAL HILL, N.S.W. By Asahel Bush Getty Oil Development Company
(Minerals Division).
The Mineral Hill "volcanic-hosted massive-sulphide" type copper-lead-zinc deposits are located in central N.S.W., Australia. Mineralization is of two types, discordant near-vertical stockworks (stringer type mineralization) , primarily in pyroclastics and conformable massive-sulphide lenses (mostly oxidized) in overlying sediments. The major stockwork zone (the Parkers Hill stockwork) is delineated by a vertical tube-like zone of high copper-leadzinc assays, quartz veining, and the following vertically zoned hydrothermal assemblage: Upper section - Quartz + chlorite + adularia + bornite + chalcopyrite galena + sphalerite ± tetrahedrite * biotite. Lower section - Quartz + chlorite + sericite + pyrite + chalcopyrite + galena + sphalerite Three types of fluid inclusions are found in quartz veins in the Parkers Hill stockwalls: Type I, liquid filled; Typed II, two phase inclusions giving homogenization temperature of 75° to 351°c and salinities from 0 to 22 wt.% equiv. NaCl; Type IIIf vapour filled inclusions indicating the solution was boiling. Physical-chemical interpretation of the zoned mineralogyf fluid inclusion evidence, whole rock chemistry, sphalerite and chlorite compositions and stable isotopes indicate that alteration and metal precipitation involved an increase in pH and/or decrease in temperature in the ore solution. Considerations of mass balance during alterationmineralization, an antipathetic relationship between adularia and sericite, and the pervasive nature of the alteration argue against wall rock reaction alone as an explanation for these chemical changes. A generalized mass transfer computer model has been constructed for a boiling hydrothermal system to trace the simultaneous interaction of (a) 46 solution species (b) loss of C0~, H 2 S, CH. and H 2 to the vapour phase (c) loss of heat via the vapour phase (df progressive precipitation of up to 17 mineral species.
- 9 -
The calculations demonstrate that ore mineral precipitation and emplacement of the zoned alteration sequence at Parkers Hill is best explained by a boiling process. Boiling of a ore solution will increase the pH, decrease temperature, and increase the concentration of metal solutes all of which will tend to take the solution in the direction of mineral precipitation. The common occurrence of vapour phase inclusions in a variety of deposit types suggests the importance of boiling as an ore forming mechanism, particularly in the case of non-sulphide minerals such as Sn and W oxides and native Au and Ag. Exploration guides to deposits which have been formed by boiling are of two types (1) Remote Indicators: vapour phase fluid inclusions, advanced argillic alteration and phreatic breccias (all forming above the level of first boiling) (2) Proximal Indicators: mineralization, vapour phase fluid inclusions and higher pH alteration phases such as adularia, albite and biotite.
VOLCANOGENIC MINERALIZATION AT GALWADGERE, NEAR WELLINGTON By J. Foldessy and K.L. Williams University of Sydney# A small base metal deposit at Galwadgere, 25 km SE of Wellington, N.S.W., occurs within a volcanic phase of the Middle Palaeozoic sequence of the Lachlan Fold Belt. It has been fairly intensively explored over the past 15 years, so far without establishing the presence of an orebody of economic grade. However, drilling has provided unusually comprehensive exposure of the major rock types. This study was undertaken to investigate the principal lithological and stratigraphical characteristics of the mineralization environment. Available drill core was re-logged and sampled for petrographic studies of thin and polished-thin sections, emphasising textural, modal and grainsize analyses. XRD techniques were used to investigate the structural state of clay mineral components and the compositions of chlorites from various lithological units. Further chemical data were obtained by electron microprobe analysis of chlorites, biotites and feldspars and by a limited program of XRF whole-rock major-element analysis. Re-interpretations of the drill hole data were then compiled to sections and level plans and key features of the depositional and mineralization environments were reconstructed. Some modifications to previously accepted stratigraphic correlations are proposed. Sediments on the western margin of the prospect, previously assigned to the Devonian Cunningham Formation, are interpreted as equivalent to the upper portion of the Silurian Mumbil Formation. There is no evidence for a
-10-
significant depositional hiatus between these sediments and the volcanics hosting the mineralization, or for thrust movement on a fault zone coinciding approximately with the lithological boundary. The acid volcanic host rocks are correlated, following Crawford (1972), with the Silurian Glenski Formation rhyolite and dacite tuffs. The lower boundary of the Gleneski Formation is placed at the top of the uppermost greywackes of the Mumbil Formation. A sequence of more basic volcanics in the eastern portion of the prospect is correlated with the Cuga Buraa Volcanics. The Gleneski Formation at Galwadgere consists of three major lithological units: a
)
The basal unit consists of the products of an initial volcanic eruption and contemporaneous sedimentation. Basal breccias are succeeded by shales and siltstones containing thin, discontinuous bands of rhyolite tuffs.
