Abstracts No.25: 10th AGC Gondwana: Terranes & Resources, 1990, Hobart by GSAustralia

Geological Society of Australia

ABSTRACTS Number 25

S&ETK

^ O f l R O ^

GONDWANA: TERRANES AND RESOURCES Tenth Australian Geological Convention Hobart 1990

Geological Society of Australia ABSTRACTS Number 25

* GONDWANA: TERRANES AND RESOURCES

Tenth Australian Geological Convention

February 4-9,1990 University of Tasmania Hobart

Published by the Geological Society of Australia Challis House, 10 Martin Place, Sydney 1990

ISBN 0 7316 8367 6 ISSN 0729 01IX

Ill

Geological Society of Australia Incorporated Officer Bearers 1989-90

President

Mr I. R. Johnson

Vice Presidents

Professor D. M. Boyd Professor D. H. Green

Honorary Secretary

Dr G. F. Taylor

Honorary Treasurer

Dr P. S. Moore

Honorary Editor

Professor J. H. Roberts

Honorary Administration Officer

Mr D. H. Probert

Tenth Australian Geological Convention Hobart 1990 Organising Committee

Convenor

Mr P.W. Baillie

Secretariat

Dine & Associates

Abstracts Editor

Dr A. V. Brown

Posters

Dr D. L. Huston

Core Shed

Dr J. B. Gemmell

Program Manager

Dr C. F. Burrett

Excursions

Dr R. R. Large

Publicity Officer

Mr I. J. Satchwell

Finance Manager

Dr R. G. Richardson

Social Secretary

Ms C. A. Bacon

GSA Representative

Professor D. H. Green

Technical Manager

Dr A. J. Stolz

Geological Society of Australia Abstract Series Number 25 page Session Summary

vii

Keynote Papers — Sir Douglas Mawson Lecture Antarctica, Homo sapiens to greenhouse: agents in planetary revolution J. M. Bowler

— International Geological Correlation Program Lecture Proterozoic petroleum M. R. Walter

Scientific Papers (in program order) A1 Volcanic Environments and Associated Mineralisation A2 Fossils from Gondwana Terranes A3 Sedimentation: Modern and Ancient A4 Gondwana Geophysics A5 Exploration Models and Mineral Deposit Evaluation A6 Mafic Magmatism and Associated Mineralisation A7 Precambrian Gold Deposits A8 Role of Structure in the Formation and Deformation of Ore Deposits A9 Fault Zone Fabrics A10 Gondwanaland Context of the Tasman Fold Belt A12 Magmas and Fluids in the Subduction Environment A14 Ocean Drilling Program (see separate volume) A15 Geology and Community Education — Initiatives for the 1990s A16 Magmas, Gemstones and the Mantle A17 Antarctic Geology A18 Granites and Associated Mineralisation

5 39 57 79 106 123 143 165 180 190 202 218 229 247 257

Poster Session (alphabetically by author)

276

R.L. Stanton Symposium — New Frontiers in Ore Deposit and Exploration Studies

305

Author Index

317

vii SESSION SUMMARY

Keynote Papers Sir Douglas Mawson Lecture Antarctica, Homo sapiens to greenhouse: agents in planetary revolution — J. M. Bowler 1 International Geological Correlation Program Lecture Proterozoic petroleum — M. R. Walter 3

Session Al: Volcanic Environments and Associated Mineralisation Al.l

Environment and mechanism of formation of the Woodlawn massive sulphide deposit, NSW — W J. McKay and J.L. Walshe 5 A1.2 The formation of Rosebery-type, volcanogenic massive sulphide deposits — M. Solomon, JJL. Walshe and C.A. Heinrich 6 A1.3 Geochemistry and alteration of the hangingwall basalts to the Hellyer volcanogenic massive sulphide deposit, Tasmania — D J. Jack 7 A 1.4 Formation of the alteration pipe and stringer zone beneath the Hellyer massive sulphide deposit, Tasmania — J.B. Gemmell, R.R. Large, G.J. McArthur, C.G. Drown and R.C. Downs 8 A 1.5 The geology and mineralisation at Mt Charter — S.W. Rand 9 A1.6 Geology and geochemistry of the precious metal-rich South Hercules volcanogenic sulphide deposit, western Tasmania — Khin Zaw, R.R. Large and S. Hunns 10 Keynote Address: Volcanogenic massive sulphide deposits, ancient and modern — J.M. Franklin 11 A1.7 The western Woodlark Basin, Papua New Guinea: submarine volcanic and hydrothermal activity associated with rifting of a continental margin — R.A. Binns, S.D. Scott and PACLARK Team 14 A1.8 Geology of the Buchans orebodies, Newfoundland — J.G. Thurlow 14 A1.9 Rock alteration, mineral and oxygen isotope zonation in the Rosebery district, Tasmania — GR. Green 15 A1.10 Au-Sn mineralisation along the Henty Fault, Mt Farrell area: mopping up the Cambrian vs Devonian controversy — J. Taheri and G.R. Green 16 Al.ll ^Ar-^Ar and U-Pb geochronology of the Goonumbla copper-gold and Gidginbung gold deposits, NSW — C. Perkins, I. McDougall, J. Claou6-Long and P. Heithersay 17 A1.12 Depositional age of the Broken Hill group from volcanics stratigraphically equivalent to the Ag-PbZn orebody — R.W. Page and W.P. Laing 18 A1.13 Geologic setting of the Umuna epithermal gold deposit, Misima Island, Papua New Guinea — D.S. Clarke and I.E.M. Smith 19 A1.14 Epithermal gold and silver mineralisation in Miocene volcanic rocks, Vanua Levu, Fiji Islands — J.L. Stockley 20 A1.15 Hydrothermal alteration, mineragraphy, structure and fluid inclusions from the Zelma acid sulphate gold deposit, Sarina, Queensland — R.T. Bills 22 A1.16 Contrasting alteration styles associated with epithermal mineralisation, North Arm Volcanics, Queensland — P.M. Ashley and A.S. Andrew 23 A1.17 SHRIMP ion microprobe studies of sulphide mineralisation in active geothermal areas: the Salton Sea as compared to the Valles Caldera — C.S. Eldridge and M.A. McKibben 25 A 1.18 Factors controlling the composition of electrum in precious metal deposits — C.H. Gammons 25 A1.19 The source of the gold in western Tasmanian VMS deposits — A J. Stolz and R.R. Large 26 A1.20 The mineralogy of gold in some VMS deposits in eastern Australia — R.S. Bottrill and D.L. Huston 28 A1.21 Mineralised Archaean calderas(?) in the Whim Creek volcanic belt, Western Australia — P.L.F. Collins 29 A1.22 Primary volcanic facies of the Gossan Hill Group, Golden Grove, Western Australia: volcanic processes and timing relative to massive sulphide mineralisation — B.A. Clifford and R.A. Cas 29 A1.23 The andesite connection - stratigraphic correlations between the major mineral fields in the Mount Read Volcanics — K.D. Corbett 30 A 1.24 The Rosebery controversy: distinguishing prospective submarine ignimbrite-like units from true subaerial ignimbrites in the Rosebery-Hercules ZnCuPb massive sulphide district, Tasmania — R.L. Allen and R.A.F. Cas 31

viii A1.25 A1.26 A 1.27 A 1.28 A 1.29 A1.30

Styles of sedimentation and volcanism within the Que-Hellyer volcanics — J.C. Waters 32 Eruptive style, products and setting of Kuroko volcanics, Miocene green tuff belt, Japan — R.A.F. Cas, R.L. Allen, H. Yamagishi, Y. Ishikawa and T. Ohguchi 34 Misbehaviour of pyroclastic flows at the land-sea interface: a tale from Wales — GJ. Orton 35 Volcanic setting of epithermal gold mineralisation in the northern Drummond Basin, Queensland — B.S. Oversby, G.R. Ewers, D.E. Mackenzie, J. McPhie, D. Wyborn, S. Law and L.P. Black 36 Geology and mineralisation of the Mt Windsor volcanic belt, north Queensland — S.D. Beams and J.S.Hartley 37 A shallow-water volcanogenic massive sulphide deposit: Mount Chalmers, Queensland — R.A. Sainty 38

Session A2: Fossils from Gondwana Terranes

Keynote Address: Himalayan terranes and suspect palaeontology — John A. Talent 39 A2.1 A new Phanerozoic reconstruction basemap series — M.I. Ross, G. Young, HJ. Stagg, J.B. Willcox and C.T. Klootwyk 39 A2.2 Asian terranes derived from Gondwana — Clive F. Burrett 40 A2.3 Fossils versus palaeomagnetism as tests of palaeogeographic hypotheses — examples from Gondwana terranes analysed cladistically — Gavin C. Young 41 A2.4 Cambrian faunas in the terranes of the southwestern Pacific part of Gondwana — J.B. Jago 42 A2.5 Relationships of Cambrian-Ordovician trilobite biofacies on Tarutao Island, peninsular Thailand — J.H. Shergold, C.F. Burrett and T. Wongwanich 43 A2.6 Ordovician corals and stromatoporoids from Gondwana terranes — B.D. Webby 45 A2.7 Conodonts and the distribution in time and space of Ordovician sediments in Australia and adjacent a r e a s _ R.S. Nicoll and J.M. Totterdell 46 A2.8 Devonian-Carboniferous microvertebrates of Gondwana — S. Turner and Wang Shi-Tao 48 A2.9 Microfossils and gross structure and stratigraphy of the Silurian-Devonian Chillagoe Formation, western Hodgkinson Province, northeast Australia — B.G. Fordham 48 A2.10 Mid Palaeozoic conodont data as fundamental chronological underpinning for analysis of allochthoneity of terranes in eastern Australia — R. Mawson, J.A. Talent, M.J. Engelbretsen and G.A. Brock 50 A2.ll Plant fossils from Neopalaeozoic Gondwana terranes of the State of Rio Grande do Sul, South Brazil: review on paleobotany and palynology — M. Guerra-Sommer, M. Marques-Toigo, M. Cazzulo-Klepzig and Z.C. correa Da Silva 51 A2.12 Early Permian faunas from the Cracow area, southeastern Bowen Basin, Queensland — J J.Draper, V. Palmieri, S.M. Parfrey and J.F. Rigby 52 A2.13 Permian foraminifera of Australia — V. Palmieri 53 A2.14 Southern polar forests in a reduced light regime: evidence from the biota — J.G. Douglas 54 A2.15 Cooling the Cretaceous: evidence for early Cretaceous cool climates in Australia from growth rings in fossil wood — J.E. Francis and L.A. Frakes 55 A2.16 Tertiary macrofloras and Tertiary stratigraphy of Poole Creek palaeochannel, Lake Eyre Basin — D. Greenwood, R.A. Callen and N.F. Alley 56

Session A3: Sedimentation: Modern and Ancient A3.1 A3.2 A3.3 A3.4 A3.5 A3.6

Shelf progradation under the influence of a boundary current: the Canterbury Basin passive margin, NZ — C.S. Fulthorpe and R.M. Carter 57 Evolution of Pliocene-Recent abyssal sediment waves on Bounty Channel levees, NZ — L. Carter, R.M. Carter, C.S. Nelson and C.S. Fulthorpe 58 Avulsion and deposition in the monsoonal Gilbert River fandelta, Gulf of Carpentaria, Queensland, Australia — B.G. Jones, G.C. Nansen and N. Senapati 58 Relationship between syneresis cracks and early diagenetic concretions in the Irby Siltstone (Proterozoic), northern Tasmania — C.R. Calver 59 Late Proterozoic and early Cambrian clastic facies relationships, SW Georgina Basin, NT — P.W.Haines 60 Controls on Jurassic non-marine sedimentation in eastern Australia: evidence from the ClarenceMoreton basin — P.E. O'Brien and A.T. Wells 61

ix A3.7

Glaciation and palaeogeography of Australia at the start of the Permian — A.T. Brakel and J.M. Totterdell 63 A3.8 Palaeogeography of the late Carboniferous glacimarine Wynyard Formation, NW Tasmania — S .J.Hand 65 A3.9 A model for glacigene sedimentation in the Troubridge Basin, SA — N.F. Alley and RP. Bourman 66 A3.10 Diagnetic enrichment of iridium and other plantinoids: evidence from the Acraman impact ejecta horizon and host shales — M.W. Wallace, R.R. Keays and V.A. Gostin 66 A3.11 Aminostratigraphy and electron spin resonance studies of late Quaternary sea level change and coastal neotectonics in Tasmania — C.V. Murray-Wallace and A. Goede 67 A3.12 Contemporary evaporite sedimentation in Karinga Creek drainage systems, Amadeus Basin, NT — A.V. Arakel, R. McWatters, D. McConchie, A. Cohen, C. Pailles and T. Hongjun 68 A3.13 Limestone microfacies distribution of the Chillagoe Formation (Siluro-Devonian), Mingana area, Hodgkinson Basin, NQ and its structural implications — T. Bernecker and J.A. Webb 69 Keynote Address: Non-tropical shelf carbonates and their recognition in the geologic record: a New Zealand perspective — C.S. Nelson 70 A3.14 The Bryozoa Membranipora aciculata as an indicator of environmental changes in the Coorong Lagoon during the last 700 years — Y. Bone 72 A3.15 Differences between subtropical (Ordovician), temperate (Recent and Pleistocene) and subpolar (Permian) carbonates, Tasmania, Australia — C.P. Rao 72 A3.16 Cold-water carbonate sedimentation during late Ordovician, the Pa Kae Formation, southern Thailand: tropical deep setting versus shallow temperate origin? — T. Wongwanich and CP. Rao 73 A3.17 Cool climate bauxite — G. Taylor, E.M. Truswell, R.A. Eggleton, M.C. Brown and K.G. McQueen 74 A3.18 Depositional environments and coal facies in the Tomago and Newcastle coal measure, NSW — M. Roach 75 A3.19 Diagnetic evolution of the hydrocarbon bearing Permian sedimentary rocks of the Denison Trough, Bowen basin, Qld: evidence from the borehole GSQ Eddystone 5 — R.Ahmad, J.C. Tipper, R.A. Eggleton and J.L. Walshe 76 A3.20 Post-depositional history of the Permian sequence from the NW Denison Trough, Queensland — P.de Caritat, J.L. Walshe, R.A. Eggleton and J.C. Tipper 77

Session A4: Gondwana Geophysics Keynote Address: Late Pleistocene and Holocene sea-level change in Australia: causes and some consequences — K. Lambeck 79 A4.1 Variations in rifting style around Australia during Gondwana breakup: the structural and sedimentary record of rifted continental margins — J. Braun and C. Beaumont 81 A4.2 Subsidence of the eastern Australian platform during the Mesozoic — K. Gallagher 81 A4.3 Crustral structure in central Australia determined by inversion of travel time — H. McQueen 104 A4.4 Thermal history of Western Australian Gondwana basins — M.F. Middleton 82 A4.5 Post-Permian subsidence and tectonics, Vulcan Sub-basin, Northwest Shelf — R. Hillis 82 A4.6 A unified model of intracratonic basin formation — Shaohua Zhou 83 A4.7 Transformational plasticity and its possible geophysical signifcance — A.C. McLaren 84 A4.8 The Tamar conductivity anomaly — W.D. Parkinson and R. Hermanto 85 A4.9 The Canning Basin induction anomaly — F.H. Chamalaun and J. Whellams 86 A4.10 CSAMT in the Eromanga Basin — L.M. Hastie, C. Cevallos and I.J. Chant 86 A4.ll Electrical conductivity structure at the contient-ocean boundary of southeast Australia — R.L. Kellett, F.E.M. Lilley and A. White 88 A4.12 Thin sheet modelling and interpretation of Tasman Sea magnetotelluric data — G.S. Heinson and F.E.M. Lilley 88 A4.13 The Wigner-Ville analysis of magnetotelluric signals — IJ. Chant and L.M. Hastie 89 A4.14 Tectonic and sedimentary process in the Tonga-Kermadec Trench and adjacent terrains: a GLORIA long-range sidescan sonar survey — W. Mayer, C. Jacobs and L. Parson 91 A4.15 Onset of aridity and dune building in central Australia: palaeomagnetic evidence — X.Y. Chen and C.E. Barton 92 A4.16 Palaeozoic apparent polar wander path and palaeolatitudes of Gondwanaland — Z.X. Li, C. McA. Powell and P.W. Schmidt 93

A4.17 A4.18 A4.19 A4.20 A4.21 A4.22 A4.23 A4.24 A4.25 A4.26

Constraints on the Gondwanan apparent polar wander path provided by preliminary palaeomagnetic results from Ordovician rocks of the Amadeus Basin — G. Thrupp 95 The upper mantle structure under northern Australia — B.L.N. Kennett, J.R. Bowman and P. Cummins 96 The laboratory study of seismic wave attenuation — I. Jackson and M.S. Paterson 96 Risk implications of the early Tasmanian earthquake sequence — M.O. Michael-Leiba 97 The faulting process of the 1988 Tennant Creek, Northern Territory earthquakes from local and teleseismic observations — J.R. Bowman, G. Gibson, G. Choy, J. Dewey, T. Jones and B. Kennett 98 Accurately-located earthquakes near Cadoux, Western Australia — V.F. Dent 99 Method for gravity and aeromagnetic interpretation — D. Boyd 99 Tasmanian crustal features — D.E. Leaman and R.G. Richardson 100 A geophysical study of the southeastern part of the Murray Basin, New South Wales — I.R. Qureshi, R.A. Spencer, R.G. Cameron, E.D. Tyne and E. Scheibner 102 Three dimensional gravity modelling of the Devonian granite complex and its relationship to mineralisation in western Tasmania — D.L. Archer 104

Session A5: Exploration Models and Mineral Deposit Evaluation A5.1 A5.2 A5.3 A5.4 A5.5 A5.6 A5.7 A5.8 A5.9 A5.10 A5.11 A5.12 A5.13 A5.14 A5.15 A5.16 A5.17

Integration of genetic theories with exploration criteria to develop viable exploration models for VMS deposits — R. Large, R. Berry, D. Huston, B. Gemmell, J. Stolz and Khin Zaw 106 Metal and textural zonation in the Hellyer massive sulphide deposit — G.J. McArthur 108 Interpretations and exploration at Mt Morgan and Mt Lyell — G.O. Arnold 108 Exploration history and evaluation of the Henty gold prospect, western Tasmania — R.H. Roberts and M.J. Fleming 109 Low-grade Archean metavolcanics in the northern Gawler Craton: geochronology, mineral potential and regional implications — W. Cowley and M. Fanning 110 The use of trace element chemistry and electron paramagnetic resonance spectroscopy as a guide to mineralisation — J.C. van Moort 112 The Broken Hill-type deposits: their geological setting, mineralisation and exploration — R. Beeson 113 Recent developments in the evaluation of the Dugald River zinc/lead deposit, Qld — W.A. Sheppard and J.V. Main 113 Volcanogenic massive sulphide potential of the Eastern Goldfields Province, Yilgarn Block, WA — P.J. McGoldrick, C.P. Swager, A. Ahmat and P. Ruxton 115 The need to estimate recoverable resources under geological constraints — P. Carrasco and J. Angus 116 Geology applied to deposit evaluation and development - some examples — D.H. Mackenzie 116 Injecting geologic uncertainty into resource estimation: the Porgera gold deposit, Papua New Guinea — N. Schofield 117 Ore reserve calculations in shear zone hosted deposits — J.L. Baxter and M.G. Yates 118 Geological modelling in the evaluation of the Kintyre uranium deposit — R.L. Andrew and G.D.Price 119 The selctive use of isotopic data in the evaluation of some Precambrian metalliferous ore deposits — NJ. McNaughton, S.D. Golding and D.I. Groves 120 A revised zoning model for the Zeehan Ag-Pb-Zn-Sn field, Tasmania — J.A. Anderson 121 Integrated data bases and expert systems in exploration geology — J.HJ. Leach and P. Dahlhaus 121

Session A6: Mafic Magmatism and Associated Mineralisation A6.1 A6.2 A6.3 A6.4

Aspects of physical volcanology of komatiites and associated nickel mineralisation, Yilgarn Block, WA — R.E.T. Hill, M.J. Gole, S J. Barnes and S.E. Dowling 123 Archaean komatiite volcanism, ground melting and synvolcanic nickel mineralisation — K.M. Frost and D.I. Groves 124 Platinum metals in Western Australia - General occurrence and their mineralogy and recovery from the Kambalda nickel deposits — D.R. Hudson 125 Ordovician magmatism in the central Lachlan Fold Belt and precious metal potential — D. Wyborn and W. Cameron 126

xi A6.5

Petrography and geochemistry of the Namdee Intrusion, Western Australia: Implications for PGE mineralisation — P.A.H. Scowan, R.R. Keays and P.R. Hamlyn 127 Keynote Address 1: Ore genesis and exploration models for platinum-group element mineralisation in layered mafic/ultramafic intrusions — R.R. Keays, P.R. Hamlyn and S.J. Reeves 128 A6.6 The major, trace and platinum group element geochemistry of the Bucknalla Complex, central Queensland — S J. Reeves, R.R. Keays and P.R. Hamlyn 129 A6.7 The geochemistry and PGE potential of selected East Kimberley and West Pilbara mafic/ultramafic intrusives and related rocks — D.A. Wallace, D.M. Hoatson, S.S. Sun and R.R. Keays 130 A6.8 Mantle plume and melting of refractory mantle: implications for PGE and Au mineralisation — S.S. Sun 130 Keynote Address 2: A review of the evidence for the high temperature hydrothermal concentration of the platinum-group elements in layered intrusions — A. Boudreau 131 A6.9 Late- to post-magmatic PGE mineralisation in the Fifield platinum province and the Ownedale intrusive complex, NSW — L.M. Barron, E. Slansky, D. Suppel, Z. Johan and M. Ohnenstetter 132 A6.10 PGE mineralisation in zoned, "Alaskan" style ultramafic intrusions in the Fifield region, NSW, Australia — B.A. Brill and R.R. Keays 133 A6.ll Petrologic and oxygen and hydrogen isotope evidence for the origin of PGE mineralisation at Fifield, NSW — A.S. Andrew, B.J. Hensen and A.C. Dunlop 135 A6.12 Experimental calibration of the olivine-orthopyroxene-spinel oxygen sensor and application to PGE deposits — C. Ballhaus and D.H. Green 135 A6.13 The role of volatiles in the formation of platinum deposits — E. F. Stumpfl 137 A6.14 Platinum group elements in the Yarawindah body, Western Australia — M. Cornelius and B.S. Fleming 137 A6.15 Chromitite-hosted platinum-group mineral occurrences in the Heazlewood River mafic-ultramafic complex, Tasmania — D.C. Peck and R.R. Keays 138 A6.16 The Giles layered basic/ultrabasic complex, central Australia: structural and penological patterns and mineralisation potential — A.Y. Glikson, C.G. Ballhaus and T.C. Pharaoh 140 A6.17 The chlorine connection and a hydromagmatic model for the concentration of the platinum-group elements in the Stillwater Complex, Montana — A. Boudreau 141

Session A7: Precambrian Gold Deposits Keynote Address: The structural and tectonic setting of Precambrian gold deposits in Australia — M.A. Etheridge 143 A7.1 Alteration zones around mesodermal gold deposits: implications for patterns of deposition and fluid sources — A.C. Barnicoat 144 A7.2 The structural and host rock setting of primary mineralisation at the Boddington gold mine — P. Symons, G. Anderson, L. Hamilton, T. Beard, R. Staley, E. Roth and S. Ho 145 A7.3 An epigenetic origin for the Lancefield gold deposit — J.M.A. Hronsky, S.E. Ho, D.I. Groves, R.P.A. Perriam and J.R. Vearncombe 147 A7.4 Proterozoic gold deposits, the Witwatersrand goldfields, and modern gold placers of Nome (Alaska) — G.N. Phillips and D.J. Sumpter 148 A7.5 Archaean gold mineralisation during late vertical movements in the granite-greenstone belts of the Yilgam Block, WA — TJ. Cudahy 149 A7.6 Geological setting of primary gold-silver deposits, Mt Gibson, WA — G.R. Brabham , S. Coxhell, A. O'Shea and A.F. Ross 150 A7.7 Precambrian mesothermal gold deposits - pumps or valves? — J.R. Vearncombe and D.I. Groves 151 A7.8 The geologic setting of early Proterozoic gold mineralisation in southwest Ghana, West Africa — B.N. Eisenlohr and W. Hirdes 152 A7.9 A comparison of the stable isotope characteristics of primary Archaean and Proterozoic gold mineralisation in Australia — S.D. Golding, NJ. McNaughton and D.I. Groves 153 A7.10 A comparison between Archaean gold deposits and lower Proterozoic gold deposits (Pine Creek Geosyncline): controls on mineralisation — G.A. Partington 155 A7.ll Fluid inclusions and stable isotope studies of Au-quartz vein deposits in the Pine Creek geosyncline, Northern Territory — A.S. Wygralak and M. Ahmad 157 A7.12 Gold deposits of the lower Proterozoic Glengarry Group, WA — J. Windh and M.E. Barley 157 A7.13 The early Proterozoic Tom's Gully gold-silver deposit, NT — S. Sheppard, R.A. Crookes, D.I. Groves, P.G. Simpson and NJ. McNaughton 159

xii A7.14 A7.15 A7.16 A7.17

Stratigraphic and structural controls on Proterozoic ironstone mineralisation, Tennant Creek, Northern Territory — M.S. Rattenbury 160 The source of mineralising fluids at Tennant Creek — R. Wedekind and R. Large 161 Granites BDF hosted gold deposit, Northern Territory — T. Ireland and N. Bryce 162 Hydrothermally hematitised conglomerates - one ore-type of the Starra Au-Cu deposits (NW Qld) — G. Davidson 163

Session A8: Role of Structure in the Formation and Deformation of Ore Deposits A8.1 A8.2 A8.3 A8.4 A8.5 A8.6 A8.7 A8.8 A8.9 A8.10 A8.11 A8.12 A8.13 A8.14 A8.15

Shear zone hosted ore deposits — J.L. Baxter and N. Hewson 165 Structure of the Cowarra gold deposit near Bredbo NSW — M.J. Rickard and K. McQueen 166 Low angle shears in central Otago, New Zealand, their regional extent, economic significance and relation to Manorburn Folds — C.N. Winsor 167 The role of fault dynamics and fluid dynamics in the genesis of vein-hosted gold deposits in lowgrade metamorphic terranes — S.F. Cox 168 Genesis of the Deborah line gold deposits, Bendigo, Victoria: structural and timing relationships of gold precipitation and its possible magmatic origin — N. Green, T.A.P. Kawk, A. Changkakoti, J. Gray and H.R. Krouse 169 Structural geology of New Zealand epithermal gold — K.B. Sporli, M.R. Gadsby and D. Clarke 170 Deformation-induced fluid pressure variations: an explanation for high fluid/rock ratios during metamorphism and ore genesis — N.H.S. Oliver, R.K. Valenta and VJ. Wall 171 Structural setting of unconformity-related U-Au-platinoid mineralisation in the South Alligator Valley, NT — R.K. Valenta 173 Timing of syndeformation base and precious metal mineralisation controlled by deformation partitioning at Peak, Cobar, NSW — M. Hinman 174 Formation of a massive sulphide orebody by syn-deformational host rock replacement in a ductile shearzone, Rosebery - Tasmania — D.G.A.M. Aerden 174 D2 fault and shear zone development in the Mount Isa area — K. Connors 175 Deformational style and strain partitioning at the Hellyer volcanogenic massive sulphide deposit — C.G. Drown and R.C. Downs 176 Fluid-inclusion studies relating to a multiply deformed, metamorphosed, volcanic-associated massive sulphide orebody, Joma Mine, Norway — A.D. Giles and B. Marshall 177 The relationship of mineralisation to structure and stratigraphy at the Balcooma volcanogenic massive sulphide deposit, northern Queensland — D.L. Huston 178 Exploration for syntectonic massive sulphide deposits based on structural and microstructural analysis of drillcore — K.C. Lawrie 179

Session A9: Fault Zone Fabrics A9.1 A9.2 A9.3 A9.4 A9.5 A9.6 A9.7 A9.8 A9.9 A9.10

Direct observations of gouge development during brittle sliding — K. Haggert, S. Cox and M. Jessell 180 The mechanics of shear band development within fault gouge — CJ. Marone 181 Shear band development in experimentally deformed synthetic gypsum rock — R. PannozzoHeilbronner 182 Possible reaction weakening and local deformation in granitic rocks — H. Stunitz and J.Fitzgerald 183 A mechanical interpretation of rate and state dependent constitutive laws for rock friction B.E. Hobbs 184 Determination of strain from automatic image analysis of deformed objects — A. Ord and P.R. James 185 Pseudotachylite with igneous quench microstructures, eastern Musgrave Ranges, NT A. Camacho and R.H. Vernon 185 Strain compatibility of minor shear structures in shear zones — J.V. Smith and D.W. Durney 186 Deformation partitioning within external thrust zones — N. Woodward, S. Wojtal, J. Paul and Z. Zadins 187 Tectono-thermal evolution of an annealed ductile shear zone activated during the Alice Springs orogeny — B. Goscombe 188

xiii

Session A10: Gondwanaland Context of the Tasman Fold Belt Keynote Address: Gondwanaland context of the Tasman Fold Belt — Christopher McA. Powell 190 A10.1 Preliminary sequence stratigraphy of the Kanmantoo Group in South Australia — C.G. Gatehouse, J.B. Jago, D.I. Gravestock and B.J. Cooper 192 A10.2 Tasmanian Precambrian massifs and the tectonic development of the Tasman Fold Belt — C.G. Elliott, D.R. Gray and N.B. Woodward 193 A10.3 Tasman Fold Belt, Victoria, Australia: Implications for Gondwanaland tectonics — D.R. Gray, CJL. Fergusson and VJ. Morand 194 A10.4 The Tasman Orogen - A view from the craton — P.R. Evans and D.A. Remus 195 A10.5 Crustal features and deformation of basins in the Tasman Fold Belt of southeast Queensland — D.M. Finlayson and C.R. Fielding 196 A10.6 Metavolcanic suites from suspect late Palaeozoic terranes of the Gympie province, southeast Australia — W J. Sivell, C.M. Stocksiek and J.B. Waterhouse 197 Keynote Address 2: Terrane history of southern South America and Antarctica — V.A. Ramos 199 A10.7 Tectonics of the Palaeozoic Shoalwater and Wandilla Terranes, northern New England Orogen, Queensland — C.L. Fergusson, R.A. Henderson and E.C. Leitch 200 A10.8 Constraints on the timing and extent of major denudation episodes associated with Gondwana break-up: fission track evidence from southern Africa — R.W. Brown 200

Session A12: Magmas and Fluids in the Subduction Environment Keynote Address 1: The role of fluids in arc-related magmas in the SW Pacific: some news and reviews — M.Perfit 202 A12.1 The origin of geochemical variation in primary magmas of the Vanuatu Arc — S. Eggins 203 A12.2 The application of noble gas geochemistry to the genesis and evolution of arc magmas — D. Patterson 203 A12.3 Helium, strontium and neodymium isotopes in mantle xenoliths — J. Stone, D.R. Porcelli, D. Vance, S J. Galer and R.K. O'Nions 204 A12.4 Trace element and ^Sr/^Sr ratios in lavas of Slamet Volcano, Java: constraints on the mantle wedge and "slab-derived" components — D. Vukadinovic and I.A. Nicholls 205 A12.5 Systematic compositional variations in eastern Sunda magmatism: is sediment involvement really necessary? — R. Varne 206 A12.6 Evidence for a subcontinental mantle source for K-enrichment in Sunda Arc lavas from U series radionuclides — G.E. Wheller, R. Varne and A J. Stolz 207 A12.7 Along-strike variations in the Taiwan-Luzon Arc — U. Knittell, M. Defant and R. Maury 208 A12.8 Partial melting of sub-arc lithosphere: a source of potassic melts and granulite residues? — J. Foden 208 Keynote Address 2: Migration of fluid and generation of basalt magmas in subduction zones — Y. Tatsumi 209 A12.9 Boninite pedogenesis and shallow mantle wedge fluids — A J. Crawford 211 A12.10 Fluids as metasomatising agents and triggers of magmatism in subduction zones — RJ. Arculus 212 A12.ll Fluids in subduction zones: experimental constraints — D.H. Green 213 A12.12 Experimental evidence for an unusually hydrous mantle-derived andesite-dacite magma, Northland, New Zealand — T.H. Green and J. Adam 214 A12.13 Fluids and magmas in the Tilba Lake volcano, southeast NSW — A.G. Purvis 215 A12.14 Strontium and neodymium isotopic studies of the Mount Read Volcanics, Tasmania — DJ. Whitford, A.J. Crawford, MJ. Korsch and S.J. Craven 215 A12.15 The subduction zone as a geochemical filter: implications for mantle evolution and the genesis of OIB—J. Woodhead 216 A12.16 A subduction component in Karoo basalts — R J. Sweeney 217 A12.17 Geochemical and geodynamical constraints on subduction zone magmatism — M.T. McCulloch and J.A. Gamble 316 238

xiv

Session A15: Geology and Community Education — Initiatives for the 1990s A15.1 A15.2 A15.3 A15.4 A15.5 A15.6 A15.7 A15.8 A15.9 A15.10 A15.11 A15.12 A15.13 A15.14 A15.15 A15.16 A15.17

Geology in the Tasmanian curriculum — R.L. Bugg 218 Geology curriculum development for the Victorian Certificate of Education (VCE) — N. Schleiger and D. Klindworth 218 The role of geoscience in West Australian schools — K.K. Sappal 219 The use of secondary school results in predicting Tertiary study performance in applied geology at QIT — L.H. Hamilton 220 Geoscience education in the next century — B.A. Tapp and W.A. Peck 220 A review of the position of geology in the K-12 curricula of the Australian states — R J. Stutchbury and R.M. Carter 221 The "involvement" component in teaching earth sciences — G. Markovics 222 The sinking of the Port Phillip and Westernport Bay areas, southern Victoria: A student seismicity exercise based on the Heath Hill Fault — N. Green and G. Gibson 222 Making earth science trips successful — P.G.L. Harlow 223 Teaching geology as an exemplar of science — T. Sprod 223 Geological education and criterion based assessment — R. Pallett 224 Recommendations from an AMIC resources workshop: Applicability to the preparation of geological resource material — B. Cook and R. Stutchbury 224 Promoting geoscience in the secondary education environment — B.A. Tapp and W.A. Peck 225 Raising geological awareness through nature appreciation — G.W. Hofmann 225 Earth science in other disciplines — I. Hawkins 227 Science education requirements for an expanding world population — E. Brennan 227 A tertiary perspective on secondary geoscience education — G.R. Taylor 227

Session A16: Magmas, Gemstones and the Mantle A16.1

Early bimodal magmatism and magma mixing from Kangaroo Island in the southern Adelaide Foldbelt, South Australia — S. Liu, P.D. Fleming and C.M. Gray 229 A16.2 Mid-Proterozoic igneous rock suites of the central and eastern Gawler craton, South Australia — J.M. Scheffler and M.J. Abbott 230 A 16.3 Origin of the Proterozoic graphite deposits of the southern Eyre Peninsula, South Australia constraints from stable isotope geochemistry — W.R. Taylor and R.F. Berry 230 A16.4 The geological context of sapphire occurrences in the Anakie region, central Queensland — P.J. Stephenson 232 A16.5 Basement morphology, Permian and Jurassic teschenites of the Gunnedah Basin — D J. Martin and N.Z. Tadros 233 A16.6 The geology, geochemistry and tectonic implications of metamorphosed mafic igneous rocks in the Wonominta Block, NSW — Bo Zhou and KJ. Mills 235 A16.7 The lower crust beneath the eastern margin of the Australian craton: xenolith evidence for the gabbro to eclogite transition — N.J. Pearson, S.Y. O'Reilly and W.L. Griffin 237 A16.8 Trace element residence sites in metasomatised mantle: implications for basalt contamination — S.Y. O'Reilly and W.L. Griffin 237 A16.9 A study of mantle xenoliths from Mt Gowrie, southeastern Queensland — Y.D. Chen and S.Y. O'Reilly 238 A16.10 Ba partitioning and anorthoclase megacryst genesis — J.F. Guo, T.H. Green and S.Y. O'Reilly 239 A16.11 Time scales of heating, metasomatism and deformation in the upper mantle — W.L. Griffin, D. Smith, S.Y. O'Reilly and C.G. Ryan 240 A16.12 Ion probe U/Pb isotopic ages of gemmy zircons from eastern Australia, including Tasmania — F.L. Sutherland and P.D. Kinny 241 A16.13 Diamoniferous lamproites of Western Australia — A.L. Jaques 242 A16.14 Comparative garnet, pyroxene, chromite and Mg-ilmenite xenocryst compositions in selected kimberlitic sources and their relevance to diamond exploration — R.R. Ramsay and N.M.S. Rock 243 A16.15 Constraints on the composition of the continental lithospheric mantle — W.F. McDonough and K.P. Jochum 245 A16.16 The Pb isotopic composition of the lower crust: the role of magmatic underplating — R.L. Rudnick and S.L. Goldstein 245

Session A17: Antarctic Geology A17.1 A17.2

Suspect terranes of the central Transantarctic mobile belt — AJ. Rowell and M.N. Rees 247 History of geological investigations in MacRobertson Land and adjoining portions of the Australian Antarctic Territory — P.W. Crohn 249 A17.3 Vertebrate fossils from Marine Plain, Vestfold Hills, Antarctica — P.G. Quilty, R.E. Fordyce, C. Jones and N. Schroeder 249 Keynote Address: Antarctic Cenozoic glacial history — PJ. Barrett 250 A17.4 Daniels Range, Antarctica: the answer to Kanmantoo conundrums — R.L. Oliver 250 A17.5 Zircon ages and the distribution of Archaean and Proterozoic rocks in the Rauer Islands — P.D. Kinny and L.P. Black 251 A17.6 A revised chronology for the Vestfold Block based on ion-probe zircon ages — L.P. Black, P.D. Kinny and J.W. Sheraton 253 A17.7 Equilibria in granulite facies calcsilicates: implications for granulite facies metamorphism in East Antarctica — I.S. Buick, S.L. Harley and I.C.W. Fitzsimons 254 A17.8 High-grade metapelitic migmatites from Prydz Bay, east Antarctica: Proterozoic metamorphism and melting — I.C.W. Fitzsimons and S.L. Harley 254 A17.9 Two stage decompression in mafic garnet-bearing granulites from Sostrene Island, Prydz Bay, east Antarctica — D.E. Thost, B J. Hensen and Y. Motoyoshi 255

Session A18: Granites and Associated Mineralisation Keynote Address 1: A comparative petrogenetic study of granites from the Lachlan Fold Belt and the Canadian Appalachians — K.L. Currie and J.B. Whalen 257 A18.1 The Marulan Batholith: an atypical group of early Devonian granitoids from the eastern Lachlan Fold Belt — P.F. Carr, B.G. Jones and B.W. Chappell 258 A18.2 The Highlands Igneous Complex, Armidale: a high K20 and high MgO syenitic to granitic suite, the problem of its source magmas — J. Kilpatrick 258 A18.3 Hornfels and migmatite abutting the Boging Granite; infiltration at work — M J. Drummond, B J . Franklin and B. Marshall 259 A18.4 The structure petrology and geochemistry of the Scottsdale Batholith, NE Tasmania — M. McClenaghan 260 A18.5 Granites and tectonics — B.W. Chappell 260 A18.6 Petrology of Proterozoic igneous charnockites from Mawson, Antarctica: high-temperature synorogenic granitoids produced by anatexis in a thickened crust — D.N. Young and D.J. Ellis 261 A 18.7 Post-Delamerian magmatism - is lithospheric thinning guilty? — S.P. Turner and J.D. Foden 262 A18.8 The precursors of rapakivi ovoids — J. Elliston 263 Keynote Address 2: Granite emplacement and temporally associated Au mineralisation — V.J. Wall and J.R. Taylor 264 A18.9 The gabbro-qz monzodiorite-alkali granite association in southern NSW - implications for intrusive related Au mineralisation — R.J. Wormold and R.C. Price 265 A18.10 Laser-Raman microprobe studies of fluid inclusions and the definition of the "copper windows" in porphyry copper-gold mineralisation (Trad Prospect, NW Luzon, Philippines) — A.G. Trudu, T.P. Mernagh and M.S. Bloom 267 A18.ll Foley's Zone, Cleveland Sn-Cu-W-Mo deposit, Tasmania — P.G. Jackson, A. Changkakoti and J. Gray 268 A18.12 Geochemistry of progressive hydrothermal alteration associated with W-Mo-Bi mineralisation in the I-type Bamford Granite, North Queensland — P. Blevin 269 A18.13 A geological and thermodynamic investigation of the hydrothermal precipitation of ferberite and scheelite — S. Jaireth, C.A. Heinrich and M. Solomon 269 A18.14 The behaviour of tin in magmatic-hydrothermal systems — J. Taylor and V.J. Wall 270 A18.15 Fractionation of the Lottah granite, Blue Tier Batholith, NE Tasmania: implications for the origin of a tin-enriched peraluminous granite — D.E. MacKenzie 271 A18.17 Origin of alkali-feldspar granites: the Lottah Granite revisited — N.C. Higgins 273 A18.18 The Mt Bischoff Sn-deposit, Tasmania — J. Wright and T. Kwak 274 A19.19 Geochemistry of late Palaeozoic felsic I- and S-type tin granites of the Mt Surprise-Cooktown area, North Queensland — D.C. Champion, R.J. Bultitude and B.W. Chappell 275

xvi

Poster Session

Gold deposits in the Northern Territory — M. Ahmad and A.S. Wygralak 276 Holocene evolution of primary dolomite forming environment in the Kingston Lake, southeast South Australia: a statistical approach — R. Ahmad 276 Physicochemical properties and hydrogeochemistry of the Holocene dolomitic carbonate sediments of the Pellet Lake, Coorong region, South Australia: a model for primary dolomite formation — R. Ahmad and P.B. Hostetler 277 The structure of the Rosebery Mine sequence, western Tasmania — R.F. Berry 278 Orthopyroxene-rich ultramafic-mafic rocks from western Tasmania and their PGE contents — A.V.Brown 279 Fuild inclusion and isotopic characteristics of the Bendigo-Ballarat versus Melbourne Trough gold deposits — A. Changkakoti, Z. Gao, T.A.P. Kwak, J. Gray and H.R. Krouse 281 Magnetotelluric deep-sounding in the Clarence Moreton basin — I J. Chant and L.M. Hastie 281 Temporal geochemical variations in Ungaran volcano, central Java — R. Claproth and P Carr 283 Remote sensing techniques for iron ore exploration in the Ophthalmia Range area, Hemersley Basin, WA — T.J. Cudahy, A.R. Gabell and M. Pal 283 Chlorite geothermometry in low-temperature (diagenetic) investigations — P. de Caritat and J.L. Walshe 284 Tectonostratigraphic terrenes of the Lachlan Fold Belt, southeastern Australia — C.L. Fergusson 285 Tectonics of the Palaeozoic Shoalwater and Wandilla Terranes, northern New England orogen, Queensland — C.L. Fergusson, R.A. Henderson and E.C. Leitch 285 Regional structural controls on gold mineralisation in the Murchison Province, Yilgarn Block, Western Australia — M.W. Grigson, J.R. Vearncombe and D.I. Groves 286 Unusual very high P-T metapelites from the Rauer Group: relics of a 1000°C Archaean granulite metamorphism? — S.L. Harley 287 Low sinuosity channel patterns in the middle Triassic Hawkesbury Sandstone, Sydney Basin — B. G. Jones, A. Griffith and B. R. Rust 287 Cambrian sequence analysis from outcrops, well logs and seismic, Amadeus Basin: a new look at basin history — J.M. Kennard and J.F. Lindsay 288 Mineralogy, ore metal distribution and zonation at Bawdwin mine, northern Shan State, Myanmar (Burma): an Ag-rich volcanic-hosted, polymetallic massive sulphide deposit — Khin Zaw 289 A fluid inclusion study of Tennant Creek ironstones: implications for ore genesis and exploration — Khin Zaw, D.L. Huston, T. Mernagh and C. Hoffman 290 Field aspects of the Ardery chamikitic intrusions, Windmill Islands, Antarctica. A dynamic magma chamber — J .A. Kilpatrick, D.N. Young and D J. Ellis 290 Structural and microstructural timing criteria used to identify syntectonic base metal deposits: the Elura and Woodcutters deposits as examples — K.C. Lawrie 291 Hornblendes from the Musgrave Ranges, central Australia: metamorphic conditions, microstructures and Ar/ Ar ages — M.A.H. Maboko, J.D. Fitzgerald, D.J. Ellis, I. McDougall and P.K. Zeitler 292 Contrasting magma types, eruptive styles and mineral deposits of the Permo-Carboniferous Featherbed Volcanics, northeastern Queensland — D.E. Mackenzie 293 The footwall precious metal zone at Que River: transition from a VMS to an epithermal mineral assemblage — PJ. McGoldrick, R.R. Large and G.W. Jenkins 293 The Raman microprobe: a tool for analysing geological samples from the surface to the mantle — T.P. Mernagh 294 Isotopic evidence for the mixing of magmatic and sedimentary components in the formation of the Dachang tin-replacement deposits, China — Minlu Fu, A. Changkakoti, J. Gray, H.R. Krouse and T.A.P. Kwak 295 The geology, petrology and alteration geochemistry of the Magpie volcanogenic massive sulphide deposit, north Queensland, Australia — I.R. Mulholland 296 Modern floodplain sedimentation in the Murray basin - Barm ah Lakes and the Great Cumbung Swamp P.E. O'Brien, R.V. Burne and G. Bickford 297 Stratigraphy, palaeogeography and mineralisation in the lower Devonian Snowy River volcanics, eastern Victoria — K. Orth and R. Nott 297 Deltas: the missing grain size component — G J. Orton and H.G. Reading 299 Mafic dyke swarms of southern Australia — A J. Parker 300 Middle Cambrian sequence stratigraphy, Georgina basin: implications for phosphate and oil — J.H. Shergold and P.N. Southgate 300 40

xvii Permian palaeogeography of Australia, BMR-APIRA palaeogeographic maps project — J.M. Totterdell and A.T. Brakel 301 Volcanic rocks from the western Woodlark Basin, Papua New Guinea, a re-activated subduction environment? — GJE. Wheller, R.A. Binns, DJ. Whitford, R.L. Chase and PJ. Michael 302 Geochemistry of the host rocks to the Scuddles volcanogenic massive sulphide deposit, Western Australia: potential of lithogeochemistry in exploration — D J . Whitford and P.M. Ashley 303 Alkalia granites from the mid and late Palaeozoic of southern NSW - petrogenetic and tectonic implications — RJ. Wormald, R.C. Price and C.M. Gray 304

R.L. Stanton Symposium: New Frontiers in Ore Deposit and Exploration Studies The role of research and development in successful exploration — S.M. Richards 305 The search for new Witwatersrand goldfields — D.A. Pretorius 305 Tin granites: their evolution from fertile sediments by partial melting and fractional crystallisation — B.W. Chappell and AJ.R. White 307 Porphyry copper deposits — G.H. Brimhall and K. Danti 308 Skarn deposits —M. Einaudi Unconventional thinking and earth Science — R.W. Hutchinson 310 Epithermal deposits: styles, analogues and exploration — R.W. Henley 311 Archean gold deposits — D.I. Groves 312 Sediment-hosted stratiform lead-zinc deposits — N. Williams 313 Massive sulphide deposits of the modern ocean floor — S.D. Scott 314

Sir Douglas Mawson Lecture

ANTARCTICA, HOMO SAPIENS TO GREENHOUSE: AGENTS IN PLANETARY REVOLUTION J.M. Bowler Museum of Victoria, Melbourne

The Earth Sciences, like every other intellectual discipline, reflect the tensions and aspirations of the society of its time. As we approach the end of the 20th Century, momentous changes are taking place in economic, social and political systems of the world. To pretend these have no impact in the Earth Sciences is to ignore the long-established reality in which the universities and other learned institutions both reflect and, in turn, influence the patterns of social, economic and political change. In attempting to assess where we stand, it is appropriate to reflect on the pathway that has led us to the present position. It is almost impossible, today, to conceive of the Earth without life. It is equally difficult to understand the magnitude and epic nature of events of just the last few million years in this planet's history. Approaching the 21st Century, well may we ask if our custodianship of this fragile earth has been based on adequate understanding of our origins, dependence on and relationships with Nature. A realistic vision of those origins and relationships, based on understanding of the geological processes, is essential to intelligent management of present resources and future planning. The role of Earth Sciences in Australia, as elsewhere in the World, is subject to change. In the major conflict emerging between conservation and development, a new sense of global consciousness is emerging. Many of those long-established values which often provided

the guiding philosophy of Earth Science, schools traditionally allied with and dependent on mining and extractive industries, are being called into question. The emerging crisis is nowhere more spectacularly evident than in Antarctica. It is no accident that Antarctica, the huge thermal buffer of the Earth's climate, was also that certain catalyst instrumental in accelerating those dramatic biological events in Africa that led to the emergence of Homo sapiens. In that process, the power of consciousness that emerged, has only in the past few thousand years exploded, sweeping across the face of the Earth. Environments that began in Miocene and Pliocene time, in a certain sense, reached their climax with the industrial revolution a mere century ago. Those events have evolved into one of the most powerful agents of change, devastatingly effecting the present, and certainly threatening the future survival of the planet. An entirely new geological force has emerged in which we, the Earth Scientists of Australia, are no longer objective observers carrying out logical, quantitative analysis, but we participate as very agents of change ourselves. In the context of these epic events, Earth Science has an immense responsibility. It alone provides the large picture so basic to human understanding of our relationships, not only with Nature but equally with each other. That is a picture we must explore and advertise.

International Geological Correlation Program Lecture PROTEROZOIC PETROLEUM M.R. Walter Private Consultant, Northbridge, NSW

IGCP Project 157, "Early Organic Evolution and Mineral and Energy Resources", was initiated in Australia and approved in 1977. The aim was "to evaluate organic geochemical, isotopic and palaeontological data in terms of the evolution of life and its relationship to episodic formation of Precambrian and lower Palaeozoic mineral deposits, particularly those of base metals and hydrocarbons". It developed from the program of the Baas Becking Geobiological Laboratory and soon established close links with other interdisciplinary organisations such as the International Symposia on Environmental Geochemistry and the Precambrian Paleobiology Research Group. Over its 11-year life it was responsible for organising, sponsoring or inspiring numerous symposia and workshops, and the results of the project are presented in proceedings volumes and special issues of journals, especially those from the Fourth International Symposium on Environmental Geochemistry (Canberra, 1979), the Dahlem Workshop on Mineral Deposits and the Evolution of the Biosphere (Berlin, 1980), Development and Interactions of the Precambrian Atmosphere, Lithosphere and Biosphere (Mexico City, 1982, jointly with IGCP 160), Precambrian Paleopedology (Raleigh, USA,1985) and the concluding meeting of the project, Early Organic Evolution: Implications for Mineral and Energy Resources (Maria Laach, West Germany,1988). Activities were focussed on four sub-projects: Precambrian weathering horizons/paleosols; Organic matter in Precambrian and early Palaeozoic sediments (with special reference to pre-Devonian oils); Antiquity and metallogenetic potential of bacterial sulphate reduction; and Fossil microbial ecosystems of the "stromatolitic" type and their modern analogues. This paper will concentrate on the second of these, on early petroleum, but data will also drawfromresultsfromthe other volumes. It is impossible to separate the results of the project from those of various individuals and the other organisations mentioned above, and it is the whole picture which will be presented, but there is no doubt that the picture would be much cruder were it not for the achievements of Project 157.

A major period of plate rifting commenced about one billion years ago and led to the formation of narrow depositional basins comparable with those of the Cretaceous proto-Atlantic and Tethyan regions. Within the Cretaceous basins organic-rich sediments were deposited as a result of limited aeration of deep waters. These are the source rocks of some of the giant petroleum fields of the Middle East and elsewhere. The climatic, tectonic and paleogeographic conditions that obtained in the later Proterozoic and Cambrian in Australia and elsewhere seem to have been comparable with those of the Cretaceous. Evidence in support of this comes from many sources, including carbon and sulphur isotope geochemistry. These conditions would have been reinforced by high atmospheric concentrations of C0 coupled with low concentrations of 0 which would have stimulated productivity and decreased oxidative degradation of organic matter. Furthermore, during the Proterozoic, there was no significant bioturbation, due to the lack of burrowing metazoans. Some small horizontal burrows occur in the very latest Proterozoic, but deep vertical burrows indicative of thorough bioturbation arefirstfound in earliest Cambrian rocks. As a result, sediments would not have been irrigated with oxygenated water even in situations where the overlying water body was oxygenated. Vast tracts of benthic microbial mats would have added to the planktonic and detrital sources of organic matter usual in the Phanerozoic. Conditions would seem to have been ideal for the formation of unusually rich petroleum source rocks. At the same time, potential reservoirs were formed as extensive well sorted sand bodies, oolid grainstones and abundant stromatolite reefs, bioherms and biostnomes. Evaporites occur at several stratigraphic levels and form potential cap sequences. Petroleum is being producedfromsuch sequences in China, Siberia and the Sultanate of Oman, and Proterozoic plays are being explored in Australia, Brazil, the USA and probably elsewhere. A potentially commercial gasfieldhas been discovered in the Amadeus Basin. Rich source rocks 1.7-1.5Ga old occur in the McArthur Basin; one occurrence is accompanied by live oil. In large areas of 2

4 the basin the source rocks are immature to mature. Both marine and lacustrine facies are present. Organic carbon contents range up to 7% with organic-rich intervals up to 200m thick. An active exploration program is underway in this basin. Proterozoic l^erogens and oils have unique geochemical characteristics; this not only has obvious exploration significance in allowing oil-source correlations but also represents a major, almost unexplored source of palaeobiological information. For instance, the biomarker record has already indicated

that the history of eucaryotic cells probably extends back to 1.7 Ga, and it is revealing something of the history of a whole kingdom of organisms, the Archaebacteria, for which there is no morphological record. IGCP157 can not take credit for all of these insights but it certainly played an important role. It facilitated the rapid worldwide exchange of information, ideas and samples and helped to turn scattered efforts into a coherent, cooperative approach.

Al: Volcanic Environments and Associated Mineralisation Convenors: GJt. Green andR. Cas

Al.l ENVIRONMENT AND MECHANISM OF FORMATION OF THE WOODLAWN MASSIVE SULPHIDE DEPOSIT, NSW W.J. McKay1 and J. L. Walshe2 2

1 Bureau Mineral Resources, Canberra Australian National University, Canberra

The Woodlawn base metal deposit is situated 50 km northeast of Canberra in Late Silurian volcanic rocks of the Lachlan Fold Belt of New South Wales. The massive sulphide resource (part of which has been mined) is in excess of 10 million tonnes of 15-20% combined PbZn, >1.5% Cu and >80 g/t Ag. Stringer-copper ore is in excess of 3 million tonnes of 1.5% Cu . The volcanic hosted sulphide deposits in the CoomaGoulburn region, including the Captains Rat deposit, appear to have formed in a shallow, meridonal marine basin bounded on the east and west by terrestrial volcanic sequences. The Woodlawn deposit formed in a linear sub-basin framed by growth-faults, which are subparallel to a northwest trending lineament (the Woodlawn corridor). The orebody has been faulted, folded and metamorphosed to lower greenschist facies conditions. However, because the deformation is heterogeneous, it has been possible to reconstruct in some detail the original morphology of the massive sulphide lens. The greatest thickness of sulphides accumulated in a 25 m deep channel in the sea-floor. Although deformed in the lower horizons, the ore in this structure is strongly zoned with three cycles of sphalerite-galena ore overlain by pyrite-chalcopyrite ore. In the open-cut mounds of chalcopyrite-rich ore that formed above a feeder zone were overlain by sphalerite-galena-rich ore. Fine scale banding of pyrite and sphalerite-galena

occurs in the upper few metres of the massive sulphide lens. Mineral compositions are strongly zoned across the orebody with chlorite, talc, phlogopite and sphalerite becoming iron poor from footwall to hangingwall. The location of the sulphides within a sub-basin, the thick accumulation of sulphides in a seafloor channel, and the metal-zonation in this channel are features that suggest the sulphides accumulated in a saline brine pool. The sulphide-silicate mineralogy is thought to reflect diagenesis in the bottom of the brine-pool and has been used to infer bottom conditions in the pool through time. Bottom temperatures in the brine-pool were initially high (120-150°C in most parts of the pool) and were above 60-80°C for much of the life of the pool but dropped sharply towards the end of hydrothermal activity. Maximum fluid salinities are inferred to have been of the order of 20-25 wt% NaCl in the seafloor channel. Elsewhere in the brine pool, salinities are inferred to have been less than about 10-15 wt% NaCl. The sphalerite-galena/chalcopyrite zonation in the seafloor channel has been interpretated in terms of fluctuating brine temperatures in this structure. At times of high temperature (>200-250°C) only chalcopyrite was precipitated. At other times cooler brines flowing into this structure were depleted in copper and only sphalerite and galena were precipitated.

A1.2 THE FORMATION OF ROSEBERY-TYPE, VOLCANOGENIC MASSIVE SULPfflDE DEPOSITS M. Solomon , J. L. Walshe and C. A. Heinrich 1

Bureau of Mineral Resources, Canberra Deptartmen of Geology, Australian National University 1

From the diversity of shapes, sizes and compositions source fluids in zinc. Gold may precipitate in the footwall among volcanic-hosted massive sulphide deposits it is by simple cooling above 300°C and in the hanging wall possible to separate out the Rosebery types (Solomon & by cooling and/or decomplexation from solutions at Walshe, 1979). The main representatives are in eastern lower temperature and higher f0 . The oxidation Australia (Green et al., 1981; McKay & Hazeldene, potential of the solutions is likely to be buffered by 1987) and in the Bathurst camp, New Brunswick. reactions between pre-ore rock and ore fluid, with the Rosebery types are larger than most VMS deposits, are presence of carbon-bearing sediments an important factor sheet-like in form, have high Zn+Pb/Cu values, and are in the early stages. The high arsenic content of some commonly hosted by sediments or sedimented tuffs; ores may result from the presence of sediments in the basins are recognisable at Rosebery and Woodlawn. pre-ore rock sequence. As the efficiency of seawater Sediments are generally common in the pre-ore sulphate reduction is lowered by exhaustion of Fe in sequence. Many are well banded, either in sulphides or the rocks, so the fluids remain oxidised and deposit sulphides alternating with silicates, though single bands barite in the later stages (Green, 1983). The relatively are only continuous at the metre scale. Cryptic cyclicity low acidity indicated by some alteration zones in felsic is present locally. The early sulphide assemblage terrains may result from the buffering effect during includes arsenopyrite and/or pyrrhotite, indicating subsurface fluid circulation of the relatively high feldspar precipitation from relatively reduced solutions, and the content of felsic compared to mafic volcanics. The silicate assemblage points to slightly acid to neutral formation of pyrite-chalcopyrite assemblages before conditions. Sulphide 8 S values are not in equilibrium. pyrite-sphalerite-galena requires high temperature (at Gold is largely in the lead-zinc hanging wall though in a least initially), salinity > 4 m Na , mixing only with few cases it also occurs in the footwall (Huston & brine pool fluid, and removal of the cooler, lead-zincbearing fraction from the basin during vent activity. Large, 1988; McKay, 1989). Forming the Stockworks — Fracturing and Physical Aspects of Ore Formation — The form, large size and evidence of basins indicates derivation stockwork formation in the footwall precedes hydrothermal alteration. Fracturing occurs when P > P, from solutions that are sufficiently saline to be negatively buoyant after mixing with ambient water, i.e. > 1.15 m + T, suggesting that fluid pressures at the base of the NaCl (McDougall, 1984, but see below). Salinities up stockworks in the early stages must exceed hydrostatic to 11.6 wL% NaCl have been found in fluid inclusions pressures by at least 5 MPa. On fracturing P falls but from Woodlawn. Small-scale layering may result from the ore fluid probably reaches the basin floor as a jet differences in particle settling rates due to variations in rather than a plume; later P may be closer to hydrostatic. grain size, density and nucleation behaviour. In addition, Lack of stockwork development indicates less vigorous plumes showing buoyancy reversal may pulsate (Turner, sytems, perhaps resulting from higher salinities of the 1966) and hence contribute to the fine layering of ore fluids (P a gph). In such systems lateral flow in sulphides and silicates. Experimental data indicate the permeable footwall rocks is more likely. periodicity of buoyancy reversal would be, say, 102 References seconds with influx of, say, 10 kg of metal at each Green, G. R., 1983. Unpubl. Ph D thesis, University of Tasmania. pulse. Chemical Aspects of Ore Formation — The large Green, G. R., Solomon, M. & Walshe, J. L., 1981. Econ. Geol. 76: 304-338. individual sizes compared to Kuroko- and Cyprus-type D. L. & Large, R. R., 1988. Econ. Geol. 83: deposits may result from trapping of most of the metal Huston, 1181-1192. content of the solutions (compared to a loss of 80% in McDougall, T. J., 1984. Deep-sea Research 31: 145-170. black smokers). Thus the metal content of the ore may W. J. & Hazeldene, R. K., 1987. Econ. Geol. 82: be a guide to metal content of the ore solutions, provided McKay, 141-164. m ^ > m ^ ^ The high 8 S values in Lower Palaeozoic McKay, W. J., 1989. Ph. D. thesis, ANU. examples confirm the importance of seawater as the Solomon, M. & Walshe, J. L., 1979. Econ. Geol. 74:797-813. sulphur source. Preliminary modelling indicates that to Turner, J. S., 1966. Jour. Fluid Mech. 26: 779-792. produce the observed metal ratios by fluid-buffered cooling from 350°C requires marked undersaturation of 2

A1J GEOCHEMISTRY AND ALTERATION OF THE HANGINGWALL BASALTS TO THE HELLYER VOLC ANOGENIC MASSIVE SULPIDE DEPOSIT, TASMANIA DJ. Jack Chase Minerals, Harare, Zimbabwe; formerly Aberfoyle, Burnie

The Hellyer hangingwall basalt contains a core lava directly above the massive sulphide with regionally high Ti/Zr > 54, higher primary MgO, Ni, Cr, and lower Y, Zr, La, and Nb than the surrounding basalt The hydrothermal system and this lava extrusion were localised by the same deep sourced structure. The basalt has been hydrothermally altered to a bright emerald green colour.This is caused by pervasive calcite-fuchsite alteration. There is accompanying hydrothermal fracturing, calcite veining and increased pyrite content Albite alteration extends out from this zone. Clearly, the basalt extruded while the hydrothermal system was still active.

Element distributions on a cross section across Hellyer show increased S (two to four times background), increased CaO (two times background), and elevated Ba in this hangingwall alteration plume. Zr, Ti0 , Y, Nb, and the rare earths remain immobile as evidenced by unchanged element ratios regardless of the degree of alteration. Mass additions and losses of mobile elements in altered rocks can be calculated by comparison with petrographically unaltered basalt with the same primary ratios. Sulphur isotopic signatures are produced from reduction of seawater sulphate and mixing with hydrothermal sulphur. 2

p 600RL

500RL

300RL-

Pre-Jack Fault cross section of Hellyer on 10900N. A lava with regionally high TiZr > 54 occurs above the Hellyer massive sulphide. The hangingwall basalt is altered to calcite-fuchsite-pyrite especially directly above the stringer zone core.

A1.4 FORMATION OF THE ALTERATION PIPE AND STRINGER ZONE BENEATH THE HELLYER MASSIVE SULPHIDE DEPOSIT, TASMANIA J.B. Gemmell *, R.R. Large , GJ. McArthur , C.G. Drown and R.C. Downs 1

Key Centre for Ore Deposit and Exploration Studies, University of Tasmania Aberfoyle Resources Ltd, Burnie, Tasmania 2

Hellyer is a large (15 million tonnes), high grade to zones of chlorite, chlorite-carbonate, sericite-chlorite, (13.1 % Zn, 6.7 % Pb, 0.3 % Cu, 160 g/t Ag, 2.3 g/t Au) and finally sericite-quartz on the margins (stringer volcanogenic massive sulphide deposit located in the envelope zone). Pyrite is a ubiquitous constituent of Cambrian Mt Read Volcanics of western Tasmania. each alteration shell. However, the alteration morphology Beneath the deposit a very well developed and preserved and zonation is not perfectly symmetrical around the hydrothermal feeder system cross cuts a thick sequence central siliceous core; eastward of the siliceous core the of massive to fragmental mafic volcanics. Footwall alteration zones are steeply dipping and terminate alteration occurs along the entire length of the deposit abruptly under the massive sulphide, while westward in an elliptical shape with dimensions of approximately they extend out under the ore body in a blanket fashion. Whole rock geochemistry indicates that most of the 1000 by 350 m and is known to a depth of 600 m below chemical variations in the alteration system reflect the the orebody. As the Hellyer hydrothermal system initiated, alteration mineralogy rather than the original lithology. pervasive alteration of the footwall mafic volcanics Distinct enrichments and depletions in major oxide and occurred. Mineralogical zoning exists within the footwall minor element geochemistry, compared to the unaltered alteration pipe, with a central siliceous core giving way footwall andesite, occur in the various alteration shells. ALTERATION ZONE

Si0

Ti0

+ Siliceous core Chlorite Chlorite-carbonate Sericite-chlorite Sericite-quartz envelope +

A1A

Fe total

MgO MnO CaO Na/) K.O

+ + + +

+ +

0 0

(+)

+ + +

+ + + + +

0 (+) (+) + = increase relative to host-rock composition; - = decrease relative to host-rock composition; o = no obvious change; + and - in brackets imply minor changes only. -

The "Alteration Index" (K 0+Mg0/K 0+Na 0+ CaO+MgO) value becomes progressively larger with increasing intensity of alteration, from the unaltered andesite (AI = 36) to the siliceous core (AI = 91). Fluids responsible for the massive sulphide mineralisation flowed through a series of stringer veins that closely post-date the pervasive alteration of the host mafic volcanics. The siliceous core of the stringer zone contains the greatest frequency of veining, with only weakly developed pyrite and minor lead-zinc veining in the chlorite, chlorite-carbonate, sericitechlorite, and sericite-quartz alteration shells. Eight stages of veining have been distinguished in the siliceous core, these include one stage of pre-mineralisation (Stage 1), three stages of syn-mineralisation (Stage 2), and four stages of post-mineralisation veining (Stages 3-6) (see table opposite). 2

205

+ + +

An analysis of metal zoning in the stringer zone indicates the presence, and alignment, of three distinct feeder systems within the alteration pipe. The largest and main feeder is located under the central portion of the deposit, with a second smaller feeder system to the south, and a weakly developed third feeder system under the far northern portion of the ore body. Structural trends of the Stage 2 veining parallel the alignment of the feeder systems. Distribution of metal values in the stringer zone surrounding the feeder systems is related to the zonation of elements in response to changing physical and chemical conditions of the hydrothermal solutions. Iron and copper are concentrated in the cores of the feeders with zinc and lead, silver, gold and arsenic, and barium becoming increasingly dispersed within the stringer zone away from the centres of hydrothermal activity.

Table 1 Characteristics mineralogy and paragenesis for each vein stage VEIN STAGE

MINERALOGY*

PARAGENESIS

Qtz

Only Qtz

Qtz, Py, Co Minor Gn, Sp, Ba, Cp, Ch

Crustiform Banding Py -> Qtz ± Ch -> Py ± Sus -> Late Co

Cp, Gn, Qtz, Py, Co, Sp Minor Ch, Ba

Mix of Sus + Qtz, -> Late Co ± Ba

Ba, Py, Co Rare Sp, Cp, Qtz

Coarse-grained Ba with mix of Sus, Co, ± Qtz

Qtz, Sp Rare Gn, Cp, Co

Qtz + Sp with rare disseminated Gn ± Cp

Qtz, Co, Ch, Rare Gn, Cp, Py

Coarse-grained tension-gash fill

Co RareQtz

Massive Co with rare Qtz

Qtz, Co, Ch Minor Py

Mix of Qtz > Co » Ch±Py

* Minerals listed in relative abundance. Qtz = Quartz, Co = Carbonate, Ch = Chlorite, Py = Pyrite, Sp = Sphalerite, Gn = Galena, Cp = Chalcopyrite, Ba = Barite, Sus = Sulphides (Gn±Cp±Sp) A1.5 THE GEOLOGY AND MINERALISATION AT MT CHARTER Sven W. Rand Aberfoyle Resources Limited, Burnie

The Mount Charter prospect comprises a small outcrop of massive barite on the contact between footwall rhyodacitic to dacitic lavas, lava breccias and volcaniclastics and hangingwall andesitic lavas and lava breccias. A limited polymict volcaniclastic unit occurs at the contact between the footwall and hangingwall rocks. These rocks are part of the Cambrian Que-Hellyer Volcanics which host the Que River and Hellyer volcanogenic massive sulphide ore deposits. The Mt. Charter prospect does not conform to a typical volcanogenic style of mineralisation. The mineralisation occurs as disseminations and stringerlike veins within the footwall rocks, with massive barite occurring as pods and veins, replacing the volcaniclastic horizon above the stringer system. Barite also occurs as veins extending into the hangingwall andesite. Hydrothermal alteration associated with the stringer system is strong sericite, pyrite and silica alteration in

the immediate footwall rocks. Away from the main stringer zone, alteration comprises moderate sericitepyrite alteration and in places chlorite and/or carbonate alteration. Alteration visible in the hangingwall andesitic rocks comprises sericite-pyrite alteration, weakening in intensity away from the contact Mineralisation associated with the most intensely hydrothermally altered rocks occurs predominantly as veins of pyrite and barite associated with anomalous concentrations of base-metals, precious-metals and arsenic. Ore minerals include pyrite, sphalerite, tennantite, galena and minor chalcopyrite. Trace minerals include gold as electrum, tetrahedrite and proustite. Sulphur isotope and fluid inclusion studies indicate that the mineralising fluids were low to moderate temperature (<«250°Q and comprised a significant component of seawater.

A1.6 GEOLOGY AND GEOCHEMISTRY OF THE PRECIOUS METAL-RICH SOUTH HERCULES VOLCANOGENIC SULPHIDE DEPOSIT, WESTERN TASMANIA Khin Zaw , R. R. Large and S. Hunns 1

Key Centre for Ore Deposit and Exploration Studies, University of Tasmania Pasminco, Rosebery, Tasmania 2

The South Hercules deposit is a newly discovered, I. Mineralised sulphide zone (variable gold and silver) 3. Massive pyrite+barite sub-zone disseminated to semi-massive base metal sulphide zone 2. Siliceous+stringer sulphide sub-zone with significant gold and silver grades which lies about 1. Sphalerite-galena+pyrite sub-zone a kilometer along strike south from the Hercules Mine, a major volcanogenic, polymetallic massive sulphide II. Carbonate altered zone (no gold or silver) 3. Blebby carbonate sub-zone deposit in the Mt Read Volcanic Arc, western Tasmania. 2. Massive carbonate sub-zone The mineralisation and alteration at the South Hercules 1. Cherty carbonate sub-zone deposit has been divided into the following zones (see Fig. 1):

Blebby carbonate subzone Q

Massive carbonate subzone Cherty carbonate subzone

Massive pyrite t barite subzone

| Siliceous ± stringery sulphide subzone

5630mN SOUTH HERCULES

f H Sphalerite - galena ± pyrite subzone

Fig. 1. Geology and mineral zonation of 5630mN and 5670mN sections, South Hercules deposit, western Tasmania.

The carbonate zones conspicuously lack precious metals with very little base metals grades. The massive pyrite+barite sub-zone contains significant Au grades and mostly occurs at the top of the sulphide lens (HI 142-2 @ 43.0 m, 22 g/t Au, 5600mN). This subzone is characterised by colloform aggregates of pyrite to compact massive pyrite with intermixed chert The siliceous+stringer sulphide sub-zone is essentially a silicified and sericitised tuff with disseminated to stringer Pb-Zn mineralisation. This sub-zone varies in texture from a siliceous tuff with disseminated sulphides to sericitic tuff with elongated blotches or blebs of sphalerite rimmed by sericite. The massive sphaleritegalena+pyrite sub-zone occurs as thin lenses. This zone is sometimes spotty in texture and grades into the siliceous + stringer sulphide sub-zone. It mostly occurs below the carbonate and massive pyrite + barite subzones and it may contain a subordinate amount of gold and silver. Microprobe analysis of sphalerite grains from the South Hercules deposit gave a range of 2.4 to 10.3 mole % FeS whereas the arsenopyrite yielded a range of 29.8-30.0 atomic As (%). 5 O isotopic composition of ls

the hydrothermal carbonates varies from +9.8 to +16.7 o/oo with a mean value of+12.9%o whereas 8 C values of these carbonates ranges from -3.5 to +0.6%o with a mean value of -2.1 %o. The present continuing fluid inclusion study indicates that two compositionally distinguishable types of fluids were involved during the formation of the South Hercules deposit; one with lowtemperature (200°C) and low-salinity (<5 NaCl equiv. wt %) and the other with high-temperature (up to 300°C), higher salinity (up to 20 NaCl equiv. wt %) and C0 rich. Thefirstlow-temperature, low-salinity fluids with no appreciable C0 are likely to be Cambrian fluids of exhalative origin. The second, high-temperature, highsalinity and C0 -bearing fluids were possiblyrelatedto Devonian deformation and metamorphism of the ores. Mineragraphic investigation on the Au mineralogy at the South Hercules deposit indicates that gold occurs essentially as electrum and the following associations of Au distribution have been recorded: (1) Individual electrum grains locked in the pyrite (2) Electrum grains distributed in the remobilised and recrystallised elongated sphalerite blebs; (a) as individual coarse-grained electrum grains in the recrystallised 13

sphalerite and (b) electrum as patches and minute blebs in the cracks of the recrystallised sphalerite (3) Electrum with galena within the recrystallised pyrite, and (4) Electrum at the margin of or within tetrahedrite grains The gold locked in the pyrite euhedra is generally finer in grain size (<5 pm to 40 |im) with a maximum mode of 16pm whereas the gold associated with other sulphides displays a range of about 6.3 jim to more than 180 |im. The grain size distribution of the gold associated with sulphides other than pyrite display a bimodal distribution with modes at 16-25 |im and 160 |im. Microprobe analysis indicates that the fineness (lOOOAu/Au+Ag wt %) for gold in the whole South Hercules deposit ranges from 427 to 965. The fineness of the gold in the pyrite euhedra varies from 616 to 966 whereas the fineness of the gold associated with the other sulphides (galena, sphalerite, tetrahedrite) ranges from 428 to 880. The fineness distribution of gold locked in the pyrite has a maximum mode of700 whereas the gold associated with other sulphides displays a bimodal distribution with modes of 550 and 700. The bimodal distribution of grain size and fineness of the gold suggests that the grains have suffered post-

depositional changes possibly related to Devonian recrystallisation. Similar grain size and gold fineness distribution have been recorded at the north-end and south-end of the Rosebery mine (Huston & Large, 1988; Khin Zaw et al., 1988) where the fine-grained gold of Cambrian, exhalative origin has been remobilised and recrystallised as coarser gold grains with higher fineness values during Devonian metamophic and metasomatic processes. The form, texture, mineralogy and fluid inclusion data on the South Hercules deposit suggest that it was formed largely by subseafloor replacement with minor seafloor exhalation, from relatively low temperature (150-200°C), neutral pH fluids. The very low copper content and relatively high Au+Ag/Pb+Zn ratio of the ores compared to Rosebery and Hercules support a low temperature environment in which gold was transported as the Au(HS)-2 complex while Pb and Zn were transported as chloride complexes. References Huston, D. L. & Large, R. R., 1988. Econ. Geol. 83: 1181-1192. Khin Zaw, Huston, D. L. & Large, R. R., 1988. Unpubl. report to Electrolytic Zinc Company of Australia: 110 pp.

Al Keynote Address: VOLCANOGENIC MASSIVE SULPHIDE DEPOSITS, ANCIENT AND MODERN J.M. Franklin Geological Survey of Canada, Mineral Deposits Subdivision, Ottawa, Canada

The study of volcanogenic massive sulphide deposits has focussed on five aspects: (1) the zone of generation of metalliferous fluid; (2) the zone of fluid ascent; (3) the immediate sub-seafloor environment; (4) the precipitation environment; and (5) the near- and farfield areas of the seafloor and water column surrounding the vent sites. New information on most of these has been gained from recent Geological Survey of Canada research on active high-temperature seafloor hydrothermal systems. The high-temperature reaction zone is well exposed about 1 to 2 km stratigraphically below VMS deposits in Cyprus, Oman and in the Snow Lake, Manitoba and Noranda and Mattagami Lake, Quebec districts. HighT interactions between modified, Mg-deficient seawater and dykes and apophyses extending from large subvolcanic magma chambers, have formed lower semiconformable alteration zones delineated by extensive silicification, epidotisation and albitisation. About 90% of the original copper and zinc contents of these fossil interaction zones entered a hydrothermal fluid, under low water/rock ratio conditions in basalt, at about 385°C (Seyfried & Janecky, 1985). The most

intense alteration is along the margins of feeder dykes and sills to the volcanic rocks which host the deposits. Highly efficient cooling of the subvolcanic magma chambers has resulted in unusually intensive magmatic fractionation. Some felsic intrusions have a carapace of mafic rocks that formed through disequilibrium fractional crystallization. The residual felsic melt, together with associated felsic volcanic products, have distinctive REE patterns with well-defined negative Eu anomalies (Campbell et al., 1982). In the basaltic sequence associated with the Galapagos sulphide deposits, anomalously abundant andesite is also the result of excessive cooling and fractionation of a highlevel magma chamber by convective heat transfer into a hydrothermal fluid (Embley et al., 1988). Finally, some very high temperature interaction between rising magma (into dykes ?) and modified seawater is evident in the volcanically active zones at Southern Juan de Fuca Ridge. Vent fluids have NaCl contents about twice that of seawater (Von Damm & Bischoff,1987); these were probably produced through super-critical phase separation which accompanied rapid contact between magma and seawater. Fluid inclusions in the stockwork

Precipitation of sulphides in the footwall zone is under the Millenbach deposits have variable, but very high salinities, also perhaps the product of super-critical largely effected by conductive cooling of the hydrothermal fluid, with very little mixing with locally phase separation (Kheang,1984). High-temperature metalliferous fluids collect and advecting seawater. Silicified pipes seem to provide move upwards in synvolcanic faults. The fluids have better thermal insulation than chlorite-dominated pipes; low specific gravities (ca. 0.5 g/cc) and rise at about consequently the latter have more extensive stringer 1-2 m/s. Only the major faults which form the margins zones than the former. Some interaction with previously of the rift valleys seem capable of reaching the fluid deposited anhydrite or unmodified seawater may allow "reservoir" in the largest seafloor hydrothermal systems a small amount of seawater-derived sulphate to enter (Kappel & Franklin, 1989). In some ancient bimodal into the hydrothermal fluid, causing a slight increase in volcanic sequences, and presently at Axial Seamount, The vertical extent of pipe development is a function caldera-margin faults focus(ed) most of the highof cross-stratal permeability of the footwall rocks. temperature fluid. Massive flows and pelagic sediments have very low Approximately 100 m below the seafloor, the fluids departfromthe slightly inclined major faults, and move permeability, and thus may have inconspicuous alteration up along secondary fractures, forming the alteration pipes. Talus piles, explosion breccia and drained pillows pipe. The latter fractures are the site of interaction and sheet flows are highly permeable, generating between large volumes of high-temperature fluid and extensive alteration zones and sub-seafloor deposition. Initial sulphide precipitation forms chimneys, which the immediate footwall rocks. The rapidly rising fluid induces draw-down around its periphery, causing large range up to about 10 m in height. Initially precipitated volumes of locally-heated seawater to circulate around anhydrite (through heating of seawater) is overprinted the margins of the discharge zone. Under the Galapagos by sphalerite, Cu-Fe sulphides and cristobalite. As the deposits, the high-temperature reaction zone in the chimneys thicken and pore space is reduced during alteration pipe has a core of silica, Fe-chlorite and early growth, the chimney becomes thermally insulted. sulphide deposition (Embley et al., 1988). High fluid A central pipe forms, lined with isocubanite, and leading pressure produced crack-seal veins. The margins of the upwards to the vent orifice. This pipe, typically 3-10 discharge zone were altered to clays, including Mg- cm in diameter, is semipermeable, enabling much of the smectite. Following the high temperature alteration high-temperaturefluidto escape into the ocean, forming event, progressively heated, locally advected, seawater "black smoke". However, some fluid leaks through the pervaded all parts of the alteration pipe, overprinting walls of the pipe, and cools rapidly to precipitate the earlier-formed high temperature assemblage with sphalerite and pyrite, which overgrow and displace lower-T, Mg-dominant minerals. Under ancient deposits, anhydrite, causing the chimney to grow by over-plating late Mg-metasomatism may have obscured much of the and inflation. The outer margins of large chimney earlier alteration, forming the Mg-dominated pipes structures become infested with vestiferimenta (tube typical of the Noranda district (Riverin & Hodgson, worms) which co-exist symbiotically with chemosynthetic (reduced-sulphur consuming) bacteria. The 1980). The alteration pipe assemblages seem to be a function latter induce formation of a marcasite-silica coating on of water depth. Pipes beneath deposits formed in deep the outer wall of the chimney. This coating greatly water are typified by Fe- and Mg-chlorite, with sericite inhibits the influx of seawater into the chimney, and (and paragonite) in their margins, and are variably, but promotes precipitation from the hydrothermal fluid by not predominantly silicified (e.g. Noranda, Mattagami conductive cooling. Preliminary age data indicate that a Lake, Snow Lake). Metamorphosed equivalents of single chimney may remain active for 50-100 years. these contain cordierite and anthophyllite cores, with In areas of sustained hydrothermal discharge, a series staurolite in both their core and marginal zones. Under of chimneys grow along fractures. These eventually deposits that formed in sequences where volcanic coalesce to form sulphide mounds as high as 50 m. textures such as hydroexplosion breccia, limited welding Discharge of high-temperaturefluidscontinues into these and exceptional vesicularity indicate depositional depths mounds. They continue to grow by internal fracturing of about 1 km or less, a broad "blanket" of carbonate and inflation, as they trap metals. Another very important alteration, commonly associated with pervasive Na- mechanism for forming large sulphide mounds involves depletion, extends for several kilometers within the discharge of hydrothermalfluidinto poorly consolidated immediate footwall strata. At Sturgeon Lake, Ontario, sediment or volcanic debris, as illustrated in the Middle the carbonate zone contains pervasive andalusite. The Valley (Juan de Fuca Ridge) and also at Escanaba carbonate species are a function of primary rock type, Trough. Strata such as turbidite and hyaloclastite beds, but immediately under the deposits, siderite is dominant. particularly where they are sealed by thin layers of Alteration pipes of this type are extensively silicified, pelagic sediment, apparentlyfillwith hydrothermal fluid. and contain abundant aluminosilicate minerals (usually Precipitation in such protected layers is by conductive andalusite and kyanite) and paragonite. Those that are cooling. These layers may also grow by inflation and metamorphosed to upper greenschist assemblages replacement, and may giveriseto "Besshi"-like tabular contain chloritoid and spessartine. sulphide bodies.

Finally, the size of VMS deposits on the seafloor The way in which a hydrothermal fluid is modified in the pipe and mound/chimney areas greatly affects the varies from just a few tonnes (EPR, Southern Juan de distribution of metals, including gold and silver. In Fuca and Axial Seamount) to around 1 million tonnes deposits that form in sufficiently shallow water to permit (Endeavour and Explorer Ridge, TAG), and to possibly boiling, (e.g. Axial Seamount) the formation of two several millions to tens of millions in the Middle Valley phases will affect their temperature, pH, and K^S activity. and Escanaba Trough areas. Size is a function of At Axial Seamount, which has very gold-rich sulphide longevity and extent of the hydrothermal discharge, and chimneys, two fluids are emerging. Normal salinity also of the ability to preserve sulphides from oxidative (3.5 wt %), high temperature (330°C) fluid transports destruction through burial, in a tectonically inactive metals and produces sulphide chimneys. However, only environment. The largest or most sustained systems in 30 m away, very low salinity, metal deficient, condensed volcanic-dominated ridges are those that formed at the vapour-phase fluid is discharging at 299°C, and forming terminal phase of excessive volcanism; these occur in deposits of pure anhydrite. Significantly, both fluids the axial graben to major elongate volcanoes, such as at contain subequal amounts of gold (150-200 ppt Au) Endeavour and Explorer ridges. Evidently the very thick and have elevated pH values (pH ca.4). Hannington carapace of volcanic rocks, and the absence of prolific and Scott (1989) have shown that the lower temperature active volcanism has promoted the development of a and increased pH conditions favour transport of gold as long-lived, stable hydrothermal system, with its source a bisulphide, rather than a chloride complex. Gold zone just above or within the cracking front of the bisulphide complexes are completely destroyed by cooling, but only slightly active magma chamber. Such oxidation; consequently gold is quantitatively a system is capable of delivering metalliferous fluid to precipitated at an oxidation boundary. Bisulphide a single area for a long time. The physical characteristics complexing also provides a zone-refining mechanism of these volcanically mature areas (inflated pillows, no for concentrating gold by late-stage low-temperature drain-out features or collapse breccias) also provide a hydrologically well-sealed environment through which fluids. Hydrothermal fluids that have undergone fractional the upward-moving fluid can pass, without mixing or dispersion. The immature areas (EPR, SJDF) are replete precipitation through cooling (possibly accompanied by boiling) become enriched in lead, barium and silver with collapse breccias and cavities, inhibiting sustained relative to copper and zinc; low-temperature chimneys, flow to the seafloor. The best cap to a hydrothermal system is a veneer of typically peripheral to the main centers of venting, are commonly enriched in these elements, as well as gold. sediments above a cooling magma chamber. Tectonic "Distal" sulphide products of hydrothermal activity quiescence, coupled with sustained heat generation, have are not abundant in modern systems, in part because of provided the energy for the largest deposits on the the highly oxidative nature of bottom seawater. Plume seafloor. In ancient environments, those areas within particulates are rapidly oxidised and partially dissolved. volcanic terrains where penecontemporaneous sediIn Archean seas and in locally restricted basins of any mentation can be demonstrated, have very good potential age, however, bottom waters were possibly sufficiently for new resources. reduced to enable the accumulation of plume particles. References The Key Tuffite unit at Mattagami Lake and the sulphidic Campbell, I.H., Coad, P.,Franklin, J.M.,Gorton, M.P., Scott, tuffs at Noranda and Snow Lake may have formed from S.D., Sowa, J. & Thurston, P.C., 1982. Can. J. Earth Sci. 19: 619-623. such fallout. R.W., Jonasson, I.R., Perfit, M.R., Tivey, M.A., The more significant product associated with the Embley, Malahoff, A., Franklin,J .M. & Smith, M.F. 1988. Can. apron to many vent areas are small mounds and thin Mineral. 517-540. beds of ferruginous silica, which have formed as a Kappel, E.S. &26:Franklin, 1989. Econ. Geol. 84:485-505. replacement of bacterial mounds and mats. These Hannington, M. & Scott,J.M., S.D., 1989. Econ. Geol. Mon. 6 (in bacteria live only where the supply of H^S is sufficient, press). in and around the chimneys. As the supply of t^S is Kheang, L., 1984. Geol. Assoc. Can., Program with Abstracts removed, the bacteria die and are replaced by silica and 9: 78. iron oxide. Low temperature, unfocused venting within Riverin, G. & Hodgson, C.J., 1980. Econ. Geol 75:424-444. a few hundred meters of the high temperature sites Seyfreid, W.E. & Janeckey, D.R., 1985. Geochim. Chosmochimi. Acta 49: 2545-2560. provides a source of silica and iron. Such beds would Von Damm, K.L. & Bischoff, J.L., 1987. J. Geophys. Res. 92: eventually form ferruginous chert 11,334-11,346.

A1.7 THE WESTERN WOODLARK BASIN, PAPUA NEW GUINEA: SUBMARINE VOLCANIC AND HYDROTHERMAL ACTIVITY ASSOCIATED WITH RIFTING OF A CONTINENTAL MARGIN R.A. Binns1*, S.D. Scott2 and PACLARK Team 1

CSIRO Division of Exploration Geoscience, North Ryde, NSW 2 Department of Geology, University of Toronto, Canada

Three cruises to the western Woodlark Basin were conducted in 1986 and 1988 by the PACLARK consortium (Geological Survey of Papua New Guinea; CSIRO Australia; Universities of Toronto and of British Columbia, Canada). The aims were to locate and study submarine hydrothermal vents and associated mineral deposits in an environment considered similar to those inferred for ancient volcanogenic massive sulphide (VMS) ore deposits. In the vicinity of the D'Entrecasteaux Islands, the Woodlark sea-floor spreading axis is propagating westwards into continental crust via a number of relatively short but deep rifts. Extension is probably related to rotation of the Solomon microplate associated with oblique convergence of the Indo-Australian and Pacific plates. Not all rifts are volcanically active. The region is characterised by rapid terrigenous and pelagic sedimentation, obscuring basement beneath some rift segments. Two separate areas of volcanic activity have been investigated by SeaBeam echo-sounding, dredging, and sea-floor photography-video (see Wheller et al., this volume, for geochemical and isotopic details). One in the east, close to the "normal" sector of the Woodlark spreading axis, comprises sheet-flows, tube-flows and talus of MORB-like basalt and Fe-Ti andesite flooring a 3200 m-deep basin (East Basin), together with basaltic andesite and andesite on adjacent Franklin and Cheshire Seamounts. The mafic volcanic rocks commonly contain magmatic Fe-Ni-Cu sulphides. Hydothermal Mn-Fe-Si oxides associated with basaltic andesite sheet-flows were dredged from Franklin Seamount, which is constructed mainly of lobate flows and is surmounted by a breached collapse caldera that implies the existence of a shallow magma chamber. A hydrothermal plume normally (CTD-hydrocast-transmissometer) was detected in

seawater over East Basin. Its source has not yet been defined, but some of the very fresh glassy basalt samples dredged there possess thin Fe-Mn oxide coatings. A bimodal association of pumiceous rhyolite and vesicular K-Mg-rich basaltic andesite/andesite occurs at the head of a 1600 m-deep basin (South Valley) near the western end of the propagating rift zone. This second volcanic region includes a cone of andesitic hyaloclastite (Dobu Seamount) rising to 350 m depth above a base of lobate and pillow flows with similar composition. The rhyolite flows crop out to 1000 m depth, are apparently older than the andesite lavas, and resemble those erupted from dormant, subaerial volcanoes associated with hot spring activity on nearby Dobu, Fergusson, and Sanaroa Islands. The locality is extensively faulted and fractured, and appears to be the source of hydrothermal plume anomalies detected in three out of four CTD-hydrocasttransmissometer tows conducted in the vicinity. No sulphides or oxide crusts were dredged, but a number of suspect outcrops were photographed. A 1385 m-deep graben in Goodenough Bay, west of Normanby Island, appears to represent further westwards propagation of the Woodlark spreading axis. Limited surveying indicates it is neither volcanically nor hydrothermally active, although small dark mounds of uncertain origin were photographed at the base of the northern fault scarp. Its walls are composed of lithified Neogene sediments, probably part of the Cape Vogel Basin. A program of submersible dives is being planned for 1991. The main target is the Dobu Seamount area which, by virtue of felsic volcanism and rapid sedimentation in a rifted continental margin, is considered the closest modern analogue in the Woodlark Basin for ancient VMS environments.

A1.8 GEOLOGY OF THE BUCHANS OREBODIES, NEWFOUNDLAND J.G. Thurlow Corner Brook, Canada

The Buchans orebodies are Kuroko-like, baritic, polymetallic and of similar tonnage and grade to Rosebery. They occur near the stratigraphic middle of the Buchans Group volcanics, a 4 km thick middle Ordovician, bimodal, submarine, calc-alkaline suite within the Newfoundland Appalachians. The ores and host rocks are characterised by prehnite-pumpellyite

facies metamorphism and a lack of penetrative fabric, resulting in excellent preservation of ores and colourful host rocks alike. Stockwork and massive sulphide ores are well represented, but the Buchans deposits are distinguished by the extensive development of mechanically transported ores. These elongate brecciaconglomerate deposits consist of sulphide and lithic

fragments which were carried downslope in polymictic, ore-grade debris flows which travelled well beyond the limits of stockwork mineralisation and related alteration. Prior to ore formation, footwall mafic/intermediate flows were brecciated with fragments undergoing some rotation and limited transport, possibly in a shatter column. Ore-related stockwork alteration is spatially associated with this event and is focused in an area of intersecting channel structures within the overlying host felsic breccia/pyroclastic sequence. Alteration consists of early widespread weak silicification followed by proximal chlorite-pyrite-base metals with peripheral sericitisation and pyritisation. Formation of the in-situ orebodies is poorly understood as the two major orebodies and their host rocks have been considerably disrupted by thrust faults and were mined out in the 1960's. Parts of these orebodies consisted of transported ores which have undergone progressive ductile shear to produce banded textures. The transported ores contain angular to subrounded high grade sulphide fragments and a variety of lithic fragments, all locally derived footwall rocks. Cobbles of subvolcanic granodiorite are locally an important component of the debris flows and are thought to have

been brought explosively to surface in pebble dykes. Synvolcanic normal faults created half-graben structures along which the elongate transported ores flowed. The orebodies consist of several debris flow events with each flow retaining its characteristic fragment composition. The stratigraphically highest debris flows are the most baritic suggesting an evolution of the composition of the source area with time. Post-Buchans Group, high level thrust faulting is the major structural event. Deformation is inhomogeneous with narrow brittle/ductile shears separating relatively thick unstrained panels. Antiformal stack structures are developed on a range of scales from local to regional. The relatively incompetent mineralised and altered rocks have served as a focus for strain and all the major orebodies are bounded on at least one side by thrusts. There is evidence that these faults inverted the earlier synvolcanic normal faults which controlled the distribution of the debris flows. Sheared mixtites which contain high grade ore clasts appear to be intruded syntectonically locally along the thrust zones. Current exploration is designed to locate the source of these clasts and in particular those which occur in intrusive sheets structurally below all the known orebodies.

A1.9 ROCK ALTERATION, MINERAL AND OXYGEN ISOTOPE ZONATION IN THE ROSEBERY DISTRICT, TASMANIA Geoffrey R. Green Department of Resources and Energy, Tasmania

The Rosebery and Hercules massive sulphide ZnPb-Cu-Ag-Au ore deposits are hosted by fine grained clastic sedimentary rocks sandwiched between pumicerich pyroclastic and epiclastic units in the Cambrian Mount Read Volcanics. Although current opinion is that the deposits represent Cambrian exhalative mineralisation, regional deformation and later granite emplacement with associated local metasomatism and mineralisation in the Devonian complicate the picture and some workers (Aerden, this volume) believe that ore formation may have occurred synchronously with deformation. Detailed petrographic, whole rock geochemical and oxygen isotope studies of the Mount Read Volcanics in a 12km strip including Rosebery and Hercules have reaffirmed several features consistent with Cambrian synsedimentary alteration and mineralisation. Exrteme alteration involving total feldspar destruction and strong Na depletion is stratigraphically asymmetric and occurs beneath the orebodies. At Hercules the alteration pipe is cored by (i) quartzchlorite-pyrite-(chalcopyrite) and passes out through (ii) quartz-sericite-pyrite-(sphalerite-galena) to (iii) rocks with carbonate- and K-feldspar-replaced plagioclase.

Similar alteration (assemblages ii and iii) is present in footwall rocks in the Rosebery Lodes-Koonya area 2.5 km S of the Rosebery orebody. Devonian cleavage development in the rocks is domainal and the alteration assemblages show clear evidence of pre-tectonic alteration. For example, in mica-rich domains pyrite is corroded, but in quartz-rich lithons it is surrounded by pressure shadows of quartz, carbonate, phyllosilicates or other sulphides. Posttectonic vein mineralisation is texturally distinct and is localised mainly in Devonian fault zones, particularly where these are underlain at a shallow depth by a granite ridge (Bamford & Green,1986; Leaman & Richardson, 1989). About 80 whole rock oxygen isotope analyses yield 8 0 values generally in the range 12.5 ± 1 %o in relatively unaltered rocks with values of up to 15%o about 1 km S of Hercules, compared with values of less in 10%o in footwall alteration zones. This distribution is consistent with mineralisation from a seawater-dominated ore fluid in the Cambrian. Syn-tectonic phyllosilicates in the Hercules area formed at temperatures of 300 to 320°C as established by the chlorite geothermometer of Walshe (1986). These 18

temperatures are independent of whole rock 8 O values and proximity to ore. 8 O values of metamorphic fluids in equilibrium with the regionally extensive volcanics would be in the range 7 to 10%o and would lead to 8 O values of 11.5 to 16.5%o for rocks in the alteration zones if alteration and mineralisation was a product of metamorphic fluid focussing. That is, the alteration zones would exhibit similar or slightly higher whole rock 8 0 values than the surrounding volcanics, the reverse of the observed trend. It is therefore clear that the Cambrian oxygen isotope zonation has been largely preserved through Devonian ls

deformation and plutonism, confirming that the technique can be a useful exploration tool in deformed volcanic sequences, such as the Mount Read Volcanics, provided it is accompanied by careful petrography. References

Aerden, D., 1990. This volume. Bamford, A.L., 1986. In Large, R.R. (ed.): The Mount Read Volcanics and associated ore deposits. Geol. Soc. Aust. Tasm. Div., Hobart: 27-29. Leaman, D.E. & Richardson, R.G., 1989. Bull. geol. Surv. Tasm. 66. Walshe, J.L., 1986. Econ. Geol. 81: 681-703.