b)
The main pyroclastics include two recognizable cycles, each beginning with the deposition of finer-grained, non-porphyritic tuffs which grade upwards into coarser varieties containing crystal phenocrysts (mostly of quartz), and remnants of pumice and glass shards in the upper horizons. The finer grained tuffs tend to be chloritic, with the chlorites being relatively ferruginous; the coarser tuffs are more sericitic and their chlorites are more magnesian. Overall sericite increases, relative to chlorite, upwards in the volcanic pile. Feldspars are present only in the uppermost units, while carbonates are most abundant in the lower part of the sequence. The tuffs are extensively altered throughout the sequence, and the term "rhyolitic" is used only in a broad sense. However, textural and grainsize variations have not been obscured.
c)
An upper volcanogenic sedimentary unit consists of tuffs, at least in part epiclastic, grading up into greywackes and siltstones.
The Cuga Burga Volcanics are chemically and petrographicaly distinctive, and show spilitic/keratophyric affinities. Mineralogical characteristics are indicative of prehnitepumpellyite rather than the more generally accepted lower greenschist facies of regional metamorphism. Sulphide mineralization consists principally of pyrite, with minor chalcopyrite and traces of sphalerite and galena. Precious metal grades are low in unoxidised ore. The sulphides occur as: a)
Disseminations in the non-porphyritic tuffs (often withi with marked stratigraphic control), and more widely dispersed in the upper portions of the mineralizaed zone. Some disseminated sulphides are marginal to transgressive quartz-sulphide veins.
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b)
Vein-type mineralization in broad fracture zones in silicified rhyolite tuff hosts. Although grades of vein-type mineralization are not usually high, the bulk of the contained copper occurs in this form.
c)
Relatively massive sulphide mineralization in lenses, up to 4 m thick, in the lowest sections of the pyroclastics. The sulphide lenses often contain tuff intercalations.
The spatial relationship between mineralization and acid vulcanism is indicative of a close genetic relationship. Chloritization is characteristic of the massive and disseminated ore zones, whereas silicification and carbonatization are more typical of the vein ores, which may reflect at least partial remobi1ization during diagenesis or regional metamorphism. Resolution of the broad features of volcanic stratigraphy at Galwadgere proved to be of material assistance to structural interpretation and evaluation of the deposit. Petrographic and geochemical studies of this nature should prove to be useful aids in the exploration of deposits of this type. REFERENCE Crawford, D., 1972. Petrological study of the Galwadgere copper district, Wellington, N.S.W. M.Sc. thesis, University of Newcastle.
BASE METAL MINERALIZATION AT CURRAWANG By E.J. Malone Jododex Australia Pty. Ltd. The Currawang East deposit is a body of polymetallic massive sulphides contained in basaltic pillow lavas. The deposit is extremely irregular in shape and distribution and is currently estimated to contain about 750,000 tonnes to 1,000,000 tonnes of average grade 1.6% Cu, 2.2% Pb, 13.0% Zn and 3 3 g/t Ag. The mineralization is associated with alteration of the host rocks, such that ore grade mineralization is usually flanked by the most intense alteration which grades through less altered rocks to unaltered pillow lavas in both the hanging wall and the footwall. The relationship of the mineralization to the alteration suggests that this is a replacement deposit, not a submarine volcanic exhalative deposit.