A1.10 Au-Sn MINERALISATION ALONG THE HENTY FAULT, MT FARRELL AREA: MOPPING UP THE CAMBRIAN VS DEVONIAN CONTROVERSY J.Taheri and G.R. Green Department of Mines, Tasmania Several small to medium sized Pb-Ag to polymetallic the granite lies at a depth of about 1 km, whereas Pb-Ag Sn-Au bearing deposits occur adjacent to the major dominated deposits occur where the granite surface is at NNE- trending Henty Fault in the Mt Read Volcanics of a greater depth. High angle reverse movement of the Henty Fault synchronous with the intrusion of the granite western Tasmania. The Henty Fault in the Mt Farrell area separates the (Berry,1989) probably created permeable ore channels Mt Black Volcanics (Central Volcanics) to the west in the area. Mineralisation at the Lakeside Prospect may be from the Farrell Slates and the Murchison Volcanics (Tyndall Group) to the east (McNeill, 1987). The rocks divided into four different stages. to the west of the Henty Fault are predominantly Stage 1: pyrite + pyrrhotite + arsenopyrite. chloritised feldspar-phyric andesite and dacite and are Stage 2: the mineralisation is divided into two substages silicified near the fault The Farrell Slates mainly consist - (2a) arsenopyrite + cassiterite + chlorite, (2b) of grey to black slates, shale, greywacke and chalcopyrite + stannite + gold with quartz, tourmaline volcaniclastic arenite. The rocks are commonly deformed and fluorite being the common gangue minerals for the both substages. and are brecciated. The mineralisation associated with the Henty Fault Stage 3: galena + sphalerite; and Zone in the Mt Farrell area post dates Devonian cleavage Stage 4: chorite+carbonates +pyrite.The mineral contents development and may be divided into two broad and/or proportion of the different ore minerals changes with increase in depth. Pyrite, galena, sphalerite, stannite categories: 1. Pb-Ag dominated deposits hosted by the Farrell and chalcopyrite contents decrease at depth, whereas Slates. The deposits occur as sub-parallel lenticular cassiterite and pyrrhotite contents increase. The main fissure lodes and veins in shears striking from NNW mineralising events appear to have been formed as a result of the boiling of a magmatic dominated, C0 to NNE (e.g. New North Mt Farrell Mine). 2. Polymetallic Sn-Au bearing mineralisation which bearing fluid at a temperature of around 340°C (fluid occurs as disseminations, fissurefillingand stringers inclusion and oxygen isotope data). The reduced adjacent to the Henty Fault in the Farrell Slates (e.g. character of the fluid and high temperature of the Lakeside Prospect) or it occurs within the Mt Black mineralisation particularly favoured tin transport through CI complexes. The temperature was also sufficiently Volcanics (Sterling Valley Sn-As prospect). The mineralisation appears to be directly associated high for CI complexes to have played a major role in Au with the intrusion of the Granite Tor Granite at a shallow transport. Boiling would have been very effective depth in the Devonian. A WSW-trending granite ridge depositional mechanism under these conditions and the extends from the Granite Tor Granite to the Pine Hill concentration of gold was probably optimised at the Granite and possibly as far west as the Heemskirk Lakeside Prospect Granite (Leaman & Richardson, 1989). A number of tin The Sterling Valley Sn-As mineralisation in the Mt and tourmaline-bearing deposits as well as Pb-Zn-Ag Black Volcanics has characteristics indicative of its vein deposits occur within the zone of the granite ridge. being a deeper analogue of the mineralisation at the Based on the recent gravity data of Leaman & Lakeside Prospect. The gold content is lower and Richardson (1989) and Archer (1989), the Sn-Au pyrrhotite is a more common mineral. Berry's (1989) mineralisation occurs above the crest of the ridge where structural synthesis of syn-mineralisation reverse fauldng 2

followed by sinistral strike slip faulting is consistent Cambrian and the Devonian fields of Gulson & Porritt with the two deposits being displaced units of an original (1987). The values from the Lakeside Deposit are the single sheeted vein system developed at both sides of most radiogenic and are consistent with a Devonian the Henty Fault If so, the extent of the left lateral magmatic source. displacement on the fault can be constrained at about In summary, the mineral zoning, petrographic 1100 m. observations, fluid inclusion, sulphur, oxygen and lead Sulphur isotope studies (e.g. Solomon et al., 1969) isotope data indicate that the mineralisation along the and the zonation of hydrothermal alteration assemblages Henty Fault zone in the Mt Farrell area is related to the in the Murchison Volcanics east of the Fault (Polya et intrusion of the Granite Tor Granite, although a country al., 1986) have been interpreted to indicate a syn-volcanic rock input for S and possibly some of the Pb and Au Cambrian origin for the mineralisation. Sulphur isotope might be important compositions reflect the gross mineralogical zonation References of the area. Values of +11.9 to +16.5%o in the Ag-Pb Archer, D., 1989. Unpubl. BSc (Honours) thesis, University of Tasmania. deposits flanking the granite ridge are similar to those in the Rosebery and Hercules volcanic associated Berry, R.F., 1989. Aust. J. Earth Science 36: 189-205. deposits. Hence the Pb-Ag deposits could contain Carr, G.R., 1988. Unpubl. Report to Billiton Aust Burnie, Tasmania. recycled Cambrian sulphur. However values of +5.2 to Gulson, & Porritt, P.M., 1987. Econ. Geol. 82:291-307. +12.0%o (80% of values between +7 to +10%o) for the Leaman,B.L. D.E. & Richardson, R.G., 1989. Bull. geol. Surv. Sn-Au deposits above the ridge crest suggest a subTasm. 66. stantial component of magmatic sulphur for these McNeil, A.W., 1987. Mt Read Volcanics Project 1:250,00 deposits. Series. Map 4. Department of Mines, Tasmania. The lead isotope compositions from the vein deposits Polya, D.A., Solomon, M., Eastoe, C.J. & Walshe, J.L., 1986. along the Henty Fault zone in the Mt Farrell area (Gulson Econ. Geol. 81: 1341-1355. & Porritt, 1987, Carr, 1988) have higher ^Pb/^Pb Solomon ,M., Rafter, T.A. & Jensen, M.L.; 1969. Mineralium Deposita 4: 172-199. than Cambrian VMS deposits and plot either within the field of the Devonian mineralisation or between the

Al.ll "Ar-^Ar AND U-Pb GEOCHRONOLOGY OF THE GOONUMBLA COPPER-GOLD AND GIDGINBUNG GOLD DEPOSITS, NSW C. Perkins *, I. McDougall , J. Claou6-Long and P. Heithersay 1

Geology Department, Australian National University, Research School of Earth Sciences, Australian National University Geopeko Exploration Ltd, Parkes 1

Establishing the timing of mineralisation and associated magmatism on both a regional and deposit scale is a fundamental research problem as well as having important implications for minerals exploration. As part of an ongoing high precision dating study of mineralisation and host rocks in the Lachlan Fold Belt, NSW, the Goonumbla and Gidginbung gold deposits have been investigated. The ^Ar-^Ar technique of KAr dating has been used to determine the age of hydrothermal phases, and U-Pb dating by ion microprobe of magmatic zircons has been utilized to establish the emplacement time of host rocks to mineralisation. The porphyry copper-gold Goonumbla deposits near Parkes, NSW, are hosted in a number of quartz monzonite pipes which intrude the trachyandesitetrachyte volcanic succession of the Goonumbla Volcanics. Ar- Ar step heating experiments on two examples of hydrothermal vein sericite texturally associated with mineralisation from the Endeavour 26 deposit yield spectra which either rise monotonically to 40

an age of438.7 ±1.1 Ma, or form a plateau-like segment age of 439.1 ± 1.1 Ma (all Âr-Âr dates are quoted at 1 s.d., with the uncertainty derived by quadratically combining precision estimates for mass spectrometer isotopic measurements and neutron fluence measurements). Step heating experiments on two samples of groundmass sericite formed as a product of regional alteration in the vicinity of the same deposit yield spectra which either rise monotonically to an age of 441.2 ± 1.1 Ma, or form a plateau age of 437.9 ± 1.1 Ma. Dates indicate that vein sericitisation was broadly contemporaneous with groundmass alteration. One of the ages on the latter variety of sericite, however, is statistically older than datesfromthe other groundmass sample and one vein sericite, which may suggest that regional alteration occurred in at least two discrete events rather than in one episode, and that the older event occurred prior to the formation of some vein sericite. ^Pb-Û dates on zircons from the Nelungaloo Volcanics, which underlie the mineralised sequence,

give an age of 438±7 Ma (2 s.d., with the uncertainty expressed as standard deviation of the mean derived from 20 age determinations). The epithermal gold Gidginbung deposit near Temora, NSW, is hosted in andesitic composition volcaniclastic rocks. ^Ar-^Ar total fusion ages on hydrothermal alunite cluster in two groups, with ages ranging from 401.0±1.3 to 405.8±1.3 Ma (1 s.d.), and 411.111.3 to 417.3±1.3 Ma (1 s.d.). Both groups of dates coincide with episodes of regional deformation (cf. Basden, 1982) which may have reset the alunite, and 417.311.3 Ma should therefore be taken as a minimum age for alteration. ^ P b - ^ dates on zircons from an andesite flow within the mine sequence yield an age of 43515 Ma (2 s.d., standard deviation of the mean derived from 26 age determinations). 23

Results of the dating so far completed indicate an association between gold mineralisation and Late Ordovician to Early Silurian magmatism (using the time scale of Harland et al., 1989). The Nelungaloo Volcanics have been shown by U-Pb dating of magmatic zircons to be broadly contemporaneous with the overlying Goonumbla Volcanics and mineralisation, and with volcanism in the Temora area. In addition, the investigation illustrates the amenability of hydrothermal sericite to ^Ar-^Ar dating. References

Basden, H., 1982. Geol Surv. NSW. Q. Notes 46. Harland, W.B., Armstrong, R.L., Cox, A.V., Craig, L.E., Smith, A.G. & Smith, D.G., 1989. A Geologic Time Scale 1989. Cambridge University Press.

A1.12 DEPOSITIONAL AGE OF THE BROKEN HILL GROUP FROM VOLCANICS STRATIGRAPHICALLY EQUIVALENT TO THE Ag-Pb-Zn OREBODY R. W. Page * and W.P. Laing 1

division of Petrology and Geochemistry, BMR, Canberra Key Centre in Economic Geology, James Cook University

In order to develop models for the tectonic and metallogenic evolution of the early to middle Proterozoic of central and northern Australia, primary depositional ages of sequences such as those enclosing the 300 Mt Ag-Pb-Zn orebody at Broken Hill need to be unambiguously determined. Despite numerous geochronological studies over the past three decades, the age and correlation of the Willyama Supergroup meta-volcanosedimentary sequence enclosing the Broken Hill orebody are not known, and the depositional origin of its immediate host rocks, the Broken Hill Group, remains a matter of contention. This is primarily because past geochronological and geological studies have concentrated on the high-grade (upper amphibolite to granulite facies) rocks adjacent to the orebodies themselves, where primary isotopic and textural features are generally modified or obliterated. The present isotopic study focuses on the stratigraphic unit laterally equivalent to the ore, traditionally termed Totosi' gneiss (now Hores Gneiss) and the Parnell Formation, at the top and middle respectively, of the Broken Hill Group (Willis et al., 1983). These two units are distinctive quartzofeldspathic, garnetiferous horizons, the upper one of which passes laterally into metasediments containing the seven stratiform Ag-Pb-Zn ore lenses of the Broken Hill orebody. At Broken Hill itself, intense deformation and granulite facies recrystallisation mask primary textures and field relations. However, north of Broken Hill at Yanco Glen and Parnell, the two horizons occur at lower amphibolite facies. Here, particularly in the Hores Gneiss, Laing et al. (1984) identified (a) textures, (b) field relations and (c) geochemical evidence

indicative of a rhyodacitic ash flow. These are: (a) abundant euhedral phenocrysts, quartz>feldspar, the quartz having embayed margins and bipyramidal b shapes; pervasive biotite clusters suggestive of flattened fiammfi; rare coarse lithic clasts; amygdales. (b) a massive non-bedded unit contrasting with the enclosing bedded, traction-current structured, siliciclastic metasedimentary succession; increasing interfingering of the above metavolcanic lithology with increasingly bedded siliciclastic metasediments, along the Hores Gneiss between Yanco Glen and Broken Hill, indicating a proximal to distal facies change. (c) feldspathic compositions different from the surrounding mature siliciclastic metasediments; along this same horizon, major-element, "immobile"-element, and REE systematics indicate coherent magmatic pedogenesis. Although Wright et al. (1987) argue that Totosi' gneisses at Broken Hill are derived from immature arkosic sediments, we consider that the abovefieldand petrographic evidence, together with the zircon morphological and U-Pb isotopic evidence outlined below, are overwhelmingly in favour of a pyroclastic rhyodacitic precursor. Zircons in the lower grade Hores Gneiss largely comprise zoned, euhedral-subhedral slender grains. Conventional, multi-grain U-Pb analyses, rather than defining a linear discordant trajectory, fall in an area of the concordia diagram, hence precluding precise age interpretation. Ion-probe U-Pb work permits differentiation of this complex pattern: there is a major population at 1670-1690 Ma, a minor group at -1780 Ma, and fewer older xenocrysts in the age range

2200 to 2700 Ma. We interpret the 1670-1690 Ma result as the age of zircon crystallisation close to the time of eruption and deposition of this part of the Broken Hill Group. This is a much younger age than that previously modelled from indicated initial Sr ratios (1800-1850 Ma; Shaw, 1968), and should these rocks be clastic arkoses (as interpreted by Wright et al., 1987) the 1670-1690 Ma result would be a maximum depositional age. The older -1780 Ma and late Archaean zircon xenocrysts, commonly mantled by 1670 Ma zircon overgrowths, are inherited components reflecting complex source(s) consistent with their Nd model ages of 2200-2300 Ma (McCulloch, 1987). The 1670-1690 Ma depositional age renders a firm chronological tie between the early Proterozoic sequences enclosing major stratiform Ag-Pb-Zn orebodies at Broken Hill, Mount Isa (1670 ± 8 Ma) and McArthur River (169 0 ± 7 Ma—Page, 1981). Whether this tie indicates a similar, syngenetic origin for the ore deposits is another matter (Laing, in prep.). At Broken Hill, the Hores Gneiss in the granulite facies (T ~800°C) has U-Pb zircon age patterns similar to the lower grade rocks, but complicated by new zircon growth and recrystallisation during the high-grade metamorphism at -1600 Ma. Consistent U-Pb monazite

ages are also close to 1600 Ma, suggesting that their blocking temperature is close to that of zircon, or that cooling to < 600°C was achieved relatively rapidly. This age for the high-grade metamorphism is some 60 Ma younger than the previously accepted Rb-Sr whole-rock age for the gneisses of 1660±21 Ma (Pidgeon, 1967; Shaw, 1968), and it probably reflects inadequate whole-rock sampling, exacerbated by incongruent Rb-Sr mineral systems of the component feldspar and mica that were clearly open in the -480 Ma Delamerian Orogeny. References Laing, W.P., Nesbitt, R.W. & Sun, S-S., 1984. Geol. Soc. Aust.Abstr. 12: 318-321. McCulloch, M.T., 1987. In Proterozoic Lithospheric Evolution. Geodynam. Ser. 17. Am. Geophys. Union: 115-130. Page, R.W., 1981. Econ. Geol. 76: 648-658. Pidgeon, R.T., 1967. Jour. Petrol. 8: 283-324. Shaw, S.E., 1968. Aust. Inst. Min. Metall. Monog. Ser. 3: 185-198. Willis, I.L., Brown, R.E., Stroud, W.J. & Stevens, B.P.J., 1983. Jour. Geol. Soc. Aust. 30: 195-224. Wright, J.V., Haydon, R.C. & McConachy, G.W., 1987. Geology 15: 598-602.

A1.13 GEOLOGIC SETTING OF THE UMUNA EPITHERMAL GOLD DEPOSIT, MISIMA ISLAND, PAPUA NEW GUINEA D.S. Clarke* and I.E.M. Smith Ep ithernial Mineralisation Research Unit, University of Auckland, New Zealand

The Umuna epithermal gold-silver deposit is a world class precious metal resource on eastern Misima Island, Papua New Guinea. Misima Island lies in the Louisiade Archipelago forming the south-eastern extension of the Papuan peninsula. The deposit occurs within a 100300 m wide zone of fractures, veins, anastomosing shears, and breccia sheets recognized over a 3 km strike length. Whereas many of the features of the Umuna gold deposit are characteristic of volcanic-associated epithermal precious metal mineralisation, the deposit is hosted by older metamorphosed volcano-sedimentary basement rocks, and Neogene granodioritic intrusive rocks. Volcanic and sedimentary rocks lie unconformably on these basement rocks, but are poorly exposed and have been lost to erosion in the vicinity of the deposit Lithologic age relationships on Misima Island have not been well defined. The Neogene granodioritic intrusions on Misima Island are typical of Papuan calc-alkaline moderate-tohigh K igneous rocks; however, granodioritic-to-dioritic igneous rocks are not abundant in these Papuan suites (Smith, 1982). Comparable rocks in western d'Entrecasteaux Islands and the Managlase Plateau of the south-eastern Papuan peninsula have higher K, Zr

and Rb, with lower Sr, V, Cr and Ni. More akin are the intermediate-to-felsic members of the late Miocene highK volcanics and co-magmatic intrusives of Woodlark Island (Ashley & Rood, 1981). The latter rocks are host to, and are broadly contemporaneous with, gold mineralisation. At least two different volcanic flow rocks have been identified on Misima Island (Williamson & Rogerson, 1983). The most common is PlioceneQuaternary hornblende andesite, typical of late MioceneQuaternary arc-type igneous suites of south-east Papua. Similar rocks occur from Mt Lamington on the northeast Papuan coast to the Calvados Islands of the Louisiade Archipelago. There is no evidence for geothermal activity overprinting these rocks on Misima Island. The other volcanic rock type is an intermediate member of a shoshonitic suite showing typical subequal Na and K contents and high Sr. This rock is comparable to mid-to-late Miocene Cloudy Bay volcanics and Fife Bay volcanics which occur along the southern coast of south-east Papua, and the more mafic members of the late Miocene Woodlark high-K volcanics. Stratigraphic correlation of both volcanic rock types on Misima is tenuous; whereas the shoshonitic

unit is also mapped as Pliocene-Quaternary in age (Williamson & Rogerson, 1983), it may be of Miocene age. An analogy can be drawn between Misima Island gold mineralisation-shoshonitic volcanics-high K intrusions, and the similar Miocene association on Woodlark Island (Clarke, in prep.). The Miocene-Quaternary igneous rocks on Misima lie within the northern of two belts of dominantly calcalkaline to shoshonitic igneous rocks recognised in eastern Papua. The northern belt extends from the northeast Papuan coast, eastward through the d'Entrecasteaux Islands, to the Louisiade Archipelago, and probably included Woodlark Island rocks prior to rifting of the Woodlark basin. Igneous activity along this belt occurred in two periods, middle-late Miocene and late PlioceneQuaternary, corresponding to periods of uplift in the

eastern Papuan region and active rifting in the Woodlark basin (Johnson et al., 1978). The igneous activity and sub-aerial volcanism of these periods has been productive with respect to geothermal activity and associated gold mineralisation. The Umuna mineralisation appears to be a manifestation of this productivity during the earlier, Miocene, period. References Ashley, P.M. & Flood, R.H., 1981. J. Geol. Soc. Aust. 28: 227-240. Clarke, D.S., in prep. Unpubl. PhD thesis, University of New South Wales, Sydney. Johnson, R.W., Mackenzie, D.E. & Smith, I.EM., 1978. Tectonophysics 46:197-216. Smith, I.E.M., 1982. Tectonophysics 87: 315-333. Williamson, A. & Rogerson, R., 1983. Geol. Surv. PNG. Rep. 82/2:171pp.

A1.14 EPITHERMAL GOLD AND SILVER MINERALISATION IN MIOCENE VOLCANIC ROCKS, VANUA LEVU, FIJI ISLANDS John L. Stockley Dominion Mining Ltd, Townsville

Vanua Levu, the large northern island of the Fiji consist of ash and crystal tuff, lithic tuff, massive archipelago has been the site of sporadic gold and silver volcanic agglomerate and pillow lava. The volcanic production since the 1930s. Approximately 80,000 oz. rocks are part of the Miocene Natewa Group and at of bullion was produced at the Mount Kasi mine from Dakuniba are over 1000 m thick. Intrusive rocks consist about 280,000 tonnes of ore during the period 1932 to of basaltic and trachytic dykes and sills. 1946 (Colley, 1976). Recent work carried out by Range In the central part of the prospect a thick pile of Resources in joint venture with Newmont Australia has bimodal fragmental (gabbro and trachyte clasts of equal indicated the presence of approximately 100,000 oz. proportions) volcanic rocks occur. The surrounding contained gold in rock not mined previously. basaltic host rocks have been uplifted into a broad Intensive mineral exploration carried out by mainly dome. Australian companies during the period 1986 to the Many of the quartz veins display multi-phase breccia present has shown that the Miocene age basaltic rocks textures and contain crenulate sulphide+feldspar clasts. of central and eastern Vanau Levu are the hosts for high Anomalous tin and molybdenum levels occur in such level epithermal mineralisation of both the quartz+ breccias. In the banded veins, microscopically visible sericite and acid+sulphate styles. At Mt Kasi and electrum grains up to 30 mm in diameter occur with Naduna, 5 km to the south, epithermal gold+silver anhedral chalcopyrite, galena, sphalerite and barite. The mineralisation occurs in barite-rich hydrothermal gangue consists of multi-phase quartz and clay altered breccias located along fault structures which have also rock fragments. Slickensides are well developed on been the loci of high level dacite dome emplacement vein outcrops and normal faults are common features, Gold mineralisation also appears to be associated with mainly on E-W trends parallel to the veins. retrograde silica+alunite alteration and extensive argillic Rock alteration at Dakuniba consists of a pervasive alteration zones occur around the high grade (up to propylitic assemblage with intense quartz+smectite 30 g/t Au) telluride-rich enargite bearing breccia bodies. alteration along vein contacts and within the vein The newly discovered Vudibasoga district 30 km north- breccias. Illite+dickite occurs in a 200 m wide zone in east of Mt Kasi shows mineral and alteration assemblages the central part of the prospect together with the mineral similar to the Mt Kasi-Naduna field assemblage chalcedony+marcasite. Finely banded At Dakuniba 80 km east of Mt Kasi an extensive siliceous rock occurs within the low temperature system of epithermal quartz veins and vein breccias alteration zone and may represent sinter. Gold grades was discovered by Pacific Islands Gold NL in 1987 throughout the 5 km vein zone are consistently greater during exploration of previously documented gold occur- than 1 g/t and peak at 21 g/t Au. Silver levels up to rences (Colley, 1976). Gold+silver mineralised massive, 500 ppm occur. Broadly speaking the Dakuniba banded and brecciated quartz veins up to 6 m in width mineralisation is of the quartz+sericite style and could crop out over an east-west strike length of 5 km in be compared with the Golden Plateau lode at Cracow in eroded tholeiitic basalt of Miocene age. The host rocks Queensland.

In terms of the volcanic setting work at Dakuniba has shown that the epithermal quartz veins developed in normal faults forming within a thick pile of tholeiitic basalt on the southern flank of the major Natewa volcano. In the Miocene Vanua Levu was an active island arc on the northern margin of the Platform (Fig. 1; Falvey, 1987). Movement of a sinistral sense along the Fiji Transform Fault resulted in extension of the Vanua Levu crust with development of the E-W trending Natewa rift structure. Bathymetry and topography show a vertical relief in the Natewa rift of over 2400 m. This rift structure transects the Natewa volcano and the Dakuniba normal faults, in which the epithermal mineralisation is hosted, are minor features on the southernmost flank of the volcano. Later, in the Pliocene large Hawaiian type shield and rift volcanos (Bua and Taveuni, see Colley, 1976) developed astride the Miocene arc. Preliminary lead isotope studies of epithermal deposits on Vanua Levu indicate different characteristics of the acid + sulphate Mt Kasi style mineralisation to

the quartz+sericite Dakuniba style mineralisation: the Mt Kasi and Naduna systems plot in a tight cluster separate from the Dakuniba set (Fig. 2). The Mt Kasi deposit may well be magmatic related in that syn-mineral sub-volcanic dacite plugs occur close to high grade enargite+barite bearing hydrothermal breccias. No altered or mineralised intrusive rocks occur within the Dakuniba prospect area and fluid inclusion studies of mineralised veins indicate that the fluids passing through the Dakuniba fault zones were of low salinity and at relatively low temperature (180-210°C). The presence of chalcedony+marcasite in the central prospect area also indicates low temperatures of mineral deposition. References

Colley, H., 1976. Memoir No. 1 Mineral Resources Division, Ministry of Lands and Mineral Resources, Government of Fiji. Falvey, D.A., 1987. Proc. Pacific Rim Congress 1987: 667-673.

Figure 1 — Plate tectonics of the Fiji Platform. 15.8

mrumen mcasiou

15.7

Si\ 15.6

QL fs. OOJ 15.5

15.4 18.7

Mt KBSI 18.8

DRKUNIBn

18.9

206 204 p b /

19.0 p b

Figure 2 — Lead isotope ratios, Vanua Levu, Fiji.

A1.15 HYDROTHERMAL ALTERATION, MINERAGRAPHY, STRUCTURE AND FLUID INCLUSIONS FROM THE ZELMA ACID SULPHATE GOLD DEPOSIT, SARINA, QUEENSLAND R.T. Bills WMC Limited, Research Laboratory, Preston, Victoria

The currently unworked Zelma gold deposit located diorite and andesite stocks and dykes while subordinate south of Mackay in North Queensland has a reported but poorly constrained kaolinite-quartz-laumonitite is underground and opencut reserve of 21,000 t at one of the last recorded events. A frequency-grade distribution plot highlights three 11.4 g/t and 30,000 t at 4.7 g/t respectively. Recent dewatering of the old workings which historically main gold populations with the 0.5 to 10 g/t interval produced in excess of 4000 oz. of gold, provided an associated with the breccia pipe while the greater than opportunity to integrate surface mapping and core 10 g/t interval, which includes 2 m at 1029 g/t from a logging with underground studies. This has indicated drill intersection below the 3 level stope is mainly that the mineralisation at Zelma is hosted by a confined to a sporadically developed inner silica -pyrite subvertically plunging, heterolithic breccia pipe shoot This shoot is characterised by more uniformly composed of volcanic fragments sourced from over rounded, multi-brecciated fragments which have a matrix 200 m in the underlying stratigraphy. Crudely bedded of comminuted rock flour, silica, pyrite and clays. The and sorted apron breccia is subaerially deposited in up diffuse interdigitating nature of the contact between this to a 300 m radius from the vent and hosts the bulk of the and the enclosing breccia pipe preclude ascertaining its geometry, although the old ballroom stopes are broadly lower grade open cut reserves . A structural synthesis of the deposit indicates the elliptical and plunge vertically, subparallel with the breccia pipe is coincident with the intersection of NNE pipe. In detail, the gold grades vary sympathetically trending faults and NNW striking shears. Both the with the extent of silicification and more particularly paragenetic and spatial relationship of the faults to the the frequency offineanastomosing hydrofractures which breccia indicate they were instrumental in controlling are preferentially developed in the silicified breccia fill. the geometry of the pipe which is upwardly flared but Free gold occurs along the selvedges of these fractures, strongly elongate along a NNE trending axis. often with native tellurium and calaverite (AuTe ). Gold Furthermore, a thicker sequence of apron breccia on the is also found intergrown with ilmenite, rutile and south side of the pit also suggests dip-slip movement on sometimes in embayments of subhedral pyrite. Other these faults. Truncation of the vent by a late strike-slip associated metals include chalcopyrite, tennantite, fault with dextral displacement precludes ascertaining bornite and minor marcasite, galena, guanajuatite the dimensions of the vent. However, assuming the (Bi^Se^j), chalcocite and covellite. Furthermore, the fault roughly bisects the vent, both the long and short Zelma ores lack silver phases, have very high gold fineness and an antipathetic relationship with those areas axes would not exceed 160 and 60 m, respectively. which exhibit intense matrix replacement by pyrite. Regional mapping together with Landsat data The gangue minerals include sericite, quartz, pyrite, highlights the proximity of a district scale arcuate lineament to Zelma and other similar satellite breccia pyrophyllite and lesser fuchsite, apatite and chlorite. Geochemical profiles from the few drillholes that occurrences. This lineament transects the gently dipping acid to intermediate Upper Devonian to Lower sample the silica-pyrite ore shoot indicate gold has a Carboniferous host volcanic succession, but in turn is sympathetic correlation with copper, tellurium, antimony cut by post mineralisation Carboniferous to Mesozoic and in the upper levels only, arsenic. At the surface diorite and andesite dykes. A significant proportion of these are also the best pathfinder elements while at the mine stratigraphy is occupied by these dykes, with depth both copper and barium apparently increase. most of them intruding and dilating pre-existing Detailed mineragraphy from throughout the gold zone structures and attesting to a complex and active structural indicates a paragenesis of early skeletal titanomagnetite progressively replaced by ilmenite/rutile which is environment. Hydrothermal alteration includes a mainstage event intimately associated with free gold and blebby pyrite. This is closely followed by native tellurium and that is comprised of early, district scale sericite-illite which in the mine is transitional to later quartz- calaverite, basemetals of bornite, galena and late pyrophyllite and silica-pyrite alteration. Essentially xenomorphic chalcopyrite, tennantite and subhedral monomineralic pyrophyllite, best developed within the pyrite. Microthermometry from primary ancj/or pseudobreccia down to at least the 3 level and to a lesser extent in the core of the NNW shears which cut the breccia secondary fluid inclusions hosted in the auriferous silicapipe, but diffract around the inner silica-pyrite zone, pyrite zone indicate trapping temperatures for the haliteenvelops all pre-existing alteration minerals. Post bearing, C0 -bearing and dilute aqueous inclusions are mineralisation propylitic alteration is confined to the between 250° and 330°C with a mean of290°C. Aqueous 2

inclusions from quartz veins that cut the breccia in the upper levels have temperatures in the range 170° to 190°C, which together with a spectrum of intermediate temperature-salinity inclusions suggest mixing of two diverse fluid types Preliminary thermodynamic modelling of the Zelma hydrothermal system indicates that muscoviteequilibrated, hot saline and metal bearing brines mixed with low pH, dilute and C0 -bearing groundwaters is a viable mechanism to produce the observed metal and alteration assemblages. This involves channelling metal bearing brines, as depicted by the halite-bearing fluid inclusions up a pre-existing breccia plumbing system during regional sericitic alteration. An overall lack of exotic clasts in the Zelma breccia suggests regional, predominantly phreatic activity which is broadly controlled by the lineament but locally focussed, as at Zelma, up planes of weakness such as the NNE faults. Co-existing pyrophyllite, quartz, 2M muscovite and C0 -rich and saline inclusions mark a zone of apparent isothermal mixing between groundwaters and brines. 2

Moreover, large-scale convection of the acid fluids caused extreme base leaching of the feldspars within the volcanic pile. Local temperature inversions from the influx of cooler groundwaters cause the fluids to become supersaturated with respect to quartz and deposit crystalline silica which, in a restricted area such as the breccia pipe produce zones of throttling. This periodic self sealing, isothermal mixing and overpressuring followed by rupturing and hydrofracturing not only reworked specific areas of the pipe but also produced multibrecciated, discrete bonanza gold shoots associated with the silica-pyrite alteration. As the heat source waned and meteoric waters dominated, monomineralic pyrophyllite enveloped the pipe and together with entrapment of lower temperature liquid rich fluid inclusions marked the collapse of the hydrothermal system. This research would not have been possible without the generous support of the WMC study leave scheme and also Epoch N.L. for the unrestricted access to the deposit and various unpublished company reports.

A1.16 CONTRASTING ALTERATION STYLES ASSOCIATED WITH EPITHERMAL MINERALISATION, NORTH ARM VOLCANICS, QUEENSLAND P.M. Ashley and A.S. Andrew 1

University ofNew England, Armidale CSIRO Division of Exploration Geoscience, North Ryde 1

Two contrasting voluminous hydrothermal alteration systems displaying many characteristics of epithermal precious metal deposits occur in the Triassic North Arm Volcanics in southeast Queensland. The alteration systems, at North Arm and Mt Ninderry, are 5 km apart, cover areas of 1.5-2 km and appear spatially related to possible former caldera structures. Hydrothermal effects have been imposed on a gently folded sequence of highK sequence of andesite, dacite and rhyolite lavas, fragmentals and shallow intrusives, together with minor epiclastic sediments which were deposited in a terrestrial, continental margin environment. At the North Arm Au-Ag prospect, potentially economic mineralisation occurs in numerous subparallel quartz-rich vein, stockwork and hydrothermal breccia systems. Although generally narrow, several ore grade Au and Ag zones occur and show an average Ag:Au ratio of 6.3. Contents of As, Sb, Hg, Se and Mo are anomalous, but base metal values are little enriched over levels in adjacent volcanics. No substantial horizontal or vertical metal zonation has been recognised. Quartz is the dominant mineralisation phase; it displays a variety of textures including growth banding, bladed pseudomorphs after calcite, vughs and fluid inclusions containing highly variable liquid-vapour ratios. Several of the phenomena imply thatfluidboiling accompanied quartz deposition. Other minor or trace minerals include calcite, sericite, pyrite, marcasite, arsenopyrite, Ag-Au2

Sb alloys, freibergite, naumannite, chalcopyrite, sphalerite, galena and molybdenite. Pyrite and marcasite are cryptically zoned and locally strongly arsenian (up to 8 wt. % As) and also contain anomalous Cu, Sb, Ag, Se. As may occur in pyrite as submicron layers of arsenopyrite (cf. Fleet et al., 1988) and reflect changes influidaS and a^ during mineral growth. Mineralisation has occurred in the range 200-330°C from fluid inclusion and mineral assemblage criteria. Fluids were evidently neutral to slightly acid, boiling, dilute and C0 -bearing. About mineralised structures, alteration is of the adularia-sericite type of Heald et al. (1987) with phyllic or K feldspar-phyllic grading outwards to distal propylitic. The most intensely rocks show local minor volume decrease, otherwise alteration was isovolumetric and has involved depletion of Na, Ca, Sr (Mg, Mn) and enrichment of S, As, Au, Ag, Sb, Hg, Mo, K, Rb, H 0, C0 (CU, Si, Ba). All hydrothermally altered rocks are depleted in 0 compared to least-altered North Arm Volcanics from the district. Altered rocks and vein quartz have a mean 8 0 value of 5.2%o whereas least-altered rocks have a mean value of lA%o. Hydrogen isotope values from phyllic altered rocks have a mean 8D value of -94%o. Under conditions typical of alteration-mineralisation (« 270°C), calculated oxygen isotopic conditions for hydrothermal water in equilibrium with phyllic alteration and quartz veins range from -0.5 to -4.8%o (Fig. 1). 2

Using the mean 8D, a value of 8 O of-13%o is calculated Sr, Ba) and relative constancy of Al, Ti, P, Zr, Nb, Ga, for unexchanged Triassic meteoric water (Fig. 1). The V, REE (Sr, Ba, Pb). Altered rocks are also enriched in difference between the results for unexchanged water 0 compared to those from the North Arm deposit and and water in equilibrium with quartz veins and alteration from least-altered North Arm Volcanics, with a mean assemblages is ascribed to exchange between heated 8 0 value of 9.7%o. 8D values on clays (mean -105%o) meteoric fluids and the igneous rocks of the North Arm are slightly depleted compared to those obtained from Volcanics. The depleted O values in the North Arm phyllic altered rocks at North Arm. The calculated values hydrothermal system are therefore interpreted as for water in equilibrium with these clays confirm a indicating a modified meteoric water source for the relationship with a probable Triassic hydrothermal event fluids responsible for the alteration-mineralisation involving a dominant meteoric water component (i.e. process. The isotopically light values for the water are potentially the same as at North Arm), rather than an consistent with them being derived from a relatively origin of the system by Tertiary to Recent weathering high-latitude position, conforming with the interpreted processes (Fig. 1). palaeolatitude of eastern Australia in the Triassic (e.g. Data from Mt Ninderry suggest that the system Scotese & Denham, 1988). Sulphur isotope values for represents a "primary supergene" type of acid sulphate hydrothermal pyrite at North Arm (-9.7 to -0.9%o S^S) alteration, i.e. one that has formed above a boiling zone imply (a) migration of *S-enriched hydrothermal fluid by meteoric oxidation of I^S and condensation resulting to higher in the system to yield relatively depleted S^S in the generation of low pH (<4) fluids. Isotopic values in the deeper parts, or (b) a mixed source involving compositions of clays from Mt Ninderry are consistent magmatic (rock) S and an isotopically light source (e.g. with the alteration zone having formed at 100-150°C. biogenic or diagenetic sulphides in organic-bearing It is plausible that the Mt Ninderry alteration system sediments) intercalated in the North Arm Volcanics. overlies, at a depth of 100-400 m, a fossil boiling zone, The Mt Ninderry alteration system is essentially with attendant potential for bonanza-type epithermal barren with respect to precious and base metals, but is precious metal mineralisation. This concept has yet to modestly enriched in As, Sb, Hg, Bi and Mo. It contrasts be tested by serious exploration. Such a system may with North Arm in that it is manifestly an acid sulphate have characteristics similar to the North Arm deposit, (Heald et al., 1987) or advanced argillic hydrothermal or in fact may be an extension of the latter, being a less system. Rocks have been intensely altered over a vertical eroded manifestation of the same geothermal regime. interval of 200 m to quartz-alunite-kaolinite (-dickite) References assemblages, with surrounding kaolinite-quartz and Fleet et al., 1988. Geol. Soc. Aust. Abs. 22: 241-245. peripheral propylitic zones. Intense mineralogical Heald et al., 1987. Econ. Geol. 82: 1-26. reconstitution has been accompanied by up to four-fold Scotese, C.R. & Denham, C.R., 1988. User's manual for volume decreases, with addition of S, H p and volatile TERRA MOBIUS™: Plate tectonicsfor the Macintosh 43 pp. metals, depletion of Fe, Mn, Mg, Ca, Na, Rb, Zn (Si, K, ls

• SMOW

Recent waters temperate Australia

5"o7»

Figure 1 — 8D vs 5 0 plot of waters in equilibrium with alteration phases at North Ann and Mt Ninderry Field I = N u Arm phyllic alteration; Field II = Mt Ninderry advanced argillic alteration. The range of values for Recent «,„,„ V* temperate AustraliafromChivas & Bird (pers. comm. 1988). waters trom ,8

25 A1.17 SHRIMP ION MICROPROBE STUDIES OF SULPHIDE MINERALISATION IN ACTIVE GEOTHERMAL AREAS: THE SALTON SEA AS COMPARED TO THE VALLES CALDERA C.S. Eldridge and M.A. McKibben 1

Geology Department and RSES, Australian National University Department of Earth Sciences, University of California, Riverside

1 2

Active sulphide mineralisation in modern geothermal systems provides a natural laboratory for studying the processes of ore formation. This investigation encompases two such systems with contrasting geologic settings and sulphide precipitation mechanisms. The Salton Sea geothermal system is hosted by evaporitebearing lacustrine sediments, is associated with mafic sills and rhyolitic domes and comprises two brine types stratified by density. It appears that the hot, dense and sulphide-bearing brines have never reached the surface and mineralisation occurs in the zone of mixing between these fluids and cooler, more dilute, overlying waters. SHRIMP sulphur isotopic studies have found that vein minerals are bimodal in their isotopic compositions, spanning a total range of -24 to 9%o, evaporitic anhydrite ranges from 3 to 1 8 a n d stratiform pyrite from -48 to -9%c. The polymetallic vein mineralisation generally appears isotopically well behaved, displaying the correct order of enrichment from pyrite through sphalerite or pyrrhotite to chalcopyrite and then galena, however some isotopic heterogeneity (up to 10%o) has been found from margins to cores of monomineralic veins. Isotopic heterogeneity also occurs within the sedimentary sulphur-bearing minerals where it has been discovered that stratiform pyrite may be isotopically zoned from core to rim by as much as 39 (-48 to -9) %o in less than a millimeter. Moreover, intergrown stratiform anhydrite and pyrite have not approached isotopic equilibrium at conditions reaching greenschist to lower amphibolite metamorphic conditions. Isotopic disequilibrium between vein and stratiform sulfides, which may differ

by as much as 29%o even when immediately adjacent, would indicate that precipitation of the vein minerals did not involve local recycling of stratiform pyrite sulphur. The ion microprobe data, combined with conventional analyses of dominant aqueous sulphur species, imply that sulphide for mineralisation derives instead from dissolution of evaporitic anhydrite at depth and partial reduction of the sulphate at temperatures near to 300°C. The Valles caldera geothermal system differs from that of the Salton Sea in that its stratigraphy is dominantly volcanic and the hydrothermal fluid has apparently boiled. Consequently, the sulphur isotopic systematics of minerals from the Valles caldera drill core are quite different from those observed in the Salton Sea samples. Isotopic variation in vein pyrite of the Valles caldera boiling zone is nearly as great as that noted in Salton Sea sediments, however, it trends the other way from isotopically heavy to light (core to rim) varying within a single cube by as much as 22 (4 to -18) %oin a distance of 75 microns. Such radical isotopic shifts are thought to be due to preferential oxidation of H^S during boiling and precipitation of pyrite from the residual, -depleted fluid. Isotopic heterogeneity in pyrite examined from two other levels in the Valles caldera system is minimal (0±3%o) suggesting that sulphide deposition was not brought about by boiling in these zones. These data would suggest then that ion microprobe analyses may help in the exploration for zones of boiling-induced precipitation and hence for areas of precious metal deposition.

A1.18 FACTORS CONTROLLING THE COMPOSITION OF ELECTRUM IN PRECIOUS METAL DEPOSITS C. H. Gammons Department of Earth Sciences, Monash University

Naturally occurring electrum displays a wide range in composition from nearly pure gold to nearly pure silver. The composition of any particular electrum grain (including "native" gold) is no accident, but rather is a function of the chemical conditions attending ore deposition, modified, in some instances, by supergene effects. For this reason, electrum is an extremely useful mineral in the study of ore deposits. The composition of electrum in hydrothermal solutions saturated with argentite (or some other silver-

bearing sulphide or sulphosalt mineral) is buffered by the prevailing fugacity of S (g), via the following reaction: 2

gcl

+ 1/2 S (g) = AgjS 2

where Ag represents the Ag component in the electrum solid solution. An increase in / S drives the above reaction to the right, resulting in a more Au-rich electrum. In turn, the prevailing S fugacity varies in a cl

very systematic manner, depending on the temperature and chemical composition of the hydrothermal fluid. Consequently, the Au-content of electrum increases with increase in temperature, increase in Kfi concentration, decrease in pH, and shift in redox conditions towards the aqueous sulphide/sulphate boundary. In the presence of argentite, electrum grains will usually be silver-rich (X = 0.3-0.7), with Au-rich electrum (X > 0.8) stable only at very high / S . For solutions undersaturated with argentite or other Ag-bearing phases, electrum composition becomes a more complicated function of the bulk Au/Ag ratio of the ore fluid, the form of Au and Ag in solution (e.g., chloride vs bisulphide complexes), and the specific depositional mechanism involved. Calculations based on chemical and mass balance constraints indicate that ore solutions with high initial Au/Ag ratio (>1) nearly always produce Au-rich electrum, whereas solutions with low Au/Ag ratio may produce either Au-rich or Ag-rich electrum, depending on the specific reactions involved. In the latter case, Au-rich electrum is favored by depositional mechanisms which involve redox reactions, a decrease in pH, or a decrease in the activity of dissolved sulphide (e.g. pyritisation of wallrock), whereas silver-rich electrum is favored by conductive cooling, an increase in pH, or processes which cause Au

quantitative deposition of both metals (e.g.,fluidmixing or boiling). Due to a combination of circumstances, silver-rich electrum is generally more common at relatively low temperature (<300°C). However, the so-called "electrum geothermometer" is of little or no use unless evidence exists for the coprecipitation of both electrum and argentite, and unless an independent means of determining the S fugacity of the ore fluid is available (e.g. Fe-content of sphalerite). Nonetheless, these conditions are not uncommon, especially for epithermal deposits (see, for example, the numerous publications by Shikazono and coworkers on the composition of electrum grains from the epithermal precious metal deposits of Japan). On the other hand, there are numerous examples in the ore deposit literature where electrum clearly was not coprecipitated with argentite. In these cases, electrum composition becomes a function of many parameters (as outlined above), and may show no obvious trend with temperature or / S^ For example, silver-rich electrum (X < 0.1) was deposited from high temperature solutions (> 300°C) in the Ag-Ni-AsCo veins of Cobalt, Ontario, whereas gold-rich electrum (X > 0.9) was deposited from relatively low temperature solutions (200°C) in the "Carlin-type" disseminated gold deposits of the western United States. 2

A1.19 THE SOURCE OF THE GOLD IN WESTERN TASMANIAN VMS DEPOSITS Joe Stolz* and Ross R. Large Key Centre for Ore Deposit and Exploration Studies, University ofTasmania

Volcanogenic massive sulphide deposits from mean values are comparable with those of unaltered western Tasmania have significantly higher gold grades modern volcanics of similar composition (Fig. 1). than other VMS deposits from eastern Australia and Hydrothermally altered equivalents of these volcanics elsewhere. The dominant controls on gold grades in (recognised by varying degrees of sodium depletion) VMS deposits are probably temperature and composition generally have somewhat higher Au concentrations. of the hydrothermal fluids. However, the concentration However, surface samples have lower Au contents and of gold in the source rocks being leached by the appear to have been leached by groundwater.The hydrothermalfluidsmay also affect thefinalgold grades Tasmanian ultramafic rocks have significantly lower gold (mean 0.5 ppb) than unaltered peridotite xenoliths in the deposit. The major VMS deposits of western Tasmania are from basaltic lavas, but similar concentrations to hosted within the Central Volcanic Complex of the ophiolitic ultramafic rocks. High-Mg, low-Ti volcanics Mount Read Volcanics or overlying andesites of the from the western Tasmanian ultramafic-mafic complexes Que-Hellyer Sequence (Corbett & Lees, 1987). have similar gold contents (mean 1.6 ppb) to mafic Additional potential source rocks for leaching of metals rocks from the Central Complex or Western Sequence include, Late Proterozoic basalts and sediments of the of the Mount Read Belt. In contrast, relatively high-Ti Crimson Creek Formation, ultramafic-mafic rocks from basalts of the Crimson Creek Formation are the most a tectonically emplaced ophiolite complex (Berry & Au-enriched primary volcanics in the Dundas Trough Crawford, 1988), and Precambrian schists and quartzites. (up to 23 ppb Au). These volcanics were probably Background gold concentrations were determined on erupted during the first rifting phase in the Trough (Varne & Foden, 1987), and may underlie much of the representative samples of these potential source rocks Mount Read Volcanic Belt. using a radiochemical neutron activation technique The relatively high gold concentrations in the (detection limit 0.1 ppb). The gold concentrations in the least altered Mount Crimson Creek basalts indicate the presence of an Read Volcanics (mainly samples of andesite, dacite and anomalous relatively Au-rich potential source rock rhyolite) are in the range 0.9 to 1.3 ppb Au, and the beneath the Mount Read Volcanics which may be

responsible for the relatively Au-rich character of the Tasmanian VMS deposits. The temperature and composition (e.g. pH, /02, aH S, mNaCl) of the hydrothermal fluids, and the consequent solubility of metal species will provide a powerful, and probably dominant, control on the final concentrations of base and precious metals in a specific massive sulphide deposit (Huston & Large, 1989). Source rock composition will only be important if the hydrothermalfluidhas 2

an appropriate chemistry to allow significant Au solubility. References

Berry, R.F. & Crawford, A.J., 1988. Aust. J. Earth Sci. 35: 523-533. Corbett, K.D. & Lees, T.C., 1987. Aust. J. Earth Sci. 34: 45-67. Huston, D.L. & Large, R.R., 1989. Ore Geol. Rev. 4:171-200. Varne, R. & Foden, J.D., 1987. Tectonophysics 140:275-295. 12

Intraplate basalts

0 c c a n n o o r

basalts

Lo -Ti high-Mg basalts w

Mount <* basalts

Rea

Crimson Creek basalts

Unaltered peridotites

Pcridotites

f r o m

ophiolites

West Tasmanian peridotites

Figure 1 — Plots comparing the average Au abundances of western Tasmanian volcanics and ultramafic rocks with rocks of similar composition from a variety of settings.

A1.20 THE MINERALOGY OF GOLD IN SOME VMS DEPOSITS IN EASTERN AUSTRALIA R. S. Bottrill and D.L. Huston 1

Department ofResources and Energy, Tasmania Key Centre for Ore Deposit and Exploration Studies, University of Tasmania 1

In a number of Eastern Australian Volcanogenic the baritic ore (fineness 730-760). At the south-end of massive sulphide deposits, gold is present in significant Rosebery a crosscutting pyrrhotite-pyrite body (related amounts and its mineralogy is dependent upon the to Devonian granite-related metasomatism) contains gold genesis of the host ore. It can be shown to be related to of coarser grainsize and higher fineness (Zaw, in press) the mode of transport and deposition of gold, the average In the probably volcanogenic but highly metagrade and the subsequent deformational history. morphosed and deformed Pb-Zn deposits of Broken In relatively undeformed Kuroko-style deposits, gold Hill (NSW) gold is rare but has been described as very can be concentrated within (1) the copper-rich stringer fine inclusions, of apparent highfineness,in tetrahedrite or stockwork zone at the base of the sulphide mound and in cobaltite (Stillwell, 1940; Stillwell & Edwards, (equivalent to the oko and keiko ores), (2) the Pb-Zn- 1944). It has also been detected by microprobe as rich massive sulphide zone in the centre of the mound substituting in a silver sulphosalt (?pyrargyrite or (kuroko-type ore), and (3) the baritic cap near the top of stephanite). the sulphide mound. The compositional variations found in electrum in In undeformed, copper-rich, stringer-type ores (e.g. the undeformed ores can be related to a model of gold Mt Chalmers, Qld), gold occurs as free gold (or electrum) transport as both chloro- and thiocomplexes. For gold of relatively coarse grain size and relatively high fineness transported as a thiocomplex in "typical" Pb-Zn (800-1000), associated with chalcopyrite or as inclusions depositing hydrothermal fluids, between 200-275°C, in pyrite. the predicted range of fineness (100-900) agrees well In the moderately deformed Mt Lyell Cu-Au deposit, with the observed range. For gold transported as a primary gold of high fineness is rarely preserved; most chlorocomplex in "typical" copper-depositing appears to have been remobilised and re-equilibrated hydrothermal fluids, above 300°C, the observed range with the ores to give electrum with finenesses of 650- of fineness (>800) agrees with a model for fluids with 900. Most electrum is closely associated with both atomic Ag/Au around 10 . pyrite and chalcopyrite, and some is associated with During deformation, gold is released from the sphalerite. The grainsize range is very wide. inclusions within, and crystal lattices of, pyrite, The highly deformed and metamorphosed Balcocma arsenopyrite, tetrahedrite and other minerals, and Cu deposit (Qld) has a much finer grain size but a migrates into grain boundaries and fractures, coarsening similar fineness range (670-790). The principal and re-equilibrating with chalcopyrite, galena and other associates are chalcopyrite, pyrite and bismuth minerals. silver-bearing minerals to form electrum or relatively In undeformed Pb-Zn rich VMS deposits (e.g. silver-rich gold. The grainsize is related to the gold Hellyer, Tasmania) gold occurs as solid solution in tenor as well as the degree of deformation. A silver-rich auriferous pyrite, arsenopyrite and probably tetrahedrite rim on the gold or electrum is commonly present, and and other sulphides within massive sulphide ore; or indicates some late-stage mobility of silver. rarely as free electrum (± galena) in the baritic cap References (Huston et al., in press). Huston, D.L., Bottrill, R.S., Creelman. R.A., Zaw, K., The moderately deformed Rosebery Zn-Pb-Ag Ramsden, A.R. & Rand, S., in prep. Econ. Geol. deposit contains gold (electrum) in several distinct Stillwell, F.L.,1940. Proc. Australas. Inst. Min. Met. associations: (1) within pyrite in Pb-Zn ores, with Stillwell, F.L. & Edwards, A.B., 1944. Proc. Australas. Inst. Min. Met. variable fineness (320-700); (2) with pyrite, galena and tetrahedrite in distal pyrite mineralisation (fineness Zaw, K., in prep. Econ. Geol. 610-730); and (3) with chalcopyrite, galena or barite in s

29 A1.21 MINERALISED ARCHAEAN CALDERAS(?) IN THE WHIM CREEK VOLCANIC BELT, WESTERN AUSTRALIA. Peter L. F. Collins Department of Geology and Geophysics, Curtin University ofTechnology

The Whim Creek volcanic belt in the western Pilbara Block of Western Australia, is an arcuate belt of volcanosedimentary rocks, some 100 km long and 5-20 km wide, that accumulated about 2950 Ma. The belt consists of a bimodal rhyolitic — basaltic volcanic suite and volcaniclastic and sedimentary rocks that unconformably overly a "contintental" basement of granitoids and older metamorphosed supracrustal sequences. The Whim Creek volcano-sedimentary sequence is symmetrically distributed about the Caines Well Batholith. A basal formation of predominantly sub-aerial(?) felsic volcanic rocks (Mt Brown Rhyolite) flanks the batholith but also crops out as domal volcanic piles that spread across the belt, e.g. between Whim Creek and Mons Cupri. The felsic volcanics are overlain by volcaniclastic rocks (Mons Cupri Volcanics), followed by a submarine sedimentary facies (Rushall Slate) which have accumulated in basinal structures on the flanks of the felsic volcanic centres. The older formations are unconformably overlain by a widespread blanket of subaerial basaltic rocks (Negri Volcanics) that crop out along the southern and eastern margins of the belt, furthest from the Caines Well batholith, and are in turn locally overlain by terrestrial sediments. Three volcanogenic massive sulphide deposits are known within the belt, at Mons Cupri, Whim Creek and Salt Creek, and are hosted by submarine sedimentary sequences that followed the main phase of felsic volcanism. The two largest deposits at Whim Creek and

Mons Cupri, have formed on the northern and southern flanks, respectively, of one of the centres of felsic volcanism. Epithermal(?) vein mineralisation is associated with or follows the final phase of subaerial basaltic volcanism. Several large circular and arcuate features within the Whim Creek volcanic belt have been identified on aerial photographs and Landsat TM imagery. Coincident features are also interpreted on image-processed aeromagnetic data which is depicting basement lithologies rather than the Whim Creek belt stratigraphy. The Mons Cupri and Whim Creek massive sulphide deposits are each within a circular feature that also enclose many of the surrounding smaller base-metal mineral deposits. The circular structures within the Whim Creek belt are 5-6 km in diameter and are comparable in size to interpretedresurgentcalderas that host Kurokotype massive sulphide deposits in the Hokuroku Basin in Japan. The Whim Creek volcanic belt is not as deformed, nor suffered the same degree of metamorphism, as the greenstone belts within the Yilgarn Block, and its present configuration is approximately the same as when the volcanic pile accumulated. Hence, if calderas had developed during Archaean Whim Creek volcanism then remnants of such structures may be preserved, and now represented by the circular (and arcuate) features identified within the volcanic belt and in the basement rocks. Late epithermal mineralisation may reflect a resurgent phase in caldera development

A1.22 PRIMARY VOLCANIC FACIES OF THE GOSSAN HILL GROUP, GOLDEN GROVE, WESTERN AUSTRALIA: VOLCANIC PROCESSES AND TIMING RELATIVE TO MASSIVE SULPHIDE MINERALISATION Bretan A. Clifford1 and Ray A. Cas2 1

Australian Consolidated Minerals, Perth, Western Australia 2 Department of Earth Sciences, Monash University

The Scuddles and Gossan Hill massive sulphide deposits are hosted by the Archean Gossan Hill Group at Golden Grove, Western Australia. The Gossan Hill Group outcrops over a strike length of 25 km on the NW flanks of the Warriedar Fold Belt, with steep dips providing a cross-section through the volcanicsedimentary succession. Critical evaluation to differentiate between primary and secondary textures allows a detailed facies analysis to be completed in these rocks which have been subjected to hydrothermal alteration, lower greenschist facies metamorphism and deformation.

The Gossan Hill Group is composed of a facies association which includes the products of proximal rhyodacitic to basaltic volcanism, mass-flow sedimentation and concentrated hydrothermal discharge, in a deep subaqueous setting. Primary volcanic facies comprise an estimated 20-25% of the volume of the preserved Gossan Hill Group and are dominated by andesits to rhyodacite lavas of calc-alkaline affinities. Tholeiitic basalt flows are present but are of limited volumetric significance. Primary volcanic facies are limited to composite lava flows, associated autoclastic breccia deposits and

syn-volcanic intrusives. Composite lavaflowsvary from low aspect ratioflows,laterally continuous on a scale of kilometres, to viscous, high aspect ratio flows. Row geometry of felsic lavas is locally controlled by differential syn-depositional subsidence, suggesting relatively low viscosities and vesicularities are generally low. Autoclastic breccias are associated with the majority of lavas and in several cases exceed their associated lava and intrusive components in volume. Abundant, widespread associated dykes suggest fissure eruption over a broad area. Primary pyroclastic facies are absent, with the possible exception of distal water-lain airfall deposits. The non-explosive style of proximal volcanism, low vesicularities of lavas and evidence for relatively low viscosities of felsic lavas suggests suppression of volatile exsolution, due to hydrostatic pressure constraints in the environment of eruption. The inferred deep

subaqueous setting required for these hydrostatic pressure constraints is consistent with the documented mass-flow sedimentary facies association and the occurrence of volcanogenic massive sulphide mineralisation. The timing of proximal volcanism relative to the formation of the massive sulphide deposits is of significance. Two phases of proximal volcanism are recognised, however, hydrothermal exhalative activity appears related only to the second phase of eruptive activity. Massive sulphide mineralisation occurs over a wide stratigraphic interval preceding the second phase of proximal volcanism. While hydrothermal activity is evident during this period, the intense activity related to the formation of the Scuddles and Gossan Hill deposits is limited to the stratigraphic interval immediately preceding the initiation of the second eruptive phase.

A1..23 THE ANDESITE CONNECTION- STRATIGRAPHIC CORRELATIONS BETWEEN THE MAJOR MINERAL FIELDS IN THE MOUNT READ VOLCANICS Keith D. Corbett Department of Resources and Energy, Tasmania

Tasmania's rich mineral heritage derives largely from the Mount Read Volcanics, a 200 km long by 20 km wide belt of Cambrian calc-alkaline felsicintermediate volcanics abutting Precambrian basement to the east and Cambrian sedimentary sequences to the west. The rocks are altered, deformed, internally complex, and difficult to date, and their pedogenesis and tectonic setting remain controversial. Three major stratigraphic-lithological subdivisions can be recognised. Most prominent is the Central Volcanic Complex (CVC), comprising mainly felsic (feldspar-phyric) volcanics with only minor sedimentary lenses. On the southern side of the major NNE-trending Henty Fault Zone (HFZ), the CVC interfingers on its western side with a sedimentary sequence of epiclastic tuffs, shales and greywackes referred to as the Yolande River Sequence (YRS, Corbett et al., 1989; formerly "Western Sequence"). Large tabular bodies of quartzfeldspar porphyry intrude this sequence. Overlying and flanking the CVC both north and south of the HFZ (and also overlying the YRS south of the HFZ) is a younger, sediment-rich, sparsely fossiliferous sequence with abundant epiclastic tuffs, volcaniclastic conglomerates, some quartz-feldsparphyric lavas, and locally-developed large basalt-andesite accumulations at or near the base. This younger sequence is referred to as the Dundas Group and correlates north of the HFZ, and as the Tyndall Group (plus local andesite) south of the HFZ. The Rosebery and Hercules orebodies apparently lie in the stratigraphically lowest part of the CVC (Fig. 1), overlain by a thick felsic lava sequence on Mt Black.

The Hellyer and Que River orebodies are apparently younger, and lie within a submarine andesite-basalt complex (Que-Hellyer Volcanics) in the lower part of the Dundas Group, overlain by the fossiliferous Que River Shale and a felsic tuff-greywacke sequence (Southwell Subgroup: Corbett & Komyshan, 1989). The multiple orebodies of the complex LyellComstock field lie within a very large (5 x 1.5 km) alteration zone, mostly within the upper part of the CVC. However, exhalative VMS bodies at Comstock actually occur within a 500 m-thick unit of andesitic breccias lying between the CVC and the Comstock Tuff (with associated fossiliferous limestone) of the Tyndall Group. These exhalative bodies, and a large associated sinter-type chert body, might well be related to the "footwall-type" mineralisation which constitutes the bulk of the Lyell field. The stratigraphic position of the andesite sequence at Lyell (Fig.l) strongly suggests correlation with the Que-Hellyer Volcanics, and raises the possibility of a major belt-wide mineralising event at about the basal Tyndall Group position, associated with andesitic volcanism. The silica-rich alteration and associated gold and sulphide mineralisation at the Henty Prospect is located within upper Tyndall Group volcaniclastics, on the footwall of the Henty Fault. Other alteration zones and minor mineralisation occur in the Comstock Tuff and in the underlying andesites of the Anthony Road area. These occurences, and those at Comstock, Que River, and Hellyer, emphasize the prospecti vity of the younger (post-CVC) parts of the MRV pile, and suggest that

previously poorly-rated Tyndall Group rocks should be re-evaluated. The larger andesite accumulations probably represent separate stratovolcanoes — e.g. Que-Hellyer, Lynch Creek, Agglomerate Hill near Comstock, Anthony Road. The rocks are medium to high-K, and moderately to strongly enriched in LREE. Abundant andesitic

intrusives in the CVC south of the HFZ have similar REE patterns and are probably feeders to these andesites. References Corbett, K.D., Calver, C.R., Everard, J.E. & Seymour, D.B., 1989. Queenstown 1:25000 Map. Corbett, K.D. & Komyshan, P., 1989. Mt Read Volcanics Project Geological Report 1.

ROSEBERYHERCULES

HELLYERQUE RIVER

HENTY

MT. L Y E L L COMSTOCK

Owen Congl.— Upper Dundos Group

Owen Congl.

Owen Congl. Volcaniclastics M«nty gold

Owen Congl.

Volconlclastics Southwell Subgroup a i

y6urtln<" Ocrvia \ ond.

1 I

Stitt Qtzlte White Spur Formation rJAnd. ^ M t Black - dacltes HW qtz-phyrlc* Ro««b«ry Footwall Pyroclastlcs

Comstock Tuff

LYNCHFORDMINERS RIDGE Owen Congl. (Pioneer

Volcaniclastics Comstock Tuff

it boiolb)

Anthony Rood Andesite9

Upp«r Animal Cntk C'wacks Lower Animal Cre«V Cwock* Central Volcanic Complex CVC

Smelters

Lynch Creek 'Bosolts*

Andesite' Comtteck

< Juk«» y Ply \ CVC wDc.\Princ«0

PHne* Ly.N CVC

Miners Ridge Bosolt (tholeiite)

YRS

STRATIGRAPH1C CORRELATION -

Tyndall Group Correlates

/ W V \ A

ly«N

Yolande^ River Sequence

Owen Congl. nj\Tu\f\ Volconiclastics

Comstock Tuff

frQm R. ShqU

QueHellyer Volconlcs (And sail aa

JUKESDARWIN

MRV MINERAL FIELDS

A1.24 THE ROSEBERY CONTROVERSY: DISTINGUISHING PROSPECTIVE SUBMARINE IGNIMB RITE-LIKE UNITS FROM TRUE SUB AERIAL IGNIMB RITES IN THE ROSEBERYHERCULES ZnCuPb MASSIVE SULPHIDE DISTRICT, TASMANIA Rodney L. Allen and Ray A.F. Cas Department of Earth Sciences, Monash University All major volcanic-associated massive sulphide felsic lavas and massive pumiceous units, and contains deposits occur in marine volcanic and sedimentary rocks, only rare discontinuous sediment units, but importantly and where the ore deposits occur in mixed subaerial- also several massive and disseminated sulphide deposits. subaqueous rock successions, the deposits invariably Corbett (1981) suggested the Central Volcanic Complex occur in the subaqueous facies. Mapping of subaqueous may be the remanants of a largely subaerial volcanic versus subaerial facies can therefore be an important chain with a series of marine embayments hosting the geological exploration tool for these deposits. massive sulphides. In the Rosebery-Hercules area, Green Sedimentary rocks are by far the easiest rocks to interpret et al. (1981) and Lees (1987) suggested that caldera in terms of depositional environment due to the immense collapse in a largely subaerial and partly welded ignimamount of work that has been done on modem and brite pile led to marine incursion and massive sulphide ancient sedimentary environments. By comparison mineralisation within the caldera. We have built on the diagnostic criteria for environmental interpretation of work of Corbett, Green and Lees to determine the extent primary volcanic facies are poorly developed. Massive of subaqueous conditions and criteria for distinguishvolcanic complexes with a paucity of sediments have ing the subaqueous volcanics from subaerial ones. We commonly been regarded as eroded subaerial volcanic conclude from the distribution of hyaloclastites, successions. The massive Central Volcanic Complex of subaqueous "ignimbrite-like" pumiceous mass flows, the Mt Read Volcanics, Tasmania, is dominated by and volcanogenic sediments that subaqueous

environments are more extensive in the Central Volcanic phyllosilicate alteration of the pumice lenses. A scenario Complex than has been previously considered. is suggested in which the rocks were heterogeneously The large volume, laterally extensive, pumiceous altered soon after deposition. The phyllosilicate-altered units in the Rosebery-Hercules district have similarities pumice patches were flattened (and foliated?) by in distribution, thickness and appearance to subaerial diagenetic compaction, resulting in false welded fiamme welded and non-welded intracaldera ignimbrites. Some textures. However, in the surrounding more competent of these rocks are altered, variably pumiceous lava, quartz-feldspar-rich altered domains pumices remained hyaloclastite and intrusive hyaloclastite. However, most relatively uncollapsed, thereby preserving non-welded are true pumiceous mass flows with individual flow pumice textures. units up to at least 150 m thick. Despite a paucity of We suggest that rather than implying subaerial interbedded sediments, the following features indicate conditions, a paucity of sediments in volcanic complexes these pumiceous massflowswere emplaced entirely in primarily implies high eruption rates relative to rates of a subaqueous environment: (1) normal grading of flow ambient sedimentation (in both subaerial and submarine tops; (2) local thick overlying massive to laminated volcanic successions). Paucity of sediments can also be vitric ash; (3) fine pumice breccia texture with minor due to the construction of volcanic centres that rise interstitial ash matrix is common (moderate sorting); above flanking basin floor sedimentary environments. (4) absence of tractional reworking in thick piles of Several different volcanic settings have been originally unconsolidated pumice; and (5) absence of proposed for volcanic-associated massive sulphide undoubted welding. deposits, including deep and shallow marine pyroclastic Apparent welded pumice textures are common in calderas, lava-dominated submarine caldera volcanoes, the Rosebery-Hercules rocks and consist of: (1) sub- submarine lava shield volcanoes, and lava-dominated millimetre fine fibrous or streaky texture (visible with submarine graben basins. We suggest that submarine hand lens and microscope), which is interpreted as pyroclastic caldera settings have been over-emphasised normal non-welded tube-vesicle pumice texture; (2) dark in the literature, due to misinterpretation of volcanic chlorite- or sericite-altered lenses, generally 1-5 cm facies, and to a common assumption that silicic long, set in a pale coloured more siliceous matrix, and volcanism is mainly explosive and caldera generating aligned roughly parallel to regional bedding. The dark regardless of whether the setting is subaerial or lenses are indeed flattened pumiceous material. subaqueous. Despite this, the Rosebery-Hercules However, pumice in the more siliceous matrix preserves massive sulphide district remains one of the most excellent delicate unflattened round-vesicle and tube- plausible candidates for a pyroclastic caldera-related vesicle pumice textures, implying the rocks are not district, more plausible in fact than the frequently cited significantly welded. All pumice clasts are attenuated type example of the Hokuroku district of Japan. or crenulated by the regional cleavage where the cleavage References is strong. However, an earlier mica foliation discordant Corbett, K.D., 1981. Econ. Geol. 76: 209-230. to the regional cleavage and parallel to the chlorite- or Green, G.R., Solomon, M. & Walshe, J. L., 1981. Econ. Geol. 76: 304-338. sericite-altered pumice lenses is also present This fabric 1987. Unpubl. MSc thesis, University of Tasmania: is interpreted as a diagenetic compaction fabric or early Lees,164T.C., pp. bedding-parallel tectonic fabric, and is post- or synA1.25 STYLES OF SEDIMENTATION AND VOLCANISM WITHIN THE QUE-HELLYER VOLCANICS John C. Waters Department of Earth Sciences, Monash University

The Que-Hellyer Volcanics (Komyshan, 1986), are a suite of Late Middle Cambrian mafic to felsic lavas and volcaniclastics in excess of 1 km thick. These volcanics lie approximately 90 km to the south of Burnie, in north-western Tasmania. The Que-Hellyer volcanics lie within the Dundas Group and are considered part of the Mount Read Volcanics (Corbett, 1989). The QueHellyer Volcanics host both the Que River and Hellyer "Kuroko style" polymetallic volcanogenic massive sulphide deposits. The stratigraphy of the volcanic sequence in the area of the Hellyer and Que River mines has been

previously defined (Fig. 1), but elsewhere, especially to the west, little is known. In the area of the Hellyer deposit extensive drilling combined with the lack of a penetrative deformation and low grade prehnitepumpellyite regional metamorphism has allowed detailed sedimentological and volcanological studies. Three kilometres to the south at the Que River deposit the geology is complicated by more intense and wide spread hydrothermal alteration and a greater degree of structural complexity. Coherent volcanics are found throughout the sequence, but appear to dominate around the ore position and in the hangingwall. In the footwall

to mineralisation coherent lavas appear volumetrically either explosive fragmentation or non-explosive less important until the base of the sequence which is fragmentation of originally vesiculated glassy shards. marked by the "Lower Basalt". The composition of The style of sedimentation changes from these these lavas varies from dominantly andesitic in the sporadic and rapidly deposited massflows,to low energy footwall, to dacitic then basalt at and just above the pelagic sedimentation for the deposition of the Que mineralised horizon (Whitford et al., 1989; Jack, 1989). River Shale after the emplacement of the Hellyer Basalt Within the Que-Hellyer sequence these lavas are The deposition of an approximately 100 m horizon of dominantly shallow intrusives to extrusives. The mafic black shales and minor sandstones represents a hiatus in lavas form both sheet and pillowed flows, possibly volcanism after which the style and products of indicating variations in extrusion rates, composition volcanism changed. The overlying Southwell Subgroup and distance from source. Dacitic lavas occur primarily represents a period of dominantly felsic volcanism with as "domes" and autoclastics and are relatively restricted evidence of explosive activity. spatially to about the horizon hosting mineralisation. The general lack of products of explosive volcanism Fragmentation of these lavas to produce the abundant below the Que River Shale may be a function of the volcanic breccias, both in the footwall and the composition of the lavas combined with their possible hangingwall to the mineralisation, was via auto- emplacement in deep water under high confining brecciation and quench fragmentation, coupled with pressures. Although the lack of such products does not later resedimentation. rule out the possibility of emplacement in shallow water A break in volcanism represented by resedimented depths, the absence of abundant large scale tractional volcaniclastics occurs immediately above and below sedimentary structures, the presence of the black, anoxic the horizon hosting mineralisation at Hellyer. These Que River Shale, and the mass flow characteristics of volcaniclastics consist mainly of massflowsof material clastic units favour a relatively deep water setting. sourced dominantly from the locally fragmented basaltic References through to dacitic lavas, with a minor component of Corbett, K.D., 1989. In C.F. Burrett & E.L. Martin (eds): basement lithics and fragmented ore. These mass flows Geology and Mineral Resources of Tasmania. Geol. Soc. Aust. Spec. Pub. 15: 86-118. are very poorly sorted and chaotic with grain size ranging D.J., 1989. Unpubl. MSc thesis, University Tasmania. from fine to medium sands up to cobble size. Some of Jack, P., 1986. In Large, R.R. (ed.): The Mount Read the individual units appear very diffusely graded. The Komyshan, Volcanics and Associated Ore Deposits. Geol. Soc. Aust, volcanic debris consists partly of vesicular to non Tasmanian Hobart: 53-55. vesicular vitric material. This vitric component has Whitford, DJ., Div., McPherson, W.P.A. & Wallace, D.B., 1989. straight to cuspate margins which could be produced by Econ. Geol. 84: 1-21. QUE RIVER SHALE

massive to laminated black shales. Hellyer Basalt massive to pillowed lavas, hyaloclasntes. peperttcs and pillow breccias ooiymichc mass flow brecoas and fine to coarse volcaniclastics Dacitic lavas, massive to amobrecciated Andesitic breccias, volcaniclastics. hyaloclasutes and minor lavas

QUE - HELLYER VOLCANICS

Lower Basalt massive to aoioorecciated and. hyaloclasutes

ANIMAL CREEK GREYWACKE

lithic rich micaceous ssc. tnterbedded saaies. volcaniclastics.

Figure 1 — Schematic stratigraphic column for the Que-Hellyer Volcanics (modified after Wallace, 1989) (not to scale).

A1.26 ERUPTIVE STYLE, PRODUCTS AND SETTING OF KUROKO VOLCANICS, MIOCENE GREEN TUFF BELT, JAPAN Ray A.F. Cas , Rodney L. Allen , Hiromitsu Yamagishi , Yohei Ishikawa and Takeshi Ohguchi 1

2 3

Dept. Earth Sciences, Monash University Geological Survey, Hokkaido, Sapporo, Japan Mining College, Akita University, Akita, Japan 1

A re-evaluation of the volcanic-sedimentary successions hosting Kuroko volcanic hosted massive sulphide (VMS) mineralisation in the Hokuroku Basin of the Miocene Green Tuff Belt, northern Honshu, Japan, indicates that evidence for the 3.5-4.5 km deep-marine, highly explosive caldera centres proposed as hosts to the VMS by some authors, is not convincing. First, there is no evidence for ring fracture faults, ring dyke systems or caldera subsidence structures, the only known faults being N-S trending basement faults that parallel the regional tectonic trend of the extensional Green Tuff Belt. Secondly, there are differences of opinion on the water depths indicated by the benthonic foraminiferal assemblages. Examination of the volcanic successions further indicates that footwall successions overwhelmingly consist of lava domes, intrusive domes, sheet lavas, and in situ and resedimented autoclastics (autobreccias, hyaloclastites), but true pyroclastic debris is minor. At the level of the mineralisation and in the hanging wall, pumiceous pyroclastic debris occurs and is abundant in the hanging wall in some drillholes. However, this pumiceous debris is not welded, and occurs in thick, variably graded, massive, mass-flow sedimentation units. There is no evidence that the pyroclastic debris was erupted in situ, nor is there evidence of intracaldera ignimbrites. Furthermore, theoretical considerations preclude explosive expansion of volatilised freshwater, seawater or magmatic water at water depths equivalent to the critical point of water (= 2 km for F.W. and 3 km for

S.W.), due to the effects of increasing hydrostatic pressure with water depth. Practical limiting water depths for explosive subaqueous eruptions are about 1 km. These constraints suggest that the hanging wall pyroclastic debris was derived from shallow-water to subaerial basin margin volcanic centres. Alternatively it may have been derived from the tops of intrabasinal lava volcanoes, represented by the footwall volcanics (as proposed by Ishikawa), which grew to shallow enough water depths whereby explosive activity was possible from the summit region. The resultant pyroclastic debris was then resedimented by mass-flows. The thickness of the pumiceous mass-flow sedimentation units is consistent with large volume, pyroclastic eruptions. Much of the pumiceous deposits may have originated as subaerial pyroclastic flows which entered water and transformed into water-supported mass-flows. Some of the deposits contain significant rounded debris, indicating residence influvialor shoreline environments. Excellent coastal exposures in the Late Miocene, northern part of the Green Tuff Belt in Hokkaido, are also dominated by lavas and in situ and resedimented autoclastics. Unequivocal in situ pyroclastics are very localised. Lava and autoclastic piles are cut by and sourced from contemporaneous feeder dyke complexes that are compositionally identical to the lavas and autoclastics they cut. Yamagishi has also proposed that the volcanic centres were submarine lava and hyaloclastite dome complex (rhyolitic, dacitic) and shieldlike (basaltic, andesitic), lava and hyaloclastite dominated volcanoes.

A1.27 MISBEHAVIOUR OF PYROCLASTIC FLOWS AT THE LAND-SEA INTERFACE: A TALE FROM WALES G J. Orton Department of Earth Sciences, Monash University

The behaviour of subaerial pyroclastic flows upon entry into water has been the subject of much speculation. Some authors maintain that pyroclastic flows interact explosively with water to generate littoral vents. In contrast, others workers maintain that flows hold on to their integrity at the air-water interface, continue to flow underwater as gaseous particulate flows, and even retain enough heat to weld for some distance past shorelines. The large ash flow sheets from the Ordovician of North Wales, UK, initially appeared to fall into the second category. The Garth tuff of the Capel Curig Volcanic Formation and the lower outflow sheet of the Pitts Head Tuff in particular provide notable case studies of ash flow tuffs which although erupted and deposited subaerially can be traced towards marine settings. They have frequently been cited (e.g. Howells et al., 1979; Kokelaar et al., 1984: 253) as examples of both subaqueous pyroclastic flows and subaqueous welding. In trying to reconstrunct depositional settings, authors have historically focussed attention on the better exposed epiclastic successions which overly tuff horizons, and falsely assumed that "even quite major eruptions commonly caused little or no change in facies or composition of the 'normal' sediments occurring immediately beneath and above the volcanic horizon" (Kokelaar et al., 1984: 265). However, more detailed analysis of sedimentary facies and depositional environments within subjacent epiclastic successions, as well as identification of more distal, largely unwelded representatives of several ash flow tuffs, illustrate that both pyroclastic flows disintegrated shortly after traversing the land-water interface. The Garth pyroclastic flow entered the sea from a wide, low gradient braidplain and persisted as a continuous sheet for at most 5 km past the coastline. Therafter, its only distinct expression consists of isolated pods of welded tuff; these are thought to represent the dense unmixed centres of flow lobes stemming from preferential incorporation of water along the clefts of a polylobateflowfront The post-emplacement formation of low-gradient coastal profiles, combined with locally abundant supplies of unlithified sediment from disrupted portions of the ash flow tuffs, was conducive to the construction of barrier islands and sand bars by wave processes. These depositional features retreated landward over the welded subaerially-deposited pyroclastic material during a marine transgression after tuff emplacement, thereby confusing early researchers. The Pitts Head pyroclastic flow, in contrast descended from

an intensely faulted basin margin and small, high gradient alluvial fans directly into marine environments. Along coastlines, evidence for vigorous interaction of the pyroclastic flow with basin waters includes: (1) development of a thick, disorganised, collage of various admixtures of unwelded tuff and sandstone along the flow base; (2) extensive rheomorphic folding of primary flow foliation ascribed to upward streaming of water vapour generated at the tuff/sediment interface; and (3) intrusion of large sand dykes into the centre of tuffs during late stages of flow emplacement. Subaqueous products from these mixing processes vary geographically from a single, partially welded pyroclastic flow deposit to a series of thinner, largely unwelded, pumiceous or crystal rich debris flows. The contrasting patterns of flow disruption within and between the two pyroclasticflowsacross the critical air-water interface are related both to distance of coastline from source vents and the topography or hydrodynamic roughness of subjacent landscapes. These would control flow velocities, amounts of fluidisation, thickness of the turbulent basal boundary layer, and the related 3-dimensional morphology of the flow front Due to continued ingestion of water along flow bases and flow fronts, the subaqueous correlatives rarely retained sufficient heat to weld in depths of water greater than deflated flow thickness. Results from this study have wider implications for interpretations that might be drawn from ancient subaqueous volcaniclastics in other other regions. Firstly, the unparalleled spectrum of hydrodynamic processes which can govern the rate of interaction between pyroclasticsflowsand ambient sediment/water mixtures, and resultant contemporaneous generation of a complete variety of products from one parent flow, will wreck havoc with both correlation of pyroclastic successions between isolated outcrops and determination of the eruptive history in any one basin. Secondly, the development of non-diagenetic welding fabrics within subaqueous settings indicates immediate proximity to volcanic vents, which might be important in exploration for some massive sulphide deposits. References

Howells, M.F., Leveridge, B.F., Addison, R., Evans, C.D.R. & Nutt, M.J.C., 1979. Geol Soc. London, Spec. Publ. 8: 611-618.

Kokelaar, B.P., Howells, M.F., Bevins, R.E., Roach, R.A. & Dunkley, P.N., 1984. Geol. SocLondon Spec. Publ. 16: 245-269. t

A1.28 VOLCANIC SETTING OF EPITHERMAL GOLD MINERALISATION IN THE NORTHERN DRUMMOND BASIN, QUEENSLAND B.S. Oversby , G.R. Ewers , D.E. Mackenzie , J. McPhie , D.Wyborn , S. Law and L.P. Black 1

Bureau ofMineral Resources, Canberra 19 Donaldson St., Norman Park, Qld; formerly Queensland Department of Mines 1

Following the discovery of gold mineralisation at assigned to an older volcanic-sedimentary sequence, Pajingo, 50 km SSE of Charters Towers, in 1984, the unit "DCv", by Law et al. (1989) and Ewers et al. northern Drummond Basin has been the scene of intense (1989). Rocks of the mixed, lower Late Carboniferous exploration for epithermal-type gold mineralisation. This sequence were intensely faulted, and probably folded, exploration resulted in the discovery or reassessment of before deposition of the quartz-rich rhyolitic ignimbrites several occurrences, two of which now support mining that dominate the upper sequence (Bulgonunna operations; however, the regional geological setting of, Volcanics as now redefined). Rb-Sr isotopic dating and controls on, mineralisation remained poorly indicates that granitoids spatially associated with these understood. Since late 1987, BMR and the Queensland two volcanic sequences range in agefrom325 to 285 Ma. Unit "DCv" is still recognised as an older, more Department of Mines have collaborated with private enterprise in integrated regional- and deposit-scale deformed sequence dominated by andesitic pyroclastic research aimed at better understanding the nature and rocks and lavas, and derived volcanigenic sedimentary timing of mineralisation processes, and at clarifying rocks. It is separated from the lower Late Carboniferous their relationships to the geology in general, and to the volcanic-sedimentary sequence over most of the region Late Carboniferous Bulgonunna Volcanics in particular. by Lower (to Middle?) Carboniferous sedimentary and Epithermal gold mineralisation in the northern subordinate volcanic rocks (mainly dacitic to rhyolitic Drummond Basin is of the adularia-sericite type (Heald ignimbrites) of the Drummond Basin, including the et al., 1987). Systems are characteristically low in sulphur Scartwater, Mount Hall, Raymond and Star of Hope (generally <2% sulphides, mainly pyrite), and dominated Formations. "DCv" is probably part of the lower by pervasive silicification of variable intensity, quartz volcanic-rich depositional "cycle 1" of the Drummond veining which is typically banded and chalcedonic, and Basin, and equivalent to parts of the Mount Wyatt argillic/phyllic (mainly sericite and mixed-layer clays) Formation, St Anns Formation, and Silver Hills and propylitic alteration. Some also contain adularia, Volcanics (Olgers, 1972). The Late Carboniferous and some contain recognisable surface-deposited silica volcanic-sedimentary and volcanic sequences may sinters (Cunneen & Sillitoe, 1989; White et al., 1989). represent the decline and end of sedimentation in the Hydraulic fractures and breccias are common. Fluids northern Drummond Basin, and the onset and associated with the most-studied systems are typically progressive dominance of subaerial volcanism. Epithermal gold occurrences in the northern of low salinity (normally <5 wt% NaCl equivalent), predominantly of meteoric origin, and characterised by Drummond Basin are typically hosted by volcanictemperatures in the range 200-300°C. Most occurrences sedimentary sequences other than the Bulgonunna are aligned to define either northeasterly or northerly Volcanics — principally by rocks assigned to "DCv". trends which may represent fracture zones and, if so, Only the Ukalunda group of occurrences (Golding & important controls offluidcirculation. Individual quartz Wilson, 1984) appears on structural grounds to be veins and alteration zones in some deposits trend directly related to the Bulgonunna Volcanics. Apart from the apparent structural control mentioned above, northwest; this may be a conjugate shear direction. Sericite alteration at Pajingo has been dated by K- there is a preference for localisation of mineralisation in Ar at 330±10 Ma (Etminan, 1989). BHP prospects at sedimentary-volcanic rocks which probably had a Conway, Bimurra, and Bluegum (about 100 km SE of relatively high degree of permeability and chemical Pajingo and 40 km north of Mount Coolon) have given reactivity to circulating fluids. These rocks may also sericite K-Ar ages of 318 ± 13 Ma, 321 Ma, and 294 ± have been the main source of gold and/or, perhaps more 10 Ma respectively. These ages span the period from importantly, the sulphur for its transport (Seward, 1973) during one or more of the late Palaeozoic magmatic late Early Carboniferous to earliest Permian. Recent U-Pb zircon dating and field investigations episodes in the northern Drummond Basin. indicate the presence of two Late Carboniferous volcanic References sequences, each dated at about 300 Ma: a lower, more Cunneen, R. & Sillitoe, R.,H., 1989. Econ. Geol. 84:135-142. deformed, mixed volcanic-sedimentary sequence, and Etminan, H., in press. BMR 1989 Yearbook. an upper, less deformed, felsic volcanic sequence Ewers G., Mackenzie D.E., McPhie J., Oversby B.S., Wyborn D. & Law S., 1989. BMR Research Newsletter 10:1-4. equated with the Bulgonunna Volcanics as defined by S.D. & Wilson, A.F., 1984. Proc. AusIMM Perth Malone et al. (1964). The lower, as yet unnamed, Golding, and Branches, Regional Conference: 1-9. sequence includes dacitic and quartz-rich rhyolitic Heald, P.,Kalgooorlie Foley, N.K. and Hayba, D.O., 1987. Econ. Geol ignimbrites, a variety of lavas, and probably fluviatile 82:1-26. clastic sedimentary rocks; some of these rocks were

37 Law, S., Mackenzie, D.E., McPhie, J., Oversby, B.S., Wellman, P. & Wyborn, D., 1989. NQ Gold '89 Coherence Abstracts: 47-50. Malone, E.J., Corbett, D.W.P. & Jensen, A.R., 1964. BMR Aust. Rep. 64: 78 pp.

Olgers, F., 1972. BMR Aust. Bull. 132: 78 pp. Seward, T.M., 1973. Geochim.Cosmochim. Acta 48:121-134. White, N.C., Wood, D.G. & Lee, M.C., 1989. Geology 17: 718-722.

A1.29 GEOLOGY AND MINERALISATION OF THE MT WINDSOR VOLCANIC BELT, NORTH QUEENSLAND Simon D. Beams1 and John S. Hartley2 1

Terra Search Pty Ltd, PO Box 981, Hermit Park, Townsville 2 Consultant Geologist, 44 Anne Street, Charters Towers

The Mt Windsor Volcanic Belt, in the Charters Towers region of North Queensland,is approximately the same size and age as the Mt Read Volcanic Belt of WesternTasmania. Similar styles of massive sulphide mineralisation are associated with the volcanics of both belts. The Seventy Mile Range Group is the volcanosedimentary host for the North Queensland mineralisation. The stratigraphy commences with a thick sequence of interbedded micaceous sandstones and siltstones of the Puddler Creek Formation which contains no volcanic lithologies and is derived from a granitic/ metamorphic basement. These are followed by the Mt Windsor Volcanics which are a complex suite of rhyolitic, rhyodacitic and andesitic to basaltic lavas, fragmentals and volcaniclastics. The overlying Trooper Creek Formation is transitional with interbedded basalt, andesite to dacite lavas and fragmentals, volcaniclastics and non-volcanic arenites and black shales. The predominantly fine-grained, non-volcanic sedimentary units of the Rollston Range Formation occur at the top of the sequence. Features such as mafic pillow lavas, graded volcaniclastic units, extensive interbedded epiclastic sediment indicate sub-aqueous deposition for the bulk of the Seventy Mile Range Group. A graptolite fauna places the Trooper Creek Formation in the Lower Ordovician, the lower units are possibly Cambrian in age. The sequence is intruded and metamorphosed by Upper Ordovician to Devonian granitoids of the Ravenswood Batholith. Much of the southern and western sections of the belt are covered by Tertiary to Recent fluviatile sediments and laterite. The succession generally strikes east-west and youngs to the south, it is characterised by low grade slate belt type deformation of lower greenschist facies, with simple open folding about a subvertical slaty cleavage trending east-west Structural complications occur locally, notably in the Highway Synclinorial Zone, in the central part of the belt Volcanogenic Cu-Pb-Zn-Ag-(Au)-Ba mineralisation occurs along the length of the belt The most prominent deposit is Thalanga. Other significant prospects are Liontown, Highway, Handcuff, Waterloo, Reward and Magpie. The mineralisation at Thalanga, Liontown and

Handcuff is mostly in stratiform lenses of banded sulphides with barite, fine grained silica+carbonate interlayered with siliceous volcanic sediments and rare Fe/Mg rich chemical sediments. The massive sulphide lenses and associated sediments occur at breaks within the predominantly rhyolitic volcanic pile. The chemical sediments and banded predominantly Zn rich sulphides are interpreted as volcanic exhalative in origin, formed contemporaneously with the host volcanic sequence. Handcuff, Waterloo and Liontown are low total sulphide systems containing only minor massive pyrite and characterised by fine grained, iron poor sphalerite. At Reward a large pipe-like massive pyritechalcopyrite body is transgressive to the stratigraphy and is interpreted as being emplaced syn- or postcleavage development at the time of peak metamorphism. However, the close association with stratabound/stratiform sulphides (hosted by volcaniclastic units) still suggests a volcanogenic affiliation. Nearby at Highway, gossanous barite-silica pipe-like breccia bodies may be the oxidised equivalents of a Reward type pipe. Magpie is a small massive sulphide deposit hosted in andesite to basalt composition lavas. Intrusion of granodiorite and gabbro has subsequently metamorphosed the altered volcanic sequence to cordieriteandalusite schists and recrystallised the massive sulphides. Feldspar destructive hydrothermal alteration is associated with all the deposits. Sericite, silica and pyrite dominate the alteration envelopes. Many of the original textures in the stratigraphic footwall are obliterated, whereas fresh feldspar bearing volcanics occur in the hanging wall sequences overlying the deposits. A steadily increasing knowledge of the geology of the Mt Windsor Volcanic Belt, built up from prospect and regional scale mapping, has allowed interpretation with reference to genetic models of volcanic-hosted massive sulphide deposits. Explorationists have combined this geological understanding with systematically collected bedrock geochemical and (mostly electrical) geophysical data to discover all the known massive sulphide deposits, apart from Liontown, in a little over ten years.

A1 .30 A SHALLOW-WATER VOLCANOGENIC MASSIVE SULPHIDE DEPOSIT: MOUNT CHALMERS, QUEENSLAND Rodney A. Sainty Pancontinental Mining Ltd, Kalgoorlie, WA

Many recent studies have adopted the view that a (R. Allen, pers. comm.), most examples exhibit non- or deep-sea environment of 1km or more is essential for poorly-collapsed tube pumice texture indicating non- or the formation of volcanogenic massive sulphide (VMS) only slight welding, but an interval with strongly welded, deposits. Ascending high-temperature (300-350°) ore- spherulitic pumice was identified above the mine pit forming fluids would, it is reasoned, boil and form The unit extends 4.5 km southwards to host the old disseminated or vein-type deposits rather than massive "North Star" gold mine. ores, if the depth was shallower (Ohmoto & Takahashi, The Upper Chalmers Sediment (UCS) overlies the 1983). Thus, the deep-water setting proposed for the ignimbrite and is about 20 m thick; the massive to Miocene Kuroko deposits, based on foraminifera (Guber bedded silty ash and volcanic wacke may be redeposited & Merrill, 1983) and the lack of evidence in fluid co-ignimbrite ash. Brachiopods, gastropods, bivalves inclusions for boiling (Pisutha-Arnond & Ohmoto, and bryozoans have been found atfivelocalities 1-2 km 1983), has often been applied to all VMS deposits. north and south of the mine. Occasional cojoined valves The Permian Mount Chalmers Cu+Au-rich massive indicate only limited fossil reworking. The identified sulphide deposit (Large & Both, 1980) is located near assemblage and its preservation indicates water depths Rockhampton in Queensland. Detailed mapping, under- of 50-300m, with a 100-200 m depth "most likely" taken as part of an exploration programme by Pancon (J.B. Waterhouse, pers. comm.). Strong bioturbation, and Outokumpu, has outlined the regional stratigraphy and Planolites and ?Teichichnus are again present. A shallow water depth implies that the ore-forming and revealed the presence of shallow-water fossils, burrows, and a patchily-welded ignimbrite within the fluids boiled beneath the sea-floor, which may account for the well-developed footwall stringer zone. Fluid host sequence. The association of benthic fossils and VMS deposits inclusions have been found to be unsuitable for study is rare, and provides a shallow depth estimate (R.R. Large, pers. comm.). However, Hedenquist & independent of controversial volcanological criteria. Henley (1985) show that evidence of boiling may not Certain pyroclastic rocks at other deposits have suggested always be preserved in inclusions, and Ramboz et al. shallow-water settings (e.g. Sainty, 1986; Groves et al., (1988) state that "the vapour resulting from fluid boiling 1988), but this deposit also offers a younger fossiliferous at low pressure (less than 200 bars) is in fact rarely trapped in inclusions". Therefore, a lack of vapour-rich sequence. The stratigraphy is flat-lying with open folding; a fluid inclusions does not preclude boiling in a shallowbroad regional syncline extends southwards from the water setting as is normally assumed. Mount Chalmers mine. Ignimbrites with wispy, lenticular pumice have The Lower Chalmers Sediment (LCS) host unit is usually been regarded as reliable indicators of subaerial 30-60 m thick and can be traced 6 km south of the to only very shallow water settings, and areas featuring mine. Strong burrowing and bioturbation is present in these rocks have often been dismissed as nonprospective an upper ashy interval at the top of the mine pit. Burrows for VMS deposits. Their presence, however, at Rosebery comprise Planolites and ?Teichichnus (C.R. Fielding, (Sainty, 1986) and at Mount Chalmers indicates that M. Banks, pers. comm.). This assemblage is typical of such sequences warrant re-assessment the Cruziana ichnofacies, generally diagnostic of the References sublittoral zone which extends to 200 m (Frey & Frey, R.W. & Pemberton, S.G., 1984. In Walker, R.G. (ed.): Facies Models. Geo science Canada, Reprint Ser. Geol. Pemberton, 1984). The causal fauna are unlikely to Assoc. Can. have been transported by canyons, fans or strong currents Groves, D.A., Morton, R.L.& Franklin, J.M., 1988. Can,. J. into a deep environment because the host is fine-grained Earth Sci. 25: 280-291. and lacks channelling or turbidite structures. Guber, A.L. & Merrill, S., 1983. Econ. Geol. Mon. 5:55-70. The hangingwall is a stratified sequence comprising Hedenquist, J.W. & Henley, R.W., 1985. Econ. Geol. 80: ignimbrite, ashy sediment, epiclastic andesite breccia, 1379-1406. and rhyolite. A 3 x 2 km x 200 m-thick flow or dome of Large, R.R. & Both, R.A., 1980. Econ. Geol. 75: 992-1009. glassy Ellrott Rhyolite occurs 2 km southeast of the Ohmoto, H. & Takahashi, T., 1983. Econ. Geol. Mon. 5: 39-54. mine. The North Star Ignimbrite is 750-120 m thick and Pisutha-Amond, V. & Ohmoto, H., 1983. Econ. Geol. Mon. 5: 523-558. overlies the LCS at the top of the mine pit, 50 m above Ramboz, C., Oudin, E. & Thisse, Y., 1988. Can. Mineral. 26: the massive sulphide (excluding intrusives). It contains 765-786. wispy, lenticular pumice to 100 mm, abundant (25%) Sainty, R.A., 1986. In Large, R.R. (ed.): The Mount Read Volcanics and Associated Ore Deposits. Geol. Soc. Aust., plagioclase crystals and fragments, and sporadic lithic Tasm. Div., Hobart. clasts to 30 mm within a siliceous matrix. In thin section

A2: Fossils from Gondwana Terranes Convenor: C. F. Burrett

A2 Keynote Address: HIMALAYAN SUSPECT TERRANES AND SUSPECT PALAEONTOLOGY John A. Talent Earth Sciences, Macquarie University

Through the activities of one Indian geoscientist, the Phanerozoic record for the Himalayan region has been polluted over the past 25 years by injection of a broad spectrum of disinformation. This includes giving vague and misleading details for localities (•phantom* localities) often with impossible stratigraphies, re-use ('recycling' of specimens [up to four times, with different 'localities' each time], re-use ('cannibalisation') of previously published illustrations figured by other scientists from localities found elsewhere around the globe, and reports of biogeographically bizarre biota and temporally incredible associations. A significant proportion of the reported materials can be said to be 'fingerprinted' because of the peculiarity of the associations or highly characteristic modes of preservation unique to specific localities elsewhere in the world. The above activities enshrined in an oeuvre of at least 405 publications [138 solo, the remainder with 117 co-authors including 56 non-Indian] span all major phyla and all periods of time Cambrian to Pleistocene, excelling the activities of John Darsee.

Clearly, it is essential that allfieldbooks, laboratory registers and all published specimens including relevant thin sections and micro-slides be made available to examining commissions so that field data pertaining to the various specimens and collections specified as spurious or dubious can be checked. Until now such requests have been resisted. Unless these materials are made available, the verdict is obvious. Especially interesting would be attempted tabling of materials identified as recycled or illustrated by pictures from other people's monographs. Regrettably, all contributions authored or coauthored by this particular person should be ignored and syntheses that have accepted his 'data' as reliable should be used with extreme caution. Unless this is complied with, the fabric of stratigraphic alignments based on manifestly dubious or suspect palaeontology makes recognition of suspect terranes in the Himalayan region well nigh impossible. The next generation will need to tread warily through these dangerous 'data' and syntheses — as though through a landscape liberally studded with landmines.

A2.1 A NEW PHANEROZOIC RECONSTRUCTION BASEMAP SERIES M.I. Ross1, G. Young2, HJ. Stagg2, J.B. Willcox2 and C.T. Klootwyk2 1 2

Rice University, Houston, Texas, USA Bureau of Mineral Resources, Canberra

A map series has been constructed for the Phanerozoic Eon which focuses on the movement of Australia and its environs. This map series is intended for use by the Palaeogeographic Maps Project at the BMR for creating palaeogeographic maps. The Mesozoic and Cainozoic series follows Royer et al. (in press). We add an attempt to palinspastically restore the terranes of the northern Australian margin (PNG). Also included is a schematic model for the development of New Zealand, and of back-arc basins since the Late Cretaceous on the eastern margin (Tomlins & Ross, 1988). The early breakup history of the southern

margin is also examined in light of new evidence (based on seismic data) of an early phase of stretching parallel to and concurrent with the breakup on the northwest shelf. A new fit position for the South Tasman Rise is also suggested, west of Bass Strait, that resolves the previous problem of overlap with the western Ross Sea shelf. Break up of the Western and Northwest margins is also examined in light of the Triassic reef complex recently drilled in the region (Williamson, 1989). The Palaeozoic series updates the maps of Scotese & McKerrow (in press, 1990). Emphasis is placed on the movements of eastern Gondwana, with cross checking

based new interpretations derived from the compilation the IAGA Global Palaeomagnetic Database. Identification and positioning of terranes (N. China, S. China, Indonesia, and Shan Thai) on the northern margin based on the recent papers published in McKerrow & Scotese (1990), and Klootwyk et al. (1988). All parameters ("poles of rotation") for the reconstructions were calculated on a Evans & Sutherland PS300 3D graphics computer for a fixed radius earth. The Mesozoic and Cainzoic motions are calculated relative to the mantle framework ("Hot spots"). The Palaeozoic series are relative to the palaeomagnetic pole.

References

Klootwyk, C.T., Idnurn, M. & Giddings, J.W., 1988. BMR yearbook. McKerrow, S. & Scotese, C.R. (eds), 1990. Geol. Soc. London Mem. 2, in press. Royer, J.Y. & Sandwell, D.T., in press. J. Geophys. Res. Scotese, C.R. & McKerrow, S., 1990. Geol. Soc. London Mem. 2, in press. Tomlins R.T. & Ross, M J., 1988. Unpubl. Palaeogeographic Mapping Project Progress Report 45. Williamson et al., 1989. APEA Journal 29: 328-344.

A2.2 ASIAN TERRANES DERIVED FROM GONDWANA Clive F. Burrett Geology Department, University of Tasmania

Numerous Phanerozoic terranes are now recognized in Asia and there is very good evidence that many of these were attached to Gondwana in the early Palaeozoic and rifted off in the middle to late Palaeozoic. For instance, the Shan-Thai (or Sibumasu) Terrane was, on the basis of a considerable new body of palaeontological data, almost certainly adjacent to the north west continental margin of Australia during the Cambrian, Ordovician and probably Devonian. Sedimentological evidence from the upper Carboniferous-lower Permian Phuket Group shows that it was glacimarine, again suggesting a Gondwana connection. North and South China were also probably attached to Gondwana in the Cambro-Ordovician and stayed

near to the Australian sector of Gondwana in the Devonian. However, the Indo-China Terrane has not, as yet, yielded biogeographically diagnostic faunas and its position in a Gondwana reconstruction remains speculative. Biogeographic data are considered to be very good at defining terranes and terrane movement during periods of high endemism. Unfortunately, palaeomagnetic data from many Asian terranes are clearly associated with collisional and post-collisional thermal events, but where good data are available, a satisfactory reconstruction may be made. Reference Long, J. & Burrett, C.F., 1989. Geology 17: 811-813.

Figure 1 — Reconstruction for the Late Devonian. Orthographic projection. Arrows show declinations and numbers are palaeolatitudes with errors. Major continents: AF = Africa, ANT = Antarctica, AU = Australia, IN = India. Terranes: H = Hexizoulang (as separate terrane) HI same but fused to N. China, HK = Hindu Kush, IR = Iran, K = Kazakhstan, L = Lhasa, N = North China, Q = Qiantang, S = South China, SI = Siberia, ST = Shan-Thai (= Sibumasu) T = Tarim Orientation of Shan-Thai based on palaeocurrent datafromthe Phuket Group provided by Mr. John Hills (from Long & Bun-ett, 1989).