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BASE METAL MINERALIZATION AT THE C.S.A. MINE, COBAR, N.S.W. By R.P.Singer Cobar Mines Proprietary Limited. The C.S.A. Mine, 11 km NNW of Cobar in western N.S.W., exploits one of a number of Cu/Pb/Zn and Cu/Au orebodies which occur at various stratigraphic levels within sediments of the Cobar Group over a strike distance of 30 km. All appear to have similar geometry and are characterized by single or multiple shoots with a great vertical dimension (>1000 m), narrow width (5-30 m) and variable length (100-400 m) . All dip east with the cleavage and plunge north and are discordant to the west-dipping, west-facing host sequence of slates, s ilts tones and greywackes. The C.S.A. Lodes comprise numerous subparallel zones of sulphide concentration within a low grade mineralized envelope. Three distinct lode systems can be recognized (Western, Eastern (including D-Zone) and the probably related but little understood QTS Zone. Each system comprises a number of shoots and the systems are separated by unmineralized sediments. The total width of the mineralization (including the QTS) is approximately 550 m. A number of ore types are recognized including a siliceous, sub-massive pyrite-chalcopyrite ore ("Western type"), a less siliceous, low sulphide, stockwork veined chalcopytire-pyrrhotite ore ("Eastern type") and a massive sphalerite-galena ore with pyrrhotite or pyrite ("Cu-Zn type"). These can form distinct shoots of one ore type or mixtures of ore types. Some ore lenses show a weak zonation generally with Pb/Zn ore on the western side and Cu-ore to the east. Reversals however are common. Accompanying the mineralization in the Eastern and Western Systems are zones of siliceous and chloritic alteration. Silicification is strongest in the Western System while the Eastern ore bodies have a wide halo of Fe-chlorite alteration. Zones of Mg-chlorite with or without talc and accompanying brecciation also occur parallel to cleavage and are commonly associated with Pb-Zn orebodies. The QTS Zone comprises a number of generally narrow high grade shoots, both Pb/Zn and Cu, intersected in drilling between 400 m and 500 m east of the C.S.A. No. 2 Shaft. Mineralization has been intersected over a strike length of 800 m. Widths of shoots range from less than 1 m to 15 m. Best intersections recorded to date are 15 m of 6.1% Cu and 7 m of 3.3% Cu, 5.6% Zn. Ore types are similar to those of the "Eastern" and "Cu-Zn types" though significant differences occur. Drilling to date is too widely spaced to allow continuity of shoots or the overall geometry to be determined.
-13The present geometry of the C.S.A. orebodies and their accompanying alteration suggests an emplacement by processes involving the introduction of sulphides from an external source into structurally favourable sites at or after the time of cleavage formation. GEOLOGY OF THE ELURA Ag-Pb-Zn DEPOSIT, COBAR, N.S.W. By B.L. Schmidt Electrolytic Zinc Company of Australasia Limited,,
The Elura Pb-Zn-Ag deposit is situated 4 3 km NNW of Cobar, N.S.W., and contains in excess of 27 million tonnes of ore at a grade of 5.8% Pb, 8.4% Zn and 130g/t Ag in a crudely elliptical body 115 x 210 m in plan and over 500 m deep that towards the top divides into two apophyses, one of which reaches the surface. The deposit is hosted by the C.S.A. Siltstone, a distal turbiditic unit within the Amphitheatre Group of the Lower Devonian Cobar Super-Group, that was deposited in the area of maximum depression towards the eastern margin of the basin. Deposition was terminated by the Mid Devonian Tabberabberan Orogeny that caused moderately intense deformation with areas of distinct fold styles, and metamorphism that reached lower greenschist grade. Cobar Super-Group sediments were overlain by a mildly deformed sequence of shallow water Upper Devonian clastics. The Elura orebody is situated in a peneplained area of very poor outcrop where most data have been obtained from artificial exposures. Amphitheatre Group sediments are irregularly deformed and a weak to moderately well developed axial surface cleavage has formed. The orebody occupies the core of a small 100-200 m wide anticline in an overall south plunging synclinorial structure. Structure in the immediate vicinity of the orebody is shown by a slump marker horizon. Away from Elura the host rock comprises dominantly a quartz + muscovite + chlorite + albite + carbonate assemblage, but within 100-200 m of the orebody wall rock alteration caused the destruction of albite. Mineralogical changes are locally more extreme adjacent to the orebody, with silicification, sericitization, ankeritization, and chloritization with