A23 FOSSILS VERSUS PALAEOMAGNETISM AS TESTS OF PALAEOGEOGRAPHIC HYPOTHESES — EXAMPLES FROM GONDWANA TERRANES ANALYSED CLADISTICALLY Gavin C. Young Bureau of Mineral Resources, Canberra

The problem of integrating biological and nonbiological data sets as they apply to palaeogeographic reconstructions is reconsidered. Traditionally, quantitative (e.g. palaeomagnetic) evidence has overridden qualitative (e.g. biogeographic) evidence in cases of conflict. Devonian vertebrates are widespread in Gondwana (Young, 1987), and occurrences in the Appalachians (Avalon Terrane) and Turkey provide two examples where respect to palaeomagnetic and other evidence indicating palaeoposition for these putative Gondwana terranes. With increasingly large data sets such an ad hoc case-by-case approach is not appropriate, but the view that quantitative palaeomagnetic evidence is superior to qualitative biogeographic and palaeoclimatic data (Van der Voo, 1988) is not accepted. It is argued that previous attempts to integrate qualitative and quantitative data (e.g. by quantifying biogeographic data using similarity coefficients) have been misdirected, and that integration of disparate data sets is largely a problem of data representation. Complex data comprising evidence for palaeogeographic reconstruction may be represented with synthetically or analytically. The former integrates large and disparate data sets in a coherent hypothesis at the expense of testability; the latter exposes inconsistencies in the evidence to further analysis and resolution by accessing new data, and is thus falsifiable. Most palaeogeographic map reconstructions fall within the first category—they synthesise pictorially the evidence of palaeogeography of an area, but in conveying a complex historical hypothesis they are deficient in their inadequate representation of historical change, in synthesising data which may not be time equivalent, and in failing to display all empirical data on which the reconstruction is based. Apparent polar wander path (APWP) representation of palaeomagnetic data exemplifies the second category. Any data (e.g. biogeographic, palaeoclimatic) providing evidence for palaeolatitude can potentially be analysed using APWP representation, but evidence concerning barriers or connections between regions (e.g. areas of endemism, overlap assemblages) cannot However such evidence can be represented and analysed cladistically. Cladistic analysis assumes that data relevant to the problem under investigation can be organised hierarchically (Brady, 1983). Given appropriate geological definitions, various geological, geophysical, and biological data relevant to the fragmentation or fusion history of regions assumed

to have a separate history can be organised hierarchically (Young, 1986; in press). Cladistic analysis of pre-Jurassic terrane interactions is directly comparable to analysis of biological taxa, because all data relevant to fragmentation or fusion history are derived from those terranes. For post-Jurassic interactions of continental blocks, crucial evidence concerning palaeogeographic history (seafloor spreading data) is derivedfromintervening oceans rather than terranes themselves. Such evidence may lack hierarchical organisation, but the view that Jurassic and younger plate motions can be read like a book from seafloor spreading data contradicts the principle that all history is hypothesis. Alternatively seafloor data may be considered as attributes of plates of lithosphere, but because of continuous contact at their margins, plates are not amenable to cladistic analysis. The oldest identified magnetic anomaly at a continent-ocean boundary gives a minimum age for onset of seafloor spreading, but other data are required to fix the age of continental breakup. Both types of evidence may be applied to cladistic analysis of continental terranes. For any suspect terrane the potential number of geological observations, and the subset of observations relevant to fragmentation or fusion history, is infinite. A cladistic approach predicts that all data in this infinite subset can be organised hierarchically to conform to a particular cladogram, which represents the unique fusion or fragmentation history of the areas concerned. Within cladistics elaborate techniques are available for assessing competing hypotheses using the criterion of parsimony, and for handling complexities in conflicting data sets which far exceed those currently exposed to criticism and refutation in the literature of palaeogeographic reconstruction. Computer parsimony analysis programs can handle digitised data from the three major subdisciplines (palaeomagnetism, palaeoclimatology, biogeography) which provide evidence for palaeogeographic reconstructions. Some of these techniques are illustrated in a consideration of Asian terranes thought to have been derived from the northern margin of Gondwana. References Brady, R.H. 1983. In Platnik, NJ. & Funk, V.A. (eds): Advances in Cladistics, 2: 49-60. Van der Voo, R., 1988. Geol. Soc. Am. Bull. 100: 311-324. Young, G.C., 1986. Geol. 94: 523-537. Young, G.C., 1987. Am. Geophys. Un., Geophys. Mon. 41: 41-50. Young, G.C. in press. Geol. Soc. Mem. 12: 243-255.

A2.4 CAMBRIAN FAUNAS IN THE TERRANES OF THE SOUTHWESTERN PACIFIC PART OF GONDWANA J.B. Jago Department of Applied Geology, South Australian Institute of Technology

Cambrian faunas are a critical factor in determining Cambrian of the Western Tasmanian Terrane is richly the tectono-stratigraphic history of the Upper fossiliferous with faunas ranging from middle Middle Precambrian-Lower Palaeozoic terranes comprising part to very Late Cambrian age. Polymeroid and agnostoid of the present south-west Pacific part of Gondwana. trilobites dominate the faunas but inarticulate and The Cambrian faunas of northern Victoria Land articulate brachiopods, sponges, helcionellids and (Antarctica), New Zealand, Tasmania and Victoria dendroids are common at some localities. A correlation provide the earliest reliable correlations of the chart of the fossiliferous sequences of the Western sedimentary and volcanic sequences within these Tasmanian Terrane is given by Jago & Brown (1989). terranes, which were marginal to the main Australian- Although almost all the faunas occur in siltstones and shales, many of which are associated with turbidites, Antarctic continent. The best known and most extensive Cambrian faunas several different contemporaneous faunal assemblages can be recognised in the Middle and Late Cambrian from these areas are those of northern Victoria Land and Tasmania. In northern Victoria Land, fossiliferous assemblages. The Late Middle Cambrian faunal Cambrian sequences occur within the Bowers assemblages are shown in Figure 1. Similar patterns are Supergroup (Bowers Terrane). The great bulk of the found in the Late Cambrian assemblages. The Cambrian faunas of Victoria are found in the known fossiliferous horizons of the Bowers Supergroup are found in the Mariner Group which is essentially a Heathcote and Mt Wellington Belts. They are mainly of regressive sequence starting in the Late Middle Cambrian Middle and Late Cambrian age, but have been little with shales, passing up through shales with limestone studied. However, unlike the other areas under lenses into shallow water sandstones with minor discussion, shelly fossils of Early Cambrian age are limestones. The faunas reflect the changing depositional known from the Heathcote Belt (Vandenberg & conditions and change from a Middle Cambrian Wilkinson 1982). In the areas discussed, it is the Late Middle and cosmopolitan agnostoid-rich fauna near the base to a more endemic Chinese-Australian fauna higher up. The Early Late Cambrian faunas, particularly the former, youngest known trilobites are of immediately post- which are by far the most common in the Cambrian Idamean age (Shergold & Cooper, 1985) but trace fossils sequences. This may indicate a period of high sea level (Cruziana facies) are found higher in the sequence. or it may reflect the time of maximum rifting and Probable Tremadoc fossils are found within the terrane dispersal. The Late Cambrian shallowing Robertson Bay Terrane of northern Victoria Land, which upwards sequences of northern Victoria Land and the suggests that Cambrian faunas could also be present. Adamsfield Trough of Tasmania may reflect terrane The Takaka and Buller Terranes contain the Lower convergence (see also Brown et al., 1988 and papers Palaeozoic rocks of north-west Nelson in the South referred to therein). Island of New Zealand (Cooper & Bradshaw, 1985). In References terms of Cambrian faunas, the Takaka Terrane contains Brown, C.M., Tucker, D.M. & Anfiloff, V., 1988. Tectonophysics 154: 309-333. mainly late Middle Cambrian faunas but some Late Cambrian faunas are known. The oldest known fossils Cooper, R.A. & Bradshaw, M.A., 1985. Lower Paleozoic of Nelson-Westland, Excursion 3, Hornibrook Symposium, of the Buller Terrane are of Lancefieldian age which Christchurch, December 1985. suggests that the Buller Terrane sedimentary rocks may Corbett, K.D., 1989: Geol. Soc. Aust. Spec. Publ. 15:175-181. extend down into the Cambrian. Jago, J.B., 1973: Lethaia 6: 405-421. In Tasmania there are two Early Palaeozoic terranes, Jago, J.B. & Brown, A.V., 1989. Geological Society of the Eastern and Western Tasmanian Terranes. Only the Australia, Special Publication 15: 74-83. Western Tasmanian Terrane contains Cambrian faunas. Shergold, J.H. & Cooper, R.A., 1985. BMR J. Aust. Geol The oldest known fossils in the Eastern Tasmanian Geophys. 9: 91-106. Terrane are of late Arenig age. The exact tectono- Vandenberg, A.H.M. & Wilkinson, H.E., 1982. Geol. Soc. stratigraphic nature of the Western Tasmanian Terrane Aust. Spec. Publ. 9: 36-^7. is still a matter of dispute (see Corbett, 1989). The

Fauna A (cosmopolitan-agnostoid) Agnostoid trilobites - Ptychagnostus, Diplagnostus, Lejopyge, etc. Polymeroid trilobites - rare or absent Dendroids, hydroids, inarticulate brachiopods (acrotretids), sponges, phyllocarids Fauna B (cosmopolitan-agnostoid/polymeroid) As for Fauna A but with the addition of cosmopolitan or widespread polymeroid trilobites including Centropleura, Pianaspis and dolichometopids Fauna C — Agnostoids plus endemic polymeroids Abundant agnostoids (Peronopsis, Clavagnostus, Tasagnostus, Valenagnostus); pagetiids Abundant polymeroids (agraulids, asaphiscids, nepeids, etc.) Rare dendroids, some inarticulate brachiopods, echinodermata Fauna D — Endemic polymeroids — Inarticulate brachiopods — Orthid brachiopods Fauna E — Orthid brachiopods Figure 1 — Late Middle Cambrian faunal assemblages of Tasmania. The different assemblages shown should be regarded as "end-members", with gradations between the different faunas (partly after Jago, 1973).

A2.5 RELATIONSHIPS OF CAMBRIAN-ORDOVICIAN TRILOBITE BIOFACIES ON TARUTAO ISLAND, PENINSULAR THAILAND J.H. Shergold1*, C.F. Burrett2 and T. Wongwanich2-3 1

Bureau of Mineral Resources, Canberra 2 University of Tasmania, Hobart Geological Survey of Thailand, Bangkok

The Tarutao Formation on Tarutao Island, peninsular Thailand, is a sequence of siliciclastics which straddles the Cambrian-Ordovician boundary as currently defined in Australia (Wongwanich & Burrett, 1983; Wongwanich et al., 1983; Jones et al., 1971). It is principally exposed on wave cut platforms and beach cliffs at Malaka Creek to the north and south of Laem Hin Ngam on the west coast of Tarutao, and south of Ao Talo Dang on the southeast coast (Akerman, 1986). The base of the Tarutao Formation is not exposed. The Tarutao Formation essentially contains two trilobite biofacies each comprising two assemblages of local distribution. The oldest, originally described by Kobayashi (1957)fromAo Talo Topo, and subsequently revised by Shergold et al. (1988, locality 6) contains:

Micragnostus sp., Eosaukia baruvasi Kobayashi, Lichengial tarutaoensis (Kobayashi), Lophosaukia cf. jiangnanensis Lu & Lin, Parakoldinioidia thaiensis (Kobayashi) Quadraticephalus planulatus (Kobayashi), Tsinania (Shergoldia) nomas (Shergold), and an undertermined leiostegiid. A related assemblage, having a slightly different faunal composition, has been more recently collected from the lower part of a section at Ao Talo Dang (Shergold et al., 1988, locality 3). This contains Hoytaspis? thanisi Shergold et aland Szechuanella? damujingensis (Lo), together with species of Asioptychaspis?, Eosaukia, Fatocephalus?, Lophosaukia, Mansuyites, Parakoldinioidia, Prosaukia, and a shumardiid.

These two assemblages constitute the saukiid- palaeogeographic distribution of the younger Tarutao tsinaniid biofacies of the North China Province biofacies is more restricted than that of the preceding (Shergold, 1988), of latest Cambrian age. This extends late Cambrian. It is located off the western and northern across the Sino-Korean Platform, along the western margins of the continent within northern tropical margin of the South China (Yangtze) Platform (W latitudes. Early Tremadoc trilobites have not yet been Yunnan and N Vietnam), through Shan-Thai (Sibumasu identified on Tarutao Island: either rocks of appropriate of Metcalfe, 1988: Burma, W Thailand, N Malaya) age were never deposited, or, more likely, they have not (Burrett & Stait, 1985), parts of Iran, Afghanistan, SE been discovered because of faulting and/or inaccessTurkey, and potentially Khirgiz Kazakhstan; and across ibility. Since these rocks embrace the critical the Australian Platform, extending into northern Victoria Richardsonella/Troedssonia and Yosimuraspis trilobite Land, Antarctica. This distribution therefore extends zones of the Sino-Korean Platform (Cordylodusproavus along the northern, western and SW margins of Gond- through C. lindstromi), a Cambrian-Ordovician wana as reconstructed by Burrett & Stait (1986, fig. 6). boundary cannot be correlated with precision at any of The younger biofacies also comprises two closely the levels currently being considered for this horizon. related, but geographically separated, assemblages. The The Tarutao Formation is overlain conformably by oldest of these occurs to the north of Malaka Creek, carbonates of the lower Thung Song Formation which opposite the National Park Headquarters, and was briefly contain rare trilobites including ?Rossaspis and described by Stait et al. (1984). It contains species of Leiostegium and late Tremadoc and middle Arenig Asaphellus and Micragnostus, Pseudokainella nautiloid and conodont assemblages (Stait & Burrett, malakensis Stait et al., Rossaspis? bunopasi Stait et al., 1984; Stait et al., 1987). harpididfragments,and as yet undescribed material, all References of late Tremadoc age. Elements of this assemblage are Akerman, T., 1986. Unpubl. thesis, University of Tasmania. 101pp. also known at locality 398.6 on Ko To Sen (off NE Burrett, C.F. & Stait, B., 1985. Earth Planet. Sci. Lett., Tarutao) and in the south of Tarutao. 75:184-190. The upper part of the section (Shergold et al., 1988, B., 1986. In McKenzie, K.G. (ed.): locality 3) at Ao Talo Dang contains similar species of Burrett, C.F. & Stait, Symposium on Shallow Tethys 2: 65-77. Asaphellus, Pseudokainella and Rossaspis?, associated Jell,International Mem. Mus. Victoria 46: 53-88. P.A., 1985. with an undetermined metagnostid, species of P.J., Shergold, J.H. & Druce, E.C., 1971. J. geol. Soc. Scotoharpes?, Jiia, a possible second pilekiid and a Jones, Aust. 18: 1-32. hystricurid. It appears to be slightly younger than the Kobayashi, T., 1957. J. Fac. Sci. Univ. Tokyo, 10(3):3 67-382. previous assemblage. However, the presence of Kobayashi,T., \91\.J.Fac.Sci. Univ.Tokyo, 18(1): 129-299. Pseudokainella, Rossaspis, Asaphellus and the harpidid Kuo, Hung-chun, Duan Ji-ye & An Su-lan, 1982. Dept. Geol., suggests a late Tremadoc (Cordylodus augulatus/C. Changchun College, Changchun: 1-31. rotundatus and Chosonodina herfurthi Zone) age for Legg, D.P., 1976. Geol. et Palaeont., 10: 1-58. Metcalfe, I., 1988. In Audley-Charles, M. & Hallam, A. (eds): both. Gondwana &. Tethys: 101-118. Similar faunas are known on the Sino-Korean Yi-yuan, 1986. In Chen Jun-yuan (ed.): Aspects of the Platform, in northeastern Hebei, Liaoning and Jilin QianCambrian-Ordovician Boundary in Dayangcha, China: Provinces of China, where they occur in the Pseudo255-313, 67-82. kainella and Asaphellus Zones of Kobayashi (1971) Shergold, J.H.,pis.1988. Geol. Mag. 125(4): 363-380. and Kuo et al. (1982), and the Wanliangtingia J.H., in press. BMR Bull. 131. Assemblage Zone of Zhou & Zhang (1985) and Qian Shergold, Shergold, J.H., Burrett, C.F., Akerman, T., & Stait, B., 1988. (1986). In Australia, a fauna with similar composition New Mexico Bur. Mines Min. Resour. Mem. 44: 303-320. has been recorded at Digger Island, Victoria (Jell, Shergold, J.H., Gorter, J.D., Nicoll, R.S., & Haines, P.W., in 1985), but is assigned an earlier age (possibly Oneotodus press. BMR Bull. 237. bicuspatus-Drepanodus simplex). Elements of the Thai Stait, B. & Burrett, C.F., 1984. Geol. Mag. 121: 115-124. fauna, eg. Rossaspis, occur in the Canning Basin (Legg, Stait, B., Burrett, C.F. & Wongwanich, T., 1984. N. Jb. Geol. Palont. Mh. (1): 53-64. 1976), but require biostratigraphic re-evaluation. Wyatt, D. & Burrett, C.F., 1987. N. Jb. Geol. Palont. In the Amadeus and western Georgina Basins a Stait,Ah.B.,174(3): correlative biofacies of similar age but different Wongwanich, T.373-391. & Burrett, C.F., 1983. J. geol. Soc. Thailand composition, based on Psilocephalina, Hystricurus, 6(2): 21-29. Koraipsis and Kayseraspis (Pacoota Sandstone Wongwanich, T., Wyatt, D., Stait, B., & Buirett, C.F., 1983. Assemblage 2 of Shergold, in press), occurs widely. In Workshop on stratigraphic correlation of Thailand and the carbonate province of the eastern Georgina Basin a Malaysia, Hat Yai: 77-95. third contemporary biofacies is based on dikelo- Zhou Zhi-yi & Zhang Jin-lin, 1985. In Stratigraphy and Palaeontology of Systematic Boundaries in China. cephalinids. Cambrian-Ordovician Boundary 2: 63-163. According to the early Ordovician Gondwana reconstruction of Burrett & Stait (1986, fig. 7), the

A2.6 ORDOVICIAN CORALS AND STROMATOPOROIDS FROM GONDWANA TERRANES B.D. Webby Department of Geology & Geophysics, University of Sydney

The Australian-New Zealand sector of Gondwana the assemblages of solitary rugosans (Calostylis, (its shelf margin and offshore terranes) is the only Ningnanophyllum, Yohophyllum and Streptelasma, and confirmed part of the supercontinent to exhibit a tabulates (Catenipora and various heliolitines) from the reasonably complete Middle-Upper Ordovician coral South China Platform. The 'transitional' successions of and stromatoporoid faunal succession, and is South-East China (Jiangnan island belt), at the outer consequently thought to have lain in the equatorial margin of the South China Platform, have an admixture zone. The Gondwana shelf margin of Tasmania has of elements of these warm- and cool-water provinces typical North American-type, warm-water coral (Palaeophyllum, tetradiids and Yohophyllum). assemblages (Favistina, Palaeophyllum, tetradiids, The Upper Ordovician faunas are significantly more Foerstephyllum sndNyctopora), and the offshore central diverse and widely distributed. In Northern, North-West New South Wales 'island arc' has similar associations and South-East China the early Ashgill platform margin (Favistina, Cyathophylloides, Palaeophyllum, tetradiids, and fold-belt assemblages include warm-water elements Billingsaria and Nyctopora). There are similarly rich such as the clathrodictyid stromatoporoids, Favistina, labechiid and clathrodictyid stromatoporoid faunas in Cyathophylloides, Palaeophyllum, Foerstephllum, Tasmania and central New South Wales. As with the Calapoecia, tetradiids and sibiriolitids (these latter with corals, they prove to have few species in common, restricted Siberian, Arctic, Altai, Monglolian and indeed the main differences are the presence of Kolyma connections), but also of agetolitids and the Cystistroma and Alleynodictyon in the central New South ?cool-water European genus Sarcinula. Agetolitids are Wales successions, and abundant representatives of known elsewhere only from Central Asia, Alaska and in Stromatocerium, Pachystylostroma and Aulacera in Ashgill (Fauna IV) assemblages in central New South Tasmania. Wales and North Queensland. Other isolated remnants such as the Tam worth terrane The late faunas of North-West China are similarly of NE New South Wales has North American 'Red characterized by tetradiids and agetolitids (but no River-type' coral faunas (Favistina, Cyathophylloides, stromatoporoids). Their occurrences are in marked Palaeophyllum and Crenulites), and the Broken River contrast to the distinctive and diverse assemblage of Embayment of North Queensland has records of solitary rugosans which appears as a component of the Agetolites, Catenipora and Plasmoporella suggesting Hirnantia fauna on the South China Platform. The closer Asian-Alaskan links. The Arthur Marble of New solitary rugosans include Bodophyllum, Streptelasma, Zealand has an Upper Ordovician assemblage of Grewingkia, Crassilasma and many others (at least 12 Favistina, Proheliolites and Plasmoporella. The genera), and there are also a few tabulates (halysitines, stromatoporoids appear to have a more restricted Protoheliolites and Amassia) - no stromatoporoids. The (possibly more equatorially limited, or circulation or Hirnantia fauna has been widely interpreted as reflecting depth controlled) distribution because these isolated a cold-water environment but in this association with terrane remnants contain corals but no stromatoporoids. solitary corals was probably formed in less extreme Also the uppermost Ordovician part of the succession conditions. It seems more likely to have formed in in central New South Wales (upper part of the Malachi's middle to low latitudes (but not close to the equator) Hill Formation) of Ashgill (Fauna IV) age has no during the main glacioeustatic shallowing of the stromatoporoids. platform, at the climax of the Late Ordovician glaciation Various Ordovician palaeogeographic recon- (Rong & Chen, 1987). If so, then at this time, the South structions have been proposed for the discete blocks in China PLatform may not have been gready displaced Asia, with relationships adjacent to or part of 'greater' from the North China Platform. Gondwana. Some constraints are provided by the There are no confirmed records of Ordovician corals Middle-Upper Ordovician coral and stromatoporoid or stromatoporiods from other 'greater' Gondwana distributions of China and South-East Asia. In the early segments such as IndoChina, Tibet, India, Iran, Arabia, Middle Ordovician (Llanvirn) the North China platform the Mediterranean region, or from the margins of Africa, was the site of early deversification of labechiid South America and Antarctica. Some of these regions stromatoporoids, and elements spread to the adjacent lay in higher latitude, cold water environs. But in the Shan Thai Block (Malaysia) immediately, but did not fold-belt regions of England, Wales and Eire, and in reach New South Wales and Tasmania until much later, Central Asia (Tadzhikistan) — like the East European suggesting less close links with the Eastern Australian Platform (Baltic) — there are occurrences of solitary sector of Gondwana. rugosans (particularly streptelasmatinids and The later Middle Ordovician faunas of North China calostylinids) halysitines (Catenipora) and heliolitines (Favistina, Cyathophylloides, tetradiids, Foerstephyllum (Protoheliolites) but no stromatoporoids. These represent and labechiids) are provincially markedly different from assemblages of the cooler, intermediate latitude

Euroasiastic province of Kaljo & Klaamann (1973). Coral-stromatoporoid associations of warm-water, low latitude aspect, characterized by labechiids, clathrodictyids, the compound rugosans Favistina, Cyathophylloides, Palaeophyllum and Crenulites, the tetradiids and oth^r tabulates such as Saffordophyllum, Foerstephyllum, Billingsaria, Nyctopora and Calapoecia, ocur in the Australian sector of Gondwana, on the North China Platform and, based only on its stromatoporoid record, the Shan Thai Block. New Zealand (corals but no stromatoporoids) and the margin (Jiangnan island belt) of the South China Platform (corals plus clathrodictyids) may also lie in this belt. The coral part of these assemblages have been characterised as

belonging to the warm-water, equatorial AmericanSiberian province by Kaljo & Klaamann (1973). Other •greater* Gondwana fold-belt (continental shelf, microcontinent and island-arc) successions, from their coral and stromatoporoid distribution, also belong to this province. They include the Altai Sayan mountain region of the Soviet Union, Kazakhstan, Mongolia and Scotland. References Kaljo, D.L. & Klaamann, E.R., 1973. In Hallam, A. (ed.): Atlas of Palaeobiogeography. Elsievier: 37-45. Rong Jiayu & Chen Xu, 1987. Acta Palaeont. Sinica 26: 507-535.

A2.7 CONODONTS AND THE DISTRIBUTION IN TIME AND SPACE OF ORDOVICIAN SEDIMENTS IN AUSTRALIA AND ADJACENT AREAS Robert S. Nicoll* and J.M. Totterdell Bureau of Mineral Resources, Canberra

Preparation of a revised biostratigraphic chart for the Ordovician of Australia (Webby & Nicoll, 1989) has led to a reappraisal of the geographic extent and stratigraphic distribution of Ordovician sediments on the Australian craton and the contiguous or proximal continental blocks of Gondwanaland. On the Australian Block, Early Ordovician sedimentation occurs in two parallel trans-cratonic seaways (Larapinta Seaway, Bonapart-Wiso-Georgina Depression) and the Arafura and Tasman Shelves and the associated marginal slopes and volcanic arc complexes. Late Ordovician sedimentation is preserved on the Tasman Shelf and adjacent slope and to the remenant western part of the Larapinta Seaway. There was either no Late Ordovician sedimentation on the Arafura Shelf, or that sediment was eroded prior to the Late Devonian. Reconstruction of the continental elements formerly attached, or adjacent, to the Tethyan margin of eastern Gondwana (Australia and Greater India) is obfuscated by post Triassic deposition and tectonic events. Continental reconstructions, principally those of Burrett & Stait (1987), Metcalfe (1988) and Scotese et al. (1988) have placed several larger continental blocks adjacent to this margin in the Ordovician. The positioning of

these blocks suggested in this report differs considerably from earlier studies and reflects gross stratigraphic and biogeographic considerations rather than palaeomagnetic data. Extension of the trans-cratonic seaways of the Australian Block on to the Tethys margin blocks is not clear on examination of presently available data. Similarities of the Larapinta Seaway sediment sequence are seen in peninsular Thailand and Malaysia (Metcalfe, 1988). The sedimentary record of the North China Block is very similar to the preserved sequence in the Bonaparte Basin and on the Arafura Shelf. Proximity of these areas is indicated by the distribution of both trilobites (Shergold, 1989) and conodonts (Nicoll, in Webby & Nicoll, 1989). References Burrett & Stait, B., 1986. In McKenzie, K.G. (ed.) International Symposium on Shallow Tethys 2: 65-77. Metcalfe, I.f 1988. In Audley-Charles, M.G. & Hallam, A. (Eds.). Gondwana and Tethys: 101-118. Scotese, C.R., Gahagan, L.M. & Ross, M.L., 1988. University of Texas, Technical Report 90. Shergold, J.H., 1989. BMR Rec. 1989/31: 1-24. Webby, B.D. & Nicoll, R.S., 1989. BMR Rec. 1989/32:1-42.

EARLY ORDOVICIAN A .AUSTRALIAN V PLATE

NEW ZEALAND N

LATE ORDOVICIAN B AUSTRALIAN , PLATE

Marginal slope-graptoli tic shales and deepwater sands Shelf Land NEW ZEALAND

Figure 1 — Reconstruction of Eastern Gondwanaland in the (A) Early and (B) Late Ordovician showing land areas, major seaways and shelves, and continental margin slope deposits.

A2.8 DEVONIAN-CARBONIFEROUS MICROVERTEBRATES OF GONDWANA S. Turner1* & Wang Shi-Tao2 2

1 Queensland Museum, Brisbane Institute of Geology, CAGS, Baiwanzhuang Rd, Beijing, China

In the last few years, increasing interest has been Material from the Late Devonian and Early shown in microvertebrates or "ichthyoliths", which are Carboniferous of New South Wales, Queensland, now being investigated systematically for their Victoria and Western Australia (Turner, 1982, 1983, biostratigraphic uses (Turner 1988, 1989). Little work 1989), contains a similar fauna to that from southern has yet been done on the use of Upper Devonian-Lower China — Muhua, Dapoushang from Guizhou Province, Carboniferous vertebrate microfossils in biostrati- Guanxi, Guangdong and Hunan Provinces (Wang & graphy. Fresh examination of many of these fossils, Turner, 1985; Wang 1989). These microfaunas, some described first in the last century, suggests that compared with other Gondwana faunas from Thailand the sharks, in particular, will prove useful zone fossils. (Long, 1989), the Middle East, Morocco, as well as Upper Devonian (Famennian) shallow water marine with those from elsewhere, provide a useful standard and so-called non-marine (brackish?) microvertebrate for assessing future sites. Microvertebrate assemblages faunas are remarkably similar worldwide but differ across the D/C boundary are proposed—a phoebodont/ significantly in content from most assemblages known xenacanth/protacrodont shark assemblage (typical of from macrofauna. Differences between microfauna from the uppermost Famennian) and an "bradyodont"/ place to place appear to be related to facies. eugeneodont/petalodontiform shark assemblage Chondrichthyan remains predominate, especially signifying the incoming of the Carboniferous. "cladodont" teeth (ctenacanth, stethacanth, symmoriid, References denaeid, coronodontid, cladoselachian), xenacanthid, Long, J.A., 1989. J. Vert. Paleo., in press. phoebodontid and protacrodontid teeth and scales; Turner, S., 1982. / . Vert. Paleo. 2(2): 117-131. palaeoniscoid, crossopterygian, acanthodian, sometimes Turner, S., 1983. Vert. Paleo. 2(3): 38. dipnoan and placoderm remains are found. Lower Turner, S., 1989. In Rich, P., Baird, R. & Thompson, E.M. (eds): The Fossil Vertebrate Record of Australasia. 2nd Carboniferous microfaunas differ from the earlier ones; ed.. In press. the constitution of microfauna to macrofauna at any one Turner, S. (ed.), 1988. Ichthyolith Issues no. 1. Snap Printers, site is not so dissimilar. Placoderms are generally absent Brisbane: 16 pp. New chondrichthyan microremains, especially Turner, S. (ed.), 1989. Ichthyolith Issues no. 2. Snap Printers, "bradyodont" (e.g. helodont, cochliodont), eugeneBrisbane: 18 pp. odont and petalodontiform, appear. Palaeoniscoid bony Wang, S-t, 1989. In Ji Q. et al. (eds): Upper Devonian-Lower fish predominate and crossopterygian fish microCarboniferous boundary section ofDapoushang, Guizhou remains are often common; acanthodian remains are Province, south China. Science Press, Beijing. generally rare and tend to be acanthodid. Certain sharks Wang, S.t. & Turner, S., 1985. Vert. PalAsiatica 23:223-234. - Ageleodusy helodonts - appeared earlier in the Eastern Gondwana Province and along the Gondwana shoreline.

A2.9 MICROFOSSILS AND GROSS STRUCTURE AND STRATIGRAPHY OF THE SILURIANDEVONIAN CHILLAGOE FORMATION, WESTERN HODGKINSON PROVINCE, NORTHEAST AUSTRALIA Barry G. Fordham Geological Survey of Queensland, Brisbane

The "Chillagoe Shelf', was considered the "miogeosynclinal" precursor to the "eugeosynclinal" Hodgkinson Basin by early workers. The Chillagoe Formation occupied its entire 300 km length and was tentatively estimated to be 2-3 km thick and to dip overall to the east below the Hodgkinson Formation. Vaguely defined faulting and folding explained its outcrop width of up to 14 km. More recently, John F. Fawckner interpreted the Chillagoe Subprovince as a

series of imbricate thrust slices. The dominant younging direction in most slices was westerly but the Hodgkinson Province younged overall to the east. Fawckner formally delineated several belts of distinctive lithology that had previously been included in the Chillagoe Formation but mapped few faults apart from those bounding the stratigraphic units. Consequently, there was still little basis for estimation of internal disruption and thickness of the Formation.

Conodont biochronology has now shown that the Chillagoe Formation may be less than 1 km thick. In the Mungana area, in the middle of the Subprovince, the Formation is repeated at least a dozen times in elongate slivers defined by a subparallel-to-anastomosing set of faults bearing little field expression. Erection of this gross structural framework for the Mungana area has enabled the first realistic reconstruction of the stratigraphy of the Chillagoe Formation. Moreover, the gross sequence appears, based on a reinterpretation of broad lithologic units mapped by GSQ personnel (Robert J. Bultitude, Jan Domagala, Paul J.T. Donchak, and Rohan W. Halfpenny), to be relatively invariant for 50 km, from Mount Lucy in the south to Rookwood in the north. Broadly, the Mungana sequence is: thin-bedded radiolarian chert, with thin intercalations and thicker sequences of lime-mudstone and lime-wackestone, capped by mafic volcanics (Llandoverian); thin-bedded to massive lime-mudstone and lime-wackestone with prominent horizons of finely laminated sponge-spicule chert (Wenlockian—Lochkovian); massive crinoidal lime-packstone (Pragian) overlain by a highly variable sequence of, sometimes, interfingering lithologies, namely, intermediate flysch (term used nongenetically), marl, granule to boulder conglomerate to sedimentary breccia, crinoidal lime-packstone, and thin-bedded radiolarian chert (lower Emsian); and intermediate flysch with minor thin-bedded radiolarian chert (undated; ?Hodgkinson Formation). North of Mungana, fault blocks contain proportionally more flysch and less limestone. Nonetheless significant portions of the Mungana stratigraphy can still be recognised in this 25 km segment and a small number of biochronologic determinations suggest similar timing. Further north, the Subprovince reappearsfrombeneath Mesozoic cover 15 km south of the Mitchell River. From here northwards, overall comparison with the Mungana sequence is weak although the age range is apparently identical and certain marker beds and characteristic limestone sequences are coeval. The similarities between these northern parts of the Sub-province and the Mungana portion are, in fact, somewhat surprising given that the 25 km segment straddling the Mitchell River is dominated by flysch (with more mudstone than further south) and the most northerly 90 km segment is dominated by mafic volcanics. The poorly outcropping southernmost portion of the Sub-province near Mount Garnet contains coeval limestones. The Chillagoe Formation is not only much thinner than previously supposed but its boundaries (it is, as yet, only defined by a type area) can now be delineated within fault blocks on stratigraphic criteria rather than expedient placement at faults. Little can presently be proposed for its lower boundary. The underlying

Ashgillian Mountain Creek Conglomerate is exposed only in the northern Sub-province where an untectonised contact is yet to be found. The upper boundary, with the Hodgkinson Formation, is apparently conformable and gradational and can probably be defined in any of a number of fault blocks on the eastern margin of the Sub-province. Microfossils also appear to place useful constraints on palaeo-environmental interpretations of the Chillagoe Formation. The lower part of the Mungana sequence includes, interbedded with cherts rich in radiolarians, limestones with a moderate diversity of microfossil groups (bryozoans, conulariids, acrotretide and torynelasmatine brachiopods, phyllocarids, and byroniids), as well as a healthy variety of, for example, ostracodes and conodonts — the Mixed and D. obliquicostatus Biofacies of James E. Barrick, typical of middle to outer platforms. The thick lime-wackestone-dominated middle part shows a marked reduction in both recovery and diversity of microfossils (especially with regard to bryozoans and conulariids, and consistency of recovery of other groups): conodont yields (usually <l/kg) and diversities are actually lower than Barrick's P. unicostatus Biofacies, typical of inner platforms; ostracodes, when present, are simple and smooth-walled; small carbonised organic (algal?) filaments and associated forms are common; and other microfossils are rare, although scolecodonts appear to survive beyond the environmental range of conodonts. Interbedded spiculites are almost exclusively dominated by monaxons although very rare marine palynomorphs (chitinozoans) also occur as do acritarchs (leiosphaerids) and spores, whose actual provenance must, for the present, remain suspect Overall the microfossils of this part suggest very restricted shallow conditions, yet the included thin-bedded limestones and cherts have been considered, by some, deep water. By the Lochkovian, occasional limestone bands are relatively rich in conodonts and suggest somewhat more open conditions although the relatively nondescript composition of Early Devonian conodont facies prevents detailed palaeo-environmental assessment. Similar assemblages occur in the overlying crinoidal limepackstones. Rare interbedded marls have a relatively diverse macrofauna (sponges, bryozoans, brachiopods, trilobites) and contain abundant though only moderately diverse sponge spicules (monaxons are still common). With regard to regional metamorphism, conodonts in the Sub-province have a base CAI value of 5, consistent with prehnite-pumpellyite to lowergreenschist grade. Preliminary results suggest that this may increase in the northern portion. Contactmetamorphic aureoles, of course, produce local increases.

A2.10 MID PALAEOZOIC CONODONT DATA AS FUNDAMENTAL CHRONOLOGICAL UNDERPINNING FOR ANALYSIS OF ALLOCHTHONEITY OF TERRANES IN EASTERN AUSTRALIA. Ruth Mawson, John A. Talent, Michael J. Engelbretsen and Glenn A. Brock Earth Sciences, Macquarie University

Definition of allochthonous and suspect terranes and their apparent movements is impeded inter alia by problems such as: the difficulty in accurately identifying and tracing marker horizons or features useful in the recognition of displaced terranes, the paucity of palaeomagnetic data for such terranes, the failure to recognise the importance of zoogeographic similarities and contrasts when considering large scale displacements, and the failure to establish precise time-alignment between now widely spaced sequences. Precision in time-alignment between geographically widely separated sequences for mid Palaeozic time in eastern Australia can, at the present time, best be provided by conodont data. Such data obtained in association with sedimentological and structural data should provide greater precision in tectonic analysis. Aspects of the pattern of tectonostratigraphic terranes in eastern Australia (Fig. 1) as hypothesised by Talent (1988), especially concerning allochthoneity of terranes, are examined in this light. For example, in the MolongMonaro Terrane the origin of debris-flow lobes in the vicinity of Nubrigyn and Red Hill can be traced by dating both the limestone matrix as well as the constituent components of the megabreccias. Similarly, careful sampling of the debris-flow lobe at Limekilns, known as the Jesse Limestone, has shown the limestone matrix between the blocks at the very base of the section to be of serotinus age and clasts within the basal matrix to yield older conodonts of dehiscens Zone thus indicating the age of at least some of the source rocks on the former Canberra-Molong carbonate platform to the west. A progress report on Figure 1 — Hypothesised tectonostratigraphic terranes, transforms conodont data from other localities in the and faults with possible translocations of 100 km or more in Molong-Monaro Terrane, the Broken River eastern Australia (from Talent, 1988). Terrane, the Yarrol-New England Terrane, the Tamworth Terrane, the Tabberabbera Terrane and be overcome to a great extent by using conodonts for the Melbourne Terrane (Fig. 1) is presented in the light high precision control on the time of emplacement and of identifying the possible source and relationships of the possible source of such terranes. such terranes. Reference Problems inherent in the discrimination of alloch- Talent, J.A., 1988. In McMillan, N.J., Embry, A.F., & Glass, thonous and suspect terranes and the analysis of changes D.J., (eds): Can. Soc. Pet. Geol. Mem. 14 313-320 in patterns of sedimentary tectonic events can, therefore, BROKEN RIVER TERRANF

Canberra

Picunim i Ed«n-MOf« Transform

ftaraian Ba

A2.ll PLANT FOSSILS FROM NEOPALAEOZOIC GONDWANA TERRANES OF THE STATE OF RIO GRANDE DO SUL, SOUTH BRAZIL: REVIEW ON PALAEOBOTANY AND PALYNOLOGY M. Guerra-Sommer, M. Marques-Toigo, M. Cazzulo-Klepzig and Z.C. correa Da Silva* Institute of Geosciences, UFRGS, Porto Elegre, Brazil.

The Parana Basin is an intracratonic basin situated are scarce in this unit and they are represented by in the central-eastern portion of South America and Lycophyta casts (Lycopoliopsis (l) Brasilodendron (?), filled up with Palaeozoic, Mesozoic and, locally, Cordaites and Glossopteris leaf impressions are Cenozoic rocks. complementary forms in this association. The The lithostratigraphic classification of the Brazilian palynological composition of the sequence show a strata used in this paper is the one proposed by Schneider gradati ve change in relation to the proportion of triletes et al. (1974). spores and saccate pollen grains. The Striatiti are the The Neopalaeozoic Gondwana sedimentary sequence most representative pollen grains increasing in number that occurs in the State of Rio Grande do Sul, South toward the top of the sequence (Lueckisporites, Brazil, is composed of rocks of the Itarar6, Guat£ and Protohaploxypinus, Striatopodocarpites, Vittatina, Passa Dois Groups. Staurosac cites, among others). There also occurs algae The Itarar6 Group comprises continental and marine (Botryococcus) in the lower part of the sequence and a sequences which are irregularly distributed and occur in marine microplancton (Actitarcha) in the upper part, restricted areas. In the continental sequence the represented by Michrystridium and Veryachium. megaflora is composted mainly of elements of Dispersed cuticles represented by abundant fragments Glossopteris flora represented by Rubidgea, of Lycophyta epiderms are also found in the basal Gangamopteris and Glossopteris, and of some relictual sequence. forms as Botrychiopsis plantiana. The palynological The Irati Formation comprised (i) dark grey/black association is composed mainly of pollen grains from and brown oil-shales, (ii) light to dark grey shales (nongymnospermic vegetation (Potonieisporites, bituminous), and (iii) carbonates. The oil-shales are Caheniasaccites, Cannanoropollis, Plicatipollenities, supposed to have a dominantly bacterially-degraded Protohaploxypinus, Vittatina) followed by trilete spores algal origin deposited in a lacustrine environment and from Pteridophyta (Lundbladispora, Vallatisporites, the presence of Botryococcus suggests deposition under Punctatisporites, Granulatisporites) as well as algae fresh-brakish water conditions. The megafloristic (Botryococcus) and Incertae sedis (Portalites, association of the Irati Formation is represented by rich Pilasporites, Tetraporina). lignoflora composed mainly of pycnoxilic trunks of The Guat£ Group is composed of Rio Bonito and gymnospermic affinity, showing diaphragmatic pith Palermo Formations. The facies associations of the Rio (Polysolenoxylon, Scleromedulloxylon, ArachnoBonito Formation are typical of fluvial systems and medulloxylon, Septomedulloxylon). Detached roots of include coal bearing strata. An important change in the Vertebraria-type are common in this lignoflora. The floral association is evidenced by the dominance of palynological assemblage is dominated by Striatiti pollen Glossopteris leaves in the "Glossopterid complex". grains, algae remains (Botryococcus) and triletes spores Herbaceous sphenopsids (Phyllotheca) and fronds of are poorly represented. The most important genera pteridophytic and/or pteridospermic affinities became among the Striatiti are Lueckisporites, Staurosaccites more representative in the association. Arborescent Lunatisporites Protohaploxypinus). The Disaccitrilete Lycophyta were identified in some horizons on the top pollen grains (Limitisporites, Falcisporites), the of the late coal seams. The spores and pollen assemblages Monosaccites (Potonieisporites, Plicatipollenites) and of the coal seams and associated rocks are characterized the Trilete spores (Convolutispora) are also common predominantly by pteridophytic spores. The main genera genera. Megaplants records of the Estrada Nova are: Punctatisporites, Leiotriletes, Calamospora, Formation are restricted to Lycophyta trunks Granulatisporites, Horriditriletes, Lundbladispora, (Lycopoliopsis (?), Brasilodendron (?) and lepidoCristatisporites, Vallatisporites, among others. The dendron leaves. Fragments of glossopterids are scarce gymnospermic grains are represented mainly by and badly preserved. Data on palynology of this Scheuringipollenites, Vesicaspora, Caheniasaccites, Formation are not available. Cannanoropollis, Potonieisporites, Protohaploxypinus, Thefloristicchanges registered from the bottom to Vittatina^ Cycadopites, Fusacolpites, Striato- the top of the sedimentary sequences may be related to podocarpites. Algae and/or related elements, such as an environmental-climatic evolution. Botryococcus, Tetraporina, Pilasporites and Portalites, Reference occur in several seams. Schneider, R.L., Muhlmann, H.,Tommasi, E., Medeiros, R.A., Daemon, R.F. & Nogueira, A.A., 1974. Cong. Bras. Geol. Rocks of the Palermo Formation are supposed to 28, Porto Alegre, Anais 1: 41-66. have been deposited in continental areas that developed into shallow water platform Megafloristic plant remains y

A2.12 EARLY PERMIAN FAUNAS FROM THE CRACOW AREA, SOUTH EASTERN BOWEN BASIN, QUEENSLAND J.J. Draper*, V. Palmieri, S.M. Parfrey and J.F. Rigby Queensland Department of Mines, Brisbane

The Camboon Volcanics and Buffel Formation formably overlies the Camboon Volcanics, is richly exposed in the Cracow area, south eastern Bowen Basin, fossiliferous and was deposited on a very uneven contain faunas long considered to be of Early Permian volcanic terrain in a series of faulted depressions and age (e.g. Wass, 1965). Waterhouse (1987, 1988), raised blocks as a variety of sedimentary rocks which however, proposed a late Carboniferous age for most of include volcaniclastic sandstone, conglomerate and the Camboon Volcanics and the lower part of the Buffel breccia, skeletal limestone, calcareous siltstone and Formation which he had raised to subgroup status sandstone, siltstone, mudstone, and impure limestone. (Waterhouse, 1983). Given the limited aerial extent of The 120 m thick section of Buffel Formation intersected the different rock types and the rapid facies changes in stratigraphic bore GSQ Mundubbera 11 contains which occur, the subdivision into formations serves Waterhouse's 'formations' in the area where they were little practical purpose so the term Buffel Formation is proposed. The lowermost 48 m containing volcaniclastic retained for rocks overlying the Camboon Volcanics sandstone, siltstone, and coal is equivalent to the and underlying the Mount Ox Subgroup. Data from "Fairyland Formation' to which Waterhouse(1987,1988) stratigraphic drilling and surface mapping do not support ascribed a Kasimovian (early Late Carboniferous) age Waterhouse's interpretation of the sequence with a ?late based on the Echinalosia curtosa-Tabellina denmeadi Carboniferous-Early Permian age indicated for the Zone contained within the unit. In GSQ Mundubbera 11, Camboon Volcanics and an Early Permian age for the a microflora from carbonaceous mudstone associated Buffel Formation. with the coal at the very base of the intersected interval The Camboon Volcanics comprise andesite, dacite, included Pseudoreticulatispora pseudoreticulata indicatrhyodacite, basalt, and rhyolite as flows and tuffs; minor ing an age no older than Stage 3 (Wood, 1984). Parfrey volcaniclastic and carbonaceous sedimentary rocks are (1984) assigned the unique macrofauna in the interval also present. Unconformably underlying the Camboon to the Paraplatyschisma-Tomiopsis denmeadi Zone Volcanics are the Early Carboniferous Torsdale beds which she tentatively correlated with the Tiverton which, unlike the Camboon Volcanics, are intruded by Formation. H. woodwardi occurs between 18 m above isotopically dated Late Carboniferous granitic rocks. the base and 0.5 m from the top of the interval and again Therefore, the base of the Camboon Volcanics can be indicates an age no older than latest Sakmarian. The no older than Late Carboniferous. Glossopteris has been upper 0.5 m of the interval contains a richer microfauna found at a number of localities in the upper part of the assigned to the Howchinella rigida n.sp. Zone which is Camboon Volcanics, and a carbonaceous mudstone 50 m also found in the Cattle Creek Formation; an Early below the top of the unit in stratigraphic bore GSQ Artinskian age is attributed to this Zone (Palmieri, in Mundubbera 10 contains a microflora which is no older prep.). Overlying the "Fairyland Formation" is 23 m of than Stage 2 (Wood, 1984). Isotopic dates(Ar-Ar) of 281 Ma (plagioclase) and 294 Ma (total rock) were limestone (intruded by microdiorite) equivalent to reported by Runnegar (1979) for samples from the top Waterhouse's 'Dresden Limestone' within which of the unit in the Cracow area (no error ranges were Waterhouse (1988) suggested an unconformity. The published). These floras and isotopic dates are from fauna below the supposed unconformity was placed in terrestrial volcanics and indicate an age of ?late the Echinalosia ejecta-Azygidium mitis Zone with an Carboniferous to Early Permian for this portion of the inferred age of Gshelian (late Late Carboniferous) (Waterhouse, 1987) whereas, the uppermost Dresden Camboon Volcanics. In the Biloela-Thangool area, rocks mapped as Limestone contains the Acantholosia domina band to Camboon Volcanics contain marine fossils. Parfrey which Waterhouse (1987) assigned a provisional, basal (1986) described a brachiopod and bivalve fauna from Permian age. It was not possible to extract identifiable six localities in volcaniclastic sandstones interbedded fossils from the limestone in the borehole. However, with volcanic rocks, and correlated the fauna with the the limestone is straddled by the H. rigida n.sp. Zone upper Buffel Formation and upper Tiverton Formation which supports the lithological evidence that the in the northern Bowen Basin. A foraminifer fauna from sequence is continuous. the Thangool area is equivalent to the Howchinella The upper 49 m in GSQ Mundubbera 11, is woodwardi Zone (Palmieri, in prep.) which is also interbedded siltstone, limestone and mudstone equivalent found in the lower Cattle Creek and Tiverton Formations. to the "Elvinia Formation" of Waterhouse (1983) who The H. woodwardi Zone indicates an age no earlier than placed the macrofauna in the Echinalosia preovalis latest Sakmarian. curvata Zone to which he ascribed a Tastubian age In outcrop, the Buffel Formation, which uncon- (Waterhouse, 1987). Parfrey (1984) placed the

macrofauna in the Echinalosia-Tomiopsis ovata Zone References and correlated it with the Cattle Creek and Tiverton Parfrey, S.M., 1984. GSQ Record 1984/34. Formations as well as a number of units elsewhere in Parfrey, S.M., 1986. GSQ Publication 387: 57-67. eastern Australia. The interval contains the H. rigida Runnegar, B.N., 1979. Alcheringa 3: 261-285. n.sp. Zone which is replaced in the uppermost 15 m by Wass, R.E., 1965. 7. Proc. Roy. Soc. NSW 98, 159-167. J.B., 1983. GSA (Qld) Field Guide: 26-51. the Pseudonodosaria serocoldensis Zone to which a Waterhouse, J.B., 1987. Palaeont. Abt. A198: 129-233. Late Artinskian age is Aassigned. The Buffel Formation Waterhouse, Waterhouse, J.B., 1988. Proc. Symposium, UNE, Nov. 1988: is unconformably overlain by the Pindari Formation in 87-92. the borehole. Wood, G.R., 1984. GSQ Record 1984/30.

A2.13 PERMIAN FORAMINIFERA OF AUSTRALIA V. Palmieri Queensland Department of Mines, Brisbane

"On the right bank of the Piper River, not far from Texas foraminiferal faunas is confirmed for the a place called Lilydale... ." With these words spoken warmer, more temperate water faunas. Furthermore, a century ago, Thomas Stephens described the locality in the Late Permian, a similarity is postulated with of the first Permian foraminifera found in Australia. some peripheral Tethys foraminiferal faunas based Since then, various authors have contributed to on the presence of genera found in the Late Murgabianbiostratigraphic studies of the Permian foraminifera Early Midian of the Transcaucasus (Pronina, 1988), of Australia. In discussing Permian foraminifera of Iran (Zaninetti et al, 1979), and China (Lin, 1980). Australia, the following aspects are relevant: This follows and confirms the assumption of Nicoll 1. The shallow Permian seas were not always in direct (1985) of the existence of warm water sediments in connection and only when they became a continuous the Noonkambah Formation (Canning Basin, WA), sea were the foraminifera able to migrate and disperse, which were found to contain a Permian conodont thus appearing in younger sediments, even though fauna. they were not present in intermediate sediments The succession of zones proposed here (see next (Crespin, 1958). page) is intended to provide a base for the establishment 2. The constant, periodic changing depicted by the of intrabasinal and interbasinal correlations for the Permian foraminiferal faunas of Australia indicates marine Permian deposits of Australia. Both government institutions and private industry that they can be used not only biostratigraphically by virtue of phylogenetic links, but also as sea level have provided substantial material through their cycle indices (see Ross & Ross, 1985, for comparison). stratigraphic drilling programmes. 3. Australian Permian foraminifera contain forms References which are considered to be ancestral to many Permian Crespin, I., 1958. Bur. Min. Res. Bull. 48. and Mesozoic genera distributed elsewhere in the Gerke, A.A., 1960. NIIGA Trudy 120. world. In accordance with the evolutionary change, a Kalasnikov, N.V. et al., 1981. Akad Nauk SSR: 1-152. number of new taxa at generic and specific levels had Lin, JX, 1980. Acta Microp. Sin. 13: 101-120. Nicoll, R.S., 1984. Proc. GSA/PESA Canning Basin Symp.: to be proposed. 439-442. 4. Similarity with Permian foraminiferal faunas from Pronina, 1988. Rev Paleobiologie 2: 89-96. central Siberia (Gerke, 1960) and Novaya Zemlia Ross, C.A.G.P., J.R.P., 1985. Geology 13: 194-197. (Kalasnikov et al., 1981) has been established, especi- Zaninetti, L.&etRoss, al., 1979. Notes Lb. Paleont. Univ. Geneve ally for the cold water faunas; whereas the similarity 5(1). with the Late Pennsylvanian and Early Permian of

Table 1 — In ascending stratigraphic order the following zones (Oppel Zones or Consecutive Range Zones) are proposed: ZONE

FORMATION

AGE

Tezaquina clivuli

Grant Gr., Lyons Gr., Nangetty Gl., Holmwood Sh., Lochinvar Fn

Asselian-Sakmarian

Ammodiscus oonahensis

Quamby Ms, Lake Philipson Beds, Holmwood Sh., Lyons Gr., Grant Gr., Allandale Fn

Sakmarian

Howchinella woodwardi

Fossil Cliff Mb, Callytharra Fn, Nura Nura Mb, Darlington Ls, Glencoe Fn, Brumby Ms, Camboon and Lizzie Creek Volcanics, Lower Cattle Creek Fn, Lower Tiverton Fn, Rammutt Fn, Rutherford Fn, Lower Buffel Fn

Late Sakmarian

Howchinella rigida

Jimba Jimba CL, Tiverton Fn, Cattle Creek Fn, Buffel Fn, Bundella Fn, Berriedale Ls, Farley Fn

E. Artinskian

Pseudohyperammina radiostoma

Noonkambah Fn, Byro Gr., Cattle Creek Fn, Stuart Range beds(?), Snapper Point Fn, Tiverton Fn

Ar tins ki an

Pseudonodosaria serocoldensis

Noonkambah Fn, Byro Gr., Wandrawandian SL, Upper Cattle Creek Fn, Upper Tiverton Fn, Upper Buffel Fn

Late ArtinskianKungurian

Ammodiscus corrugatus

Upper Aldebaran Ss.-Freitag Fn, Mount Ox Gr., Gebbie Fn, Lower South Curra Ls(?)

Kungurian-Ufimian

Hillella echinomarginata

Ingelara Fn, Catherine Ss, Barfield Fn, Lower Peawaddy Fn, Lower Flat Top Fn, Blenheim Fn

Ufimian-Kazanian

Lunacammina maioris

Peawaddy Fn, Flat Top Fn, Upper South Curra Ls, Gigoomgan Ls

Kazanian

Robustopachyphloia mantuanensis

Peawaddy Fn (Mantuan Productus Bed), Black Alley Sh., uppermost Flat Top Fn

Kazanian

A2.14 SOUTHERN POLAR FORESTS IN A REDUCED LIGHT REGIME: EVIDENCE FROM THE BIOTA J.G. Douglas Geological Survey of Victoria, East Melbourne

Very thick (3000-4000 m) non marine sediments accumulated in a great trough forming part of a postulated Antarctica-Australia land mass during the 40 million year time span of the Early Cretaceous Period. The beds are best known in the Otway and Gippsland Basins of south eastern Australia, where there is a plant fossil record through the sequence and sporadic fossil fauna localities. Geophysical and geomagnetic measurements supplement geological data, much of which has been obtained by extensive deep drilling programmes. The data given are concerned with the environmental implications of a postulated 75°S - polar latitude for these basins during part at least of the Early Cretaceous, and perhaps even for the early part of the Late Cretaceous. It falls into two main parts. Firstly an outline

of the biota that existed at the time, and suggested environmental scenarios; secondly a progress report on ongoing experiments and findings, especially those related to an implied long polar night situation. The fossil fauna of these Early Cretaceous beds includes four orfivenominated dinosaurs and associated vertebrates: lung fish, actinopterygians (bony fish), turtles, pterosaurs (flying reptiles), and plesiosaurs. There are birds (represented only as feathers) and freshwater fish (about 10 have been named. Over 80 insects have been described, almost all from the Koonwarra Fish Bed locality. Vegetative remains described include a wide range of land plants; several fungi (principally epiphyllous, or leaf habitat) about 10 hepatics, a moss or two, half a dozen other cryptogams, (Sphenopsida, Isoetales and Lycopsida) and 40 ferns. About 50 plants

from extinct groups (Bennettitales and "Pteridosperms" are on the list, and of the higher plants, half a dozen ginkgos' and angiosperms. The very few conifers described are consideraly supplemented by cuticular residues from perhaps 50 species (personal preparations). Although all these forms amount to less than 300 species, many emanate from a singlespecialised and very restricted microenvironment (the Koonwarra Fish Beds). The fossil record, of fortuitously preserved fragments, shows then the presence of an extremely rich biota, in some areas at least quite comparable if not exceeding present day equivalents. Palaeoenvironments have been interpreted from this, and other, (especially sedimentological) data, but much is contradictory and speculative. Douglas (1969) presented an overgeneralised statement that, "there was a temperate climate, and a moderately high rainfall (perhaps 30-45 inches), with a seasonal dry period". Recently, Rich & Rich (1989) opted for "cool, seasonal non tropical climate", which may not be so different; Felton(1989) preferred "subtropical to cool temperature humid", which Francis & Frakes (1988) found evidence of "episodic cold winters... high summer temperatures", and Gregory et al. (1988) postulated the catchment area with even more severe, "annual mean temperatures less than 5°C and possibly below freezing". Again however, all not beyond the generalised statement of Douglas. A basic situation remains however; if the palaeomagnetic determinations are covered, and the

absence of a continuous ice sheet such as currently prevails is conceded, near polar conditions entail several months of continuous night. Evidence for a polar night regime may be obtained from the biota. As present day plants adjust to their environment, so too should any "plants of the night" have physiologically and morphologically adapted to that environment The plant anatomy, especially cuticular anatomy, and comparison with similar species from lower or even equatorial latitudes may provide some answers. Research is underway to evaluate overall aspects of the floral assemblages, especially in respect to light regime effects on modern analogues of ancient floras. Data obtained so far indicate that there seems to be a remarkable anatomical similarity between the southeastern Australian and more equatorial assemblages (e.g. Rajmahal Group, India). Effective comparison is however inhibited by the difficulty in finding well documented southern hemisphere lower palaeolatitude assemblages dated to a satisfactory precision. References Douglas, J.G., 1969. Mem. Geol. Surv. Vict. 28. Francis, J.E. & Frakes, L.A., 1988. Abs. 3rd IOP Conf. Melbourne Gregory, R.T. et al., 1988. Earth & Plan. Sci. Lett. Rich, P.V. & Rich T.H., 1989. Abs. 28th Inter. Geol. Cong. Washington.

A2.15 COOLING THE CRETACEOUS: EVIDENCE FOR EARLY CRETACEOUS COOL CLIMATES IN AUSTRALIA FROM GROWTH RINGS IN FOSSIL WOOD Jane E. Francis* and L.A. Frakes. Department of Geology and Geophysics, University of Adelaide

Growth rings in petrified wood from the Eromanga Basin contain an unique record of seasonally-cold climates in central Australia during the early Cretaceous. The wood originally came from forest trees that grew on high-latitude land around the southwest margins of the basin, and which were subsequently transported and buried in marginal sandstones and basinal mudstones (e.g. Bulldog Shale). The trees were of coniferous type, Gondwanan ancestors of the living podocarps and araucarians. Analysis of growth rings in the wood shows that two distinct populations of trees were preserved within the same stratigraphic assemblage. One group has narrow rings less than 1mm in diameter, whereas the other group has rings of average width 2 mm. The trees with narrow rings grew under cool climate conditions with short growing seasons, probably at higher elevations inland. The second population has widerrings,indicating

that the prevailing climate was slightly warmer. However, their rings show greater annual variability as a result of a more changeable climate from year to year. This is characteristic of trees growing at the lower forest border, in this case probably along basin shorelines. Evidence from sediments in early Cretaceous highlatitude sites in Australia, such as ice-rafted boulders, glendonite nodules and oxygen isotopes, shows that the climate was seasonally cold, with warm summers suitable for plant growth and cold winters often below freezing. The patterns of growth recorded in the tree rings are consistent with this climate scenario. Similar evidence observed in northern high-latitude regions indicates that this seasonally-cold climate was an important feature of early Cretaceous global climates and not a local effect.

A2.16 TERTIARY MACROFLORAS AND TERTIARY STRATIGRAPHY OF POOLE CREEK PALAEOCHANNEL, LAKE EYRE BASIN D. Greenwood*, R.A. Callen and N.F. Alley Department of Mines and Energy, South Australia

Fossilised plant remains are found in two Tertiary also have features in common with the Eocene sedimentary basins in central South Australia. The macroflora further south at Golden Grove. The Eyre smaller of these basins is the Billa Kalina Basin, lying Formation sediments have been cut into by a further set between the Denison/Willouran Divide and the Stuart of channels, infilled by sandy sediments intertonguing Range Divide and the much larger Lake Eyre Basin, with green magnesium clays and dolomite. This younger which occupies most of northeastern South Australia channel set is correlated with the Etadunna Formation, and adjacent parts of Queensland, Northern Territory deposited sometime during the Late Oligocene to and New South Wales. A significant palaeochannel Miocene, and containing a poorly preserved but (Poole Creek Palaeochannel) related to the Lake Eyre distinctive flora, different from that of the Eyre Basin was produced by a river flowing southwards Formation. Subsequently both sequences were silicified from the Willouran Ranges. The Lake Eyre Basin and carbonate-cemented. contains two important rock units: A total of 19 taxa were recorded including form 1. The carbonaceous, sandy, fluvial Eyre Formation, species that have assisted in unravelling the stratigraphy: deposited at various times from Late Paleocene "Gymnostoma" cone, "coarse reticulate leaf' "daisy", through to probably Late Eocene, and usually about "Banksieaeformis HI", "Casuarina" cone, and "parallel 70-80 m thick, resting uncomfortably on Middle veined leaf'. Eight forms are in common with the Middle Eocene Nelly Creek site, the most important Cretaceous rocks; and 2. The Etadunna Formation, unconformable on the Eyre being "coarse reticulate leaf', "entire pinnatifid leaf', Formation, consisting of fine green clastics and white "serrate pinnatifid leaf' and "daisy". Forms in common dolomite, laid down at various intervals during the with other Eocene sites in Australia are "Brachychiton" Late Oligocene to Late Miocene (Wells & Callen, leaves, "Banksieaeformis II" leaves, "Gymmostoma cones". "Banksieaeformis I" resembles leaves of 1986). Two silicified Tertiary macrofloras (Greenwood et Banksieaephyllum cuneatum from the Middle Eocene al., 1989) occur in very similar sandy channel deposits Maslin Bay macroflora. "Eucalyptophyllum" was also of different ages in the Poole Creek Palaeochannel. The recorded. best-preserved flora is in a younger facies of Eyre References Formation, and is probably of Middle Eocene age, as Greenwood, D.R., Callen, R.A. & Alley, N.F. 1989. S. Aust. Dep. Mines Energy unpubl. rep. indicated through correlation with unsilicified macrofloras bearing well-preserved spores and pollen Wells, R.T. & Callen, R.A. (eds), 1986. Geol. Soc. Aust., Aust. Sed. Gp, Field Guide Series 4. on the edge of Lake Eyre South. The Poole Creek floras

A3: Sedimentation: Modern and Ancient Convenor: C.P. Rao

A3.1 SHELF PROGRADATION UNDER THE INFLUENCE OF A BOUNDARY CURRENT: THE CANTERBURY BASIN PASSIVE MARGIN, NEW ZEALAND

Craig S. Fulthorpe* and Robert M. Carter Geology Department, James Cook University Multi-channel seismic profiles from the Canterbury Basin reveal the widespread importance of bottom current activity in shaping the development of a Neogene sediment prism on top of a broad platform with a gentle seaward dip. The platform was formed above a condensed section of late Oligocene to early Miocene greensand and pelagic calcarenite, overlying the midOligocene Marshall Paraconformity. Dip profiles show a general evolution of the depositional setting from a starved platform, to a ramp, to a shelf with a distinct shelf-break. The Neogene sediment prism contains numerous examples of channel-like features (Fig. 1) which extend up to 12,000 m horizontally and 1000 m vertically. Individual features can be traced between dip-profiles, and many are sub-parallel to the present coastline and shelf edge. The cross-bedded fill of individual "channels" characteristically progrades southwest. In many, and perhaps most, cases the "channels" in fact correspond to residual space left between a landward-prograding, off-shelf sediment drifts and the adjacent ramp/shelf foreslope. Erosion, perhaps coupled with periodic mass-

failure, probably characterised parts of the currentundercut ramp/shelf foreslope. Progradation of the shelf was by the addition of successive sediment drifts. Below reflector Pink (ca. 11.5 Ma), the foreslope-parallel channels are concentrated in the central part of the basin, while above Pink they exist primarily in the northeast where, in places, they may still be present along the modern shelf foreslope. The seismic architecture of the Neogene prism results from the interplay of an abundant western sediment source and an eastern boundary current system. Present day circulation involves northward flow along the east coast of the South Island, i.e. is in the opposite direction to the inferred boundary current. The palaeooceanography of the basin therefore may have involved a Miocene phase of strong southern flow e.g. through the Mernoo Gap; alternatively, the cross-stratified sediment drifts may have been deposited as antidunes. In either case, boundary current activity has manifesdy played a crucial role in the sedimentary evolution of the Canterbury Basin Neogene shelf prism.

TWO-WAY TRAVELTIME (S)

0.5

1.0-

1.5

2.0-

Figure 1 — Typical residual channel-like seismic feature resulting from the landward progradation of an off-shelf sediment drift.

A3.2 PLIOCENE-RECENT ABYSSAL SEDIMENT WAVES ON BOUNTY CHANNEL LEVEES, NEW ZEALAND Lionel Carter1, Robert M. Carter2*, Campbell S. Nelson3 and Craig S. Fulthorpe2 1

New Zealand Oceanographic Institute, Wellington, NZ; 2 Geology Department, James Cook University 3 Department of Earth Sciences, Waikato, NZ

Levees bordering the Bounty Channel, 900 km east of New Zealand, accommodate a 400 m thick sequence of abyssal sediment waves that have formed since the Pliocene. These bedforms, with amplitudes of 2-17 m and wavelengths 0.6-6 km, were formed by turbidity currents as indicated by their restriction to levee backslopes, the frequent occurrence of turbidites in cores, and the preferential but not exclusive development of waves on the left bank in accord with the southern hemisphere Coriolis deflection. The wave field was instigated c. 2.6 mybp in the middle Pliocene, when glacially lowered sea-levels

allowed New Zealand rivers to discharge directly into the Bounty Channel and its attendant canyons. The field grew vertically through the coalescence of small waves into larger bedforms accompanied by migration of the wave crests across and up levee backslopes at an average rate of 5.6 m/ky. Wave growth decreased in the late Pleistocene, probably in response to the progressive containment of turbidity currents within the steadily deepening Bounty Channel. Wave growth occurred predominantly during glacial periods. During interglacial periods of quiescence, as at present, the wave field was draped by calcareous biopelagic ooze.

Figure 1 — Cross section (viewed up-chennel) through the Bounty Channel and Bounty abyssal fan, mouth of Bounty Trough. Note well developed sediment waves on north bank levee.

A33 AVULSION AND DEPOSITION IN THE MONSOONAL GILBERT RIVER FANDELTA, GULF OF CARPENTARIA, QUEENSLAND, AUSTRALIA B.G. Jones1, G.C. Nanson2 and N. Senapati3 2

1 Department of Geology, University ofWollongong Department of Geography, University ofWollongong 3 Comalco Aluminium Ltd, Weipa, Queensland

The laterally extensive Cenozoic fluvial to deltaic Gilbert River fandelta accumulated under tropical monsoonal conditions with macrotidal influences in its lower reaches. The fandelta covers an area of about 11,000 km2 but has a maximum thickness of only 120 m. The constant gradient over the lower fandelta

surface and the adjacent offshore Gulf of Carpentaria means that during low stands of sea level erosion was minimal, the river acted as a transport corridor, and pedogenic carbonates developed in the adjacent floodplain under the more temperate climatic conditions. In the sand-dominated braided upper reaches of the

Gilbert River the marked seasonalflowhas led to clogging of the channel by sand during waning river flow. Such conditions promote gradual avulsion of the active channel across the adjacent floodplain. In the lower fluvial-dominated reaches of the Gilbert River the main channel becomes smaller due to overbank loss of water into smaller distributary streams. Scouring of the channel is an important feature during the early flood stages in this area but the channel refills with sand during the waningflowdue to tidal retardation of the low velocity. Channel-fill sequences in the low to moderate sinuosity river system are generally 4-6 m thick consisting of two-thirds channel sand and one-third overbank mud. Scouring along the thalweg of the active channel is minimal during a monsoonal flood whereas sand on the channel margins shows extensive erosion and deposition during the cycles. The lower part of the resultant channel profile thus consists of reworked lag gravel and coarse-grained flat-and cross-bedded sand. Slightly sinuous, flat crested dunes with wave lengths of 50-250 m and an amplitude rarely exceeding 1 m migrate down the active channel under high flow conditions. However, they have a low preservation potential and result in the accumulation of 5-15 cm thick, low-angle planar cross-beds. As the river widens its bed and migrates laterally, the coarse sands are

succeeded by finer grained deposits laid down on the channel margins and in second order channels. These smaller channels have lowerflowvelocities and contain shorter wavelength dunes with a higher preservation potential. Planar cross-beds still dominate but trough sets are present locally. Where secondary channels reenter the main channel large (up to 3 m) cross-beds prograde into the latter. Thus a typical vertical profile becomes finer upwards (to medium sand) but the set size increases upwards as the channel becomes clogged with sand and preservation of individual dunes is enhanced. The channel sands are capped by overbank mudstone and splay sand lenses. Most channels are active for 500-1500 years with avulsion resulting in confined channel sequences rather than the sheet channel sands more typical of perennial braided streams. Sand body geometry within the fluvial part of the fandelta is elongated in a downfan direction. Towards the coast fluvial sands overlie and interfinger with tide dominated organic-rich supratidal, shoreline and shallow marine sequences. In older buried analogues the intersection of shoreline and channel sands with supratidal and shallow marine sequences would provide suitable reservoir and source rocks for petroleum accumulation.

A3.4 RELATIONSHIP BETWEEN SYNERESIS CRACKS AND EARLY DIAGENETIC CONCRETIONS IN THE IRBY SILTSTONE (PROTEROZOIC), NORTHERN TASMANIA C. R. Calver Department of Resources and Energy, Hobart

A dolomitic member within the upper Proterozoic Irby Siltstone at Sisters Beach displays a variety of unusual mesoscopic early diagenetic structures (Calver & Baillie, in press). The unit consists predominantly of dolomitic mudstone, siltstone, and nodular dolomite deposited in a low-energy sublittoral shelf environment Storm-deposited layers, consisting of wave-ripple crosslaminated dolosiltite beds with erosional bases, constitute a minor, more proximal facies. Early diagenetic features in the quiet-water sediments include substratal shrinkage (syneresis) cracks, concretions and an unusual compound structure composed of concretions nucleated on shrinkage cracks. 'Molar-tooth structures' (Smith, 1968), abundantly preserved in dolomitic mudstone beds, are the most common mode of preservation of early substratal shrinkage cracks. These are thin, spar-filled veins strongly contorted by compaction. In a few beds the veins resisted compaction, remaining as narrow, spindle-shaped bodies orthogonal to bedding with primary lamination strongly deflected about them. Associated small clastic dykes appear to be sedimentfilled syneresis cracks. At many horizons throughout the sequence, permeability anisotropy produced by the

cracks while they were still open allowed early cementation in the sediment immediately surrounding them. The resulting upright, tabular concretions with thin medial veins were invariably sufficiently rigid to resist compactive deformation. These compound structures occur together with normal, bedding-plane early diagenetic concretions in complex nodular beds previosly termed 'hieroglyphic' dolomites. Formation of the upright concretions centred on shrinkage cracks apparently slightly preceded formation of the bedding-parallel concretions. The spectrum of diagenetic fabrics appears loosely related to primary lithology, ranging from siltfree lutites with abundant molar-tooth structures and no concretions, to muddy laminated dolosiltites with abundant concretions and no vestige of syneresis crack development Intermediate between these are rocks in which the interplay between syneresis and early cementation allowed development of the compound structures and complex 'hieroglyphic' fabrics. The rocks are tectonically deformed, and a strong preferred orientation of the shrinkage-crack derived structures in the bedding-plane appears to be entirely of tectonic origin. Analyses of strain suggest the original

distribution of the shrinkage cracks in the bedding- References plane was uniform, ruling out the involvement of an Calver, C. R. & Baillie, P. W., in press. J. Sed. Petrol. 60. Smith, A. G.,1968. /. Geol 76: 426-443. external stress field in their development A3.5 LATE PROTEROZOIC AND EARLY CAMBRIAN CLASTIC FACIES RELATIONSHIPS, SOUTHWESTERN GEORGINA BASIN, NORTHERN TERRITORY Peter W. Haines Northern Territory Geological Survey, Darwin

In the Barrow Creek area of the southwestern Georgina Basin, the late Proterozoic and Early Cambrian sequence comprises a complex of generally undeformed clastic sediments unconformably overlying an early to middle Proterozoic basement. Most of this sequence was previously united as the Central Mount Stuart Formation (Smith & Milligan, 1964; Offe, 1978). However, several workers (Daily, 1974; Walter, 1980) have suggested the existence of a disconformity at the base of the Cambrian and between Proterozoic diamictites and younger sediments. This study now recognises the existence of at least five distinct packages of late Proterozoic to Early Cambrian sediment in the area, each separated by regional unconformities or disconformities. The Central Mount Stuart Formation, as expressed at its type section, essentially relates to only one of these units. Publication of new stratigraphic names used below is in progress (Bagas et al., in prep.). The basal unit, the Amesbury Quartzite, comprises a thin (20-30 m) transgressive marine facies which may correlate regionally with basal transgressive units in other late Proterozoic to Palaeozoic basins in central Australia. It is unconformably overlain by, and in places completely removed before deposition of the Boko Formation, a massive diamictite of glacial origin containing faceted and striated cobbles and boulders in a matrix of red-brown mudstone. Matrix composition allows a tentative correlation with the Marinoan glacials of the Adelaide Geosyncline and elsewhere. The Amesbury Quartzite and Boko Formation are only locally preserved beneath the unconformity at the base of the succeeding Central Mount Stuart Formation, and appear to have undergone some tilting prior to this erosional event. The Central Mount Stuart Formation is a deltaic sequence of sandstones, mudstone and minor conglomerate which shows increased marine influence and thickening (up to 800 m) towards the south and southeast of the area. Palaeocurrent measurements and facies distribution patterns indicate that sediment was derived from uplifted basement areas to the north and west The degree of marine influence and textural and mineralogical maturity also increase up-section at many individual localities. Sedimentation was controlled by northeast-southwest oriented faults and locally the

sediments were deposited over a very irregular basement topography, which led to the development of conglomeratic wedges adjacent to basement scarps. The relative position of a late Proterozoic fossil horizon in various sections indicates that the age of the basal unconformity becomes successively younger to the north. The conglomeratic Andagera Formation, which occurs within valleys and around the margin of the Davenport Ranges to the northwest, is considered to be a lateral equivalent that is largely of fluvial origin. A minor disconformity separates the Central Mount Stuart Formation from overlying marine clastics and minor carbonates containing trace fossils which indicate an Early Cambrian age. This sequence contains a minor internal disconformity and has been Neutral Junction formations (formally combined as the Donkey Creek beds, Walter, 1980). The Octy Formation (up to 90 m) comprises shallow marine crossbedded sandstones with a strongly bipolar palaeocurrent pattern, in which the dominant current direction, probably representing shoreward tidal currents, is from the east. The Neutral Formation (average 40 m) comprises glauconitic, and often strongly bioturbated,finesandstone, siltstone, shale and minor limestone representing a somewhat deeper and quieter marine setting. Syndepositional tectonic activity including block faulting and graben formation, was largely responsible for sediment supply and controlled the distribution of sedimentary facies in this part of the Georgina Basin, particularly during the deposition of the Central Mount Stuart Formation. The peak of tectonic activity coincides with the inferred timing of the Petermann Ranges Orogeny in the southwestern corner of the Northern Territory. References

Bagas L., Haines P.W., Wyche S., Simons B.A. & Morris D.G., in prep. NT Geol. Surv. Explan. Notes SF53-6. Daily B., 1974. Geological Society of Australia Specialist Group in Biostratigraphy and Palaeontology Prog. & Abst., Hobart, 1974: 4-8. Offe, L.A., 1978. BMR Aust. Explan. Notes SF53-5. Smith K.G. & Milligan E.M., 1964. BMR Aust. Explan. Notes SF53-6. Walter, M.R., 1980. BMR Aust. Report 214.

A3.6 CONTROLS ON JURASSIC NON-MARINE SEDIMENTATION IN EASTERN AUSTRALIA: EVIDENCE FROM THE CLARENCE-MORETON BASIN P.E. O'Brien* and A.T. Wells Bureau of Mineral Resources, Canberra

A large part of eastern Australia is covered by the Mesozoicfluvialand lacustrine rocks of the Eromanga, Surat and Clarence-Moreton Basins. This study examines sedimentation changes in the Jurassic Bundamba Group of the Clarence-Moreton Basin in order to understand the processes that caused them. The study aims to test the hypothesis that most sedimentation changes in the Jurassic sediments of eastern Australia were caused by eustatic sea-level changes. The Clarence-Moreton Basin formed by thermal relaxation of the crust after Permian to Triassic transtension. Strike-slip faults that controlled transtension remained active throughout basin development introducing variations to subsidence rates. The Late Triassic to Early Jurassic fluvial sediments of the Bundamba Group contain abundant plant debris and thin coals and lack red palaeosols indicating humid climates during deposition so that changes cannot be ascribed to large climatic fluctuations. Most changes in sedimentation in the ClarenceMoreton Basin result from tectonic effects (Table 1). Tectonic rejuvenation is reflected in re-arrangement of palaeocurrent patterns, influxes of coarse detritus from uplifted basement blocks and increased proportions of labile grains in the sandstones because of steeper slopes in the basin hinterland. Increases in quartzose detritus indicate erosional maturation of the hinterland. Low sandstone- shale ratios in units indicate relativerisesin base level because the rising base level reduces stream gradients favouring aggradation, so lowering the

avulsion-aggradation ratio. Lower stream gradients also favour a change from braided streams to meandering and anastomosing patterns. Threerisesin relative baselevel occurred during Bundamba Group deposition, two resulting from increased subsidence (Table 1). The third caused simultaneous changes in adjoining basins. It probably stemmed from a eustatic sea level rise. Palaeocurrent data indicates that any connection to the sea was well north of the Clarence-Moreton Basin. Facies assemblages suggest that subsidence caused by variations in in-plane tectonic stresses did not cause this change. The replacement of shaley units deposited in response to base levelrisesby sheet-like sandstone units probably reflects the build up to steeper, near-graded stream profiles. After base level rises, streams aggrade to increase slope. Increased slope may cause a change from meandering to braided stream patterns and graded streams transport all their detrital load so that avulsionaggradation ratios are relatively high, resulting in relatively high sandstone-shale ratios. Falls of regional base level cause regional down cutting. Large amounts of down cutting are only visible locally in the Bundamba Group suggesting that regional falls did not take place. Therefore, sedimentation changes in the Bundamba Group of the Clarence-Moreton Basin indicate that eustatic sea-level changes can have a major influence on fluvial systems in intractonic basins. However, not every major change can be automatically ascribed to sea-level fluctuations.

62 Table 1— Sedimentation changes in the Bundamba Group, Clarence-Moreton Basin. Formation

Characteristics

Controlling Processes

Aberdare & Lay ton's Range Congs. Raceview Fm.

Alluvial fan & valley fill conglomerates passing basinwards into mixed load fluvial sediments. Lithic sandstones. Radial palaeocurrents.

Initial rapid subsidence. Flat basin floor and steep edges.

Ripley Road & Helidon SS.

Sheet-like, bedload stream deposits. Quartzose sandstones. S to N palaeocurrents in E, W to E in the W.

Landscape matured and streams reach grade after initial subsidence. Subsidence possibly low so high Avulsion/Subsidence.

Gatton SS.

Sheet-like, bedload stream deposits. Diachronous change to lithic sandstones. Palaeocurrents all S to N. Some basement blocks emerge briefly.

Tectonic rejuvenation of basement areas, tilting of the W part of the basin.

Ma Ma Creek Mbr Koukandowie Fm.

Lacustrine and suspended-load fluvial sediments. Chamositic oolite, brackish water acritarchs. Change mappable through adjacent basins.

Base level rise, probably sea level rise or tectonicdamming of the distal end of drainage system.

Heifer Creek Mbr, Koukandowie Fm.

Sheet-like, bedload stream deposits. Quartzose sandstones. S to N palaeocurrents.

Re-establishment of graded system after base level rise. Mature landscape providing quartzose detritus.

Koukandowie Fm above Heifer Creek Mbr (E only).

Mixed-load & bed load stream deposits. Lithic sandstones. S to N palaeocurrents.

Tectonic rejuvenation and slightly increased subsidence in the east (Logan Sub-basin).

The major influences on fluvial sedimentation styles are: 1. Source area lithologies and topography that control the volume, composition and calibre of sediment supplied to the rivers. 2. Climate that controls discharge, sediment composition and volumes, and vegetation cover. 3. Basin subsidence rates influence rates of deposition and river slopes, and avulsion to aggradation ratios determine sandstone-shale ratios and sandstone body geometry. 4. Base level changes related to sea level or tectonism, control stream long profiles and hence aggradation or erosion. 5. Groundwater systems in the basin can draw off water from a river or crop out to produce lakes and swamps, so changing the fluvial style.

A3.7 GLACIATION AND PALEOGEOGRAPHY OF AUSTRALIA AT THE START OF THE PERMIAN Albert T. Brakel* and Jennifer M. Totterdell Onshore Sedimentary & Petroleum Branch, Bureau of Mineral Resources, Canberra

This paper presents the results of the BMR-APIRA Palaeogeographic Maps Project for the palynological Stage 2 interval, which is considered to be earliest Permian or latest Carboniferous by various workers. In New South Wales, and possibly elsewhere, the base of this time slice co-incides with a hiatus. The interval saw the climax of the late Paleozoic glaciation, hence its most outstanding feature is the domination of the southern and western parts of the continent by extensive ice sheets andfloatingice shelves. The largest of these occupied Western Australia south of the Fitzroy Trough, and extended into central Australia east of Alice Springs. Another ice cap covered the region between Adelaide, Oodnadatta, northwestern Cooper Basin and Wilcannia, while a third ice sheet occupied most of Tasmania and Victoria. These regions may have been connected during the coldest stadia. Valley glaciers existed along the margins of the Sydney and Galilee Basins, and possibly in north Queensland and the Kimberley region. Shallow marine basins at the time were the Grantleigh Trough,

Carnarvon, Canning, Bonaparte, northern Sydney, Arckaringa, and Murray basins. The last two were probably connected southwards to a Red Sea-type sea filling a rift valley along the southern Australian margin, on the site of the later break with Antarctica. The main fluvial basins were the Werrie, Pedirka, southern Cooper, and Galilee Basins, and probably the Denison Trough. The most striking feature of eastern Australia was an immense belt of bimodal volcanism stretching from west of Sydney to Townsville. To the east, much of the belt was parallelled by eroding highlands, and east of these in turn, from Taree northwards, a deep water turbidite regime prevailed. Subduction, which was active here in the Late Carboniferous, was terminated when plate movements underwent a radical re-adjustment at about the start of the Permian. The Gympie Terrane, also with a deep water facies, was not in its present position, but formed part of an exotic terrane out in the Eopacific; some of the other blocks in the New England Orogen may prove to be exotic as well.

£ P E R M IAN Palynoiogical 5 LAND

LOG/MS

LEU

Unclassified

warn Y///////A

Eros ion a/ Glacial

'LDF*

F/uv/a!

LDL

lacustrine

LDU

Deposi+/ona/' unclassified

COASTAL •CD P-'\.

LOG/MS

Paralic

MARINE ^.MS1MB A' llllllllll LOG/MS

Shallow

'ZOOnt)

Bafhyal

abyssal

'(>200m) l/o iconic

area

1000 km

ATB,JMT(BMR)

A3. 9 PALAEOGEOGRAPHY OF THE LATE CARBONIFEROUS GLACIMARINE WYNYARD FORMATION, NORTHWEST TASMANIA Stephen J. Hand Geology Department, University of Tasmania

The Wynyard Formation (fig.l) consists largely of diamictites, rhythmites and glacifluvial sandstones and conglomerates. Diamictites can be separated on the basis of sedimentary structures, clast orientation fabric and sorting into three rock types, tillite, debris flows and dropstone diamictite. The Wynyard Formation is Late Carboniferous (palynological stage 1) in age (Truswell, 1978). Clast composition of diamictites was used to establish a stratigraphy within the Wynyard Formation. The formation was divided on the basis of clast composition into eight clast lithozones. The rocks of clast zone one were subdivided into two distinct members. The gently folded, west dipping rocks of the Doctors Rocks Member outcrop at Wynyard and are unconformably overlain by rocks of the Seabrook Member which outcrops at Wynyard and in the deeply incised river valleys to the Southwest. Clast zones 2-8 consist of rocks of the Parrawe Member which conformably overlie rocks of the Seabrook Member and outcrop at and south of the valley of the Hellyer River. Each of the three members corresponds to a separate glaciation. The range of depositional environments within the Wynyard Formation was reconstructed by comparing the lithofacies associations of rocks of the Wynyard

Formation with those found in modern glacial environments. Rocks of the Doctors Rocks and Seabrook Members were found to be identical to those being formed at present in front of fjord-tidewater glaciers with a melting/freezing base. Rocks of the advance phase of the Parrawe Member were found to be identical to those being formed at present in front of fjord-ice shelf glaciers with a melting/freezing base. Rocks of the retreat phase of the Parrawe Member were of the same glacial regime as those of the Doctors Rocks and Seabrook Members. Ice movement directions and clast provenance studies suggest that the glaciers of the Doctors Rocks and Seabrook Glaciations travelled from the Zeehan area towards Wynyard in a NNE direction. Similar studies suggest that the glacier of the Parrawe Glaciation travelled from the Cradle Mountain area in a NNW direction, intersecting the retreating glacier of the Seabrook Glaciation (Fig.l). These studies indicate that the Carboniferous glacial rocks at Zeehan and a possible nunatak at Cradle Mountain may be related to the rocks of the Wynyard Formation. References Truswell, E.M., 1978. Tasm. Geol. Surv. Bull. 56 1 46°30'

h41°00' „10km „ N

// \ \

Figure 1 — Outcrop of Wynyard Formation (shaded) in NW Tasmania and postulated extent of fjord system in Upper Carboniferous (stage 1) time. !42°QQ'

Zeehan

A3.10 A MODEL FOR GLACIGENE SEDIMENTATION IN THE TROUBRIDGE BASIN, SOUTH AUSTRALIA Neville F. Alley * and Robert P. Bourman l

1 2

Department of Mines and Energy, SouthEducation, Australia South Australian College of Advanced

The Late Palaeozoic Troubridge Basin extends from fabrics indicate that ice movement through the Basin the northern Coorong across Fleurieu Peninsula to was generally in a northwesterly direction. Local Kangaroo Island and southern Yorke Peninsula. Its divergence of flow to the north (Backstairs Passage) boundaries are not tectonically controlled but rather and to the west (Inman River Valley) were the result of reflect remnants of a much more extensive blanket of ice being channelled along major valleys or depressions. glacigene sediment (Cape Jervis Formation), much of The topography was submerged beneath the ice mass which is now preserved in major and minor erosional during the glacial maximum. Deglaciation commenced with vertical downwasting troughs. Only 50 m of the Cape Jervis Formation are encountered in outcrop; up to 300 m have been of the ice mass into what is now the coastal lowland and Gulf St Vincent. Ice-proximal conditions prevailed in intersected by drillholes. The Cape Jervis Formation comprises three main the area of the modern coastline, where large subaqueous fades: (1) lodgement till at the base of the sequence, to subaerial fans were built into ice-dammed lakes or only a few metres thick, (2) ice-contact, fluvioglacial incorporated intofluvioglacialoutwash plains, and where and glaciolacustrine clay, silt, sand, gravel and numerous thin sheets of flowtills were shed from the diamictons (flowtills) up to 100 m thick, and (3) glacio- stagnating ice. The decay of the ice was accompanied by eustatically marine clay, silt and sand up to 200 m thick. The beds often overlie glacially polished and scoured bedrock, controlled transgression of the sea into the isostatically exhibiting a suite of features that include grooves, striae, depressed lowlands, perhaps along the margin of the ice friction cracks, small rock crag-and-tail forms, a variety sheet. The transgression probably came from the north of small stoss-and-lee features and plastically moulded via the Arckaringa, Pedirka and Cooper basins. Subsequent glaciomarine sedimentation occurred in brackish rock surfaces. A model of deposition to account for the stratigraphy water turbid withfinestransported in by large volumes and sedimentology of the glacigene sediments requires of meltwater. Glaciomarine sediments representing thefinalphase only the advance and decay of one ice mass. This ice was a continental-scale, wet-based glacier that produced of deglaciation in the Basin have been dated by foramsignificant erosion of the bedrock during advance and iniferal and palynological evidence as earliest Permian considerable deposition of proglacial sediments during (Sakmarian to Asselian) in age. The actual age of the glaciation is unknown and may even extend back into stagnation. Studies of glacial erosional features, erratics and till the Carboniferous. A3.ll DIAGENETIC ENRICHMENT OF IRIDIUM AND OTHER PLATINOIDS: EVIDENCE FROM THE ACRAMAN IMPACT EJECTA HORIZON AND HOST SHALES Malcolm W. Wallace *, Reid R. Keays and Victor A. Gostin 1

Department of Geology and Geophysics, University of Adelaide Department of Geology, University of Melbourne 2

Geochemical investigations on the Late Proterozoic Acraman impact ejecta horizon and its host shales (Bunyeroo Formation) in the Adelaide fold belt have provided strong evidence for diagenetic mobilization of the platinum group elements (PGE), including Ir. Both the ejecta horizon and the green shales that envelop it have strong PGE enrichments (Ir up to 2.0 ppb, Pt up to 270 ppb) relative to the host red shales (average red shale background of 0.019 ppb Ir and 0.89 ppb Pt). Several PGE anomalies (0.073-0.45 ppb Ir, 3.11-314 ppb Pt) also occur in thin green shale beds at other stratigraphic levels within a predominantly red

shale sequence. In addition, isolated green reduction spots red shales have PGE enrichments. All thin green shale horizons and green reduction spots analyzed, regardless of their stratigraphic position, have relatively high levels of Ir and other PGE. The non-extraterrestrial, diagenetic origin of the PGE at other stratigraphic levels away from the impact ejecta horizon is indicated by their restriction to reduced green shales in a predominantly red shale sequence, their association with enrichments in Cu-V-Zn-Ni, and their nonchondritic PGE interelement ratios. The impact ejecta horizon has a more chondritic PGE geochemistry

consistent with a meteoritic origin. A redox precipitation model is proposed to explain the PGE anomalies in the green shales. The discovery of significant PGE mobility within low temperature diagenetic environments indicates the

potential for economic accumulations of PGE by diagenetic processes and has implications for the recognition of meteoroid impact-related events in the sedimentary record.

A3.12 AMINOSTRATIGRAPHY AND ELECTRON SPIN RESONANCE STUDIES OF LATE QUATERNARY SEA LEVEL CHANGE AND COASTAL NEOTECTONICS IN TASMANIA C.V. Murray-Wallace * and A. Goede 1

The N.W.G. Macintosh Centre for Quaternary Dating, University of Sydney 2 Department of Geography and Environmental Studies, University of Tasmania 1

Numerous Tasmanian coastal stratigraphic sequences and landforms provide evidence for late Quaternary sea level change and neotectonics. The high elevation of marginal marine strata and coastal landforms, relative age relations and neotectonic significance, although anticipated by some (van de Geer et al., 1979; Bowden & Colhoun, 1984) has remained problematic in detail. Problems of interpretation in early studies were compounded by the practice of correlating coastal sediments with the tectonically overprinted Mediterranean sea level record (Jennings, 1959). The subsequent delineation of global, glacio-eustatic sea level curves, however, extending from the middle Pleistocene to the Holocene, provided a framework to infer preliminary ages for these coastal features (Chappell, 1983; Chappell & Shackleton, 1986). Geomorphological, lithostratigraphic and palynological evidence in conjunction with radiocarbon dating (Colhoun et al., 1982), indicated that the ages of many of these sequences exceeded the practical limits of radiocarbon dating (ca. 50,000 years). On this basis, many of these features were ascribed Last Interglacial ages (substage 5e of the oxygen isotope record). This event has been dated globally at 125,000 ± 10,000 yr BP by uranium-series disequilibrium (Stearns, 1984). The timing of these high sea level events remained poorly understood however, as available dating methods were not applicable to specific mineralogical components of the sequences in Tasmania, and many of the techniques' assumptions were not upheld in their stratigraphic context. The application of amino acid racemisation and electron spin resonance methods in the delineation of a late Quaternary glacio-eustatic and neotectonic history from several settings in Tasmania, is summarised in this paper. The relevance of these methods for dating the Tasmanian deposits, centres on their applicability to a wide range of fossiliferous materials, the potentially wide time span they cover and their small sample requirement This study represents the first systematic attempt at directly dating the biota from these deposits. At Mary Ann Bay in the Hobart region, moderate to finely comminuted molluscan fossils occur in a wellbedded marine unit exposed within an actively eroding

cliff. Whole Pec ten meridionalis andfragmentsof Fulvia tenuicostata occur within a laterally persistent, tabular cross-stratified unit, comprising clean, fine grained quartz sands. The shell bed occurs some 22 m above present High Water Mark (HWM), and a Last Interglacial age is indicated by amino acid racemisation and electron spin resonance. Richly fossiliferous, unconsolidated quartz sands, termed the Mella Sands (Gill & Banks, 1956) occur extensively within two prominent topographic depressions, west of Smithton, along the north west coast of Tasmania. At Broadmeadows, Mowbray Swamp and Montagu, Last Interglacial intertidal facies occur at elevations ranging between 11 to 13 m above HWM. Mollusca from the deposits are well preserved with original pigmentation and nacreous lustre on many individuals. The three sites are located between 3.5 to 6 km from the sea. The Last Interglacial age of these deposits is also supported by amino acid racemisation and electron spin resonance. With the exception of tectonically uplifted sites, the height of the Last Interglacial sea surface around the Australian coastline is consistently below the 6 m level commonly cited (Murray-Wallace & Belperio, in press). Confirmation of the Last Interglacial age of the Tasmanian sequences therefore, strengthens the notion that they have been uplifted. Other high level occurrences of emergent coastal landforms and sediments at levels of +20 to +22 m HWM have been recorded from King Island (Jennings, 1959) and Hinders Island (Sutherland & Kershaw, 1971). Assuming a constant rate of uplift and a +4 m level for the Last Interglacial sea surface, indicates uplift rates of 0.06 m/ka for the northwest coast of Tasmania and 0.15 m/ka locally at Mary Ann Bay. The uplift mechanism, however, remains problematic. Bowden & Colhoun (1984) attributed the high level occurrences of interglacial strata and erosional landforms to regional uplift in response to either hot spot activity or crustal underplating during the northward drift of the Australian continent Features consistent with such a tectonic setting include crustal doming reflected in the geometry of the Stumpys Bay Sands, and higher than average geothermal gradients along the northwest coast (e.g. moundsprings). In contrast,

Holocene marginal marine strata in Tasmania have nowhere been found more than 2 m above HWM. As a result, there is insufficient resolution to assess whether these neotectonic processes are extant. In conclusion, concordant results from amino acid racemisation and electron spin resonance confirm that several high level occurrences of marginal marine strata along the coasts of NW and SE Tasmania were deposited during the Last Interglacial. The anomalously high elevation of these strata with respect to globally established levels for the Last Interglacial sea surface is attributed to neotectonic uplift, although the mechanism remains problematic. Uniformitarian explanations with recourse to the pre-Quaternary record provide no clear evidence supporting a history of tectonic uplift Relative lithostratigraphic relationships hint at the likelihood of differential tectonic uplift occurring at a local scale, suggesting that Quaternary uplift of portions of Tasmania did not occur en bloc. Evidence from Holocene sequences does not provide sufficient resolution to

resolve whether these tectonic processes are still occurring. References Bowden, A.R. & Colhoun, E.A., 1984. In B.G. Thorn (ed.): Coastal geomorphology in Australia. Academic Press: 313-342. Chappell, J., 1983. Search 14: 99-101 Chappell, J. & Shackleton, J.N., 1986. Nature 324: 137-140. Colhoun, E.A., van de Geer, G. and Mook, W.G. (1982). Quat. Res. 18, 108-126 Gill, E.D. & Banks, M.R., 1956. Rec. Q. Vict. Mus. 6: 1-41. Jennings, J.N., 1959. Rec. Q. Vict. Mus. 11: 1-39. Murray-Wallace, C.V. & Belperio, A.P., in press. Quat. Sci. Rev., Aust. Spec. Issue Steams, C.E., 1984. In W.C. Mahaney (ed.): Quaternary dating methods. Elsevier: 53-66. Sutherland, F.L. and Kershaw, R.C., 1971. Pap. Proc. R. Soc. Tasm. 105: 151-175. van de Geer, G. Colhoun, A.E. & Bowden, A.R., 1979. Geol. Mijnb. 58: 29-32.

A3.13 CONTEMPORARY EVAPORITE SEDIMENTATION IN KARINGA CREEK DRAINAGE SYSTEM, AMADEUS BASIN, NORTHERN TERRITORY A. V. Arakel1*, R. McWatters1, D. McConchie2, A. Cohen3, C. Pailles1, and T. Hongjun4 3

1 Queensland University of Technology, Brisbane; 2 University of New England, Lismore Campus Geological Survey of Israel, Jerusalem, Israel; 4 Chengdu College of Geology, Sichuan, P.R. China

Karinga Creek drainage system represents the southeastern compartment of a 500 km long drainage valley, identified as the Central Australian Groundwater Discharge Zone, extending from Lake Hopkins in Western Australia, through Lakes Neale and Amadeus, to the Finke River in the Northern Territory. The Karinga Creek drainage system is comprised of numerous playa lakes and ephemeral drainage creeks, which accept water mainly from shallow groundwater aquifers in Mesozoic shales, calcrete buildups and local sand dunes. Recent sedimentological and hydrogeochemical studies in the area indicate that the playa-lake sediments act as a host to a variety of evaporite deposits, which are deposited from highly concentrated brines enriched in magnesium and potassium elements. Calcium, magnesium and potassium sulfates comprise the bulk of primary evaporite mineral facies in the playa beds. Sedimentary features and geochemical signatures of the surficial evaporite-mineral deposits indicate that diagenetic alteration at the sediment-water interface plays a major role in modification of the primary evaporite-mineral assemblages through diurnal and seasonal cycles.

Consideration of the hydrogeological setting, physiographic features of the playa chain and the contained evaporite-mineral facies in the Karinga Creek drainage system has enabled subdivision of the study area into major drainage domains. Evaporite sedimentation in these domains is believed to be influenced largely by the sources of solute input, the local hydrologic gradients, and the extent of vadose diagenetic alteration at the sediment-water interface. The correlation of petrochemical data with elemental concentration pattern in the playa solutions, indicate that although a regional geochemical evolutionary pathway may be apparent for the Karinga Creek drainage system, yet a simple evaporative concentration model does not provide complete explanation for evaporite mineral facies pattern recorded from the study areas. The investigations in the Karinga Creek drainage area include assessments of brine and evaporite-mineral resources and feasibility of producing high value industrial salts by integrated solar evaporation and secondary processing techniques. A resume of current and proposed research and development activities in the area will be presented.