commensurate chemical changes. The orebody is surrounded by 5-80 m wide haloes of megascopic siderite porphyroblasts and base metal sulfide enrichment. Three ore types are distinguishable on the basis of gangue mineralogy and content, and Fe-sulfide mineral abundances. Siliceous and massive ores are similar except that the former contains more abundant SiC>2 and less CC>2. Both contain above average abundances of Ag, As, Sb and Hg,but only traces of pyrrhotite. Pyrrhotite ore contains abundant pyrrhotite and siderite, little quartz and below average contents of Ag, As, Sb and Hg. Ore types have relatively distinctive compositions so that the orebody is compositionally and mineralogically zoned. Minerals in the orebody in approximate order of abundance are pyrite, sphalerite, siderite, pyrrhotite, quartz, galena, arsenopyrite, chalcopyrite and a suite of minor minerals including
-14barite, Ba-feldspars, muscovite, chlorite, tetrahedrite, tennantite, enargite etc. Much of the silica in the orebody appears to be silicified rock. Two megascopic preferred orientations are present, one a sub-vertical north-south striking gross mineralogical 'sulfide' layering and the other sub-horizontal dominantly gangue filled fracture of the 'dilation' layering. Four dominant varieties of pyrite are present and appear to have been deposited in sequence. Arsenopyrite commonly replaces early pyrite and both minerals are replaced by sphalerite, galena, chalcopyrite and pyrrhotite, which tend to be in equilibrium with each other, and also quartz, siderite, chlorite, muscovite, tetrahedrite etc. Pyrrhotite replaced pyrite with accompanying increase in the FeS content of sphalerite during a prograde event, and the FeS content of sphalerite in pyrrhotite ore decreased during a later retrograde event. Mineral assemblages in the orebody and host rock indicate formation probably in the range 254 -3309:at an estimated pressure of 2.5-3.5kb. The development of cleavage in minor sulfides in the halo and the relationship of siderite porphyroblasts to cleavage indicate that mineralization was deposited prior to the termination of cleavage formation and was metamorphosed, possibly soon after deposition, thereby obscuring many depositional features. The extent to which features in the orebody and host rock are of metamorphic origin is unknown. Two genetic models have been proposed, but the lack of unequivocal diagnostic criteria prevents determination of the 1 'correct genesis. A modified syngenetic model proposes that the mineralization was originally stratiform and forcibly intruded into a favourable structural site during deformation. Post-emplacement metamorphism is required to explain a variety of features which appear unlikely to have been generated in such a way. An alternate epigenetic model proposes structurally controlled replacement with contemporaneous alteration and is better able to explain many of the features without recourse to extensive metamorphic modification. The deposits near Cobar have some similarities to the Elura mineralization but many differences of detail. For example, the commodities produced were dominantly Cu and Au, and wall rock alteration involves extensive chlorite formation at the expense of muscovite. Genetic models proposed are similar to those for Elura, with the replacement model possibly providing a better explanation of the observed features.
-15COMMONALITY AND DIFFERENCES
IN ORES
OF THE COBAR S U P E R - G R O U P , N.S.W, By Brian Marshall and S.R. Sangameshwar Department of Applied Geology, N.S.W. Institute of Technology.
Mineral deposits in the Cobar Super-Group fall into two types (Gilligan and Suppel, 19 78). Type (b) deposits extend from Nymagee through Cobar to Elura; they are characterized by a dearth of volcanicity and supposed exhalation in a deep water environment. In addition, type (b) deposits are in rocks that have undergone cleavage-forming deformation and metamorphism and, at least in part, have experienced these processes? they have indistinguishable Pb isotope compositions, a similar spread of A3kS values and extensive wall rock alteration developments; they exhibit regional stratigraphic and minescale zonation of elements and mineral species: they engross diverse mesoscale relations which may be selectively adduced to support simplistic genetic models but are commensurate with the many processes that have contributed to the present state of the ore. Despite the above commonality, macroscale relations between ore bodies, cleavage and bedding permit recognition of subtypes. These are: (i)
The Nymagee subtype in which tabulate ore bodies are grossly concordant with foot wall bedding and discordant with cleavage.