A3.14 LIMESTONE MICROFACIES DISTRIBUTION OF THE CHILLAGOE FORMATION (SILURO-DEVONIAN), MUNGANA AREA, HODGKINSON BASIN, NORTH QUEENSLAND AND ITS STRUCTURAL IMPLICATIONS T. Bernecker* and J.A. Webb Department of Geology, La Trobe University

The northernmost sedimetary province of the Tasman Orogenic Belt in eastern Australia is the Hodgkinson Basin. The Chillagoe Formation outcrops for 250 km along the western margin of this basin; its western boundary is the Palmerville Fault which separates Palaeozoic sediments and igneous rocks from the Precambrian basement. In the Mungana area the Chillagoe Formation (Early Silurian-Early Devonian) consists predominantly of limestones and intercalated chert, as well as minor siliciclastic sediments and basalt Towards the east the Chillagoe Formation is faulted against the Hodgkinson Formation (Early-Late Devonian), a very thick sequence of siliciclastic turbidites with occasional limestone clasts and some larger bodies of limestone. During the last ten years the interpretation of the depositional environment of the Chillagoe limestones has been subject to controversy. Because the limestones outcrop mainly as prominent steep-sided bluffs up to 100 m high, it has been argued that these bodies represent large allochthonous blocks formed during the break-up of a carbonate shelf (Green et al., 1988). Angular contacts between several carbonate horizons, the presence of apparently slump-folded, thin-bedded lime-mudstone units and the extensive occurrence of coarse breccias seem to support the idea of deep marine sedimentary processes. Detailed sedimentological mapping and carbonate microfacies analysis in a relatively well-exposed portion of the Mungana area has now revealed that in fact marker horizons and intervening sequences can be traced laterally for several kilometers throughout this area. Furthermore, the facies boundaries delineated bear little relationship to the distribution of the limestone bluffs. The Chillagoe limestones can be subdivided into ten carbonate microfacies, dominated by wackestones with a variety of bioclastic associations. Overall, the Chillagoe Formation has been deposited on a carbonate platform. Indicative of accumulation in a shallow marine environment are abundant Ajnphipora-beds, a variety of bryozoans and stromatoporoids with associated megalodont bivalves and coated grains. Corals in growth position are widespread and quite diverse: Silurian sediments seem to be dominated by encrusting tabulate forms of alveolitids and favositids, often associated with various species of halysitids and, less common, heliolitids. Branching forms include syringoporids and thamnoporids. Rugose corals are represented by tryplasmids, and later in the Devonian by large colonies of Xystriphyllum. A unique tabulate coralstromatoporoid boundstone/basalt association is also evidence for sedimentation in relatively shallow water.

Stromatactis, a feature believed to be characteristic of mud-mounds (Wallace, 1987), does occur but poor outcrop has obscured any evidence of topographic buildups. A true reef-facies is not developed. A deeper shelf facies comprises peloidal wacke-/ giainstones, crinoidal wacke-/grainstones and marls rich in siliceous sponge spicules, bryozoan fragments, brachiopods and trilobites. The cherts are also interpreted as relatively deep water shelf accumulations. Mapping of marker-beds and determination of microfacies distributions has yielded evidence for the repetition of several lithological horizons throughout the area. Conodont biostratigraphy has recently shown (B.Fordham, this volume) that the Chillagoe Formation is cut by numerous faults running parallel to bedding; these are responsible for extensive tectonic brecciation. This faulting is now interpreted as a major thrust system repeating slivers of various thicknesses and age ranges. Thrusting has been previously recorded within the Hodgkinson Basin. Earlier work in the Mitchell and Palmer River areas (Fawckner, 1981; Hammond, 1986) reported evidence for major thrust faults, which separate the eastern margin of the Chillagoe Formation from the Hodgkinson Formation, as well as the Precambrian basement from the Palaeozoic cover (Shaw et al., 1987). Since lithologies do not change rapidly and only less than 1000 m of limestone and chert cover an agerange of approx. 40 million years, sedimentation from the Early Silurian to the Early Devonian proceeded extremely slowly and must have taken place under fairly stable conditions. During the Emsian, uplift in the western hinterland resulted in the supply of coarse grained siliciclastic material, which was transported onto the carbonate platform along with limestone clasts eroded from uplifted areas of the shelf. These clasts range in age from Early Silurian to Early Devonian. The input of terrigenous material marks the onset of the deposition of the Hodgkinson Formation, which is interpreted as a relatively deep basinal turbiditic facies several kilometers thick (Bultitude et al., 1987). References

Bultitude, R.J., Green, P.M. & Domagala, J., 1987. Qld.GvtMining J. 88: 187-191. Fawckner, J.F., 1981. Unpubl. PhD thesis, James Cook University. Green, P.M., Domagala, J. & Bultitude, R.J., 1988. 9th Aust. Geol. Com. Abs. 21: 162-163. Hammond, R., 1986. Unpubl. PhD thesis, James Cook University. Shaw, R.D., Fawckner, J.F. & Bultitude, R.J., 1987. Aust. J. Earth Sci. 34: 69-94. Wallace, M.W., 1987. JSed. Petrol. 57: 695-700.

A3 Keynote Address: NON-TROPICAL SHELF CARBONATES AND THEIR RECOGNITION IN THE GEOLOGICAL RECORD: A NEW ZEALAND PERSPECTIVE Campbell S. Nelson Department of Earth Sciences, University ofWaikato, Hamilton, New Zealand

Geologists have traditionally regarded shallowmarine carbonate formation as a low-latitude (sub)tropical phenomenon. However, evidence now indicates that carbonate deposits can develop and become widespread on shelves at any latitude and that the principal limiting factor is the rate and extent of terrigenous sediment input, not temperature. Consequently the paleoclimatic significance of limestones becomes less straightforward. Coincidentally, however, many of the structural, lithological, faunal, floral, mineralogical, geochemical and diagenetic attributes contributing to the fades of modern shelf carbonates in sub(tropical waters are distinctive from those occurring in temperate and cooler waters. For example, hermatypic coral reefs do not develop beyond about the mean annual surface-water isotherm of 20°C, near 30° latitude, and their associated rimmed shelves or platforms support a wide variety of in situ and grain- through mud-supported deposits, usually aragonite-dominated, of chlorozoan and/or chloralgal type, including common non-skeletal carbonate components such as ooids and aggregates, in a largely constructive diagenetic regime fostering grain preservation and chemical precipitation. In contrast, non-tropical carbonates are associated mainly with more

open shelves or ramps accumulating typically (lowand/or high-Mg) calcite-dominated skeletal grains tones and packstones of bryomol or foramol type, under the influence of potentially more destructive diagenetic influences, including metastable grain dissolution, maceration and biodegradation. Despite the potential for post-depositional modification of the above primary fades during passage into the rock record, much of the diagnostic evidence survives, either directly or indirectly. The broad bipartite division of shelf carbonate deposits appears to have existed throughout the Phanerozoic, although many limestones with non-tropical affinities probably remain to be interpreted as such. Presently they are mainly reported from sequences of Ordovician, Permian and mid-late Cenozoic age, all periods of known major continental glaciation and accentuated latitudinal climatic gradients. Diagnostic features for the recognition of ancient non-tropical carbonate deposits are evaluated with reference to the Cenozoic limestones of New Zealand (Table 1). Aspects of the economic geology of non-tropical limestones are also briefly considered, particularly in relation to their potential for porosity development and fluid migration, and the occurrence of associated non-carbonate authigenic mineral deposits.

Table 1 — Comparison of properties typical of the traditional tropical shelf carbonate model (A) with those for nontropical limestones as exemplified by both the modem (B) and Cenozoic (C) carbonate deposits of New Zealand.

B Environmental or facies parameter

Warm-water shelf carbonate model

New Zealand modern shelf carbonates

New Zealand Cenozoic shelf limestones

Latitude

between 30°N and 30°S

between 35°S and 49°S

between 35°S and 60°S

Climate zone

subtropical-tropical

cool-warm termperate

cool-warm temperate

Mean annual water temperature

above 23°C

13-19°C

below 20°C

Minimum water temperature

about 14°C

9-12°C

about 5°C

Water salinity

normal to hypersaline

normal marine

Level of CaC03 saturation ?supersaturated

saturated-supersaturated

undersaturated-saturated

undersaturated-

open, strong storm swell

generally open,

Water circulation common storm

Tectonic regime Shelf gradient

restricted-open

stable less than 0.5 m/km

and tidal influence

and/or tidal deposits

stable-unstable

0.25-2 m/km

more than 0.5 m/km

Environmental or facies parameter

Warm-water shelf carbonate model

B New Zealand modern shelf carbonates

New Zealand Cenozoic shelf limestones

Reef structures

common (mainly coralgal)

absent (local oyster banks)

rare (mainly oyster)

Sedimentation rate

10-100 cm/100 y

low but variable, commonly palimpsest and/or relict deposits

less than 5 cm/100 y, many diastems

CaC0 3 content

over 90%

50-100%

Terrigenous grains

rare

rare-abundant

Glauconite

rare

commonly present, particularly in skeletal chambers

common, pelletal and in skeletal chamber

Evaporite minerals (including dolomite)

common

absent

absent (locally rare dolomite)

Non-skeletal carbonate grains

common-abundant

absent

Major skeletal grain types

calcareous green algae corals (ahermatypic) benthic foraminifers molluscs calcarous red algae

bryozoans

bryozoans molluscs foraminifers, mainly benthic, barnacles echinoderms calcareous red algae brachiopods

Skeletal grain assocxiations

chlorozoan chlor algal

bryomol foramol

Algal mats

common

absent, or not preserved

absent absent-locally common as matrix

molluscs, chiefly bivalves foraminifers, mainly benthic barnacles calcareous red algae serpulids corals (ahermatypic) echinoderms brachiopods

bryomol foramol

Carbonate mud

common- abundant

absent-rare, flushed and by-passed

Main origin of carbonate mud

floral disaggregation and inorganic precipitation

physical abrasion, bioerosiion skeletal abrasion and and maceration of skeletons bioerosion

Primary sediment mineralogy

aragonite > Mg-calcite

Environment of alteration of metastable carbonate grains

subaerial

Environment of major lithification

submarine and subaerial

Timing of cementation

Mineralogy of major cement Major sources of cement dissolution

> aragonite

possibly submarine

probably submarine

unlithified

subsurface burial

some early, mainly late diagenetic

early diagenetic

calcite, often ferroan

aragonite or Mg-calcite

inter granular

sea water and dissolution

of calcitic skeletons

of aragonite grains Carbonate petrography

mud-, wacke-, pack-, grain- and boundstones

grains tones

grain- and packstones

A3.15 THE BRYOZOA MEMBRANIPORA ACICULATA AS AN INDICATOR OF ENVIRONMENTAL CHANGES IN THE COORONG LAGOON DURING THE LAST 700 YEARS Yvonne Bone Department of Geology & Geophysics, University of Adelaide

The eastern margin of the Coorong lagoon is the site of small but laterally extensive carbonate buildups. These ubiquitous buildups are composed of intergrown Bryozoa and serpulids. The anascan, cheilostome Bryozoa is Membranipora aciculata (MacGillivray, 1891), and it is the dominant buildup former, and the only Bryozoa present This is the first report of Bryozoa buildups within the Coorong, and of Af. aciculata in South Australia. The buildups consist of sheet-like and circular mounds up to 400 mm in diameter and 300 mm in height. Their lateral gradation into calcretized areas and their laminated appearance probably explains why they have not been recognised previously, as their appearance does prompt the assumption that they too are calcrete. X-ray diffraction analyses of M. aciculata shows that it is a high-Mg calcite bryozoan, analogous to similar Bryozoa, including other Membranipora sp., currently living on the adjacent Lacepede Shelf. Dating by C indicates that the main growth phase of the buildups was a Late Holocene phenomenon, which took place approximately 700 years BP. This coincides with the Medieval Warm Epoch. However, when considering temperature in isolation from other environmental factors, stable isotope analyses do not support a higher temperature. But, if all environmental factors are 14

considered concurrently, then it appears that this most active buildup phase was a time of warmer temperature, lower salinity and high water level than those pertaining today in the same region. The limited assemblage of only two species forming the buildups suggests that the Coorong was already an inhospitable environment even at the time of colonisation. The atypical bryozoan monospecificity suggests an element of chance was involved in this colonisation and that the Coorong lagoon was already an inhospitable environment. As environmental factors changed, partially due to the influence of Europeans, there was a decline in productivity and subsequent reduction in the size of the active buildups, along with a reversal of the dominant organisms. Today, only sparsely distributed and exceedingly small colonies of Af. aciculata survive in this stressed, marginal marine setting. Thus, this study of the Coorong indicates that the rate of natural environmental change can be accelerated by man over a geologically meaningful time, and emphasises the fragility of its maintenance. Indeed, given the present interest in climatic change and the alarmist views on global warming, this study provides documented evidence for a warmer climate 700 years BP.

A3.16 DIFFERENCES BETWEEN SUBTROPICAL (ORDOVICIAN), TEMPERATE (RECENT AND PLEISTOCENE) AND SUBPOLAR (PERMIAN) CARBONATES, TASMANIA, AUSTRALIA C. Prasada Rao Department of Geology, University of Tasmania

Recent carbonates are forming in tropical, temperate and polar settings. Tasmanian carbonates are now forming in a cool temperate region. Extensive subtropical (10°N) and subpolar (80°S) carbonates formed in Tasmania during Ordovician and Permian times respectively. Subtropical Ordovician carbonates are similar to modern warm-water ones and contain Chlorozoan Biota, diverse non-skeletal grains, abundant micrite and dolomite, some sparry calcite and evaporites. Covariance between Mn and Sr (also Na) indicates stabilisation of aragonite to calcite in a semi-closed diagenetic system. The Sr/Na ratios are around >3 as in modern warmwater carbonates. A small 8 0 difference (2%o) between marine calcite and meteoric calcite indicates pronounced low latitude meteoric diagenesis. 18

Modern and Pleistocene temperate carbonates are mainly composed of bryomol fauna with marine calcite cements. Sr, Na and Sr/Na ratios (-1) indicate calcitic mineralogy of fauna and cement with some aragonite or vaterite. Positive correlation of Mn and Fe with Mg suggests submarine lithification and a slow rate of sedimentation. The 8 0 and 5 C field is distinctly different from that of tropical carbonates and follows the trend of sea-floor diagenesis involving upwelling waters. Permian subpolar carbonates contain abundant glacial erratics and faceted dropstones of widely varying composition. Fauna is less diverse than warm-water Permian counterparts but is abundant in Eurydesma, brachiopods, pelecypods, bryozoa and crinoids. Marine and mixing-zone calcite cements formed in water 18

temperatues <3°C. Covariance between Mn and Sr (also Na) indicates open flow diagenesis. The Sr/N ratios are around 1 due to the calcitic mineralogy of the fauna and the cements. The 8 18 0 and 813Cfieldfalls in the mixing

zone due to extensive melt-water influx and is distinctly different from tropical carbonates (Fig.l). The covariance of Sr/Mn with 813C reveals appreciable water/ rock interaction by melt-waters.

Permian, Tasmania Present

•

Original

Q CL

o n

5I 30

-25

-20

-15

-10

-5

6 0%o PDB Figure 1 — Isotopic differences between tropical (Recent), subtropical (Ordovician) and subpolar (Permian) carbonates

A3.17 COLD-WATER CARBONATE SEDIMENTATION DURING LATE ORDOVICIAN, THE PA KAE FORMATION, SOUTHERN THAILAND: TROPICAL DEEP SETTING VERSUS SHALLOW TEMPERATE ORIGIN? Thanis Wongwanich and C. Prasada Rao Department of Geology, University of Tasmania

The Late Ordovician (Upper Caradoc to Ashgill) Pa Kae Formation (up to 126 m thick) overlies the peri tidal Ordovician Thung Song Group and is comformably overlain by Silurian deep-water black graptolitic shales and chert with radiolarians. This formation was deposited during a period of intense glaciation in higher latitudes of Gondwanaland. It is thinly bedded (10-15 cm), red stromatolitic limestone that alternates with laminations of unfossiliferous silty mudstones. Stromatolites occur as flat laminations at the bottom of bedding to columnar stromatolites at the top of each bed. Oncolites and thrombolites occur in some lower horizons. Major allochems are bioclasts, pellets, lumps and intraclasts with ferromanganese nodules (up to 15 mm), rare quartz silts and glauconites. Fauna comprises of abundant ostracodes, very small (~3 mm) blind (no eyes) and large-eyed pelagic trilobites,crinoids, calcispheres and brachiopods with

other diverse fauna such as straight nautoloids, conodonts, gastropods and other planktonic and pelagic organisms. Microdolomite locally occurs as submicron crystals within micrite and replaces burrows and nonluminescent calcite cement. Hardgrounds occur with bored surfaces encrusted by ferromanganese layers. Early diagenetic microneptunian fissures contain laminated micrite and spar. Cold-water origin is indicated by the mainly original calcitic mineralogy of the fauna and of micrite and sparry calcite, occurrence of glauconite, visually impaired trilobites and absence of oolites. The Sr and Mn concentrations are similar to those of other coldwater carbonates. The heaviest 8 18 0 value (-3.1%oPDB) of marine calcite when compared with low latitude worldwide tropical Late Ordovician shallow marine calcite 8 ls O values (—5%oPDB) indicates 10°C lower temperatures during deposition of Pa Kae Formation.

A deep environment is suggested by ferromanganese nodules, blind and big-eyes trilobites, absence of subaerial features and a temperature drop of 10°C in a tropical setting, which corresponds to depths of 200-300 m depending on the magnitude of coldwater incursion by upwelling. Therefore, stromatolites formed in deep waters (>200 m) by microbial processes. Oxidizing conditions existed as evidenced by red colour, high range of Fe (185-20,295 ppm) and Mn (234-4145 ppm) concentrations and high Fe/Mn ratios (2-8) and lack of pyrite. However, Recent algal stromatolites are restricted to intertidal and shallow subtidal settings and

Warm Shallow Carbonate \

Marine Surface Water

Formation •

^ C

( 1 4.

+•

^ ^ ^

V ' '

^—_ ——

+. . . ^

><T

^ / \l \

Pa K a e

+ 6 -

vO *

u /

f/-

x 0

/ < r

Spar in veins Calcareous mudstone

• +

^ \ \ \ «\ \

Micrite Spar in voids

\ 6130%, PDB

• •

they require light for their growth. In addition, the occurrence of a diverse fauna including shallow marine brachiopods, crinoids and gastropods in a non-turbiditic sequence and absence of deep-water fauna such as radiolarians argue for a shallow mid latitude cold setting into which pelagic trilobites, adapted to cold-waters, migrated along with cold currents. The 813C values (up to +3.4%o PDB) in the marine calcites are much heavier than shallow marine tropical Late Ordovician marine calcite values (~+l%<?PDB) due to cooler temperatures and biogenic fractionation involving photosynthetic fixation in photic zone in surface waters (Fig.l).

ORDOVICIAN Sea Water

'+2 6180%O

PDB

Figure 1 — Isotopic fields of cold and warm shallow marine carbonates and their comparison with Pa Kae Formation. TEC = temperature equilibrium calcite.

A3.18 COOL CLIMATE BAUXITE Graham Taylor1, E.M. Truswell2, R.A. Eggleton1, M.C. Brown3 and K.G. McQueen3 1

Centre for Australian Regolith Studies, Canberra 2 Bureau of Mineral Resources, Canberra 3 University of Canberra, Canberra

The Monaro region of the southeastern highlands of NSW is substantially covered by Palaeocene to Oligocene basalts whichflowedover a landscape similar in character and relief to that in the region now. The pre-basaltic and early basaltic landsurface contained a number of rivers and lakes which have been preserved by the basalt. The sediments deposited in these lakes and rivers are preserved beneath the basalt flows and contain abundant fossil wood, leaves and pollen. Many of the basalt flows have weathered surfaces which have also been preserved by subsequent flows.A basaltic weathering profile from Coal Pit Creek (Gordon, 1989) is typical of those developed throughout the region. It consists of:

0-0.05 m

bright red zone massive and pisolitic hematite, goethite, maghemite, and minor smectite. It is the metamorphosed upper part of the profile. 0.05-0.25 m purplish red zone of pisolitic hematite, maghemite, gibbsite, and goethite. It is the upper bauxitic hardpan, slightly metamorphosed. 0.25-0.4 m dark red massive and homogenous gibbsite, goethite, hematite, quartz, and anatase. This is the transition to the underlying clay zone. 0.4-5.5 m mottled red/yellow clay zone of massive gibbsite, hematite, goethite, and minor anatase

and quartz. This zone contains an increasing number of increasingly fresh basalt corestones downward and the fabric becomes less massive and more basaltic. progressively fresher basalt

very narrow (mean thickness <1 mm) rings suggesting cool to cold climates when compared to extant forests. The mean sensitivities (<3) suggest that the forests responded minimally to climatic variation and the lack of false rings indicate few if any climatic extremes. All 5.5- m the rings have very few late-wood cells, the result of a The typical bauxitic weathering profiles may contain rapid termination to the growing season, probably due up to 50% A1203. They are widespread in the lower to falling temperatures.A large and diverse leaf flora parts of the basalt section across the whole Monaro but collected from one pre-basaltic Palaeocene deposit are most common in the central areas of the province. suggests cool to cold and humid climates in the region Palaeomagnetic data indicate the profiles developed at the time (R. Hill, pers.comm.).Bird & Chivas (1988) have sampled kaolinite from sub-basaltic weathering over 1 to 5 Ma (R. Musgrave pers. comm.). The palynomorphs collected from the sub- and inter- profiles in the region and examined their stable isotope basaltic sediments are representative of the geochemistry which also indicates cold climates during Lygistepollenites balmei Zone and include Podocarpus, the Palaeocene.The juxtaposition of the fossil bearing Phyllocladus, Dacrycarpus, Araucaria, some deposits and the weathering profiles clearly suggest that Proteaceae, Nothofagus, and Casuarinaceae. These substantive weathering profiles of a bauxitic nature can suggest that the climate was uniformly wet develop in climates other than the 'tropical* or (1500-2400 mm mean annual rainfall) with mean 'monsoonaT conditions geomythology would have us annual temperatures between 10° and 20°C and a lower believe is necessary for their formation. The critical limit of 0°C. It is likely that temperature extremes were factor in their production under the described conditions outside this range but would generally have been are abundant water, tectonic stability and time. mitigated by the high rainfall regime under which the References Bird M J. & Chivers A.R., 1988. Chemical Geology plants grew. (Isotope Geoscience Section) 72: 249-165. The fossil wood is dominated by gymnosperms with u 1989. Unpubl. report for Special Studies in both araucarians and podocarps having been identified Gordon Science, Canberra CAE. with a few angiosperms. All the wood has well defined rings indicating seasonal climates. All the trees had

A3.19 DEPOSITIONAL ENVIRONMENTS AND COAL FACIES IN THE TOMAGO AND NEWCASTLE COAL MEASURES NSW MJ. Roach Key Centre for Ore Deposit and Exploration Studies, University of Tasmania

The Tomago and Newcastle Coal Measures provide an oportunity to examine the relationship between coal facies and depositional environments in a sequence comprising marine, deltaic, fluvial and alluvial clastic sediments and associated coal seams. Three deep fully cored boreholes from the western portion of the Newcastle Coalfield were used in this study. Together they form a composite section of 900 metres from within the Wallis Creek Formation at the base of the Tomago Coal Measures through to the Triassic Narrabeen Group. Clastic depositional environments were interpreted from detailed vertical profile logging. Peat forming environments were determined from standard maceral analysis (AS 28561986) using the system of Diessel (1986). Coal seams from the Wallis Creek Formation occur in close association with coasening upwards grainsize profiles, interpreted as prograding delta lobes. The intense bioturbation associated with these sequences supports the interpretation that these coal seams were formed in a lower delta plain depositional setting. Coal seams from this interval contain more than 80% vitrinite,

most of which is desmocollinite, and plot on the coal facies diagram of Diessel,(1986) with low Tissue Preservation Index (TPI) values and high Gelification Indices (GI). Coal seams from the Four Mile Creek and Dempsey Formations and the Lambton Sub-Group of the Newcastle Coal Measures are often associated with fining upwards and alternating grainsize profiles in epiclastic sediments which suggests a fluvial mode of deposition. An upper delta plain or alluvial depositional setting is interpreted for the coal bearing portions of this interval. Seams contain intermediate proportions of vitrinite and inertinite with the vitrinite fairly evenly divided between telovitrinite and detrovitrinite macerals. The higher proportion of inertinite than in the Wallis Creek Formation indicates that drier, more oxidising conditions prevailed while the proportion of structured macerals, telinite, telocollinite and semifusinite indicate that a significant proportion of the peat was composed of woody plant material. These factors are reflected in the coal facies diagram as intermediate values of TPI and GI.

Moon Island Beach Sub-Group coal seams were deposited in close association with coarse grained clastic sheets, deposited by high energy braided streams. An alluvial plain depositional setting is inferred for these sediments and this view is supported by the coal petrographic composition with high proportions of inertinite macerals and correspondingly low proportions of vitrinite. Inertodetrinite is a major component of the total inertinite content, this is due to insitu degradation of fusinite and semifusinite under highly oxidising conditions. Wallarah seam deposition was the last major period of peat formation in the Newcastle Coal Measures. It clearly represents peat deposited under harsh conditions of oxidation with a total inertinite content of approximately 90% and thick accumulations of inertodetrinite. Low TPI and GI values reflect this conclusion. The overall regressive sequence from the base of the Tomago Coal Measures to the top of the Newcastle

Coal Measures, as interpreted from the clastic sedimentation patterns, is reflected by the change in coal petrographic composition. The transition from reducing conditions, during the deposition of lower delta plain coal seams at the base of the Tomago Coal Measures, to highly oxidising alluvial plain conditions at the top of the Newcastle Coal Measures is accompanied by a decrease in vitrinite content from 80% to 10% and a corresponding increase in inertinite. In many ways coal compositional changes are more sensitive indicators of changes in depositional environments than the accompanying changes in clastic sediments and deposition patterns. The coal facies diagram of Diessel (1986) serves as a simple but effective means to discriminate peat forming environments from standard maceral analysis data. Reference Diessel, C.F.K., 1986. Adv. Study Syd. Basin, 20th Symp.

A3.20 DIAGENETIC EVOLUTION OF THE HYDROCARBON BEARING PERMIAN SEDIMENTARY ROCKS OF THE DENISON TROUGH, BOWEN BASIN, QUEENSLAND: EVIDENCE FROM THE BOREHOLE GSQ EDDYSTONE 5 R. Ahmad*, J. C. Tipper, R. A. Eggleton, and J. L. Walshe Basin Research Group, Department of Geology, The Australian National University

The Borehole GSQ Eddystone 5 is located at Lat. 25°00'S andLong. 148°29'E within the Denison Trough, Bowen Basin, Queensland. It has encountered a 1000 m Permian sedimentary sequence of eight formations. These sedimentary formations are, in ascending order, the Reids Dome Beds (RDB), Catde Creek Formation (CC), Aldebaran Sandstone (ASS), Freitag Formation (FF), Ingelera Formation (IF), Peawaddy Formation (PF), Black Alley Shale (BAS), and the Bandana Formation (BF). The general lithologies of these formations are sandstone, siltsone, mudstone, shale, and minor amounts of limestone all of which have undergone intense diagenetic alteration, and several of them contain oil and/or gas. In order to reconstruct the diagenetic evolutionary history of the sediments of the above formations, samples collected from them were analyzed by different methods, such as: XRD for bulk mineralogy and clay mineralogy, texture and structure of the sandstone and siltsone; and SEM for diagenetic mineralogy and texture. XRD analysis of bulk sediments as well as the < 2 iim fraction shows the presence of quartz, Naplagioclase, K-feldspars, micas, calcite, dolomite, ankerite, siderite, dawsonite, gypsum, pyrite, analcite, chlorite, kaolinite, illite (I), smectite (S), and I/S interstratified clays. Based on the results of thin section petrography and extensive SEM investigations, a number of diagenetic events and their approximate paragenetic sequence (relative timing of occurence) were recognized (Fig.l). Generally, overgrowth quartz (OQ),

recrystallised chert, calcite, dolomite, ankerite, siderite, dawsonite, pyrite, chlorite, kaolinite, illite, smectite, and I/S interstratified clays are the diagenetic minerals. Smectite and/or I/S clays were formed as result of dissolution and recrystallization of the detrital feldspars. These clay minerals subsequently underwent further illitization, thereby giving rise to the formation of illite and/or modified I/S clays. The abundance of I/S clays and their contents of percent illite layers vary among the formations. This is higher in the RDB, CC, ASS, and the FF relative to the overlying IF, PF, BAS, and the BF. Formation of OQ is a common diagenetic event in these sediments, and is very pronounced in the ASS and FF respectively. Dissolution of feldspars and detrital chert were probably the main sources of Si-ions for the OQ that was formed prior to the significant reduction of primary porosity. Authigenic calcite occurs mainly as a replacement product of the matrix materials, detrital and overgrowth quartz. This calcite was subsequently replaced, in part, by ankerite and siderite which is quite remarkable in the CC, ASS, FF, and BF. Ca-ions were probably derived from fossil shell materials especially from the carbonate-rich layers in the PF, dissolution of feldspars and/or from other external sources. Dissolution of detrital feldspars created secondary porosity. Formation of OQ and the pore-filling by authigenic carbonates and clay minerals accompanied by compaction caused a significant reduction in primary porosity in most of the sandstone and siltsone except the ASS that still retains in relatively higher amounts.

The higher abundance of I/S clays and their higher contents of illite layers in the RDB, CC, ASS, FF relative to the overlying IF, PF, BAS, and the BF along with other related factors suggest that diagenesis in the former group was largely influenced by seawater flushing, possibly due to the Early Permian and Midlate Permian transgressions. On the other hand, the Compaction Decompaction Dissolution of feldspars and recrystallization to smectite, I/S and kaolinite Illitization of smectite and I/S Recrystallization of chert Formation of overgrowth quartz Formation of authigenic chlorite Replacement of matrix, quartz and feldspars by calcite Pore-filling by dawsonite Replacement of calcite by ankerite, siderite and dolomite Formation of pyrite

abundance of kaolinitite, smectite and Fe-rich carbonates, and the absence or very low presence of illite and/or I/S clays in the latter group probably suggest that the diagenesis in them was influenced predominently by meteoric water flushing during post Mid-late Permian time.

Early

Late ?

Figurel — Parqagenetic sequence of diagenetic events in the Permian sedimentary sequence encountered in the Borehole GSQ Eddystone 5.

A3.21 POST-DEPOSITIONAL HISTORY OF THE PERMIAN SEQUENCE FROM THE NORTHWESTERN DENISON TROUGH, QUEENSLAND P. de Caritat*, J.L. Walshe, R.A. Eggleton and J.C. Tipper Department of Geology, Australian National University

The post-depositional history of the Permian different recognised stages. The scale of diagenetic sedimentary rocks from the Denison Trough has been homogeneity appears to be very small (from a few m to investigated by means of petrography, SEM, XRD and a few cm). Therefore, the detailed diagenetic paragenesis microprobe analysis. Approximately 100 samples from of each sample tends to be unique. Generally, however, two fully-cored boreholes, GSQ Springsure 18 and 19, either of the two following simplified diagenetic paths were collected from sandstone (Reids Dome Beds, is found: Aldebaran Sandstone, Freitag Formation, Catherine (1) early and minor quartz overgrowth, followed by early poikilotopic calcite cementation and Sandstone), mudstone (Reids Dome Beds, Cattle Creek replacement which effectively isolated the sands Formation, Ingelara Formation, Peawaddy Formation, from further diagenetic evolution, or Black Alley Shale, Bandanna Formation) and (2) early and minor clay, carbonate, or sulfide carbonate-rich (Mantuan Formation) intervals. precipitation, followed by early extensive quartz The sandstones are all sublitharenites (average overgrowth development (± chert cementation). composition Q73F8R19), and were deposited in fluvial Then, Fe-Mg-carbonate (siderite/ ankerite) cementto nearshore marine environments. Their diagenetic ation and replacement took place, subsequently history is typically complex, comprising at least 17

followed by partial dissolution of carbonate cement and of detrital feldspars (s.l.), extensive kaolinite pore fill and, locally, some calcite cement precipitation. This second path is far more common than the first one for the sandstones of the Denison Trough. The two underlined stages are the most widespread in the samples studied. Sandstones that followed path (2) ("sands 2") were much more affected by physical (and chemical) compaction than those that followed path (1). Diagrams of %minus-cement porosity versus %cement indicate that porosity reduction in "sands 2" occured mostly as a result of mechanical compaction rather than chemical cementation. Carbonate dissolution and kaolinite precipitation can be related either to the infiltration of relatively acidic meteoric waters at unconformities (Early Jurassic), or to the generation of organic acids in adjacent maturing shales. Several types of detrital ferromagnesian grains (biotite, amphibole, pyroxene) have locally undergone chloritisation during diagenesis, and the chemical 9

composition of the diagenetic chlorite has been used to compute the temperature of formation of that mineral. A sharp peak in the temperature frequency distribution at about 100-110°C suggests that chloritisation took place at maximum burial depth (3-4 km), i.e. during the Middle to Late Triassic. Semi-quantitative XRD analysis of fine-grained samples shows the following trends with depth: (1) an increase in illite (30-80%) in the interstratified illite/ smectite (I/S) clays; (2) a shift from random to "allevardite-type" ordering in the I/S; and (3) a decrease in smectite proportion. Organic matter contained in the shales is of type III, indicating terrigenous origin and primordial gas generation potential. The reflectance of the vitrinite in the samples analysed is low (R = 0.6 %). Burial history and thermal maturation modelling, using those constraints, imply that the geothermal gradient must have been low during most of the basin history (post-maximum burial) at the locations of GSQ Springsure 18 and 19. o

A4: Gondwana Geophysics

Convenors: F.E.M. Lilley and D.E. Leaman

A4 Keynote Address: LATE PLEISTOCENE AND HOLOCENE SEA-LEVEL CHANGE IN AUSTRALIA: CAUSES AND SOME CONSEQUENCES Kurt Lambeck Research School of Earth Sciences, Australian National University

Evidence for sea-level change in Late Pleistocene and Holocene time abounds around Australia's margin but the patterns of change are not the same everywhere. Spatial variability has been observed for which there has not yet been a wholly satisfactory quantitative explanation. How much, if any, is indicative of vertical tectonics? How much is the result of the Earth's response to the changing surface loads as mass is exchanged between ice sheets and the oceans? A comprehensive model of the temporary and spatial variability of sealevel is important for understanding a variety of geophysical, geological, geomorphological and biological processes. To understand what is happening to sea-level today and to provide a basis for understanding what may happen tomorrow, it is desirable to have a model of past change that serves as a test for various hypotheses. The understanding of how the Earth responds to the changing surface load provides an example of isostasy at work for which the rates of the process can be observed. Hence it becomes possible to estimate the Earth's rheological parameters that in turn assist in understanding other solid-Earth processes such as mantle convection. Also, for geophysics and geology, sea-level is the reference relative to which vertical tectonics are measured and it becomes important to be able to separate tectonics from sea-level change. For glaciology the Late Pleistocene and Holocene sea-levels provide constraints on models of the disintegration of the last great ice sheets; of the volume of meltwater and of the timing of melting. Do the sealevel observations constrain models of Antarctic deglaciation? For geomorphology, the knowledge of sea-level change is important for understanding coastal response to changes in this level. Are regional variations in the Late Holocene highstands indicative of a modification or redistribution of the intensities of coastal processes through time. The pioneering work on Late Holocene sea-levels in Australia was by R.W. Fairbridge who identified highstands at a few metres above present level along the Western Australian coastline. Agreement on

Fairbridge's sea-level curve was not universal, even within the Australian region. Southern and southeastern Australia seemed to be devoid of such highstands whereas northern Queensland did contain such evidence. Thanks to careful work by a number of geomorphologists in the past decade, a distinct spatial pattern of Late Holocene sea-level change is emerging and the reality of the existence of highstands, with regionally variable highstands, is now widely accepted. Less agreement occurs on the reason for this variability. The Earth's response to changes in surface loads plays a central role in understanding sea-level change. Because of the irregularly shaped coastlines and the existence of shallow coastal shelves, the loading of the crust by the meltwater released in Late Pleistocene time is not uniform and the Earth's response becomes variable. In addition, the sea-level change is influenced also by the unloading of the crust beneath the disintegrating ice sheets. What is required is a global formulation of the sea-level change in terms of these redistributions of surface loads and Earth response. Such a model has been developed and tested for several regions of the world, particularly in the Australian region and in northwestern Europe. Conclusions that can be drawn from matching the model results with observations of sea-level change include the following: 1. There has been a significant volume (~10 km ) of meltwater, which originated from Antarctica, added into the oceans between 18,000 years and 6000 years ago; 2. This meltwater was added into the oceans at about the same rate as meltwater from the Arctic ice sheets; 3. Sea-level at the time of the last glacial maximum is not everywhere at the same depth below the present sea-level because of the Earth's adjustment regional variations of more than 30 m can occur as in the Australian region (Fig. 1); 4. The post-glacial marine marine transgression will not be the same everywhere. In particular, the sealevel rise at offshore sites will appear to lag behind coastal sites by as much as 1000 years (Fig. 2). 7

8. Antarctic melting did not cease at 6000 years ago but continued at a reduced rate up to recent times (Fig. 5). 9. The viscosity of the upper mantle beneath the Australian margin and offshore region is about 2 X 1 0 Pas and that of the lower mantle is about 1 0 Pas. 10. The effective lithospheric thickness for the same region is about 50-80 km. 11. There is no need to invoke recent (past 6000 years) vertical tectonic movements along the Australian coast with the possible exception for the Perth region.

5. Holocene highstands will generally develop along the Australian coastline but amplitudes will vary significantly according to coastline geometry (Fig. 3). 6. The timing at which sea-level first reached the present value also exhibits significant variability, occurring earlier upstream in the extensive tidal flats of the Ord River, for example, than near the coast 7. Holocene highstands do not develop at islands and reefs that lie well offshore. This aspect of the model, together with (4), is significant for understanding reef growth rates (Fig. 4).

2 2

North Queensland, Latitude 23.5 S

North Queensland, Coral Sea coast

-110

8 © cb

Shoalwater B Towns vi lie

J2 0

Dunk-Goold Is. -o—

Yule Point "| King-Flinders Is.

155

152 153 longitude

time (x1000 years) BP

-6 -4 -2 time (x1000 years) BP

Figure 2. Predicted sea-levei during the late post glacial marine transgression.

Figure 3a. Predicted Late Holocene sea-levels along the North Queensland Coast.

-10

Figure 1. Predicted sea-level at 18000 years BP along latitude 18 S, North Queensland shelf. Spencer Gulf, SA.

-8

-6

North Queensland 1 1 -

8 J2

C. Spencer o

Adelaide

IIS

-4

Kingscote -6 -4 -2 time (x1000) years BP

Figure 3b. Predictions of Late Holocene sea-level in South Australia

-8

•

B—-

50 km o—- 8 0 k m

#—. • •

140 km

180 km ' an i — i J -6 -4 -2 0 time (x 1000 years) BP

Figure 4. Predicted Late Holocene sea-level as a function of distance from present shoreline

-100

-20

-15 -10 -5 time (x1000 years) BP

Figure 5. Equivalent sea-level (Volume of melt water/ocean surface area) for late Pleistocene and Holocene melting.

A4.1 VARIATIONS IN RIFTING STYLE AROUND AUSTRALIA DURING GONDWANA BREAKUP: THE STRUCTURAL AND SEDIMENTARY RECORD OF RIFTED CONTINENTAL MARGINS J. Braun * and C. Beaumont 1

Research School of Earth Sciences, Australian National University Department of Oceanography, Dalhousie University, Halifax, Canada 1

We suggest that the geometrical and temporal evolution of continental rift zones and, ultimately, the morphology of passive continental margins are controlled by the distribution and nature of weaknesses in the continental lithosphere inherited from previous tectonic activity. The thermal state of the lithosphere prior to and during continental rifting as well as rates of sedimentation/erosion also affect the structural character and stratigraphy of the rifted continental margin. Results

from a 2-Dfiniteelement thermo-mechanical model of the continental lithosphere help us to assess the relative importance of each of these processes. As Australia separated from the rest of Gondwana, a large number of continental margins formed in a variety of styles. The available data on the surface morphology, sediment stratigraphy and basement structure of several Australian margins are used to illustrate the above hypothesis.

A4.2 SUBSIDENCE OF THE EASTERN AUSTRALIAN PLATFORM DURING THE MESOZOIC K. Gallagher Department of Geology, La Trobe University

Widespread subsidence and sedimentation occurred during the Late Palaeozoic-Mesozoic across the eastern Australian platform. The subsidence history of this region, which includes the Clarence-Moreton, Eromanga and Surat Basins, has been quantified using standard back-stripping techniques on over 40 wells. Some consistent features are revealed and these are examined in terms of relevant basin formation mechanisms. The Eromanga and Surat Basins both show a linear form of subsidence during the Jurassic, although the rate of subsidence in the eastern Eromanga region is about half that which occurred in the Surat Basin. In the ClarenceMoreton Basin the subsidence generally appears to have been more irregular and rapid than in the basins to the west Both the Eromanga and Surat Basins show a marked increase in subsidence rate in the Early Cretaceous for between 10-20 my and then the sedimentary record abruptly ceases. Significant erosion occurred over much of the platform area with as much as 2 km of section being removed from the ClarenceMoreton and Surat Basins in the Late Cretaceous. It is considered that more erosion occurred in the eastern part of the platform. This is supported by the preTertiary subcrop geology which shows younger strata towards the west and it is possible that subsidence continued in the Eromanga Basin while the region to the east was being eroded.

It is suggested that the areally extensive subsidence of the eastern Australian platform is consistent with the model recently proposed by Mitrovica et al. (1989). In this model, the widespread subsidence is related to subduction induced convection below the continental platform. Seaward tilting of the platform over length scales of over 1000 km can occur with the most rapid subsidence occurring adjacent to the margin. When convergence ceases, uplift and erosion occurs as thermal equilibrium is attained. The model is not wholly consistent with the evolution of the Eromanga Basin and here an additional mechanism needs to be invoked. A simple thermal model is favoured, and the rapid increase in subsidence during the Early Cretaceous is attributed to an excess sediment load at the surface, rather than a deeper lithospheric process. The interpretations imply that a variety of different tectonic subsidence mechanisms controlled the evolution of the Mesozoic cratonic sedimentary basins in eastern Australia and the convergent plate margin off the eastern coast of Queensland had a significant influence on the subsidence and sedimentation in this region. Reference

Mitrovica, J.X., Beaumont, C. & Jarvis, J.T., 1989. Tectonics, August 1989, in press.

A43 CRUSTAL STRUCTURE IN CENTRAL AUSTRALIA DETERMINED BY INVERSION OF TRAVEL TIME RESIDUALS — Herbert McQueen see page 104

A4.4 THERMAL HISTORY OF WESTERN AUSTRALIAN GONDWANA BASINS M.F. Middleton Geological Survey of Western Australia

Regional vitrinite reflectance maps have been published by Middleton and Hunt (1989) for Permian horizons in Western Australian sedimentary basins. These maps form a basis of deducing thermal history of the Western Australian (Permian to Quaternary) sedimentary basins by thermal basin modelling techniques. During the presentation of this paper, (i) maps of palaeo-heat flow through time based on the vitrinite reflectance maps, and (ii) the relationship of tectonics and basin formation models to the heat flow history of the western part of the continent, will be presented. The vitrinite reflectance data was converted to heat flow using geohistory analysis of selected petroleum wells. The palaeo-geothermal gradient was modelled from the geohistory and vitrinite reflectance data using

Lopatin and other similar techniques. The heat flow is determined by assuming thermal conductivities within the various basins. The principal findings are that: (i) high regional heat flow occurred on the western margin of the basins during the Late Permian and Triassic; (ii) low regional heat flow occurred in all the basins until the Late JurassicEarly Cretaceous when continental breakup occurred and again caused high regional heat flow; (iii) low heat flow then continued to the Quaternary; and (iv) the Late Tertiary-Quaternary high heat flow predicted by various workers for some basins (e.g. Canning Basin) are not valid on the basis of the present analysis. Reference Middleton, M.F. & J.W. Hunt, 1989. Int. J. Coal Geol, in press.

A4.5 POST-PERMIAN SUBSIDENCE AND TECTONICS, VULCAN SUB-BASIN, NORTHWEST SHELF R. Hillis School of Earth Sciences, Flinders University

Tectonic subsidence plots have been calculated for well, extrapolated well and significant off-well (seismically-based) locations in the Vulcan Sub-basin and adjacent highs (western Timor Sea). The subsidence plots have been normalised to a common datum, the distinctive "near top Permian" marker, and corrected for sediment compaction, palaeobathymetry, eustatic sea-level change and loading effects. An intra-cratonic setting, prior to Gondwana rifting affecting the region, is proposed for the observed rapid Triassic subsidence. This is supported by the remarkably uniform thickness and facies of the early Triassic Mount Goodwin Formation (a distinctive weakly reflective shale overlying the 'near top Permian' marker). In contrast, syn-rift sequences in extensional basins exhibit rapid lateral facies and thickness changes. Intracratonic basin subsidence mechanisms such as deep crustal metamorphism (Middleton, 1980) and/or instrusion (Klein & Hsui, 1987), and/or regional tectonic compression (Lambeck, 1984) are considered appropriate. Recent ODP data show Callovian-Valanginian rifting, as seen in the Vulcan Sub-basin, was a precursor to formation of the present continental margin (ODP Leg 123 Scientific Party, 1989). The fully corrected subsidence plots show relatively little tectonic subsidence during the Callovian-Valanginian rift phase, even in the depocentre of the Swan Graben, where the

Callovian-Valanginian interval reaches its maximum thickness. This is atypical of a normal passive continental margin. Assuming an extensional origin for the margin, this absence of tectonic subsidence is considered to support White and McKenzie's (1989) suggestion that formation of Australia's Western and Northwestern Margins was associated with extensive volcanism — so-called 'wet' rifting. In their model, volcanism occurs where rift zones cut through regions of anomalously hot mantle. Hotspots, some 100-200°C above normal mantle temperatures, will generate extensive volcanism during otherwise passive rifting. The addition to the crust of igneous material, the density of which has been modified by adiabatic decompression, inhibits syn-rift subsidence. The model predicts that extensive volcanic wedges will underlie the continental margin. While the wedges of thick oceanward dipping reflectors described by Hinz (1981) may constitute such volcanic sequences, drilling is required to definitively test the4 wet' rift model. Until such drilling is attempted support can be sought from deep crustal geophysics (seismic refraction and gravity) and penological analysis of volcanics recovered from the shelf. Confirmation that rifting was 4 wet* would have significant implications for the origin of marginal plateaux such as the Scott Plateau. Their relatively thick crust and lack of subsidence may be due to igneous

underplating associated with 'wet' rifting. It is worth noting that it will be difficult to characterise such a heavily intruded region of thinned and faulted continental crust as purely continental or oceanic crustal material (as debated by Stagg & Exon, 1979; Veevers, 1979). The post-Valanginian Cretaceous subsidence of the Vulcan Sub-basin is modelled as typical post-rift thermal subsidence. The predicted exponentially decaying thermal subsidence history for a wet' rift is the same as that for a 'dry' rift. Subsidence plots show major Palaeocene/Eocene subsidence rejuvenation which does not coincide with an obvious phase of fault activity on seismic sections. The synchronous major change in the rate and direction of movement of the Australian plate (as witnessed by magnetic anomalies in the Southern Ocean) may have induced compression and subsidence of Australia's passive northern margin by a mechanism similar to compression-driven intracratonic basin subsidence (Lambeck, 1984). 4

Despite the lack of datable borehole returns over the relevant interval, the initiation of Miocene-Recent subsidence (apparent on the plots), and associated fault activity appears to pre-date the Pliocene collision of the Australian plate with the South East Asian plate along the "Timor edge". In view of the possibility that this phase of tectonism pre-dates collision, it is very tentatively ascribed to the action of slab-pull driven tension on the old passive margin as it approached the subduction zone. References Hinz, K., 1981. Geol. J. E22: 3-28. Klein, G. deV. & Hsui, A.T., 1987. Geology 15: 1094-1098. Lambeck, K., 1984. Aust. J. Earth. Sci. 31: 25-48. Middleton, M.F., 1980. Geophys. J. R. Astron. Soc. 62:1-14. ODP Leg 123 Scientific Party, 1989. Geotimes, March 1989: 16-19. Stagg, H.M.J. & Exon, N.F., 1979 Geol. Soc. Am. Bull. 90, 795-797. Veevers, J.J., 1979. Geol. Soc. Am. Bull. 90: 797-798. White, R. & McKenzie, D., 1989 J. Geophys. Res. 94(B6): 7685-7729 .

A4.6 A UNIFIED MODEL OF INTRACRATONIC BASIN FORMATION Shaohua Zhou Department of Geology and Geophysics, University of Adelaide

This paper consists of two parts. Thefirstpart presents a mathematical model of intracratonic basin formation associated with large scale lithospheric deformation in which the lithosphere is regarded as a strong and relatively rigid plate overlying a much weaker semifluid asthenosphere. By introducing two special stress potential functions (<(> and \|/) for converting stress equilibrium equations to simpler partial differential equations, the general deformation problems of an elastic plate are governed by the following equations: f

3fy

f ^ O - 0 .

material. Solving the three-dimensional deformation problem defined above in the plane wavenumber domain, the solution can be cast analytically in the Fourier transformed form. Based on the solution of <|) and \|/, the deviatoric stresses^ x' y' f xy ,0xz , ayi') and the strains (e , e , e ) are given by N

d a z =

3 ( < l > + V , / )

a x z =

d^ dy' °

y z =

O x y - a x y ^ O ) ^ , R=f ( H M ^ z

f ^ l ^ o

in which |x is the shear modulus of the material. The details of the derivation of the above equations and further discussions will be presented elsewhere where f = pg is the body force in the z direction (submitted to Geophys. J. R. Astro. Soc.) The theory is (positive downward), r is the density change and g is the exact and applicable in principle to all modes of linear gravity acceleration, A is the 3-D Laplace operator, lithospheric deformation associated with sedimentary P is the surface load, P is the reaction force or active basin formation. To accommodate the complicated rheouplift force from the substratum, o is the average logy of the continental lithosphere, the theory has been combined deviatoric stress of the horizontal plane, h is extended to anelastic plate, in which only the viscoelastic the plate's thickness and v is the Poisson's ratio of the (Maxwell) and viscous effects are considered. ^(Vl*=0-Vlz=h) - J(4>lz=0-4>lz=h)=^ z

84 The second part applies the previous model to discuss initiate basin development It is shown that the wellvarious mechanisms of initiating a surface depression known McKenzie's model of basin formation which and their effects on basin development in the interior of entails the extension and the associated thinning of the the continental lithosphere. The mechanisms of lithosphere requires additional uplift stress from the sedimentary basin formation can be classified by the deep mantle. Without the vertical stress it is difficult to origins of the driving forces. There are basically three create a stress concentration area needed to cause a surface depression if the horizontal stresses are only types of forces: 1. Internal density increase represented by f which derivedfromplate boundaries. The interaction between results from a variety of geological processes (such the horizontal tectonic force and the vertical uplift force as phase transition or metamorphism, concentration determines whether there is a surface uplift or of heavier rocks due to inherent inhomogeneity of subsidence, depending on which effect is dominant On the lithosphere, intrusion of dense mantle material the other hand, horizontal tectonic compression is shown and thermal contraction or cooling of the crust or to be capable of creating a topographic low at the surface for possible basin development by uplifting the upper mantle); 2. Horizontal tectonic extension or compresssion surrounding areas, although it has been claimed on the represented by a which is generally derived from basis of the conventional thin plate theory that tectonic plate subduction or collision at the boundaries or it compression could not bend or buckle the crust to form a surface depression with geosyncline dimension. can be generated by topographic variation; 3. Surface sediment loading represented by P which is Surface sediment loading contributes substantially to often limited by the available budgets of source the basin subsidence, but its effect on the isostatic movements is restricted by the lithosphere rheology. sediments. It is one of the advantages of the theory presented here The idea of Airy isostasy or extended regional isostasy including the effect of the whole lithosphere flexure that it deals with not only the isostatic movements due to surface sediment loading but also the dynamic effects tends to give only a marginal approximation of the of various driving forces on basin development, and actual isostatic adjustments due to surface load. An that all possible mechanisms are incorporated into a accurate method has been developed in this paper, which unified theory which provides a basis of comprehensive can calculate the exact effect of any three-dimensional investigations of any intracratonic basin formation. sediment load overlying the surface of any layered Based on the unified theory, a number of numerical lithospheric structure. This paper has, however, only provided a framework experiments using simple models has been conducted to investigate the contributions of the driving forces to of the study of sedimentary basin formation. It should basin subsidence. To summarize the model experiments, be mentioned that all these results are only applicable to the internal force created by density increase plays a the continental lithosphere with a linear rheology, but dominant role in basin development, while thermal further developments and extensions are possible and contraction or cooling of previously heated rock bodies various complicated models of basin formation can be in the lithosphere produces a significant impact on basin built on the basis of the simple models presented in this subsidence. Horizontal tectonic extension and paper. compression are viable and important mechanisms to 0

A4.7 TRANSFORMATIONAL PLASTICITY AND ITS POSSIBLE GEOPHYSICAL SIGNIFICANCE A.C. McLaren Research School of Earth Sciences, Australian National University

In the materials science literature, three (and, perhaps, four) apparently distinct types of inter-relationships between phase transformations and plastic flow can be identified. These are: 1. A plasticity due to a phase transformation which is stress assisted; 2. A phase transformation causes a significant reduction in grain-size leading to superplastic behaviour. 3. A small external stress applied during thermal cycling about a phase transformation produces a large deformation, although such a stress is too small to induce any plastic strain in either of the

phases at temperatures near the transformation temperature T . The strain produced when the specimen is cycled about T is considerably greater than that ascribed to any normal deformation processes, often reaching several hundred percent prior to fracture. It is important to note that this is a transient phenomenon. The deformation ceases once the phase boundary stops moving. However, in the first reported experiment of mechanical weakening associated with a phase transformation (Sauveur, 1924), a polycrystalline rod of iron was placed in an apparently stable temperature c

gradient which spanned the transformation temperature. When subjected to a torque or a tensile stress, the rod deformed in a relatively small region where the temperature was near T . It may be that the phase boundary was oscillating in this region, for reasons which are not obvious. If the temperature gradient was definitely stable, then this form of transformation plasticity must be differentfromthat observed when the specimen is thermally cycled around T . Transformation plasticity of the first type had not be§n considered as an important geophysical process. However, several interesting speculations have been made about the possible role of the other types of transformation plasticity in producing zones of local weakness in the vicinity of phase transformations in the mantle and leading to possible decoupling of mantle c

layers both in the vicinity of subducting lithosphere (Ruff & Kanamori, 1983) and on a mantle-wide scale (Sammis and Dein, 1974; Vaughan & Coe, 1981; Parmentier, 1981). However, before the role of the several types of transformation plasticity in mantle dynamics can be further explored, much more information is needed regarding the conditions under which these modes of deformation occur, as well as a much better understanding of the underlying mechanisms, particularly in Earth materials and good analogues. Recent deformation experiments and transmission electron microscope observations suggest that the third type of transformational plasticity may be directly linked to the nucleation of mobile dislocations by the stresses associated with a moving phase boundary.

A4.8 THE TAMAR CONDUCTIVITY ANOMALY W.D. Parkinson* and R. Hermanto University of Tasmania, Hobart

The Tamar Lineament follows an approximately that it is the location of a conducting body. Superimposed on the effect of such a conductor is straight line from near the mouth of the Tamar River to near the Tasman Peninsula. The pre-Carboniferous the effect of the oceans suirounding Tasmania. At long geology is very different on the two sides of this line. periods, of the order of 100 minutes or more, the effect On the northeast side Devonian sediments are of deep- of the oceans dominates, and all induction arrows point water fades, typified by the Mathinna Beds. On the towards the SSE. As the period decreases induction southwest side Devonian and older sediments are of arrows at sites to the northeast of the Tamar Lineament shallow-water origin, such as the Owen conglomerates turn to a WSW direction, and those at sites to the southand Gordon limestone. The granites in the northeast west of the lineament turn to an ENE direction, both side are slightly older than those of the southwest side. pointing towards the Lineament for periods of less than There are also geophysical differences. The magnetic 30 minutes. The effect of the ocean is more marked near anomaly structure is smoother in the northeast and the south and east coasts, where the effect is earthquakes are less frequent. This paper reports a superimposed on the effect of the conductivity anomaly. striking conductivity anomaly coinciding with the For example, at Fingal, about 30 km from the east coast, the induction arrow points almost south, even for Tamar Lineament Magnetic fluctuations within a certain period band short periods. A striking feature of the anomaly is that it extends are usually found to be polarised so that the vectors representing them lie in a plane. The plane is often a great distance to the west The influence is clearly horizontal, but if it is inclined at an appreciable angle visible at Bionte Park, in the centre of the island. It this indicates a gradient of conductivity underground, appears to be wider in the north, where its western limb the plane tilting upwards towards the better conductor. may curve to the west, passing under Deloraine and This can be indicated on a map by an "induction arrow" Mole Creek. However, because of the effect of Bass whose direction is the direction of upward tilt, and Strait, this is difficult to verify. Along most of its length whose length indicates the length of tilt. More detailed it is sufficiently close to a two-dimensional structure information can be obtained if the electric field in the that it can be modelled as such. A new interpretation ground is measured as well as the magneticfield.This method was used in which the single site induction is the principle of the "magneto-telluric" technique, arrows are combined to form a synoptic model of the which amounts to a probing of the conductivity as a effect of the conductor on a given imposedfield.This function of depth. can then be inverted to a model of the electric currents Field measurements were made in Tasmania at more flowing underground. The results indicate a broad than 40 locations with a single EDA fluxgate sheet of current about 40 km wide with a sharp eastern magnetometer. At six of these, electricfieldsalso were edge and a more gradual tapering to the west. This measured so that the magneto-telluric technique was interpretation technique gives little information applied. It was found that induction arrows in eastern about the depth of the conductor, except to indicate Tasmania point towards the Tamar Lineament, indicating that the top is probably less than 5 km deep. The

magneto-telluric observations indicate a depth of about 2.5 km. Some geologists consider that the Tamar Lineament is part of what has been called the "Tasman Line", which consists of rifts and transform faults. It runs from Carine through the Flinders Ranges, then via a transform fault (the "Gambier-Beaconsfield Fracture") to Bass Strait, then southwards as the Tamar Fracture Zone. It is interesting that several conductivity anomalies lie on or near this line.

It is not easy to identify the cause of the high conductivity associated with the Tamar anomaly. The presence of semi-conductors, except graphite, is precluded by the absence of a significant magnetic anomaly coinciding with the conductivity anomaly. The most likely cause seems to be fractured rock saturated with highly conducting fluids. Further magneto-telluric observations should define the conductivity and depth more precisely and so help in identifying the cause of the high conductivity.

A4.9 THE CANNING BASIN INDUCTION ANOMALY F.H. Chamalaun* and J. Whellams Flinders University, Bedford Park, SA.

An array comprising seventeen three-component fluxgate magnetometers was deployed across most of the Canning Basin. The magnetometers recorded the geomagnetic field fluctuation for 3 months from July 1985. Although the geomagnetic field was relatively quiet during the period of recording, sufficient data were obtained to delineate a large induction anomaly and to study the effect of a conductor on aeromagnetic surveys. The fluctuation data were analysed in terms of Fourier amplitude maps and in terms of induction vectors. Both approaches show that a conductor at shallow depth is associated with the Fenton Fault, and follows the Fenton Fault from Broome to the south west. The most likely origin for the conductor appears to be the halite deposits that are associated with the Silurian Carribuddy Formation, and which were mobilized in the Devonian. The electrical currents induced by the external geomagnetic field fluctuations produce time varying magnetic fields of internal origin surrounding the conductor. In an aeromagnetic survey

the internal component is added to the external fluctuation. The normal procedure of using a single base station to correct for external fluctuation in the aeromagnetic data does not correct for the internal component, which is spatially inhomogeneous. Using the array data and interpolating thefieldbetween stations, we simulated an aeromagetic survey with the conductor present Contour maps of the simulated field show that the conductor produces errors as large as 100 nTesla for events and tens of nTesla for the daily variation. The simulation included flying tie lines and these too have large misfits that cannot be removed by the usual procedures. As the precision of aeromagnetics improves and there is a greater need for improved interpretation of aeromagnetic data, errors caused by electrical conduction structures will become more important. To correct for induction errors, arrays of base stations are needed, or alternatively, gradient measurements could be employed.

A4.10 CSAMT IN THE EROMANGA BASIN L.M. Hastie*, C. Cevallos and I.J. Chant Physics Department, University of Queensland

The results of two controlled source audiomagnetotelluric (CSAMT) surveys over the Kenmore oil field in the Eromanga Basin of Central Queensland, using equipment and techniques developed at the University of Queensland (Hastie, 1989; Hastie et al., 1989) will be discussed. This method was able to detect a resistivity contrast of about 20 Qm at about 1400 m depth, and so to delineate the structure of the oil reservoir. The CSAMT results show important differences from the seismic results of this area. While the full value of this information still has to be properly tested and evaluated, it seems that information useful in resolving stratigraphy important to oil exploration can be obtained,

and the method is, in this case, an effective supplement to seismic surveying. This advantage is enhanced by the relatively low cost of this type of survey. Because the method responds to changes in electrical resistivity properties rather than acoustic velocity variations, it should generally be expected to add additional information on the geological structure. In order to achieve the desired accuracy for this type of survey, the interpretation was carried out using a resistivity well log to control the inversion process. The upper kilometre is forced to a set average resistivity based on the well log, thus providing some immunity from the highly variable near surface resistivity

variations. The upper section is relatively uniform and producer was predicted to occur at or near well 11, the structure is comparatively simple, enabling the which was different from the seismically identified high. modelling to achieve good results at the depths of Well log results subsequently showed it to be the highest interest. logged occurrence of the oil producer. The structure In the accompanying contour plot (Fig. 1), the depth suggested by the CSAMT survey extends much further to the resistive layer at the top of the Hutton sandstones to the west than the seismically identified structure and is shown in the black contours. The control was provided is more curtailed to the east. The non-producing wells by well 7, and the mean deviation of the CSAMT at Kenmore North 1 and the nearby field of Black results from the well log depths to the top of the Hutton Stump gave no resistive anomaly at the level of the was 8 m. Wells 8 and 5 show the most discrepency due Hutton sandstones. to their close proximity to the edge of the field, as References defined by CSAMT. Note that wells 10 and 11 were Hastie, L.M., 1989. NERDDC Final Report. drilled after the first and second surveys respectively, Hastie, L.M., Cevallos, C. and Chant, I.J., 1989. Explor. Geophys. 10: 339. and the well log results were predicted by the CSAMT interpretations to within 14 m. The high point of the KENMORE NORTH 1

Figure 1 — The solid contours show the sub-ground level depth in metres to the resistive layer at the top of the Hutton sandstones identified with the presence of oil. The grey contours show the depth to the next resistive layer when the Hutton is not detectable. The edge of the oil field as defined by the oil-water contact would be expected to lie outside the area defined by the dark contours. A spot depth in metres is shown beside the point representing the location of a recording site.

A4.ll ELECTRICAL CONDUCTIVITY STRUCTURE AT THE CONTINENT-OCEAN BOUNDARY OF SOUTHEAST AUSTRALIA R.L. Kellett ' , F.E.M. Lilley * and A. White 1 2

Research School of Earth Sciences, Australian National University now with Zonge Engineering and Research Organisation, Fullarton, SA School of Earth Sciences, Flinders University of South Australia 1

Electromagnetic induction occurs naturally in the Earth on a large scale and is caused by sources such as magnetic storms which have associated strong electric currents in the Earth's upper atmosphere. As a result of this electromagnetic induction process in the Earth, measurements of magnetic storms made at the Earth's surface are influenced, often strongly, by the Earth's electrical conductivity structure nearby: that is, within scale-lengths of hundreds of kilometres of the point of observation. Such surface measurements of magnetic storms are recognized, near the coastlines of many continents, to show a characteristic pattern known as the "Geomagnetic Coast Effect". A longstanding point of discussion has been whether the Geomagnetic Coast Effect is adequately explained by the seawater adjoining a coastline, or whether a contrast in electrical conductivity is also required between the material of the continent and the material of the seafloor. This paper describes a study of the Geomagnetic Coast Effect of southeast Australia, for which an unusually comprehensive data set is held, with observations on the Australian continent, on the continental slope off the coast of NSW, and on the floor of the Tasman Sea at a series of sites reaching across to New Zealand. The data set consists of magnetotelluric and magnetometer sites from the Tasman Project of Seafloor Magnetotelluric Exploration, and from the more recent Continental Slope Experiment. The closeness of the station spacing across the continental margin shows clearly the location of maximum vertical magnetic field over the continental slope, and the dependence of the horizontal magnetic

fields on bathymetry. The magnetotelluric impedances show a strong anisotropy which changes polarity across the coast The observations cover both modes (termed E-polarisation and B-polarisation) of two-dimensional electromagnetic induction. Numerical modelling of two-dimensional conductivity structures shows that the ocean, with known sedimentary basins and a simple one-dimensional crust and mantle structure, produces most of the features seen in the magneticfielddata, but not in the seafloor electric field data. Modelling by an inversion procedure based on systematic search produces, as best-fitting model for all the data, one which has on the continental side of the coastline a typical non-shield continental conductivitydepth profile, with conductivity increases at 200 km and 400 km; while on the oceanic side of the coastline the best-fitting model has the same increases in conductivity, but at depths 100 km and 300 km. This change in the deep structure across the continent-ocean boundary, indicated by the best-fitting model, is consistent with seismological information. The structure is interpreted to be a result of the process of passive or "Atlantic-type margin formation, by which the southeast Australian coastline formed during the opening of the Tasman Sea, between 75 and 53 Ma before present. A depth of 100 km for the base of the oceanic lithosphere corresponds well to the age of rifting some 80 Ma ago, allowing the oceanic lithosphere to cool and thicken for this period of time. The contrast with the continental profile suggests an electrical asthenosphere relatively deeper beneath southeast Australia.

A4.12 THIN SHEET MODELLING AND INTERPRETATION OF TASMAN SEA MAGNETOTELLURIC DATA G.S. Heinson* and F.E.M. Lilley Research School of Earth Sciences, Australian National University

The Tasman Project of Seafloor Magnetotelluric Exploration (TPS ME) took place between December 1983 and April 1984. Seven magnetotelluric and two additional magnetometer sites spanned a range of tectonic features across the Tasman Sea. These include the passive margin of South East Australia, the fossil spreading ridge, the Tasmantid Seamount Chain and the Lord Howe Rise, an area of submerged continental

crust. Initial analysis by Ferguson (1988) indicated strong three-dimensional induction effects present in the observed data. It was concluded that the most probable causes were the continental margin effect and changes in bathymetry. Sea-water is several orders of magnitude more highly conducting than the underlying crust and upper mantle. Three-dimensional induction effects due to sharp changes in conductivity at the ocean-continent

interface and mare gradual changes in bathymetry may persist over distance-scales of several thousand kilometres. A method is presented of modelling the salt water of the Tasman Sea and adjoining oceans as a thin sheet of variable lateral conductance, which overlies a series of uniform layers representing the solid Earth. The ocean is electrically thin at low frequencies in comparison to the electromagnetic skin depth, which is a measure of the penetration of electromagnetic energy through the Earth, both within the ocean itself, and in the underlying upper mantle. The sheet can thus be considered to have negligible thickness and so obeys basic electromagnetic boundary conditions. The horizontal electric fields and the vertical magnetic field components across the sheet are continuous, however the horizontal magnetic field components are discontinuous across the sheet by a factor dependent on the spatial changes in conductance of the sheet and the horizontal electric field flowing within the sheet. As the bathymetry of the Tasman Sea and the conductivity of sea-water are well known, the lateral changes in conductance of the ocean layer can easily be calculated. The theory and a suitable computer algorithm were developed by a group led by J.T. Weaver, at the University of Victoria, B.C., Canada. Many of the features present in the TPSME data are reproduced by this method. The electromagnetic response of both the thin sheet and the underlying one-dimensional layered space may be calculated, and this leads to a greater understanding of induction processes in the ocean and of the coupling between the ocean layer and the underlying solid-Earth. It is possible to remove the strong three-dimensional induction effects from the observed data, and so the TPSME data are solely a measure of the response of the Earth directly beneath

Conductivity (S/m) .001 100

.01

Original Model

High Conducting Layer

De-Distorted Model 1000

therecordingsite. One- and two-dimensional modelling techniques are then applied to determine the conductivity structure beneath the Tasman Sea. One of the principal aims of the TPSME was to accurately delineate the thickness and nature of an asthenosphere, which is important for mantle dynamics. A high conducting layer, which may indicate a partial melt, was found beneath a resistive lithosphere using one-dimensional modelling techniques on the observed data. When the three-dimensional effects due to induction in the ocean are removed from the observed data, this high conducting layer is imaged at a greater depth. Reference Ferguson I .J., 1988. Unpubl. PhD thesis, Australian National University.

A4.13 THE WIGNER-VILLE ANALYSIS OF MAGNETOTELLURIC SIGNALS I J. Chant* and L.M. Hastie Physics Department, University of Queensland, Brisbane

Wigner-Ville time-frequency analysis of some standard magnetotelluric data shows breakdown of stationarity to be a major contributor to impedance scatter and that time-frequency signal analysis techniques may be used to improve estimates of the impedances. The magnetotelluric (MT) method measures the tensor ground impedance as a function of frequency by utilizing naturally occurring ionosphere radio noise. The method most commonly used is to measure the orthogonal horizontal electric (E) and magnetic (H) vectors at the earth's surface as a function of time. A Fourier transform technique is then used to obtain the frequency spectrum (Dekker & Hastie, 1981). This method assumes that any breakdown in stationarity will not adversely effect the E/H ratio used in determining

the impedance tensor. Repeated measurements, showing non-normal scattering of the impedance data cast doubt on this assumption (Tzanis & Beamish, 1987; Chant, 1986). The modified Wigner-Ville distribution (WVD) is a broad spectrum time-frequency analysis technique (Boashash, 1988) given by the expression:

W(tJ)=

I z(t +

j)z\t-j)emTdt

where z(t) is the analytic signal associated with the real signal s(t).

Some MT data from a survey at Peaks Crossing, Queensland (Chant & Hastie, 1988), were analysed using the WVD. The WVD of each channel showed the presence of non-stationarities in the raw data. The impedances calculated for each frequency were found to vary during the sampling period. The effect of this variation on data analyzed by the normal method is to smear the power across a frequency band, resulting in biased estimates of the impedances. Figure 1 shows the mean and median of the logarithmic impedances for each frequency taken across the WVD time windows from a typical data set. The resulting impedances are much smoother than those produced using the standard FFT method and converting to the same frequency bands using the mean and the median (Fig. 2). A good indication of the effectiveness of the technique is the coincidence of the mean and medians of the impedance data (Fig. 1) which can only occur for normally distributed impedances. Comparison of the

two figures shows the bias that has been introduced by the standard Fast Fourier Transform method. We have shown that time-frequency analysis, and in particular the Wigner-Ville transform, may result in significant improvements in the quality of MT impedance data, though more advanced filtering techniques may prove necessary in extreme cases of non-stationarity. Time-frequency filtering can also be used to separate the various signals found in MT data to allow further study of source and polarisation type biases. References

Boashash, B., 1988. IEEE Trans ASSP 36: 1518 Chant, I.J., 1986. Unpubl. MSc, University of Queensland. Chant, I.J. & Hastie, L., 1988. Explor. Geophys. 19: 417 Dekker, D.L. & Hastie, L.M., 1981. Phys. Earth & Planet Int. 25: 219 Tzanis, A. & Beamish, D., 1987. /. Geophys. 61: 97.

100 10 r

a £ '1.0

CO .1 C/LUD a:

.01 r

.001

FREQUENCY (Hz) Figure 1 — Mean and median Wigner-Ville derived impedances. 100 I 10

E a £ ' 1 . 0 rl > GO .1

CO LU GC

.01

.001

FREQUENCY (Hz) Figure 2 — Mean and median FFT derived impedances.

A4.14 TECTONIC AND SEDIMENTARY PROCESS IN THE TONGA-KERMADEC TRENCH AND ADJACENT TERRAINS: A GLORIA LONG-RANGE SIDESCAN SONAR SURVEY W. Mayer *, C. Jacobs , and L. Parson 1

University of Canberra, Canberra Institute of Oceanographic Sciences, Godalming, UK 1

A GLORIA side-scan sonar mosaic of portions of seafloor at the outerrise.There is evidence of slumping the Tonga and Kermadec Trenches was collected in in at least some of the gullies. Sediment transport paths May 1988 during Cruise 33 of Charles Darwin. The that are less well exposed on the sonogram also occur mosaic covers a 45 km wide swathfrom22°S to 27°30 S on the seamount's eastern flank and on the eastern side located partly over the trenches and partly over the of Osbourn Seamount. The channels can be traced over forearc, with additional coverage of part of the outer the respective sediment aprons that surround the two rise and the Louisville Seamount Chain. The sonar data seamounts. In the insonified area seismic records indicate has been supplemented by airgun, 3.5, and 10 kHz that sediment cover is thickest where the two sediment profiles. The bathymetry is compiled from published aprons coalesce, with possibly up to 400 m of material. charts and new information obtained on this survey. A small, isolated, and previously unmapped seamount The trench axis follows a sinuous path that varies in is located close to the Kermadec Trench at 26°25'S. The directionfromnorth/south to 030°. It reaches depths in GLORIA images indicate a volcano with an irregular excess of 10,000 m but shoals to less than 6000 m topography lacking a flat summit area. where the subducting Osbourn Seamount interrupts the The Louisville Ridge seamounts have a prominent trench to form a sill. Where the Pacific Plate descends magnetic signature. Two small magnetic highs on the into the trench the bending lithosphere has fractured to landward slope, associated with bathymetric highs may produce a characteristic horst and graben topography. be indicating accreted seamount fragments. If so, this GLORIA images the trenchward facing scarps as narrow, accretion appears to be a very episodic phenomena linear areas of strong backscatter, subparallel to the whose record is soon erased. trench axis. In the trench and on its outer wall the faults The most prominent features imaged over the forearc are spaced 1-4 km apart and have vertical displacements are a series of narrow, closely spaced linear features of up to 600 m. Their throw tends to increase towards with high backscatter occurring between 23°S and the trench axis. Their slightly sinuous traces can be 24°30'S. These occur on one of the steepest parts of the followed laterally for up to 50 km. The floors of the slope below the trench-slope break. Their trends closely grabens are up to 4 km wide and are inclined towards parallel the Tonga Trench as it swings from a north/ the trench at angles equal to or less than that of the south direction near Osbourn Seamount to a direction regional slope (about 5° for the outer trench wall and east of north at Horizon Deep. The 10 kHz records 1.5° for the outer rise). Sediment cover over the outer show these features in profile as rounded, antiformal rise and on horst blocks amounts to about 200 m and structures with amplitudes from 30 m up to 100 m. may be slightly thicker in some grabens. They are interpreted as a series of slumps mobilized on GLORIA images the northernmost extension of the an oversteepened slope which have failed on sinuous Louisville Seamount Chain, including the Osbourn fronts up to 10 km in length to pile up against each other Seamount and the adjacent seamount in the chain whose behind a high on the lower slope. Above the trenchsummit is located some 85 km to the southeast. Two slope break GLORIA has imaged a number of areas of prominent ridges extend from the summit area of the uniform, even-grained, low backscattering. These are latter towards the southwest and west and rise from interpreted as sediment-filled basins on the Tonga 5200 m on the outer rise to 3200 m. These are imaged Terrace. on GLORIA as areas of strong backscatter interrupted In the northern part of the survey area there are two by irregular darker tones. The former are interpreted as irregularly shaped features of high backscattering. These acoustic basement outcrops while the darker areas are have tentatively been interpreted as erosional aprons sediment ponds filling depressions. Two prominent lava surrounding diapirs. The larger of these is about 15 km flows have also been imaged by GLORIA as irregular across. The diapiric structures have been mass-wasted areas of uniform backscattering intensity. They occur and show darkly-imaged channel formations on their along the slopes of the tworidgesand have their origins slopes; the latter are confirmed by 3.5 kHz records. at the summit area of the seamount. Both flows have a Trench-parallel normal faults, dipping both towards highly irregular surface, the south flow may, in addition, and away from the trench have also been recognised in be covered by a relatively thin veneer of sedimentary the forearc region, but are less-prominently developed material. The southwestern flank of the seamount is than normal faults on the trench's outer wall. Faults dissected by a number of prominent canyons. They are with an orientation normal to the trench have only been narrow at the edge of the summit plateau, widening at noted on the Tonga Platform. their lower end to about 2 km before emerging onto the ,

A4.15 ONSET OF ARIDITY AND DUNE BUILDING IN CENTRAL AUSTRALIA: PALAEOMAGNETIC EVIDENCE X.Y. Chen & C.E. Barton * 1

Research School of Pacific Studies, Australian National University Bureau of Mineral Resources, Canberra 2

Playa sediments provide one of the few sources of information about the Quaternary history of Australia's extensive arid regions. In such environments palaeomagnetism can often provide a valuable chronological framework where other methods fail. Sediments from Lake Amadeus, a groundwater discharge playa in central Australia, comprise a surface playa sequence (the Winmatti Beds), varying between 0.6 m and 3 m in thickness, overlying a long sequence of fairly uniform shallow water fluvial-lacustrine clays (the Uluru Clay). The latter extends down to at least 15 m (the maximum depth cored during the study), and possibly down to 65 m below the present playa surface. The Brunhes-Matuyama boundary (0.73 Ma) was identifiable in the upper sediments of all the cores studied, typically at a depth of between 1 m and 2 m. Below this there are two plausible chronological interpretations of the palaeomagnetic data. Deposition rates are very low: typically no more than 1.5 cm Ka in the Winmatti Beds, and down to 0.2 cm Ka in the Uluru Clay. These rates suggest that the fluvial-lacustine phase may have lasted for at least 5 million years. The boundary between the Uluru Clay and the Winmatti Beds bears a close correspondence to the boundary between the Blanchetown Clay (a lacustrine sequence) and the Tyrrell Beds (a playa sequence) in ancient Lake Bungunnia, in what is now the Murray Basin in southeastern Australia. However the transition appears to occur at about 0.9 Ma (or possibly 1.6 Ma) at Lake Amadeus, whereas it has been dated palaeomagnetically at less than 0.7 Ma (probably 0.5 Ma) in Lake Bungunnia. Records from other locations in southeastern Australia likewise support a major change from wetter to dryer conditions at, or shortly after, the Matuyama-Brunhes polarity transition. Thus the current evidence from Lake Amadeus indicates that in central Australia the onset of aeolian and saline gypseous deposition, characteristic of arid-zone facies, may have pre-dated the corresponding change in the 1

southeastern part of the continent by about 0.4 million years, or possibly by as much as 1.1 million years. Another major difference in climatic conditions between central and southeastern Australia is apparent at the transition from the Tertiary to the Quaternary (taken to be the Gauss-Matuyama boundary at 2.5 Ma). In the Lake Bungunnia sequence the G/M boundary coincides with a disconformable horizon within the Blanchtown Clay that implies a major change from relatively dry to wetter conditions. Similarly, in Lake George the G/M boundary is associated with evidence for reactivation of the lake under wetter conditions. However, in Lake Amadeus the G/M boundary occurs within the homogeneous Uluru Clay and does not correspond to any significant change in sedimentary environment, or, by implication, the climatic regime in central Australia. In common with most playas in central Australia, Lake Amadeus is completely surrounded by a ring of gypseous dunes derived from material deflated from the surface of the playa. This contrasts markedly with the clay-rich dunes (lunettes) that occur on the down-wind sides of dry lakes in southeastern Australia. The difference is attibuted to a combination of the anticyclonic wind pattern that dominated early Quaternary arid landforms over much of Australia, and differences in the character of deflation products from groundwaterdischarge playas (central Australia) and surface-drainage playas (southeastern Australia). Magnetostratigraphic evidence from Auger Island, in the middle of Lake Amadeus, suggests that the oldest gypseous dune formation on the island must have predated the start of the Jaramillo subchron at 0.98 Ma (or possibly the start of the Brunhes epoch, 0.73 Ma), thus making these the oldest dated dunes in Australia. The age of the Auger Island dunes is probably characteristic of other gypseous dunes around the margins of playas in central Australia.

A4.16 PALAEOZOIC APPARENT POLAR WANDER PATH AND PALAEOLATITUDES OF GONDWANALAND Z.X. Li *, C.McA. Powell & P.W. Schmidt 1

School of Earth Sciences, Macquarie University CSIRO Division of Exploration Geoscience, North Ryde, NSW 1

Available Palaeozoic palaeomagnetic data from Gondwanaland were classified into four different categories using a qualityfilter.Class A poles are of the highest quality, and can be used to calibrate the apparent polar wander path (APWP) by defining the position and age of the palaeopole. Class B poles define the shape of the APWP, but there is some uncertainty about the age of the palaeopoles. Class C poles may lie on the APWP, but there is large uncertainty about the reliability of the palaeopoles, and class D poles are unsatisfactory for determining the APWP. Only poles of class A and B quality were used to calibrate and/or define the APWP of Gondwanaland. We consider that the APWP for the Cambrian- Ordovician and the Devonian-Permian intervals are reasonably well defined. However, due to the insufficient number of high-quality palaeomagnetic data for the latest Ordovician to Early

Devonian interval, the polarity of the Early Palaeozoic poles is still uncertain. Such an uncertainty has led us to construct two possible APWP for Gondwanaland (Fig. 1). By comparing these two APWP with the migration path of glacial centres during Palaeozoic, APWP A is favoured. Figure 2 shows the palaeolatitudes of Gondwanaland during the Palaeozoic according to the favoured APWP. It shows that during early to mid-Palaeozoic, there was enormous latitudinal movement for western Gondwanaland but little such movement for eastern Gondwanaland. During the Carboniferous, eastern Gondwanaland shifted quickly to the southern polar region, whereas western Gondwanaland drifted towards the equator and collided with Laurussia. This collision formed the late Palaeozoic supercontinent — Pangaea.

•

AUSTRALIA

A AFRICA • C

Figure 1 —

SOUTH AMERICA

•

INDIA

•

ANTARCTICA

Proposed Palaeozoic apparent polar wander paths ( A P W P s ) of Gondwanaland. Poles with

calibrating the A P W P s because of their anomalous positions.

were not used for

CAMBRIAN

EARLY-MIDDLE

0RD0V1C1AN

M1D-LATE<?>

SILURIAN

EARLY TO MID-SILURIAN

SILURIAN-DEVONIAN

EARLY TO MID-CARBONIFEROUS

MID-LATE DEVONIAN

PERM0-CAR80NIFEROUS

Figure 2 — Palaeolatitudes of Gondwanaland during the Palaeozoic suggested by the APWP A and the formation of the Late Palaeozoic Pangaea. Bold lines of latitude represent palaeoequators.

A4.17 CONSTRAINTS ON THE GONDWANAN APPARENT POLAR WANDER PATH PROVIDED BY PRELIMINARY PALAEOMAGNETIC RESULTS FROM ORDOVICIAN ROCKS OF THE AMADEUS BASIN G. Thrupp Earth Sciences, Macquarie University, NSW

Although the definition of the Devonian to Early north and upward. It too is exclusively of normal polarity, Carboniferous segment of the Gondwanan Apparent post-dates folding, and probably was acquired recently. Polar Wander Path (APWP) has been improved by In about half of the samples from the four wells and recent palaeomagnetic studies in the Lachlan Fold Belt from one of the surface sections, a characteristic of southeast Australia, the Ordovician-Silurian segment component that differs markedly from the recentfieldis remains poorly constrained. A large and quite rapid defined adequately by thermal demagnetisation. The rotation of Gondwana, during Ordovician-Silurian time, mean direction of the majority of the characteristic about an Euler pole close to northern Australia is implied components is northeast or southwest and nearly by a recently proposed Palaeozoic APWP. horizontal; both polarities are represented. Increased Exceptional fossil preservation and very low dispersion of directions with compensation for attitude conodont colour alteration indicies in the Ordovician of bedding indicates that this magnetisation was acquired sedimentary rocks of the Larapinta Group in the southern after tectonic tilting. Thus, despite the evidence from part of the Amadeus Basin suggest that the rocks may conodont CAIs for very little heating of the Larapinta retain original remanent magnetisation. Although Group in the central and southern Amadeus Basin (CAIs acceptable outcrop of the Larapinta Group rocks is rare less than 1.5), the rocks have been remagnetized. and surface weathering is often severe, continuous core Presumably, the remagnetisation mechanism in the samples from several wells in the Amadeus Basin central Amadeus Basin is chemical. In the Arunta Complex and Ngalia Basin, north of provide pristine Ordovician sedimentary rocks that are the Amadeus Basin, major deformation that affected well-suited for palaeomagnetic analysis. From two surface localities, a total of 120 samples rocks as young as Early Carboniferous is associated have been collected from 21 sites, and and from four with the culmination of the Alice Springs Orogeny wells a total of 205 samples have been collected. The which culminated during mid-Carboniferous time (e.g. surface samples were collected from a drainage ditch Bradshaw & Evans, 1988; Mortimer et al., 1987). Pole along the Stuart Highway near Maloney Creek, and the positions derived from remagnetisation attributed to Storm Creek region south of the Levi Range. The core this orogenesis indicate that by mid-Carboniferous time isfromfour vertical BMR wells: Henbury 6, Rodinga Australia was close to the south pole (Li et al., 1989). 6, Mt Liebig 2, Lake Amadeus 1. The dip of bedding In contrast, the post-folding, shallow component of was measured from the drill core. Because the regional magnetisation in the Larapinta Group of the central structural trends of the Amadeus Basin are very Amadeus Basin was acquired when central Australia consistent, the samples were oriented by assuming was close to the equator. The pole position is close to continuity of surficial regional strike at depth. the Late Devonian-Early Carboniferous segment of the Consistency of paleomagnetic results confirms that the Gondwanan APWP. This result is consistent with the constant strike assumption is a reasonable approx- geologic evidence (e.g. Playford et al., 1976; Bradshaw & Evans, 1988) that folding occurred in the Amadeus imation. Detailed incremental thermal demagnetisation Basin in the Late Devonian. experiments are completed on 102 samples. A post- References tilting, north-upward component is removed between Bradshaw, J.D. & Evans, P.R., 1976. APEA J. 28,: 267-282. 50 and 250°C in most samples from the two surface Li, X.Z., Powell, C.McA., Thrupp, G.A., Schmidt, P.W., 1989. J. Struct. Geol (in press). localities and from samples from two wells; it is most G.E., Cooper, J.A., James, P.R., 1987. LiXhos 20: likely due to recent weathering and/or acquisition of Mortimer, 445-467. thermoviscous remanent magnetisation. A high- Playford, G., Jones, B.G., Kemp, E.M., 1976. Alcheringa 1: temperature characteristic remanent magnetisation in 235-243. surface outcrop samples from reddish calcarenite layers in the Stokes Siltstone Formation is also dominantly

A4.18 THE UPPER MANTLE STRUCTURE UNDER NORTHERN AUSTRALIA B.L.N. Kennett*, J.R. Bowman and P. Cummins Research School of Earth Sciences, Australian National University

A number of arrays of portable seismic data loggers have been deployed in the Northern Territory and Queensland to record events from the earthquake belt running through Indonesia and New Guinea. The propagation paths from these sources give a good seismic sampling of the upper mantle under northern Australia, which can be exploited to determine the nature of the seismic structure with depth and horizontal position. The portable arrays have varied considerably in geometry and aperture and have provided an opportunity to look at the propagation patterns on a variety of scales from a dense set of digital stations with aperture around 50 km, to a relatively sparse analogue array with an aperture of 1000 km. The combination of different events and arrays gives quite good coverage with cross control available in the general area of the Gulf of Carpentaria. By combining the recordings from a number of events, a high data density can be achieved in composite record sections for P wave arrivals which

allow clear identification of the phases associated with prominent '400' and '650' km transitions in the upper mantle. The composite record sections and the character of the recordings of individual events are wellfittedby a model in which there is a slow variation in the depth of the major discontinuities on a scale of about a 1000 km. Superimposed on this broad scale variation are smaller scale features with scales of 200-400 km and amplitudes of a percent or so. These give rise to variability in travel times and amplitude anomalies associated with focussing and defocussing of energy by horizontal velocity gradients. On a number of paths there are strong reflections from the upper mantle discontinuities, the typical velocity contrast would be estimated to be about 5% near 400 km and 4% near 650 km. Beneath northwestern Australia there is strong support for a '200 km discontinuity* with a contrast around 2-3% but this becomes less pronounced for more easterly paths.

A4.19 THE LABORATORY STUDY OF SEISMIC WAVE ATTENUATION Ian Jackson* and M.S. Paterson Research School of Earth Sciences, Australian National University

The zone of relatively low seismic wave velocities and high attenuation in the Earth's upper mantle is a persistent feature of seismological models, especially for oceanic and tectonically active continental regions. The traditional interpretation in terms of partial melting is by no means secure, as there are a variety of alternative mechanisms likely to be operative in the solid state. Improved understanding of anelastic relaxation in the upper mantle, and its implications for rheology on longer timescales, requires the conduct of experiments under laboratory conditions which simulate closely those of seismic wave propagation in the earth. Of particular importance is the design and execution of experiments at seismic frequencies (<1 Hz), most available information on the elasticity of rocks was obtained at much higher ultrasonic frequencies (>1 MHz). Pressure and temperature are also very important variables in the respective contexts of suppression of crack porosity and the facilitation of thermally activated relaxation mechanisms. An approach based on the observation of forced torsional oscillations of an assembly comprising a series combination of a rock cylinder and an elastic standard has been pursued. The twist induced in each part of the assembly exposed to the same harmonically varying torque is measured by a sensitive capacitative displace-

ment transducer. The relative amplitudes and phase of these signals provide comparative determinations of the shear modulus G and internal friction Q , the latter being a measure of dissipation. These experiments are conducted under conditions of high pressure (to 300 MPa) and temperature (to 1000°C) for low-amplitude oscillations (strain clO" ) with periods of 1-100 s. To date this equipment has been deployed mainly in studies at room temperature. The influence of strain amplitude, oscillation period, pressure and grain size on the shear modulus and internalfrictionare well illustrated by representative results, plotted below, for a suite of essentially monominerallic calcite rocks. The insensitivity of shear modulus G and internal friction Q to strain amplitude indicates that the behaviour is linear and therefore directly applicable at the even lower strains of teleseismic wave propagation. Both modulus and internal friction are strongly pressure sensitive below 100 MPa, reflecting the closure of intergranular porosity. At high pressures (and room temperature) a close approach to ideal elasticity is realised ( Q < 0.001). The insensitivity of Qr to variation of either oscillation period or grain size suggests that the residual anelasticity at high pressure is dominated by an intracrystalline mechanism with a wide range of characteristic times. 1

More recently, progress has been made towards the quantitative determination of anelasticity under conditions of both high pressure and temperature. A

(0 3 3 "O O E co o JZ C/)

28 ' 27 • 26 ;

•

Solnhofen limestone Period 10 s

- f i1

_ O

CARRARA MARBLE Period 30 s ,7 Strain amplitude 5 x 1 0

5 0.002 /N

•

i • -6 -7 lg (strain amplitude)

-5

2 1 lg (period/s)

-o 100 200 Pressure / MPa

o 300

I Calcite rocks

r(0 0.001

0.001 • -

CO 0.001 c ok. c 0.000

Carrara marble

31 30 29 28 27

o 0.001-

comparison of the anelasticity of a Norwegian dunite with that of high grade alumina to 1000°C at 300 MPa will be presented.

k. 0) c

•

2 3 1 lg (grainsize/micron)

A4.20 RISK IMPLICATIONS OF THE EARLY TASMANIAN EARTHQUAKE SEQUENCE M.O. Michael-Leiba Australian Seismological Centre, Bureau of Mineral Resources

During the period April 1883-January 1892 an estimated 1701 earthquakes with intensity-deduced Richter magnitudes, MI > 4.0 were felt in northeastern Tasmania and the islands off its coast. There were two main episodes of activity. The first occurred between April 1883 and April 1886. During this period the monthly seismic energy release generally exceeded 1012 Joules and five of the 1675 earthquakes had MI > 6.0. Several years of relative quiescence followed. Then in January 1892, the largest event in eastern Australia took place. It had a Richter magnitude, MI, of 6.9 and was

followed by a MI 6.0 aftershock eight minutes later. Activity returned to a relatively low level following these earthquakes. These events occurred on the edge of the continental shelf in the west Tasman Sea in a previously quiescent region. Large earthquakes have taken place in a similar environment off South Australia and Western Australia. This suggests that the coastal areas of eastern Australia are not immune to the effects of large and potentially damaging earthquakes which would be difficult to predict on the basis of seismicity patterns.

A4.21 THE FAULTING PROCESS OF THE 1988 TENNANT CREEK, NORTHERN TERRITORY EARTHQUAKES FROM LOCAL AND TELESEISMIC OBSERVATIONS J. Roger Bowman , Gary Gibson , George Choy , Jim Dewey , Trevor Jones and Brian Kennett 1

Research School of Earth Sciences, Australian National University Seismology Research Centre, Phillip Institute of Technology National Earthquake Information Center, United States Geological Survey, Denver, USA Mineral Resources Department, Suva, Fiji 1

All earthquakes that occurred in the Tennant Creek On 22 January 1988 three earthquakes of M 6.3, 6.4 and 6.7 occurred in a twelve hour period near Tennant fault zone from 1987 to March 1989 and were large Creek, Northern Territory and produced 32 km of surface enough to be recorded by national and global networks rupture. Low-angle (20-30°) thrust faulting is seen on are relocated using the method of joint hypocenter two main scarps separated by a 7 km gap. The Lake determination (JHD). Arrival times from the WRA array Surprise scarp forms a north pointing chevron of about and portable stations are combined with the national 24 km length with the west arm trending west-southwest and global network data in order to provide temporally and the east arm, east-southeast. The 8 km long uniform coverage of the fault zone and calibration of Kunayungku scarp lies on a northwest extension of the teleseismic station corrections. The JHD locations for eastern Lake Surprise scarp. However, there is no surface the mainshocks are consistent with the interpretation of breakage connecting these two scarps. On the faulting progressing from west to east The spatial Kunayungku and eastern Lake Surprise scarps, the distribution of the largest aftershocks in 1988 and 1989 surface on the south side of the scarp was thrust over the is similar to that observed for smaller aftershocks that north side, whereas on the western Lake Surprise scarp, were well located with only portable array data. The the north block was thrust over the south. 1987 earthquake series occurred in a cluster only several Relocations of the three mainshocks using data from km across in the gap between the two main scarps. the Warramunga (WRA) seismic array, situated 30 km Broadband displacement and velocity records of P east of the scarp, indicate that mainshock hypocentres waves recorded at teleseismic distances are analyzed to moved progressively from west to east. These results determine the rupture characteristics of the three Tennant suggest that the Kunayungku scarp was produced by Creek mainshocks and the largest (mb 5.8) aftershock. the first mainshock, and the west and east ends of the The analyis provides estimates of focal depths, moment, Lake Surprise scarp were produced by the second and focal mechanism, radiated energy and associated stresses third mainshocks, respectively. for the mainshocks. With broadband data we are able to Data from portable seismographs operated in the resolve a complex rupture history for each mainshock. fault zone are used to locate more than 200 aftershocks. The first earthquake is comprised of two subevents with The aftershock zone is about 40 km by 10 km in plan the same focal mechanism (strike 100°, dip 35°, slip and is elongated parallel to the trend of the surface 90°) but distinct depths of 6.5 km and 4.5 km. This ruptures. Focal depths range from near surface to about rupture propagated updip and to the northwest Although 8 km. In the western and eastern portions of the fault the second mainshock consists of three subevents at zone, aftershocks occur only south of the Kunayungku depths of 3.0-3.5 km, its waveforms exhibit no and Lake Surprise scarps, respectively. In the central resolvable directivity. The focal mechanism of the first section, in contrast, aftershocks lie primarily to the two subevents (strike 290°, dip 70°, slip 120°) is well north of the western Lake Surprise scarp. In all sections constrained by nodal P arrivals at stations TATO and the shallowest earthquakes lie closest to the scarps, MAJO. The focal mechanism of the third subevent of whereas deeper events lie most distant. The aftershocks the second mainshock (strike 250°, dip 50°, slip 115°) form inclined zones delineating the primary fault surfaces is less well constrained by the broadband data, but the ruptured by the mainshock. In the east and west, the north-dipping nodal plane of this mechanism is fault planes dip to the south, consistent with the sense consistent with surface deformation and aftershock of surface rupture. In the central segment, on the other distribution. The third mainshock was also a complex hand, a group of aftershocks with well constrained focal rupture, consisting of three subevents with the same depths clearly define a north-dipping plane, also mechanism (strike 100°, slip 45°, dip 80° and depth of consistent with the sense of surface rupture for this 4.5 km). Rupture propagation was to the southeast The scarp segment. The aftershock data would allow a south-dipping nodal planes for the first and third second, south-dipping plane in the central section that mainshocks are identified as fault planes by the sense of may be continuous with the fault plane on the eastern thrusting seen at the surface and by the distribution of Lake Surprise fault, but that does not break the surface. aftershock hypocentres. For the second mainshock, the This suggests that conjugate fault planes ruptured during first two subevents may have ruptured along the souththe second mainshock, consistent with broadband dipping plane that did not break the surface, whereas waveform modelling results. the third subevent ruptured the north-dipping plane. s

99 The third subevent had low energy release and may have broken along a preexisting fault plane expressed at the surface as a quartz ridge. The depth of the largest

aftershock is estimated at 3.0 km from the broadband records.

A4.22 ACCURATELY-LOCATED EARTHQUAKES NEAR CADOUX, WESTERN AUSTRALIA V.F. Dent Bureau of Mineral Resources, Mundaring Geophysical Observatory, WA.

Southwest Western Australia has been the site of numerous significant earthquakes. An M> 7.2 earthquake occurred near Meeberrie (near Geraldton) on 29/4/1941. Since then there have been major earthquakes at Meckering (14/10/68, ML 6.9), Calingiri (10/3/1970, M> 5.7) and Cadoux (2/6/79, M> 6.2). While the surface rupture, where it exists, allows the location of a main event, the foreshocks and aftershocks have to be located instrumentally. There are significant errors in these locations, particularly for the older events. In order to accurately locate some aftershocks of the Cadoux event, and in particular, to attempt to determine their depths, a number of portable seismographs were positioned in the wheat belt area of Western Australia in late 1983. From these stations, it has been possible to locate a number of relatively small events with a relatively high degree of accuracy. For some events, it has been possible to get accurate depth determinations. The well-located earthquakes show a strong correlation with the rupture caused by the major Cadoux earthquake of 2/6/79. They are roughly parallel to it, and on its western side. The earthquakes which are accurately depth-located show a depth range of 0 to 4 km. There is a trend of increasing focal depth with

distance westwards away from the surface rupture, supporting the interpretation of a westerly-dipping fault zone. The relationship between the Cadoux aftershocks and the fault zone is not so apparent from the many other aftershocks which were located without the benefit of close stations, and of modern computer techniques. However, some of these events have been relocated using more modern techniques, and the relationship between the events and the fault zone is enhanced. Denham et al. (1987), on the basis of three years of aftershock data, postulated that initial (i.e. the first six months of) seismic energy was released in the centre of the fault zone, and that subsequent energy was released at the extremities. The improved data presented here neither contradict nor reinforce this postulate. The improved data do show an interesting concentration of events about 16 km south-east of Cadoux, which is on the south-eastern side of the fault zone.

References Denham, D., Alexander, L.G., Everingham, I.B., Gregson, P..J., McCaffrey, R. & Enever, J..R., 1987. AusL J. earth Sci. 34: 507-521.

A4.23 METHOD FOR GRAVITY AND AEROMAGNETIC INTERPRETATION David Boyd Department of Geology and Geophysics, The University of Adelaide

Because of the immense amount of diverse data to be considered the full interpretation of regional gravity and aeromagnetic data is an exceedingly complex undertaking. Although a preliminary study of the data can be done quickly a serious study of a large area will take a long time and because of the complexity of the problem, the work must be well structured if it is to be effective. A methodical approach is required to help both the interpreter in conducting his study and the reader in following the logic and arguments which it is necessary to do to understand fully the conclusions reached in the interpretation. The four principles of philosophic method set out by Descartes — (i) accept only that of which you are sure ; (ii) divide the problem into parts; (iii) start with the easiest parts; (iv) review the work frequently — are as

applicable to geophysical interpretation as they are to the solution of abstruse philosophical problems. The four principles are developed as follows to show how they provide direction to the geophysicist dealing with the wide scope of a regional interpretation. 1. Although the philosopher may be able to accept only data of which he is absolutely sure, the geophysicist who works with field data cannot start from such certainty; the problem of the quality of the data however lies at the root of many problems in interpretation. Shortcomings of the geophysical data (thought of both in terms of accuracy, noise level and adequate sampling of the data) must be understood by the interpreter if he is not to be misled by the maps on which his work is based. Both the aims and the explanations of the interpretation depend on the observations and concepts

100

of the geologists: if the geological record is incorrectly presented to the geophysicist the outcome of the interpretation will be flawed and may be totally misleading. 2. The problems of regional geophysics can only be handled by dividing the problem into parts. The survey itself can be divided into at least six stages, namely planning, observations, reduction, presentation, interpretation and report preparation. The interpretation, or more correctly conclusions and recommendations derived from the interpretation, are based on the analysis of the data (a mathematical process resulting in the identification and outlining of individual bodies) and synthesis of the geophysical and geological data to produce fresh information and insight into the geology. Regional aeromagnetic maps are broken down into recognisable sub units on which more detailed studies are undertaken if this is justified by the aims of the interpretation; a full and complete analysis and interpretation is beyond the scope of a lifetime and some priorities must be imposed on the interpretation or the work could go on indefinitely. 3. In some surveys valuable information can be obtained very easily and quickly by a study of the contour maps and images. This information should be

set out systematically before passing on to tackle the problems considered to be of greatest scientific or economic importance. If in doubt tackle the easiest problems first 4. It is normal practice in interpretation to review progress frequently, often daily, weekly and monthly because so many individual parts of the interpretation are inter-related; as one matter is understood it may clarify unresolved problems encountered earlier. Even after a survey including the follow up phase, has been completed, the reports and the maps should be scanned every year or two, for the aims of an interpretation are partly determined by current knowledge, and economic and political climate. New mineral discoveries, change of access to the area, or new markets may change significantly the way in which you look at the anomalies on a map. Interpretation is a multidisciplinary activity which calls for a wider range of skills than can normally be found in one individual. Interpretation is most successfully done in an environment where cooperation and team work is encouraged and in a society in which the spread of information is facilitated by enlightened regulation.

A4.24 TASMANIAN CRUSTAL FEATURES D.E. Leaman * and R.G. Richardson 1

Leaman Geophysics, Hobart Department of Resources and Energy, Tasmania 1

The triangular extension of Australian continental crust known as Tasmania has a thickness of about 28 km. It is composed of siliceous, metamorphic sequences but only parts of the Rocky Cape and Tyennan Blocks are yet known to be more than 15 km thick. Within the Proterozoic rocks of W and SW Tasmania several apparently discrete blocks can be recognised. Each presents a variation in rock suites and structural associations. Named blocks include the Forth, Cradle Mountain and Prince of Wales Range Blocks. Like most Precambrian rocks these are virtually non magnetic but significant density variations are indicated which are not predicted by exposed siliceous lithologies. The best defined density change, with limited depth extent (<6 km), is associated with the convex western face of the Prince of Wales Block SE of Macquarie Harbour. Marginal structures involve fault systems and some rare occurrences such as the eclogite along the Collingwood River. It is possible that the entire Prince of Wales Block has been overthrust on Tyennan material and that other members of the thrust stack are exposed further east near Adamsfield or concealed near National Park and Weld River. A similar pattern may relate Proterozoic blocks across N Tasmania. This would imply

a general Alpine structural sequence involving the entire area of the island. Two principal stages are suggested in this development; latest Precambrian and Devonian although the generally westward motion of the latter now dominates visible structures and relationships. The sense of earlier movements was probably eastward. The Arthur Lineament generates large gravity and magnetic responses and marks the western edge of a late Precambrian and middle Cambrian mobile zone. The eastern edge is flagged by the Mt Read Volcanic piles on shallow Tyennan basement Trough materials accumulated in two cycles exceed 12 to 15 km in thickness. The initial trough was sinistrally offset prior to Mt Read volcanism to juxtapose terranes near Cape Sorell. Within the West Tasmania Mobile Zone (WTMZ) Cambrian and Devonian structuring is overprinted and Cambrian thrust slices carry ultramafics. A comparable pattern has been deduced for the Huon Mobile Zone (HMZ) extending NW from the south coast The combined trough sequence is only 8 km thick. The W side appears to be a thrust stack while the axis of the Derwent valley marks the eastern side. Ultramafics occur on the western periphery and are inferred internally. The age and structuring of

101 sequences is identical to the WTMZ. This structural pattern implies structural introduction of the Cambrian ultramafics which were subsequently further deformed; but which were present only as components in thrust slices and never as a covering mass across the Proterozoic heart of the island. The Tamar Lineament, long considered the primary crustal suture in Tasmania—erroneously it now appears, is the eastern margin of the Tamar Mobile Zone (TMZ) which is up to 40 km wide. Along the north coast materials comparable to the other zones are involved. Elsewhere the TMZ, like much of the HMZ, is concealed by post Carboniferous rocks. Much of the movement in this zone occurred centrally or on the W margin and the N-S wall of the batholithic granitoid complex of E Tasmania was either displaced or controlled by it where these intersect. Granitoid roots lie at least 9 km deep. Granitoids are marginal to all zones and widely distributed with respect to late Precambrian-Devonian mobile zones or many of the Proterozoic blocks. This implies that most of the crust beneath Tasmania was involved in the orogenic process supporting the implication of repeated thrust slices across its width.

The absence of marked anomalies across the TMZ suggests that the effective basement beneath the flyschstyle Mathinna Beds of the NE is of upper Proterozoic type comparable perhaps to the Rocky Cape, Badger Head or Jubilee Blocks. Jurassic dolerite centres were controlled by basement structures and the NW-SE trending aspects were rejuvenated during Tertiary extension. The central plateau was uplifted along the S edge of the TMZ and depressed along the Derwent margin of the HMZ. Tertiary basins in N Tasmania occupy en echelon relationships within the TMZ. In S Tasmania they are concentrated along the Derwent axis and in the zone between this axis and N-S splays from the W side of the TMZ. Apart from reactivation of the Macquarie HarbourBirch Inlet structure Tertiary activity was minimal elsewhere. Trend patterns inferred from gravity and magnetic data account for most sites of recent seismic activity. The figure summarises regional structures and trends as inferredfromcurrent analysis of magnetic and gravity data. This work is incomplete.

102

A4.25 A GEOPHYSICAL STUDY OF THE SOUTH-EASTERN PART OF THE MURRAY BASIN, NEW SOUTH WALES I.R. Qureshi *, R.A. Spencer , R.G. Cameron , E.D. Tyne & E. Scheibner 1

University of New South Wales Department of Minerals and Energy, Sydney 1

A preliminary appraisal of the gravity and magnetic data pertaining to the south-eastern part of the Murray Basin is presented. The area lies between latitudes 34°S and 36°S and between longitudes 144°E and 147°E and covers four 1:250,000 sheets of Hay, Narrandera, Deniliquin and Jerilderi. The Geological Survey has recently completed an aeromagnetic coverage of this area in collaboration with BMR and have supplemented the existing BMR gravity data with measurements at 12 km spacing along selected profiles. The new measurements have improved the definition of anomaly magnitudes and gradient zones. The main features of these data are shown in figures 1 and 2. The studied area covers parts of the eastern and southern boundaries of the Murray Basin with the Lachlan Fold Belt Since the basin contains a thin platform cover of up to 600 m of Cainozoic sediments, the gravity and magnetic anomalies are largely indicative of the physical character of the underlying basement. A distinct change in trend from the general northerly direction of the Fold Belt to NE in the centre of studied area marks the probable extension of the central Victorian Magmatic Province under the basin (Fig. 1). The prominent gravity gradient zone in the east marks the western boundary of the outcropping granites of the Wagga-Omeo Metamorphic Belt (Scheibner, 1987). An east west gradient zone runs in the southwest The Bouguer anomaly maxima lie within a narrow range of ±50 GU which is considered to be an appropriate background value. The Minima on the other hand reach -420 to -470 GU at the eastern boundary but only -300 to -330 GU over the rest of the area. It is very likely that these latter minima are also produced by buried granites of varying ages — some of which have been intersected

in boreholes. The anomalies at C and D (Fig. 1), both of magnitude 280 GU, can be produced by shallow granites extending in depth to about 8 km (using a density contrast of 0.1 g cm ).The picture of the area as seen in the total field magnetic intensity is very complex though overall anomaly change lies within ±200 nT except in the south-western corner. The trends in the eastern part are N to NW and change abrupdy to NE in the central part — forming a distinct arcuate feature concave to the east This change accords with the change in gravity anomaly trends. The accurate feature may mark the Kiewa Line (Brown et al., 1988). Complex anomaly patterns overlie the outcropping granites and buried granites (as inferred from gravity lows) in the eastern half of the area. Generally the trends of the gravity and magnetic anomalies associated with granites are concordant. The main gravity low running NE in the west has dominant magnetic signatures only at its ends and it is likely that the inferred granite body is made up of plutons of differing magnetic character. The magnetic anomaly at E (Fig. 1) is associated with a gravity high and may have a granodiorite as its source. The gravity lows at the southwestern boundary are accompanied by broader zones of magnetic gradient and may thus be associated with deeper granites. The alignment of several magnetic anomalies along a north-south zone in the west may mark a major fault zone as suggested by Brown et al. (1988). References 3

Brown, C.M., Tucker, D.H. & Anfiloff, V., 1988. Tectonophysics 154: 309-333. Scheibner, E., 1987. In Monger, J.W.H. & Francheteau, J. (eds.): Am. Geophys. Un., Geodynamic Series 19:133-165.

103

147

Gravity gradient zone

—

Magnetic gradient zone

Gravity high with trend

Magnetically defined zones

Gravity low with trend

Profile AB (see Fig. 2)

Figure 1 — A sketch of prominent gravity and magnetic anomaly features in the southeastern Murray Basin.

r- 200

200 TMI

100 Z> CD

-100

0 1 BOUGUER

-100 *>

D i -200 •js o i_ -300 — Q) z: D cn -400 O = CD -500 "1

-0

- - 1 0 0

200000

250000

300000

350000

400000

AMG Easting (metres)

450000

Figure 2 — Gravity and magnetic anomaly profiles along line AB (see Fig. 1).

500000

-200

550000

104

A4.26 THREE DIMENSIONAL GRAVITY MODELLING OF THE DEVONIAN GRANITE COMPLEX AND ITS RELATIONSHIP TO MINERALISATION IN WESTERN TASMANIA D.L. Archer RGC Exploration Pty Limited, Renison

Three dimensional gravity modelling in Western Tasmania shows that subsurface extensions of the outcropping Devonian granites are interconnected at depth, and are likely to constitute one major parent intrusion. The Heemskirk, Meredith, High Tor and Pine Hill plutons comprise parts of this large intrusion referred to as the Devonian granite complex. A shallow granite ridge is modelled to extend west from the High Tor Granite, below the Mount Read Volcanics, outcropping as the Pine Hill Granite near Renison. The ridge extends further west towards Zeehan where it dips steeply forming a deep trough before outcropping as the Heemskirk Granite. The cross-cutting granite ridge widens progressively from below the Mount Read Volcanics, near Rosebery to the High Tor Granite and continues to widen for a substantial distance further to the east The Meredith Granite is connected to the granite ridge at Pine Hill by a deep NNW trending granite trough. Many features of Western Tasmania geology support the presence of a shallow granite ridge. Structural interpretation using landsat imagery, aeromagnetics and gravity data identified ENE trends superimposed on a more dominant N-S orientation. Such trends maybe a structural relict of the granite ridge intrusion. Furthermore, the distribution of Devonian Sn, Ag, Pb and Zn mineralisation along this linear ENE trending zone which cross-cuts, and is superimposed over

Cambrian mineralisation trends within the Mount Read Volcanics, is explained by the presence of an underlying granite ridge. The distribution of contact metamorphic fades, porphyry dyke swarms and drill hole intersections again support the existence of a granite ridge at depth. Intrusion of the Devonian granite complex into high levels of the crust, would have greatly influenced the tectonic evolution of Western Tasmania during the Devonian and Early Carboniferous. Definition of the Devonian granite surface from 3D-gravity modelling has important implications for the exploration of genetically and spatially associated mineralisation. The modelling of a cusp on the granite ridge just east of Zeehan and the distribution of mineral assemblages and sulphur isotope data within the Zeehan mineral field supports the presence of two mineralising hydrothermal systems separated by a deep granite trough. East of the granite cusp and west of the Pine Hill outcrop, the granite ridge is extremely narrow in cross-section confining mineralisation to a linear zone above it West of Pine Hill, the ridge widens and mineralisation is controlled to a greater extent by major fault/ridge intersections. With the granite surface successfully defined by 3D-gravity modelling, delineation of areas on the ridge which bisects favourable host stratigraphy and major structures will identify areas with high exploration potential.

A43 CRUSTAL STRUCTURE IN CENTRAL AUSTRALIA DETERMINED BY INVERSION OF TRAVEL TIME RESIDUALS Herbert McQueen Research School of Earth Sciences, Australian National University

Teleseismic travel time anomalies for events recorded on portable seismometer arrays across the margins of the Amadeus basin in central Australia record the effects of inhomogeneous crustal structure in the lithosphere beneath the arrays. Observations for sources at a range of different azimuths show systematic variations and provide sufficient redundancy in sampling different paths through the same lithospheric block to allow the extraction of a picture of the velocity structure down to a scale of the order of the station separation, in this case 5-10 km. These residuals have been interpreted using forward modelling and ray tracing techniques by Lambeckm et al. (1988) and Lambeck & Burgess (in prep.). Because this method involves trial and error evaluation of a suite of preconceived models it is open

to the possibility of subjective bias in the class of models chosen for testing, and it is very time consuming to evaluate enough cases to counter this possibility and arrive at an optimum range of models. The previously processed residuals are here used as input to a numerical inversion procedure to obtain a direct estimate of the lithospheric velocity field along three sections. The fundamental assumption of the method is that rays from a particular source have nearly identical paths in the source region and pick up all their travel time variability from inhomogeneities near the receivers where raypaths are most widely spread. The model parameterisation consists of a rectangular mesh of constant slowness cells extending about 100 km beyond each end of the line in the plane of the nearly

105

linear arrays and to a depth, in most cases, of 100 km. The model structure is assumed to be two dimensional in the plane of the N-S trending arrays, although the incident teleseismic rays are traced through a fully threedimensional mesh of cells. This assumption is justified by the strong east-west strike of surface and gravity structure in the vicinity of the lines. The maximum effective depth of the model is at the base of the region of dense ray crossovers. Cell slownesses are iteratively refined using a modified SIRT (simultaneous iterative reconstruction technique) algorithm which distributes consistent slowness anomalies through the area illuminated by the incident raypaths. Velocity structure is then recovered from the cell slownesses based on an assumed background velocity gradient, and can be combined with an emporical velocity-density relation to generate gravity profiles for comparison with observations. Of the lines analysed, two (the Redbank and Arunta lines) are across the northern margin of the Amadeus Basin and one (the Musgrave line) is across the southern margin. In all cases, the most prominent feature of the solutions is a sharp interface between slow and fast regions dipping away from the basin and beneath the adjacent basement blocks. The fastest region, on the upper side of the interface, corresponds to a belt of high grade metamoiphic rocks where they outcrop at the surface. The interface between slow and fast regions extends to at least 50 km depth in all cases, and the ontrast is marginally weaker on the Musgrave line. Its

average dip appears to be around 50-60° on the northern lines and steeper, perhaps 60-80° on the Musgrave line although it is less steep near the surface. On the northern lines the interface can be correlated with the Redbank Thrust identified in deep seismic reflection data (Goleby etal., 1989). Secondary features are also observed, including a possible knee on the dipping interface on the Musgrave line and a m ore complex structure on the Redbank line suggesting that a steeper fault cuts through the hanging wall of the thrust there. Where the lines extend into the Amadeus Basin, there is an increase in average crustal velocity towards the centre of the basin consistent with shallowing of the Moho toward the central arch in the gravity field. Derived gravity profile predictions resemble observations surprisingly well in their style and approximate magnitude and provide an extra confirmation of the general features of the models generated by the inversion. The results support a thick skinned thrusting model for the deformation of the basin margins and suggests a degree of symmetry between the Alice Springs Orogeny in the north and the earlier deformation at the southern edge of the Amadeus Basin. References Goleby, B.R., Shaw, R.D., Wright, C., Kennett, B.L.N. & Lambeck, K., 1989. Nature 377: 325-330. Lambeck, K., Burgess, G. & Shaw, R.D., 1988. Geophysical J. 94: 105-124.

106

A5: Exploration Models and Mineral Deposit Evaluation Convenors: J. Angus and R. R. Large

A5.1

INTEGRATION OF GENETIC THEORIES W I T H E X P L O R A T I O N CRITERIA T O D E V E L O P VIABLE E X P L O R A T I O N M O D E L S FOR V M S DEPOSITS

Ross Large*, Ron Berry, David Huston, Bruce Gemmell, Joe Stolz and Khin Zaw Key Centre for Ore Deposit and Exploration Studies, University of Tasmania

The development of viable exploration models depends on the integration of genetic ideas with useable exploration criteria. The power of an integrated exploration model is fully realised through its use in a predictive manner to search for variations in the known and well documented styles of mineralisation. Exploration models are often used in a very restrictive sense with no regard for the wide variability of deposit types, even within the one class of ore deposit. Recent research and exploration in the Mount Read Volcanics of western Tasmania have revealed a wide variability in deposit styles, and demonstrate the inherent shortcomings in applying narrow, overly constrained, exploration models. Five distinct styles of volcanic-related mineralisation have been defined within submarine volcanic settings such as the Mount Read Volcanics. • sea-floor Zn-Au (-Pb-Ag-Cu-Ba) polymetallic massive sulphides • sea-floor Cu-Au massive sulphides and associated stockworks • footwall Au-Ag-base metal-sericite-silica ± adularia veins and disseminated stockwoik systems • deep level Cu-Au-pyrite-chlorite disseminated and stockwork zones • magnetite-hematite-pyrite±Cu ± Au vein and replacement systems associated with granitic intrusives. The sea-floor massive sulphides represent the best exploration target for high-grade base-metal and gold mineralisation. Zinc-barite-rich massive sulphide deposits (e.g. Rosebery, Que River, Hellyer) have mean grades of 21% Pb+Zn and 2-4 g/t Au with the best grades concentrated close to the stratigraphic hangingwall of the deposit. Copper-rich massive sulphides (e.g. Mt Chalmers) exhibit a concentration of gold (with copper) in the footwall stringer zone and lower massive sulphide ore. Gold-base metal - silica - sericite ± adularia mineralisation has been recognised in the footwall stringer system of the Que River deposit (McGoldrick et al., this volume) and at the South Hercules deposit

(Khin Zaw et al., this volume). This style of mineralisation may represent a submarine version of an epithermal stockwork feeding a sea-floor massive sulphide deposit. Understanding the style of deformation in a given area and its effect on the ores is a critical aspect of exploration model development (e.g. Huston, this volume). In western Tasmania, folding and shearing in the Devonian has produced an array of deformation styles in the massive sulphide ores and related stringer zones; varying from broad open folding (Hellyer Drown & Downs, this volume), through upright tight to isoclinal folding (Que River - Large et al., 1987) to sheath folding and thrust stacking (Rosebery - Berry, this volume). The deformation has contributed to important changes in the geophysical and metallurgical characteristics of the ores, in addition to an increase in thickness of high-grade ore zones in fold hinges, and in the spatial relationship between the ores and associated footwall alteration zones. The key factors contributing to the gold-lead-zincrich nature of deposits in the Mount Read Volcanics are considered to be: • Tectonic environment — the Central Volcanic Complex (CVC) has developed as a series of coalescing volcanic centres along a narrow rift structure. High heat flow associated with rifting created long-lived convective hydrothermal systems. • Source rocks — the Cambrian volcanics represent an adequate source of gold and base metals. Anomalously high gold values in the Crimson Creek high-Ti basalts (stratigraphically below the CVC) may have been a significant factor (Stolz & Large, this volume). • Chemistry of fluids — the hydrothermal fluids were buffered by the volcanics to allow maximum transport of gold and base-metal complexes. Switchover of gold transport from AUC1 to Au(HS) within the developing sulphide mounds provided a mechanism for gold-zinc refining and upgrading of the outer and upper zones of the orebodies (Huston & Large, 1989). 2

107

Figure 1 shows a simple representation of the geophysical and geochemical techniques. exploration model for the priority 1 high-grade zinc- References barite-rich massive sulphide deposits, emphasising the Berry, R.F., 1990. This volume. tectonic environment, genetic model and geological, Drown, C.G. & Downs, R.C., 1990. This volume. geochemical and geophysical criteria relevant to explor- Gemmell, J.B., Large, R.R., McArthur, G., Drown, C.G. & ation. The Hellyer deposit is a type example for this Downs, R.C, 1990. This volume. D.L., 1990. This volume model (McArthur, this volume; Gemmell et al., this Huston D.L. & Large, R.R., 1989. Ore Geol. Rev. 4:171-200. volume). Separate models have been developed for the Huston, Khin Zaw, Large, R.R. & Hunns, S., 1990. This volume. other styles of volcanic-related mineralisation in western Large, R.R., P., Berry, R. & Young, G, 1988. Tasmania. An important aspect of these models is that Econ. Geol.,McGoldrick, 681-693. because they cover a diversity of styles of volcanogenic McArthur, G.,83:1990. This volume. mineralisation, each defines a separate exploration target, McGoldrick, P., Large, R.R. & Jenkins, G., 1990. This volume. requiring a different exploration approach, and specific Stolz, J. and Large, R.R., 1990. This volume.

MODEL 1: Z i-Au-(Pb-Ag-Ba) polymetallic VMS TARGET

• • • •

high priority typical grades 15% Zn, 3 g/t Au, 8% Pb. 160 g/t Ag, 0.4% Cu typical tonnage 2-20 mt examples — Rosebery, Que River, Hellyer (Tas.)

GEOPHYSICAL PARAMETERS

• non-magnetic • moderate to poor conductor depending on cpy, py content • high density contrast

ENVIRONMENT

• Proterozoic to Recent calc-alkaline volcanic piles • continental margin setting with eruption through thick continental crust • subduction related tectonics but with VMS within local rift structures • compositional variation from rhyolite-andesite basalt • deep submarine volcanic setting in the rift • target ores are distal from the hydrothermai heat engine, usually within upper-most or lateral primary volcanic stratigraphy • underlain by massive pyroriairirs or lavas

GEOCHEMICAL PARAMETERS

• zoning from FW to HW : Fe Cu Pb, Zn, Ag, Au —> Ba • higher silver 100-300 g/t • good Ba-Zn-Au correlation • zinc ratio 60 to 80 • variable As (250 ppm • 5% in gold ores)

ORE FLUID CONDITIONS

• moderate temperature 200-250°C • neutral to alkaline pH 5-6 • variable fO, reaching high values at top of ore (pyrite field) • long-term hydrothermai system promotes mound refining • moderate salinity <4 wt% • moderate to high aH^S • switchover from AuC^ to Au(HS) at base of sulphide mound z

CAUSE OF GOLD DEPOSITION

• increasing SO/H,S ratio as fluid moves to top of mound • dropping temperature

108

A5.2 METAL AND TEXTURAL ZONATION IN THE HELLYER MASSIVE SULPHIDE DEPOSIT G. J. McArthur Aberfoyle Resources Limited, Burnie

The Hellyer volcanogenic massive sulphide deposit, located in the Cambrian Mt. Read Volcanics of western Tasmania, was discovered by Aberfoyle in 1983. Early surface drilling and subsequent detailed underground infill drilling has revealed a large, folded and faulted, but massive body that requires a thorough 3-dimensional understanding of metal location to undertake realistic mine planning. Investigation of metal zonation within the Hellyer deposit followed the successful development and implementation of a customised resource modelling technique. Several features of the deposit had a major influence on the way the modelling technique developed. These were: the large across dip dimension, extremely variable dip and strike, generally sharp geological orebody contacts with virtually no interfingering waste, polymetallic mineralization containing seven elements of economic significance, continuous zones of enrichment and a major fault system cutting acutely across the deposit with a measurable displacement. The customized technique makes use of a natural stratigraphic coordinate system to mimic the major continuity around the folding and displaced across the principal fault system. Kriging is used to interpolate the grades for the seven elements plus density into model cells restricted to the massive sulphide envelope as interpreted in three dimensions by the mine geologists using the structure contour method. The resulting 3-D grade model when viewed in cross section, longitudinal section, plan and plan projection shows that metal zonation closely follows the pattern commonly displayed in other VMS deposits. Immediately above the central feeder system (as characterised by siliceous alteration and strongest development of stringer mineralisation in the footwall), Fe as pyrite and Cu as chalcopyrite are concentrated. The upper and outer regions of the massive sulphide body are enriched in Pb, Zn, Ag, Au and As, with Ag

and As particularly showing a sharp increase in grade at a correlatable horizon. Ba is concentrated in the massive barite cap overlying the sulphides and as crosscutting veins in a zone around the major fault system. A thin and discontinuous highly siliceous cap at the hangingwall shows the maximum concentration of Au. The Cu/Zn ratio traditionally used in the Noranda camp to detect the focus of mineralisation is also applicable at Hellyer showing highest magnitude immediately above the feeder core. Macroscopic ore textures have been classified into six major categories: banded, massive, fragmental reworked, "shrinkage shadows", boxwork veining and recrystallized. These textures have been extended into the model by using arbitrary texture indices built from the geologists' core logging. Each texture type (apart from massive) tends to be concentrated in particular horizons of the sulphide pile. The Fe and Cu-rich core is typically recrystallised and the upper enriched zone banded. Symmetric crustiform boxwork veins cluster just beneath the enriched zone and the unusual "shrinkage shadow" texture in the lower half of the enriched ores. Fragmental ores show a minor concentration at the footwall. Despite deformation in the Devonian, the metal and textural zonation pattern developed during the mineralising event was largely preserved but strain has been strongly partitioned into the enriched zone surrounding the less ductile core. Although there is currently a lack of supporting detailed mineragraphy, the zonation observed is consistent with the metal refining and redistribution model put forward for the Kuroko ores (Eldridge et al., 1983). References

Eldridge, C.S., Barton, P.B. & Ohmoto, H., 1983. Econ. Geol. Mon. 5: 241-281.

A5.3 INTERPRETATIONS AND EXPLORATION AT MT MORGAN AND MT LYELL G.O. Arnold RGC Exploration Pty Limited, Canberra

The Mt Morgan and Mt Lyell mines exploit the largest gold and copper deposits known in the Palaeozoic of Australia, and have parallels between their histories of exploration and interpretation. Both were discovered by prospectors in the 1880s and were studied in detail prior to the 1960s. Both were long regarded as structurally-controlled epigenetic mineralisation. H. Conolly made imaginative and influential

interpretations of both deposits in the 1940s and early 1950s, and succeeded in discovering ore extensions at Mt Morgan. Since the late 1960s, the Mt Morgan and Lyell deposits have been generally regarded as examples of volcanogenic sulphide mineralisation, though Mt Morgan also had a conflicting interpretation as a porphyry-copper-related hydrothermal breccia. In 1984 and 1985, there were reappraisals of the

109

geology at both mines by geologists of Gold Fields No ore has been discovered in the vicinity of Mt Morgan, Exploration (now RGC Exploration) together with though prospects have been found with examples of consultant R.H. Sillitoe. At Mt Lyell, field evidence mineralisationfittingeach of the three proposed models. Major deposits such as Mt Morgan and Mt Lyell indicates that silica-hematite-barite alteration associated with high-grade copper ore-bodies is localised by the have influenced exploration in eastern Australia in regard Great Lyell Fault. The volcanic host rocks (Mt Read to notions of prospectivity of the region and appropriateVolcanics) are reverse faulted against a substantially ness of the particular mineral deposit models which younger sequence (the Owen Conglomerate) by this provide strategy and focus for search. However, as fault, and the silica-hematite-barite alteration illustrated by their histories of interpretations, Mt Morgan transgresses the fault and affects the younger sequence and Mt Lyell have complex geology, difficult to interpret as well as the volcanics. At Mt Morgan, mapping and with certainty. And there is no reason to assume the last core logging indicated that the ore-body is a structurally- word has been written on them now. The explorationist controlled replacement body related to an adjacent faces a further uncertainty in the application of deposit intrusive complex, the Mt Morgan Tonalite. The quartz- models to particular prospects where data are typically pyrite ore-body is discordant to bedding and has limited. There are numerous prospects in eastern alteration which overprints the adjacent Mt Morgan Australia whichwere evaluated as porphyry copper Tonalite and retrogresses contact metamorphic mineralisation in the 1960s, as volcanogenic massive sulphide mineralisation in the 1970s, and as epithermal assemblages associated with the intrusion. Since H. Conolly discovered ore extensions at Mt gold mineralisation in the 1980s. From this skeptical view point, explorationists should Morgan using a now-discredited theory, none of the interpretations at Mt Morgan or Mt Lyell has led to the best regard mineral deposit models as no more than discovery of more ore. These interpretations have tools, similar to geophysical or geochemical techniques, influenced the expending of large amounts of money potentially useful, but also able to be abused. Multiple and effort on exploration, and have had profound effects working hypotheses should be entertained more often on the intellectual climate in which exploration was than they are. However, in practice, the cultural and conducted. At Mt Lyell, discoveries of ore were made psychological advantages of strongly-held convictions between 1955 and 1972, mainly due to the use of about the applicability of a favoured ore deposit model electrical geophysics and changed criteria for ore grades. can outweigh all other considerations. A5.4 EXPLORATION HISTORY AND EVALUATION OF THE HENTY GOLD PROSPECT, WESTERN TASMANIA R.H. Roberts and M J. Fleming RGC Exploration Pty Limited, Hobart

The Henty Gold Prospect is located 30 km north of Queenstown on the West Coast of Tasmania. The prospect is a joint-venture between RGC (Tasmania) Limited (64.7%), operators, and Little River (Resources) Pty. Limited (35.3%). It is covered by a 997 ha Mining Lease No. 16M/89. The geology of the area is dominated by the Henty Fault striking NNE, which separates the Cambrian Central Sequence Volcanics in the west from the Tyndall Group Volcanics to the east. Gold mineralisation at Henty occurs in siliceous zones and minor massive sulphide lenses hosted within a steeply dipping pyritic alternation envelope in the footwall of the Henty Fault The Henty Prospect represents the culmination of 23 years of exploration within former Exploration Licence No. 9/66, which at one time covered 637 sq kms. Exploration was initially for Mt Lyell-style of mineralisation and commenced in the Henty area in 1968-69 when a quarter-mile spaced E-W grid was mapped, soil sampled (Cu, Pb, Zn), and covered by ground geophysics. No significant anomalies were obtained.

Discovery in 1972 of copper mineralisation associated with old workings just to the north of the current Henty Prospect led to more detailed follow-up in 1972-73 including, re-gridding at a 200 m spacing, mapping and gradient array induced polarisation IP. Major IP anomalies were identified at the old workings, at the current Henty Prospect and further to the south. Costeaning of a coincident IP and soil geochemical anomaly in 1973-74 revealed a lens of semi-massive base metal sulphide mineralisation. Six diamond drill holes were drilled at the IP targets with only one hole intersecting a thin massive sulphide lens. A further three holes were drilled in 1982-83, two of which intersected thin massive sulphide mineralisation. Routine gold assaying commenced at this time and significant gold values were found in the massive sulphide intersections plus a second zone of silicapyrite mineralisation. A review in 1984, instigated by the need to reduce the licence area in accordance with new Department of Mines regulations, checked for gold in a previously unassayed zone in drill hole no. HFZ-5, (drilled in

110

1974) which returned 4 m horizontal width at 10 g/t Au. This result initiated a major search for gold mineralisation at Henty. Major drilling programmes were undertaken between 1984 and 1988 to evaluate the deposit. However, although the drilling outlined a very continuous broad mineralised zone, it showed that the Spatial distribution of the high-grade intersections was extremely erratic, even in areas with a 50 m drill spacing. The style of mineralisation also varied dramatically between holes such that a workable model for the structural occurrence and geological continuity of the high-grade zones could not be identified. Surface drilling was serving to indicate the presence of the zone, but was not allowing it to be evaluated. Little could be quantified except that within a designated resource area, with a strike length of 600 m and occurring within 350 m of surface, there was a 50% chance of obtaining a significant intersection of at least 4 g/t Au average over a minimum horizontal width of 2 m. Based on this information an initial inferred resource was estimated at 500,000 tonnes at 10 g/t Au. Before a measured resource estimate could be made it was clear that a great deal more data would be required and the true potential of the deposit would only be realised by intensive underground development and sampling. Consequently in February 1988 it was decided to drive a decline into the upper section of the deposit, and to explore and evaluate it with a 25 m drill pattern from the decline and a 200 m sill drive along the

mineralisation. This work would test the continuity of the high-grade intersections, provide a clear geological model for ore reserve estimation, allow bulk metallurgical samples to be taken, and investigate ground conditions along the mineralisation. The decline began on 24th November, 1988, with the portal located at the site of the 1973-74 costean; 850 m of decline and 212 m of sill development were completed on 14 October, 1989. Every round fired in the sill (approx. 3 m x 3 m x 3 m ) was geologically mapped and channel-sampled in detail. A main zone containing high-grade gold mineralisation could be traced along the entire length of the sill. However, it is off-set along its length by numerous small faults which typically cause displacements of 1-2 m. Several different styles of mineralisation occur within the main zone, and a strong correlation exists between the style of mineralisation and its related gold grade. Surface drilling continued throughout the period of undergrounddevelopment, exploring the deposit at depth, where the best intersection to date is 10 m horizontal width at 55.8 g/t Au. To the end of September, 1989, 106 holes plus 12 wedges have been drilled totalling 27,018 m. Exploration and evaluation expenditure has totalled $6.9 million of which $3.7 million was spent on underground development A resource estimate is presently underway and will be followed by a feasibility study in early 1990.

A5.5 LOW-GRADE ARCHAEAN METAVOLCANICS IN THE NORTHERN GAWLER CRATON : GEOCHRONOLOGY, MINERAL POTENTIAL AND REGIONAL IMPLICATIONS Wayne Cowley1 and Mark Fanning*2 2

1 Geological Survey of South Australia, Adelaide Research School of Earth Sciences, Australian National University

The Late Archaean - Early Proterozoic Mulgathing and Sleaford Complexes of the the Gawler Craton principally consist of deformed high crustal level granitoids and layered gneisses, and supracrustal sequences that have been metamorphosed to the granulite facies. The supracrustal sequences are represented by the Christie Gneisses of the Mulgathing Complex which occur in the northwest of the craton and contain a wider range of lithologies (banded iron formations, calc silicates and pillow basalts) than the Carnot Gneisses of the Sleaford Complex in southern Eyre Peninsula. Both of these gneissic groups have intermediate pressure granulite facies assemblages and record Rb-Sr total rock isochron ages that indicate that this prograde event, termed the Sleafordian Orogeny, occurred at 2400-2450 Ma. A notable exception in the Mulgathing Complex is meta-basalt which contain relatively undeformed relict pillows that have been metamorphosed only to amphibolite grade conditions.

Subsequent deformation and metamorphism has been superimposed on the Mulgathing and Sleaford Complexes by the Early Proterozoic Kimban Orogeny, which for the most part reached a peak at middle amphibolite grade. In the Mt Woods Inlier, however, granulite facies assemblages have been attributed to the Kimban events. The voluminous, essentially undeformed Gawler Range Volcanics (GRV) cropout extensively in the central part of the craton and postdate the Kimban Orogeny with a U-Pb zircon age of 1592 Ma. In 1982, diamond drilling by Esso Exploration intersected -450 m of acid and basic volcanics in DP-1 whilst searching for an Olympic Dam-style ore body in the northern Gawler Craton near "Millers Creek". Penological and geochemical studies indicated tenuous affinities with the GRV. The volcanics in DP-1 have been described on the basis of petrographical evidence as interlayered pink and grey dacitic tuffs and lavas and

Ill green-grey andesitic-basaltic lavas and breccias, over- on the basis of the 2560 Ma zircon age, but with a T lying brecciated and massive vesicular trachyte. of 2450 Ma. These data suggest that there may be a Subsequent limited total rock geochemical analyses younger magmatic episode within the sequence that has have been used to show that the dacite samples are not previously been recognised, represented by rhyodacitic in composition and that the basalt and rhyodacite dykes. Sm-Nd analyses of four andesite trachyte have andesitic compositions. Strong sericitic samples further confirm these interpretations. Two of and chloritic alteration is ubiquitous and fine carbonate the andesite samples have E of -0.2 and +0.4 at 2560 veining is common. Sulphides are sparsely distributed. Ma and T of 3045 and 2940 Ma. The third sample has The rocks exhibit a variable degree of foliation, but similar Nd isotopic characteristics to those inferred for despite this deformation, in places relict volcanic the rhyodacite dykes, whilst the fourth has a T model amygdales, feldspar phenocrysts, spherulites and age of 2728 Ma, intermediate between these two "end fragmental textures can be seen. members" and may represent a mixed magma or mixing In an effort to verify a proposed correlation between of sources. the DP-1 volcanics and the GRV we initiated a U-Pb The recognition of the volcanic rocks in DP-1 as zircon study. Zircon grains were separated from a Archaean substantially changes the view of the ancient sample of rhyodacite (373.6-376.5 m depth) that contains geological history of the Gawler Craton. These volcanics twinned plagioclase and embayed quartz phenocrysts. have been little affected by either the Sleafordian or The zircon grains are generally subhedral squat crystals Kimban Orogenies, the basement block intersected by that are colourless to very pale hyacinth in colour. DP-1 having constantly resided at relatively shallow Most crystals show little internal structure, though in crustal levels whilst the nearby Mulgathing Complex some there is interpreted to be growth zonation. Four and Mount Woods Inlier suffered granulite facies multi-grain and two single grain conventional U-Pb metamorphism. The volcanics are inferred to be now zircon analyses are discordant and define a discordia fault-bounded against these low crustal level blocks to regression line yielding an upper concordia intercept of the southwest and northwest, whilst to the east the 2558 ± 6 Ma and a lower intercept of 66 ± 22 Ma. The volcanics are bounded by the basement around Olympic single grain analyses are the more concordant, the zircon Dam and lie in the projected path of the "Olympic Dam samples having lost 22-23% of their radiogenic Pb. Tectonic Corridor". This suggests the possible presence Despite the significant discordance and thus the of a deep seated, potentially mineralising fracture system necessary extrapolation to an upper intercept with providing a source for Cu, U, Au and rare earths. concordia, the close agreement of all six analyses to the Volcanic rocks of similar age (2560 Ma) have not discordia trend suggests that the zircon grains been recognised in the Yilgarn and Pilbara Blocks in crystallised at 2558 ± 6 Ma. The lower intercept Western Australia. Cratonisation in these regions was approaches the origin inferring that the Pb loss occurred essentially complete by 2600 Ma with felsic plutonism near the present day. continuing sporadically until c.2500 Ma in the Yilgarn. Rb-Sr and Sm-Nd total rock analyses have Although direct comparisons cannot be made, there subsequently been carried out to further refine the may be potential for base metal sulphide mineralisation Precambrian history for this weakly deformed sequence in the DP-1 volcanics analogous to the Fe + Cu + Zn ± of bimodal volcanics. The Rb-Sr data for four rhyodacite Pb sulphide accumulations hosted by acid to basic samples show a linear trend on an isochron plot, imply- volcanics and volcaniclastic and pelitic sediments in ing an age of c.1900 Ma with an initial ratio of 0.712. the Yilgarn. Though the DP-1 volcanics are unHowever, the Sm-Nd data indicate a more complex mineralised, there is pervasive chloritic and sericitic sequence of events. Three rhyodacite samples, including alteration suggesting that mineralisation may occur the sample from which the zircon grains were separated, where there has been more intense hydrothermal activity. have Em in the range -1 to +1 at 2560 Ma and depleted The presence of weakly deformed volcanics with mantle model-ages (T ) of 2846,2960 and 3083 Ma. some affinities to the GRV, but demonstrably Archaean This confirms the Archaean age for the zircon in the in age highlights the necessity for further isotopic studies rhyodacite samples, i.e. the zircon grains are not an of isolated volcanic outcrops that have previously been inherited feature and furthermore the parental material lithologically correlated with the GRV. Additionally for the volcanics may have separated from a depleted the Nd model ages suggest that crustal material older mantle as long ago as 3000 Ma. Two other rhyodacite than 2600 Ma may be present in the northern Gawler samples, including the samples recording the apparent Craton. 1900 Ma Rb-Sr isochron, have E of +6 as calculated DM

112

A5.6 THE USE OF TRACE ELEMENT CHEMISTRY AND ELECTRON PARAMAGNETIC RESONANCE SPECTROSCOPY AS A GUIDE TO MINERALISATION J.C. van Moort Geology Department, University of Tasmania

Non metamorphic auriferous quartz shows a pronounced development of the electron paramagnetic resonance (EPR) peak at a characteristic effective spectroscopic splitting factor g = 2.0025. Barren quartz does not show this paramagnetism. This relation was worked out in detail for the Devonian Beaconsfield reef in Tasmania; the Devonian shear zones at Fosterville, Victoria; the Tertiary gold deposits in Waihi and Karangahake in New Zealand, and Rodalquilar in Spain; and the Cambrian gold-bearing volcanogenic massive sulphide deposits in western Tasmania. Statistical evaluation of the Beaconsfield and Fosterville deposits indicates that the gold content of the quartz (as determined by fire assay over lm length of core) correlates highly significantly with the concentrations of the trace elements Kb (Rb and Li, if present) and also with the intensity of the EPR peak as determined on microsamples of vein quartz.

The concentration Kb (Rb and Li) is caused by the presence of submicroscopic flakes of muscovite in auriferous quartz. The paramagnetism of the quartz is caused by substitution of Ge and possibly As in the Si0 lattice in combination with the ever-present A1 substitution. The first two elements can only be trapped in the quartz lattice during very rapid browth. Analyses of Ge and As in carefully cleaned quartz leads to the location of centres of (palaeo) hydrothermal activity. The relation with the Au content is of a secondary nature. The paramagnetism of the quartz is annealed out during metamorphism. The trace element content, however, remains and can be used in metamorphic terranes as a substitute for the fast EPR method. The table below summarises the chemical relation for some of the Beaconsfield samples. Reference 2

van Moort, J.C., Cohen, D.D., Russell, D.W. & Katsaris, A., 1990. Nuclear instruments and physical research (in press). Table 1 — Postion, intensity of the EPR signal and chemical composition of microcrystalline vein quartz, Beaconsfield. Concentrations in jigg" . From van Moort et al. (1990). 1

Code DDH letter

depth* m

A T U J K L

102.7 2.1 65 176.5(i) 1.5 tr 306.7(i) 4.5 tr 377 3.7 tr 690 21.1 73 728 4.9 158 490 tr 4.8

tr tr 41 12 27 24 20

tr tr tr tr 10.7 tr tr

tr tr tr tr tr tr 4.5

29 5 10 27 31 15 9

3.2 tr tr tr tr tr tr

513.4 513.6 514.5 518.8 519.4 552.1 695.5 701 892.2(i)

7.0 82 12.0 197 tr 11.0 6.2 39 10.0 129 12.0 62 10.7 504 7.7 293 50 631

tr tr 11 11 tr 20 tr 412 24

tr tr tr tr tr tr 8.0 9.2 84.1

4.0 109 2.7 9 3.4 49 3.0 8 3.2 9 3.2 18 3.0 52 4.4 48 tr 100

22.2 2.5 tr tr tr 7.8 4.3 tr 1.7

766.5 788.7 802

5.3 136 3.0 tr 21.6 17

13 20 40

tr tr tr

3.3 3.7 tr

16 8 5

tr tr 2.4

1.0

SI B15 B15 Bll Bll Bll B4A

Q ore zone

Z B4A P B4A Y B4A X B4A D B4 B4 C Bll G H Bll R ore zoneB15

M N 0

Bll Bll Bll

STD clear quartz, unknown origin

EPR cm

Au (by fire assay)

<0.005 <0.005 <0.005 <0.05 0.4 0.1 <0.005

tr -

22.9 2.5

tr -

tr tr 4.3 -

1.7

109 109 11.5 52 1.3 0.7 52 354 15

2.4

<0.05 <0.05 <0.05

* the depth indicates where the samples have been taken from drill core. The intersection of the ore zone is represented in the middle of the table.

113

A5.7 THE BROKEN HILL-TYPE DEPOSITS:THEIR GEOLOGICAL SETTING, MINERALISATION AND EXPLORATION R. Beeson Billiton Australia, Melbourne

The Broken Hill-type deposits are a distinct form of lead-zinc-silver deposit, that cannot be readily accommodated in either the stratiform, sediment-hosted or volcanogenic massive sulphide types. They are characterised and distinguished from other lead-zinc deposit types by the clastic and chemical sediments of their rock package, and by the form of the associated volcanism. Key distinguishing parameters are the changes that occur, both upwards through the stratigraphy and laterally towards the position of the lead-zinc ores, in the grain size, volcanic component and composition of the host rock sequence, and particularly in the components of the chemical sediment package. The last includes variations of iron formation fades, and the distributions of tourmaline rocks and manganiferous sediments. In addition the distribution of types of base metal mineralisation, particularly that of copper versus lead+zinc, follows a systematic pattern within the clastic and sediment package.

Two distinct sub-types are present within the Broken Hill type deposits, hosted by either calc-silicate rocks (Broken Hill, Zinkgruvan, Balmat), or by iron formations (Gamsberg, Aggeneys, Pegmont, Boquira, Dammberg). The sub-types have distinct rock associations and stratigraphic positions within the overall package. Geological, geophysical and geochemical exploration data all contribute significant information to assist in the search for new deposits of this type. The identification of position within the characteristic stratigraphic package on the basis of geology, mineralogy and geochemistry highlights parts of the sequence with potential for hosting mineralisation. Geophysical techniques, particularly magnetics and EM, can identify targets in many but not all cases, and are of considerable importance in unravelling complex stratigraphy.

A5.8 RECENT DEVELOPMENTS IN THE EVALUATION OF THE DUGALD RIVER ZINC/LEAD DEPOSIT, QUEENSLAND W.A. Sheppard and J.V. Main 1

CRA lsa CRA Exploration, Exploration, Mount Brisbane

The Dugald River zinc-lead deposit is located approximately 85 km NE of Mt lsa in northwest Queensland, Australia. The lode occurs within black slates of Middle Proterozoic age (Blake, 1987). The lode gossan extends 2400 m in a N-S direction, occurs partly within a steeply dipping shear-zone and locally cross-cuts the stratigraphy at a low angle. The sulphide lode extends to at least 1025 m vertically below surface. Recent drilling has defined a high-grade shoot at the southern end of the lode down to 300 m below surface. This shoot extends to 650 m below surface and is open below this depth. The lode is typically up to 5 m thick but can exceed 15 m thick within the shoot, being thickest at depth. Indicated resource is approximately 15 million tonnes at 14.6% zinc, 2.0% lead, 65 g/t silver in the high-grade zone from surface to 700 m depth. Discovery of the Dugald River gossan pre-dates 1881 as Jack (1898), who described it as a lead silver

prospect, noted the presence of pits and trenches on his visit of 1881. However, only in 1937 did significant prospecting and exploration commence. This work culminated with the first three holes into the deposit being drilled by the Queensland Geological Survey in 1939. These holes were aimed at testing the lead-silver rich part of the gossan below the oxidation base. In 1948, the Zinc Corporation, a predecessor company to CRA Limited, purchased the three freehold leases covering the Dugald River gossan and drilled a further 22 holes in the interval up to 1953. Most of these holes tested the lead-rich, near-surface southern shoot In the following 30 years a further 24 holes were drilled to intersect the lode, but these were widely spaced and were aimed at testing the entire extent of the lode at a nominal. 400 m spacing, this phase of exploration ceased in 1980 with the completion of hole DR49 (which intersected the lode at 1025 m vertically below surface), and

114

with the realisation that the lode was a zinc, rather than lead-silver body. Geological resource at the end of this work was 60 Mt x 10% zinc, 1% lead, 1 oz/t silver. Since the recommencement of exploration in mid1987, the focus of the drilling has been to establish the boundaries, geometry, mass and grade of a highgrade shoot at the southern end of the lode. A further 107 intersections of the lode have been made. Most are above 300 m with eight intersecting at depths greater than 300 m below surface. Thirty of these tested the oxidised part of the lode for supergenely enriched silver. Geological studies of the 1970s and early 1980s interpreted the lode as a stratiform, shale-hosted body of mineralisation hosted in a sequence of weakly deformed, right-way-up sediments. It is now thought that while the metals may be syn-depositional/syndiagenetic, it is the syn-deformational remobilisation of sulphides which has produced the high-grade mineralisation and that has controlled its location and geometry. The hangingwall stratigraphic sequence can now be shown to be overturned (Neudert, pers. comm.) and to both fine and contain increasing carbonate up sequence. Metamorphism reached upper green schist to lower amphibolite fades. The oldest sediments in the stratigraphic package hosting the lode are quartzites. The precursor lithology in interpreted to be clean well-sorted sands deposited in a shallow-water, moderate-energy environment. These pass upwards through finer sandy units and silts (now micaceous schists) into black silts and muds (now black slates). The lode occurs within the transition zone from these slates into the stratigraphically youngest argillaceous limestone. Calc-silicates which occur east and west of the lode are now identified as the product of syn-deformational alteration of the quartzitemicaceous schist-slate sequence. The Dugald River host rocks have previously been assigned to the Corella Formation (Blake, 1987). However, similarities between the revised Dugald River sequence and the host rocks of the Mt Isa and Hilton orebodies cast doubt on this and indicate a probable common host sequence, age and genesis for the three deposits. Regional strikes are N-S. Steep westerly dips predominate. Two major layer-parallel faults separate the immediate Dugald River host rocks from the

Knapdale quartzites to the west and the calc-silicates to the east. The package is folded in a syncline, the axis of which occurs east of the lode. A 15 m wide zone of massive ferruginous gossan and lead-rich gossan phases with breccia textures at the southern end of the lode is the surface expression of the high-grade shoot. Elsewhere slatey gossan predominates. Complete oxidation can extend to 30 m below surface but is generally less than 20 m deep. Within the sulphide zone the higher grade shoot contains three mineralisation types. Massive breccia mineralisation dominates the lode footwall. This ore consists of rounded fragments of vein quartz and silicified slate in a matrix of massive pyrrhotite, pyrite and/or sphalerite, this breccia is best developed below 200 m. Slatey mineralisation occurs predominantly in the hangingwall of the lode and is strongly sphaleritic with prominent pyrite aggregates but rare pyrrhotite. This mineralisation style includes massive sphalerite, sphalerite of wispy aspect and occasionally finely laminated pyrite. Pyrrhotite breccia forms the third mineralisation type. This breccia shows a similar textural range to that of the slatey mineralisation, but it has no consistent location with regard to footwall or hangingwall of the lode. Above 250 m depth the shoot dips approximately 75° to the west and plunges at a moderate angle northwards. Below 250 m depth the lode dips 45°-60° west and the shoot appears to increase in lateral extent To the south of the shoot the lode is predominantly pyrrhotitic, while to the north, slatey mineralisation predominates. It appears that the slatey mineralisation and pyrrhotite breccia mineralisation pre-date the massive breccia (R. Valenta, pers. comm.). Although there are fine-grained pre-deformational sulphides to support a syn-depositional/syn-diagenetic origin for the Dugald River zinc-lead metal, the location and geometry of the high-grade shoot zinc-lead mineralisation are the result of remobilisation during deformation. The host rock stratigraphy shows a similarity to the general sequence at the Mt Isa and Hilton deposits with the degree of deformation of the Dugald lode being the essential difference between it and the Mt Isa and Hilton deposits. References Blake, D.H., 1987. BMR Bulletin 225. Jack, R.L., 1898. GSQ Bulletin 10.

115

A5.9 VOLCANOGENIC MASSIVE SULPHIDE POTENTIAL OF THE EASTERN GOLDFIELDS PROVINCE, YILGARN BLOCK, WESTERN AUSTRALIA. P. J. McGoldrick *, C. P. Swager , A. Ahmat and P. Ruxton 1

' GSWA, Kalgoorlie Regional Office; GSWA Perth Billiton International, Port Moresby 2

Recent improvements in world base metal prices has stimulated renewed interest in exploration for volcanogenic massive sulphide (VMS) deposits. While the Eastern Goldfields Province (EGP) of the Yilgarn Craton (GSWA Memoir 3, in press) contains world class reserves of Ni, historically, with the notable exception of Teutonic Bore, it has produced only minor amounts of other base metals (Blockley, 1971; Marston, 1979). In contrast, elsewhere in the world VMS deposits in Archaean greenstone belts have been a major source of Zn and Cu, and to a lesser extent Ag and Au (Franklin et al., 1981). For example, the Abitibi belt in Canada contains dozens of clustered deposits ranging in size from a few thousand to more than ten million tonnes. Felsic and intermediate volcanics and related rocks are the immediate hosts to most of the Canadian Archaean deposits (ibid.), even though volumetrically they comprise only a small proportion of outcropping rock types. Cursory comparisons reveal similar abundances of such rocks in the EGP, hence, a shortage of potential host rocks cannot account for the paucity of known VMS deposits in the EGP. Furthermore, the last surge of base metal exploration in the EGP, during the 1970s, resulted in a number of technical successes in the search for VMS style mineralisation. For instance, Suisse-Aluminium Australia located large massive pyrite lenses containing only low Cu and Zn grades in felsic volcanics NE of Kanowna (Dept Mines WA, Open File M531). Unfortunately, the only economic deposit discovered at this time was Teutonic Bore in the northern part of the EGP. Teutonic Bore was a smallish deposit (1.4 mt, 16.4% Zn, 4.2% Cu, 1.2% Pb and 203 g/t Ag) with all the features of a typical syngenetic VMS deposit (Greig, 1984; Hallberg

and Thompson, 1985; Agnew, Clough and Vaughan, in prep.). Despite this lack of success during the 70s, we believe that a number of recent developments give cause for catious optimism that significant VMS deposits remain to be discovered in the EGP. These include a much better understanding of the geological evolution and tectonic setting of the EGP, resulting from detailed regional mapping by the GSWA and BMR, and thematic studies by Tertiary institutions and mining companies. Most of the EGP will be mapped at 1:100,000 scale for the first time within five years and precise ion-probe ages are slowly becoming available. Concurrent with this mapping the GSWA will undertake geochemical and petrographic studies of the felsic volcanic complexes to the E and NE of Kalgoorlie. Technological advances eg., low-cost multi-element geochemistry, air-core drilling, novel geophysical techniques, will permit different exploration strategies to be utilised. Finally, refinements in our understanding of the genesis of VMS deposits, resulting from studies of ancient and modern (eg., "black smoker") deposits will enable new and better exploration models to be applied to the search for these deposits. References

Blockley, J., 1971. GSWA Min. Res. Bull. 9. Franklin, J., Lydon, J. & Sangster, D., 1981. Econ. Geol 75th Anniv. Vol.: 485-627. Greig, D., 1984. Proc. Aus. IM.M 289: 147-156. Hallberg, J. & Thompson, J., 1985. Econ. Geol. 80: 19531964. Marston, J., 1979. GSWA Min. Res. Bull. 13.

116

A5.10 THE NEED TO ESTIMATE RECOVERABLE RESOURCES UNDER GEOLOGICAL CONSTRAINTS P. Carrasco and J.G. Angus RGC Exploration, Canberra

Investigations at a number of mining operations in Australia and the South-west Pacific have illustrated the sensitivity of resource estimates to "the support effect", mining constraints, estimation method, and the geological model. For selective mining operations they have illustrated the dependence of selection efficiency on the spatial structure of the grade, the method of estimating block grades, and local geology. They have demonstrated the sensitivity of the estimated "relative economic benefit" to mining selectivity and estimation method. For selective gold mining operations "in-situ resource" estimates generally bear little relationship to the corresponding recoverable resource. This is because they do not allow for selection inefficiencies and do not relate to a realistic degree of selectivity. "Manual" estimates assume an unrealistically high level of selectivity (the size of selection unit effectively being that of a drill core sample), while "ordinary kriging" estimates often assume an unrealistically low selectivity. For one gold deposit, which exhibited a high nugget effect and low grade-continuity, the estimated recoverable resource differed from a "manual" in-situ estimate by up to 40% in both tonnage and grade terms. The "Relative Economic Benefit" of mining an epithermal gold deposit was calculated on the basis of (1) ordinary kriging estimates of 50 x 50 x 5 m blocks and (2) multi-gaussian kriging of 2.5 x 2.5 x 5 m blocks (and selective mining of such blocks). The "economic benefit" calculated on the latter basis was more than 200% higher than that calculated on the former — the former unfortunately being the sort of basis on which feasibility studies are most commonly performed. The effectiveness of grade control is a function of selection capacity, the precisions and accuracies of sample preparation and assaying, the estimation

A5.ll

technique, the geological constraints applied, and the spatial structure of the grade. In one low-grade gold deposit, characterised by high nugget effects and low grade-continuity, the regression relationship between estimated block grades (from grade control sampling) and "real" block grades was such that, at an estimated grade of 0.5 g/t, the average bias between the two varied from +100% to +20%. The higher bias was estimated for discontinuous, spotty, mineralisation, employing a "manual" selection method. The lower bias was calculated for more coherent mineralisation — selection being based on estimates by ordinary kriging. To minimise the amount of ore sent to waste in such circumstances a regression correction must be employed when applying cut-offs to block estimates. For many gold deposits characterised by high nugget effects, sectional interpretations based solely on the raw assay grades of small core or chip samples can, because of the "support effect", give a misleading picture of the spatial distribution of ore-grade mineralisation on a scale relevant to the choice of mining; method or to geological modelling. Unless this effect is appreciated, this can lead to adoption of an incorrect geological model or even an inappropriate mining plan. Five deposits studied were each comprised of a number of geological domains of contrasting alteration styles, lithologies, or degree of weathering. Essential to a reliable resource estimate in each case was to compare the spatial structures of the grade within each domain and, where different, to determine the geostatistical characteristics of the boundaries separating them. A particularly important boundary in this respect is the outermost boundary (or envelope) of economic mineralisation.

GEOLOGY APPLIED TO DEPOSIT EVALUATION AND DEVELOPMENT — SOME EXAMPLES D.H. Mackenzie CRA Exploration Pty Limited, Canberra

Geology applied to successful development and mining of an ore deposit must provide simple quantifiable answers to the geologically complex combinations of dimensions, tonnes, grade, rock quality and treatment characteristics which are unique to each deposit Sound specific knowledge of these parameters and awareness of the business are the starting points.

"Every mine ... is a problem special in itself' (H.C. Hoover). The Broken Hill orebody at ZC Mines (now part of Pasminco) has seven ore lenses which exhibit a wide range of parameters. There is bimodality of grade distribution in every ore lens with a deficiency in the critical cutoff grade zone around 7-10% Pb + Zn. There

117

is wide variation in milling and concentrating characteristics between grade classes and ore lenses. These were key geological factors which contributed to severe losses sustained in 1986-87 when metal prices were also at an all time low. The geological realities were not fully conveyed by geologists, nor applied realistically at the planning stage, nor appreciated adequately by management At Cleveland tin mine the pre-1970 interpretation held that the multiple sulphidic ore lenses were segments of one lode bed which had been folded tightly and then imbricated along steep bedding-parallel thrusts to form a shallowly plunging body. Inferred cumulative movement on the thrusts amounted to hundreds of metres. This concept governed where and how one might explore for new ore. Mapping and re-interpretation in new openings in the mid-1970s showed that bedding thrusts did not exist The most important structures were a series of inconspicuous low angle sinistral strike slip faults with movements of less than 100 m. This simple observational concept assisted in renewed exploration and development of faulted continuations of the ore lenses down dip for an additional 200 m depth, rather than down plunge. The Mount Fubilan porphyry copper deposit at Ok Tedi in PNG has a leached auriferous cap 10-290 m thick which overlies the oxide copper zone. The gold cap was treated by the cyanide leach-CIP process which is sensitive to above average levels of cyanide soluble

copper, in such minerals as chalcocite, bornite, digenite and malachite. Presence of cyanide soluble copper reduces recovery of gold and increases the use of expensive peroxide to neutralise the tailings. In mining the leached cap it was found that there were erratically distributed small (1 m ) siliceous pods which had resisted leaching and contained cyanide soluble copper minerals. Grade control grid drilling at 10 by 10 m had not been adequate to locate these pods. Their presence adversely affected recoveries and dictated a significant change to grade control practice to direct the pods to the copper flotation circuit. The Jason lead-zinc property in the Yukon was evaluated when a resource of about 4 Mt of 14% (Pb + Zn had been indicated. Careful relogging of drill core suggested that a raw grade of 13.5% Pb + Zn was realistic. The presence of 1.4% Pb + Zn as non recoverable oxide which would also lower the recovery of the remaining sulphide lowered the effective grade. In addition extensive porous, leached and partially oxidised ground in and adjacent to the ore zone suggested that at least a 20% mining dilution was likely, thus reducing the available grade to about 60% of the original estimate, a significant change to the economic viability especially in such a remote region. The lesson is that the geologist needs to know the business he or she is in; needs to know the deposit; must recognise the implications of this knowledge; and must tell someone about it clearly and simply. 3

A5.12 INJECTING GEOLOGIC UNCERTAINTY INTO RESOURCE ESTIMATION: THE PORGERA GOLD DEPOSIT, PAPUA NEW GUINEA Neil Schofield FSSI Consultants (Australia) Pty Limited, Sydney

Geologic features may play an important role in resource estimation particularly if such fratures form natural boundaries to mineralisation that can be relatively easily recognised and followed during mining. However, at the exploration and prefeasibility stage of a project, there may be insufficient information available to confidently define and map such important geologic features. Part of the process of resource estimation therefore is to build a model of the mineralisation which incorporates the geologic information about certain geologic features but also accounts for the uncertainty inherent in the geometry of such features. Probably the most commonly used method of modelling geologic boundaries to mineralization is to define a unique "hard" boundary either by digitisation or with polygons. The boundary is interpreted to separate significantly different styles of mineralisation which exist exclusively on either side of it This approach to modelling is suitable to a relatively small number of highly continuous and very predictable geologic features such as faulted boundaries.

For less predictable features such as the Domain 77 mineralisation geometry at Porgera, this polygonal modelling approach is inappropriate because it does not take into account the uncertainty inherent in the knowledge of this geometry. Domain 77 mineralisation is extremely high grade and strongly structurally controlled compared to the surrounding, more pervasive lower grade mineralisation and constitutes around 11 percent by volume of the mineralised zone of interest Indicator Kriging theory in geostatistics provides a methodology for analysing mineralisation with complex geometry and building models which incorporate a direct measure of the uncertainty related to such geometry. The imposition of "hard" boundaries is not required. The results of indicator kriging may be directly interpreted as the probability that a particular mineralisation style exists at a given point location in the deposit or in the case of a block location, the proportion of that mineralisation style contained within the block. The application of the indicator kriging approach to the analysis of Domain 77 at Porgera provided maps of

118

the mineralisation which demonstrate clearly the effect of geologic uncertainty in the estimates of insitu tonnage and grade. In most open pit mining situations, these maps provide a good basis for assessment of the reserves. However, for underground mining envisioned at Porgera,

conditional simulation may provide more useful maps of the mineralisation geometry. References

Handley, G.A. & Bradshaw, P.W.D., 1986. Gold86:416-424. Journel, A.G., 1983. Math, Geol. 15: 445^68.

A5.13 ORE RESERVE CALCULATIONS IN SHEAR ZONE HOSTED DEPOSITS John L. Baxter and Matthew G. Yates Continental Resource Management Pty Ltd, Rivervale, WA

The fallacy of calculating ore reserves by cross sectional or long sectional methods in shear zone hosted deposits is clearly demonstrated by analysis of both strike-slip and dip-slip terrains. Most geostatistical techniques designed for the estimation of ore reserves depend on a relatively uniform distribution of grade and thickness within an ore deposit environment in shearhosted mineralisation this rarely occurs. The distribution of mineralisation is controlled by the kinematic and geometric variation within the shear zone. While the kinematic variation is dependant primarily on the temperature and consequently the depth of formation of the deposit, the geometric variation appears to be relatively insensitive to this function. Shoot mineralisation will be defined wither by the intersection of secondary shear planes (brittle deformation) or the stretching lineation of the mylonitic fabric (ductile deformation). In genral, application of the shear zone model to ore reserve estimation tends to reduce tonnage, however the average grade is usually increased substantially. The geometry within shear zones is fractal. Ore reserves aren't. It is essential that the geometric analysis be referred back to a plane. The model we have employed successfully in both strike-slip and dip-slip arrays is to establish the geometric relationship within the shear zone and project this onto a plane which contains the intersection lineation between secondary shear planes in the brittle or brittle-ductile domain or the stretching lineation in the ductile domain. In general these planes are similar, but if mineralisation is in secondary shear planes there may be some necessity to have more than one plane or reference. It is essential to make some estimate of the number of intersection shoots that occur in the brittle domain. This can be done by analysing

grade results in open pit or underground openings and ideally is supported by geological mapping. When estimating resources from drilling this is sometimes difficult However, it can be undertaken using oriented drill core to produce satisfactory results. When calculating the ore reserves within shear hosted mineralisation it is important to ascertain whether the lodes are located within a shear plane or whether it is at an intersection of secondary planes. In some deposits only one or other of parts of the array will be developed. For example, at Sand King in the Eastern Goldfields, only the extension vein array has developed. In this case it is necessary to understand not only the geometry but the size of each of the extension arrays. We have had success in estimating a proportion of the shear array that contains mineralisation by mapping existing mine openings, estimating the proportion of the lode that is mineralised using statistical analysis of drill data and mine openings, or as a last resort the percentage of drill holes through a shear array which contain substantial mineralisation. In the case of shoots it is more difficult It is invalid to exclude high grade values from ore reserves as shoots within the shear array are likely to contain a substantial porportion of the metal resource. The number of shoots that occur within a deposit can be determined either from mapping or drilling. Ideally a hole drilled parallel to the predicted movement direction in a brittle or brittle-ductile environment will identify the number of shoots likely to be encountered, this is one of the most difficult aspects of estimating ore reserves in shear zones hosted deposits, however it is a requirement that this geometric and spatial analysis be undertaken before ore reserves is shear hosted deposits are calculated.

119

A5.14 GEOLOGICAL MODELLING IN THE EVALUATION OF THE KINTYRE URANIUM DEPOSIT R.L. Andrew and G.D. Price CRA Exploration Pty Limited, Belmont WA

The Kintyre uranium deposit consists of high grade stratigraphic model prevailed throughout 1987. uranium veins containing pitchblende in a deformed Detailed structural mapping, relogging of drill core, metasedimentary host sequence. The early recognition reinterpretation of drill sections and compilation of of Kintyre as a high grade vein deposit associated with geological flitch plans followed in 1988. These data, a Proterozoic unconformity suggested first of all that together with evidence from adjacent orebodies other similar deposits should occur in the Rudall suggested that Kintyre was located within an Fj antiform province. The Mt Cotten uranium-copper prospect and that the ore was both stratabound and strongly 100 km to the south contains an ore grade uranium folded by F folds. Infill drilling to test the folded intersection and regional exploration is in progress to stratabound model confirmed the vein sets to be almost locate additional uranium ore of this style. The uncon- planar, as originally modelled. Although apparently formity-vein type model, from Canadian and Northern concordant within the dipping limbs of the structure, Territory experience, also dictated that evaluation uranium veins are in fact axial planar in S and discordant drillholes be closely spaced to obtain an optimum to bedding in the main antiformal closure. The recognition of this S fabric on several scales accorded resource estimate. The evaluation of the Kintyre uranium deposit since well with subsequent drilling information at Kintyre. It is the S j fabric which primarily controls the vein 1985 was strongly influenced by the evolving interpretation of both lithological and structural control as sets of uranium ore at Kintyre and at its satellite oredrilling progressed. In 1984, before any drilling bodies. Local realignment of veins within the steeply commenced, the potential roles of both bedding (S^ dipping S cleavage occurs on a small scale and is not and a steep, penetrative cleavage (S^ in controlling evident on a scale of 5 m and above. Deep diamond drilling below the Kintyre orebody mineralisation were recognised but not understood. Initial drilling was successfully directed along the in 1989 confirmed the presence of an antiform, but in a azimuth of the S and S intersection at a target lying recumbent rather than an upright style as previously below outcropping uranium silicates. The ore con- interpreted. This difference in style had no effect on the figuration on thefirstdrill section suggested that several orebody model. Throughout most of the Kintyre orebody, sets of high grade uranium veins were concordant with the Sj fabric essentially coincides with bedding so the bedding and separated by intervals of barren host rock. geometry of the 1985 interpretation has not changed This model of stratigraphic control conflicted in detail materially. On the scale of 200 m at Kintyre, the vertical with observations on drill core showing individual pitchblende-bearing veins to lie within the penetrative projection of uranium ore at a 0.5 kg/t U 0 cutoff cleavage (S^, cross-cutting the bedding (S^. As diamond closely follows the strike of the major lithologies and in drilling on several sections 25 m apart advanced during particular the host chlorite schist/metachert unit The 1986, the stratigraphic model became more compelling early model of gross stratigraphic control was therefore as uranium ore was traced along strike east and west of correct on the scale of the entire orebody. In retrospect, the initial azimuth selection for the the original drill section. Preliminary resource calculations were based on a stratigraphic model, but it Kintyre drilling grid was ideal for the evaluation drilling of the orebody and as a basis for estimating the tonnage became evident that the potential for S to produce steeply dipping ore shoots should be tested. A pattern of and grade of the resource. Geological modelling has inclined, closely spaced drilling perpendicular to S and played an important overall role in understanding the oblique to the Kintyre grid was designed to intersect a ore controls and in guiding the evaluation of the Kintyre known high grade vein set. This structural drilling uranium resource. demonstrated no S control on a macro scale so the 2

120

A5.15 THE SELECTIVE USE OF ISOTOPIC DATA IN THE EVALUATION OF SOME PRECAMBRIAN METALLIFEROUS ORE DEPOSITS N. J. McNaughton-, S. D. Golding' and D. I. Groves*

Key Centre for Strategic Mineral Deposits, University of Western Australia University of Queensland 2

Research over recent years has built up a substantial database of isotopic information on Western Australian Precambrian gold and base-metal deposits. Together with case-histories from the literature, it is now timely to review the usefulness of specific isotopic data in developing exploration models and in the evaluation of the economic potential of metalliferous ore prospects. Lead isotopic data for galena or pyrite can discriminate between epigenetic and syngenetic sulphide bodies in the highly prospective Archaean Norseman-Wiluna greenstone belt Sulphide Pb isotopic data may indicate: (i) the source region for metals (i.e. greenstones/mantle, lower or middle crust); (ii) relative timing of mineralisation with respect to other deposits in the region; and (iii) if remobilisation of Pb (and Au) occurred at younger times due to structural reactivation (McNaughton, 1987; Dahl et al., 1987). Together with reliable geochronology, penological and geochemical information, Pb isotopic studies of Precambrian ore systems constrain genetic models and hence exploration models for specific deposits. The isotopic signatures of gangue and wallrock alteration carbonates (C and O isotopes) and quartz veins and wallrocks (O isotopes) from greenstone-hosted Au deposits reflect the source of the fluid phase and fluid-rock interactions. Carbonate formation is also common during alteration which predates mineralisation, and the isotopic signature of the introduced carbon is often diagnostic of the alteration style and source of carbon, and different to that for Au-related carbonate alteration (Golding et al., 1987; Groves et al., 1988). This is important for the recognition of Au-related alteration, as distinct from other alteration styles, during prospect assessment. In some cases, mineralisation of a particular structural style in an ore mining district may have a distinctive isotopic signature (e.g. Golden Mile, Victory-Defiance, Golding et al., 1987; Groves et al., 1988). In other examples, there may be systematic variations in the isotopic composition of carbonates around Au lodes. For example, in massive, carbon-poor rocks such as granitoids, a carbonate alteration halo surrounding Au-mineralised shear zones show a systematic isotopic trend to higher 8 C away from mineralisation, and this trend can be recognised from diamond drill-hole samples on 100 m spacing about known economic mineralisation (e.g. Lawlers, Cassidy et al., 1988). In massive mafic rocks, such as dolerites, wallrock silicates proximal to mineralisation generally have high 8 0, whereas vein quartzes have low SO within high-grade ore zones. For distances up to 100 m 13

from mineralisation, wallrocks show a systematic lowering and vein quartzes show a rise in 5 0 (e.g. Mt Charlotte [Golding & Wilson, 1987]). Both features provide directional indicators to mineralisation and a distinctive isotopic signature for high-grade mineralisation. Marine carbonates in Proterozoic terrains have a distinctively high 8 0 which becomes progressively lower in carbonates which have interacted with mineralising epigenetic fluids (Golding et al., 1990), and provides a distinctive signature in alteration haloes on >lkm scale around known Au deposits. Volcanic massive sulphide deposits, with modified seawater as the mineralising fluid, are characterised by distinctively low 8 0 in gangue silicates associated with mineralisation, and a large alteration halo (>1 km; e.g. Kuroko ores, Green et al., 1983) which shows a systematic increase in 8 0 towards background values with increasing distance from mineralisation. Further, the light isotopic signature associated with mineralisation is largely preserved through metamorphism and is detectable in metamorphosed greenstone terrains. In summary, selective stable isotopic analyses on diamond drill-hole samples during prospect assessment have the potential to assist in the recognition of alteration haloes around important mineralisation, and to provide directional indicators to the location of mineralised zones if they are narrowly missed during drilling. Sulphide Pb isotopic data may distinguish between syngenetic and epigenetic mineralisation, and help constrain genetic models and hence exploration strategy in Precambrian terrains. Relative to diamond drilling, selective isotopic data is inexpensive and a cost-effective adjunct to deposit evaluation. References 18

Cassidy et al., 1988. Geol. Dept & Extension, U.WA. Publ. 12: 185-194. Dahl et al., 1987. Geol. Dept & Extension, U.WA. Publ. 11: 189-201. Green et al., 1983. Econ. Geol. Mon. 5: 395-411. Groves et al., 1988. Nature 331: 254-257. Golding et al., 1987. Geol. Dept & Extension, U.WA. Publ. 11: 215-238. Golding et al., 1990. This volume. Golding & Wilson, 1987. Geol. Dept &. Extension, U.WA. Publ. 11: 203-213. McNaughton, 1987. Geol. Dept & Extension, U.WA. Publ 11: 181-188.

McNaughton et al., 1988. Geol. Dept & Extension, U.WA. Publ. 12: 195-207.

121

A5.16 A REVISED ZONING MODEL FOR THE ZEEHAN Ag-Pb-Zn-Sn FIELD, TASMANIA John A. Anderson University of New England!Aberfoyle Resources Limited, Norwood, SA

Asymmetric lateral zoning from pyrite to siderite veins ("lodes") across the Zeehan field was interpreted to demonstrate an easterly vector of fluid flow from a source within the stanniferous Heemskirk granite (Twelvetrees & Ward, 1910). Both & Williams (1968) proposed a later cupola source for an outlier of stanniferous pyrite lodes whereas Solomon (1981) suggested the fluids of a single symmetrical hydrothermal system interacted with limestones to produce the asymmetry. Three stratabound cassiterite deposits were recently delineated within the stanniferous outlier (Anderson, 1989). Substantive evidence of a cupola locus (Anderson, 1986) and a structurally controlled and vertically zoned hydrothermal system enables the pattern to be explained by a revised three-dimensional model of vertical zoning. The Devonian magmatic-hydrothermal event was facilitated at all scales by a wrench fault regime. The Heemskirk and Meredith stocks intruded opposite margins of a NW zone on D folding and sinistral faulting at the truncation of NNE D sinistral faults. The ENE Heemskirk-Renison granitoidridge(Large, 1986) and a swarm of "Red" phase dykes coincide with D extension and wrenching orientations respectively. D primary shearing reactivated the Riedel component of D creating a rhomboidal pattern of dilational jogs which became the loci of cupolas, fluid flow and mineralisation. A medial D trend of stanniferous and pyritic outliers, largest lead lodes and cupola indicators is interpreted to overlie a 7 km NNW granitoid ridge rising from the subcropping NE corner of the Heemskirk stock. Cusps peak 500-1500 m beneath the present surface at 3-4 km 2

spaced D intersections. Incorporating S isotope data of Both et al. (1969), mineralising magmatic fluids are interpreted to have flowed southwards both along the ridge and towards the Heemskirk fieldfromone source at the ridge's northern end. Situated within the strongest D -D disruption and cupola aureole, the stratabound deposits abut D -infilling pyrite stannite siderite lodes and pyrite silica cassiterite lode-filled jogs. Delicate replacement of Precambrian basalts and evaporatic mesitites constitute a small resource. The principal Severn deposit is hosted by Cambrian volcaniclastics as replacements of carbonatesmeared bedding slips or stockwork-veined dilational jogs within the drag zone of a sinistral-normal D fault The single mineralisation sequence produced a vertical zonation of lower pyrrhotite cassiterite replacements overlain by pyrite cassiterite lodes and replacements than a lode sequence of pyrite stannite to sphalterite+exsolved stannite to upper siderite then late galena Ag-sulphosalt breccia infill. The vertical transition from pyrite to siderite persists across the field at an east-dipping sub-horizontal level, causing the illusion of asymmetric horizontal zonation. References l

Anderson, J.A., 1986. Rec. BMR Aust. 1986/10: 1-2. Anderson, J.A., 1989. Geol. Soc. Aust. Spec. Publ 15:434-438. Both, R.A., Rafter, T.A., Solomon, M. & Jensen, M.L., 1969. Econ.Geol. 64: 618-628. Both, R.A. & Williams, K.L., 1968. /. Geol. Soc. Aust. 15: 217-244. Large, R.R., 1986. Rec. BMR Aust. 1986/10: 40. Solomon, M., 1981. Econ. Geol. 76: 194-208. Twelvetrees, W.H. & Ward, L.K., 1910. Bull. Geol. Surv. Tasm. 8.

A5J7 INTEGRATED DATA BASES AND EXPERT SYSTEMS IN EXPLORATION GEOLOGY Joseph H J. Leach and Peter Dahlhaus Bailorat Centre for Remote Sensing, Bailorat CAE

One of the major problems facing modern geological exploration teams is the vast volume of data that has already been collected or which can be very rapidly collected from instrument sources. This problem will be greatly exacerbated in the future as the flow of data from various instrument sources increases. In particular, detailed geophysics and modern imaging scanners can collect more data in a few days than a conventional exploration team would collect in a year. The first response to the beginnings of this high data flow was the use of multivariate statistical analysis. This has been used successfully since the late 1960s. However, the

recent increase in the volume and types of data have made it difficult to meaningfully apply such analyses to the full available data set. This problem has led to the development of geological expert system software of increasing complexity. The first of these systems was Prospector, the development of which began in 1975. Prospector has "logic rules" linked in a hierarchial network.The rules are statements of evidence and hypotheses derived from interviews with experienced geologists. It was found to give poor results in practice both because uncertainty was propagated through the system by "fuzzy" logic

122 and because of the limitations of the original knowledge base: it did not transfer well from the USA to Australia because its rules were based on American experience. More recent geoscience systems are muPROSPECTOR and PROSPECTOR III, both microcomputer versions of Prospector; GEO VALUATOR, for mineral resource appraisal; EXPLOR, for oil exploration; and FINDER, used to identify massive sulfide deposits through geochemical data. GEOMYCIN and "The Deciding Factor" avoid the problem of limitations in the original knowledge base by establishing "shells". These do not contain a knowledge base but consist of operative mechanisms for knowledge representation and probability updating.

and verifying new ideas. Such a system needs to be built from the data base up rather than from the model down. Some such data base systems are already being developed overseas. SPANS is an example of a system used by the geological survey of Canada to combine remote sensing data, geochemical sample data, and geological maps to assess high probability areas for mineral exploration. The Intelligent Geological Information System (IGIS) project at Ballarat CAE is designed to take this approach to its logical conclusion. It consists of three stages. The first stage is to develop an integrated geological data base. This is done by converting all data, including published map data, to an image format. These data sets are then geocoded and stacked to form an integrated All of these systems are developed on the same philosophy: a rule based structure which requests specific image data set. Such a data set can be accessed by the geologist as image layers but the system has no inherent information be retrieved from a data set Although such intelligence. This is the current stage of the project. systems are valuable as training tools, they have a During the second phase of the project, a rule based number of drawbacks in operation. These include: software system will be added which will be able to 1. They still do not make optimal use of the vast data recognise separate geological entities within the data set available to the modern exploration team. base and distinguish between those features that have 2. The rules may not be adequate to cope with a novel been previously mapped and recognised and those that geological occurrence. 3. If used too rigorously, they could inhibit the develop- have not. This system would be interrogated in ment of intuition and judgment in young geologists. geological english (e.g. show plutons, dykes etc.). During the final phase of the project, a further This could seriously limit the industries ability to "expert" layer or geological knowledge base will be find new types of deposits. added. This layer would contain exploration model There is a philosophical point also. In all of these parameters and could be used to interrogate the data set systems the judgment lies with the machine and the at this level (e.g. show areas prospective for epithermal human is used for data retrieval. This seems to us to be gold). the reverse of the optimal arrangement. The optimal Whether or not the college succeeds in fully system would be one which not only allows the geologist developing IGIS, systems such as this will become access to an extensive geological knowledge base but also facilitates his access to the field data base, allowing necessary and routine exploration tools in the early years of the next century. him to "browse" through the data following hunches

123

A6: Mafic Magmatism and Associated Mineralisation Convenors: 5-5. Sun and C. Ballhaus

A6.1 ASPECTS OF PHYSICAL VOLCANOLOGY OF KOMATIITES AND ASSOCIATED NICKEL MINERALISATION, YILGARN BLOCK, WA R.E.T. Hill *, MJ. Gole , SJ. Barnes and S.E. Dowling 1

' CSIRO, Perth Geochemex Aust. Inc., Perth Hunter Resources, Perth 3

The stratigraphy of the Norseman-Wiluna greenstone and covers >7000 km. It exhibits petrological belt of the Archaean Yilgarn Block of Western Australia characteristics similar to those of the lenticular bodies. contains intervals in which komatiites are the The Formation is comprised of a lower zone of olivine predominant lithology. At several locations these are cumulates dominated by olivine adcumulate and an hosts to significant nickel sulphide mineralisation. upper zone of layered olivine-orthocumulate-gabbro Komatiite lithologies range from olivine adcumulates units. The features of the olivine adcumulates are indicathrough orthocumulates to spinifex textured flows. In the northern half of the belt there are two different tive of crystallisation at low degrees of supercooling styles of komatiite volcanism. In the Agnew-Wiluna from rapidlyflowingkomatiite lava. The differences in area olivine adcumulate bodies, lenticular in cross shape between the types of adcumulate bodies reflect section, form zones of thickening within regionally the nature of theflowover which the komatiites flowed. The adcumulate lenses crystallised in thermal erosion correlatable units of olivine orthocumulates and spinifextextured flows, e.g. Mt Keith, Perseverance. In contrast, channels akin to lunar rilles, that formed during the olivine adcumulate forms extensive sheets exemplified continuous eruption of superheated lava onto a by the Walter Williams Formation in the Siberia-Menzies dominantly felsic volcanic flow. In contrast, the area in the central part of the Norseman-Wiluna belt geometry of the adcumulate sheets results from the The olivine adcumulate bodies are extrusive and form prolonged unrestricted sheetflowof lava on to a more integral parts of komatiite lava sequences. The lenses refractory mafic volcanic floor. exhibit gradational contacts with laterally equivalent Several of the olivine adcumulate lenses host spinifex-textured flows, mineralogical, textural and disseminated nickel sulphides. These deposits are compositional layering; and cyclic variation in olivine believed to result from the precipitation of olivine and composition. Their lower contact is consistent with sulphides in cotectic proportions fromflowingkomatiite intermediate-felsic volcaniclastic rocks. At Perseverance lava. This origin is supported by the consistency in and Honeymoon Well, the lenses transgress felsic flow nickel tenor (0.5-0.7 wt % Ni), and patterns exhibited rocks and rest upon lower komatiite sequences. Where by the distribution of PGE. These deposits have intact, such as at Mt Keith, the upper zones of the previously been classified as "Intrusive-Dunite bodies are characterised by branching olivine harrisites Associated", however it is now clear that they are and zones of cyclic fractionation exemplified by the extrusive in origin and differ from deposits such as those at Kambalda in their mechanism of emplacement sequence olivine, clinopyroxene, plagioclase. The Walter Williams Formation is 600-800 m thick in a different volcanogenic environment

124 A6.2 ARCHAEAN KOMATIITE VOLCANISM, GROUND MELTING AND SYNVOLCANIC NICKEL MINERALISATION K.M. Frost* and D.I. Groves Key Centre for Strategic Mineral Deposits, University of Western Australia

The Archaean Yilgarn Block of Western Australia contains concentrations of Fe-Ni-Cu sulphides that occur in two main associations: (i) well stratified disseminatedmatrix-massive sulphides in the basal zones of komatiite flow units (e.g. Kambalda, Widgiemooltha, Windarra), and (ii) disseminated to blebby sulphides in coarsegrained, olivine adcumulate, dunite bodies (e.g. Mt Keith, Betheno, Agnew). The well preserved spinifex and cumulate textures in the komatiite hosts prove an extrusive origin, but the poor exposure and lack of detailed research on the dunite hosts led Marston et al. (1981) to infer an intrusive origin. However, recent studies in the Agnew area have challenged the latter suggestion by showing a lateral correlation between dunites and komatiite flows (e.g. Barnes et al., 1988), and petrological/geochemical comparisons between the host rocks for the two deposit types suggest a common komatiitic parent (Donaldson et al., 1986). This is particularly important in view of the suggestion that significant ground melting accompanied the emplacement of the mineralised komatiitic dunite bodies (e.g. Hill et al., 1987). This paper presents the results of recent field-based studies on the Kambalda-type deposits in the Kambalda, St Ives and Widgiemooltha areas, which indicate that localised ground melting occurred beneath mineralised komatiite lavas (Frost & Groves 1989; Evans et al., 1989). The numerous nickel ore shoots that comprise the Kambalda nickel field are considered the type-examples of komatiitic peridotite-hosted nickel deposits, and they display clear evidence for the segregation and accumulation of dense immiscible sulphide-rich melts including: (i) the restriction of sulphides to the basal sections of lowermost komatiite flows, (ii) stratification of ore zones with progressively lower disseminatedmatrix-massive sulphide zones, and (iii) sulphide and oxide mineralogy and chemistry compatible with crystallisation from high-temperature komatiite magmas. The deposits also show a strong volcanic control on the distribution of nickel sulphides, particularly the confinement of mineralisation to highly elongate footwall embayments (i.e. trough structures) at the base of laterally extensive komatiite sequences. Gresham & Loftus-Hills (1981) noted major differences between the volcanic associations of mineralised and unmineralised sections of the lowermost one to four flows. Ore environments are characterised by thick, highlymagnesium, cumulate-dominated flows, partially confined to trough structures, and contrast with thinner flows and abundant interflow sulphidic sedimentary units in unmineralised flanking environments. Lesher et al. (1984) interpreted ore environments as active lava

channels in which there was substantial turbulent flow of komatiite melt along a primary topographic depression, whereas flanking-ore environments represented relatively lower volume flow and quiescent flow conditions in levee-type positions. The source of sulphur and the timing of sulphur saturation relative to emplacement of the komatiite hosts are critical to resolving the genesis of the Kambaldatype nickel deposits. However, it is difficult to differentiate between mantle- and crustally-derived sulphur in any Archaean deposits, and sulphur isotope ratios of sulphides from nickel ores and sulphidic sediments are indistiguishable. Lesher & Groves (1986) concluded from theoretical and experimental considerations that primary komatiite magmas are unlikely to be sulphur saturated on eruption, and further suggested that the assimilation of sulphidic sedimentary rocks initiated sulphur saturation and the separation of immiscible sulphide-rich melts. Renewed interest in this mechanism as an ore-forming process relates to the fluid dynamic studies of Huppert et al. (1984), that predicted extensive melting of floor rocks by low viscosity and turbulently convecting komatiite lavas. There is little doubt that the parent magmas of the komatiites were contaminated with continental crust during ascent (e.g. xenocrystic zircons and Sm-Nd mixing arrays), but evidence for in-situ contamination during eruption and lateral flow is largely indirect (e.g. absence of sedimentary units in ore environments). However, there is now convincing evidence for ground melting of footwall lithologies in ore zones. For example, interspinifex sulphides in hanging-wall ore positions were probably formed by the melting and selective replacement of the groundmass interstitial to former olivine spinifex-textured blades by sulphiderich melts in the overlying komatiite flow (Groves et al., 1986). Further, detailed mapping of Foster nickel ore shoot, St Ives, shows about 5 m of truncated stratigraphy (sulphidic sedimentary and basaltic rocks) directly underlying massive sulphides (Evans et al., 1989). Most importantly, ocellar units in the lower komatiite flows are interpreted as modified sediment melts (Frost & Groves 1989, McNaughton et al., 1988). The latter evidence is potentially the most conclusive for proving ground melting of sulphidic sedimentary rocks by komatiite lavas. Thin felsic units (0.1 to 5 m thick) with distinctive ocellar structures, termed ocellar units, occur towards the top of the lowermost komatiite flows in the Kambalda, St Ives and Widgiemooltha areas. Ocellar units show arestrictedoccurrence, parallel to mineralised troughs, in komatiite flows at the transition from ore to

125

flanking-oie environments. Ocelli, the main component many younger Fe-Ni-Cu deposits, the Kambalda-type of these units, are spherical (0.1-2 cm in diameter), nickel deposits appear to have a non-magmatic source albite-phlogopite-rich and enclosed in a chlorite-rich of sulphur. matrix. Frost & Groves (1989) demonstrated that the References intricate coalescence structures and relict skeletal Barnes, S.J., Hill, R.E.T. & Gole, M.J., 1988. J. Petrology, 29: 305-331. textures, in conjunction with element partitioning between ocelli and matrix, favour a silicate liquid Donaldson, M J., Lesher, C.M., Groves, D.I. & Gresham, J.J., 1986. Mineral. Deposita 21: 296-305. immiscibility origin. D.M., Cowden, A. & Barratt, R.M., 1989. In Ocellar units have high concentrations of in- Evans, Prendergast, M.J. & Jones, M.J. (eds): Magmatic Sulphides compatible elements (e.g. REE, Zr) at levels that are —The Volume. IMM, London: 215-219. inconsistent with fractionation from komatiite lavas. Frost, K.M.Zimbabwe & Groves, D.I., 1989. In Prendergast, M.J. & The mineralogy and bulk chemistry of these units, Jones, M.J. (eds): Magmatic Sulphides — The Zimbabwe particularly high Zn contents (up to 1000 ppm Zn), are Volume. IMM, London: 207-214. diagnostic of sulphidic sedimentary rocks. An important Gresham, J.J. & Loftus-Hills, G.D., 1981. Econ. Geol. 76: difference is the low levels of Fe, S and chalcophile 1373-1416. trace elements (e.g. Cu, Ni, Co) in ocellar units, that is Groves, D.I., Korkiakoski, E.A., McNaughton, N.J., Lesher, C.M. & Cowden, A., 1986. Nature 319: 316-319. elements that would strongly partition into immiscible sulphide-rich melts. Therefore, geochemical data support Hill, R.E.T., Gole, M.J. & Barnes, S.J., 1987. Excusion Guide Book No. 1. Geological Society of Australia, Western the interpretation that ocellar units are modified Australia Division, Perth: 74 pp. sediment-derived melts. The chemical signature of the Huppert, H.H., Sparks, R.S.J., Turner, J.S. & Arndt, N.T., molten sediment was preserved at least initially by a 1984. Nature 309: 19-22. lack of mixing with the komatiite host; this seems likely Lesher, C.M., Arndt, N.T. & Groves, D.I., 1984. In Buchanan, given the large chemical and viscosity contrasts. D.L. & Jones, M.J. (eds): Proc. 3rd Nickel Sulphide Field However, most sediment was probably assimilated in Conference, Perth, Western Australia. IMM, London: the turbulently convecting lava channel, and only a 70-80. limited amount collected along the flanks and was Lesher, C.M. & Groves, D.I., 1986. In Friedrich, G.H. et al. preserved as ocellar units. (eds): Proc. 27th International Geological Congress, Moscow. Springer-Verlag, Berlin,: 43-62. Recent field-based and geochemical studies on the Marston, R.J., Groves, D.I., Hudson, D.R. & Ross, J.R., 1981. Kambalda-type nickel sulphide deposits thus suggest Econ. Geol. 76: 1330-1363. that ground melting of sulphidic sedimentary units and McNaughton, N.J., Frost, K.M. and Groves, D.I., 1988. Geol. possibly footwall basalt occurred in ore environments, Mag. 125: 285-295. and these observations are in accord with theoretical and fluid dynamic studies on komatiite lavas. As for A63 PLATINUM METALS IN WESTERN AUSTRALIA — GENERAL OCCURRENCE AND THEIR MINERALOGY AND RECOVERY FROM THE KAMBALDA NICKEL DEPOSITS D.R. Hudson CSIRO, Division of Exploration Geoscience, Perth

Australia has a considerable, but largely unrealised potential for the discovery and production of platinum group elements (PGE). The western half of the continent is underlain by Archaean granite-greenstone terrains, well-exposed in the Yilgarn and Pilbara Blocks, that contain komatiite-associated nickel sulphide deposits and numerous partly-dismembered layered gabbros, several of which contain significant Ni-Cu sulphide mineralisation. Proterozoic gabbros occur within the Kimberley and Pilbara regions of Western Australia and in central Australia and many contain minor PGE mineralisation. Platinum group minerals (PGM) and gold have also been identified in samples taken from outcrop loaming and drainages in the Pilbara and, in particular, alloys of iridium, osmium and ruthenium, with minor platinum, have been reported from along 10 km of strike near Karratha.

Early PGE production in Australia was based on recovery of osmiridium from mainly alluvial deposits derivedfromultramafic rocks in northwest and southern Tasmania and from placer deposits near Fifield in New South Wales. The Tasmanian deposits produced some 1000 kg to 1965, but in recent years production has been meagre. Historic production of isoferroplatinum and associated PGM from the Platina Lead and other deposits near Fifield totalled about 650 kg, and in recent years there has been considerable exploration activity in the area, both for placer deposits and primary mineralisation. Australia's most significant PGE production has come as a consequence of the mining of komatiite-associated nickel sulphide ores at Kambalda. Mining commenced in 1967, and to 1988 the deposits had produced about 600 thousand tonnes of nickel metal. Current production is reported at somewhat less than 40 thousand tonnes of

126

nickel per year, which is associated with some 80 kg of platinum and 500 kg of palladium. The average composition of in situ Kambalda ore (calculated to 100% sulphide) has been estimated as (in %) Ni, 14.4; Cu, 1.1; Co, 0.3; S, 39.9; (in ppb) Pt, 1630; Pd, 2104; Os, 537; IT, 293; Rh, 240; Ru, 1074; Au, 1721; and Ag, 5710. This estimate equates reasonably well with reported recoveries of palladium, but suggests a much higher value for platinum than is presently stated in production figures. The relative abundance, compositional variability, and geologic occurrence of PGM have been determined by a study of samples from a number of geologic environments in the Kambalda nickel deposits. These results have been integrated with findings from a study of PGM in amalgamation residues from the gold recovery circuit of the Kambalda nickel mill, to give an overall picture of the nature and distribution of PGM at Kambalda. The major platinum minerals are sperrylite and moncheite, and the major palladium minerals are sudburyite, merenskyite, stibiopalladinite, palladoarsenide, michenerite, and testibiopalladite; palladium also occurs in solid solution in palladian melonite. Irarsite is the only iridium mineral recognised, and no discrete phases have been observed for the other platinum-group

elements. Sperrylite occurs within massive pyrrhotitepentlandite ores and is particularly abundant in ores that are rich in chalcopyrite. A high proportion of the Pt in the Kambalda nickel deposits is believed to occur as sperrylite, but not all sperrylite is sufficiently coarse in grain size to enable gravity concentration. Nevertheless, sperrylite is the dominant platinum-group mineral recovered in gravity concentrates. Discrete palladium minerals are most abundant in stringers of sulphide in the footwall to the ore, in crosscutting sulphide veins, or in reaction zones associated with hydrothermal veins and porphyries. Within the massive and matrix ores, coarse discrete palladium minerals are rare, and it is concluded that palladium occurs in solid solution or as finely dispersed submicroscopic grains in sulphide minerals, predominantly pentlandite. The occurrence, within the ore zones, of Pt as sperrylite and Pd dispersed in pendlandite is believed to reflect a primary magmatic distribution. However, the presence of sudburyite, moncheite, merenskyite, michenerite, testibiopalladite, and palladian melonite in stringers and reaction zones indicates that their formation may be related to postmagmatic processes, in particular metamorphic segregation of sulphides and the interaction of ore sulphides with younger hydrothermal fluids.

A6.4 ORDOVICIAN MAGMATISM IN THE CENTRAL LACHLAN FOLD BELT AND PRECIOUS METAL POTENTIAL Doone Wyborn and Warrington Cameron 1

Bureau of Mineral Resources, Canberra Geology Department, Australian National University 1

In the central Lachlan Fold Belt, the Ordovician geology is dominated by widespread mafic volcanic sequences which have built up from earlier submarine conditions to later emergent volcanoes, and associated limestone reefs. Magmatism is concentrated in four main belts from west to east: (1) Fifield-Nyngan; (2) Parkes-Narromine; (3) Orange-Wellington; and (4) Sofala-Rockley. There is no geological evidence to support the notion that these were once joined as a single continuous island arc that has been subsequently sliced up by strike slip faulting. The magmatic composition of all belts are K-rich or shoshonitic, and are dominated by basalt rather than andesite. In fact andesite is quite rare, and most of the nomenclature that has been used for these rocks in the past should be amended to take account of this fact. The basalts plot on a mantle normalised diagram (spidergram) with distinctively high Ba, K, Sr, and P, and low Nb, Zr, and Ti. Basalt in the Cabramurra-Adelong-Temora-West Wyalong region is not shoshonitic (it is tholeiitic), but there is evidence accumulating that these latter rocks are of Silurian age, and should not be included in the magmatic system.

In places, such as in the subvolcanic cores of major volcanic centres, the magmas have fractionated into highly evolved types that host low-sulphur porphyry copper/gold deposits, or are the source for skarn-type gold mineralisation. More mafic intrusive bodies that are not necessarily part of the cores of the major volcanic centres commonly show classical development of mafic and ultramafic cumulates, with olivine and clinopyroxene initially dominating (dunites and wehrlites), followed by hornblende clinopyroxenites and monzogabbros. The platinum and palladium contents of shoshonites from theregionthat have been analysed so far are high (Pt+Pd typically 20 to 40 ppb, or 10 to 20 times higher than tholeiitic rocksfromthe Lachlan Fold Belt), whereas sulfur contents are low (typically <200 ppm). Fractionation has probably proceeded in a state of sulphur undersaturation in the magma, so precious metal contents would build up in the magma with fractionation. Chondritic ratios of Pt/Pd in many rocks suggest the high abundance levels are primary. Values up to 140 ppb Pt+Pd have been obtained in hornblende clinopyroxenite cumulate rocks with mg numbers around 70.

127

Some of the intrusions investigated so far have cumulate sequences over 1km thick, giving a potential source easily large enough for the formation of a significant PGE resource given appropriate concentration mechanisms. The widespread occurrence of Ordovician shoshonites, and their high levels of PGE's, has major implications for the tectonic development of the Lachlan Fold Belt, and of the composition of the underlying mantle. The Ordovician shoshonites were replaced in

the Silurian by the appearance of tholeiitic nocks instead, and by the widespread melting of continental crust to form granites. It is suggested that a sulphur-poor subcontinental lithosphere is the source for the shoshonites, but this was replaced by asthenospheric upwelling in the Silurian, to source the tholeiites. This major upper mantle overturning brought hot asthenosphere in contact with the crust, and conductive heating provided the high geothermal gradient that gave rise to the granites.