(ii)
The C.S.A. subtype in which elongated, flattened, ellipsoidal ore bodies are dominantly concordant with cleavage and discordant with bedding.
(iii)
The Elura subtype in which a plug-like ore body is grossly discordant to bedding and cleavage.
Assuming exhalogenesis, subtype differences must reflect the content (by volume) and geometry of the primary mineralization; the contrasting mechanical behaviour of mineralization and the overlying and underlying sedimentary packages; the interplay of chemical and mechanical remobilization processes during the protracted period in which metamorphic dewatering took place and ductile and brittle structures progressively evolved in the host rocks. It is suggested that: a)
The Nymagee subtype comprised stratiform mineralization in an argillaceous host above a competent sandstone package. Mechanical and chemical remobilization occurred but were insufficient to reconstitute gross geometric relations.
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b)
The Elura subtype comprised thick areally restricted lenticular mineralization in a dominantly fine grained distal turbidite sequence. An early domal structure nucleated about the mineralization and focussed the tectonically driven piercement effect. Cleavage development extended the vertical axis and enhanced the plug-like discordance. Chemical remobilization has modified ore distribution but is subordinate to mechanical processes.
c)
The C.S.A. subtype comprised developments of stratiform mineralization linked by feeder mineralization above a major basement shear zone. Deformation and waxing metamorphism induced mechanical and chemical remobilization and overprinting by cleavage and shear zones. Following cessation of ductile processes, probably during waning metamorphism, (meta-)hydrothermal fluids formed mainly dilational veins and vein systems concordant with cleavage. This chemical event has left a dominating imprint on the C.S.A. subtype.
Thus, all of these evolutionary complex ores bear the mesoscale imprint of the same set of processes. An understanding will only come from quantifying the relative input of the processes and establishing the time-sequence of their operation. Reference Gilligan L.B., Suppel, D.W. (1978) Mineral deposits in the Cobar Super-Group and their structural setting. New South Wales Geological Survey, Quarterly Notes 33: 15-22.
S T R A T I G R A P H Y , STRUCTURE AND GEOCHEMISTRY OF KANGALOOLAH VOLCANOGENIC S U L P H I D E D E P O S I T S
By Carol Simpson Macquarie University, A sequence of Upper Silurian-Lower Devonian volcanics and epiclastics in the Abercrombie-Cordillera area are considered to have been deposited in a volcanic rift developed on continental crust at the southern termination of the Hill End Trough. The four lithologic associations recognized, namely, silicic, pyroclastics, volcaniclastics, andesites and spilites are intimately associated, and the complex stratigraphic relationships between them have not been clearly resolved. Contemporaneous accumulation of very fine-grained clastics occurred throughout the depositional history.
-17Two phases of deformation have been recognized in the Abercrombie-Cordillera area, and are considered to be episodes within a single deformational event, probably occurring during the Upper Devonian-Lower Carboniferous. The first phase of deformation produced variably plunging folds, and an axial surface slaty cleavage/foliation. Differences in fold style and orientation are attributed to rotation of ealry-formed sub-horizontal folds to their present dominantly steeply plunging attitudes, during progressive strain. The second phase of deformation produced kink folds and a crenulation cleavage. Small lead-zinc-copper orbodies which occur sporadically along a volcanic/pelite boundary, are considered to be cigarshaped lenses, with long axes steeply plunging, parallel to the mineral lineation and minor fold axes over much of the area. Mineralogical and chemical changes in alteration zones around sulphide mineralization have been documented from three drill holes. The dominant mineralogic changes are the total absence of feldspars in altered rocks, and the abundance of chlorite immediately associated with sulphides. The clearest discriminant elements or groups of elements are Na 2 0 which is consistently depleted, and FeO, Pb, Zn, Cu, Rb/Sr and K 2 0/Na 2 0 which are consistently enriched, in alteration zones.
MOLYBDENUM MINERALIZATION
AT MOUNT PLEASANT, N.S.W.