A6.5 PETROGRAPHY AND GEOCHEMISTRY OF THE NARNDEE INTRUSION, WESTERN AUSTRALIA: IMPLICATIONS FOR PGE MINERALISATION P.A.H. Scowen *, R.R. Keays and P.R. Hamlyn 1

Research School of Earth Sciences, Australian National University Dept of Geology, University of Melbourne ORE Pty Ltd, Heidelberg Victoria 2

The Narndee Intrusion is a large mafic layered values of Pt, Pd and Au are 100 ppb, 63 ppb and 10 ppb intrusion situated -50 km south of the Windimurra respectively. There is no correlation between the S and Intrusion in the Yilgarn Block of Western Australia. It Cu contents of the rocks. The Kockalocka rocks show is roughly shaped like a fish hook and crops out over an no correlation between Cu and Pt+Pd or between Cu area of -700 km . Alteration/weathering and meta- and Au and rocks from the Milgoo section show only a morphic effects are highly variable with the rocks very weak positive correlation between Cu and Pt+Pd ranging from virtually pristine to completely altered. and Cu and Au. Original igneous textures are usually preserved even in Cyclic variations in the geochemical data suggest highly altered rocks. The most continuous outcrop occurs that the Narndee Intrusion formed from numerous small in the southwest of the intrusion along the Kockalocka pulses of magma. The very low Ir content of rocks at the section where at least 5 km of stratigraphy is exposed. base of the Kockalocka section could be the result of The Kockalocka section is divided into three zones: a crystallisation from a magma that had already undergone Lower Zone, -300 m thick, of alternating peridotite and extensive fractionation. The lack of correlation between gabbronorite units, a Gabbronorite Zone, -2100 m thick, S and Cu is probably due to secondary processes that of alternating pyroxenite and gabbronorite and a Cyclic led to S loss. Hoatson & Keays (1989) have suggested Zone, -2700 m thick, consisting of seven cyclic units of that once a magma is sulphide-saturated, the Pt+Pd peridotite-olivine gabbronorite-gabbronorite. Exposure content is independent of Cu and Ni. Therefore, the lack in the north is poor. Isolated blocks of the intrusion are of correlation between Cu and Pt+Pd in the Narndee separated by large areas of no outcrop, however, three rocks may indicate that the magma was sulphidesequential drill holes provide -1000 m of core hereafter saturated. It appears that the magma responsible for the referred to as the Milgoo section. studied portion of the Narndee Intrusion had previously XRF, electron microprobe and neutron activation undergone extensive fractionation, was sulphidesaturated or very close to sulphide-saturated and that analyses (for PGE+Au) have been completed along the Kockalocka and Milgoo sections. Both sections show magma chamber processes were such that sulphides of cyclic variations in Cr, Mg#, Ni, S, Cu, Pt, Pd and Au. variable PGE enrichment precipitated continuously over Cr, Ni and Mg# are strongly (positively) correlated and a wide stratigraphic interval rather than in a narrow are highest in the ultramafic rocks. Ir tends to follow Cr, horizon or reef. Ni and Mg# although all rocks at the base of the Reference Kockalocka section, even the peridotites, have very low Hoatson, D.M. & Keays, R.R., 1989. Submitted to Econ. Geol. Ir (-0.01 ppb). Concentrations of Pt, Pd and Au are variable but show a strong positive correlation. Peak 2

128

A6 Keynote Address 1 ORE GENESIS AND EXPLORATION MODELS FOR PLATINUM-GROUP ELEMENT MINERALISATION IN LAYERED MAFIC/ULTRAMAFTC INTRUSIONS Reid R. Keays , Paul R. Hamlyn ' and Shane J. Reeves 1

1 2

Department of Geology, University of Melbourne OREPty Ltd, Heidelberg, Victoria 2

The generation of platiniferous horizons in layered intrustions involves S-saturation of a previously Sundersaturated magma in a magma chamber. The recognition of this requirement can be used to develop exploration strategies for this style of mineralisation. The potential of a magma to form a Platinum Group Element (PGE) deposit is determined by its sulphur capacity which in turn is determined by temperature. Most First Stage Magmas (FSM) such as MORB are Ssaturated during ascent to the surface; as a result, they have low PGE, and high S. High temperature magmas (e.g. komatiites and picrites) do not become S-saturated until a late stage and as a result have much higher PGE contents than S-saturated magmas. There is strong evidence that the parental magmas to the lower portions of layered mafic/ultramafic intrusions hosting economic platiniferous horizons (Bushveld Complex, Stillwater Complex, and Great Dyke) as well as many of the mafic/ultramafic complexes (e.g. Munni Munni) in the Pilbara and Kimberley regions of Western Australia were either Siliceous High Magnesium Basalts or boninites. The former may have formed as a result of AFC processes in which komatiitic magmas assimilated felsic crust (Sun & Nesbitt, 1989). The major platinumbearing layered intrusions all post-date the formation of Archaean greenstone belts, the principal hosts of Komatiitic rocks. Cratonisation, which preceded intrusion of the layered complexes, may have caused subsequent komatiitic melts to pool, and interact with, the base of the crust. Boninites are Second Stage Magmas (SSM) derived by the partial melting of the refractory source region of S-saturated FSM. These magmas inherit the PGEenriched sulphide droplets left behind by the latter and as a result are PGE-richand, most important, very low in S. As an example, whereas MORB contain an average of 0.6 ppb Pd, low Ti picrites from King Island, Australia, contain an average of 20 ppb Pd. (Hamlyn et al., 1985). The most characteristic chemical feature of boninites is

their very low Ti0 content, most of which was removed along with S as an incompatible element in the FSM. As incompatible elements, Pd, Pt and Au build up along with S and Cu during the course of fractionation of S-undersaturated magmas; however, once a magma becomes S-saturated, the Pd and Pt are rapidly removed by the first immiscible magmatic sulphide droplets to separate from the magma because of their very high partition coefficients (e.g. Dpd = 20,000). Whereas Ssaturated magmas have very low Pd/S ratios, Sundersaturated magmas have Pd/S ratios. Because they contain variable proportions of trapped interstitial silicate melt, cumulates formed from these magmas preserve the Pd/S ratios of the magmas. Hence, cumulates lying below mineralised horizons have high Pd/S ratios whereas those above have low Pd/S ratios. These principles can be used in exploration for PGE mineralisation in layered intrusions as will be shown using examples from the Great Dyke, Zimbabwe, Narndee Intrusion, Western Australia, and Bushveld Complex, South Africa. Each of these intrusions exhibit markedly different PGE patterns which are attributed to different magmatic processes and timing of S-saturation in each of the intrusions. Our approach is to document PGE patterns in the entire environment rather than focusing on the immediate ore horizons as has been done in many previous studies. It thereby avoids many of the interpretative problems caused by later "hydrothermal" redistribution of the PGE on the thin section scale resultingfromthe activities of late stage fluids. References 2

Hamlyn, P.R. & Keays, R.R.,1986. Econ. Geol. 81:1431-1445 Hamlyn, P.R., Keays, R.R., Cameron, W.E., Crawford, A.J. & Waldron, H.M., 1985. Geochim. Cosmochim. Acta 49: 1797-1811 Sun, S.S., Nesbitt, R.W. & McCulloch, M.T., 1989. In Crawford, A.J. (ed.): Boninites and Related Rocks. Allan and Unwin.

129

A6.6 THE MAJOR, TRACE AND PLATINUM GROUP ELEMENT GEOCHEMISTRY OF THE BUCKNALLA COMPLEX, CENTRAL QUEENSLAND Shane J. Reeves , Reid R. Keays and Paul R. Hamlyn 1

Department of Geology, University of Melbourne ORE Pty Ltd, Heidelberg, Victoria 2

Mantle normalised metal plots (Barnes et al., 1987) The Bucknalla Complex, which was previously known as the Westwood Layered Intrusion (Carrigg et for both the mineralised and unmineralised rocks of the al, 1989), is a small (10 km ), layered, tholeiitic, mafic- Bucknalla Complex display very similar trends (Fig. 1). ultramafic intrusion located SO km southwest of Both plots display the anomalous low iridium content, Rockhampton. The complex comprises clinopyroxenite, PPGE enrichment and the clear control of sulphides on olivine clinopyroxenite, wehrlite, troctolite, hornblende the distribution of the PGE and Au. Pd/Ir ratios are gabbro, gabbro, anorthosite, leucogabbro and dolerite, extremely high (1800-9300) indicating extreme containing relatively primitive mineral composition fractionation of the PGE. These trends may, in part, assemblages ( A n ^ Fo ^ mg#-Cpx ^ & generally reflect PGE abundances inherited from the source (i.e. An , mg#-Cpx and Fo ). The Complex is a saucer- relatively low degrees of partial melting) but were shaped lopolith (2200 m x 6 km at maximum exaggerated by the extraction of the IPGE during the stratigraphic intersection) which intruded Lower Permian early stages of fractional crystallisation and by the spilitic pillow lavas, cherts and tuffs, of the Rookwood precipitation of a PPGE-enriched sulphide component. Volcanics, during the Lower Permian (Murray, 1975) The Complex is known to host minor Pd-Pt-Au-Cu and which has been tilted vertically and in a northeast mineralisation, located in a small shaft along the central direction. Clifford (1987) found the complex to consist ridge of the intrusion. Two styles of mineralisation have of over 15 laterally discontinuous igneous units ranging been recognised; (i) Pd phases and electrum associated in thickness from 1-50 m. Plagioclase is a cumulus with monomineralic clinopyroxenites, late crystallizing phase throughout the intrusion while orthopyroxene is amphibole and secondary copper sulphides; and (ii) Pd absent until the very uppermost levels of the stratigraphy. and rare Pt phases associated with intercumulus primary The chromium composition of magnetite, analysed by magmatic copper sulphides and secondary deuteric electron microprobe, has been found to mimic whole- sulphides. Phases recognised include electrum (Au-Ag rock mg# and is a good measure of the degree of alloy), palladoarsenide, palladoantimonide, palladium fractionation of the rocks. sulphide, michenerite (PdBiTe ) and sperrylite (PtAs ). PPGE (Pd & Pt), Au, S and Cu values for the Pd phases have only been observed in rocks in which intrusion are high while DPGE (Ir & Ru) are low (from deuteric alteration has significantly altered the sulphides, 120 analyses) with Pd, 2-70 ppb; Pt, 3-40 ppb; regardless of the PGE tenor of the rocks, suggesting Au, 1-20 ppb; Ir, 0.01-0.07 ppb; Ru, 0.2-0.6 ppb; S, that the PGMs have formed by an accretionary process 150-400 ppm; and Cu, 40-600 ppm. Pt, Pd and Au following alteration and mobilisation of magmatic PGEdisplay good correlations with Cu, particularly at bearing sulphides. more elevated levels, while Ir and Ru are better corre- References lated with whole rock Ni and Cr. Pd, Pt, Au, Cu & S Barnes et al.,1987. Geoplatinum '87. Elsevier: 113-143. are elevated in rocks which have intermediate whole- Carrigg, J.A., Reeves, S.J., & Mclver, R.G., 1989. In Field conference guidebook, Rockhampton region. Geol. Soc. rock mg# (47-60). These trends suggest that the PGE Aust. Q'ld division. are, to some extent, controlled by fractionation and that M.J., 1987. Unpubl. Honours thesis, James Cook the high melting point PGE (Ir,Ru) are precipitated Clifford, University of North Queensland. with the early crystallising phases, olivine and clino- Murray, C.G., 1975.1:250,000 Geological Series-Explanatory pyroxene, whereas Pt, Pd and Au are concentrated by Notes. Rockhampton, Queensland. Aust Govt Publishing sulphides. Service. 2

130

A6.7 THE GEOCHEMISTRY AND PGE POTENTIAL OF SELECTED EAST KIMBERLEY AND WEST PILBARA MAFIC/ULTRAMAFIC INTRUSIVES AND RELATED ROCKS D.A. Wallace *, D.M. Hoatson , Shen-su Sun and R.R. Keays 1

Bureau of Mineral Resources, Canberra Department of Geology, Melbourne University 1

The Precambrian shields of Western Australia have the other West Pilbara intrusions in having a sulphura number of mafic/ultramafic layered intrusive undersaturated ultramafic zone and enrichment of PGE complexes and extrusive equivalents which either at high stratigraphic levels. The West Pilbarra SHMB contain PGE, or have the potential for PGE mineral- parental magmas have similar PGE contents (e.g. Pt isation. Integrated chemical and isotope studies of and Pd — 15 ppb) to Stillwater and Bushveld parent representative suites from the western part of the Pilbara magmas. In contrast to the West Pilbara complexes, parent Block and from the Halls Creek Mobile Zone reveal significant differences between parent magma magmas of the Early Proterozoic mafic/ultramafic compositions in both terrains. These differences are of complexes in the Halls Creek Mobile Zone have an importance to their PGE potential and to the type of affinity to low-Ti continental-type tholeiite. They show mineralisation which could be expected in these areas. Fe and Ti enrichment trends with differentiation, are In the West Pilbara late Archaean parent magmas LREE-enriched and are Nb-depleted (La/Nb -2-3). Of are siliceous high-magnesian basalts (SHMB), which these however, the Woodward Dolerite has a distinctive MORB-type character (MgO > 8-9%; Ti0 0.5-0.6%; can be identified as two basic types of komatiite: (a) komatiite flows characterised by near chondritic A1 0 /Ti0 26), suggesting its derivation from greater Al 0 /Ti0 (-21), Ti/Sc and flat heavy rare earth than 25% melting of depleted mantle pyrolite. With the possible exception of the Woodward (HREE) patterns. Complexes investigated of this type are either hypabyssal (Cooya Pooya Dolerite), or Dolerite the Halls Creek magmas appear to have been sulphur saturated prior to emplacement Hence they extrusive (Negri Volcanics); and (b) komatiite flows depleted in A1 (Al 0/ri0 -10; would not be regarded as favourable for Merensky i.e. about half chondrite concentrations), Sc Reef-type PGE mineralisation. Feeder dykes of the Halls (14-20 ppm), Y (10-14 ppm) and enriched in light rare Creek intrusives as well as the Woodward ^Dolerite earths (LREE) relative to HREE (La,, /Sn^ — 2.7; La,/ may, however, have potential for Noril'sk-type Ni-CuLi^ — 9). Rocks characterised by this type are from PGE sulphide mineralization, or remobilised layered intrusions (Munni Munni, Andover, Dingo, hydrothermal mineralisation. Mount Sholl). The Munni Munni Complex differs from 2

A6.8 MANTLE PLUME AND MELTIING OF REFRACTORY MANTLE: IMPLICATIONS FOR PGE AND Au MINERALISATION Shen-su Sun Bureau of Mineral Resources, Canberra

Current popular hypotheses of PGE mineralisation in layered mafic-ultramafic complexes include anomalous enrichment of PGE in their mantle sources, specific magma types (e.g. boninite, komatiite) and very efficient concentration of PGE by small amounts of sulphide droplets from large quantities of silicate melt (high R factor) through convective mixing. These hypotheses are evaluated in terms of (1) chemical composition of the parent magmas and their PGE contents; (2) PGE and S abundances in the mantle; (3) transport mechanisms for the PGE. Parent magmas of early Precambrian PGE-bearing layered complexes are commonly siliceous, high magnesian basalts (SHMB), whereas PGE-enrichment in younger complexes are commonly associated with an early high-temperature phase of continental flood

basalts and picrites. Thermal considerations, radiometric ages of xenocryst zircons and chemical and isotopic modelling indicate that light REE-enriched Archaean SHMB were most likely derived from komatiite that experienced crustal contamination. They have Ti/Pd, Pd/S, Pd/Cu and Pd/Au similar to values estimated for komatiites and primitive mantle (-3 x 10 , -2 x 10 , -1.5 x 10 and ~4 respectively). Thus PGE mineralisation in Archaean layered complexes is not due to PGE enrichment in the mantle sources, but related to Sundersaturation caused by large degrees of partial melting. Sulphide saturation and the efficient concentration of PGE is induced by such processes as crystal fractionation, crustal contamination and magma mixing. 5

131

S-undersaturation of mafic-ultramafic magmas can potential for the establishment of large magma chambers, be achieved either by (1) high temperature (>1400°C) sulphide assimilation from the shelf sediments, and and/or large degrees (>25%) of melting of the convecting involvement of the refractory lithospheric mantle (Pd < mantle (with fairly constant Pd ~4, Pt ~6, Au ppb 4, Au < 1 ppb and S < 50 ppm?) in magma generation. and S -250 ppm from Archaean to the present) or, (2) Despite the high PGE potential of boninitic magmas smaller degrees of melting of refractory mantle (with (Pd -15, Au -2 ppb, highly S-undersaturated) there is Pd, Pt < 4, Au < 1 ppb and S « 250 ppm) of the a paucity of economically significant magmatic PGE continental lithosphere or wedge above the subduction occurrences associated with them. Their unstable forearc zone. The first type includes komatiite and continental tectonic environment is probably not favourable for flood basalt and picrite related to mantle plume activities, establishing large magma chambers and efficient Swhereas the second type includes boninite, low-Ti saturating and PGE concentration mechanisms. tholeiite, some lamprophyre and shoshonite related to However, some shoshonite lavas are found in more subduction zone processes. Most of these fertile magmas stable continental environments and rather large intrusive have Pd~Pt concentrations of -15 ± 5 ppb. complexes are known (e.g. Fifield, NSW). Boninite, Komatiites, flood basalts and picrites in rifting low-Ti tholeiite and shoshonite terrains are prospective environments of cratons have the following advantages for PGE and Au mineralisation related to metamorphic, for hosting PGE mineralisation: the stable craton offers hydrothermal and supergene processes. A6 Keynote Address 2: A REVIEW OF THE EVIDENCE FOR THE HIGH TEMPERATURE HYDROTHERMAL CONCENTRATION OF THE PLATINUM-GROUP ELEMENTS IN LAYERED INTRUSIONS A. Boudreau Department of Geology, Duke University, Durham, NC, USA

Stratabound platinum-group element (PGE) deposits, such as the Merensky Reef of the Bush veld Complex or the J-M Reef of the Stillwater Complex, are commonly associated with pegmatitic textures and relatively abundant amounts of hydrous minerals and graphite. While it has long been recognised that these deposits show evidence for the involvement of a high temperature volatile fluid phase, it is only recently that studies have begun to look at the role of these fluids as either having modified an orthomagmatic sulphide deposit or, more radically, having been responsible for the PGE-sulphide mineralisation itself. It is evidence for the latter possibility which is reviewed here. The presence of discordant, platiniferous pipes and pods which occur in both the Stillwater and Bushveld complexes demonstrate that the PGE and sulphur may be redistributed within the cumulate pile late in the solidifcation history. More specifically, these features imply that high temperature fluid migration through a solidifying cumulate pile is not limited to channeled migration in veins and fissures, but instead can be pervasive on a scale large enough to mobilise and concentrate the minor PGE and sulphur component of the cumulate sequence to the degree observed in the pipes. Within stratabound deposits such as the J-M Reef, mineralisation is not uniformly distributed along strike, but is locally absent or concentrated in massive sulphide pods, and may be discordant to layering. While it has been suggested that local occurrences of such features as massive sulphide pods may be the result of reworking of a magmatic deposit, their mineralogical and chemical similarity to the stratabound disseminated mineralisation

implies that one cannot distinguish magmatic from hydrothermal sulphides from noble and base metal contents alone. Analysis of volcanic gases show enrichments in noble metals over melt abundances by several orders of magnitude. In addition, the reported positive correlation of PGE content of the vapor with CI points to Clcomplexing as the mechanism of transport. The hydrous minerals of the stratabound deposits, particularly apatite, are unusually Cl-rich. Furthermore, Pt + Pd are much more soluble than the other PGE in Cl-complexing fluids, and hence the CI association is consistent with the high (Pt+Pd)/Ir ratios characteristic of these deposits. If the cumulates below the deposits are the source rocks for the reef mineralisation, then these rocks should show some evidence of that loss. Indeed, cumulates below the J-M Reef contain lower concentrations of S and chalcophile elements than do rocks stratigraphically above the J-M Reef. The conventional interpretation is that the deposit marks the point at which melt became saturated in immiscible sulphide melt. However, this stratigraphic distribution is also consistent with the loss of the ore elements to an upward-migrating vapour. In addition, the reported lower sulphur fugacity reported for some Stillwater Ultramafic assemblages is consistent with S loss. Selective loss of trace metals from the Ultramafic series is suggested by the occurrence of molybdenite and/or laurite as part of the sulphide assemblage in the cumulates below the J-M Reef: a vapor scavenging metals from the footwall would tend to leave the footwall enriched in such insoluble sulphides of Mo, Ir, Os and Ru relative to the more soluble

132

elements S, Cu, Ni, Pt and Pd. These observations are suggestive, but more work needs to be done to test this hypothesis. In contrast to conventional magma mixing models, there have been several models which suggest that aspects of the cumulate stratigraphy can be explained by the mixing of vapor to a melt ± crystal assemblage. For example, the addition of volatiles will shift cotectic boundaries such as to favor mafic minerals over plagioclase, and can explain why pegmatitic zones or zones containing relatively abundant hydrous minerals typically contain a single mafic cumulate mineral. Buildup of volatiles in cumulate pile is seen in increasing development of pegmatites below the reefs. Other features of layering, conventionally considered to be evidence of increased magmatic turbulence (e.g., the increased presence of modal layering and slump structures), can be interpreted to be the result of increased diffusivity and decreased viscosity accompanying volatile buildup within the cumulate pile.

The stratigraphic position of the major PGE zones is equivocal between orthomagmatic and hydromagmatic origins for the PGE deposits. The location of the major economic reefs above the ultramafic sequences would be expected if the ultramafic cumulates served as the source rock for the ore elements. Ultramafic rocks typically have higher PGE abundances than do basalts and more evolved rocks, making them better source material for the PGE than the basaltic magmas from which they crystallised. Ultramafic zones also tend to be more orthocumulate (have more trapped intercumulus melt) than the stratigraphically higher rocks and hence have the permeability to allow vapor exsolving from the interstitial melt to migrate. In conclusion, the origin of stratabound PGE deposits in layered intrusions should be considered very much open to debate. If the reef mineralisation was the result of an upward migrating volatile fluid, rather than sulphide saturation in the (now crystallised) supernatant magma, then evidence for this loss should be found in further study of the rocks below the reefs.

A6.9 LATE- TO POST-MAGMATIC PGE MINERALISATION IN THE FIFIELD PLATINUM PROVINCE AND THE OWNEDALE INTRUSIVE COMPLEX, NSW L.M. Barron *, E. Slansky , D. Suppel , Z. Johan and M. Ohnenstetter 1

Geological Survey of New South Wales, Sydney, Australia GIS BRGM-CNRS, Centre de Recherches sur la Synthase et la Chimie des Mineraux, Orleans, France 1

Situated about 380 km west of Sydney, the Fifield Platinum Province contains 15^40 Alaskan-type complexes intruding the metasedimentary. rocks of the Cambro-Ordovician Girilambone Group in central New South Wales Australia. In spite of the low specific gravity of the country rocks, the complexes are located within a gravity-high NNE belt some 80 km wide and over 500 km long, called The Parkes Terrace with Fifield about halfway along. The Parkes Terrace is set in the Girilambone Wagga Anticlinorial zone of the Paleozoic Tasman Geosyncline, with Siluro-Devonian sediments and acid volcanics in the Tullamore Syncline (to the east) and the Murga Syncline (west). There are several N-S thin arcuate discontinuous horizons of foliated serpentinite through the region of the Parkes Terrace and these mark the locus of a subduction zone active during the Ordovician, a permanent plane of crustal weakness that partly reopened in the Devonian as a result of the pronounced crustal thinning which produced the gravity-high Parkes Terrace and allowed the emplacement of the Alaskan complexes. The known complexes are clustered in a band on the west half of the Parkes Terrace in the Fifield region, but the continuation of this band (to the many inferred buried complexes in the north) involves a low angle crossing over to the east half of the Paikes Terrace. Near Fifield, these circular to elliptical intrusives, emplaced during the Siluro-Devonian as conduits to volcanoes, are

composed of monzodiorite, gabbro-norite, hornblendite, hornblende/olivine/biotite/magnetite clinopyroxenite, pyroxene peridotite and dunite. Platinum at Fifield was discovered in 1887, with early production of 650 kg of impure alluvial platinum and 325 kg of alluvial gold, mostly from the Platina Deep Lead with lesser amounts won from Burra Burra, Fifield and Gillenbine Tank Leads, eluvial and alluvial deposits and Tertiary gravel remnants. A general model is proposed for the PGE mineralisation, ranging from high temperature primary magmatic, through remobilisation and concentration of host rock PGE during localised but wholesale metasomatism/zonerefining of the various host rocks at elevated temperatures, followed by limited vapour overprinting at moderate temperatures. The concluding stages are weathering/lateritic concentration in situ, followed by eventual erosion and concentration in at least two ages of alluvial channels. The solutions which induce the metasomatic processes are derived from the strategic emplacement and crystallisation of hydrous monzodiorite magma rich in fluorine and chlorine. This magma type, which appears to be introduced into the ultramafic crystal mushes at the conduit stage of development of the complexes, is very primitive. At the Owendale and Hylea Complexes, the monzodiorites are both hypersthene olivine modal and normative as well as extremely mafic and have a shoshonitic affinity. As a

133

result of the metasomatic event, the different ultramafic host rocks are modified according to their original mineralogy, but with certain adjustments indicative of the monzodiorite source. A schematic framework for the various stages of platinum mineralisation is suggested as follows, with the asterisk '*' indicating the mineralisation identified to date, while d = dunite, p = pyroxenite: MO Primary magmatic; M0 , M0 ?* Ml Metasomatic and remobilised into 30-150 m pipes; Ml *,Ml M2 Metasomatic and remobilised into minor monomineralic pegmatoidal 10-100 cm units; M2 *, M2p* M3 Limited msitu remobilisation and overprinting by Sb and As, decomposition of alloys; M3 , M3 * ML* Lateritic concentrations, degree of modification not known MP*Placer concentrations, at least two ages. The Ml and M2 PGM venues show no appreciable high-temperature sulphide association while M3 shows only rare sulphides, but the sulphide association of MO is not known because these PGM have not yet been found in a hard rock setting. The Pt/Pd ratio for all of these stages is generally greater than one. The minor element chemistry in the alluvial nuggets, when compared to that of the hard rock PGM, clearly indicates that the hard rock source for the nuggets has not yet been found. The Owendale circular zoned intrusive complex, located near the intersection of two major tectonic structures (Gilmore Suture and Lachlan River lineament), has received detailed study for the PGMs located in the new type of PGM occurrence within units (M2 ): pegmatoidal, biotite- and magnetite- poor clinopyroxenites which form irregular lenses and veinlike bodies within biotite- and magnetite-rich d

clinopyroxenites. The mineralised "P-units" are not enriched in base metal sulphides with respect to the surrounding barren clinopyroxenites, but their bulk Cr levels and the Cr levels in magnetite are both enriched above the host pyroxenite. Early PGM are inclusions and interstitial to clinopyroxene while the latest PGR rim other PGM, fill mineral cleavages and fractures cutting magnetite with subsolidus ilmenite exsolution lamellae (one of the last magmatic phases in the clinopyroxenite). The PGM are only exceptionally associated with very late base metal sulphides which are also very rare. The mineralogical study of PGM in the P-units indicates that the mineralisation was generated from a sequence of mineral assemblages which were characterised by equilibrium crystallisation at the early stages of evolution (M2 ), and were replaced by disequilibrium conditions during the latest mineralisation periods in M3^. The early PGM (erlichmanite, (Pt-Fe) alloys, coopente) were formed from a fluid-rich system slightly before the end of clinopyroxene crystallisation. The association OsSj+P^e is next replaced by an equilibrium assemblage OsS +PtS. Cooperite is extensively replaced by geversite then sperrylite through limited vapour overprinting by Se and As (M3 ), accompanied by the lower temperature formation of PtjFe coupled with a rise in the activity of iron which induced the slightly later stabilisation of pyrrhotite and associated rare chalcopyrite, pyrite, sphalerite, cobaltite, cobalt- and cobaltian-pentlandite. The presence of the equilibrium association OsS +PtS implies a maximum temperature of 860°C for M2 with fS = -1, while the composition of cobaltrich pentlandite indicates the highest stability limit at 670-710°C for M3 . Thus the temperature range of the formation of the Kelvin Grove M2 -M3 mineralisation can be estimated at about 850 to &50°£. p

A6.10 PGE MINERALISATION IN ZONED, "ALASKAN" STYLE ULTRAMAFIC INTRUSIONS IN THE FIFIELD REGION, NSW, AUSTRALIA. B.A. Brill and R.R. Keays Department of Geology, University of Melbourne

The Fifield region was Australias largest producer of alluvial Pt (> 600 kg). The Pt was derived from primary Pt mineralization in zoned ultamafic intrusions of Early Devonian age (Pogson & Hillyard 1981). The intrusions have a concentric zoning of the lithologies and show similarities to Alaskan style complexes. The complexes occur near the intersection of a major N-S trending tectonic boundary (Gilmore Suture) and a regional cross-lineament (Lachlan-River Lineament), (Barron et al., 1987) within the Girilambone-Wagga Anticlinorial zone of the Lachlan Fold belt (Scheibner, 1976). The intrusions are known to extend NNW for 320 km and it has been suggested that the mafic-

ultramafic intrusions may be part of an Early Devonian belt of granitoids and shoshonitic volcanics which extend from Victoria to NSW (Wyborn et al., 1987). Typical features of the intrusions are that they seldom outcrop, are strongly magnetic and that Pt is the only PGE present in significant proportions. Primary sulphides are rare and mineralisation occurs over wide intervals of bedrock. The Kars complex, located south of Fifield, has a dunitic-wehrlitic core and grades outward into olivine clinopyroxenite, clinopyroxenite and hornblendite. The intrusion is cut by dykes of diorite, aplite and hornblende monzonite. A later phase of monzonite intruded

134

pyroxenite and formed a pyroxenite-monzonite breccia at the contact. The pyroxenites contain abundant accessory magnetite. Coarse and euhedral magnetite is also present associated with coarse biotite in veins (referred to as 'primary', i.e. probably late magmatic magnetite) and contrasts with fine grained magnetite which has formed during serpentinisation of the dunites. A typical feature of the Kars complex is the abundance of pegmatoidal pods and veins of dunitewehrlite and the magnetite-biotite veins which correlate with the highest Pt grades. Discrete phases of Pt-Fe alloy and stibiopalladinite have been observed within a magnetite vein as inclusions in pyroxene and also directly associated with the magnetite. A similar situation was found at the Owendale complex, the largest of the intrusions located at Fifield. The circular shaped complex consists of monzonitegabbro-norite withringsand arcuate lenses of pyroxenite, dunite, hornblendite, peridotite and olivine-serpentinised equivalents. Even though the highest grades from the complex were recorded within 'P' units (Barron et al., 1987) which represent highly irregular veins of clinopyroxene, our detailed study of drillcore by NAA showed that anomalous Pt also coincides with the presence of pegmatoidal wehrlite-dunite and ^replacement) dunite. All the described features give abundant evidence of intensive fluid activity. A model is that each intrusion represents a stratiform igneous complex that was diapirically intruded before it was totally solidified. The intrusions were formed from S-undersaturated, PGE-rich magmas that became S-

saturated during the course of fractional crystallisation and precipitated PGE-rich sulphides. Disruption of the intrusions and passage of high temperature fluids resulted in redistribution of the PGE. Elements such as Pd, Au and S may have been flushed out of the system entirely. Significant mineralisation occurs within the weathering profile overlying primary Pt-zones. Our studies showed that an excellent laterite profile is developed over bedrock. The PGE, especially the peaks of Pt, occur above the base metal peaks at the top of the ferruginous zone above the strongly leached zone which is the source for the enriched zone of base metals at its base (Figure 1). The Pt in this zone has either been chemically enriched by downward migration of Pt in aqueous solution or it represents a zone of residual and mechanical enrichment. Our initial results indicate significant residual enrichment. A moderate correlation exists between PGE's and Se (S) contents indicating that the enrichment occurs within a primarily, PGE and S enriched zone. References Barron, L.M., Suppel, D.W., Johan, Z. & Ohnenstetter, M., 1987. GSA Symposium: Platiniferous horizons in Layered intrusions, University of NSW, 1987. Pogson, D.J., Hilyard, D., 1981. Results of isotopic age dating related to Geological Survey - Records 20 (2): 251-273 Scheibner, E., 1976. Explanatory notes on the Tectonic Map ofNew South Wales. NSW Geol. Survey, Sydney: 283 pp. Wyborn, D., Turner, B.S. & Chappell, B.W., 1987. Aust. J. Earth Sci .34: 21-45.

Figure 1 — Location of the Pt peak at the top of the ferruginous zone in a laterite profile at Bulbodney Creek, Fifield.

135

A6.ll PETROLOGIC AND OXYGEN AND HYDROGEN ISOTOPE EVIDENCE FOR THE ORIGIN OF PGE MINERALISATION AT FIFIELD, NSW A.S. Andrew *, B J. Hensen and A.C. Dunlop 1

CSIRO Division of Exploration Geoscience, North Ryde University of New South Wales, Kensington 2

PGE mineralisation occurs in pyroxenites and initial 0 values of 5.5,5.5 and 4.1%o respectively. The peridotites within intrusive complexes with affinities to result of the subsolidus equilibrium exchange was to Alaskan-type intrusions at Fifield, NSW (Suppel & enrich the pyroxene and biotite in 0 and to deplete the Barron, 1986). The Owendale Intrusive Complex (OIC) magnetite. Open-system post-magmatic hydrothermal and adjacent Tout Intrusive Complex (TIC) contain a alternation would be associated with recognisable variety of rock types including peridotite, dunite, oxygen, and perhaps hydrogen, isotope signatures and pyroxenite hornblendite, monzogabbro, monzodiorite departures from equilibrium oxygen isotope and monzonite. Magmatic layering is variably developed fractionations between coexisting mineral pairs (Gregory in most rock types except peridotite and dunite. The & Criss, 1986). OIC and TIC are geochemically indistinguishable and Based on mineral-melt fractionations from Dunn are both shoshonitic (Agnew, 1987). At the Kelvin (1986), the calculatied 0 of the source magma at Fifield Grove prospect (OIC) Pd- and S-poor mineralisation is is 5.7%o, identical with that estimated for an average predominantly in cross-cutting clinopyroxenite veins mantle composition. Hydrogen isotope values from (0.1 to >1.0 m across) within olivine-bearing pyroxenite, hornblende and biotite are within the range measured biotite pyroxenite, pyroxenite and hornblendite. on phlogopites from kimberlites and from hornblende Orthopyroxene is not present in these rock types. The in gabbros elsewhere. Using mass balance calculations veins are commonly monomineralic with textural zoning the oxygen and hydrogen isotope values constrains any determined by crystal size and the enrichment of biotite contamination of the magma with crustal material with and magnetite in adjacent host rocks. an average 0 of 10%o to less than 5%. Oxygen and hydrogen isotope values have been On the basis of the stable isotope evidence and by determined on mineral separates from host rocks anology with similar examples from the Isle of Rhum (pyroxenite, hornblendite), veins and selvedges to place (Butcher, 1985) we suggest that the mineralised veins constraints on the relationship of mineralisation to are magmatic in origin. magmatism. Oxygen isotope values for pyroxene (8 0 References = 5.0 to 6.6), hornblende (6 0 = 5.8 to 6.6), biotite(8 0 Agnew, P.D., 1987. Unpubl. Honours thesis, University of New South Wales: 202 pp. = 5.1 TO 7.0) and magnetite (8 0 = 2.0 to 4.1) and hydrogen isotope values for hornblende (8D = -73 to Bottinga, Y. & Javoy, M., 1973. Earth Planet. Sci. Lett. 20, 250-265. -63) and biotite (8D = -73 to -56) show only minor Bottinga, T. & Javoy, M., 1975. Rev. Geophys. Space Phys. variations with no clear difference between mineralised 13, 401-418. and unmineralised samples. Oxygen isotope fraction- Butcher, A.R., 1985. Geol. Mag. 122, 503-518. ation between mineral pairs reflect high temperature Dunn, T., 1986. J. Petrology 27, 987-997. crystallisation from a magma at about 1200°C and Gregory, R.T. & Criss, R.E., 1986. Mineral. Soc. Am. Reviews subsolidus equilibrium exchange to 550°C (Bottinga & in Mineralogy 16: 91-127. Javoy, 1973,1975). Suppel, D.W. & Barron, L.M., 1986. Quart. Notes of Geol. Closed-system behaviour is implied by covariation Survey of New South Wales 65: 1-8. of 0 in pyroxene, biotite and magnetite with inferred 18

A6.12 EXPERIMENTAL CALIBRATION OF THE OLIVINE-ORTHOPYROXENE-SPINEL OXYGEN SENSOR AND APPLICATION TO PGE DEPOSITS C. Ballhaus* and D.H. Green Department of Geology, University of Tasmania

The oxygen state of the parent magma is an important variable in the formation of PGE deposits in layered and concentric Alaskan-type intrusions. The oxygen fugacity determines the stability of a sulphide phase, the capacity of a magma to dissolve sulphur, the timing of oxide crystallisation, and the complexing behaviour

of PGE in late-magmatic hydrothermal fluids. Most stratiform sulphide-PGE horizons are characterised by the associations olivine-orthopyroxenechromite or orthopyroxene-chromite-accessory quartz, hence permitting calculation of f0 via the transfer reactions 6 fayalite + 0 = 3 ferrosilite + 2 magnetite, 2

136

and 3 ferrosilite + 0 = 2 magnetite + 6 Si0 . To ferromagnesian silicates. Apart from this, however, our calibrate the magnetite activity/composition relations results may advance the understanding of PGE in complex Cr-AI spinels, we have equilibrated olivine mineralisation in Alaskan-type intrusions (Barron et (FO ) orthopyroxene (En )-spinel [Cr/(Cr+Al) from al.; Brill & Keays, this volume) and stratiform Merensky0.19 to 0.80] mixtures at a temperature range of 1000 to type PGE deposits. In Alaskan-type intrusives, unusually 1250°C and pressures of 10 and 25 kbar. All runs are Mg-rich silicates, magnetite-rich spinels up to Crcarried out under fluid-saturated conditions in a piston- magnetite, and the absence of an early-magmatic cylinder apparatus, using a modified double-capsule sulphide phase may indicate oxidising crystallisation technique. Depending on the f0 conditions sought, the conditions equivalent to MnO-Mn^, at which a sulphide samples are contained in an inner capsule of graphite, phase becomes unstable (Carroll & Rutherford, 1988). San Carlos olivine, or Pd Ag . The inner capsule is Consequently, PGE under these conditions will packed in synthetic oxide buffer material within a welded crystallise as alloys and intermetallic compounds and outer noble metal capsule. Oxygen fugacities are will be transported as chloride complexes. controlled using a range of solid buffers (W0 -WC-C, The results can also be applied to the Merensky reef Ni-NiO, MnO-Mi^O,, and Fe 0 - Fe 0 ). in the southwestern Bushveld Complex at Rustenburg The parameter most sensitive to changes in P0 is where the occurrence of oxygen-invariant mineral the Fe 7£Fe ratio in spinel (Fig.l). Within the range of assemblages permits direct calculation of oxygen partial experimental conditions the ratio is temperature pressures. The reef in that part of the Bushveld consists independent if the spinel is equilibrated along an f0 -T of a pegmatoidal orthopyroxenlte layer rimmed by two path determined by a buffer. It is also virtually constant chromitite stringers. The f0 -sensitive mineral over the range of spinel compositions, at least at low to parageneses are (1) orthopyroxene-chromlte-(ilmenite)moderate f0 . At NNO and above, Fe 7LFe falls slightly quartz, (2) olivine-orthopyroxene-chromlte, and (3) with increasing Cr(Cr+Al). For a given bulk olivine-orthopyroxene-chromhe-Al spinel-graphite. The composition, increasing oxygen fugacity also increases latter two assemblages are common in pothole the Cr/(Cr+AI) ratio in spinel. The effect of pressure is disturbances, whereas quartz-normative mineralogies to lower the Fe contents in spinel markedly in the predominate in normal undisturbed Merensky reef. intermediate f0 region, especially for aluminous Application of our results gives oxygen fugacities compositions. The redox state also affects silicate ranging from about one log unit above NNO in normal compositions. Typical minimum mg-numbers of olivine Merensky reef, to slightly less than QFM in potholes. equilibrated with the Mn0-Mn 0 buffer are 0.93, and Graphite, a common accessory and occasionally major 0.96 for the HM buffer, while oxygen fugacities phase in potholes of the Merensky reef (Ballhaus, 1988), equivalent to or less than NNO leave mg-numbers largely must be post-magmatic (~700°C) in order to be in equilibrium with spinel at these f0 conditions. unchanged. Extrapolation of the experimental results to natural Oxidation states calculated here are substantially higher assemblages has to be made with caution. Most natural than previous "intrinsic" f0 measurements suggest spinels are reset to temperatures around 700 to 800°C (Elliot et al., 1982). and enriched in iron due to cation exchange with 2

1.0 g 0.8

Q. </>

CO 0

0.6

Z^tU-i

HQ4

open symbols 1 GPa filled symbols 2.5 GPa

0.4 NNO

KHfcrt a

0.2 flHji^H-J-H 0.0

0.2

WCWO i

0.4

0.6 Cr/(Cr+AI) spinel

0.8

1.0

137

A6.13 THE ROLE OF VOLATILES IN THE FORMATION OF PLATINUM DEPOSITS E.F. Stumpfl Institute of Mineralogy and Petrology, Mining University, Leoben, Austria

Evidence for the significant role played in the formation of platinum group element (PGE) deposits now comprises macroscopic (field/underground), microscopic, analytical and experimental data from a variety of deposits and environments. There are also indications that chromite in ophiolitic environments has formed during a phase of intense volative activity. The relevance of traditional concepts such as magma mixing and scavenging of PGE by sulphide droplets, which accumulate in well-defined horizons, needs to be reconsidered in the light of new evidence. In some layered igneous complexes, such as Bushveld and Stillwater, as well as some of the complexes in Finland, potholes (disturbances of the stratigraphic succession with up to several hundred metres diameter and distinct pegmatoid features) are widespread. These are not products of magmatic erosion, but foci of intense volatile activity, with hydrothermal alteration and high graphite contents (Ballhaus, 1988). Underground development on the Stillwater JM-Reef has revealed a highly irregular distribution of mineralisation. Microscopic features include the observation that PGE mineralisation at Penikat, Sompujarvi and Konttijarvi in Finland, occurs both with and without sulphides and/or chromite. It is, however, invariably linked to hydrous silicates. Clearly, scavenging of PGE by sulphides cannot have been the decisive factor for PGE concentration in these cases. In the Duluth Complex, there are examples of PGE-contents occurring independently of sulphides maxima. Graphite and hydrosilicates are widespread in mineralised layers. In ophiolites, platinum group minerals (PGM) have been observed as inclusions in chromites, but also as products of serpentinisation in the silicate matrix. PGM inclusions in chromites are frequently associated with a wide spectrum of hydrous silicates, including phlogopite, sodium-rich amphibole, albite and copper sulphides.

Zircon inclusions have been reported in chromites from ophiolites in the Alps. In Troodos, Cyprus, the composition of silicate inclusions in chromite changes towards more alkali- and H 0-rich as one proceeds upwards from the mantle peridotite towards the cumulates (McElduff & Stumpfl, 1989). This is linked to an increase in absolute PGE contents. At New Norcia (Yarrawindah Brook), Western Australia, mobilisation of PGE during high-grade metamorphism of the Western Gneiss Terrain has resulted in PGE concentrations in excess of 1 g/t over thicknesses of 50 metres. Thermodynamic calculations (Mountain and Wood, 1988) show that PGE are soluble in hydrothermal solutions as a variety of complexes over a wide temperature range; the natural corollary to these findings is the concentration of PGE in laterites, and in Kupferschiefer-type sediments. The stability of various PGM associations (sulphides, sulpharsenides) has recently been investigated by Makovicky et al. (1989). The temperature intervals obtained (400-700°C) correspond to hydrothermal conditions suggested for natural PGM associations in environments such as the dunite pipes of the Bushveld and Complex. The accumulated evidence points towards the integral role played by volatiles in the formation of PGE deposits world-wide, a role which commences prior to the deposition of discrete PGM and is by no means confined to hydrothermal redistribution of "magmatically" formed platinum group minerals. References 2

Ballhaus, C. 1988. Econ. Geol. 83: 1140-1158. Makovicky, E., Karup-Moller, S., Makovicky, M. & RoseHansen, I., 1989. Min. Petrol. (in press). McElduff, B. & Stumpfl, E.F., 1989. Min. Petrol, (in press). Mountain, B.W. & Wood, S.A., 1988. Econ. Geol. 83: 492-510. f

A6.14 PLATINUM GROUP ELEMENTS IN THE YARAWINDAH BODY, WESTERN AUSTRALIA M. Cornelius* and B.S. Fleming Reynolds Australia Metals Ltd, Perth.

The Yarawindah intrusive body (approx. 4 km x 0.75 km) forms part of the high-grade gneiss terrain of the Yilgarn Block, and is situated approximately 130 km NE of Perth within the Jimperding Metamorphic Belt It consists of mafic and ultramafic rocks of

gabbronoritic, olivine-gabbronoritic and harzburgitic composition, metamorphosed to amphibolite and tremolitic-serpentinite. The lower contact is not exposed but is assumed to be a tectonic contact with gneisses of the Western Gneiss Terrain. The upper part of the body is conformably intercalated with quartzites and minor

138

psammites. Up to 50 m of laterite and saprolite are developed over most parts of the body. The intrusion carries significant sulphide mineralisation, comprising pyrrhotite, chalcopyrite, pentlandite and pyrite. The sulphides have largely been remobilised; primary intercumulus sulphides are rare. Primary and secondary mobilised sulphides contain discrete platinum group minerals (PGM). The most important are sperrylite (PtAs ), michenerite (PdBiTe) and testibiopalladite (Pd(Sb,Bi)Te). Sulphides and PGM are associated with hydrous minerals such as amphibole, chlorite and serpentine, as well as quartz and garnet. The association of Pt and Pd with remobilised sulphides and hydrous metamorphic minerals is ascribed to post-magmatic hydrothermal transport of PGE and sulphides during medium to high-grade regional metamorphism. Exploration drilling by Reynolds Australia Metals also revealed a PGE-enriched zone in the laterite and saprolite overlying Pt-Pd anomalous hard-rock. Combined Pt and Pd contents in saprolite-hosted anomalies vary between 0.5 ppm and 20 ppm and 2

generally exceed the Pt-Pd values of the underlying hard-rock. Pt-Pd mineralisation in the weathered zone are lensoidal and generally exceed the lateral extent of the hard-rock source. On a small scale (10 m) Pt-Pd values in the saprolite/laterite bodies show a strong variance, with values generally increasing towards the centre of the anomalies. There is a Pt-Pd depleted zone developed between the hard-rock/saprolite interface and the overlying saprolite anomaly. The lower boundary of the Pt-Pd anomaly is also reflected in changes of the mineralogical composition of the saprolite. The saprolite can be subdivided into two mineralogical zones. An upper goethite and a lower chlorite-tremolite dominated zone, both containing varying amounts of kaolinite, talc and quartz. The Pt-Pd mineralisation is restricted to the lowopart of the goethitic zone. The enrichment of Pt and Pd in the laterite and saprolite is attributed to a fluctuating palaeo-watertable, and mobilisation, transport and precipitation of Pt and Pd from meteoric fluids.

A6.15 CHROMITITE-HOSTED PLATINUM-GROUP MINERAL OCCURRENCES IN THE HEAZLEWOOD RIVER MAFIC-ULTRAMAFIC COMPLEX, TASMANIA David C. Peck and Reid R. Keays Department of Geology, University of Melbourne

The Heazlewood River complex (HRC) is the largest of several allochthonous mafic-ultramafic complexes occurring in Cambrian sedimentary belts in western Tasmania. The HRC comprises six distinctive layered ultramafic sequences, four dyke suites, low-Ti tholeiitic basalt and boninite. The ultramafic rocks are predominantly olivine- and orthopyroxene-bearing cumulates in which chromite is an ubiquitous accessory phase. Three types of chromitite are recognised in the complex. Thin schlieren of chromitite occur in the western part of the complex and consist of Cr-rich (type I) or Al-rich (type II) spinel. Type III chromitite occurs in the eastern part of the HRC, and includes pods and layers of massive chromitite that are spatially associated

with gabbroic dykes and interlayered with an unusual xenolith-bearing plagioclase peridotite. Type I and II chromitite precipitated from chromite-saturated ultramafic magma and may reflect periods of replenishment Type III chromitite developed in response to the interaction between gabbroic intercumulus liquid and ultramafic magma at the top of the crystal pile. This interaction generated a magma having only chromite on the liquidus through magma mixing and supercooling. The chromitites locally contain ppm-levels of platinum-group elements (PGE). Pt, Rh and Ru display the highest abundances, Os and Ir rarely exceed 100 ppb, and Pd is severely depleted relative to the other PGE (see table).

Selected PGE Analyses of Chromitite from the Heazlewood River Complex (data are in ppb) Sample

TOTAL

IPGE:PPGE

R72 R90 225 260 300032 300038

96 160 40 8 1700 34

110 96 41 10 1800 29

520 560 400 66 2000 59

240 130 22 78 150 70

1000 210 3 550 450 590

72 18 11 24 12 14

2038 1174 517 736 6112 796

0.55 2.28 13.4 0.13 8.99 0.18

139

The chromitites display a large range in the ratio of associated with Cr. This association may reflect the Os+Ir+Ru (IPGE) to Rh+Pt+Pd (PPGE). This is shared preference of these elements for octahedrally coillustrated on the chondrite-normalised PGE plots ordinated sites in melts and minerals (Westland, 1981; (Figure 1). The chromitites display a number of different Murck & Campbell, 1986). The precipitation of both trends including the negative sloping pattern typical of the IPGE and Cr will be promoted by fractional ophiolites and the positively sloping pattern crystallisation which can reduce the availability of characteristic of the Merensky reef. The PGE tenor of octahedral sites in a magma due to increasing Si contents the chromitites reflects varying degrees in the (Dick and Bullen, 1984). development of a bimodal platinum-group mineral The PPGE enrichment in the chromitites is attributed assemblage including inclusions of IPGE minerals to collection by S and As. The high aS associated with (predominantly laurite, RuS ) in chromite, and later chromitite in the HRC (and in many other ultramafic formed Pt- and Rh-bearing sulpharsenides, arsenides intrusions) can locally be accounted for by magma and alloys. Similar PGM assemblages are reported in mixing. However, mixing between S-poor and S-rich chromitites from ultramafic intrusions representing a magmas cannot explain many chromitite occurrences wide range of tectonic settings. These assemblages (e.g. type I and type II chromitite). In these examples a reflect the decoupling of the IPGE and PPGE in primitive reduction in the S-carrying capacity of the melt may magmas, with the former showing a strong preference reflect extensive depletion of Fe * during chromite for chromite and the PPGE for later-formed sulphides crystallisation. The low Pd abundances of chromitite and arsenides. from the HRC can be explained by the selective removal Results from the present investigation provide some of Pd by reducing fluids involved in the serpentinisation constraints on the geochemical behaviour of the PGE of the complex. during chromitite formation. IPGE-enrichment was References observed in both massive and disseminated chromite Dick, H.J.B. & Bullen, T., 1984. Contrib. Min. Pet. 86:54-76. indicating that chromitite formation is not critical to the Murck, B.W. & Campbell, I.H., 1986. Geochim. Cosmochim. Acta 50: 1871-1887. precipitation of these elements. The IPGE appear to be very insoluble in primitive magmas and are strongly Westland, A.D., 1981. C/AfAf Spec. Vol. 23: 5-18. 2

Figure 1 — Chondrite-normalised PGE plots for chromitites from the HRC.

140

A6.16 THE GILES LAYERED BASIC/ULTRABASIC COMPLEX, CENTRAL AUSTRALIA: STRUCTURAL AND PETROLOGICAL PATTERNS AND MINERALISATION POTENTIAL A.Y. Glikson , C.G. Ballhaus and T.C. Pharaoh 1

Australian Bureau of Mineral Resources, Canberra Department of Geology, University of Tasmania British Geological Survey, Nottingham, UK 2

Relicts of layered basic/ultrabasic intrusions occur within felsic granulites and are intruded by granite throughout most of the Musgrave Block, from the Everard Range in the east to isolated outcrops northwest of the Jameson Range in the west, a distance of over 500 km along strike (Fig. 1). Principal earlier work has been summarised by Nesbitt et al. (1970), Daniels (1974) and Thomson (1977). Two structurally distinct provinces are identified, including (1) strongly deformed intrusions rich in ultrabasic components (Kalka, Mt Davies, Michael Hills and Hinckley bodies) and (2) gently dipping to flat lying instrusions consisting almost exclusively of gabbro (Blackstone, Cavanagh and Jameson bodies) (Fig. 2). The layered intrusions have been emplaced within felsic granulites which underwent earlier deformation (D ), and their intrusion resulted in deformation (D ). In turn the layered bodies are intruded and partly metamorphosed by granite gneiss which resulted in folding (D ) and are faulted along mylonitic shear zones such as the Hinckley Fault. Extensive recrystallisation of the gabbro into basic granulites has occurred along margins of the intrusions. The deformation resulted in fragmentation of originally contiguous intrusions into elongated slices whose correlation is hampered by lateral and vertical heterogeneities of the magma chambers. A detailed study of the 1600 m-thick Wingellina Hills intrusion for this body, involving pulses of ultrabasic magma injected into a chamber of crystallising olivine gabbro and x

24°

—i128° —

gabbronorite. Significant variations in the ratio of ultrabasic/basic components are observed in the Hinckley and Michael Hills intrusions. Thick successions of peridotite/pyroxenite (Murray Range) and pyroxenite/ gabbro (Latitude Hill) represent centres of ultrabasic magma injection. The injection of hot ultrabasic magma into cooler chambers resulted in supercooling and intraplutonic quenching which expanded the field of metastable crystallisation of chromite. Changes in f0 associated with magma mixing also enhance the formation of chromite, minor concentrations of which occur in pegmatoid pyroxenites. The study of mineral parageneses, including opx-cpx-spinel symplectites along olivine-plagioclase boundaries, indicates isobaric cooling under pressures around 6 kbar. The thickness of the magmatic successions of the Giles Complex and the common intercalation of gabbroic, pyroxenitic and peridotitic units, provide a geological setting in which chromite and platinum group element mineralisations can be expected to occur. The identification of sulphide mineralisation along contacts and transition zones between basic and ultrabasic units requires detailed inch-scale studies of selected successions. References 2

Daniels, J.L., 1974. Geol. Surv. W. Aust. Bull. 123 Nesbitt, R.W. Goode, A.D.T., Moore, A.C. & Hopwood, T.P., 1970. Spec. Publ. Geol Soc. S. Africa 1: 547-564 Thomson, B.P., 1977. Aust. Inst. Min. Met. Mon. 5:451^60.

136°

132°

A M A D E U S BASIN

.. .Cauldron subsidence granite and acid / r • ivolcanics; layered bimodal volcanics: — cj IOOO Ma andca 1040 Ma Giles Complax.layered basicultra basic intrusions.ca 1000-1050

Retrograded acid gneiss, schist. nugmatite.Kulgeran granite; ca 1170-1050 Ma High-grade granulites; ca 1550-1200 Ma ...... Acid to basic gneisses: amphibolite \ / / \ transitional granulite; ca 1650 Ma Kulgeran granites; ca 1120-1075 Ma

OFFICER BASIN 28°

300 j km

i 6/662/T

Figure 1 — Geological sketch map of the Musgrave Block, portraying its principal structural and lithological domains.

141

128° ~ Ml Mo«

~/ s ~

' 0 IEHMANN HILLS / r

—

\ x

. ft / ' Os 1

•I ^

20 km , kII/I

Giles u n e a Complex uu

I / /// J ' I I(basic - -u/trabasic rocks) # gneiss, felsic 'A ' ( 1 ' I ' ' ' i1 B » 7-C „ • • / ^Ml Daisy Bates | ^ Major road

Minor road or track Major

drainage

Generalised

'uuuJrhy.'Sli-

Ml Mprph«U

- ion

sand dunes

Height (m) above S.L.

;

l ' N X \\V,V v N

. i • c-

• V

Figure 2 — Principal outcrops of the Giles Complex and associated granulites and gneisses in the Tomkinson Range, WA andSA.

A6.17 THE CHLORINE CONNECTION AND A HYDROMAGMATIC MODEL FOR THE CONCENTRATION OF THE PLATINUM-GROUP ELEMENTS IN THE STILLWATER COMPLEX, MONTANA A. Boudreau Department of Geology, Duke University, Durham, NC, USA

It has become increasingly apparent that the pedogenesis of platinum-group element (PGE)-enriched sulphide deposits in large layered intrusions such as the Stillwater and Bushveld complexes must have involved the interaction of a volatile-rich fluid phase at some point in their formation to produce the pegmatitic textures, graphitic assemblages and the high temperature, Cl-rich hydrous minerals associated with these deposits. Apatite is a useful indicator of fluid activity as it is stable at magmatic temperatures and does not readily reequilibrate on cooling. Because of the markedly different partitioning behavior of CI (vaporphile) and F (magmaphile), variations in the Cl/F ratio of apatite allow insights to the postcumulus evolution and migration of vapor through the cumulate sequence of layered intrusions. Apatite compositions from sill and dyke rocks below the Stillwater Complex, which are believed to be

representative of the parental magmas of the complex, typically contain > 1.2 wt % F and < 3.0 wt % CI. The stratigraphic trend of the Cl/F ratio in the cumulate sequence is characterised by an initial rapid increase in this ratio in the Basal series and lower Ultrmafic series toward chlorapatite end-member compositions, the apatites typically containing < 0.4 wt % F and > 6.0 wt% CI. A change from Cl-rich to more F-rich apatite compositions occurs within Olivine-bearing zone I (OB I), the host zone of the platiniferous J-M Reef. Although apatite compositions are variable above the JM Reef, F-rich compositions predominate and are similar to those of the sill/dyke rocks. Below the J-M Reef, the chlorapatite-bearing cumulate section is characterised by the lack of stratiform disseminated sulphide zones, with sulphide mineralisation occurring instead as podiform accumulations of disseminated and locally massive sulphide associated with pegmatites.

142

Insofar as the sill/dyke rocks are representative of migrates upward and carries with it the vapor-compatible the parent melt Cl/F composition, the initial Stillwater elements which were originally concentrated in a minor magma(s) was not unusually Cl-rich. Further, the fluor- cumulate sulphide fraction, e.g. S, Pt, Pd, Cu, Ni, As, apatites at the base of the Stillwater imply that the Cl- Te. The upward migration of vapor is limited at any rich apatite higher in the section are not the consequence time to the level at which the interstitial melt is vaporof an infiltrating, Cl-rich country fluid, nor can they be saturated; vapor migrating above this level must remodeled by conventional fractional crystallisation dissolve in the hotter, vapor-undersaturated schemes. intercumulate melts and both enrich the melt in CI and To explain the enrichment of CI below the J-M precipitate sulphide, which is relatively insoluble in Reef, the lack of sulphide zones below the reef, and the silicate melt. As the cumulate pile continues to solidify, association of the Reef with a break in the Cl/F ratio of the vapor-saturation boundary moves upward. Upward apatite, it is proposed that the origin of PGE-enriched migration of the ore front may be limited by zones in layered intrusions is broadly analogous to the (a) encountering a stratigraphic discontinuity, as might formation of roll-front uranium deposits. In a roll-front occur at the level at which a fresh injection of primitive uranium depost (Fig. 1 A), oxidised groundwater moving melt occurred, (b) by the complete degassing of the through a permeable aquifer transports U, which is footwall, causing the sulphide front to become frozen at soluble in oxidised solutions. Where this oxidised ground a level within the cumulate pile, or (c) by reaction with water encounters sediments containing organic material, the cumulates. The buildup of a mineralisation front the solution reacts with the organic material, becomes may occur several times as the solidification zone reduced, and precipitates U which is insoluble in reduced advances, and can explain multiple mineralised zones aqueous solutions. The destruction of the organic in any intrusion. However, high grade PGE zones high material during oxidation at the reaction front causes in an intrusion would be unlikely owing to (a) the loss the roll front to migrate in the direction of fluid flow of CI during degassing at the top of the chamber, (b) the cumulates become more adcumulate with stratigraphic over time. An analogous situation can occur during the height, limiting the amount of degassing possible from solidification of a large layered intrusion (Fig. IB). The the cumulates, and (c) the PGE behaving as compatible solidification of interstitial melt deep within the cumulate trace elements for melts saturated in sulfur, hence pile leads to saturation in a vapor phase, which cumulates are a progressively poorer source rock for preferentially takes up CI. On exsolution, this vapor the PGE with increasing stratigraphic height t

143

A7: Precambrian Gold Deposits Convenors: DJ. Groves and G. Davidson

A7 Keynote Address THE STRUCTURAL AND TECTONIC SETTING OF PRECAMBRIAN GOLD DEPOSITS IN AUSTRALIA Michael A. Etheridge Research School of Earth Sciences, ANU, Canberra and Tectonex Geoconsu Itants, Canberra

Precambrian gold deposits in Australia span a period of >2 billion years, occur in virtually every significant rock type, are associated with a wide range of other metals, and are found in a broad spectrum of structural and tectonic environments. Do they, however, have any structural and/or tectonic features in common, and, if so, are their common features of any use in understanding genetic processes and designing exploration programs? The great majority of the deposits are hydrothermal, were deposited in the temperature range 150° to 400°C, and formed at significant but moderate depths (3-10 km) in the crust That is, they are predominantly mesothermal. In broad terms, mesothermal deposits depend on the provision of a suitable hydrothermalfluid,a gold source, energy to drive fluid migration, and active structures to channel fluid flow. This paper concentrates on the structural influences on these parameters in Precambrian gold deposits, outlining the key structural principles, and introducing examples from a range of Precambrian terranes and tectonic settings. At the deposit scale, structural setting or control simply involves the provision of high permeability pathways and hydraulic gradients, which result in the channelling of a gold-enriched fluid through a small volume of chemically receptive host rock. The critical factor for both permeability enhancement and the establishment of suitable local hydraulic gradients is deformation-induced dilatency. Dilatency results from virtually any deformational style, provided that the fluid pressure is significantly above hydrostatic. Since fluid pressures in most deformational environments are generally close to lithostatic, dilatency is to be expected during rock deformation. Variations in the amount of dilatency in a deforming rock mass depend principally on two factors, local strain-rate and the geometry of structures. The principles of localised dilatency in shear/ fault zones and folds have been widely published in the past few years and provide the framework for predicting the location of zones of enhanced dilatency, fluid flow and consequent prospectivity. However, an enriched gold fluid must generally

have passed through a relatively large volume of rock to scavenge its gold, thus requiring large scale recirculation of thefluid.Fluid recirculation, especially in a high fluid pressure environment, requires a complex and dynamic interplay of rock deformation, dissolution and precipitation by the fluid, and unfamiliar time/ depth/fluid-pressure relations. In the late Archaean greenstone terranes, which have produced the bulk of Precambrian gold in Australia, it is widely accepted that most of the gold was precipitated from a regional metamorphic fluid in active shear/fault zones. The emphasis has recently shifted from prograde to retrograde metamorphic settings, and awayfromearly structures to those (usually wrench-related) which occur late in the deformational sequence. Increased attention is also being given to the role of late intrusives of both crustal (K-rich granites/porphyries) and mantle (lamprophyres) origin. In detail, however, some deposits apparently did form early in the deformational sequence, probably associated with prograde metamorphism (e.g. Sons of Gwalia, Harbour Lights, ?Golden Mile), whereas others formed tens of millions of years later, well after the metamorphic peak (e.g. Lady Bountiful) and some clearly occur in late wrench structures (e.g. Mertondale). Deposits occur in virtually every rock type present in the Yilgarn province, from virtually undeformed to thoroughly mylonitised. Despite some consistency of fluid chemistry and associated alteration, there appears to be no simple, common process or model that provides a predictive tool for exploration for Yilgarn gold deposits. Most of the recent discoveries have been in "the shadow of the headframe", with relatively few greenfields discoveries that have required the application of concepts derived from research. Until recently, Proterozoic gold deposits in Australia were few, and models involving syngenesis/diagenesis in reduced, commonly Fe-rich sediments prevailed. However, as the number and geographic spread of deposits has increased, the importance of mesothermal processes has become apparent Gold in the Proterozoic ranges from about 1850 Ma (Pine Creek, ?Granites) to

144

possibly as young as 650Ma (Telfer), occurs in granitoid contact aureoles (Telfer, Pine Creek), a range of compiessional (Jabiluka, ?Tennant Creek) and extensional (Olympic Dam) structures, and in a wide range of rock types. Gold in the Proterozoic occurs more commonly in polymetallic deposits (Au-Cu-Bi at Tennant Creek, Au-Cu in the Cloncurry/Selwyn region, Au-Cu-U-REE at Olympic Dam, Au-Pt-Pd at Coronation Hill). There is some evidence that important Au deposits and deposit types in the Proterozoic occur on or are controlled by reactivation of earlier formed structures. Even more than in the Archaean, deposit models, unless extremely broad, are almost as numerous as deposits, and they are of limited use in understanding specific deposits or guiding regional exploration. Despite the increasing number of impressive discoveries in the Australian Proterozoic, it pales as a gold resource compared to the Archaean and the Palaeozoic, especially on an areal basis. Mesothermal fluid activity was clearly widespread in a number of terranes which are relatively gold-poor, although the same terranes are commonly rich in base metals (e.g., Mount Isa, Broken Hill). However, the fluids involved are commonly significantly more oxidised and saline than those responsible for gold transport in the Archaean, reflecting the widespread occurrence of evaporitic, carbonate and other oxidised rocks in the Proterozoic sequences. Suchfluidsare more suitable for base metal than for precious metal transport. There are extensive, low to medium grade metamorphic terranes with similar tectonic, structural and metamorphic environments to those of the Archaean and Proterozoic, but which contain

little gold. It is not clear whether this is a gold source or transport problem. Returning to the questions posed in the opening paragraph, the following conclusions emerge. • Precambrian gold deposits have little in common with regard to their structural and tectonic environments, except that their locations are generally structurally controlled, and the fluids are broadly mesothermal. Broad tectonic differences between, for example, the late Archaean and the early to middle Proterozoic provide some basis for their contrasting prospectivities for gold, and for some of the contrasts in deposit styles. • Deposit models that are sufficiently constrained and detailed to be predictive are of limited use in directing exploration, especially at the grass-roots level, because the deposits themselves have such a wide range of characteristics, and the models are likely to be specific to the deposit(s) from which they were derived. More generalised models are insufficiently predictive to direct exploration, but they do provide an important conceptual framework, within which to analyse individual deposits or target areas. • Exploration is best underpinned by a thorough understanding of the geology of the target area. For example, generalised models of structural control (e.g., dilatent jogs, Riedel shears) are of little use unless the structural geometry and history of the target area are thoroughly understood. Careful geological mapping that attempts to determine the 3D spatial and temporal relationships between rock types, rather than simply to catalogue their surface distribution, should be the basis for such exploration.

A7.1 ALTERATION ZONES AROUND MESOTHERMAL GOLD DEPOSITS: IMPLICATIONS FOR PATTERNS OF DEPOSITION AND FLUID SOURCES. A.C. Barnicoat Key Centre for Strategic Mineral Deposits, University of Western Australia

Amphibolite facies gold deposits occur in brittleductile shear zones, and are characterised by mineralogically simple assemblages. That such deposits are syn-tectonic is suggested by the parallelism of stretching lineations and ore shoots, and confirmed by presence of less deformed quartz veins and alteration zones cutting strongly deformed ones. The synmetamorphic nature of the deposits is indicated by the low variance of the alteration assemblages and by the pattern of assemblages, which represents an increase in aC0 with increasing alteration. The dissimilarity in assemblages between greenschist facies gold deposits (characterised by some of the minerals dolomite, chlorite, muscovite and albite) and amphibolite facies deposits (which contain some of the minerals diopside, tremolite, biotite, cordierite, grossular-rich garnet and zoisite) is due not to widely 2

differing alteration processes and fluids but rather to differences in P-T conditions. This can be demonstrated by examining activity diagrams such as aCa * against aMg + and aCa * against aK+ and aSi0 . These diagrams, constructed from thermodynamic data, allow prediction of the consequences of the interaction of a fluid with host rock of varying compositions and/or at varying temperature (cf. Bohlke, 1989). Using activity diagrams, limits on the composition of the altering/mineralising fluid can be made fairly simply. With that knowledge of the (approximate) fluid chemistry, it is possible to predict the minerals in equilibrium with thefluid.This mineralogy may be that of the primary source, or of a secondary source with which thefluidmay have equilibrated on its way to the alteration site. At temperatures close to those of the source (primary or secondary), alteration will occur 2

145

only where the fluid interacts with rocks with a mineralogy quite distinct from the source. As the temperature difference between the source and alteration region increases, disequilibrium caused solely by the change in temperature will become significant, and rocks similar in bulk composition, if not mineralogy, to the source will undergo alteration. The composition of fluid inferred from the activity diagrams is such that the fluid may have been in equilibrium with either a mafic or granitoid source which may have been primary or secondary. Under amphibolite facies conditions, the mineralogy (though not the mode) of both of these rock types will be similar (qtz-plag-hb±bi)\ many granitoids contain ksp in addition to bi. Fluids in equilibrium with both of these rock types will thus be of similar composition at a given temperature and the presence of absence of biotite will be more significant in controlling the K content of the fluid than ksp. The above has two implications. Firstly, the bulk solute composition of the fluid does not enable distinction between a granitic (magmatic) and mafic (metamorphic) source for the fluids. Secondly, at a temperature close to those of the source, afluidgenerated from either type of source will be relatively close to equilibrium with mafic and felsic rocks, but strongly out of equilibrium with other, more exotic compositions

such as BIF and ultramafics. Alteration will thus be most pronounced where the fluid passes from mafic or granitoid rocks into a chemical environment defined by lithologies such as BIF or ultramafics. In general, this means that alteration (and mineralisation if causally related) will be important near the lower contacts of BIF and ultramafics with mafic and granitoid lithologies. The precipitation of gold under these conditions is likely to be driven by alteration reactions in a similar fashion to greenschist facies deposits (e.g. Phillips, 1984). Changes in the fluid chemistry induced by alteration, such as decreases in sulphur activity and pH, will cause a decease in gold solubility as Au(HS)"2. Recent experimental data (Shenberger & Barnes, 1989) show that above 250-300°C, gold solubility in fluids buffered by plausible assemblages (such as py-po-mt and ksp-mus-qtz) either increases only quite slowly or actually decreases, in contrast to extrapolations made from the data of Seward (1973). References Bohlke, J.K., 1989. Econ. Geol. 84: 291-327. Phillips, G.N., 1984. In Foster, R.P. (ed): Gold '82. A.A. Balkema, Rotterdam: 389-416. Seward, T.M., 1973. Geochim. Cosmochim.Acta, 37:370-399. Shenberger, D.M. & Barnes, H.L., 1989. Geochim. Cosmochim. Acta, 53: 267-278.

A7.2 THE STRUCTURAL AND HOST ROCK SETTINGS OF PRIMARY MINERALISATION AT THE BODDINGTON GOLD MINE P. Symons*1- G. Anderson1, L. Hamilton1, T. Beard1, R. Staley1, E.Roth2 and S. Ho2 2

1 Boddington Gold Mine, Boddington, Western Australia Key Centre for Strategic Mineral Deposits, University of Western Australia

The Boddington gold deposit is situated 100 km SE of Perth in the Saddleback greenstone belt, an Archaean sequence of metavolcanic and metasedimentary rocks with the Western Gneiss Terrain, Yilgarn Block. Zircon U-Pb geochronology indicates eruption ages of 2670 to 2650 Ma for the volcanic rocks (Wilde and Pigeon, 1986). Primary gold mineralization occurs within variably hydrothermally-altered, felsic and intermediate volcanic rocks and intrusions underlying an extensive lateritic gold deposit containing reserves of 60Mt@ 1.6 grams Au/tonne (e.g. Symons et al., 1988). Bedrock gold resources have been identified within the Pipeline, A Breccia, H, D West, Supergene, and Diorite prospects (Fig. 1). Interpretation of aeromagnetic and radiometric data indicates that the regional structure is dominated by a sinistral strike-slip shear zone/fault system, with the Boddington mineralisation situated along a major flexure. The western limit of the mineralised host rock sequence is defined by a zone of intense deformation. Granitoid plutons occupy an area of structural disruption to the east On a prospect scale, both alteration intensity

and gold mineralisation are spatially related to areas of high fracture density and brecciation in a brittle-ductile shear regime. Brecciation is the dominant control at the A Breccia, Pipeline, and F Supergeneprospects, with a fracture-vein association evident at the D West and the Diorite prospects. The regional sub-vertical foliation overprints and locally reactivates both mineralisation and alteration assemblages. Reactivation is evident in the H prospect, where brecciation is overprinted by pyritic, quartz-clinozoisite schists within the ductile shear zone. Bedrock lithologies include aphyric and porphyritic diorite, andesite and schistose, felsic to intermediate volcanic and pyroclastic rocks with subordinate dacite, basalt and ultramafic rocks. Hydrothermal alteration ranges from the incipient albitisation of plagioclase to complete textural destruction. Hydrothermal minerals within the diorite/andesite suite include biotite, actinolite and clinozoisite/epidote. Alteration zonations are poorly developed, but where they occur hydrothermal biotite and the alteration of plagioclase to albite, sericite and clinozoisite is ubiquitous. Hydrothermal actinolite is

146

restricted either to the groundmass of intermediate al., 1988). Minimum trapping temperatures were 200°C intrusions (including diorite and hypabyssal equivalents, to 300°C, but are interpreted to be in the order of400°C and adjacent andesites), or to distinctive sulphide-bearing at 0.8 to 1 kbar. Ore fluid composition is best represented veins. Clinozoisite/epidote is widely distributed, by the I^O-NaCl-CaC^ system, with metals transported occurring within sulphide±actinolite-bearing veins, in as chloride complexes. This contrasts with the typical distinctive stringer veining within the Pipeline prospect, Archaean mesothermal gold deposits of the Yilgarn and vein breccias. Metamorphic grades are greenschist Block, in which ore fluids were low salinity (up to 3 wt facies, with hydrothermal minerals typically re- % NaCl equiv.), HjO-CO^rich (up to 25 mol. % C0 ) crystallised into distinctive metamorphic aggregates. fluids transporting metals as reduced sulphide complexes This contrasts with the retrograde nature of alteration (Ho, 1986). associated with Archaean gold deposits in greenschist The primary mineralisation displays features facies terrains elsewhere in the Yilgarn Block. Post- consistent with a dioritic porphyry copper model, and is mineralisation dolerite dykes intrude the entire sequence. considered to be an example of a structurally-controlled, Gold mineralisation is dominantly associated with Archaean gold-rich porphyry ore system. In contrast to chalcopyrite, pyrrhotite, and pyrite in quartz± the widespread mesothermal "gold-only" deposits clinozoisite± actinolite veins. Other ore mineral phases, elsewhere in the Yilgarn Block (e.g. Groves & Phillips, in decreasing order of abundance, include molybdenite, 1987), this potentially represents a new type of arsenopyrite, scheelite, bismuth and galena. In contrast exploration target in Archaean greenstone belts. to gangue minerals, ore mineral zonations are distinct References Massive sulphide veinlets, adjacent to diorites within Groves, D.I. & Phillips, G.N., 1987. Ore Geol Rev. 2:287-322. the Diorite prospect, consist of complex intergrowths of Ho, S.E., 1986. Unpubl. PhD thesis, Univ.West.Aust.: 90p. chalcopyrite and pyrrhotite, with relatively minor pyrite Ho, S.E. et al., 1988. Unpubl. report to Boddington Gold Mine: 9p. and sphalerite. In peripheral zones, pyrite is the dominant Symons, P.M. et al., 1988. Geol SocAust. Inc. Abstr. 22: iron sulphide. 56-61. Initial fluid inclusion studies indicate that ore Wilde, S.A. and Pidgeon, R.T., 1986. Aust J. Earth Sci. 33: mineralizing fluids were highly saline (up to 40 wt % 491-501. NaCl equiv.) and probably magmatic in origin (Ho et 2

BODDINGTON GOLD MINE AREA

147

A7J

AN EPIGENETIC ORIGIN FOR THE LANCEFIELD GOLD DEPOSIT

J.M.A. Hronsky l * t S.E. Ho \ D.I. Groves \ RP.A. Peniam 2 and J.R.Vearncombe1 1

Key Centre for Strategic Mineral Deposits, University of Western Australia 2 Western Mining Corporation, Belmont, WA.

The Lancefield gold deposit, located in the northeastern part of the Eastern Goldfields Province, Yilgarn Block, Western Australia, is hosted by an Archaean greenstone sequence. The regional geology of the mine area is dominated by several moderately east-dipping, north to north-easterly trending, high-strain zones that are sub-parallel to lithological contacts. Major rock types in the mine area include granitoid, ultramafic rocks, metabasalt, metaconglomerate and metawacke. Gold mineralisation is hosted by two essentially tabular lode zones. Main Lode and West Lode, oriented sub-parallel to the local lithological sequence. These lode zones are discrete (typically 1-10 m wide), highstrain zones within a heterogeneous zone (typically >100 m wide) of both high- and low-strain rocks, termed the Lancefield deformation zone. This deformation zone comprises ultramafic rocks in the footwall of the lodes and metabasalt and metadolerite surrounding the lodes, which are largely strongly altered metasedimentary rocks (see below). Foliation intensity increases towards the lode margins, and reverse fault displacement, best indicated by the offset of transgressive felsic dykes, has occurred in the plane of the lodes. Complex internal faulting within the lode zones is common, with the local development of structures resembling duplexes. The main ore-shoots on both lodes occur within major, steeply plunging flexures of the lodes and plunge sub-parallel to theseflexures.These major flexures occur adjacent to, and plunge sub-parallel to, the truncation of the lode-hosting rock units by the major fault contact with the structurally underlying ultramafic sequence. This structural geometry is interpreted to be a consequence of the interaction of the Lancefield deformation zone with a rigid granitoid body which protrudes into the footwall ultramafic sequence in the mine position. Other subsidiary ore positions along strike from the main ore shoots can also be related to steeply plunging flexures of the lodes. In addition, low amplitude, subhorizontally plunging flexures of the lode are an important subsidiary control on grade variation within dominantly ore-grade areas (i.e. ore shoots) within the lodes. The lode zones typically comprise one or more siliceous units within strongly altered mafic schist (after metabasalt or metadolerite). Both the siliceous units and the altered mafic schist host gold mineralisation, with the siliceous units typically (but not exclusively) being the most important host. Minor mineralisation is associated with cross-cutting quartz veins within the lode zones. Geometrical relationships between the siliceous unit(s) and the altered mafic schist are often complex and controlled by internal faulting. Locally,

there are lateral transitions from altered mafic schists into siliceous units. In the simplest situation, a central siliceous unit is flanked symmetrically by altered mafic schist, but locally the lode zone comprises altered, veined mafic schist with no associated siliceous unit. Minor cross-cutting felsic dykes are mineralised where they are cut by the lodes. The siliceous lode units have a dominant mineralogy of quartz-sulphide-carbonate, with or without minor muscovite, chlorite and biotite. Typically mm to cm scale segregations of quartz-dominant and sulphidedominant domains define an anastomosing fabric, subparallel to that in the surrounding mafic schists. Commonly, sulphide-dominant layers will asymmetrically wrap augen-shaped, siliceous domains. Commonly, this fabric is partially or totally overprinted by recrystallisation, resulting in a coarse (cm scale) segregation of sulphidic and siliceous domains. In the parts of the siliceous units that have not been extensively recrystallised, fabric-parallel quartz veins are common. These are distinguished by the occurrence of fabricparallel inclusions of wall-rock and, locally, the occurrence of symmetrically layered quartz veins with fibrous grain-growth normal to the vein wall. Commonly, pyrite-rich layers show a symmetrical layering, defined by variations in grain size, about a discrete plane. This implies incremental pyrite growth about this plane. Much of the material forming the siliceous units within the lode zones is interpreted to have had a carbonaceous shale protolith. Evidence for this includes the ubiquitous presence of accessory carbonaceous material, rare remnant framboidal pyrite and elevated contents of Zn and Cu (mean values 2000 ppm and 400 ppm, respectively), elements commonly enriched in exhalative interflow sediments. There is no evidence for relict sedimentary textures within these siliceous units, and although Zn and Cu are moderately correlated neither correlates with Au. Within the siliceous units, there is a contrast between less siliceous rocks with high phyllosilicate and carbonaceous material contents in the sub-grade of the lode and the intensely silicified rocks of the ore zones. Geochemical data indicate that A1203, Ti0 2 and Zr, as in other Archaean gold deposits, are relatively immobile within Main Lode, and that there is a broadly linear relationship between increasing gold grade and decreasing content of these elements. This relationship is consistent with progressive addition of hydrothermal components to host rocks in the mineralised zone, as recorded in other Archaean epigenetic gold deposits (e.g. Phillips & Groves, 1983; Skwarnecki, 1987). Gold-related carbonation extends both into the

148

hangingwall and footwall of the lodes. Fluid inclusion data indicate a low salinity Kp-C02-CHA ore fluid and depositional P-T conditions of about 2 kb and 245-330°C. Carbon isotope data for carbonates from within the lode zones show an extreme variation in 13C, characteristic of fractionation between C0 2 and CH4 in the ore fluid. As the lodes contain significant carbonaceous material, these data are interpreted to indicate introduction of a low salinity Hp~C0 2 fluid to a reducing carbonaceous environment with resultant CH4 production and variable COJCliA in the fluid. Both fluid inclusion and carbon isotope data are inconsistent with a syngenetic exhalative model for the gold deposit Oxygen isotopic data for carbonates and quartz associated with the mineralisation are within the range of 18 0 from 9.3 to 16.5% and are similar to data from other, clearly epigenetic, Archaean deposits (e.g. Golding & Wilson, 1987). These values are significantly higher than would be expected if seawater was the major source of mineralising fluids. Studies of sulphide morphology and geochemistry indicate that, although both diagenetic and epigenetic populations are present, gold is associated with epigenetic pyrite and arsenopyrite that formed late

in the paragenetic sequence. The model age derived from the least-radiogenic lead isotopic data from the pyrites indicates that the mineralisation has a similar age (ca. 2.65 to 2.60 Ga) to other epigenetic gold deposits in the Eastern Goldfield Province (Dahl et al., 1987). This deposit has previously been considered as an example of a stratiform, syngenetic gold deposit. However the mineralisation is not stratiform, nor even stratabound in detail. Ore-rock spatial relationships together with structural, textural and geochemical evidence, collectively indicate an epigenetic origin for the Lancefield gold deposit. References Dahl, N., McNaughton, N.J. & Groves, D.I., 1987. GeolDept &. Univ.Extension, Univ. West Australia, Publ. 11: 189-201. Golding, S.D. & Wilson, A.F., 1987. Geol. Dept & Univ.Extension, Univ.West Australia, Publ. 11: 203-213. Phillips, G.N. & Groves, D.I., 1983. J. Geol. Soc. Aust. 30: 25-39. Skwarnecki, M.S., 1987. Geol.Dept & Univ.Extension, Univ.WestAustralia, Publ. 11: 1009-1135.

A7.4 PROTEROZOIC GOLD DEPOSITS, THE WITWATERSRAND GOLDFIELDS, AND MODERN GOLD PLACERS OF NOME (ALASKA) G. Neil Phillips1* and David J. Sumpter2 2

1 Box 228, Cotteslow, WA Windfall Gold Co., Nome, Alaska

The Witwatersrand gold deposits of South Africa have been widely described as fluvial-alluvial fan gold placers in atypical "Proterozoic-style" basin of ca. 2400 Ma. As such, they would represent far more gold than all other Proterozoic deposits combined: furthermore Witwatersrand-style targets would be highly attractive for exploration in Proterozoic basins in Australia. Regrettably, some of these data still quoted for the Witwatersrand basin are no longer accurate, particularly the age and likely control on the mineralising process. Published zircon dating indicates that the Ventersdorp metavolcanics overlying the Witwatersrand are 2699±16 Ma implying the Witwatersrand is clearly Archaean (Armstrong et al., 1986). More importantly, the Witwatersrand succession was in place at the time of major hydrothermal gold introduction evidenced in other Archaean successions including Western Australia, Zimbabwe and Canada at ca. 2650-2700 Ma (McNaughton & Dahl, 1987; Marmont & Corfu, 1988). Sedimentological studies have confirmed that significant parts of the auriferous Upper Witwatersrand may be fluvial, but that the main reef packages accounting for >30,000 tonnes Au best fit marine transgressions (Cadle et al., 1987; Phillips et al., 1988; Bailey et al., 1989). The quest for modern analogues for the

Witwatersrand deposits provides little comfort to the placer model for gold: instead, modern marine gold placers are rare and/or small, and the major gold placers are today found in degrading stream sections as linear mineralised zones (unlike the planar aspect and inferred marine setting of the Witwatersrand reefs). Nome, Alaska, is the only recorded marine gold placer of any size (ca. 6-10 million ounces of gold produced from the immediate district), and gold has been mined in a variety of geological settings (Sainsbury, 1975). Lode gold deposits occur 5 km and more from the coastline as a series of fault-related veins in graphitic schists and marbles (Read & Meinert, 1986), residual placers occur near many of the lodes, stream placers form downstream from lode deposits, buried placers underlie the coastal plain in old alluvial fans, the sand on the present beach is gold-mineralised, and further gold placers now covered by the ocean are being mined offshore. Nome provides a coherent picture of primary lode gold as a source of fluvial and marine gold placers, and in all cases mining has demonstrated linear gold deposits whether fluvial or marine. The key parameters in the development of the Nome gold fields are an abundant proximal gold source, an active process of burial and preservation, and only limited gold mobility after burial (low temperature groundwaters and permafrost).

149

Thus these deposits at Nome, although the only significant modern marine gold placers, do not provide a ready analogue for the Witwatersrand nor do they suggest that the processes occurring during marine sedimentation in the Witwatersrand would have been particularly effective in sorting and concentrating gold. Nome provides little evidence that marine processes are effective in substantially upgrading low concentrations of gold into high grade deposits. However, all this does not mean the Witwatersrand marine intervals were not critical to gold mineralisation: clearly they have been most important in concentrating and/or preserving heavy minerals in the narrow reef package intervals (Phillips & Myers, in press). References Armstrong, R., Compston, W., Retief, E.A. & Welke, H.J., 1986. Geol. Soc. S. Africa, ExtdAbst.: 89-92.

Bailey, A.C., Law, J.D.M., Cadle, A.B. & Phillips, G.N., 1989. EGRU Info. Circular, Univ. Witwatersrand: 212. Cadle, A., Bailey, A., Law, J. & Phillips, N., 1987. Soc. Econ. Paleont. Mineral, meeting, Texas 5: 11. McNaughton, N.J. & Dahl, N., 1987. Recent advances in understanding Precambrian gold deposits. Univ. West. Aust. Geology and Extension: 29-49. Maimont, S. & Corfu, F., 1988. Geol. Soc. Aust. ExtdAbst. 22: 45-50. Phillips, G.N. & Myers, R.E., in press. Gold 88. Phillips, G.N., Myers, R.E., Law, J.D.M., Bailey, A.C., Cadle, A.B., Beneke, D., Borrego, P.M.D.A., Giusti, L., Ingle, L., Kerr, S.J., Palmer, J. A., Ramos, Z.C.D.N. & Roberston, N.S., 1988. Geol. Soc. Aust. 22: 319-324. Read, J.J. & Meinert, L.D., 1986. Econ. Geol. 81:1760-1774. Sainsbury, C.L., 1975. US Bureau Mines Open File Rep. 7375: 108 pp.

A7.5 ARCHAEAN GOLD MINERALISATION DURING LATE VERTICAL MOVEMENTS IN THE GRANITE-GREENSTONE BELTS OF THE YILGARN BLOCK, W.A. T.J. Cudahy CSIRO, Division of Exploration Geoscience, Perth.

Mesoscopic structures have been mapped at numerous field and mine localities across the Yilgarn Block. These structures provide evidence for the nature of megascopic structures (e.g. regional folds and crustal shear zones), the chronology of Archaean structural development, the stress/strain conditions at the time of their formation and their possible tectonic environment The types of mesoscopic structures mapped, include: (i) mineral foliations/lineations; (ii) asymmetric and symmetric folds; (iii) fractures, faults and veins (conjugate, en echelon and tensional); (iv) pressure shadow features; (v) slickensides on fault surfaces; (vi) evidence of cataclasis; and (vii) "boudinage" structures. Particular attention was paid to the overprinting and/or anisotropic relationships between the different structural/geological elements, as well as their orientation. Hydrothermal gold mineralisation in the Yilgarn Block is generally associated with narrow, steeply dipping zones of brittle-ductile to brittle deformation. A cross-section of a typical mineralised zone would comprise relatively unstrained and massive country rock (devoid of mineralisation) grading into foliated host rock (possibly weakly mineralised), which then rapidly increases in strain development towards the mineralised fault zone (highly strained). The attitude of the enveloping foliation may be at an angle to that of the fault zone. However, closer to the fault zone, the foliation will often curve into, and parallel the fault plane; the foliation becoming intimately associated with the phyllosilicate foliation with the fault zone. Smaller mineralised fault zones often show no enveloping foliation at all.

The mineralised fracture/fault zone is characterised by faults, veins cataclastic materials, movement striations, pressure solution removal surfaces and mineral growth (including foliation development). Brecciated quartz fragments and lithons of wall rock, which were caught up in the dynamic development of the fault zone, often show asymmetric foliation patterns and/or pressure shadow crystallisation. Shear zone fabrics (CS) may be present but are difficult to discriminate from crenulation fabrics. Slickensides of quartz and other minerals are commonly seen on fault surfaces. Thrust blocks are also evident Late stage asymmetric folds generally fold the fault zone foliation, i.e. post-dates foliation development Their axial plunges indicate the intermediate strain axes. Undeformed late-stage mineralised veins often crosscut the fault zone. These generally have indications of stress/strain/movement that are consistent with earlier-formed fault-zone structures. The mesoscopic structural information shows that: 1.The deformation associated with hydrothermal mineralisation occurred during the last increments of major strain in the Archaean structural development; 2. The deformation associated with mineralisation was progressive, i.e. early foliation development (ductile) followed by brittle-ductile fault-zone movement (fracturing, faulting, brecciation, pressure solution and recrystallisation) with late sage veins and asymmetric folding; 3.There are variations in the local and regional patterns of stress/strain development during the mineralisation event, ranging from subvertical movements to strike slip movements, e.g. active fault structures during

150

mineralisation in the Ora Banda region were experiencing sinistral strike-slip movements, whereas in the Lawlers, Leonora and Laverton regions, the movements were dominantly oblique to vertical; and 4.There are close spatial, geometric and temporal relationships between the deformation associated with mineralisation and the intrusion of felsic porphyry and felsic dykes and felsic/silicic plutons, e.g. the mineralised zone footwall is invariably closer to a neighbouring pluton and the fault-zone itself generally exhibits a normal-sense of displacement The Archaean structural and tectonic history can be deducted from the evidence of mesoscopic structural characteristics/relationships. Archaean deformation was dominated by regional horizontal, ENE-WSW compressive stress. Thefirststructures to develop were thrusts stacking the greenstone stratigraphy. This was followed by regional folding, major strike-slip shearing, foliation development, and regional greenschist facies metamorphism. Still within this compressive environment, felsic/silicic magmas began to rise along pathways that tapped greenstone/granite source rocks deep in the crust. These pathways were zones of extension located between the compressive zones of ?waning crustal, strike-slip shear. Moderate to steep dipping zones of ductile strain began to develop in response to the upwelling granitoids which concentrated the developing strain peripheral to their margins. The shear movement along these high-strain zones acted in

response to the jostling activity between the upwelling plutons and the regional NE-SW subhorizontal compression. Synchronous with, or immediately following the intrusion and crystallisation of the felsic/ silicic plutons, the deep-seated felsic porphyry and lamprophyre dykes were intruded. These dykes rose up along the same crustal pathways as the felsic/silicic plutons, often exploiting the developing zones of highstrain at higher levels of the crust when solid plutons obstructed their passage. With decreasing tectonic/ plutonic activity, the deformation evolved from ductilebrittle behaviour to more brittle behaviour as a result of the decreasing temperatures and possibly higher fluid pressures. Decreasing temperatures resulted from reduction in the geothermal gradient accompanying waning igneous activity. The possible increase in fluid pressures may have been caused by the volatiles released from mantle and/or magmatic sources. These volatiles contained the gold bearingfluidswhich found favourable dilantant sites for deposition within the active brittleductile fault zones. Much of the later vertical crustal motions were caused by isostatic adjustments between the felsic plutons and denser greenstone packages. In conclusion, the Archaean structural history is one of progressive strain development. A subhorizontal ENEWSW directed compression was active throughout this time but was complicated by vertical activity (igneous and isostatic) and the effect of strain-anisotropies.

A7.6 GEOLOGICAL SETTING OF PRIMARY GOLD-SILVER DEPOSITS, MT GIBSON, WESTERN AUSTRALIA G.R. Brabham , S. Coxhell , A. O'Shea and A.F. Ross 1

Reynolds Australia Metals Ltd, Perth Forsayth NL, Perth 2

The Mt Gibson Mine is located 300 km NE of Perth in the Archaean Retaliation Belt, the southernmost greenstone belt in the Murchison Province (Lipple et al., 1983). The Belt consists of sheared and folded remnants of a mafic sequence and an overlying felsic sequence with banded iron formation (respectively the Gabanintha and Windaning Formations) (Watkins & Hickman, 1988). They are considered to be stratigraphically higher than most units that host the major gold deposits in the Murchison succession. Large scale mining of a lateritic gold resource commenced in 1986 and continuing exploration has discovered numerous zones of primary mineralisation beneath the laterite blanket (Gee, in press). The auriferous laterite covers an arcuate area of 6 km x 0.5 km and is locally termed the Gibson Anomaly. On current information it accounts for over 15 tonnes of gold (production and reserves). The Anomaly lies within the attenuated southern portion of the Retaliation Belt and correlates with a low

magnetic linear trend evident on enhanced magnetic imagery. Drilling over the length of the Anomaly has outlined a 300-700 m wide deformation zone in the mafic sequence (Holek, 1989). Lithologies are: amphibolite, metabasalt, metadolerite, serpentinite, minor cherty interflow sediments, and quartz-feldspar phyric rocks. Outside the basement deformation zone most lithologies display a weak regional metamorphic schistosity (S ) defined by amphibole orientation. Inside the zone, numerous 10-50 m wide shears are characterised by an overprinting tectonic fabric (S ) in all lithologies, accompanied by pervasive alteration assemblages e.g. mafic schists (quartz-tremolite/actinoliteclinozoisite/epidote-biotite), felsic schists (quartzmuscovite-biotite), ultramafic schists (tremolite-chloritehypersthene-antigorite) (C. Rugless, pers.comm.) Felsic schists are derived from numerous quartzfeldspar phyric lenses within the deformation zone. Some of these lenses exhibit chilled margins and x

151

irregular contacts indicative of an intrusive origin. The Hornet-Enterprise Deposits are the first primary deposits to be intensively explored by core drilling. Mining of the Hornet Deposit reveals a classic weathering profile with steeply dipping elongate sulphide lodes overlain by supergene enriched oxide ores, a depleted saprolite horizon and an auriferous laterite cap. The near vertical ore lenses are up to 700 m long x 15 m wide and consist of sulphide-rich schists, controlled by an anastomosing shear fabric (S ) in both mafic and felsic lithologies. Limbless folds, mineralogical banding and transposed layering are indicative of a ductile deformation regime. A later deformation event has producedflexuresand faulting of S shears and ore lenses. Primary gold mineralisation occurs as 5-50 |im, free grains and as electrum, sited on sulphide cracks and grain boundaries. The sulphide assemblage comprises, in order of decreasing abundance, pyrite, pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, galena and traces of bismuthinite. High grade ore lenses are pervasively silicified and consist of a quartz-chloriteepidote-biotite-amphibole-sulphide assemblage with occasional native bismuth. Silver/gold ratios of sulphide ore average 3/1. Trace antimony, tungsten and mercury minerals have been reported (Townend, 1988). Mineralisation and S fabrics are accompanied by the following gangue assemblages. In mafic hosts these 3

typically comprise quartz, high Ti biotite, hornblende! tremolite, epidote, garnet, diopside±sphene. Felsic porphyry hosts show development of quartz, sericitemuscovite, epidote, biotite, cordierite and garnet Whilst no ore lenses have yet been discovered in ultramafic lithologies, where affected by S shears these rocks comprise tremolite, chlorite, cummingtonite, hypersthene and antigorite. Discrete zones of cordierite and anthophyllite occur in mafic lithologies. Other minerals noted in the alteration system are tourmaline, gahnite and chrome spinel. In summary, the Hornet-Enterprise Deposits display features consistent with a major hydrothermal system emplaced in a ductile shear zone. Characteristics of the Au-Ag mineralisation are: pervasive biotite alteration; pervasive silicification and boudinaged quartz veining; no significant carbonate alteration; high Ag/Au ratio; and elemental associations (Cu, Pb, Zn, Bi, As, S). References 3

Gee, R.D., in press. Geol. of Mineral Deposits of Aust. and PNG. AJJM.M. Holek, P., 1989. Unpubl. Report to Mt Gibson Joint Venture Lipple, et al., 1983. Geol. Survey of WA Ninghan Sheet Explanatory Note. Townend, R., 1988. Unpubl. Report to Reynolds Australia Metals Ltd. Watkins K. & Hickman, A., 1988. Geol. Survey WA£ull.l31.

A7.7 PRECAMBRIAN MESOTHERMAL GOLD DEPOSITS — PUMPS OR VALVES? Julian R. Vearncombe* and David I. Groves Key Centre for Strategic Mineral Deposits, University of Western Australia

Mesothermal gold deposits, such as those in principal control on deformation is fluid pressure with Archaean greenstone terrains, are commonly located in failure after the fluid pressure exceeds a critical level; brittle-ductile structures active during the late the system acts as a valve, such that fluid motion at deformation events of terrain evolution. In at least some failure is followed by a decrease in fluid pressure, hydrocases, reactivation of earlier mechanically weak thermal self-sealing, a new build-up of fluid pressure structures can be demonstrated to be a controlling factor. and a repetition of the cycle. In this multiply-repeated Controversy over the precise structural control on these cyclic system, fluid pressures are localised and deposits largely revolves around their interpretation as abnormally high suggesting that fluids are channelled either suction pumps orfluid-pressure activated valves. along pre-existing structures and not simply intragranular In the former, fluid pressures are below lithostatic and wallrock fluids at sublithostatic fluid pressures. An rock failure is due to high differential stresses. Fluid important implication of this conceptual model is that motion in the suction pump conceptual model is from there may be reactivation of structures at orientations wallrock into the dilatant fracture, and is cyclically not predicted by conventional Andersonian fault criteria. repeated with each failure as the stresses exceed the Thus foliation-parallel veins in ductile shear zones and critical rock strength. An important implication of this the presence of deposits in steep reverse faults are conceptual model is that only those structures compatible with the valve conceptual model. predicted by conventional Andersonian fault criteria are The two conceptual models have important expected. implications for exploration. In the suction pump model In the conceptual model involving thefluid-pressure mineralised sites would be restricted to dilational sites activated valve the effective least principal stress may such as extensional jogs in a strike-slip fault. In the be tensile, that is in conditions of low differential stress fluid-pressure activated valve model compressional sites and extremely high fluid pressures. In this system the are the most prospective.

152

A7.8 THE GEOLOGIC SETTING OF EARLY PROTEROZOIC GOLD MINERALISATION IN SOUTHWEST GHANA, WEST AFRICA. B.N. Eisenlohr * and W. Hirdes 1

Key Centre forfurStrategic Mineral Deposits, UniversityHannover, of WesternW. Australia Bundesanstalt Geowissenschaften und Rohstojfe, Germany

Rocks of early Proterozoic age (ca. 2100 Ma) form a large part of the West African craton and crop out extensively in Ghana, the Ivory Coast, Burkina Faso and Mali. Although gold mineralisation is found in all these countries, the major historical and current producers are all located in Ghana. Gold mining in Ghana dates back to ancient times (Kesse, 1984) and Ghana, formerly known as the Gold Coast, continues to be Africa's largest producer outside southern Africa. The origin of the gold mineralisation and the tectonic history of the terrane is controversial (Leube et al., in press; Ledru, P. et al., 1988) and these are discussed in light of recently acquired structural geologic data. Regional and Structural Geology The Lower Proterozoic supracrustal rocks in Ghana are subdivided into the Birimian and Tarkwaian system. The Birimian comprises a sequence of mostly finegrained sedimentary/volcanoclastic rocks which separate a series of four, roughly equal-spaced, northeast trending belts of tholeiitic volcanic rocks. The Tarkwaian consists of coarse clastic sediments (grits, arkoses and conglomerates) derived from Birimian and granitoid source rocks which were deposited in graben-like trough within the volcanic belts (Leube et al., in press; Strogen, 1988). Both Birimian and Tarkwaian rocks are deformed, metamorphosed, and intruded by granitoid, but the number and timing of deformation events and granitoid emplacement are controversial. Birimian volcanic rocks contain per-aluminous, relatively Na-rich, granitoids that may be penecontemporaneous with volcanism, in contrast to the Birimian volcanoclastic/sedimentary rocks that contain meta-aluminous granitoids emplaced during deformation. Granitoids only locally intrude the Tarkwaian rocks. The amount of volcanic rocks, Tarkwaian sedimentary rocks and granitoids present in each of the belts varies. At a broad scale a low-strain and a high-strain structural domain are recognised. In the low strain domain, rocks contain a northeast-trending subvertical foliation (S ) that is subparallel or at a small angle to bedding and a subhorizontal foliation/bedding intersection lineation (L ). This foliation is welldeveloped in the fine-grained sedimentary/volcanoclastic rocks. The rocks are tightly folded but the lack of marker beds or consistent fold vergence variations precluded the identification of regional folds. Regional folds are evident, however, in Tarkwaian rocks as marker beds are present. Significantly, pebble clasts in the conglomerates do not appear to have been deformed prior to deposition. Rocks belonging to the high strain domain occur predominantly along the northwest margin x

of the volcanic belts. Fabrics are particularly well developed along the northwest margin of the Ashanti Belt where repetition of stratigraphy, overturned bedding, the presence of a strong foliation (S ) and a southwest plunging stretching lineation (L ) indicate that Birimian volcanic rocks were thrust obliquely onto the Tarkwaian rocks. Deformation in both Birimian and Tarkwaian rocks was accompanied by greenschist facies metamorphism. The overall structure of the belts is synclinal, as suggested by the presence of the youngest rocks in the centre of the belt. The structural features described above indicate that Birimian and Tarkwaian rocks were deformed jointly in a single progressive event involving a northwest-southeast directed compression. Early formed structures include the development of upright folds with sub-horizontal to gently plunging axes. Ongoing deformation resulted in the formation of higher strain zones located at the northwest margins of the volcanic belts, particularly the Ashanti Belt where thrusts are evident. Gold Mineralisation Two main types of gold occurrence are recognised: (i) gold associated with quartz reefs and/or altered and veined country rock; and (ii) gold located in quartz pebble conglomerate. In the reef/lode deposits gold is located free within laminated quartz veins or refractory within arsenopyrite, and to a lesser extent pyrite, that is associated with veined and altered sedimentary and volcanic rocks. The mineralised structures generally range in width between 0.5 and 5 m, strike northeast and crosscut the foliation (S ) in the host rock at a small angle. Both quartz veins and lodes are associated with finely disseminated C and up to 4% C-rich bands located on either the hangingwall and/or footwall of the vein/lodes. The general aspects of these gold deposits are similar to epigenetic mesothermal gold deposits described for Archaean terranes (Groves & Phillips, 1987; Barley et al., 1989) but some differences exist; these include the abundance of arsenopyrite and C, a relatively low Au fineness (approx. 820) and a spatial association with Mn (Leube & Hirdes, 1988). Although reef/lode gold mineralisation is found along most belt margins all major mines, with one exception, are located along the northwest margin of the Ashanti Belt which is the most tectonised margin in the terrane. Economic concentrations of gold in quartz pebble conglomerate have only been found in the Tarkwaian located within the Ashanti Belt Gold occurs in lenticular strips approximately 0.5 to 2 metres thick, 50-150 m wide and 200-600 m long within the Banket Series. 2

153

Gold is present as fine 10-15 |im size grains within the conglomerate matrix and show no sign of mechanical abrasion (Junner et al., 1942). Faults that crosscut the mineralised conglomerates may contain gold and some pyrite mineralisation. This occurrence of gold and the well developed crystal form of gold in the conglomerate points to a degree of gold remobilisation during deformation and metamorphism (Junner et al., 1942). The intimate association of gold with the conglomerate and the complete lack of gold in other rock types of the Tarkwaian strongly supports a palaeoplacer origin for the gold. Conclusion The gold in the Tarkwaian conglomerates has generally been regarded as originating from Birimian lodes that were uplifted and eroded after afirstphase of deformation. Subsequently, both Birimian and Tarkwaian rocks were deformed during a second deformation phase. However, the lack of structural evidence for a deformation event that affected Birimian rocks only, the stratiform conglomerate-hosted gold ore lenses that are part of the folded succession and the crosscutting nature of the reef/lode gold mineralised structures militate against such an event sequence. Deposition of the clastic sequence in grabens in a active tectonic environment with emerging volcanic belts and granitoids followed by a deformation event adequately explain the observed stratigraphic and structural relationships. This event sequence implies a distal or epithermal gold source, possibly related to early granitoid intrusion, for the

conglomerate hosted gold. Reef/lode gold mineralisation is hosted in late structures and largely confined to the strongly tectonised northwest margin of the Ashanti Belt The authors would like to thank the directors of the Bundesanstalt fur Geowissenschaften und Rohstoffe (BGR), Hannover, W. Germany and the Geological Survey Department (GSD), Accra, Ghana for their kind permission to give this talk. The structural geologic study presented in this talk was financed by the BGR and formed part of a long-term technical cooperation between the BGR and GSD. References

Barley, M.E., Eisenlohr, B.N., Groves, D.I., Perring, C.S. and Vearncombe, J.R., 1989. Geology 17: 826-829. Groves, D.I. & Phillips, G.N., 1987. Ore Geol. Rev. 2:287-322. Junner, N.R., Hirst, T. & Service, H., 1942. Gold Coast Geol Surv. Mem. 6: 75 pp. Kesse, G.O., 1984. In Foster, R. P. (ed.) Gold '82. A.A. Balkema Rotterdam: 645-659. Leube, A. & Hirdes, W., 1988. Geol. Soc. Aust. Abstr. 23: 149-152. Leube, A., Hirdes, W., Mauer, R. & Kesse, G.O., in press. Precambrian Res. Ledru, P., Milesi, J.P., Vinchon, C., Ankrah, P.T., Johan, A. & Marcoux, E., 1988. In International Conference and Workshop on the Geology of Ghana with Special Emphasis on Gold, Programme and Abstracts, Accra, Ghana: 26-27. Strogen, P., 1988. In International Conference and Workshop on the Geology of Ghana with Special Emphasis on Gold, Programme and Abstracts, Accra, Ghana: 41.

A7.9 A COMPARISON OF THE STABLE ISOTOPE CHARACTERISTICS OF PRIMARY ARCHAEAN AND PROTEROZOIC GOLD MINERALISATION IN AUSTRALIA S.D. Goldings N. J. McNaughton* and D.I. Groves*

Dept. Geology and Mineralogy, University of Queensland, St Lucia 4067 QLD Key Centre for Strategic Mineral Deposits, University of Western Australia 2

Recent studies suggest that Archaean mesothermal gold deposits are developed in second order structures related to major craton-scale faults which also controlled the distribution of regional mantle-derived carbonation and calc-alkaline porphyry and lamprophyre dyke swarms (Groves et al., 1988; Eisenlohr et al., 1989). However, there is considerable controversy regarding the source of gold-bearingfluids,with models that stress crustal metamorphism ± reworking mantle components (Groves et al.., 1989; Perring et al., 1989), devolatilization of felsic magmas (Burrows et al., 1986; Cameron & Hattori, 1987) and deep circulation of meteoric waters (Nesbitt, 1988). There are fewer published studies of Proterozoic gold deposits, which include stratabound to stratiform deposits in banded iron-formation (BIF) or carbonaceous and calcareous siltstones, and discordant quartz-vein deposits in a range of host rocks. The stratabound to

stratiform nature of many Proterozoic gold deposits has led some authors to propose a syngenetic-exhalative origin for these deposits (Nicholson & Eupene, 1984; Davidson et al., 1988), although others have recently presented geologic and geochemical data that support an epigenetic rather than syngenetic origin for some stratabound deposits (Goellnicht et al., 1988). The origin of the discordant quartz-vein deposits is less controversial, although there is still no consensus regarding fluid sources, circulation paths, and depositional mechanism for these deposits. Herein, the available stable isotope data for veins and alteration assemblages from primary gold deposits of late Archaean to Proterozoic age are reviewed and the constraints these data provide on the source of ore fluids are discussed. Published oxygen isotope data from Archaean mesothermal gold deposits of the Norseman-Wiluna

154

Belt (Golding & Wilson, 1987; Golding et al., 1989)) carbonate species and/or a variable input of organic and seawater-derived carbon (Golding & Wilson, 1987; indicate that' (i) auriferous veins and their immediate alteration haloes Golding et al., 1989). were fluid dominated, whereas variable, lower fluid/ Reconnaissance carbon isotope data for Proterozoic rock ratios have affected district-scale alteration gold deposits suggest that host rock sources (i.e., organics assemblages; and marine carbonate) predominated, although a (ii) temperature gradients were not significant either significant magmatic component in the orefluidscannot be discounted. laterally or vertically during mineralisation; Most Archaean gold deposits have 8*S values around (iii) the fluid reservoir was homogenous at the mine scale and had relatively high 8 0 values of 4 to 9 %o 0 %o CDT, although the giant Golden Mile deposit has distinctive negative 8 S values which have been SMOW; and (iv) correlated 5 C - 8 O variations in gold-related interpreted to be due to fluid-wallrock interaction carbonates at the district-scale probably reflect source- (Golding & Wilson, 1983; Phillips et al., 1986). Most conduit heterogeneities and support multi-source ore Archaean sulphides of diverse origin have 8*S values in the range - 4 to +4 %o (Lambert & Donnelly, 1989) systems. Published and new reconnaissance oxygen isotope and thus the sulphur isotope data for gold-related data for Proterozoic gold deposits in the Northern sulphides cannot be interpreted unambiguously. In contrast, a wide range of sulphur isotope Territory and Paterson Province, Western Australia, may indicate a connate and/or low-temperature compositions have been recorded for sulphides from Proterozoic sedimentary and igneous rocks, probably metamorphic origin for orefluids,but are also compatible with a magmatic component in these mineralising system because of the transition to an oxidising hydrosphere (Goellnicht et al., 1988; Etheridge, 1984). Salinity- and increased importance of sulphate-reducing processes temperature relationships for at least one deposit (i.e., (Lambert & Donnelly, 1989). Sulphur isotope data for Telfer (Goellnicht et al., 1988), together with lead Proterozoic gold deposits are generally relatively isotopic studies provide additional evidence for the enriched, and the overlap of 8 S values of goldmixing of magmatic and basinal waters. In contrast mineralised and other veins unambiguously related to with Archaean gold deposits, where wallrock silicates granitic intrusions is consistent with a magmatic source. Stable isotope data, together with geologic and fluid proximal to mineralisation typically exhibit a positive 5 0 anomaly (Golding & Wilson, 1987; Golding et al., inclusion studies suggest that many Proterozoic gold 1989), the presence of marine carbonates and other deposits are granitoid-related, whereas the late-Archaean sedimentary rock types with high primary 8 0 values greenstone-hosted deposits mainly reflect hydrothermal in Proterozoic terrains results in distinctively lower activity associated with manfle-crustal outgassing. 8 O values in alteration haloes about some epigenetic References Burrows et al., 1986. Nature 321: 851-854. ore deposits. There are only reconnaissance hydrogen isotope data Cameron & Hattori, 1987. Econ. Geol. 82: 1177-1191. 1988. Geol. Soc. Aust. Abs. 22: 85-90. available for both Archaean and Proterozoic primary Davidson etet al., al., 1989. Mineral. Deposita 24: 1-8. gold deposits (Golding & Wilson, 1987; Golding et al., Eisenlohr Etheridge (1984): BMR Research Newsletter 2, 3. 1989; Large & Wedekind, 1988). The few data that are et al., 1988. Geol. Soc. Aust. Abs. 22: 79-84. available suggest a metamorphic and/or juvenile source Goellnicht Golding & Wilson, 1983. Econ. Geol 78: 438^50. for Archaean deposits with a possible connate component Golding & Wilson, 1987. Geol. Dept. & Extension, UWA in Proterozoic ore fluids. Publ. 11: 203-213. Carbon isotope studies of carbonate minerals from Golding et al., 1987. Geol. Dept. & Extension, UWA Publ. 11: regional carbonate alteration styles and wallrock 215-238. alteration zones about gold mineralisation in the Golding et al., 1989. Econ. Geol. Monogr. 6: 368-380. Norseman-Wiluna Belt suggest that the mantle-derived Groves et al. (1988): Nature 331, 254-257. carbon reservoir in regional fault-controlled alteration Groves et al. (1989): Econ. Geol. Monogr. 6, in press. zones in the most likely source for C0 in auriferous ore Lambert & Donnelly, 1989. Geol. Soc. Aust. Spec. Publ. 13, in press. fluids (Golding & Wilson, 1987; Golding et al., 1989; Large & Wedekind, 1988. Geol. Soc. Aust. Abs. 22: 73-78. Golding & Wilson, 1983; Phillips et al., 1986). The Nesbitt, Geology 16: 1044-1048. provinciality of carbon isotope data, even for Archaean Nicholson1988.& Eupene, 1984. AIMM Conference Proc.: gold deposits which are spatially and probably genetic377-396. ally related to the same fault zone, negates an exclusively Perring et al., 1989. Econ. Geol. Monogr. 6, in press. juvenile (i.e., magmatic or mantle) origin for the ore Phillips et al., 1986. Econ. Geol 81: 2008-2015. fluid components and may reflect dissolution of different 18

155 A7.10 A COMPARISON BETWEEN ARCHAEAN GOLD DEPOSITS AND LOWER PROTEROZOIC GOLD DEPOSITS (PINE CREEK GEOSYNCLINE): CONTROLS ON MINERALISATION G.A. Partington Key Centre for Strategic Mineral Deposits, University of Western Australia

In the past few years there has been major advances in the understanding of the genesis of Archaean gold deposits (e.g. Perring et al., 1988) which have included an understanding of the tectonics and geochemistry of these type of deposits and has led to several new genetic theories. Lower Proterozoic gold deposits in the Pine Creek Geosyncline are at present receiving renewed attention with the opening of old mines such as Moline and Cosmo Howley and new discoveries such as Goodall. Syngenetic or remobilised syngenetic models have been postulated in the past to explain the formation of these gold deposits (Nicholson & Eupene, 1984). However regional mapping carried out by the author for Northern Gold NL in the Moline, Pine Creek and Howley districts, interpretation of aeromagnetic enhancements and detailed mapping along the Howley Ridge suggest that these models may not adequately explain the genesis of these deposits. The aim of this paper is to describe the characteristics of the gold mineralisation in the Pine Creek Geosyncline by using the Howley District as an example and, with comparisons to the better understood Archaean gold deposits (see table), suggest possible controls on mineralization. Gold mineralisation in the Howley District occurs in alteration haloes ± quartz vein systems; in laminated quartz veins in fractures or shear zones, in vein stockwork sets associated with antiformal structures, in small highgrade quartz veins associated with shear systems adjacent to granitoids; and as alluvial deposits. The mineralisation is confined to early Proterozoic lithologies of the Mount Partridge Group, South Alligator Group, Finnis River Group and Zamu Dolerite (see table). There appears to be no dominant host rock lithology as gold mineralisation is found in all host rock types. However the larger gold deposits appear to be associated with aeromagnetic anomalies caused by iron-rich lithologies. Gold mineralization is also associated with elongate alteration zones consisting of biotite, sericite, tourmaline, chlorite, pyrite, arsenopyrite, pyrrhotite, quartz and carbonate. Mass balance calculations in altered Zamu dolerite indicate that Au, As, Ag, K, Rb, Ba, Ga, Zr, S have been enriched while Na, Sr, Fe, Ni, Cu and Zn are depleted (Oepen et al., 1988). Arsenopyrite thermometry was used to constrain the temperature of formation mineralization and gave temperatures ranging from 250-330°C (Oepen et al., 1988). Gold mineralisation has a heterogeneous distribution in the Pine Creek Geosyncline and is confined to elongate zones, or "Lines" (e.g., Howley Line) associated with regional folds or shear zones. The main regional structural controls in the Howley area are a series of

reverse-slip N-trending ductile-brittle shear zones which are spatially associated with overturned asymmetric Fx folds. These shear zones have an en echelon pattern and postdate Fx folding but are deformed by open westerly plunging F2 folds. The larger deposits also generally occur where the reverse-slip shear zones coincide with a system of NW-trending strike-slip ductile shear zones. On a deposit scale the mineralization generally occurs in quartz veins parallel to shear fabrics; in stockwork zones which occur as tension fractures formed synchronously with shearing; and veins also parallel to either bedding (S^, a regional axial planar cleavage (Sj) or as disseminated gold within sheared alteration zones. The cross-cutting or replacement nature of most of the mineralisation suggests that mineralisation postdates the deposition of the Finnis River Group of sedimentary rocks and the intrusion of the Zamu dolerite. Tension fractures hosting gold mineralisation in shear zones are commonly refolded, boudinaged and overprinted by further shearing suggesting that mineralisation was synchronous with shearing and postdated Fx folding. Granitoid intrusion dated at ca 1780 Ma (Stuart-Smith et al., 1987) also appears to have been synchronous with to post shearing. The upper age of mineralisation is constrained by the deposition of Middle Proterozoic sedimentation. Many of the features of the gold deposits in the Howley District suggest that they could not have been formed by syngenetic processes (see table). These data also argue against remobilised syngenetic models for their formation. Rather, the gold mineralisation is structurally controlled occurring in brittle-ductile structures at the greenschist/ amphibolite fades boundary and hence probably has an epigenetic origin. Structural models proposed for Archaean gold mineralisation may therefore apply (e.g., Eisenlohr et al., 1989; Vearncombe et al., 1988.) The Lower Proterozoic gold mineralisation also has geochemical signatures and alteration styles common to Archaean gold deposits suggesting that the mineralisation may have had similar transporting mechanisms and geochemical controls (e.g., Groves & Phillips, 1987). It is also possible that similar genetic models may apply. Three of the models proposed for Archaean gold mineralisation which may explain the formation of the Lower Proterozoic gold deposits in the Pine Creek Geosyncline are: (1) Metamorphic (e.g., Groves & Phillips, 1987); (2) Granitoid (e.g., Burrows & Spooner, 1988); and (3) Multi-source (e.g., Eisenlohr et al., 1989) It is clear that further study of these deposits is required if successful exploration is to continue. With the recognition that the Lower Proterozoic gold deposits

156

in the Pine Creek Geosyncline are probably epigenetic Oepen, P.S., Friedrich, G. & Kater, G., 1988. Geol. Soc. Aust. Abstracts 22: 204-206. and structurally controlled rather than syngenetic many areas believed to be low priority exploration targets in Perring, C.S., Barley, M.E., Bettenay, L.F., Cassidy, S.D., Golding, S.D., Groves, D.I., Hallberg, J. A., McNaughton, the past may now be considered prospective. N.J. & Rock, N.M.S., 1988. Geol Soc. Aust. Abstracts 22: References 296-301. Burrows, D.R. & Spooner, E.T.C., 1988. Geol. Soc. Aust. Stuart-Smith, P.G., Needham, L. Bagas & Wallace, D.A., Abstracts 22: 302-306 1987. Pine Creek 1:100,000 BMR. Map Commentary. Eisenlohr, B., Groves, D.I. & Partington G.A., 1989. Vearncombe, J.R., Barely, M.E., Eisenlohr, B., Grigson, M.W., Mineralium Deposita 24: 1-8. Groves, D.I., Houston, S.M., Partington, G.A. & Groves, D.I. & Phillips, G.N., 1987. Ore Geol. Rev.: 287-322. Swarnecki, M.S., 1988. Geol. Soc. Aust. Abstracts 22: Nicholson, P.M. & Eupene, G.S., 1984. Darwin Conference, 19-23. AIMM: 377-396.

Table 1 — Comparison of Archean and Lower Proterozoic gold mineralisation. Archean epigenetic gold mineralisation

Lower Proterozoic Gold Mineralisation

Metamorphism

Mid-greenschist to low-amphibole

Greenschist to low-amphibole

P-T data

2-3 kbar; 250-350°C

2-3 kbar; 250-330°C

Tectonic environment

Au deposits were formed in a brittle ductile regime in the crust during rift closure

Au deposits were formed in a brittle-ductile regime in the crust associated with basin closure

Structural control Subsidiary faults and shear zones parallel to oblique-slip, strike-slip and reverse-slip shear zones. Mineralisation occurs as laminated veins, sheeted vein systems and tension fractures

N-trending reverse-slip shear zones spatially associated with overturned F1 folds and NW-trending strike-slip shear zones. Mineralisation occurs in alteration haloes, laminated quartz veins and in vein stockwork sets

Age

Syn-post granitoid intrusion, syndeformation and syn-post peak metamorphism, ca. 2.6 Ga

Syn-deformation, syn-pre granitoid intrusion and syn-post metamorphism, ca. 1.8 Ga.

Alteration

K-mica, ankerite-dolomite, chlorite, quartz, pyrite, arsenopyrite. Elements added Au, Ag, As, B, Sb, W, S, K, Rb, Li, Ca, Sr, Ba

Biotite, sericite, tourmaline, chlorite, pyrite, arsenopyrite, pyrrhotite, quartz and carbonate. Elements added Au, Ag, As, B, K, Rb, Ca, Sb, Ce, W, Sn, Ba, S

Host rocks

Mafic/ultramafic volcanic rocks, mafic/ultramafic sills, BIF, granitoids and rare sediments

BIF, shales, clastic sedimentary rocks, volcanic rocks and dolerite sills

157

A7.ll FLUID INCLUSIONS AND STABLE ISOTOPE STUDIES OF Au-QUARTZ VEIN DEPOSITS IN THE PINE CREEK GEOSYNCLINE, NORTHERN TERRITORY Andrew S. Wygralak and Masood Ahmad Northern Territory Geological Survey, Darwin

The Early Proterozoic metasedimentary and solid phase. Types (a) and (b) are spatially closely metavolcanic rocks in the Pine Creek Geosyncline related and are primary. Types (c) and (d) are secondary contain a large number of small to medium size and are confined to healted fractures. Most of the type (0.5-10 mt), low grade (3-8 g/t Au), Au-quartz vein (a) inclusions decrepitated or homogenised into liquid deposits, e.g. the Enterprise, Union Reefs, Fountain H p or C0 phase, some homogenised by fading of Head, Spring Hill, Zapopan, and Moline deposits. Sn- meniscus. Types (b), (c) and (d) homogenised into liquid quartz and Pb-Zn-Cu-quartz veins within this area have phase. The averages of the fluid data are: close spatial relationships with Au-quartz veins and Type (a) — T 292°C, salinity 7.6 wt.% eq. NaCl, CH together define a temporal vein formation sequence in 9 mole %; Type (b)—T 206°C, salinity 13.5 wt.% eq. which Sn precipitation was followed by Au and finally NaCl; Type (c) — T 130°C, salinity 19.0 wt.% eq. by Pb-Zn-Cu. Almost all mineralised veins are located NaCl; and Type (d) — T 158°C, salinity 30.1 wt.% eq. in close proximity to or within the contact aureole of NaCl. Pressure calculated from temperatures of homogenisation (or decrepitation) and densities of C0 1780-1710 Ma granitoid intrusions. Four paragenetic stages were identified in the Au- phase ranged between 500 and 1500 bars and is in quartz veins: (i) white quartz, pyrite, arsenopyrite, agreement with the contact metamorphic mineral pyrrhotite, chalcopyrite, gold; (ii) bluish-grey quartz equilibria indicating pressure of less than 2 kbar. with sulphides as above and gold; (iii) milky-white, 8*S values for sulphides rangefrom+4%o to+10 vuggy quartz with pyrite and trace gold; and (iv) carbon- suggesting precipitation in the reduced sulphur field ate veinlets with trace gold. Alteration of vein walls is and point to a magmatic sulphur source. 8D values in not pronounced and contacts are sharp. The altered wall fluid inclusion water range from +27%o to -57%o and rock contains quartz, chlorite, sericite, carbonates and calculated value of 8 0 in fluid ranges from +5.5%o to sulphides. +10.3%o suggesting a mixed magmatic and metamorphic Four types of fluid inclusions were distinguished: source. 8 C values in fluid inclusions range from (a) H/)+C0 +CH with vol.% of C0 +CH commonly -31.1 %o to +12%o and may reflect dual (magmatic and between 30-70%; (b) I^O liquid + vapour with vapour organic) carbon sources with possible additional Eh phase > 20%; (c) H 0 liquid + vapour with vapour effect caused by presence of CH . phase < 15%; and (d) F^O liquid + vapour + soluble 2

A7.12 GOLD DEPOSITS OF THE LOWER PROTEROZOIC GLENGARRY GROUP, WESTERN AUSTRALIA Jacqueline Windh and Mark E. Barley Key Centre for Teaching and Research in Strategic Mineral Deposits, University of Western Australia

The Lower Proterozoic Glengarry Group hosts a age. The base of the Glengarry Group consists of a number of recently discovered gold mines, including succession of coarse clastic sedimentary rocks Horseshoe Lights (production to 12/87 of 1.5 Mt at (conglomerate, arkose and sandstone of the Juderina, 4.2 g/t Au; remaining resource 2.7 Mt at 2.8 g/t Au, Doolgunna and Karalundi Formations). These rocks 27 g/t Ag, 3.4% Cu); Fortnum (total reserve plus resource are overlain by the Narracoota Volcanics, a succession 4.0 Mt at 3.2 g/t Au), Nathan's Deep South (resource of ultramafic to mafic submarine flows that is up to 954,000 t at 2.8 g/t Au); and Labouchere (resource several kilometres thick and is locally capped by 925,000 t at 3.6 g/t Au). Further significant gold intermediate to felsic tuffaceous rocks. The ultramafic showings within the Glengarry Group are currently and mafic rocks have low Ti/Fe and Zr/Fe, and have being evaluated. Preliminary results of an investigation been compared to boninites (Hynes & Gee, 1986). The of the stratigraphic and structural setting of these gold volcanic rocks are overlain by the Thaduna Formation, a thick turbidite sequence of bedded greywacke and deposits are presented below. The Glengarry Group lies unconformably upon the mudstone with only minor coarser clastic rocks. The northern margin of the Archaean (i.e. >2.5 Ga) Yilgarn Horseshoe Range BIF overlies the Thaduna Formation, ciaton, and is overlain by the 1.8-1.5 Ga (Gee, 1987) and is in turn overlain by chloritic siltstone, quartzose Padbury Group; it is therefore of Lower Proterozoic siltstone and sandstone, and quartz pebble conglomerate

158

of the Labouchere Formation. Gold occurrences are the anticline is traversed by a W-dipping reverse shear known in all of these formations; the significant deposits, zone. Gold mineralisation occurs dominantly in however, generally occur in or near the volcanic rocks. association with veins hosted by lenticular pods of The Glengarry Group has a complex structural jaspery-chert (microcrystalline, purple to red quartzhistory; it has undergone tectonism both prior to, and haematite-magnetite rock) within mafic volcanics that after, deposition of the Padbury Group (Hynes & Gee, locally contain fine-grained disseminated magnetite. 1986). The southernmost parts of the Glengarry Group Mineralised veins are WNW-trending, steeply dipping, are in depositional contact with the underlying Yilgarn and consist of quartz, coarse-grained pyrite, minor craton, dip gently northward, and are only weakly chlorite and carbonate, and trace chalcopyrite. The origin deformed. Further north, in the vicinity of the gold of the jaspery-chert (syngenetic or epigenetic) is mines, the Glengarry Group contains a pervasive NNW- uncertain, but the association of the gold with crossto NW-trending, steeply SW-dipping cleavage that is cutting quartz veins is indicative of an epigenetic origin associated with S- to SE-plunging folds and W- to SW- for the mineralisation. Subsidiary gold mineralisation dipping reverse shear zones. Later W- to WNW-trending also occurs with cross-cutting veins in feldspathic crystal faults re-orient the main cleavage and truncate shear tuffs and associated with disseminated pyrite within zones and folds. Cleavage re-orientation near the foliated volcanic rocks. faults is inconsistent, indicating both dextral and sinistral Nathan's Deep South is hosted by chloritic slates senses of displacement and thus suggesting that and siltstones interbedded with quartz pebble the faults have been reactivated. Juxtaposition of: conglomerates of the Labouchere Formation. The mine (1) Narracoota Volcanics to the south against younger is 3 km south of the Fortnum Fault, is traversed by Thaduna Formation to the north across the Fortnum (or several late NW- to WNW-trending faults, and is 800 m Dome) Fault; and (2) younger (1.6 to 1.1 Ga) Bangemall east of a W-dipping reverse shear zone which juxtaposes Group to the north with Glengarry Group to the south mafic and ultramafic volcanic rocks with the Labouchere across the fault at Horseshoe Lights, suggests a Formation. Economic gold mineralisation occurs most component of north-side-down displacement to these commonly in very thin (5 to 20 mm) bedding-parallel late faults. Most of the gold mines occur on or near a seams of massive, coarse-grained euhedral pyrite with W- to SW-dipping reverse shear zone near one of the interstitial quartz, carbonate and trace chalcopyrite, late W- to WNW-trending faults. Each of the mines is within the chloritic siltstones. Local truncation of the briefly discussed below in terms of its stratigraphic and pyrite seams against cross-cutting fractures, with structural setting. continuation of the hosting bed across the fracture, Horseshoe Lights is a Cu-Au-Ag deposit which indicates that the pyrite seams are not themselves beds occurs about 200 m south of a W-trending fault. It but are of replacement (diagenetic or epigenetic) origin. consists of two highly weathered massive sulphide It is not yet known whether chlorite in the siltstones is (chalcopyrite-pyrite) bodies which occur on adjacent of primary (metamorphic) origin or was introduced due SW-dipping reverse shear zones that transect the to alteration. Disseminated magnetite alteration occurs Narracoota Volcanics/Thaduna Formation contact. in siltstones above and below the main mineralisation. Genetic models for this deposit are controversial. Quartz pebble conglomerate adjacent to the mineralised Evidence consistent with a syngenetic exhalative origin siltstones contains quartz-carbonate (ankerite and calcite) for the mineralisation includes: (1) the massive nature veins with trace to several per cent sulphide (pyrite and of the sulphide orebodies; (2) the high Cu to Au ratio of chalcopyrite). These veins have been weakly to strongly the ore; (3) the location at the top of a fractionated folded and both cut cleavage and contain cleavage; submarine volcanic succession overlain by turbidites, they are therefore syndeformational. The evidence for a and (4) the occurrence of thin, bedding-parallel cherty replacement origin for the auriferous pyrite seams, their horizons of probable exhalative origin in both tuffs and association with syndeformational veins, and the siltstones at the volcanic-sediment contact. Evidence occurrence of NW-trending faults through the main supporting an epigenetic, shear-controlled model for its mineralised zone together indicate an epigenetic origin formation includes: (1) the present location of the for Nathan's gold mineralisation. orebodies along shear zones; (2) the orientation of ore Mining has just commenced at Labouchere, and shoots parallel to the elongation lineation; (3) the little data have yet been recorded. Labouchere occurs 2 evidence of mineralisation in both the volcanic and km north of the Fortnum Fault, withinfine-to mediumsedimentary rocks; and (4) hypogene haematite grained sedimentary rocks of the Labouchere Formation. alteration, which has not been documented in syngenetic Gold mineralisation is associated with silicification massive sulphide deposits. The high degree of supergene textures and disseminated pyrite in light grey to pink alteration to the orebody makes resolution of this genetic massive chert. The origin of the chert (syngenetic, controversy difficult; fluid inclusion and isotopic studies metamorphic-replacement, surface silicification) is not are. in progress. yet known. The association of the mineralisation with Fortnum occurs on the south side of the WNW- cross-cutting silicification and veining in sheared trending Fortnum Fault, which truncates a S-plunging hangingwall lithologies suggests an epigenetic origin anticline that exposes uppermost Narracoota Volcanics; for the mineralisation.

159

Each of these four gold deposits is unique in its style of mineralisation. However, a number of characteristics common to all of the deposits (with the possible exception of Labouchere) include: (1) proximity of mineralisation to a W- to WNW-trending fault; (2) occurrence of mineralisation on or near a W- to SWdipping reverse shear; (3) proximity of mineralisation to a volcanic-sediment contact; (4) associated sulphides limited essentially to pyrite and minor to significant Cu sulphides; and (5) evidence of relatively oxidising fluids in the vicinity of the gold mineralisation (haematite and/or magnetite alteration). These gold deposits all show evidence of an epigenetic origin. Their spatial association with the relatively late W- to WNW-trending

faults suggests that mineralisation may have been late in the tectonic history of the Glengarry Group. This contribution is a compilation of work completed by the authors, and by numerous exploration and mine geologists over a number of years. In particular, the authors wish to acknowledge the work of geologists from the follwing companies: Homestake Australia Ltd, Horseshoe Gold Mining Proprietary Ltd, and Dominion Mining Ltd.

References

Gee, R. D., 1987. Geol. Surv. W.A. 1:250 000 Peak Hill Sheet Explanatory Notes. Hynes, A. & Gee, R.D., 1986. PrecambrianRes. 31:107-132.

A7.13 THE EARLY PROTEROZOIC TOM'S GULLY GOLD-SILVER DEPOSIT, N.T. S. Sheppard *, R.A. Crookes , D.I. Groves , P.G. Simpson , and NJ. McNaughton 2

Key Centre for Strategic Mineral Deposits, University of Western Australia Carpentaria Gold Pty Ltd, Berrimah, N.T. 2

Tom's Gully is a high grade gold-silver deposit located 90 km ESE of Darwin in the Northern Territory, at latitude 12°50'S, longitude 131°34'E. The deposit was discovered in 1986 by Carpentaria Exploration Company Pty Ltd in the northern part of the Pine Creek Inlier, in an area not previously known for any significant gold ocurrences. In addition this deposit has a geometry dissimilar to other epigenetic gold deposits in the inlier. Production commenced in October 1988. As of July 1989, the deposit consists of an open cut reserve of 333,000 tonnes at 9.6 g/t and an underground indicated resource of 638,000 tonnes at 9.6 g/t Gold-silver mineralisation at Tom's Gully is contained within an east-striking quartz vein, hosted by the Wildman Siltstone, a lower unit in the Pine Creek Inlier stratigraphy. In the mine area, this formation consists of finely laminated pale grey and dark grey siltstone and shale, with disseminated pyrite. Pyrite also forms bedding-parallel and discordant veins up to 3 mm thick. A bedding-parallel cleavage in the siltstones and shales, and a 4-5m thick duplex zone just above the vein, suggest early movement on bedding planes and some thrusting to the northwest, respectively. The mine sequence was then folded about upright, open to closed folds plunging at 10-20° to the SW, with a poorly developed, near-vertical axial surface cleavage. These folds formed during the regional folding and lower greenschist facies metamorphism at 1885-1870 Ma (D, of Needham et al., 1988). The mine sequence was hornfelsed during intrusion of the adjacent Mount Goyder Syenite/Mount Bundey Granite composite pluton, which also produced andalusite (3-10 mm long) and cordierite (<2 mm in diameter) porphyroblasts in the sedimentary rocks. At least 15 lamprophyre dykes have been identified

in the mine area; some of these intrude the Mount Goyder Syenite and Mount Bundey Granite. The dykes are mostly 0.3-1.0 m thick, vertical or subvertical, strike approximately parallel to the fold axes and are thus perpendicular to the ore vein.The majority of dykes do not penetrate the vein; those that do are thought to occupy fault planes. Preliminary Rb-Sr data suggest a date of 1812±36 Ma for the pluton and that the lamprophyres are the same age as the syenite and granite. Three styles of gold-silver mineralisation are present at Tom's Gully. 1. The bulk of mineralisation at Tom's Gully is contained within an east striking quartz vein, which has been emplaced into a listric fault zone dipping at 30-35° south at the surface, and about 15° south at depth. The vein extends for 800 m along strike, although only the eastern 400-450 m is auriferous. Average thickness of the reef is 1.5 m, but it varies between zero and four metres thick. Where the reef is absent the enveloping fault zone can still be traced by inspection and basemetal anomalies. The fault zone is composed of crushed and altered siltstone with variable amounts of carbonate and quartz-pyrite stringers up to 0.6 m above and below. Preliminary drilling indicates the vein continues for at least 1000 m down-dip.The vein is offset by several SW trending faults of unknown displacement, although one of these is apparently accompanied by rotation, resulting in a steepening of dips at the eastern end of the vein, from 25° to 45°. 2. Fault zone ore. This comprises fault dragged and rotated blocks of the quartz vein, and mineralised fault breccia within the Crabb Fault 3. Mineralisation is sporadically found in sericite-chlorite altered ?tuff beds in the hangingwall, where these are within several metres of the vein. Gold

160

and silver appears to be associated with 1-5 mm thick at about 2:1. In contrast the gold:silver ratio shows pyrite veins in joints and fractures. considerable variation, particularly down-dip. In the top 20 m the gold:silver ratio is about 2:1, decreasing Quartz in the mineralised part of the reef is mostly to 1:1 at 60 to 70 m depth, and below this varies a fine-grained, laminated, bluish-grey variety with from 1:1 to 1:2. abundant sheared inclusions or laminae of carbonaceous The vein displays a strong down-dip lineation defined siltstone, 1-3 mm thick. The quartz becomes progressively more milky toward the western end of the by elongate quartz (recry stallised aggregates) along most vein, coincident with a gradual decrease in precious and of its strike length, suggesting syn- to pre-tectonic base metal values. Sections of clear quartz within the emplacement of the vein. The strong layer-parallel fabric vein are always barren. These reflect the various stages in the ore and dog-tooth quartz oriented perpendicular to the shallow-dipping vein are consistent with deposition of quartz deposition. Detailed core logging indicates at least four stages during thrusting. Field relationships imply that quartz vein deposition andassociated gold-silver mineralisation of sulphide deposition in the vein: (i) disseminated to massive pyrite and very minor, is broadly synchronous with the episode of syenite and fine-grained arsenopyrite, apparently brought in with granite intrusion, and related lamprophyre dykes. the quartz. Also part of this phase is a quartz-pyrite Radiogenic isotope studies are in progress to test the possibility of a genetic association. stockwork forming a halo to the vein. Currentlyfluid-inclusion,ore-petrology and thermo(ii) Coarse-grained euhedral pyrite, as veins between or cutting quartz laminae and as coarse infill of cavities dynamic studies are designed to determine the nature of ore fluids, the depositional conditions and the cause of in the quartz. (iii) Coarse-grained euhedral arsenopyrite as veins ore deposition. Field observations point to a specific replacing carbonaceous siltstone laminae and pyrite, association between gold and arsenopyrite that is (?)late or in fractures within the quartz. It is this phase of in the paragenetic sequence. Both are best developed in sulphide deposition which contains the majority of carbonaceous siltstone laminae in the vein, suggesting that reduction of a relatively oxidised ore fluid may the gold-silver mineralisation. (iv) Late stage, minor carbonate-pyrite veining in the have induced precipitation of the sulphides, gold and vein and surrounding sedimentary rocks. Gold in silver. the quartz vein occurs as particles of electrum up to Reference 25 (im in diameter within arsenopyrite. The pyrite: Needham, R.S., Stuart-Smith, P.G. & Page, R.W., 1988. Precambrian Research, 40/41: 543-564. arsenopyrite ratio in the vein is reasonably constant A7.14 STRATIGRAPHIC AND STRUCTURAL CONTROLS ON PROTEROZOIC IRONSTONE MINERALISATION, TENNANT CREEK, NORTHERN TERRITORY Mark S. Rattenbury Key Centre for Ore Deposit &. Exploration Studies, University of Tasmania

High-grade gold, copper and bismuth ore deposits within the Early Proterozoic Warramunga Group are almost exclusively associated with massive magnetite or hematite±quartz±chlorite "ironstones". Ivanac (1954) established strong structural controls on the positions of the ironstones in fold hinges, along reverse faults and cleavage planes. Mineralisation of the ironstones is also commonly associated with stratiform and relatively oxidised horizons incorporating various hematite shales. The Warramunga Group has undergone one major east-west folding episode. Fold axes are regionally subhorizontal and cylindrical, but on a several hundred metre scale or less the fold axes can change markedly in plunge angle. The rapid plunge variations are probably due to local non-cylindrical folding rather than fold interference with earlier or later north-south trending folds. The folds are upright, with rare marginally overturned limbs, very open to close, with hinge shapes falling between concentric and parallel. The folds have a well defined axial plane cleavage which shows little

variation from an average steeply north-dipping orientation. Section balancing considerations and the disharmonic fold shapes indicates the folds propagated from buried reverse faults and the deeper structure of the Warramunga Group may be dominated by a large thrust fault system. Strike-slip along NW-SE trending faults occurred in conjunction with kink folding, locally deforming the regional cleavage and truncating the eastwest fold structures. The oxidised horizons occur at discrete stratigraphic levels above and below a folded stratiform porphyry within the Black Eye Member of the Carraman Formation. The Argo horizon occurs 180-300 m stratigraphically above the porphyry and associated ironstones include those worked by the Peko and Argo mines. The Nobles Nob horizon occurs 220-380 m below the porphyry and includes the Nobles Nob mine, the Golden Forty mine and the Burnt Shirt mine. The Juno horizon occurs 820-1040 m below the porphyry and includes the Juno mine. A greater frequency of ironstone/hematite

161

shale horizons west of the Tennant Creek township, copper and bismuth. Regions of rapid fold plunge change including the TC8 mine, may be due to regional are not considered to be important structural controls of westward stratigraphic thinning and/or facies changes. ironstone formation, as has been suggested by Ivanac The White Devil mine occurs at a hematite shale/ (1954). The intermittent distribution of the ironstones ironstone horizon but the stratigraphic relationship to along the east-west lines across the Tennant Creek goldfield may reflect the dimensions of the circulating the stratiform Great Western porphyry is not clear. Ellipsoidal ironstone lodes commonly occur within hydrothermal cells which have redistributed iron from minor fold anticlinal hinges with long and intermediate the Warramunga Group sediments. Alternatively, lateral axes contained within the cleavage plane. Reverse faults lithologic and chemical variations within the hematite have acted as conduits for the mineralisingfluidsaround shales/oxidised horizons may have resulted in a variable some ironstones, although many stringer alteration zones capacity to oxidise the iron-saturatedfluids,influencing are parallel to the steep, east-west regional cleavage. the size and occurrence of the ironstones at various The inferred deep thrust system in the basal Warramunga stratigraphic levels within the Warramunga Group. Group may have locally acted as conduits for the fluids Reference and also tapped potential source lithologies for the gold, Ivanac, J. F., 1954: Bur. Min ResBull. 22: 162 pp. A7.15 THE SOURCE OF MINERALISING FLUIDS AT TENNANT CREEK Richard Wedekind* and Ross Large 1

Western Mining Corporation Limited, Kalgoorlie Key Centre for Ore Deposit and Exploration Studies, University of Tasmania

Gold-bismuth-copper mineralisation found at Tennant Creek is hosted by massive magnetite! chlorite±muscovite±talc±hematite±carbonate lodes. Economic mineralisation clearly overprints the ironstone lodes, but because gold, copper, and bismuth are exclusively and intimately associated with them, the process of ironstone lode formation and overprint of economic mineralisation is considered part of a single evolving system. To help characterise the composition of fluids responsible for economic mineralisation and ironstone lode formation, a study of the oxygen and hydrogen isotope composition of chlorite sampled from within and immediately adjacent to several of the ironstone lodes (Warrego, Juno, TC8, Argo, and Eldorado mines) has been undertaken. The calculated oxygen and hydrogen isotopic composition offluidsin equilibrium with chlorite forms a well definedfieldon a conventional hydrogen versus oxygen isotope diagram. The results overlap with the compositional fields of magmatic, metamorphic and seawater, but the trend of positive slope in the data suggest the fluid was derived from formation waters. The probable temporal association of lode formation with folding and regional metamorphism suggests the fluids were derived from the sediments through the conversion of water-rich clay minerals to illite and chlorite. Within the isotopic range of Tennant Creek fluids there is a separation of Warrego fluid compositions from those of Juno, Argo, and TC8 that is consistent with the different mineralogy and zonation observed in these deposits. This distinction could result because the Juno-type fluids were either:

1) more evolved, having interacted with the host rocks to a greater degree and probably were also hotter, more saline, and derived from a deeper source, or 2) water-rock ratios during ironstone lode formation were lower. The stronger gangue mineral zonation observed in the Juno-type ironstone lodes (magnetitechlorite —> talc-magnetite —> dolomite-talc) appears to reflect a rock dominated system which is consistent with option 2. The relatively broad range in the calculated fluid compositions does not preclude mixing of another component during mineralisation. The results for the largest and third largest gold producers at Tennant Creek (Warrego and Juno mines respectively), show relatively high 8 0 and low 8D which could indicate a trend of increased mixing with either metamorphic or magmatic fluids during economic mineralisation. This 'exotic' component is consistent with fluid inclusion evidence of compositional differences betweenfluidsinvolved in ironstone lode formation and mineralisation (Zaw & Huston, unpublished data). Metals may have been derived from the same source as the 'exotic' fluids, or they may have been leached from the sediments. Modelling of the sediment leaching process indicates that while there is sufficient iron and copper available within the Warramunga Group sediments, gold at least would require a contribution from an external source i.e. either directly from a magma or through leaching of the basement. Sulphur isotope analyses of mineralisation in the Warrego mine indicate that an igneous sulphur source is most likely and because of the complete absence of sulphides within the Warramunga Group, it is likely this was also derived from an external source. 18

162

A7.16 GRANITES BIF HOSTED GOLD DEPOSIT, NORTHERN TERRITORY Trevor Ireland and Nicholas Bryce* North Flinders Exploration, Wayville, SA.

The Granites Goldfield, some 550 km NW of Alice which are markedly elongate in a subvertical direction, Springs, is located towards the eastern margin of the and subparallel to the fold axes and mineral lineations. Early Proterozoic Granites-Tanami Complex. During These metasomatic infiltration zones are characterised the Barramundi Orogeny (1880-1850 Ma) the Granites- by addition of Ca and variably Si0 , S, Au, Co , As & Tanami inlier was regionally metamorphosed to Lower Fe. Veins and alteration zones show variable timing Greenschist facies, while metamorphic grades up to with respect to structural features, but most show effects hornblende hornfels facies occur in probable tectonic of the D, deformation episode, indicating that the bulk (thermal) zones, as at the mining lease. The oldest rocks of mineralization occured early during the deformation exposed in this terrain are the Mount Charles Beds history (Valenta &Wall, 1989). Alteration often overprint (Blake et al, 1979). This sequence hosts all known gold high grade minerals, indicating that all mineralisation occurrences in the region. Of the two mining centres, did not predate high grade metamorphism. the Granites line of mineralisation, with a strike length The high vein densities in ferromagnesian schists, of 9 km, occurs in a pelite-dominated thin-bedded iron- such as the Host Unit, relates to the competence contrast formation bearing schist, termed the Host Succession. and strain incompatibility between these rocks and their The evidence suggests that the gold deposits are pelite dominated envelope (Valenta & Wall, 1989; broadly stratabound but structurally controlled. A Purvis, 1989). In essence, thepelites (especially graphitic lithographic-metasomatic-metamorphic model is offered and sulphide-rich variants) were able to deform in a for the geology and gold mineralisation at the Granites. ductile manner and accommodate high strain(s) rate, The strata in the Granites camp have undergone at whereas the ferromagnesian schists behaved more brittly least three recognizable deformation episodes (D to and developed dilatant fractures. The fracture system, D ) (Marjoribanks,1985). These episodes display folds developed during early deformation, provided the with steep to vertical axes, which are occasionally cut permeability and hence a pathway for fluids. Regions of by faults with variable orientations and by rare gentle veining, alteration and mineralisation are structurally folds. These latter structures are associated with controlled and best developed proximal to major dilatant retrograde metamorphism. On a large scale, the generally zones. This contact separates rock masses of differing bedding-parallel D to D package of structures bends deformation styles and rheology and is likely to have around the present shape of the lease adjacent Granites behaved locally as a "detachment" during early granite, indicating that at least the last stage of granite deformation. Heterogenesis strain at this contact is likely emplacement postdated these structures. The earlier to have been an important factor in the localisation of intrusives, however, are strongly foliated and locally dilatant zones within the Host Unit. The chemical transgressive. The apparent rapid decrease of meta- features of the ferromagnesian schists, particularly the morphic grade away from the granite probably indicates Host Unit, which lead to the localisation of mineralthe indifferent susceptibility of the greywackes which isation, are the high FeO content and the extreme Fe/ "sandwich" the Host succession. (Fe+Mg) of such hosts, their reduced character and Gold mineralisation is hosted by a highly reduced generally low original CaO content (with some graphitic beds), pelite dominated succession In summary: which contains iron-formations and other ferro- 1. A greenschist facies detachment zone developed at magnesian schists, and, as mentioned, is deformed and the contact between the graphitic and non graphitic metamorphosed under hornblende hornfels facies sediments is a model being considered (Purvis, 1989). conditions. Ore grades are localised mainly in the Host 2. High T-low P metamorphism is typical of early Unit, a stratigraphically and compositionally distinctive Proterozoic origins, and would induce fluid migration iron formation. But additionally significant mineralearly in the deformation event. These fluids could isation occurs in amphibole rich units in the hangingwall result in metasomatism and associated gold mineraland footwall sections. The Host Unit exhibits a coherent isation (Scrimgeour, 1989). internal stratigraphy of thin bedded Ca-poor amphibole- References rich (commonly magnetite bearing) garnet-amphibole Blake, D., Hodgson, I. & Mahling, P., 1979. BMR Bulletin and garnet-biotite rich schists. In the Host Unit, 197. mineralisation correlates with sectors where the iron Marjoribanks, R., 1985. Unpublished report, to NFM Ltd. formation is thick; has the highest vein frequencies; is Purvis, A., 1989. Unpublished report to NFM Ltd. often sulphide bearing; and exhibits the most marked Scrimgeour, I., 1989. Honour thesis, University of Adelaide. metasomatism. Mineralisation occurs in zones of mainly Valenta, R. & Wall, V., 1989. Upublished report to NFM Ltd. bedding-parallel veining, defining tabular orebodies 2

163

A7.17 HYDROTHERMALLY HEMATITISED CONGLOMERATES — ONE ORE-TYPE OF THE STARRA Au-Cu DEPOSITS (NW QLD) Garry Davidson Key Centre for Ore Deposit and Exploration Studies, University of Tasmania

The Starra Iron-Formation Member consists of distinctly irregular contacts are also present, in which deformed banded iron formation, tourmalinite, and pebbly ironstone partially encloses conglomeratic related ironstones. It crops out intermittently over a 610 quartzite. km area and is one of the lower members of the Area 222 consists of two folded mineralised hematite evaporitic to shallow-marine, Mid-Proterozoic, Staveley lenses, amidst a much larger area of variably hematitised Formation, in the Mount Isa Eastern Succession (110 km breccia. At depth, the breccias are seen to be relatively south of Cloncurry). The magnetic Western Hematites unstrained polymictic rudites, which are extensively include a 6.5 km long segment incorporating the Starra hematite-altered. The highest Au and Cu grades correlate mineralised zones, and the non-magnetic barren Eastern with the most intense hematite alteration, and with Hematites are part of the iron formation. The Starra increases in magnetic susceptibility. Shear is a 1 km thick north-south zone of high strain Clasts consist of calcareous arenite and siltstone commencing immediately east of the Western Hematites, (variably scapolitic), with lesser porphyritic rhyolite, which separates the Staveley Formation from the alloch- chloritised vesicular basalt, milky vein quartz, carbonthonous Gin Creek Block (Switzer et al., 1988). aceous shale, tourmaline-bearing siltstone, and most Four mineralised mag-hem-qtz-py-cpy±siderite significantly, fine-grained barite-bearing hematitic ironstone orebodies are known, with bedding- ironstone. Poor to moderate sorting, sub-angular to conformable form and individual strike-lengths less than angular shapes, and sub-imbricate orientations charac400 m. They are Areas 222, 244, 251 and 257, named terise the breccias, which have a gravel to cobble-sized for their regional grid northing. These ores are commonly framework. Interbedded calc-arenite intervals separate oxidised to -200 m depth, with a well-defined sequence individual conglomerate beds, and are considered good of Cu minerals resulting from weathering (Kary & evidence for waterlain conglomerate deposition. Harley, in press). Areas 251,257, and 244 are all zones At petrographic scales the tectonothermal overprint of similar primary style, sharing an ironstone ore-host, is slight; cleavage is poorly developed, quartz is only and footwall magnetite-hematite-sulphide network- mildly undulose. With increasing alteration the primary veining. All are strongly deformed, which has lead to a quartz sand matrix (with syntaxial overgrowths) and controversy over a syngenetic versus epigenetic origin detrital mica is replaced by anhedral hematite, quartz, (Davidson et al., 1988; Laing et al., 1988). muscovite and chlorite. The margins of clasts become Area 222 mineralisation is unlike the other bodies: progressively bleached, transitional to massive hematiteit stratigraphically overlies them, is breccia-hosted, and chlorite rock with a ghosted clast fabric. Progressive far less deformed. It contains textures which are also alteration of clasts is compelling evidence that the common throughout the Eastern Hematites, where hematite, quartz and chlorite was not a part of the 2-20 m thick ironstones are interbedded with lensoid conglomerate bed-load, but were introduced postpolymict conglomerate possessing features of prograding depositionally. Small primaryfluidinclusions (too small alluvial fans. This has provided crucial new insights to for thermometry) observed the hydrothermal quartz, the ore genesis. contain a small vapour bubble, halite, and an unidentified Breccia textures in the ironstones of the entire area daughter salt: a minimum 26.5% NaCl (weight % vary from indisputably tectonically-related, to the equivalent) for the introduced fluid is indicated. Barite-bearing siliceous ironstone fragments are dominant texture, that of discrete polymict clasts in a hematitic matrix, which are of less determinate origin. important evidence for the pre-tectonic origin of Tectonic breccias in the Eastern Hematites are most ironstone in the Starra area. The host-breccias have the intensely developed in fold transposition zones, and are form and structure of sedimentary conglomerates. On identified by (1) a similarity between adjacent clasts, the basis of textures, the deposits originated as preand between the clasts and the host rock; (2) the presence tectonic, polylithic, channel or piedmont-related debris of rootless thickened fold hinges, wedges and lenses; flows, although the three dimensional geometry of the and (3) an alignment of clasts with the cleavage. In resulting units is not known. Hydrothermal fluids many ironstone-hosted breccias, including those of Area ascended along secondary faults, and dispersed into the 222, these criteria cannot be satisfied. The least deformed porous conglomerate horizons, reacting with the minerals of these contain polymict fragments with no obvious of these aquifers. The preferential alteration of clast elongation, in a hematite matrix, which under high margins is evidence that the deposited hematite did not strain transformed into cleavage-parallel silicate lenses merely fill conglomerate voids, or originate as part of in a cleaved massive hematite matrix. The ironstone the bedload, but reacted with sedimentary components breccia-sediment contacts are usually planar, although progressively, in many instances culminating in 2

164

References wholescale hematite replacement The invading hydrothermalfluidwas highly oxidised, Davidson G.J., Large R.R., Kary G. & Osborne R., 1988. Geol. Soc. Abs. 22: 85-90. saline, and saturated with respect to silica and iron; it is Rubenach M. & Switzer C.K., 1988. Geol. Soc. the best guide to thefluidswhich formed the other ore- LaingAbs.W.P., 114-115. bodies of the area. Evidence has been previously Kary G. 21: & Harley R. in press. Austral. Inst Min. & Metall. presented (Davidson et al., 1988) favouring an exhaled special vol. on Australian ores. origin for these. Switzer C.K., Laing W.P. & Rubenach M. 1988. Geol. Soc. v

Abs. 23: 212-214.

165

A8: Role of Structure in the Formation and Deformation of Ore Deposits Convenors: S.F. Cox and R.F. Berry

A8.1 SHEAR ZONE HOSTED ORE DEPOSITS John L. Baxter and Nick Hewson Continental Resource Management Pty Ltd, Rivervale, WA

Structurally controlled mineral deposits, particularly gold, are often closely related to shear zone geometry. It is of fundamental importance that when structural analysis of mineralised environments is undertaken, that both kinematic and geometric factors be considered. The kinematic factor in shear zone hosted mineralisation can be primarily related to the rheology of the rock suite being deformed and the temperature at which the shear is induced. There is a substantial body of data which now indicates that temperature is the prime control on the style of deformation within shear zones. Brittle deformation occurs where the rocks are relatively cold and can contain deformed products where clay has not re-crystallised. This style of deformation is common in epithermal terrains and appears to have a temperature boundary in the vicinity of 150°C. Ductile deformation occurs at the greenschist/amphibolite transition. Here there is development of a foliation by crystal-plastic and inter-granular deformation processes. In general this temperature is in the vicinity of 350°C. There is a zone between purely brittle deformation and purely ductile deformation within which most of the rock suites that contain ore deposits occur. It is known as the brittle-ductile transition zone. Within this zone rocks will deform in both brittle and ductile manners, depending on the rheology of the rocks. Recognition of kinematic indicators is of substantial importance when interpreting the geometry of the shear zone. In brittle domains ore shoots will be controlled by the intersection of secondary shear planes within the array, while in ductile deformation the elongation of the ore deposit will be parallel to the stretching lineation within the mylonite zone. Shear zones are rarely confined to a single plane, but develop within a family of planes which anastomose through the crust The geometry of these planes can be predicted from geometric analysis. The most important identifier is whether the shear array is in a coaxial conjugate or a non coaxial shear array. Conjugate shear arrays can be incredibly complex as each arm of the conjugate array may contain a non coaxial shear. The angle between the conjugate array, which is normally

about 60°, will vary depending on whether the rocks are in the brittle domain (<60°) or in the ductile domain (>60°). When there is the suspicion that a coaxial conjugate shear array is present the direction of movement on the shear plane should be analysed with care and flattening surfaces and extension surfaces clearly identified. The geometry of the non coaxial system, including the approximate angles between secondary shear planes, remains the same irrespective of the scale being reviewed. Within the non coaxial shear array there are five shear planes which may or may not all occur within a single shear system. As well as the shear planes there is often a flattening plane and an extension plane. The movement on the shear plane can be calculated from the geometrical analysis. In Western Australia, mineralisation has been identified in strike slip, reverse and normal dip-slip and oblique-slip arrays. It is therefore essential that the geometry of a shear hosted deposit be identified prior to any assessment of ore reserves. The majority of the mineralisation contained in a shear occurs within zones of dilation which can occur in a variety of locations within a shear zone. In many shear arrays there is a dilatant zone at the intersection of secondary shear planes. This brecciapipe style mineralisation is common in Archaean Shields and can lead to clusters of deposits within an environment. Dilation also develops within extension vein arrays which can be controlled by coaxial or non coaxial deformation. Transtensional shears, which are often Reidel shears within the main shear array, can be dilated when fluids are active. Dilation can occur with perturbations on the shear plane caused by lithology change which can induce shear hosted mineralisation. Within reverse shear arrays dilatancy of the secondary shear planes, particularly the reidel shear planes, can occur. Our observations indicate that the P or pressure shears are rarely dilated within this system. Sibson (1989) has pointed out that the location of the earthquake within the shear array is a significant component of determining whether there will be dilatancy on a bend or not This aspect of shear zone hosted mineralisation requires further analysis.

166

Earthquakes induce shear zones and focusfluidsduring mineralisation. When a shear is operating with a particular sense of movement, and if this sense of movement becomes reversed due to a change in the stress regime, then planes which were deformed during the first event will be dilated during the reactivation. This is a common observation within the goldfields of Western Australia. In certain deposits (e.g. Reedys, in the Murchison Province) a large proportion of the mineralisation has been developed late in the shear history by small secondary movements within the shear plane. If the main shear plane jogs to a new orientation within the shear array, a dilatant zone may occur. If the shear is essentially strike-slip these dilatant zones can produce breccia-pipes which may penetrate substantial distances within the earths crust Jog dilatancy of this type has been identified very clearly in some Archaean deposits (e.g. Slippery Gimlet at Ora Banda) and some epithermal deposits in north Queensland. In ductile shear zones where boudins are developed, dilation occurs adjacent to the boudins in the pressure shadow areas, parallel to the extension direction. Reference

SOUTHERN SHOOT

GOLDEN KILOMETRE

Sibson, 1989. J. Struc. Geol. 1.

KEY TO STEREONETS

Shear / Foliation Quartz vein Fracture / Joint 0 Mani shtar direction R Retdel shtar (fraction SOUTHERN SHOOT T Ttnsion (fraction P Pressure TH Thrust plants DGK Mani shtar (fraction — GOLDEN KLIOMETRE PGK Prtssurt shtar direction RG'K ReWel prhat (fraction OC Oextrat conjugate SC Sinistral CONJUGATE SET 1 CT Tension direction MC Normal conjugate Figure 1 — Example of dip-slip and strike-slip mineralisation; Southern Shoot and Golden Kilometre Mine, Mt Pleasant, Western Australia.

A8.2 STRUCTURE OF THE COWARRA GOLD DEPOSIT NEAR BREDBO, NSW M J. Rickard * and K. McQueen 1

Australian National University, Canberra Canberra College of Advanced Education, Canberra 1

The Cowarra gold deposit consists of structurally The deposit differs from many other Ordovician slatecontrolled, narrow and anastomosing pyrite-pyrrhotite hosted deposits (e.g. in Central Victoria) in its high rich veins and pods developed in at least three separate sulphide content and the style of structural control. At least three major deformation events can be lodes within deformed Ordovician greywacke and slates. Gold occurs mostly as small inclusions in the sulphides. recognised in the Ordovician rocks of the surrounding

167

region and main phase folding has resulted in tight to isoclinal folds with a general northerly plunge. The Cowarra lodes occur on the eastern limb of a regional anticline within a zone of parasitic folds. These are overturned to the west with short inverted limbs. A strong axial planar cleavage is commonly paralled to bedding except in fold hinges and quartzite beds where it fans markedly. The mesoscopic folds display a remarkable plunge variation across the major limb, changing from zero in the synclinal core to 80° in the anticlinal hinge. The ore-bodies form a series of lenticular sheets in a narrow zone of shears in a midlimb position where the plunge is generally 35°N but locally varies from 10°S to 85°N. The overall orientation of the lodes is parallel to the local plunge and there is a thickening of the main lode where the plunge decreases in the centre of the deposit The shears containing the mineralisation are lm to 2m-wide zones of schistose material, developed parallel to cleavage in fold limbs, especially inverted limbs adjacent to massive quartzite hinges. Many fold hinges are faulted out, with the shear zones passing from one limb to the other in plan and section, thus producing an anastomosing pattern. Beds from normal and inverted limbs are adjacent within some shear zones. There are no S-C structures or riedel shears as commonly found in other types of shear zones. Ore veins pinch and swell but lie mainly parallel to bedding in the shear zones.

Thinner veinlets also feather out along the cleavage planes. Minor quartz, carbonate and chlorite accompany sulphides in the veins. Numerous quartz veins occur in the host rocks but these do not carry significant mineralisation. Seven types have been recognised, the most important of which are thick quartz lenses and veins parallel to the shear zones and thin veinlets along the cleavage. Flat quartz "tension" gashes occur only on fold limbs in massive quartzite and these are folded in places. Three types of faults occur in the mine including faults parallel to and within the shear zones, steep faults with narrow gouge zones which cross-cut the shears, and moderate to flat reverse faults. The latter two types post-date the mineralisation. The mineralised shear zones appear to have developed as ductile shears during continued compression following the main phase of isoclinal folding. The form of these zones was strongly influenced by the highly anisotropic nature of the interbedded slates and greywakes and the pre-existing fold structures. Goldsulphide bearing fluids were channeled into the shears during subsequent decompression or fluid overpressuring. Decompression probably accompanied granite intrusion in the area and these granites appear to be the source of the mineralising, hydrothermal fluids. Later kinking and faulting disrupted the ore bodies slightly.

A83 LOW ANGLE SHEARS IN CENTRAL OTAGO, NEW ZEALAND, THEIR REGIONAL EXTENT, ECONOMIC SIGNIFICANCE AND RELATION TO MANORBURN FOLDS Colin N. Winsor Geology Department, University ofOtago, Dunedin, New Zealand.

Gently dipping mesoscopic shears and quartz veins are a ubiquitous feature in the Otago Schist Belt (Fig. 1). A prominant set of low angle N dipping mesoshears often parallels Manorburn Generation (Norris, 1977), F axial planes although locally transecting folds. South dipping mesoshears are also present, constituting a conjugate set Both thrust and normal movement is indicated, related to regional compression and later extension. In western portions of the Belt two sequential F fold subphases (i.e. F and F ) are recognised, but in more eastern segments F folds dominate and mask the earlier folds. F folds with general N dipping axial planes are considered to postdate F^ folds with S dipping planes, as: 1) the former transect vergence boundaries with associated mesofolds which exhibit S dipping axial planes, 2) F axes are subparallel to the stretching direction (L ), while F axes are oblique, 3) a crenulation related to F transects F folds, and 4) vein timing and L rotation, supports this history. The existence of two post-S ductile subphases and dominance of N to NE dipping shears, is significant in 3

terms of gold mineralisation, which is distributed throughout the Schist Belt (Craw & Norris, 1988). Important factors influencing the distribution of mineralisation sites associated with low lying shears are: 1. Geometry of F axial planes; that is, in areas where axial planes are N to NE dipping, there is greater potential for imbrication. 2. Variations in the intensity of F folding. Where this folding is more prominant, late syn- to post D veins are likely to develop parallel to axial planes. 3. Lithology control; a finely laminated schist with extensive strike length, moderate width and adjacent to coarsely layered schist, is well suited to strain concentration, macroshear formation and imbrication of the finely laminated schist. Notably only one macroshear (the Hyde-Macraes Shear Zone, McKeag et al., 1989) is recognised for which these requirements are met The general absence of macroshears is due to the strongly foliated, segregated and coarsely layered nature of the Otago Schist and the spatial localisation of F folds. 3

168

References McKeag, S.A., Craw, D. & Norris, R.J., 1989. Mineral Craw, D. & Norris, RJ., 1988. Proc. of conference: New DeposUa 24:124-131. England Orogen Tectonics and Metallogenesis. The Norris, R J., 1977. Geol. Soc. NZ Annual Conf. Abst. University of New England, Armidale N.S.W.

Figure 1 — Location map, portion of the Otago Schist. Displaying (1) areas of known gold mineralisation, study areas 1, 3, 4, 5 & 7, Conroys Gully and Whites Reef; (2) sites of F vergence boundaries, study areas 1,4, 5, 6, and Blackstone Hill; and (3) regions of known low-lying macroshears including the Hyde- Macreas Shear Zone and areas 1 & 2. 3

A8.4 THE ROLE OF FAULT DYNAMICS AND FLUID DYNAMICS IN THE BENESIS OF VEIN-HOSTED GOLD DEPOSITS IN LOW-GRADE METAMORPHIC TERRANES S.F. Cox Research School of Earth Sciences, Australian National University

Vein-hosted gold deposits in the Bendigo-Ballarat Zone of the Lachlan Fold Belt have similarities in overall structural style, mineralogy, and fluid chemistry that are shared with many mesothermal gold deposits in low-grade metamorphic terranes worldwide. Examples from the Bendigo-Ballarat Zone are used to demonstrate that not only do these deposits have structurallycontrolled locations and geometries, but fault dynamics and fault-regulated fluid dynamics can play a major role in the genesis of this important class of deposits. The vein deposits in the Ordovician Castlemaine Supergroup of central Victoria have formed during fault activity which was broadly synchronous with, but mostly late during regional crustal shortening and low-grade metamorphism of the quartz-rich flysch sequence (Cox et al., 1987). Major mineralisation is restricted to narrow domains which are parallel to the regional fold and fault trend, up to about 10 km long, and usually less than 1 km wide. Vein systems have developed in fault-related and

fold-related dilatant fractures which were generated at supralithostaticfluidpressures. Major gold deposits are localised in dilatant jogs within high-angle reverse faults, in fault-modified saddle reefs and associated beddingconcordant reefs, and in extension vein networks related to these. The geometry and internal structures of vein systems and associated fault zones indicate that mineralisation has involved up to several hundred crackseal growth increments per centimetre of vein width. Repeated vein growth increments have been controlled by cyclic fluctuations in fluid pressure and shear stress which accompany repeated episodes of fault motion. Fluid inclusion studies indicate that the fluids involved in mineralisation are H 0-C0 -CH fluids, which are similar to those found in many low grade metasedimentary belts. Fluid temperatures have been close to 300°C, and pressures have been around 150 MPa. A number of factors indicate that CH production, due to reaction of water with graphite slates adjacent to vein systems, has played a key role in gold 2

169

deposition by reducing oxygen fiigacity and destabilising which is required to promote efficient gold deposition in fault zones and associated hydrofracture networks. gold complexes (Cox et al., 1990). The timing of mineralisation, the nature of the Prior to fault failure episodes,fluidmigration is impeded associated hydrothermal alteration, and the isotopic and below valved zones. This results in fluid infiltration of chemical compositions of thefluidsindicate ore genesis wall-rocks adjacent to faults, and leads to CH involving large volumes of C-O-H metamorphic fluids enrichment of the hydrothermal fluid. Fault zone failure whose flow has been channelised along high and valve breaching causes an abrupt drop in fluid permeability fault zones. The crustal scale fault archi- pressure within fault zones, especially at dilatant jogs tecture which developed during regional deformation is and saddles. This causes the secondary, CH -enriched interpreted to have controlled the large-scale fluid fluids to be driven back into the transiently low pressure migration pattern during mineralisation, focussing fault zone where they mix with the more oxidised primary hydrothermal fluids traversing the fault zone. metamorphic fluid flow towards the goldfields. The geometry and dynamics of active fault zones The development of the supralithostatic fluid pressures necessary to generate hydrofracture networks have played central roles in channelingflowof auriferous is critically dependent on the upward migration of fluids fluids, and in controlling the fluid pressure history, being throttled by an appropriate distribution of low fracture growth,fluiddynamics, andfluidmixing. This permeability zones in theflowpath. At the deposit scale has governed the coincidence of structural and this control is largely provided by local, transient geochemical traps necessary for the formation of hydrothermal sealing of fault zone segments, and leads mesothermal fault-zone hosted gold deposits in the to cyclic fault-valve behaviour (Sibson et al., 1988; Cox Bendigo-Ballarat Zone of the Lachlan Fold Belt etal., 1990). This is a critical factor which has restricted References the development of supralithostatic fluid pressures and Cox, S.F., Etheridge, M.A. & Wall, V.J., 1987. Ore Geology Reviews 2: 65-86. localisation of vein deposits to relatively small, Cox, S.F., Wall, V.J., Etheridge, M.A. & Potter T.F., 1990. discontinuous fault zones. Ore Geology Reviews, in press. Cyclic fault-valve behaviour has played a substantial R.H., Robert, F. & Poulson, K.H., 1988. Geology 16: role in controlling fluid dynamics and fluid mixing Sibson, 551-555. 4

A8.5 GENESIS OF THE DEBORAH LINE GOLD DEPOSITS, BENDIGO, VICTORIA: STRUCTURAL AND TIMING RELATIONSHIPS OF GOLD PRECIPITATION AND ITS POSSIBLE MAGMATIC ORIGIN N. Green *, T.A.P. Kwak , A. Changkakoti , J. Gray and H.R. Krouse 1

Department of Geology, La Trobe University Department of Physics, University of Alberta, Edmonton, Canada Department of Physics, University of Calgary, Calgary, Canada 1

2 3

The Bendigo gold deposits are structurally controlled vein-type lodes that occur in an Early to Middle Ordovician sequence of sandstones, siltstones and shales. Early Devonian deformation produced a series of NNW trending, tight chevron folds with a well developed slaty cleavage. A second phase deformation followed with stress directions similar to the first and a component of W over E tectonic transport The stress was taken up by fold tightening with concomittant reverse fault movement along siltstone and shale units on the fold limbs. Doming due to N-S compression or fault readjustment produced a series of plunge reversals along the fold hinges. Laminated veins developed along reverse fault zones in response to incremental opening and closing episodes not unlike crack seal vein formation. Massive saddle reefs formed in the hinge zones as a result of more complex movements on the east and west dipping limb faults. Slickenside lineation directions are sub-vertical on the laminated limbs and parallel to the fold axis in the hinge zones.

The irregular bodies of massive quartz (saddle reef type) developed in the fold closures, average 5 m by 3 m in cross-section and are continuous along strike for over 1 km in the Inner Reef of the Deborah Line. Laminated or leg type reefs are much smaller with average widths of 0.5 to 1.0 m. These laminated veins extend down the fold limbs for up to 100 m. The saddle reefs terminate in a zone of tensional vein arrays and stockworking in the surrounding competent units. Detailed fluid inclusion microthermometry, gas chromatography and laser Raman microprobe are being used to delineate fluid types, gas-liquid compositions and fluid transport mechanisms in these vein systems. The fluids were dominantly aqueous with variable carbon-dioxide-methane ratios. Preliminary results suggest that three distinct fluids were involved in the formation of the laminated and massive veins. Temperatures ranged from 130 to 360°C (av. 250°C) in the massive saddles, and 130 to 310°C (av. 230°C) in the laminated veins. Salinities ranged from 7.5 to 9.5 and 3.0 to 13.5 wt % NaCl equivalent respectively. The

170

three fluids underwent complex mixing as the vein systems evolved with boiling occurring in the massive sections of the reefs. Gold is generally found in the lower temperature, lower salinity laminated vein margins in association with ankeritic carbonate and graphitic shale relicts. The massive reefs are generally gold-poor with economic grades found only in laminated outer portions. Gold

also occurs as inclusions and veinlets in arsenopyrite and pyrite within the quartz veins and associated black shale fragments. Measured 8D of fluid inclusion waters in quartz and calculated 8 0 values suggest a metamorphic origin for the fluids responsible for depositing quartz. Carbon and sulphur isotope data of the carbonates and sulphides respectively suggest a magmatic origin. 18

A8.6 STRUCTURAL GEOLOGY OF NEW ZEALAND EPITHERMAL GOLD K.B. Sporli, M.R. Gadsby and D. Clarke Ep ithernial Mineralisation Research Unit, Department of Geology, University of Auckland

New Zealand's epithermal gold deposits are concentrated in the Coromandel area of the North Island, and were formed in late Miocene-Early Pliocene time in the eastern of two calcalkaline volcanic arcs along Northland peninsula, during migration of a newly established subduction system to its present position, and during establishment of the Alpine Fault transform regime In contrast to the eastern arc, the western calcalkaih-e arc lacks mineralisation and high level plutons, because of yet unknown tectonic controls. Differences in lithosphere structure may have an influence. Mineralisation is related to epithermal quartz veins up to 10 m in thickness, which mostly dip steeply, but are rarely vertical, either because of subsequent tilting of the veins, or becau. v the veins are following conjugate normal faults. However, control by pre-existing jointing or joint opening along fault surfaces is also important Low angle "flats" may represent listric normal faulting combined with hydraulic fracturing. Veins trend dominantly NNW in the northern part of Coromandel v

peninsula and dominantly NE in the southern part Neither direction is directly compatible with stress orientations to be expected from plate convergence vectors at the time of mineralisation. Small scale structures of the veins include dilational jogs both in strike slip and dip slip mode, changes in attitude due to linkage between en echelon segments and between veins representing octahedral shear. Some veins may be due to and were held open by dilation between blocks of country rock rotating against each other. There are two major types of vein fill, probably indicating contrasting structural/hydraulic environments: (1) Tokatea type, dominantly consisting of simple comb textures; and (2) Golden Cross type, represented by intricately banded veins with crustiform layers, and dense microcrystalline, often sulphide rich fills. Incremental growth, fracturing and cross-cutting layers in some veins indicate that crack-seal mechanisms operated in some instances. Prominent post-vein faulting records both strike slip and dip slip on E-W and N-S surfaces.

171

A8.7 DEFORMATION-INDUCED FLUID PRESSURE VARIATIONS: AN EXPLANATION FOR HIGH FLUID/ROCK RATIOS DURING METAMORPHISM AND ORE GENESIS N.H.S. Oliver *, R.K. Valenta and V.J. Wall 1

CSJJR.O. Division of Geomechanics, Mt Waverley, Victoria Department of Earth Sciences, Monash University MIM Holdings Ltd, Brisbane 2

At large scales, an apparent discrepancy exists in some terrains between the volume of fluid released by devolatilisation reactions during regional metamorphism, and the volume of fluid required to dissolve, transport, and precipitate mass in metamorphic/hydrothermal systems. This paper attempts to explain this discrepancy with a model of structurally-controlled fluid recirculation. While many metamorphic penologists have calculated high fluid/rock ratios (F/R) for greenschist- and amphibolite-facies metamorphism, there is dispute as to the validity of the calculations, and the mechanisms of fluid- and mass transfer that may explain the high F/R (e.g. Etheridge et al., 1984; Ferry, 1986). Stable isotopic and mass transfer patterns in the Mary Kathleen Fold Belt (Oliver et al., 1990) and the Waterville-Augusta region of Maine, USA (Ferry, 1986; Rumble et al., 1986) give clear indication that the amount of fluid involved in the metamorphism is far too great to have been derived from the local rocks, suggesting fluid focussing or recirculation mechanisms. The problem is even more acute for many hydrothermal ore deposits, which, on the basis of the solubility characteristics of ore components in the fluid, require apparently vast volumes of fluid and highly efficient concentration mechanisms. Stress, strain and fluid pressure variations during regional metamorphism are a natural consequence of the deformation of compositionally heterogeneous rock suites, as differing rocks have different permeabilities and rheologies. Two examples of finite difference modelling of stress variations are presented, involving a strong body in a weak matrix (Mary Kathleen Fold Belt), and intersecting faults/shear zones with different movement senses (Hilton Mine). Large strain heterogeneities are also apparent fromfieldobservations of such settings. Particular attention is drawn to strain shadow regions in the Mary Kathleen model, and intersections of antithetic faults in the Hilton model. The large spatial variations of stress and strain give rise to pronounced zones of dilatancy, rock failure, large fluid pressure gradients, and hence strong driving forces

for fluid migration. The loci of maximum dilatancy change with time depending on the geometry of the system, strain rates, rock strengths and permeabilities, giving rise to highly dynamic systems in which fluid may be pumped back and forth, circulated, or funnelled for considerable distance. Zones of structurally localised dilatancy play two important roles: (1) fluid pressure changes due to dilatancy may periodically disrupt the "normal" lithostatic fluid pressure gradient, with the development of zones of near- or sub-hydrostatic fluid pressure gradients, resulting in tortuous or even downwards fluid flow (Fig. 1, over page), and (2) dilatancy provides high permeability fluid pathways, regardless of flow direction. Although it is difficult to place absolute constraints on the magnitude of fluid pressure variations attending regional metamorphism/deformation, we suggest that large, extensive, but transient fluid pressure changes are likely in heterogeneous greenschist- and amphibolitefacies rock sequences (Fig. 2, over page). The magnitude, scales, and times of suchfluidpressure changes may be sufficient to generate irregularfluidcirculation patterns in many deforming rocks. This mechanism of dilatancy pumping can explain calculated high F/R and locally important intense metasomatism in many amphibolitefacies terrains (e.g. Mary Kathleen), and retrograde shear zones in granulites (e.g. Broken Hill; Enderby Land, Antarctica). Furthermore, for those greenschistor amphibolite-hosted ore deposits apparently requiring immense fluid volumes (e.g. greenstone-hosted gold deposits), the proposed mechanism would allow large metal source regions both above and below the depositional site. References

Etheridge, M.A., Wall, V.J., Cox, S.F. & Vernon, R.H., 1984. J. Geophys. Res. 89: 4344-4358. Ferry, J.M., 1986. J. Petrol. 27: 695-714. Oliver, N.H.S., Valenta, R.K. & Wall, V.J., 1990. J. Metamorphic. Geol., in press. Rumble, D. m, Ferry, J.M. & Hoering, T.C., 1986. Contrib. Mineral. Petrol. 93: 420-428.

172

i—i

Constant depth separation during burial

•

permeable fluid pathway e.g. shear zone

Time 1 PTf A> PfT B + P gh (upfjow) fluid Time 2 As Pf B + Pfl.iiHg57 h fluid

/0 /}/ fa

episodically brittle dilatant zone e.g. jog-related

Figure 1

£ q. o Q

v(downflow) J

C " Q flow)

Time 3 A< Pf B + P gh X f fluid

Time 4 ^ Px A > Pf B + P gh (upflowy fli liri

Consider 2 points (A and B) along a permeable fluid pathway, one being located in a zone of episodic dilation due to brittle failure. During hypothetical "burial" of this pathway (in reality the pathway would change configuration with depth), periods of dilatancy associated with failure at point A may result in a marked departure from the usual near- lithostatic fluid pressure gradient as displayed by point B. If the magnitude of the fluid pressure change associated with failure at A is sufficient, periods of downwards fluid flow may occur as indicated, p gh = hydrostatic head

Figure 2 Pf gradients Hydrostatic

Approaching hydrostatic (strong component of brittle behaviour, high F/R) and lithostatic (ductile behaviour, low F/R) Lithostatic

At various times, zones of high permeability may propogate into mid-crustal levels, permitting the development of irregular flow paths and even recirculation. This figure might represent a section of the crust after several cycles of dilatancy pumping. Although the net flow is most likely upwards over any reasonable geological interval, substantial circulation and downward flow may occur at specific times.

173

A8-8 STRUCTURAL SETTING OF UNCONFORMITY-RELATED U-Au-PLATINOID MINERALISATION IN THE SOUTH ALLIGATOR VALLEY, NT R.K. Valenta Department of Earth Sciences, Monash University

U-Au-platinoid deposits in the South Alligator Valley dextral strike-slip movement on this system may have occur around the intersections between a long-lived been up to tens of kilometres, while strike-slip movement fault system and a Proterozoic unconformity. Rocks in during and after El Sherana Group deposition probably the South Alligator Valley preserve a complex history totalled less than 5-10 km. of folding, faulting, and sedimentation, which can be U-Au-PGE mineralisation occurred in the later stages summarised as follows: of fault movement, in structurally-controlled dilation 1. Deposition of Early Proterozoic sandstones, shales, sites. The northwest-trending dextral strike-slip fault system controlled patterns of sedimentation and mineralgreenstones, and iron formations. 2. Formation of rare D isoclinal folds and a widespread isation, and deposit geometries are systematically related to area-scale fault kinematics. In contractional areas, bedding-parallel fabric. 3. Formation of the regional D anticlinal axis to the mineralisation formed in subhorizontal dilation systems; southwest of the South Alligator Valley, resulting in the while in extensional areas mineralisation tended to conformation of SW-verging minor folds and a penetrative centrate around subvertical dilation systems. It is likely cleavage within the South Alligator Valley. Bedding- that mineralisation occurred after deposition of the parallel high strain zones began to form at this time. Kombolgie sandstone. Late mineralisation is consistent They show dextral displacements in their present steep with observations of alteration distribution, deformation patterns, vein orientations and distribution of orientations. 4. Formation of minor NE-verging folds and cleavage mineralisation relative to large and small scale structures. Mineralisation has been concentrated at the earlyduring D , associated with continued formation of high Proterozoic/mid-Proterozoic unconformity for both strain zones. 5. Deposition of bimodal volcanics and high energy chemical and structural reasons. Reduced rocks below clastics of the El Sherana Group during movement on the unconformity formed a chemical/redox trap for mineralisation, while the small scale structural control the NW-trending regional fault system. 6. Gentle folding of the El Sherana Group about NW- on ore localisation was dilation resulting from irregular trending axes associated with continued fault movement faulting and folding caused by strain incompatibility and minor folding and extensional folding in basement between rocks above and below the unconformity. The combined structural and chemical trapping responsible rocks. 7. Deposition of felsic volcanics and coarse clastics of for these deposits is a critical factor in the formation of the Edith River Group, in angular unconformity with many other types of hydrothermal ore deposits. Mineralisation generally occurred around bends in the El Sherana Group. Systematic relationships between faults and unit thicknesses imply syn-sedimentary fault the main fault or on 1-2 km scale "second order" faults which appear to have had less than 100 m of displacemovement. 8. Gentle folding of the Edith River Group about NW- ment This can be explained in two ways: (1) Faults trending axes, accompanied by tightening of folds in with small movement relative to surface area can have the El Sherana Group, all during ongoing movement on more irregular surfaces and should therefore show substantial dilatancy associated with small displacethe main NW-trending fault set 9. Deposition of coarse clastics and volcanics of the ments, while faults with large displacements should tend to develop a principal slip surface which is less Katherine River Group in angular unconformity with dilatant; and (2) Minor faults show physical the Edith River Group. and orientations relative to far-field 10. Gentle folding of the Katherine River Group about characteristics stresses which should allow relatively constant high NW-trending axes, accompanied by tightening of folds permeabilities during deformation. This suggests that in the El Sherana and Edith River Groups. Deformation faults are of major importance to patterns of large in the Katherine River Group is spatially associated minor scale fluid circulation during deformation. Ore deposits with the main NW-trending fault set minor faults are often associated with restricted This sedimentation-deformation history implies on of very high permeability which act as channelways alternating extension and contraction in a northeast areas for fluid flowing under a far-field hydrodynamic direction. An alternate explanation is that basin formation gradient Strongly heterogeneous fluid/rock ratio patterns and folding about the same axis were related to an be explained by major aperture variations on the ongoing process of strike slip movement on the two can surface. These aperture variations can be produced major fault systems. The orientations of normal, reverse faultvariations in fault geometry, geometries of and strike-slip faults are kinematically consistent with by intersecting faults and rheological differences between a setting of mainly dextral strike-slip movement on the rock types cut by the fault. main northwest-trending faults. Pre-El Sherana Group x

174

A8.9 TIMING OF SYNDEFORMATIONAL BASE AND PRECIOUS METAL MINERALISATION CONTROLLED BY DEFORMATION PARTITIONING AT PEAK, COBAR, NSW Mark Hinman James Cook University, Townsville An undeveloped base and precious metal resource at Peak, containing 30 tonnes of gold, is localised in a series of lenses showing differing paragenetic details that can be timed as early-syn (and possibly pre), syn, and post relative to the major penetrative deformation event, D r Spatially the lenses of mineralisation lie in zones of anomalously high and heterogeneous strain developed by the partitioning of D2 strain around infaulted slices of rigid acid volcanic and volcanoclastic material. These anomalous strain zones, in a regionally otherwise homogeneous strain field, are revealed by significant bedding attenuation and rotation in cross section towards the cleavage, S2; the unfolding of earlier Dx folds; and the development of doubly-plunging structures by the rotation of bedding through the horizontal towards the D2 stretching direction, L i r Three phases of mineralisation with contrasting mineral assemblages and metal associations and differing timing relationships relative to D2 fabrics occur. Each lens of ore is composed of different overprinting combinations of these three phases of mineralisation. Early silver-poor, sphalerite-galena mineralisation, associated with carbonate and silica, occurs as disseminated sulphide in strongly silica replaced material and as folded and boudinaged veins. The associated sediment silicification commonly preserves an S t cleavage and/or a very primitive S2 cleavage. The relative intensification of the S2 cleavage outside the zones of

silicification on the macro-scale and in thin section indicates the early- (or pre-) D2 timing of this event. Later in D2, silica-green chlorite-chalcopyrite-pyrrhotitegold mineralisation formed as replacive veins and zones commonly in previously silicified material that behaved brittlely in response to ongoing deformation. A tectonic breccia, which developed along the contact between volcanics and sediments, has a silica-chloritechalcopyrite-pyrrhotite matrix and contains sediment clasts with rotated, well developed, S2 fabrics. Some veins of this assemblage overprint the early sphaleritegalena veins. The silica-chlorite-chalcopyrite-pyrrhotite veins, although internally deformed, are not usually boudinaged or folded. This indicates their later syn-D2 timing. Within the veins and replacive zones an internal paragenesis indicates that the chlorite, sulphide and gold is associated with weak syn-D2 deformation of the quartz that is expressed by the development of subgrains along the quartz's grain boundaries. Massive, banded, silver-rich, chlorite-associated, sphalerite-galena mineralisation is formed by replacement post-D2. Replacement fronts cut and consume D2 fabrics and all earlier phases of mineralisation. Banding is not coherent with D2 structures but instead parallels the boundaries of relic siliceous blocks and forms in response to late stage jostling between these unreplaced remnants.

A8.10 FORMATION OF A MASSIVE SULPHIDE OREBODY BY SYN DEFORMATIONAL HOST ROCK REPLACEMENT IN A DUCTILE SHEARZONE, ROSEBERY, TASMANIA Domingo G.A.M. Aerden Department of Geology, James Cook University of North Queensland

The Rosebery Pb-Zn-Ag-Au massive sulphide orebody is hosted in a narrow, east dipping, phyllonitic sediment lens, within Cambrian felsic volcanics of the central Mount Read Volcanics. These rocks were deformed and folded during a Devonian orogeny. Structural and micro-structural analysis indicate that the dominant cleavage within the host horizon and footwall volcanics formed by localised reverse shearing, late during D2, along a pre-existing cleavage (S^ and along the slightly shallower dipping bedding. Reactivation of Sl and layer parallel shearing resulted in partial detruction of S2 crenulations, unfolding of mesoscopic D2 folds and the development of a penetrative shear related cleavage SXR2 (St reactivated during D2). Extensional microcracking and foliation

boudinage occurred both in the downdip and horizontal direction within the plane of the reactivation cleavage. Microstructural timing relationships show that the orebody- and alteration minerals grew by metasomatic replacement of the sericite-quartz host rock schist. Sx and S2 cleavages are truncated and overgrown by the silicate gangue and the sulphide minerals constituing the ore. Remnants of partially replaced host rock are preserved within massive mineralisation and alteration. Ghosting of cleavage bands and less common, host rock folds may locally give the ore a false sedimentary and folded appearence. Mineralisation occurred during cleavage parallel extension and microcracking in a paragenetic sequence, as indicated by consistent replacive contacts between the minerals constituting the ore.

175 This sequence is: pyrite I - chlorite - quartz - carbonate accords with the complete discordance of the orebody - pyrite II - (sphalerite + galena + chalcopyrite). The to bedding in the host rocks and it's structurally paragenetically later minerals grew at interfaces between controlled position in a large scale boudinage structure earlier minerals and at extensional microfractures within (see Aerden, in press). The ratio and composition of the early mineralisation or within barren host rock which paragenetic minerals vary with the distance to the central gaped during ongoing extension. structural mineralisation trap in the mine, yielding a Cleavage parallel extension, hence microcracking mine scale mineral zoning characterised by a proximal and foliation boudinage, was heterogeneously distributed Cu rich zone and barite rich distal zones. across different cleavage parallel layers. Consequently, Isotope studies suggest a Cambrian volcanogenic selective mineralisation occurred of particular cleavage source for the Rosebery orebody. Consistent with a synlayers that were undergoing extension and microcracking deformational timing of mineralisation, genesis of the by the action of infiltrating hydrothermal fluids. orebody is therefore best explained assuming metaContinuous shifts and adjustments in the planar shear- morphic leaching of volcanogenic sulfides present in strain partitioning pattern through time, resulted in the volcanic footwall of the deposit, upward transport successive mineralisation of different cleavage layers of metals and sulphur along the Rosebery shearzone that consequently became progressively enriched in the and concentrated redeposition of sulphides in a structural minerals that grew late in the paragenetic sequence. trap. This mineralisation process accounts for the cleavage Reference parallel compositional banding of the Rosebery ore. Aerden, D.G.A.M., 1990. J. Struct. Geol, in press. The syn-deformational timing of mineralisation A8.ll D FAULT AND SHEAR ZONE DEVELOPMENT IN THE MOUNT ISA AREA 2

Karen Connors Department of Earth Sciences, Monash University, Victoria

In the Mount Isa Inlier, the D event is defined as a in development of centimetre scale shear bands which period of E-W shortening and vertical extension resulting have a consistent west over east sense of movement in regional, upright, N-S oriented folds and a crenulation The movement zones also contain asymmetric, noncleavage. However, evidence from this study indicates cylindrical folds which overprint both S and the that the roughly E-W shortening resulted in several mylonitic fabric. However, the strong asymmetry and generations of folds. Small to large scale, asymmetric eastward vergence of the folds suggest development in folds (F ) were followed closely by movement on close association with the mylonite. The mesoscopic shallow to moderately dipping shear zones. Within and structures are consistent with easterly-directed adjacent to the movement zones, strain was accommo- movement on the shear zones. Intense development of the L mineral lineation dated on the mesoscopic scale by development of a mylonitic fabric, shear bands, and asymmetric folds As within and adjacent to the shear zones suggests that it fault movement slowed or ceased, several generations has formed in association with shearing and defines the of upright, symmetric folds and crenulations (F , F , F ) stretching direction during movement. The lineation is developed throughout the area. Prograde metamorphism defined by elongate quartz grains, micas, amphiboles, occurred during and after E/S development, but the F , and rarely by sillimanite (largely retrogressed to white F , and F folding events are associated with retrograde mica). Considered alone, alignment of prograde metaassemblages. (Note: D refers to the second regionally morphic minerals (ie. amphibole and sillimanite) parallel recognised orogenic event, and F , F , F and F represent to L does not constrain L development, and hence the successive folding/faulting episodes which occurred shear zone movement, as syn-metamorphic. However, other microstructural relations indicate that prograde during the D orogeny.) A series of N-S trending shear zones are metamorphism began pre- or syn-S continued through preferentially developed in high strain zones on the to post-S^. Evidence for this includes: (1) Deflection steep to overturned 'lower' limbs of F antiforms. of S and development of pressure shadows around Mesoscopic (cm to 10's cm) to map scale (100's m) F relict porphyroblasts of chlorite and white mica after folds exhibit attenuation of bedding, parallelism of S cordierite, suggesting pre- to syn-S growth of cordierite; bedding, and development of a mineral/stretching and (2) granoblastic, relatively strain-free quartz grains lineation on these limbs. Mylonitic fabrics are parallel and recrystallised mica textures, both of which define to the S cleavage and may have reactivated the cleavage. S indicating that metamorphic conditions were sufficient Slightly less ductile reactivation and/or intensification for ^crystallisation to keep pace and outlast S developof S resullted in dismembered F folds and 'shredded' ment. Subsequent retrogression is commonly more bedding. In many areas progressive deformation resulted intense along the shear zones than within adjacent rocks. 2

176

Two of the largest and best known of the D faults are the Mount Isa Fault and Adelheid Thrust. The Mount Isa Fault has been active over an extended period. The main period of ductile deformation is defined by a zone which varies from a few metres to -200 m. Late hydrothermal quartz breccias containing shale and siltstone fragments outcrop adjacent to the early shear zone or scattered within low-grade units to the east The Mount Isa Fault sits within the eastern ('lower') limb of a large scale F anticline. It dips steeply to the west, parallel or subparallel to S and bedding. However, evidence suggests that the shear zone, as well as S and bedding, were less steeply dipping during movement The F anticline and shear zone are situated on the steep, west-dipping limb of a F synform, and such overprinting provides an obvious mechanism for steepening of both the fault and F axial plane. An original, more moderate, west dip for the F lower limb is also supported by the gentle, west to southwest dipping attitude of the mesoscopic Fs. The Adelheid Thrust marks the contact between the Eastern Creek Volcanics and the younger Myally Subgroup or Surprise Creek quartzites. Structural relations vary along strike and are commonly ambiguous. The shear zone is thin (several metres) in comparison to the Mount Isa Fault and is defined by finely brecciated (mm scale) fault gouge and/or a mylonitic fabric. East 2

of the King Prospect, the Adelheid Thrust is situated on the western (upper) limb of a large scale, N-S trending F anticline, in contrast to other D faults (the Mount Isa Fault occurs on the eastern limb of the same fold). Two lines of evidence support late syn- to post-F fault movement: (1) Intense development of the L mineral/ stretching lineation, which is defined by prograde metamorphic minerals, adjacent to the fault (as discussed above), (2a) the fault contact is exposed within 100 m of the subhorizontal to gently plunging F hinge zone and is not repeated on the thick (1000 m) eastern limb, and (2b) the fault transects the F fold and eventually truncates the entire western limb and hinge zone leaving only the steeply overturned eastern limb exposed. The steeply overturned beds of the eastern limb can be traced along strike for the entire extent of the map area (>21 km) and show little variation in trend. Small and large scale shear zones throughout the Mount Isa area interrupt the stratigraphic sequence and obscure the main contacts. Consistent style and timing relations suggest that many of them are associated with F and F folding during the D orogeny. Both of the largest faults have previously been attributed to different events- the Adelheid Thrust to D1 the Mount Isa Fault to D . However, despite some differences both structures appear temporally related to F -F folding. 2

A8.12 DEFORMATIONAL STYLE AND STRAIN PARTITIONING AT THE HELLYER VOLCANOGENIC MASSIVE SULPHIDE DEPOSIT Christopher G. Drown*, Richard C. Downs Aberfoyle Resources Limited, Burnie

Deformation at the Hellyer deposit is strongly with development of extensional quartz + carbonate + lithology dependent both in structural style and degree epidote veins. In both hand specimen and thin section of shortening. The Hellyer deposit is a well preserved cleavage is absent or only very poorly developed and volcanogenic massive sulphide of Cambrian age situated volcaniclastic breccia fragments commonly observed in the Que-Hellyer Volcanics of the Mt Read Volcanics. within this sequence are equant and show no evidence The stratigraphic footwall to Hellyer is andesitic in of flattening. composition. Below the orebody is a well developed Deformation within the outer sericite-silica and hydrothermal alteration plume which is mineralogically central chlorite-sericite hydrothermal shells of the zoned from an outer shell of sericite-quartz through footwall alteration plume is dominated by the developchlorite-sericite and then into an inner core of highly ment of ductile structures. A moderate to strong cleavage siliceous alteration. The hangingwall is of basaltic (Sj) defined by sericite and chlorite, and axial planar to composition with a less extensive hydrothermal the mine fold trend is developed within these zones. alteration plume characterised by the presence of fuchsite Volcaniclastic breccia fragments are flattened and (McArthur, 1986). The orebody is zoned mineralogically predeformational carbonate veins that lie at high angles with sphalerite and galena-rich ore concentrated laterally to the flattening plane are folded. Measurements of long and vertically away from a pyrite-rich core on the to short breccia fragment axes indicate that up to 70% footwall (McArthur, 1986). The regional metamorphic shortening has occurred in the most phyllosilicate-rich grade is prehnite-pumpellyite facies (Whitford, 1984) rocks while values of around 50% are more common in and in the mine area one phase of folding resulting from lithologies containing slightly more silica. In thin section cleavage is well developed and unstrained euhedral east-west compression is recognised. Deformation is of a semi-brittle nature in the grains of pyrite often display silica ± sericite beards. unaltered andesitic footwall, the major structural Synclinal hinge zones are developed where phyllosilicate elements being small scale thrust faults and shear zones contents are highest

177

The innermost zone of the footwall hydrothermal Deformation at Hellyer is dominantly brittle in the plume is characterised by intense siliceous alteration unaltered footwall andesites, the unaltered hangingwall with minor sericite and chlorite. As in the unaltered basalts, the pyrite-rich core of the orebody, and the footwall andesites, semi-brittle shear zones with siliceous core of the footwall hydrothermal alteration. extensional silica + carbonate vein arrays are the main In contrast the phyllosilicate-rich hydrothermal alteration structural elements and cleavage is only weakly zones in both the footwall and the hangingwall, and the developed. Pb-Zn-Ag-rich outer zone of the orebody have responded Within the Pb-Zn-Ag-rich zone of the orebody, to the deformation in a ductile manner. Resolution of tectonic banding parallel to and defined by alternating the principal stressfieldof shear zone vein arrays devellayers of sphalerite + galena and pyrite is well developed. oped in brittle lithologies shows the major stress direction Pyrite and carbonate veins are commonly folded where to be normal to the axial planar cleavage and sulphide they lie across the plane of flattening. Where they lie banding developed in ductile lithologies. Strain has parallel to the tectonic banding carbonate layers been strongly partitioned into the ductile zones and fold commonly display necking and boudinage whilst pyrite hinges are controlled by the distribution of phyllosilicatelayers display more brittle extensional textures such as rich alteration. East-west shortening of the footwall and tension gashes. The pyrite-rich core of the orebody orebody is greater than the hangingwall due to the lacks any obvious ductile textures and a decrease in relative size of the alteration zones, the difference in RQD in this zone suggests that deformation is of a shortening being accommodated by thrust faulting and brittle nature. shearing at the orebody-hangingwall contact. Deformation within the unaltered hangingwall basalt References is dominated by small scale thrusting and semi-brittle McArthur, G.J., 1986. In Large R.R. (ed.): The Mount Read Volcanics and Associated Ore Deposits. Symposium, Geol. shear zones with S only very weakly developed or Soc. Aust. (Tasm. Div.), Hobart: 11-20. absent, however, within the hangingwall fuchsite Whitford, 1984. In P.W. Baillie and P.L.F. Collins (eds): alteration plume the S1 cleavage is moderately developed MineralD.J., Exploration and Tectonic Processes in Tasmania. and some folding is observed. An anticlinal hinge is Geol. Soc. Aust. (Tasm. Div.), Burnie: 57-58. developed where fuchsite alteration is most intense. x

A8.13 FLUID-INCLUSION STUDIES RELATING TO A MULTIPLY DEFORMED, METAMORPHOSED, VOLCANIC-ASSOCIATED MASSIVE SULPHIDE OREBODY, JOMA MINE, NORWAY Alan D. Giles* and Brian Marshall Department of Applied Geology, University of Technology, Sydney

Detailed structural analysis and fluid inclusion studies reveal a complicated geothermobarometric history for the Joma massive sulphide deposit. Regional metamorphism and deformation have been followed by re-equilibration of Hp/NaCl inclusions during uplift. Joma mine exploits a stratiform Cu-Zn, volcanicassociated massive sulphide deposit within the dominantly low-grade Koli Nappe system of the Norwegian Caledonides. The Koli Nappe system structurally overlies the high-grade Seve Nappe system, the two systems collectively forming a volcanosedimentary allochthonous complex (Stephens, 1988). Four periods of deformation are recognised from the mine region (Odling, 1986; Marshall et al., 1987); DJ-D generated a transposition schistosity and mineral elongation lineation, D formed a crenulation foliation, and D was associated with minor kinking and faulting. Microfabric interpretation of host rock mineralogies — calc-silicate assemblages of chlorite, quartz, calcite ± actinolite ± phlogopite ± albite ± diopside — indicate that the metamorphic peak (biotite zone, intermediate pressure, greenschist facies) accompanied D^ had little declined by D , but had fallen significantly by D . Fluid 2

inclusions were studied in samples of vein quartz respectively formed during the D D , D , and D events within the ore zone. Fluid inclusion studies yield temperatures of homogenisation (Th) in the range 137-358°C, fluid salinities (derived from the temperature of melting of last ice (Tm ice)) of 0-6.7 wt % NaCl equivalents and calculated densities of 0.73-0.96 g cm . The temperatures of eutectic (first) melting (Tfm) of-19 to -23°C indicate that sodium is the dominant cation present in the fluids. Clathrate melting determinations indicate minimal presence of C0 . Plots of Th against Tm ice for the Joma samples have three main characteristics. One, although possessing largely overlapping Th ranges, two populations can be recognised in terms of Tm ice. Two, if treated as a continuous population there is a trend of increasing Tm (increasing salinity and density) with decreasing Th. Three, the Th values are generally well below the formation temperatures of metamorphic biotite. The above characteristics are present, regardless of the deformation event with which the quartz is associated. The two Tm ice populations and the other characteristics r

178

may not therefore be simply ascribed to D overprinting D D , or D overprinting preceding events. It is, however, possible that the distinct Tm ice populations are recording differences inheritedfromfluid inclusions related to the deformation events. The strictly similar population distributions of fluid inclusions in quartz from the differing deformation events suggest a post-D overprint. Such an overprint is consistent with re-equilibration under P-T conditions accompanying regional uplift. Two further lines of evidence support this notion of re-equilibration during uplift: one, although the trend of increasing Tm ice (and hence increasing salinity) with decreasing Th is similar to that exhibited by boiling of fluids, the constant vapour/ liquid ratios and large range in Th negate this; two, those fluid inclusions yielding substantial decreases in density have morphologies approximating euhedral (negative) crystal shapes which, according to Sterner & Bodnar (1989), characterise isothermal decompression. Increasing salinity with decreasing Th could reflect progressiveflushingby a more salinefluidduring upliftinduced microfracturing. It could alternatively represent fluid loss by diffusion, or partial rupturing of the inclusion during a microfracturing event, again during regional decompression. The second explanation is preferred because the need for pervasive flushing by a special fluid is removed. The initial conditions of P-T re-entrapment cannot be pinpointed. However, reentrapment pressures lie within limits provided by peak metamorphism (say 3-4 kbar for the stability field of calcareous mineral assemblages) and "minimum" corrected values (0.0-1 kbar) derived from Th data for the system NaCl-H 0. Re-entrapment temperatures occupy a cooling range of approximately 150°C. The 3

data are consistent with a model involving uplift along the path between the isochoric and isothermal limits, followed by entrapment of secondary inclusions over a limited cooling range. P-T conditions inferred from fluid inclusions in metamorphic rocks commonly conflict with values obtained by other methods. This is because primary (metamorphic) fluid inclusions are commonly masked by, and re-equilibrate to form, secondary inclusions during subsequent deformation events, including uplift Fluid inclusions in samples from structurally-controlled sites in ore may be mechanically and chemically cushioned by the ductile sulphides. If so, they are potentially capable of recording P-T conditions during successive deformation events; and of placing constraints on the remobilisation of sulphides during these events. However, the formation and resetting of fluid inclusions in vein quartz during regional uplift of the Joma district has effectively destroyed or masked their capacity to record P-T conditions during earlier events. Whether a similarly drastic overprint occurs in many or only a few metamorphic terrains is uncertain. Nor is it known whether high and low pressure terranes are equally affected. There is clearly a need for extreme caution when interpreting fluid-inclusion data from syn-kinematic vein quartz in deformed and metamorphosed orebodies. References Marshall, B., Odling, N.E., Reinsbakken, A. & Vokes, F.M., 1987.7. GeolSocAust. 19: 112-115. Odling, N.E., 1986. Terra Cognita 6: 548. Stephens, M.B., 1988. Geology Today. Jan-Feb.: 20. Sterner, S.M. & Bodner, R.J., 1989. J. Met. Geol. 7:243-260.