By M.J. Vicary C.S.R. Limited. The Mount Pleasant molybdenum prospect is situated about 30 km south of Mudgee, N.S.W. The Prospect was discovered by C.S.R. Limited during a regional stream sediment geochemical programme in 19 74 when anomalous base metal values were recorded over an area of approximately 6 km2. Follow-up geological investigations revealed the presence of pyrite and subordinate base metal sulphides, molybdenite and minor scheelite in veins and fractures within a sequence of hydrothermally altered volcanics and sediments. A little disseminated pyrite and chalcopyrite occurs locally in the host rocks. Diamond drilling at Mount Pleasant has subsequently outlined stockwork-type porphyry molybdenum mineralization spatially related to porphyritic phases of a concealed and steeply plunging acid intrusive stock. The granitic rocks are of adamellitic composition and intrude a host sequence of acid to intermediate tuffs, greywackes and quartz wackes of the Upper Silurian Chesleigh Formation and intrusive metadolerite dykes. An extensive contact metamorphic aureole is present.
-18-
Mineralization consists of pyrite and molybdenite, with associated and minor scheelite, chalcopyrite, bismuthinite, sphalerite, stibnite, galena and arsenopyrite. Extensive hydrothermal alteration occurs at Mount Pleasant. It is predominantly fracture-controlled and usually occurs as alteration envelopes adjacent to veins and fractures. Minor pervasive alteration is also present. Molybdenum mineralization is associated with potassic, phyllic and to a lesser degree argillic alteration assemblages. Propylitic alteration is characteristic of peripheral areas of the prospect.
A TIN SILICATE SKARN, DORADILLA VIA ROURKE, N.S.W. By I.R. Plimer North Broken Hill Ltd. A tin-bearing skarn horizon 15 kilometres in length, 40-110 metres wide and at least 200 metres deep is present at Doradilla, 40 kilometres SE of Bourke. The horizon does not crop out, is deeply weathered and covered by aeolian soils. The area comprises predominantly flysch-like pelitic metasediments possibly of the Ordovician Girilambone Beds, with minor psammitic units and serpentinite intrusions. The calcareous horizon is intruded by quartz-feldspar porphyry dykes and a small genetically related leucoadamellite mass. Minor narrow basaltic and andesitic dykes cut the sequence. The leucoadamellite is slightly sericitised, contains zincian fluorannite, monazite, zircon and ilmenite, is probably an "A"-type granitoid. The skarn is zoned across strike from an inner complex massive garnet-pyroxene skarn 2-17 metres wide, enveloped by a well laminated pyroxene-plagioclase-garnet-skarn 20-40 metres wide to an outer laminated skarn - calcareous pelite. The inner zone is characterized by numerous stages of mineralogical and textural overprinting. Stage I
garnet (gro > andrad) - pyroxene (diop > hed)
Stage II
garnet (stannian andradite > gro) - pyroxene (hed > diop) - vesuvianite
Stage III
calcite-malayaite-quartz
Stage IV
sphalerite-chalcopyrite-stannian magnetitequartz-cassiterite-bornite-sphene-galenaarsenopyrite
Stage V
fluorite-calcite-biotite-chlorite
Stave VI
laumontite-calcite
-19The laminated pyroxene-plagioclase-garnet skarn contains Stage I assemblages commonly with plagioclase and wollastonite and Stage II-VI are developed only along fractures or in rare patches. The outermost skarn, comprising an interlaminated skarn-calcareous pelite comprises wollastonite-plagioclase assemblages which have partially replaced a quartz-biotitesericite assemblage. The skarn formed as a result of a porous, permeable highly reactive unit being intruded by a post-orogenic high level (?) fluorine-rich A-type melt. Because of the lack of^F phase in the skarn, the lack of greisenisation and the presence of fluorannite overgrowths on earlier annite in the laucoadamellite, the tin was probably transported as OH and/or CI species. Successive skarn stages are due to decreasing T, variable f 0 2 and f s 2 and replacement, redissolution and reprecipitation of Sn from earlier phases.
Sn MINERALIZATION AT ARDLETHAN By R. Paterson Ardlethan Tin Limited. The Ardlethan tin deposits are located within granitic rocks of the Lachlan fold belt 100 km. NW of Wagga Wagga. Discovered in 1912 they were worked intermittently on a small scale until 19 56 producing 9,500 tonnes of cassiterite concentrate. In 1961 Aberfoyle Tin N.L. commenced a detailed evaluation of the deposits and in May 1964 open pit mining commenced. Underground mining of deeper ore began in early 19 79. To November 19 81 approximately 7.0Mte 0.49% Sn have been processed. Current hard rock resources are estimated to be 5 - 7 Mt at 0.4% Sn. the geological sequence at Ardlethan is interpreted as follows. A late Silurian weakly foliated S-type biotite adamellite intruded Ordovician metasediments. Subsequently an unrelated series of high level S-type garnet bearing quartz feldspar porphyries with associated rhyolitic flows and ignimbrites was emplaced into sediments and earlier hypabyssal rocks followd. An extremely fractionated late stage differentiate of the leucogranite - a quartz feldspar porphyry- was forcefully intruded into the nearby biotite adamellite. Accompanying volatiles explosively released during crystallization of this porphyry altered and hydraulically fractured the surrounding host and the porphyry dyke complex itself and deposited the cassiterite and sulphie mineralization. Cassiterite and sulphides occur as fine dissemination, as veinlet and gash network fillings, hydrothermal breccia fillings and as late stage pipes and veins.
-20-
RESERVE PAPER* DISSEMINATED Ni-Cu-Co
MINERALIZATION IN THE
ULTRAMAFIC VOLCANICS AT ROCKLEY, N.S.W. By R. A. Binns CSIRO Division of Mineralogy, North Ryde,
N.S.W.
Although previously described as 'andesitic pyroclastics1, the Upper Ordovician Rockley Volcanics cropping out south of the Bathurst granite are dominated by former ultramafic extrusives (25-28% MgO) and by magnesian basaltic pyroclastics (10-12% MgO). Some rock types bridging these also occur, together with serpentinites and less magnesian basalts or basaltic andesites. All were extensively metamorphosed under low amphibolite facies conditions. Geochemically, the Rockley sequence resembles both Arcahean komatiites and Phanerozoic boninites but is anomalously enriched in phosphorous, and in potassium at the less mafic end of the compositional spectrum. Comparisons with komatiites and boninites may be spurious (the geotectonic-stratigraphic setting appears consistent with neither), but the presence in significant abundance of ultramafic volcanics raises possibilities for hitherto unrecognized sytles of mineralization. Fine grained, disseminated aggregates of pentlanditepyrrhotite-chalcopyrite-cobaltite are widespread in the ultramafic actinolite-chlorite-talc schists, particularly those containing abundant carbonate minerals. Nickel contents (800-1400 ppm) do not correlate with sulfide abundance, and these aggregates appear produced by sulfurization of former lithophile trace elements consequent upon activity of C02"S-As bearing fluids during or prior to regional metamorphism. The possibility that ore grades might be attained locally is suggested by pods and veins of coarser grained pyrrhotitechalcopyrite-quartz, so far known only wihtin basaltic hosts but evidently formed by remobilization of disseminated sulfides during metamorphism and deformation. One occurrence is known of chalcopyrite associated with a pre-metamorphism stockwork alteration pattern in basaltic pyroclastics, suggestive of porphyry-style mineralization. Unless serpentinites are deomnstrated to occur in greater abundance as part of the extrusive sequence, the potential for massive Ni-Cu sulfides of magmatic origin appears low. Possible association of the Rockley Volcanics with previous metal mineralization has not been investigated, nor have comparisons been made with other Ordovician volcanic sequences north and west of the Bathurst granite. *To be presented in event of any scheduled paper being cancelled.
-21-
AUTHOR
Binnsf R.A.: 20 Bush, A.: 8 Degeling, P.R.: 4 Evans, P.R.: 6 Flood, R.H.: 3 Foldessy, J.: 9 Malone, E.J.: 11 Marshall, B.: 15 Packham, G.H.L 1 Patterson, R.: 19 Plimer, I.R.s 18 Powell, C.M.: 6 Sangameshwar, S.R.: 15 Scheibner, E.: 2 Schmidt, B.L.: 13 Shaw, S.E .: 3 Simpson, C.: 16 Singer, R.P.: 12 Suppel, D.W.: 4 Vicary, M.J.: 17 Williams, K.L.: 9
INDEX