A8.14 THE RELATIONSHIP OF MINERALISATION TO STRUCTURE AND STRATIGRAPHY AT THE BALCOOMA VOLCANOGENIC MASSIVE SULPHIDE DEPOSIT, NORTHERN QUEENSLAND David L. Huston Key Centre for Ore Deposit and Exploration Studies, University of Tasmania

The Balcooma prospect is one of three volcanogenic massive sulphide deposits in the Cambro-Ordovician(?) Balcooma metamorphic belt in northern Queensland. The belt is an assemblage of multiply deformed metasediments and felsic metavolcanics that has undergone amphibolite grade metamorphism. The deposit occurs in a metapelitic lens within a sequence of metagreywackes with minor interbedded volcaniclastic units. Four generations of cleavage have been recognised at the surface, and two of these (S and S3) are pervasive. The first three folding events (D to D^ are appanrently co-axial with fold axes plunging to the south-southwest at 20°. The prospect occurs on the eastern limb of an open D anticline, and is bisected by a northwest trending D tear fault that divides the prospect into two structural 2

domains. To the northeast of this fault the structure is dominated by a completely overturned D anticline/ syncline pair which is truncated by the tear fault to the south. To the southwest of the fault the structure is dominated by a moderately southeast dipping moncline that steepens down dip. Massive sulphide lenses at Balcooma are interpreted as occurring at three distinct stratigraphic horizons within the metapelite. The central horizon contains reserves of 3.5 M.t. of 3.0% Cu and occurs in the core of the upward closing syncline (antiform), whereas the upper and lower horizons are dominated by zinc-lead-rich mineralisation and occur in the limbs of folds. Chloritic alteration occurs in a pipe below the main copper-rich lens, whereas pyritic quartz-muscovite alteration is associated with zinc-lead-rich lenses. 2

179

A8.15 EXPLORATION FOR SYNTECTONIC MASSIVE SULPHIDE DEPOSITS BASED ON STRUCTURAL AND MICROSTRUCTURAL ANALYSIS OF DRILLCORE K. C. Lawrie James Cook University of North Queensland, Townsville

Successful prediction of 6 new orebodies, at Elura Mine, Cobar Trough, NSW, has resulted from determining both the relative timing and structural trapping geometries responsible for localising previously discovered mineralisation. Mapping of underground exposures suggested the potential for repetition of structural traps, and this was tested by an exploration programmme, commenced to examine potentially economic intersections discovered to the north of the existing orebodies. The new orebodies are subvertically-plunging and pipe-like massive sulphide orebodies similar to the existing orebodies. Mineralisation is localised in the lower strain anticlinal hinges of doubly-plunging F folds within a zone of overall NW-SE trending high strain that is continuous for at least 1 km. The main mineralised D structure is deformed and offset by discrete, planar, NE-SW trending D brittle-ductile high strain zones. Five variably penetrative cleavages (S ) are recognised in the study area (Lawrie, in prep.). Alteration and mineralisation occurred in several pulses within D . Subsequent deformation largely partitioned around the main orebodies, due to the competence contrast between unaltered host rocks and silicified and massive carbonate altered rock present at orebody margins and at depth. The new orebodies lie at depth and have no obvious surface geophysical or geochemical signature. To locate them it is essential to target cores of the doubly-plunging F fold hinges, which occur in an area of <60 m diameter in plan. This necessitates the collection of structural data with the drillcore in its original orientation. The constant cleavage/ variable borehole orientation method 2

for orientating drillcore (Duncan, 1984), which has proven successful elsewhere in the Cobar Trough (Hinman, pers. comm., 1989), could not be used at Elura due to the presence of the two morphologically similar and variably penetrative S and S cleavages. These fabrics differ in strike from 0 to 100° (WNW-NE respectively) with overprinting relationships visible in thin section but not clearly seen in drillcore. Mapping of D extension lineations (L^) at a distance as well as adjacent to the main orebodies in both low and high strain zones showed L to be fairly uniform and subvertical, with a slight northerly plunge (de Roo, 1989; Lawrie, in prep.). The L^lineation was similarly found to be sub-vertical and parallel to L^. The subvertical nature of the extension lineations, with the added assumption of a relatively steep cleavage orientation (>65°), permitted bedding and cleavage data to be collected. The precision and practicality of this method were tested in areas where drillcore intercepted exposed areas, and an excellent correlation was found. Measurements were made with the assistance of a core orientating frame, which permitted dip and strike readings to be recorded directlyfromdrillcore (Lawrie, in prep.). This method was found to be quicker than taking measurements relative to the core axis, with the necessary computations or manipulation on stereonets (Laing, 1989). References 2

de Roo, J. A., 1989. Econ. Geol. 84: 256-278 Duncan, A., 1984. Internal Report, M. I. M. Laing, W. P., 1989. Structural Analysis in Drillcore. Cambridge Univ. Press (New York). In press. Lawrie, K. C., in prep. Econ. Geol. (submitted).

180

A9: Fault Zone Fabrics Convenor: R. F. Berry

A9.1 DIRECT OBSERVATIONS OF GOUGE DEVELOPMENT DURING BRITTLE SLIDING Kerry Haggert* , Simon Cox , Mark Jessell 1

Geology Department, University of Melbourne CSIRO Division of Geomechanics, Mt Waverley Department of Earth Sciences, Monash University 1

2 3

A see-through apparatus for direct observation of brittle frictional microprocesses during sliding experiments on an analog material has been developed. The size of the rig allows operation while on the stage of an optical microscope, and video recordings of experiments have been used for subsequent analysis of the sequential development of microstructures. Individual framesfromthe video are converted to pixel images, which enables us to apply digital enhancement and analysis techniques using a desk-top computer. Normal and shear forces are recorded so that the mechanics of the system may be correlated with the fault zone fabrics. Sodium nitrate, which is a crystalline analog of calcite, has been used. This is brittle at room temperature, so thin samples are prepared by hot pressing at close to the melting temperature (300°). The sliding interface is prepared by cutting the foil with a blade, so it is initially straight within an essentially 2-D sample. Total shear displacements of up to 30 mm have been achieved. Four microstructural elements: fractures, twins, gouge and void space were distinguished. A description of damage development will be given by focussing on the contribution of each of the elements recognised, and their interaction with the macroscopic sliding interface. Slip remains concentrated close to the initial interface in most experiments until the stage at which disaggregation occurs. Damage does not develop smoothly and continuously during sliding, but is

concentrated in a series of discrete increments. Early in experiments fracturing is concentrated around grain boundaries, leading to loosening of grains of wall material, a reduction in size and incorporation into the gouge zone. Later on, larger partly transgranular fractures form parallel to the maximum compressive stress orientation. This damage extends into the wall material a distance comparable with the gouge zone width. Twins are observed with composition planes on average parallel with the sliding direction. The gouge zone increases in width as the displacement increases during most experiments, but trends are fairly irregular and no single growth law can be deduced. Persistent voids are supported at the margins of the gouge material during sliding. Shear force data shows a progressive weakening of the interface with increasing shear displacement. Step changes in normal force produced proportional increases in shear force, so that the friction coefficient history has no discontinuities. However the resolution of the mechanical parameters are inadequate for more detailed analysis at this stage. These experiments have probably produced the first direct, sequential, real-time observations of the purely brittle processes at a frictional interface. With further improvements in the instrumentation, an understanding of the true micromechanics of fault movement will be achieved.

181

A9.2 THE MECHANICS OF SHEAR BAND DEVELOPMENT WITHIN FAULT GOUGE Chris J. Marone Department of Geology, University of Melbourne and Division of Geomechanics, CSIRO

Fault zones commonly contain localised regions of high shear strain, in the form of shear bands or shear surfaces. Such features are often studied with the intent of deducing the displacement history of a fault or the conditions under which the fault was active, e.g. the stress field. From this point of view, it is important to understand why, and under what conditions, such shear localisation features develop. Moreover, fault zone evolution and, in particular, variations in shear band width with accumulating slip are of interest in a broader context; for example, the relationship between fault slip and mineralisation may be effected by shear band formation (or growth) if there is an associated transition from dominantly seismic to aseismic slip. Hence, we would like to identify the factors responsible for shear band formation and determine how they vary with accumulated slip, so as to cause widening or narrowing of shear bands or variations in shear band spacing. To investigate these issues an experimental investigation of microstructure development within a granular material (Ottawa sand) used to simulate fault gouge was conducted. Ottawa sand is composed of >99% quartz and the material used in this study had a grain size of 400 to 800 |xm. Four millimetre-thick layers of sand were sheared between rough steel surfaces in a triaxial testing apparatus to shear strains (y) of up to 3.3. The layers were sheared at constant effective normal stress of 100 MPa and under saturated drained conditions. Porosity (<(>) changes were measured throughout shear and microstructural observations were carried out on the deformed layers. The observations indicate that the initially porous material (=20% porosity) undergoes significant compaction until a shear strain of 0.6-0.7. In agreement with previous work, considerably more compaction is observed when the shear load is cycled from 0 to failure, compared with monotonic shear of like magnitude. For y > 0.6-0.7, the net volume change over load/unload cycles is zero or positive (net dilation). Irrespective of load cycling, shear localisation features are not found in layers subjected to y <13-1.5, whereas those subjected to greater shear strains contain distinct Riedel shear bands, 100-200 p.m in width. Particle size analyses indicate that the comminution rate decreases at about y = 1.5, after which the particle size distribution is approximately independent of y for the range studied. This "steady state" particle size

distribution agrees with that reported by others for a natural fault gouge. Analysis of the energy expended in drained shear deformation indicates that shear localisation is possible if (1) the material is dilating (i.e., d<J>/dy > 0) and (2) a transition occurs from d<t>/<±y> 0 to d ty/df< 0 during shear. Our measurements indicate that dilatancy occurs during shear loading after shear strains of as little as 0.2-0.4, but that d<t>/dy is low (<0.01) and dilatancy quickly gives way to compaction with continued shear. For shear strains > 0.8-0.9, d<t>/dy is quite large (0.15) and remains positive during shear loading while d^/dy changes from positive to negative. The satisfaction of the localisation criteria for y > 0.8-0.9 is consistent with the appearance of fully developed Riedel shear bands after y= 1.3-1.5. Furthermore, the change from d<\>/&f > 0 to < 0 occurs at the peak in the stress-strain curve (after which the shear strength diminishes to a residual value with continued shear) in agreement with the onset of shear localisation. Particle size analyses indicate that the comminution rate is independent of load cycling. In addition, although layers subjected to load cycling undergo greater compaction than monotonically sheared layers, shear localisation occurs at about the same shear strain in each case. This suggests that the transition from homogeneous to localised strain depends more on the particle size distribution than porosity, or on some combination of the two. Shear localisation is more closely related to particle size distribution than porosity -to the extent that the two are independent -which they can be, to some extent, via load cycling. Our observations indicate that shear localization is driven by the rate of dilatancy with shear strain such that below a critical dilatancy rate strain remains homogeneous whereas above this rate shear localisation is possible. As the dilatancy rate depends on porosity and shear strain, shear bands may be expected to widen or migrate within natural fault zones as the size distribution (or porosity) of the material within the shear band varies from this critical value. The orientationof the Riedel shears relative to the shear zone boundaries imply that they will be relatively short lived features. Other experiments to larger strains show that boundary parallel shears become dominant and that these features widen and migrate through the gouge zone with continued slip. 2

182

A93 SHEAR BANK DEVELOPMENT IN EXPERIMENTALLY DEFORMED SYNTHETIC GYPSUM ROCK Renee (Panozzo) Heilbronner RSES, Australian National University

Synthetic gypsum rock is obtained by "hot pressing" carried out (final strain magnitude e = 0.6, 0.9, 1.2). At room temperature and a confining pressure of a <100 |imfractionof gypsum powder (25°C, 110 MPa) in the presence of excess H^O. The resulting gypsum 100 MPa, the shear strength, T, of synthetic gypsum is cylinders have a density of about 95%; they display a 180 MPa at e = 0.05, increasing to >300 MPa at e planar shape fabric normal to the cylinder axis and a >1.00. (In coaxial tests at the same temperature and strong preferred orientation of b-axes parallel to the confining pressure, the material is only slightly cylinder axis. Shear zones of 1 mm thickness are obtained workhardening: Aa is 180 MPa at t = 0.05, and 210 by sectioning the cylinders obliquely (35° with respect MPa at e = 0.20). On the t/e chart, the material appears to cylinder axis) andfittingthem between forcing blocks to deform stably, but the Theological stability is not of Solnhofen limestone. Using a triaxial apparatus simple paralleled by deformational homogeneity. shear experiments up to variousfinaldisplacements are Within the shear zone, deformation is accommodated g

Figure 1 — Auto-correlation functions of four samples of experimentally deformed gypsum. (Pseudo-contourplots by removal of 5 significant bits). Section parallel to displacement direction, normal to shear zone boundary, shear sense dextral, shear zone boundary horizontal. A — Undeformed sample: P and T applied, but no displacement. Note asymmetry around plane of initial preferred orientation (35° clockwise from vertical). Inner contours elliptical, outer contours lozenge shaped. B— e = 0.60. Contours are rotated clockwise, position of apex is first corner when counting clockwisefromtop. Inner contours elliptical and more rotated than outer contours. C— e = 0.90. Contours are only slightly rotated with respect to previous sample position of apex is second comer when counting clockwise from top. Inner contours elliptical and less rotated than outer contours. Short side of lozenge dipping in direction of shear. D— E = 1.20. Inner contours not elliptical anymore. Inclination of short side of lozenge in direction of shear bands. g

183

by two apparently independent mechanisms: flattening shear bands). ACF analysis (shape fabric analysis by of gypsum grains, and development of shear bands. An auto-correlation function) reveals that shear band initial flattening is present in a plane normal to the formation and its precursors, such as shape change of cylinder axis, i.e. in a plane which oriented 35° clockwise grains, starts to be significant at e = 1.00 (Figs 3, 4). with respect to the shear zone boundary normal if the Note that this change-over of dominant deformation shear direction is dextral (Fig. 1). As shearing progresses, mechanisms is not mirrored by any change of bulk the gypsum grains rotate clockwise out of this plane, rheology, at least not within the range of strain however, lagging behind the direction of maximum magnitudes attained so far. In other words, the kinematic finite strain (Figs 2, 3). In view of this, it is quite development of microstructure is crucial to the underremarkable that the crystallographic preferred orientation standing of the dependence of rheological behaviour on of b-axes remains absolutely stationary with respect to dominant deformation mechanisms. the shear zone. Within the shear bands, the gypsum grains are More important than grain flattening is the formation extremely elongated, forming long continuous tails. of shear bands. As deformation goes on, shear bands These are not recrystallised, rather, the dominant mechincrease both in number and in length. They are oriented anism seems to be intracrystalline slip. Judging from in a direction of >90° with respect to the shear zone ordinary light microscopical observations, the crystalloboundary normal, i.e. parallel to the shear zone boundary graphic preferred orientation seems to be constant, but and dipping into the direction of shear. Geometrically, the amount of material is too small to be represented in they conform to the configuration of the type I S-C the b-axis pole figure. mylonites described by Lister & Snoke (1984). The References development of S planes (plane of grain flattening) is Lister & Snoke, 1984. JStruct.Geol. 6: 617-638. prior and coeval to the formation of C-places (plane of §

A9.4 POSSIBLE REACTION WEAKENING AND LOCAL DEFORMATION IN GRANITIC ROCKS H. Stunitz and J. FitzGerald RSES, Australian National University

Intermediate plagioclase and some K-feldspars are transformed directly to albite, without the formation of metastable under greenschist facies P,T-conditions in any intermediate plagioclase. The plagioclase is always altered to albite ± white the presence of an aqueous fluid. Since the localisation of shear deformation in granitoidsfrequentlytakes place mica ± hydrous CaAl-silicates. Some alteration could under such P,T-conditions, the breakdown reactions of have occurred post-kinematically, but a large portion of feldspars may be an important factor, which influences it can be inferred to have occurred syn-kinematically. the rheological behaviour and the localisation of shear The alteration takes place pervasively throughout the deformation in granitoids. The great amount of research grains, but is often concentrated alongfracturesand in already done on the deformation of granitoids revealed primary Ca-rich zones. The reaction products (prethe variability and complexity of phenomena associated dominantly albite) have a grain size similar to the with feldspars during deformation. However, no specific products of the K-feldspar alteration (5-30 ^im). The comparison of the microstructural evolution of feldspar degree of alteration of the feldspars may vary, but the in different deformed rock suites has yet been presented. deformed zones are always preferentially altered. The The deformation and associated alteration of granitoid kind of alteration is dependent on the fluid chemistry rocks under greenschist facies conditions were and thus may vary considerably. Transgranular investigated in three different sample series, one from fracturing, microboudinage and alteration precedes the Eastern Australia (Wyangala Dam Granite), and two mylonite formation in all cases. The fine grained from the Alps (Corvatsch Granodiorite and Aiguilles albite alteration products become aligned in the orientation of the penetrative foliation by deformation Rouges Granite, both Switzerland). The K-feldspars frequently show an alteration to of those fine grained albitic aggregates, which takes albiteiwhite micatquartz. The alteration is concomitant place by non-cataclastic deformation mechanisms. The or precedes the deformation and proceeds by reduction albitic aggregates are equally or more strongly deformed of fragments of the K-feldspar and increase of a than pure quartz aggregates, which are deformed by predominantly albitic matrix. The alteration products intracrystalline plasticity (rotation recrystallisation). have a small grain size of approximately 5-30 |im and Thus, the albitic aggregates, which are reaction are found along the margins of, along fractures or in products from the feldspar decomposition, may be networks throughout the primary K-feldspar grains. The weaker than pure quartz aggregates. Conversely, the absence of myrmekites suggests that K-feldspar is large primary feldspar clasts, from which the albitic

184 aggregates have formed, have deformed mainly be fracturing. The control of the small grainsize of the albite in the fine grained aggregates is not clear, but does not appear to depend on second phase particles in those aggregates. The grainsize of albite is always about one order of magnitude smaller than that of quartz aggregates in the same specimen, a factor that might have been important, if grainsize-sensitive deformation mechanisms had been active. The recrystallisation mechanism for the feldspars has not been clearly identified, but it is not rotation recrystallisation, which was found by other workers to operate under amphibolite facies P,T-conditions. The greenschist facies microstructures are more indicative

of classical nucleation processes. Some evidence for intracrystalline plasticity can be found in plagioclase (dislocations, subgrain formation), and the alteration tends to be concentrated along the regions of high dislocation densities. The general problem in these granitoids remains the assessment of the accurate P,T conditions during the deformation for the following reasons: 1. The deformation history of granitoids is usually complex, because deformation normally takes place during cooling and during a later deformation imprint. 2. P,T-sensitive index minerals for greenschist facies conditions are usually absent or scarce in granitoids.

A9.5 A MECHANICAL INTERPRETATION OF RATE AND STATE DEPENDENT CONSTITUTIVE LAWS FOR ROCK FRICTION B.E. Hobbs CSIRO Geomechanics, Mt Waverley

It is now well established that second order effects deformations. These experiments show that a step-wise are superimposed on Amontons' Law for friction for increase in shearing strain-rate is accompanied by a the sliding of one rock surface over another. These step-wise increase in shear strength followed by an second order effects mean that the coefficient of friction evolution to a new shear strength in a manner analogous depends on the velocity of sliding and on the "state" of to the stress changes observed infrictionexperiments. the interface or of the gouge within it A step-wise It is proposed that the rate and state dependent increase in velocity of sliding is accompanied by a step- constitutive laws observed in laboratory scale friction wise increase in the coefficient of friction followed by experiments have their origin in the strain-rate a slow evolution to a new coefficient of friction. The dependence of the constitutive behaviour of the gouge rate of evolution may be described by a characteristic between the sliding rock surfaces. The step-wise increase distance (or characteristic distances); the microstructures in the coefficient of friction associated with a step-wise which control these characteristic distances are a matter increase in the velocity of sliding is related to the rate for debate but a physical understanding is important in of change of stress with change of strain-rate at constant deciding how laboratory determined parameters should strain obtained from the gouge constitutive law. The be scaled up to natural faults. The purpose of this paper characteristic distances which describe the evolution of is to suggest some mechanical basis for the observed stress with continued sliding are not to be identified rate and state constitutive laws. directly with gouge microstructure or the spacing of Although it is not widely appreciated, the strength Reidel shears in the gouge but by the shear strain of rocks undergoing brittle deformation is strain-rate dependence of changes in friction angles, cohesion and dependent and thermally activated; these effects are not dilation angle in the gouge constitutive law at a given as marked as for plastic mechanisms but are well strain-rate. As such, the characteristic distances in the documented and are presumed to have their origin in frictional constitutive law scale with the thickness of stress corrosion at crack tips. The results of some new the zone undergoing shearing for a given shearing rate experiments will be presented to give an indication of and gouge material. The implications of such a scaling the magnitudes of the effects for unconsolidated sand, law for instabilities on natural faults will be discussed. for marble and for sandstone in simple shearing

185

A9.6 DETERMINATION OF STRAIN FROM AUTOMATIC IMAGE ANALYSIS OF DEFORMED OBJECTS A. O r d a n d P.R. James

CSIRO Geomechanics, MtWaverley Department of Geology and Geophysics, University of Adelaide 1

Three-dimensional finite strains fields may be shape and size of the deformed objects, and then each interpreted from the fabric of a fault zone. This requires of the 512x512 array of pixels is addressed before each calculation of the two-dimensional strain fields deter- object is uniquely recognised. mined in each of the three principal planes of the finite An equivalent ellipse is then drawn around each strain ellipsoid. The principal planes are sometimes object. Values of the magnitudes of the major and minor assumed (e.g. James et al., 1989) to be approximately axes of the ellipse, the angle of the major axis of the parallel to the foliation plane, and to planes normal to ellipse from the x axis of the image and the x and y cothe foliation and normal and parallel to the lineation ordinates of the centroid of each object are addressed by exposed in a mylonite. Many calculations of two- the section of the program which calculates the dimensional finite strain fields are therefore required to parameters for the various plots used for representing check this assumption as well as to improve the sampling strain. Fry and R / 0 diagrams and the resulting Flinn statistics within any deformed region. The various diagrams may then be plotted on the computer screen or methods used for calculating strain depend either on the as a hard copy. shapes of the initial and deformed objects or on the The method was tested for various fabrics and was relative distances and angles between displaced markers checked by comparing results obtained by automatic (see Hanna & Fry, 1979, for a review). They therefore image analyses with those for which the object pose a well-defined problem for solution using automatic dimensions were measured by hand. The two methods image analysis. were found to produce similar scientific results. The An interactive computer method is described here automatic method is far faster than the manual method by which Fry, R / 0 and Flinn diagrams are derived and results in more thought being given to understanding from a collection of deformed quasi-elliptical objects the data than in measuring them. within a few seconds. References The objects are viewed by a solid state camera with James, P.R., MacDonald, P. & Parker, M., 1989. Tectonophysics 158: 23-48. input to a frame grabber in an IBM PC-AT. The image is thresholded at the optimum value for preserving the Hanna, S.S. & Fry, N., 1979. Struct. Geol. 1: 155-162. A9.7 PSEUDOTACHYLITE WITH IGNEOUS QUENCH MICROSTRUCTURES, EASTERN MUSGRAVE RANGES, NORTHERN TERRITORY A. Camacho & R.H. Vernon * 1

Northern Territory Geological Survey, Alice Springs School of Earth Sciences, Macquarie University 2

A 5 km-thick zone of mylonitic deformation superimposes granulite facies rocks on amphibolite facies rocks in the eastern Musgrave Ranges, Northern Territory. This thrust is interpreted as an extension of the Woodroffe Thrust, which has an age of550-600 Ma. It dips shallowly to the south, and contains mylonite, ultramylonite and pseudotachylite. Mylonites at the base of the thrust grade into ultramylonites that pass abruptly into pseudotachylites. Some deformed pseudotachylites occur in the ultramylonite zone, but nondeformed pseudotachylite is rare in the ultramylonitemylonite zone, and occurs only in dolerite dykes. The pseudotachylite-bearing zone is 1 km thick, and contains approximately 5 % of pseudotachylite veining. The orientation of the veins appears to be random. Generation surfaces are uncommon, probably because

many of them were obliterated by relatively large volumes of melt. Pseudotachylites are encountered only in the granulite facies rocks. The precursors of the pseudotachylites appear to be felsic granofelses. The larger pseudotachylite veins are quasiconglomerates, in the sense of Sibson (1975). Unmelted rock and mineral fragments consist of felsic granofels, with microstructural evidence of strong, high-strain rate plastic deformation and cataclasis. Rounded relics of quartz, plagioclase, K-feldspar and orthopyroxene are present Some of the rounded plagioclase and orthopyroxene relics are rimmed by acicular microlites or dendritic overgrowths of the same mineral. Hydrous minerals are absent, in contrast with previously described pseudotachylites. The isotropic,fine-grainedfraction shows spectacular

186

examples of igneous quench microstructuies, especially skeletal and dendritic crystals of plagioclase and feathery dendritic microlites that appear to be pyroxene, although electron microprobe analysis and X-ray diffraction have so far failed to prove this. Recalculated microprobe analyses of scanned areas rich in this material are consistent with the presence of pyroxene (probably both clino and ortho). Also present are glass devitrification microstructuies (spherulites), evidence of liquid flow, and partly melted residual grains with former glassy rims of different optical properties from those of the surrounding isotropic material. The foregoing field and microscopic observations suggest that the pseudotachylites formed by melting in anhydrous conditions. The mylonites and ultramylonites have mineral assemblages indicative of the amphibolite facies, whereas the pseudotachylites have igneous assemblages indicating high-temperature crystallisation, including precipitation of orthopyroxene (e.g., as microlitic overgrowths on the rims of partly melted relics). Thus, the rocks formed as a result of very local, short-lived rise in temperature, while the surrounding rocks were much cooler. Rotated blocks of ultramylonite

occur in some of the pseudotachylites, and some pseudotachylite veins have been mylonitised, suggesting overall contemporaneity of ductile and brittle processes. Most workers have suggested that pseudotachylites represent former melts generated by frictional heating. However, Wenk (1978) suggested an origin by ultracataclasis. The microstructures of many pseudotachylites, especially the occurrence of dendritic crystals and crystallites, strongly indicate crystallisation from a melt, as do the eastern Musgrave examples. Because these pseudotachylites occur in such a wide zone, they may provide insight into the role of melt production during earthquake activity. Conceivably, they may also provide some information on magmageneration processes in anhydrous conditions. Previous workers have suggested that pseudotachylites have the same bulk chemical compositions as their parent rocks, but microprobe data show that the eastern Musgrave pseudotachylites are less siliceous than their host rocks. References

Wenk, H.R., 1978. Geology 6: 507-511. Sibson, R.H., 1975. Geophys. J. R. Astron. Soc. 43:775-794.

A9.8 STRAIN COMPATIBILITY OF MINOR SHEAR STRUCTURES IN SHEAR ZONES J.V. Smith * and D.W. Durney 1

University of Technology, Sydney, Macquarie University 2

One model which is widely used for interpreting brittle and semibrittle minor shear structures in rocks is the Riedel model. This model can be explained in terms of simple shear strain and displacement or, as is more often done, in terms of stress orientation and reorienation (e.g. Naylor et al., 1986). Geologists tend to think (with some justification) of deformation structures as being 'caused' by 'stress'. But it is necessary to examine the foundation of this argument as a rational way of explaining structures. In this paper we review the Riedel model and some of its variants in terms of boundary conditions. Some new experiments are reported which illustrate the importance of these conditions for the structures developed. In principle, the motions within any closed region can be described by a set of equations governing rheology, stress equilibrium and compatibility, and by a set of conditions acting on the boundary of the region. These conditions may be conditions either of stress or of displacement, but not both of these in the same direction. For example in elastostatic problems the conditions are "the normal stress or normal displacement and the shear stresses or the shear displacements" (Crouch & Starfield, 1983: 1). In the Reidel clay boards model, deformation in the principal plane of the model is caused by the movement or displacement of the two rigid boards which make the

two parts of the clay to move relative to each other in fixed directions. The boundary conditions in the plane of deformation are therefore conditions of specified displacement parallel to the boards and zero normal displacements and transverse shear displacement elsewhere, or what is called simple shear displacement, except that plane strain is not specified in the Riedel model. The deformation of the clay therefore belongs to a class of 'displacement controlled deformations'. The stresses on the clay will be variables that are dependent on the displacements and the changing Theological properties of the material. The deformation imposed on the clay is overall simple shear and so the maximum bulk infinitesimal shortening direction is 45° to the shear direction. In the initial isotropic state of the clay the principal axes of stress will be parallel to these strains. As the deformation proceeds, the amount of strain exceeds that which the clay can accommodate homogeneously and so minor shear structures begin to develop. The bisector of the conjugate primary minor shears so formed is parallel to the infinitesimal shortening direction. If the dihedral angle of the conjugate shears were 90°, the primary synthetic minor shears (PSMS) would be parallel to the master shear and there would be no need for the development of other structures. But, as is commonly observed in brittle and semi-brittle

187

master shear zone and the PSMS form parallel to the master shear. At this orientation they are compatible with the boundaries and no secondary structures are developed or required. The slip on these shears is oblique due to the extension across the zone. At higher displacement orientations features of extension dominate, but the influence of the shear component of the deformation can be seen in the en echelon pattern of the extensional structures. A 'stress controlled* clay board apparatus was constructed to allow displacements to occur freely in 2dimensions under known applied shear and normal forces. In the experiments conducted the displacement path was found to be dependent on the normal force applied. The boards tend to open up during the initiation of primary shears and the PSMS form closer to the master shear direction than in simple shear. These investigations demonstrate: (1) that the orientation of minor shear structures is dependent on the boundary conditions of the deformation; and (2) the sequential development of minor shear structures is controlled by internal strain compatibility requirements. The boundary conditions of the constant area Riedel experiment make it an appropriate analogue for near surface deformation of cover rocks over a reactivated basement fault. But its application to other geological situations such as deformation within basement (e.g. Mueller et al. 1988) may be questionable because there is no obvious geological constraint to keep normal displacements zero in these cases. References

materials, the dihedral angle (26) in clay is less than 90°. Consequently, the PSMS are inclined to the master shear by an angle of 45°-0, typically about 15°. Shear displacements on the PSMS (the antithetic primary minor shears remain relatively inactive) cause a bulk elongation parallel to the master shear. This can be quantified by the displacement component matrix for a simple shear (y) inclined at angle (a) to the x-axis which is I" 1 + y sin a y cos a - y sin^

y cos hx l-y sin a y cos a

(Ramsay & Huber, 1983: 290, with correction). The top left component is the stretch in the master shear direction. Such elongation is incompatible with the undeformed boundaries and so the PSMS must be accompanied by some compensating shortening deformation parallel to the master shear. The secondary synthetic minor shears (P-shears of Tchalenko, 1970 and Bartlett et al., 1981), which form symmetrically disposed across the master shear direction from the PSMS, provide a corresponding shortening in the master shear direction. Together, the primary and secondary synthetic minor shears produce an infinitesimal bulk strain that is compatible with the strain caused by the boards. But the sawtooth arrangement formed by the two shears does not allow internal compatibility for large strains and so strain, and hence stress, concentrations must be created at their junctions. Ultimately this will lead to the development of through-going shears parallel to the master shear direction. This type of analysis can also be applied to non- Bartlett, W.L., M. Friedman & J.M. Logan, 1981. Tectonophysics 79: 255-277. constant area displacement controlled experiments. In these experiments, movement between the boards is Crouch, S.L. & A.M. Starfield, 1983. Boundary Element Methods in Solid Mechanics. Allen & Unwin, London. constrained to occur in directions that are not parallel to the shear zone. As the displacement vector orientation Mueller, A.G., G. Mandl & C.H.K. Sijpesteijn, 1986. J. Struct Geol. 8: 737-752. is increased from 0° to 9O°-20 outwards, the PSMS form at progressively lower angles to the shear zone. Ramsay, J.G. & M.I. Huber, 1983. Techniques of Modern Structural Geology, Vol. 1. Academic Press. When the displacement orientation is equal to 9O°-20 the infinitesimal shortening strain is inclined at 9 to the Tchalenko, J.S., 1970. Bull. Geol. Soc. Am. 81: 1625-1640. A9.9 DEFORMATION PARTITIONING WITHIN EXTERNAL THRUST ZONES Nicholas Woodward *, Steven Wojtal , Joseph Paul , and Zintars Zadins 1

University of Maryland, College Park, Oberlin College, Oberlin University of Tennessee, Knoxville, University of Rochester, Rochester 1

Mesoscopic contractional and extensional faults dominate many external thrust zones. The fault arrays occur in the hanging walls of some thrusts and in the footwalls of others. When both contractional and extensional mesoscopic faults occur in thrust zones, foreland-dipping extensional faults overprint contractional faults. In a thrust zone from the Little Mountains fold-thrust belt of New York State, mesoscopic faults cut and offset a major thrust contact

and involved rocks from the thrust's hanging wall and its footwall simultaneously. The Little Mountains exposure indicates that minor faults were active during thrust slip and are not early-formed features which were passively translated with the remainder of a sheet In exposures where mesoscopic faults are not found cataclastic rocks are commonly found. Contractional and extensional microfaults (with R and R Riedel shear geometries) are common in external t

188

thrust zone cataclasites and resemble the mesoscopic fault patterns. Contractional microfaulting occurs dominantly in thick cataclastic zones and extensional microfaulting occurs dominantly in thin cataclastic zones. This suggests that the Riedel shear geometry faults locally increase and decrease the thickness of cataclasites along a thrust Contraction and extensional microfaulting can alternate during thrust transport, as shown by clasts of both microfault geometries in both types of cataclasites. In the Little Mountains exposure brittle forelanddipping mesofaults can be traced downward into shear band cleavage which cut the dominant (hinterlanddipping) footwall cleavage. In this exposure, dominantly

brittle behaviour in the hanging wall strata occurs adjacent to ductile behaviour in the footwall strata. The occurrence of mesoscopic fault arrays and/or cataclasites along a particular segment of a thrust fault, or their relative position in the thrust zone, are inferred to indicate the locus and the dominant mechanism of sliding for a fault segment at the time thrust motion ceased. Varying tractions along thrust segments, or changes in the relative strengths of upper plate rocks, lower plate rocks, and fault rocks, probably control the partitioning of deformation between mesoscopic fault arrays and cataclasites, or between hanging wall and foot wall deformation.

Figure 1 — Field sketch of thrust zone exposed in the ICC Quarry, Little Mountains thrust belt, Hudson Valley, New York. Devonian Coeymans Formation was thrust over Devonian Kalkberg Formation along primary detachment (T). An upper detachment (UD) separates undeformed Coeymans stratafromextended Coeymans and Kalkberg strata in fault zone. Note the late extensional fault (E) that cuts the upper detachment but ends at one of several extension faults listric into the lower detachment zone (LDZ). The down dip continuations of these listric faults are an extensional crenulation or shear band cleavage (SB) which cuts across highly deformed Kalkberg strata in the lower detachment

A9.10 TECTONO-THERMAL EVOLUTION OF AN ANNEALED DUCTILE SHEAR ZONE ACTIVATED DURING THE ALICE SPRINGS OROGENY Ben Goscombe Geology Department, University of Melbourne and Department of Resources and Energy, Hobart

The Wallaby Knob Schist Zone (WKSZ) is a wide (>5 km), east-west trending schist and mylonite zone, in the central Arunta Block, that separates granulites in the Strangways Range from upper amphibolite facies gneiss to the north. The WKSZ is dominated by amphibolite facies assemblages and preserves penological and geothermobarometric evidence of an anti-clockwise P-T path. Deformation commenced at approximately 430-500 Ma during the Alice Springs Orogeny and culminates prior to the thermal maxima (550-620°C, 4-5.6 kbar) at approximately 400 Ma. Such a P-T history was responsible for thermal annealing of the whole zone subsequent to deformation giving rise to predominandy granular textures. Despite this, many features of the earlier ductile shearing experience have been preserved. Deformation was initiated as tight to isoclinal folds (F ) that plunge 40r

70° to the east A strong fold axial planar foliation is developed in these earliest folds (S ). S constitutes the penetrative mylonitic and schistosic fabric of the WKSZ and dips 65-90° north. Mineral aggregate and mineral elongation lineations (Lj) plunge 50-90° east. Sense of shear is consistently reverse north over south and defined by S-C relationships, asymmetric porphyroblast-foliation relationships, small scale offsets and stepping on the foliation surface. Fault analysis of movement vectors (L ) within S defines a sub-horizontal, north-south direction principle compressive stress. Late-stage deformation was less pervasive and involved both tight folding (F ) and discrete small-scale thrust offsets (<10 cm) of the early foliation. F fold axial surfaces are essentially coplanar with F but have a wider range in plunge. No fold axial planar fabric is developed, though small fold axial planar pegmatitic x

veins are observed. Small scale thrusts trend more NWSE and are developed at very low angles to S The WKSZ was reactivated by normal north down movements along discrete zones during isostatic rebound in response to crustal over-thickening during the Alice Springs Orogeny. Normal movements are indicated by a number of features: 1. Slivers of early Palaeozoic Heavitree Quartzite are interleaved within the WKSZ. The present position of these supracrustal rocks, within a middle crustal level shear zone, can only be arrived at by postthrusting normal back faulting. 2. Gneisses north of the WKSZ are of lower grade than r

189 the granulites to the south. This juxtaposition must have been arrived at by normal (north down) movements greater than their initial up thrusting. 3. There is a decrease in temperature and pressure estimates of the thermal maxima from south to north across the WKSZ. Rapid decompression is indicated by petrological and geothermobarometric data in conjunction with well constrained cooling ages subsequent to the Alice Springs Orogeny. Thus the WKSZ preserves not only evidence of the initial deformation and crustal over-thickening, but also preserves evidence for the thermal and isostatic consequences of the Alice Springs Orogeny.

190

A10: Gondwanaland Context of the Tasman Fold Belt Convenor: C. McA. Powell

Keynote Address 1 GONDWANALAND CONTEXT OF THE TASMAN FOLD BELT Christopher McA. Powell School of Earth Sciences, Macquarie University

The Tasman Fold Belt (TFB) in Eastern Australia represents a 3000 km segment of a fold belt more than 15000 km long which bounded the Pacific margin of Gondwanaland throughout the Palaeozoic and early Mesozoic until the breakup of Gondwanaland about 160 Ma ago. The TFB can be subdivided into three tectonic realms, each of which has a distinctive history overlapping, in part, the history of the adjacent fold belt. The Kanmantoo Fold Belt (KFB) is the westernmost, and contains sedimentary rocks deposited along a passive margin from the latest Precambrian to the midCambrian. In the Late Cambrian and Early Ordovician extensive granite plutons were intruded, and the KFB was uplifted and thrust westward onto the Australian craton. The Lachlan Fold Belt (LFB), and its northern extension in Queensland, the Thomson Fold Belt, has a fragmentary Cambrian history represented by volcanics and sediments typical of marginal-sea or other intraoceanic settings. In the Ordovician, widespread quartzose sediments, in part derived from the uplifted KFB, were deposited in deep-water turbidite environments. At the end of the Ordovician, the LFB was segmented into two parts, the western part continued the sedimentary pattern of the Ordovician, with a gradual shallowing of the water level as quartzose sediments derived from Gondwanaland prograded eastward. The eastern part, as far west as the Wagga Metamorphic Belt, was extensively deformed in the Early Silurian, and then became a zone of crustal extension with widespread silicic volcanics and subjacent plutons. The LFB was deformed in the Middle Devonian, though the intensity of deformation varied from strong (in Victoria) to mild (in northern New South Wales). The quartzose clastic sheet which had been prograding into the western LFB in the Silurian, reached the eastern edge of the LFB by the Late Devonian. In the mid-Carboniferous, the entire LFB, and many parts of Central Australia, were deformed by folding and thrusting for the last time. The New England Fold Belt (NEFB) is the easternmost fold belt in the TFB, and contains tectonostratigraphic terranes which could be exotic. Its

Early Palaeozoic history is problematic, being represented by only a few scattered outcrops, and these could be exotic blocks embedded in subduction complexes. By the Late Silurian or Early Devonian a palaeogeography involving an east-facing magmatic arc was established, and this continued until the end of the Carboniferous. The potentially exotic tectonostratigraphic terranes all lie east of this arc, and are separated from the arc and forearc basin by the YarrolPeel ultramafic belt. Evidence is accumulating that there were large strike-slip motions in the eastern part of the NEFB, and a Late Carboniferous dextral orocline in the easternmost NEFB has been postulated. The easternmost NEFB was uplifted, but by no means stabilised, in the Early Permian, to shed sediments westward into the Sydney-Bowen foreland basin which covered the suture between the LFB and NEFB. Tentative provenance-linking suggests that the LFB and NEFB could have been close together in the Early Carboniferous, but only in the Late Carboniferous is the linkage firmly established. The terranes of the Gympie province in the NEFB may not have been accreted until the early Mesozoic. The tectonic history of the TFB can be understood in terms of a succession of palaeogeographies related to a magmatic arc which episodically stepped eastward from the Gondwanaland craton. At some intervals, marginal seas, possibly ocean-floored, separated the magmatic arc from the continental craton (Marianastyle subduction) whereas at other times the magmatic arc occupied the rim of the continent (Chilean-style subduction). The magmatic arc did not depart the eastern rim of the NEFB until the mid-Cretaceous. Transcurrent motion along the TFB appears to have been very important from the Ordovician until at least the Early Permian, so that many terranes are displaced. Some, but by no means all, of the tectonic elements of the TFB can be recognized in the trans-Antarctic and South American sectors of the trans-Gondwanan Pacific Margin. Major changes in palaeogeography and tectonic style appear to have affected all sectors in the Late Cambrian to Early Ordovician interval, and could be

191

related to microcontinent collisions postulated to have occurred in southern South America. Similarly, the endOrdovician deformation in the LFB is also recognized in South America. Deformational events, which began in the late Middle Devonian in Eastern Australia and which continued in places in the Devono-Carboniferous, climaxed in the Trans-Gondwana mid-Carboniferous deformation, which appears to be part of the worldwide Heicynian orogeny. A second widespread, and locally intense deformation occurred in the mid-Triassic. Comparison of the tectonic history of the Australian, Antarctic and South American sectors of the transGondwanan fold belt shows that although some events are of fold-belt-wide (and hence global) significance,

others have more local effect - a conclusion consistent with patterns observed in Cenozoic fold belts where zones of transpression and transtension co-exist. Correlation of the tectonic regimes of the TFB (Powell et al., 1990) with the Gondwanan Apparent Polar Wander Path (APWP), updated on the basis of new palaeomagnetic data (Li et al., 1990), shows that major changes in tectonic style occur around inflexion points on the APWP. References Li, Z.X., Powell, C.McA., Schmidt, P.W. & Thrupp, G.A., 1990. J. Struct. Geol. (in press). Powell, C.McA., Li, Z.X., Schmidt, P.W. & Thrupp, G.A., 1990. Struct. Geol. (in press).

Figure 1 — PalaeoPacific margin of Gondwanaland showing selected tectonic elements, including continental blocks of possible exotic origin now entrained in the trans-Gondwanide fold belt, A = Antarctic Peninsula, C = Chilenia, E = Ellsworth Block, G = Gympie Province, H = Haag Block, M-B = Marie Byrd Block, P = Patagonia, and T = Thurston Block. Other blocks containing exotic terranes may exist in New Zealand and its contiguous submarine plateaux.

192 A10.1 PRELIMINARY SEQUENCE STRATIGRAPHY OF THE KANMANTOO GROUP IN SOUTH AUSTRALIA C.G. Gatehouse1, J.B. Jago2* D.I. Gravestock1, B.J. Cooper1 1

South Australian Department of Mines and Energy, Adelaide 2 South Australian Institute of Technology, Adelaide

initially by carbonate (Cycle 1) and then by quartzofeldspathic siliciclastic sediments(Cycles 2 and 3). Three stratigraphic sequences have been recognized in Cycle 1, two in Cycle 2, and one in the incomplete and least well-known Cycle 3. The Kanmantoo Group was deposited in Cycles 2 and 3 (Fig. 1). Relative water depth curves, interpreted from the lithofacies and fossil succession, show that shelf and deeper off-shelf environments in the Arrowie and Stansbury Basins, were affected by relative sea-level changes in the same sense and at the same time. Each sequence commences during relative sea-level fall, and passes through low, rising and peak sea-level to finish again on a falling trend. Relative water depth curves are constructed at specific locations within a basin. The curve shown for the Kanmantoo Group is at the type locality of most formations on the south coast of Reurieu Peninsula (Daily & Milnes, 1971).

Preliminary sequence stratigraphy of the Cambrian Kanmantoo Group is outlined. The mainly siliciclastic, poorly fossiliferous sedimentary rocks (now metamorphosed) were deposited east and south, respectively, of contemporaneous red-bed dominated upper units in the Stansbury and Arrowie Basins. The geological framework of these basins was summarised by Dalgarno (1964) and Jago & Daily (1982). The older Stansbury and Arrowie shelf, ramp and slope carbonates, are sufficiently fossiliferous to test the sequence stratigraphic principles of Vail (1987) and others. This has proved successful, and an upward extension of the same techniques is applied to the Kanmantoo Group. Three major depositional cycles are evident in the Arrowie and Stansbury Basins. The lowest was deposited non-conformably on Proterozoic Gawler Craton or disconformably on Adelaidean sediments. Sedimentation took place in an extensional tectonic setting, dominated

o® 2< o

MIDDLETON SANDSTONE

z <

BALQUHIDDER

PETREL COVE FM.

FORMATION

o >o h

TUNKALILLA

TALISKER CALC - SILTSTONE /

1z <

z <

NAIRNE PYRITE MBR

o o

z <

KARINYA

< o z 5

J o

_l u. <

CD <

— lli < tr

< o

CAMPANA CREEK MEMBER B L O W H O L E CK I MILENDELLA 1 LMST MBR SILTSTONE M B R

— T R U R O VOLCANICS

NORMANVILLE GROUP (upper

MAOIGAN INLET MEMBER

ce <

Fine to coarse-grained turbidity flow deposits t local channels with coarse to granular sands Currents from northwest Mid-outer fon-basin plain setting

Muastone wirn upward - tmnning ana upwarafining sandstone beds, rare channels, bioturbation Sulphide - rich at base and top Fine-coarse-grained sandstone with mud partings, common channels, lenticular cut-and-fill with pebble conglomerate including gneiss and marble

•

Yuruga Formation on Yorke Peninsula ( sequence incomplete )

•

Lake Frome Group in the Flinders Ranges

•

Romsoy Limestone, Corrodgery Formation, Stansbury Limestone, Moonan Formation and Coobowie Limestone on Yorke Peninsula

•

Nildoftie Siltstone, Eregunda Sandstone Members of Billy Creek Formation, Wirrealpa and Aroona Creek Limestone in the Flinders Ranges

HEATHERDALE SHALE

Laminated siltstone coarsening upwards into upward thickening fine-grained sandstone, siltstone bioturbated, thicker sandstone beds channelled, slumped, cross-bedded Thick upwards-thinning fine-grained sandstone, bioturbated , sandy laminated siltstone

Grey laminated siltstone with thin sandstone laminae ana rare Deas Grey, silty, poorly laminated limestone | Worm casts, currentbedded deep water basin plain

•

Minlaton Formation on Yorke Peninsula

•

Warragee Member of Billy Creek Formation in the Flinders Ranges

•

Upper Parara Limestone, Nepabunna Siltstone, Norma Greywacke in the northern Flinders Ranges

u / /

J S

TYPE 2 Sequence Boundary

T Y P £

Upper Parara Limestone, Bunkers Sandstone, Orapannna Shale m the central Flinders Ranges

(

// //

Sequence Boundary

Upper Parara Limestone on Yorke Peninsula

•

VAIL 1967 * VAN WAGONER a/, 1967

10s of metres

c r/

•

SYSTEMS TRACTS

Fine-grained quartzose Feldspathic turbiditic sandstone fining upwards into mudstone Dewatermg structures common in some areas, thinner bedded to south Blue-block non- micaceous shale with calcareous nodules interbedded siltstone, shale beds with hyolithida and a conocoryphid trilobite Environment - deep water, strongly reducing Andesitic volcanics including lava flows, one pillowed flow and tuffoceous horizons interbedded with Heatherdale Shale Tuffoceous horizons may extend into Carrickolmga Head Formation

SHALLOW |

TYPE 1 Sequence Boundary

Banded mudsfone, calc-siltstone, morble, common sulphide-rich beds Deep offshore reducing environment Oscillating terrigenous - calcareous seaimeni supply Lenticular pyritic carbonaceous shale

RELATIVE WATER DEPTH [)EEP

100s of metres

SHALE

BACKSTAIRS PASSAGE FM.

FM.

TAPANAPPA FM.

Fine-grained quartzose sandstone, cross-bedded, current rippled , rare angulor shale clasts Shallow marine, currents from west Micaceous silty sandstone, laminated and crossbedded, climbing ripples, shale-chip breccias

WIDER CORRELATIONS

CYCLE

LITHOFACIES

CYCLE 3

STRATIGRAPHY

/ /

\ -

OBSCURED BY TECTONIC OVERPRINT OF KANGAROOIAN MOVEMENTS (DAILY a FORBES, 1969)

KEY

CYCLE 1

AGE

SEQUENCE STRATIGRAPHY-SOUTH COAST OF FLEURIEU PENINSULA

Hiqhstand

/ /

Tronsgressive \

She,f mar9in

jjj^

Lowstand

FK3.1

193

Measured sections of the Carrickalinga Head and Backstairs Passage Formations on Fleurieu Peninsula and in the Mount Lofty Ranges provide facies data for environmental reconstructions (Gatehouse et al., in press). Three members of the Carrickalinga Head Formation crop out on the south coast. A fourth (Milendella Limestone Member) occurs further north in the Karinya Syncline. Madigan Inlet Member represents the inner to middle fan sub-association of Mutti and Ricci Lucchi (1972). This lowstand systems tract follows deep water Heatherdale Shale of Cycle 1. Correlations with the Stansbury (Yorke Peninsula) and Arrowie (Flinders Ranges) Basins are indicated. Blowhole Creek Siltstone (and Milendella Limestone) Member was deposited during a relative sealevel rise (transgressive systems tract). Only minor amounts of sand entered the depositional site. It is clear here, and in the Arrowie Basin carbonates of Cycle 1, that rising sea-level causes retention of sand in continental or shelf environments. As sea-level falls the reverse applies, with sediments transported seaward off the shelf. Campana Creek Member is a prograding quartzose sand body deposited rapidly at a time of high, but falling relative sea-level (highstand systems tract). Contemporaneous faulting along the inner shelf adds a dimension of complexity, the consequences of which are not fully understood in the more basinal succession. Subsidence rates there may have exceeded sea-level fall, resulting in a Type 2 sequence boundary. The upward-coarsening Backstairs passage Formation may represent re-establishment of slower subsidence rates during tectonic quiescence (shelf margin systems tract) and re-adjustment of the rate of siliclastic supply with respect to relative sea-level.

Renewed marine transgression accompanied deposition of the Talisker Calc-siltstone and Karinya Shale, possibly the deepest marine sediments of the Kanmantoo Group. Subsequent sea-level fall was accompanied by progradation of the Tapanappa Formation, notable for exotic clasts in conglomerate filled channels, and culminating in a lowstand. This may be a global event represented by the Toyonian Regression on the Siberian Platform and Hawke Bay Event in western Newfoundland. Cycle 3 appears to repeat lowstand, transgressive and highstand sedimentation culminating in the quartzose shallow marine Middleton Sandstone. However, further study of these units, and equivalents in the Arrowie and Stansbury Basins, is required for improved sequence stratigraphic interpretation. The crucial strata are at the top of the preserved sedimentary pile; thus Cycle 3 is incomplete. Younger units, possibly deposited in these basins prior to the Cambro-Ordovician Delamerian Orogeny, have counterparts in the Warburton Basin. It is from there that a sequence stratigraphy representing the missing southern section may be forthcoming. References Daily, B. & Milnes, A.R., 1971. Trans. Roy. Soc. South Aust. 95: 199-214. Dalgarno, C.R., 1964. Trans. Roy. Soc. South Aust. 88: 129-144. Gatehouse, C.G., Jago, J.B. & Cooper, B.J., in press. Geol. Soc. Aust., Spec. Publ. 16. Jago, J.B. & Daily, B., 1982. Geol. Soc. Aust. Spec. Publ. 9: 6-12. Mutti, E. and Ricci Rucchi, F., 1972. Int. Geol. Rev. 20: 125-166. Vail, P.R., 1987. AAPG Studies in Geology 27: 1-10.

A10.2 TASMANIAN PRECAMBRIAN MASSIFS AND THE TECTONIC DEVELOPMENT OF THE TASMAN FOLD BELT C.G. Elliott *, D.R. Gray & N.B. Woodward 1

Department of Earth Sciences, Monash University Department of Geology, University of Maryland, USA 1

Tasmania's tectonic significance in the Early and Middle Palaeozoic history of the Gondwanaland margin is twofold: 1) It is the only area of the Tasman Fold Belt of Australia in which Precambrian basement is exposed and may be examined; and 2) Tasmania lies in the triple junction between pre-rifting Australia, New Zealand and Antarctica. It remains a problem, however, as to how Tasmanian geology fits into Gondwanaland reconstructions, and how it can be explained by the simple plate tectonic models proposed for southeastern Australia and northern Victoria Land, Antarctica. The distribution and role of Precambrian basement blocks or massifs in the Tasman Fold Belt in Tasmania may be explained in one of three ways. Either: (1) Precambrian rocks are autochthonous and rooted,

forming a continuous basement now locally exposed as inliers in the Palaeozoic cover. Part or all of the cover sequence may be autochthonous or may have been emplaced by thrusting or crustal extension; (2) Precambrian rocks are allochthonous fragments that have been emplaced along thrust and/or transcurrent faults. If this is the case, Precambrian sequences need not be related to each other, and need not represent true basement; or (3) Precambrian sequences in Tasmania represent a combination of the first two possibilities. Choosing between the above possibilities requires examination of the boundaries of the Precambrian blocks and comparison of structural/ tectonic features within and around them. Many Precambrian/Cambrian exposures in Tasmania are

194

bounded by steep faults, even though the massifs have traditionally been represented as having moderately to gently dipping unconformable margins. The timing of movement on those faults is uncertain, and a major problem lies in removing the very considerable effects of the middle Devonian Tabberabberan Orogeny and of Mesozoic and Cenozoic deformation related to the

separation of the major continents and the opening of the Bass Strait Nevertheless, evidence from elsewhere in the Tasmanides for lower Palaeozoic accretionary tectonics supports the possibility that Precambrian massifs have been mobile and have played more than a passive role in the assembly of the Tasman Fold Belt and of Gondwanaland.

A103 TASMAN FOLD BELT, VICTORIA, AUSTRALIA: IMPLICATIONS FOR GONDWANALAND TECTONICS David R. Gray *, Christopher L. Fergusson & V.J. Morand 2

department of Earth Sciences, Monash University department of Geology, University ofWollongong Department of Geology, James Cook University 3

The Tasman Orogenic Belt of southeastern Australia constitutes a composite Palaeozoic terrane which developed as a consequence of cratonisation by arccontinent collision. The westernmost segment, the Lachlan Fold Belt, has an Early Cambrian to Early Carboniferous history characterised by an Ordovician east-facing and migrating island arc (cf. Powell, 1983). The belt has undergone a complex amalgamational and deformational history with interplay of compressional (Late Ordovician-Early Silurian, Early Devonian, Middle Devonian, and middle Early Carboniferous) and extensional (Late Silurian, Late Devonian and Early Carboniferous) tectonics (Cas, 1983). Terrane translation and accretion of discrete allochthonous fragments which had developed adjacent to the ancient Gondwana continental margin, was along strike-slip faults in an inferred zone of oblique convergence. Subsequent overthrusting in the Middle Devonian led to suturing and stabilisation of the Gondwana margin. This Middle Devonian event led to crustal thickening, crustal anatexis and the emplacement of the post-tectonic granites. It also marks a major change in tectonic style from strikeslip faulting to major east-west compression and coincides with a gap in magmatic activity, a change in regional facies patterns from volcanism, shallow- and deep-marine sedimentation to mainly fluviatile overlap sequences, and a belt of magmatism in central Victoria. Overthrusting is probably related to rapid convergence along the inferred subduction zone east of the Benarnbra terrane (Fergusson et al., 1986). In Victoria the major features of the Tasman Fold Belt are: the laterally extensive (over 800 km present width), thick (upwards of 10 km deformed thickness)

pile of Ordovician through Devonian quartz-rich turbidites, constituting an extremely large continental margin sediment-prism; the fault-bounded Cambrian greenstone slices which geochemically resemble MORBs and boninites and are clearly of oceanic affinities (Crawford & Keays, 1978); the Silurian high temperature-low pressure metamorphism requiring geothermal gradients on the order of 65°C/km (Morand, 1989); the vast volume of granite (up to 30% total exposed surface area of the belt) typified by shallowlevel composite plutonic bodies with exposed areas less than 500 km (Chappell et al., 1988); the largely upright nature of the folding; and the dominance of steeply dipping faults characterised by strike-slip movement components in the metamorphic belt, and elsewhere generally by dip-slip reverse components accompanied by later strike-slip movement components (Gray, 1988). Juxtaposed fold-thrust zones of different age and vergence, as well as strike-slip movement components along reactivated faults, provide major constraints for tectonic reconstruction of this part of Gondwanaland in the Early to Middle Palaeozoic. References 2

Cas, R.A.F., 1983. Geol. Soc. Aust. Spec. Publ. 10. Chappell, B.W., White, A.J.R. & Hine, R., 1988. Aust. J. Earth Sci. 35: 505-524. Crawford, A.J. & Keays, R.R., 1978. Earth Planet. Sci. Lett. 41: 197-208. Fergusson, C.L., Gray, D.R. & Cas, R.A.F. 1986. Geology 14: 519-522. Gray, D.R., 1988. In Douglas, J.G. & Fergusson, J.A. (eds): Geology of Victoria: 1-36. Morand, V.J., 1989. J. Metamorphic Geol. (in press). Powc !, C.McA., 1983. Aust. J. Earth Sci. 30, 353-373. 1

195 A10.4 THE TASMAN OROGEN — A VIEW FROM THE CRATON P.R. Evans* and D.A. Remus University of NSW, Sydney

The western flank of the Tasman Orogen during the Devonian-Carboniferous was characterized by a series of foreland basins: the Darling Basin in the south; the Adavale Basin and Warrabin Trough in the centre; and the Drummond Basin in the north. These are buried by younger platform cover of the Murray and Eromanga Basins and knowledge of their form and evolution is largely obtained from studies of geophysical surveys, calibrated by petroleum exploration wells. The Adavale Basin was traversed by the BMR Eromanga-Brisbane deep crustal seismic transect, which discovered the presence of a series of west-dipping ramps or dislocations through the crust below the basin and below the Nebine Ridge to the east. Although the structural styles and sedimentary records of these basins differ from each other, they indicate that a common series of tectonic events affected the length of the orogen. The interplay and contemporaneity of horizontal compression, vertical, and strikeslip movements are interpreted as products of overall N-S sinistral slip between the craton and the orogen. The Adavale Basin and Warrabin Trough are products of an original broader depression that passed through four tectonic stages, an initial period of rifting and three phases of foreland thrusting. Crustal extension during the Early Devonian rift phase was principally in a NE-SW direction, and was initially accompanied by acid volcanism (Gumbardo Formation). In the later stages the rifts controlled deposition of deltaic and shallow marine facies (Eastwood Beds). Later in the Early Devonian the first phase of foreland loading deepened the basin such that deltaic facies in the NW of the basin merged into deep water facies to the SE (Log Creek Formation). A peripheral bulge developed in the region of the Canaway Ridge, between the Adavale Basin and the Warrabin Trough. Tectonic relaxation during the Middle Devonian caused the deltaic facies to merge upwards into the shelf and alluvial facies of the Lissoy Sandstone, which in turn was succeeded by the carbonate and sabkha facies of the Bury Limestone and Cooladdi Dolomite, respectively. Late in the Middle Devonian the second phase of crustal loading created minor deformation and further depressed the crust. Therisingterrain to the east initially restricted access to the ocean, so that Boree Salt was deposited in the deeper sector of the basin. Open marine facies re-entered the depression (upper Etonvale Formation), eventually to be replaced by the largely non-marine shales and sandstones of the Buckabie Formation.

The third and final stage of foreland thrusting, during the Carboniferous, created "Sevier"-type blind thrusts into a Triangle Zone (controlled by the presence of the Boree Salt) and thrusts to the surface (where the salt is generally absent) along the line, and to the east, of Pleasant Creek Arch. The Darling Basin is the product of a comparable series of tectonic episodes although the style of structures expressing these movements differs from the structural styles of the Adavale Basin. They are akin to the "Wyoming" style of basement uplift, rather than the "Sevier" style of thrusting that deformed the Adavale Basin. The initial Early Devonian rifts were created by NESW extension and were accompanied by limited volcanism. The first stage of foreland loading created a deepwater trough in the east of the basin in which mainly gravity flow facies (Amphitheatre Group) were deposited. The top of this cycle is represented by the Middle Devonian Winduck Group. The thick molasse of the Middle Devonian-Early Carboniferous Mulga Downs Group is a product of the second phase of foreland loading. Terminal deformation of the Darling Basin occurred during the Carboniferous, the western margin of the Ivanhoe Block marking the thrust front. The Drummond Basin became a foreland basin upon a Middle Devonian platform during the Late Devonian and Early Carboniferous and has experienced both intraCarboniferous and subsequent deformation; its history is distinct from the other foreland basins and is not considered for the present The series of tectonic pulses from Early Devonian to mid-Carboniferous affecting both foreland basins are distinct reflections of events during the Lachlan Orogeny. They indicate that there was initial (Early Devonian) net extension along a N-S corridor on the craton-side of the orogen, where rifting was accompanied by minor volcanism. An analogue is to found in the Cainozoic rift corridor through Thailand induced by sub-parallel oblique slip of India past Southeast Asia. Later in the Early Devonian both basins assumed characteristics of foreland basins in response to crustal loading from the east and with net compression from the SE. Structures created during the final stages of foreland deformation, during the Carboniferous, are comparable with foreland structures, for example to the east of the Rocky Mountains. The alignment of structures around the Adavale Basin is relatable to the alignment of deep crustal sutures and gravity trends, which presumably were developed in pre-Devonian times.

196

In both basins, erosion reduced the landscape to relatively low hills and valleys prior to deposition of Late Carboniferous and Permian sequences under the influence of another, distinct tectonic regime. Conversion from en echelon net extension to net

compression and the number of tectonic pulses could be simply explained in terms of relatively minor changes in plate motion. The major change in stress vector late in the Carboniferous or in the Permian may require another explanation.

CRUSTAL FEATURES AND DEFORMATION OF BASINS IN THE TASMAN FOLD BELT OF SOUTHEAST QUEENSLAND D.M. Finlayson1 and C.R. Fielding2 1 2

Bureau of Mineral Resources, Geology & Geophysics, Canberra Department of Geology and Mineralogy, University of Queensland

The Eromanga-Brisbane Geoscience Transect across the Tasman Fold Belt brings together diverse data sets in the study of the palaeo-Pacific margin of Gondwanaland. The Bowen-Gunnedah-Sydney Basin system is a major north-south feature of the margin crossed by the transect, whose origins are still not fully understood. The basin system evolved from the Late Palaeozoic events affecting basement rocks of the Thomson, Lachlan and New England Orogens. These events reflect processes on this margin of Gondwanaland. In this paper we consider seismic and other data from the Surat Basin region in southeastern Queensland near latitudes 27-28°S and the nature of regional events that probably contributed to the evolution and structuring of the basin sequences. The Devonian-Carboniferous tectonic regime of the New England Orogen was that of a convergent plate margin, with (west to east) a volcanic arc, forearc basin, and a continental slope and basin, the palaeo-Pacific plate being subducted westward under the craton. Although the plate margin moved farther east during the Late Palaeozoic, convergence continued. The basinforming regimes prevailing on the craton side of convergent plate margins can be gauged from three presentday examples, western North/South America, New Zealand, and Sumatra/tfava. Most major basins evolve from: (a) attenuation of the crust/lithosphere during stretching; (b) contraction of a cooling lithosphere; and (c) depression of the lithosphere by sediment or tectonic loading. The latter is dominant at most convergent margins, but extensional, compressional, and strike-slip elements are all present in the basin styles of such a regime, depending on the rate and obliquity of convergence. In southeast Queensland, seismic, gravity and magnetic data define the nature of structures in the Surat/Bowen Basin sequences and within older Palaeozoic basement Most faults in the area can be regarded as polyphase structures. To the east of the Burunga-Mooki Geosuture, gravity and magnetic data map the Late Carboniferous - Early Permian oroclinal bending within basement rocks (Texas Orocline). To the west of the geosuture, the north-south trending

Meandarra Gravity Ridge defines major basement features under the Taroom Trough/Suiat Basin. Farther west, in the central Thomson Fold Belt, major tectonism had largely ceased by mid-Carboniferous times with the deformation of Devonian Adavale Basin sequences and the uplift of the Nebine Ridge. Seismic data give a present-day image of crustal structures and tell us most about the youngest orogenic events, and progressively less about older basin-forming events. However, it is likely that reactivation of older basement structures influenced the style seen in the younger structures and, consequently, a history of likely processes can be interpreted. We here outline some of the major late Palaeozoic-Mesozoic events. To the east of Burunga-Mooki Geosuture, Early Carboniferous to Early Permian (Kuttung Formation) volcanoclastics (including thick marine mudstones and shales, and possibly up to several kilometres thick), are uplifted and identified below the Bowen Basin rocks. These pre-Permian sequences indicate a basin-forming episode prior to recognised Bowen Basin deposition and they possibly underlie the Taroom Trough. Their deformation to the east of the Burunga Fault is probably related to the strike-slip movements associated with the Late Palaeozoic oroclinal bending within the New England Fold Belt above a mid-crustal detachment Basement rocks on the western side of the Taroom Trough (Roma Shelf, Denison Trough) comprise mainly Devonian(?) metasediments (Timbury Hills Formation), thought to persist along the eastern margin of the Thomson Fold Belt. This formation was intruded by Early Carboniferous Roma Granite. The Timbury Hills Formation is underlain by a west-dipping detachment surface interpreted to extend through most of the crust Between these eastern and western margins of the Taroom Trough, is the prominent, north-south trending, Meandarra Gravity Ridge which correlates closely with normal(?) faulting in the deepest part of the trough. It is tentatively interpreted to be related to early transtensional events associated with (Carboniferous?) basin initiation (limited rifting?) and possible mafic intrusion. Within the Early Permian-Triassic Bowen Basin sequences there is a record of extensional/transtensional

197 and compression/transpressional events. In the Roma Shelf and Denison Trough areas, a major Early Permian extension episode was followed by several lesser PermoTriassic crustal movements (extension, compression, strike-slip). In the Denison Trough area, the highly conglomeratic Reids Dome beds are interpreted to have been deposited rapidly in response to crustal extension. Generally, subsidence curves derived from well data , e.g. Apple Tree 1; Bainbilla 1; Alick Creek 1; Grantham 1; Newington 1,2; and Myall Creek 1, indicate predominantly Permian extension followed by a sag phase. To also satisfy the basin geometry across the Taroom Trough, a strike-slip component of movement is interpreted in addition to an extensional movement on the eastern bounding (Burunga/Leichhardt) fault. Models of extensional basin systems require linking of the boundary fault to a detachment surface at some depth in the crust. The Burunga Fault is currently interpreted as a high-angle feature, with westward detachment in the lower crust above the Moho (36-38 km) which is only seen tentatively in the seismic data. The stratigraphic and shallow seismic record indicates that extension (transtension?) was probably restricted to the Early Permian, with thermal relaxation (sag) during Late Permian times, and onset of thrust loading from Early Triassic times onward (crustal shortening). The most substantial structuring in the region occurred during a mid- to Late Triassic convergent event with an element of transpression. Major fold trends associated with uplift and erosion of Bowen Basin sequences in the Moonie Fault/Undulla Embayment area are northeast-southwest, consistent with sinistral movement on a north-south, high-angle, fault system

along the Burunga-Mooki Geosuture. Compressional and transpressional faulting is evident in pre-Jurassic sequences. The style of faulting suggests that some fault planes may sole out within the upper crust and deep seismic data provide some supporting evidence. Farther east in the Clarence-Moreton Basin a transtensional mechanism is interpreted for Triassic troughs. Post mid-Triassic strike-slip (dextral) movement is identified on the Demon Fault in the New England Fold Belt, and sinistral transpression farther south on the eastern margin of the Gunnedah Basin. Tertiary crustal shortening reactivated basement faults within the whole of the Burunga, Moonie and Peel Fault systems. This event is also identified elsewhere in eastern and central Australia, including the Gunnedah and Eromanga Basins to the south and west, respectively. The resultant structures are significant markers of the major regional terrane boundaries. In the exposed part of the Bowen Basin, farther north, there is deformation by low-angle thrust faulting, and thrust planes steepening towards the basin margin in the Gogango Overfold Zone. The multiple late Palaeozoic- Mesozoic events in the Bowen/Surat Basin region of southeastern Queensland illustrate the nature of the likely processes on this convergent Gondwanaland margin. The early history of the foreland basin system which evolved here seems to have strong elements of high-angle and strikeslip faulting in an extensional regime. Later margin convergence reactivated Palaeozoic fault systems with elements of uplift, thrusting, transtension, and strikeslip.

A10.6 MET A VOLCANIC SUITES FROM SUSPECT LATE PALAEOZOIC TERRANES OF THE GYMPIE PROVINCE, SOUTHEAST QUEENSLAND W.J. SivelF, C.M. Stocksiek1 and J.B. Waterhouse2 1 2

School of Science and Technology, University of Western Sydney' Department of Geology and Mineralogy, University of Queensland

Contrasting geochemical trends for several metavolcanic suites from the Gympie Province imply origins of their host suspect terranes diverse in tectonic settings. To date, five petrographically and geochemically distinct mafic volcanic associations have been recognised. These include the tholeiitic metabasalt suites which comprise portions of the (pre-Middle Carboniferous) Rocksberg Greenstone, the Amamoor beds and Cedarton Volcanics (of Early CarboniferousEarly Permian age), and the (Early Permian) Highbury Volcanics of the Gympie Group. The genesis of the basic magmas parental to these suites required markedly differing source contributions from depleted asthenospheric mantle, sub-continental lithospheric mantle, and (slab-derived) subduction-related components.

Basalts from the Highbury Volcanics are island arc tholeiites (IAT) formed during an immature stage of eruptive activity in an intra-oceanic arc. They are unique among the analysed metabasites in possessing high Zr/Nb (~ 30), very high Ba/Nb, as well as low Ti/ V, Ti/Zr and Ti/Y ratios. Geochemical features of the Highbury basalts are explicable in terms of contributions solely from a depleted, convecting upper mantle source and large ion lithophile element (LILE) enriched fluids ascending from subducted oceanic crust, in a tectonic regime far removed from any (sub-) continental lithospheric influence. Arc signatures are also pronounced for highly porphyritic metabasalts from the Cedarton Volcanics. Like the Highbury basalts, the Cedarton volcanics display high Ba/Nb, low Ti/V and Ti/Y, but they have

198

very low Zr/Nb ratios 5) and higher Zr/Y, Ti/Zr and (i.e. intra- or inter-arc) rifting within an overall Zr/P 0 than the Highbury eruptives. This reflects the continental margin subduction regime. Compared to involvement of a substantial sub-continental lithospheric sources proposed for the Cedarton Volcanics, submantle component in the genesis of these rocks and continental lithospheric mantle and slab-derived source implies their generation in a continental margin contributions were reduced, due to a greater input from subduction-related setting. The mantle wedge which convecting upper mantle. Initiation of local extensional overlies subducting oceanic lithosphere beneath the tectonics within a regionally developed subduction magmatic arcs of active Cordilleran margins may system facilitated the ascent to shallow depths of comprise both (asthenospheric) convecting upper mantle convecting asthenospheric mantle, thereby diminishing and geochemically distinct sub-continental lithospheric the opportunity for involvement in magma genesis of mantle. Both of these mantle regions (in addition to a (high Nb/Zr, Zr/Y) sub-continental lithospheric mantle. slab-derived subduction component) may contribute to Enhancedfluxof upwelling asthenosphere in the vicinity the geochemical compositions of continental margin of the developing rift also had the effect of reducing the volcanic arc magmas. Compositional variation within amount of interaction of a given mantle volume with both the Highbury and Cedarton basalt suites is mainly slab-derived fluids. High rate of magma generation due to fractionation of ferromagnesian phases (olivine favoured the eruption of more primitive (higher Mg#) magmas in the rift settings. Geochemical features of + clinopyroxene), typical of volcanic arc basalts. Significant but variable sub-continental lithospheric distinctive (ophitic/hypocrystalline) Amamoor Type 2 mande contributions are also required to account for basalts are in some respects transitional between those geochemical features of metabasalts from the Rocksberg of the Cedarton and Amamoor Type 1 eruptives. In Greenstone and Amamoor beds. Two composition- terms of their relatively high Ba/Zr, Ba/Nb and La/Ce ally distinct suites of Amamoor basalts are identified. ratios, and low Ti/Y, they closely resemble the Cedarton Zr/Nb ratios for Rocksberg and Amamoor Type 1 Volcanics. Any proposed model for their origin must basalts (Zr/Nb - 10) are intermediate between those of account for their pronounced depletions in elements the Cedarton and Highbury basalts, and their Zr/Y, like Ce, P, Zr, Ti and Y, possibly due to renewed fusion Ti/Zr and Zr/P 0 ratios are higher than for the Highbury of mantle that had already yielded previous melt rocks. Most Rocksberg and Amamoor samples possess fractions. Distinctive magmatic evolutions for the Gympie overall Cordilleran margin geochemical affinities (low Zr and Y levels and generally high LILE abundances, Province mafic volcanic suites are consistent with the reflected in high Ba/Zr ratios relative to mid-ocean proposal that their host sequences of metasedimentary ridge basalts (MORB)). In addition, the majority of the and metavolcanic rocks comprise discrete suspect Rocksberg and Amamoor Type 1 basalts have (higher) terranes. In the easternmost Gympie Province, the Ti/V and Ti/Y ratios, as well as Ti/Cr-Ni distributions Gympie Group (including the Early Permian Highbury similar to MORB. For a given MgO content, they basalts of intra-oceanic island arc affinity) is almost possess lower La/Nb and Ba/Nb than the Cedarton certainly exotic with respect to the other (in part older) Volcanics. The Amamoor Type 1 basalts have distinctly sequences of overall Cordilleran-margin affinities to lower LaJY (0.7-2.4), Sr and La/Ce than the Highbury the west. Continental margin affinities of the Cedarton and Cedarton eruptives. Relatively low Sr, Aip^ and Volcanics, Rocksberg Greenstone and Amamoor beds Al 0 /Ti0 for the Rocksberg and Amamoor Type 1 suggest their closer original palaeo-geographic basalts imply significant plagioclase-controlled relationship to Gondwanaland than proposed for the fractionation at an early stage of their magmatic Gympie Group. Whereas the Gympie Group required evolution, as commonly inferred for magmas from rift- tectonic translation to the Gondwana rim, these terranes related environments (e.g., seafloor spreading ridges). were always proximal to the Gondwanaland margin. In the case of the plagioclase + olivine phyric Amamoor However, prior to docking of the Gympie "island arc" basalts, this is confirmed by observed phenocryst assemblage, these suspect terranes may already have assemblages. The apparent (though less pronounced) undergone substantial strike-slip displacement along affinities of the Rocksberg and Amamoor Type 1 basalts the continental edge which apparently comprised a with magmasfromactive continental margins, together complex system of arc and (intra- and inter-arc) rift with features transitional to those of MORB, indicate segments. that the generation of these magmas accompanied local 2

Keynote Address 2 TERRANE HISTORY OF SOUTHERN SOUTH AMERICA AND ANTARCTICA

199

Victor A. Ramos DepcurtmerUo de Geologia, University de Buenos Aires, Argentina

Southern South America and Western Antarctica has three episodes of partially coeval accretion during the latest Precambrian and Palaeozoic. Southern South America, the Antarctica Peninsula, North Victoria Land and adjacent regions all have similar tectonic histories characterised by alternating periods of rapid plate movements associated with subduction and arc magmatism, and subsequent amalgamation of terranes, followed by stationary intervals characterised by rhyolitic magmatism. The Late Proterozoic accretionary history of Southern South America began with extensive calcalkaline magmatic activity (700-500 Ma), along the Eastern Sierras Pampeanas terrane. This magmatism ceased when this Proterozoic microcontinent collided against the older 2000 Ma) Rio de La Plata craton. The same interval in the central Transantarctic Mountains is associated with the collision of the Beardmore microcontinent against East Antarctica (Borg & De Paolo, 1989). The Puncoviscana belt, northwest of the Sierras Pampeanas was a Late Proterozoic accretionary prism consisting of turbiditic sequences formed at the trailing edge of the Sierras Pampeanas block during the latest Proterozoic-earliest Cambrian (Ramos, 1988). Similarly, the age-equivalent Wilson terrane (500-550 Ma) in Western Antarctica is also a sedimentary accretionary prism formed at the margin of the craton. Both the Puncoviscana and the Wilson terranes were subsequently metamorphosed and deformed at approximately the same time. The Puncoviscana belt was also the locus of Stype granitoids and trondhjemites related to an eastdipping subduction. After the welding of the Arequipa crustal block to the Sierras Pampeans, which was already amalgamated with the Amazonian and Rio de la Plata cratons (Ramos, 1988), a stationary period began with minor rhyolitic plateaux developing along sutures in Southern Brazil (Ramos, 1976). This aggregation period was part of the worldwide event that formed the large Pangea supercontinent at the end of the Proterozoic. Rifling and dispersal of different blocks came soon after this period. The second amalgamation period occurred during the Late Ordovician - Early Silurian in Northern Argentina (the Ocloyic Orogeny) where the Arequipa and related crustal blocks were resutured to the Gondwana continent, while in central Argentina, the Precordillera displaced terrane acquired its present position (Ramos et al., 1986). The Ross Orogeny occurred in the central Transantarctic Mountains during the Late Ordovician, at the same time as the Bowers and Robertson Bay terranes were assembled together to

form the Admiralty microcontinent (Borg et al, 1987; Vetter & Tessensohn, 1987). Western Sierras Pampeanas, as well as the Arequipa block registered an important magmatic activity around 440 Ma, when tholeiitic gabbros, hornblende-bearing and other metaluminous granitoids were intruded (Rapela et al., 1989) associated with subduction toward the craton. The end of the Early Palaeozoic is marked by the accretion of Chilenia in the Late Devonian-Early Carboniferous (the Chanic Orogeny) to South American part of the Gondwana (Ramos et al., 1984). At the same time, the Admiralty microcontinent, after a period of ocean-ward subduction, was amalgamated to East Antarctica (Borg & De Paolo, 1989). The final assemblage of the second supercontinent of Pangea, was completed during the Late Palaeozoic with the accretion of Patagonia (South America, Ramos, 1987) and Weddelia (Antarctic Peninsula, Marie Byrd and related terranes, Dalziel et al., 1987) to the rest of Gondwanaland. The stationary period with low mobility and extensive rhyolite eruption (Kay et al., 1989), in Western Argentina and Chile as well as in Western Antarctica characterized the magmatic activity prior to the Mesozoic break-up of the continents. References

Borg, S.G. & De Paolo, D.J. 1989. Antalith Meeting, Washington, July 1989. Borg, S.G., Stump, E. Chappell, B.W., McCulloch, M.T., Wybom, D., Armstrong, R.L. & Holloway, J.R., 1987. Am. J. Sci. 287: 127-169. Dalziel, I.W.D., Garrett, S.W., Grunow, A.M., Pankhurst, P.J., Storey, B.C. & Vennum, W.R., 1987. In G.D. McKenzie (ed.) Gondwana Six; Structure, Tectonics, and Geophysics. Am. Geophys. Un., Washington, Geophys. Mon., 40: 173-182. Kay, S.M., Ramos, V.A., Mpodozis, C. & Sruoga, P., 1989. Geology 17(4): 324-328. Ramos, V.A., 1976. VI Congreso Geologico Argentino (Bahia Blanca), Buenos Aires. Actas I: 187-204 Ramos, V.A., 1987. Trans. Geol. Soc. South Africa 87(2): 169-179. Ramos, V.A., 1988. Episodes 11 (3): 16&-174. Ramos, V.A., Jordan, T.E., Allmendinger, R.W., Kay, S.M., Cortes, J.M. & Palma, M.A., 1984. Geologico Argentino, Actas 2: 84-106. Ramos, V.A., Jordan, T.E., Allmendinger, R.W., Mpodozis, C., Kay, S.M., Cortes, J.M. & Palma, M.A., 1986. Tectonics 5(6): 855-880. Rapela, C., Toselli, A. & Saavedra, J., 1989. In Rapela, C. & Kay, S. (eds): Circumpacific Phanerozoic Granites. Geol. Soc. Am. Mem., Bueonos Aires (in press). Vetter, U. & Tessensohn, F., 1987. Geol. Runsch. 76(1): 233-243.

200

A10.7 TECTONICS OF THE PALAEOZOIC SHOALWATER AND WANDILLA TERRANES, NORTHERN NEW ENGLAND OROGEN, QUEENSLAND Christopher L. Fergusson *, Robert A. Henderson and Evan C. Leitch 1

Department of Geology, University ofWollongong Department of Geology, James Cook University Department of Applied Geology, University of Technology, Sydney 1

The New England Orogen along the central Queensland coast contains the Wandilla terrane in the west and the Shoalwater terrane in the east and north. The Wandilla terrane is largely a tectonic melange consisting of greywacke and mudstone, with less common chert, tuff and greenstone. An Early Carboniferous age is inferred from abundant oolite debris contained within coarse greywacke. The Shoalwater terrane consists of a largely coherent, variably metamorphosed, Palaeozoic succession of quartzose turbidite sandstone and mudstone which contains trace fossils. Its age is surmised as Late Carboniferous on the basis of regional correlations. Both terranes underwent multiple deformation and at least one main episode of prolonged metamorphism. The first deformation (D ) developed from subduction accretion in a Carboniferous forearc. Early subduction-complex structures in the Wandilla terrane are dominated by steeply dipping melange with lenticlular phacoids of tuff and greywacke in a mudstone matrix, whereas in the Shoalwater terrane most beds are coherent and are part of a steeply dipping imbricate fault system. The second deformation (D ) is widespread in both terranes and formed a subhorizontal x

to moderately east-dipping cleavage and tight to open F folds. A strong down-dip stretching lineation formed in the Wandilla terrane. F folds and S -S relationships have a consistent geometry indicating an antiform to the east. These relationships reflect the near-vertical orientation of bedding, as a result of the D deformation, subsequently tilted to the east and northeast during D westward-directed thrusting. This deformation is related to renewed convergence after an episode of transform faulting during the Late Carboniferous and earliest Early Permian, and marks the beginning of the HunterBowen Orogeny. The D and D deformations only affect the Shoalwater terrane and developed as a progressive sequence during the later stages of the Hunter-Bowen Orogeny. D structures are NNWtrending upright to shallowly SW-inclined folds and axial planar crenulation cleavage that developed in overall ENE convergence. D structures are upright open folds commonly with no axial planar structure. The Hunter-Bowen Orogeny was caused by a westdipping subduction zone that also formed a magmatic chain represented by widespread Late Permian-Triassic plutonic and volcanic rocks of the New England Orogen. 2

A10.8 CONSTRAINTS ON THE TIMING AND EXTENT OF MAJOR DENUDATION EPISODES ASSOCIATED WITH GONDWANA BREAK-UP: FISSION TRACK EVIDENCE FROM SOUTHERN AFRICA Roderick W. Brown Department of Geology, La Trobe University

The chronology and nature of landscape development in southern Africa has stimulated debate since the beginning of the present century, yet no consensus has been reached. Major differences in viewpoint result largely from a lack of quantitative information concerning the timing and magnitude of denudation episodes affecting the sub-continent, particularly since the break-up of Gondwana. The major discrepancies between the various proposed landscape chronologies imply that there are fundamental problems associated with dating approaches used up until now, for example, in the correlation between land surfaces and unconformities in the offshore sedimentary record. This situation suggests that the use of a more direct dating technique may help to resolve these discrepancies. Apatitefission-trackanalysis (AFTA) is a valuable tool

for evaluating the thermal history of rocks at temperatures below ~125°C and consequently for examining the thermo-tectonic development of the upper few kilometres of the Earth's crust. As a result AFTA can be used to date major periods of denudation directly. The application of apatite fission-track analysis to rocks collected from a range of elevations along the western continental margin of southern Africa indicates that the present land surface exposes rocks that have cooled rapidly from temperatures above ~125°C to temperatures below ~60°C during the early Cretaceous. This rapid cooling of the upper crust is interpreted as the result of accelerated erosion and consequent uplift of the rock column associated with the early development of the continental margin. The timing of this episode of denudation is broadly synchronous with the break-up of

201 West Gondwana and correlates with the pattern of sedimentation derivedfromborehole data in the adjacent offshore basin. The ~125°C palaeo-isothermal surface for the preuplift crust is recorded by the apatite age-elevation profiles as a distinct break in slope (Fig. 1). The age at which this break occurs is approximately the time at which cooling began and the slope of the age-elevation profile below the break is proportional to the rate of cooling and can therefore be used to estimate the denudation rate. The present day elevation at which this break occurs allows an estimate of the amount of denudation to be made. For reasonable estimates of the palaeo-geothermal gradient some kilometers of denudation must have taken place (Fig. 2). For a palaeo-

geothermal gradient of 30°C/km the calculated amount of denudation is 3 km. In addition, the presently available AFTA data from southern Africa indicates that early Cretaceous denudation of several kilometers occurred both seaward and landward of the prominent escarpment. These data do not support the view that the escarpment represents a boundary between the interior regions characterised by moderate to low rates of dedudation from the seaward areas characterised by much higher denudation rates. An altenative view proposed here is that the present escarpment is a relict feature remaining after the early Cretaceous phase of erosion with the regional landform being predominantly lithologically controlled by the abundant dolerite sills that underly the interior plateau.

Apatite Age (Ma) 100 200 I I 1 1 | I I I I I _ • Apatite age • Mean track - ° length

s 0 o

borehole surface

s U 0) PC 1

— sea level

2 a. a

o0)

2 a

180 160 140 120 100 80 60 40 20

"3 )

PAZ

1 2 3 4 5 6 7 8 9

ML L l- ILJ_L. WMLJL 0

Mean Track Length (|im) Figure 1

Required denudation (km) Figure 2

202

A12: Magmas and Fluids in the Subduction Environment Convenors: D.H. Green and A J. Crawford

A12 Keynote Address 1 THE ROLE OF FLUIDS IN ARC-RELATED MAGMAS IN THE SW PACIFIC: SOME NEWS AND REVIEWS Michael Perfit Department of Geology, University of Florida

The chemical characteristics of most island arc mag- submarine lavas that are chemically transitional between mas and some back-arc basin magmas have commonly MORB and arc tholeiites have been erupted along the been ascribed to enrichment of a MORB-like mantle by ocean-arc boundary south of the Solomon Islands. fluids/melts derived from actively subducting lithosphere Most volatiles in basalts from the Woodlark Basin or fossil slabs. In some arcs however, these charac- are typical of N-type MORB but CI abundances are sigteristics have been ascribed to variable extents of melting nificantly enriched (Muenow et al., 1990). The extreme of a plum pudding-type mantle enriched with OIB vesicularity of Solomon arc lavas (including submarine components (e.g. Morris & Hart, 1983; Gill, 1984). eruptives) precludes determination of primary volatile During the past decade little headway has been made abundances. In the N.Fiji/Lau basins most of the basalts towards either identifying the components of arc magmas have MORB-like volatile abundances but submarine arc or the processes related to arc magma genesis. Recently, tholeiite and transitional basalt samples are more hydrous the measurement of volatiles (including rare gases), (Aggrey et al., 1989). Glasses, rocks and mineral separstable isotopes, boron and Be in arc lavas have provided ates from the Woodlark/Solomon region have He/*He some new insights into the pedogenesis of subduction ratios from 1 to 9 times the atmospheric ration (Ra). zone magmas. Pertinent data from south western Pacific Values from the spreading centre are typical of MORB arcs together with new analytical results provide (8.2-9.3 Ra) but more radiogenic (6.9 ± 0.2) in arc tholeiite samples (Trull et al., 1990). Very low He/*He additional constraints on our current models. Geochemical characteristics of late Cenozoic ratios (0.1-5 Ra) in many of the arc rocks are due to volcanic rocks from southwest Pacific island arcs define atmospheric and radiogenic contamination after eruption. at least four different rock associations that range from Although MORB-type mantle volatiles appear to dominlow-K arc tholeiites to medium-K calcalkaline rocks to ate, correlations between He, O and Sr isotopes suggest high-K silica-saturated and undersaturated varieties. slight influence of a subduction-related fluid or silicate Alkalic rocks exist in Fiji, Vanuatu, the Solomon Islands melt. New measurements of Be and B in young arc lavas and Bougainville Island and are particularly abundant in mainland Papua New Guinea and the Tabar-to-Feni have shown that they are excellent tracers of sedimentary Islands. Samples of high-K basalt and unique Na-Ti- components and the involvement of a fluid phase. Lavas basalt have been recovered from the New Ireland and from the Bismarck arc have high Be/B and B/Be relative Woodlark basins. Chemically, most of the alkalic rocks to MORB and ODB and form an array that requires have arc rather than OIB signatures (e.g. low Ti0 , mixing of a mantle component with a "fluid" derived HFS, Th/U and high LIL, Zr/Nb, Ba/La). Light REE are from sediment devolatilisation or mixing of altered slab moderately to highly fractionated relative to heavy- and sediments (Morris et al., 1990). Calculations suggest REE but there is no marked depletion in HREE as in less than 10% slab or sediment-derived fluid with high many intraplate alkalic volcanics. Sr and Nd isotopic B/Be must be added to the mantle source in order to values range from around 0.7033 and +8.7 (eNd) in arc generate the Bismarck array. This is in agreement with tholeiites to 0.7052 and +2.0 in high-K lavas plotting to calculations based on Sr- Nd systematics but not He and the right of the "mantle array". Pb isotopes are similar Pb. At present our models still rely on metasomatising to other western Pacific arcs with only slight enrichments a MORB-like mantle but we are severely limited by a in ^ P b / ^ b relative to ^ b / ^ P b . Fairly systematic lack of information on the partitioning of volatile increases in LILE and LREE correlate with increasing elements between solids, silicate melts and fluids. Al^Sr/^Sr, but each arc exhibits a moderate degree of though the newest results continue to support some type heterogeneity. Typical N-type MORB have been re- of fluid control on arc magma genesis the composition covered from the Woodlark Basin spreading centre and and derivation of this phase continues to be enigmatic. 3

A12.1 THE ORIGIN OF GEOCHEMICAL VARIATION IN PRIMARY MAGMAS OF THE VANUATU ARC

203

Stephen Eggins Department of Geology, University of Tasmania

The eruption of primitive Mg-rich lavas throughout of HFSE relative to LREE, which accompany increasing the Vanuatu Arc make it a unique example among mod- incompatible element abundance levels, require that em island arcs for the study of primitive arc magma progressive melting-out of residual HFSE and LILE genesis. The Vanuatu primitive lavas are characterised bearing phases (amphibole?) occurs as melting proceeds. by phenocryst assemblages which comprise magnesian Both existing experimental studies concerning the olivine (Mg# 80-94), magnesian clinopyroxene (Mg# stability of titanate phases in arc magma source regions = 80-94), Cr-rich spinel (Cr# -60-90), and sometimes and the high liquidus temperatures of Vanuatu Arc also calcic plagioclase (An 70-95). Primary magma primary magma compositions, however, preclude the composition estimates, which have been obtained for existence of such phases during segregation of primary each of the major volcanic centres in the arc, range from melts from the Vanuatu Arc mantle wedge. low-K tholeiitic to high-K alkaline picrites. Their Alternatively, the observed trace element systematics comparison with peridotite melting studies, reveals a could arise from competing enrichment of the Vanuatu range of conditions of melt segregation which vary Arc mantle source by two separate agents. One agent, from -4 GPa for high-K compositions to ~2 GPa for probably an aqueous fluid phase, is responsible for the low-K compositions. enrichment of LILE found in low-K tholeiites. The The fractionation-corrected trace element geo- other agent, which enriches LILE and LREE equally chemistry of the Vanuatu primary magma compositions over HFSE, dominates the source of high-K lavas and follows systematic trends which can be interpreted in has the chemical characteristics expected of an terms of variable degrees of melting of a single source. incompatible element-rich melt phase which has equilInversion of the primary magma data set, and the ibrated with a residual titanate phase. The overall comparison of results with the mantle source of N- depletion of HFSE and HREE in the Vanuatu Arc source MORB basalts erupted in the North Fiji Basin, indicates is best explained by the incorporation of geochemicallythat the Vanuatu Arc mantle is enriched in LILE, but depleted (refractory) mantle, that is residual after melt depleted in both HFSE and HREE relative to the source extraction beneath the North Fiji basin, into the sub-arc of N-MORB. Within the framework of variable melting mantle wedge. modelled, the systematic increase in LILE and decrease M22 THE APPLICATION OF NOBLE GAS GEOCHEMISTRY TO THE GENESIS AND EVOLUTION OF ARC MAGMAS Des Patterson Research School of Earth Sciences, Australian National University

The noble gases possess a unique geochemistry, characterised by chemical inactivity, low abundance, and extreme incompatibility. In addition, a useful number of noble gas isotopes are the stable products of radioactive decay (notably He and Ar from the decay of U+Th and ^K respectively). These attributes result in the noble gases being potentially powerful geochemical tracers of the movement, and behaviour, of volatiles during the genesis and evolution of magma. The application of noble gases to questions of volatile behaviour in subduction environments, to date has been mainly restricted to the measurements of He, and to a lesser extent Ar, isotopic ratios in the groundwaters and fumarolic gases of subduction related hydrothermal systems (e.g. Baskov et al, 1973; Nagao et al, 1981; Sano & Wakita, 1985: Hulston et al, 1986; Sano et al, 1987; Poreda & Craig, 1989). These, and other workers 4

have identified a widespread mantle derived excess of He with respect to He compared with atmosphere CHe/ He of hydrothermal fluids are approximately six times higher than the atmospheric ratio of 1.4 x 10^). The mantle derived He isotopic ratio observed in geothermal fluids survives gross dilution with meteoric groundwater owing to the extremely low abundance of He in the terrestrial atmosphere, a function of the continuous loss of He from the atmosphere to space. To fully utilise the potential of the noble gases in studying subduction related magmatism, it is necessary to circumvent this problem of atmospheric contamination. This is particularly true for Ne, Kr, and Xe which show relatively small variations in isotopic composition in terrestrial samples. At the Research School of Earth Sciences, ANU, a research project is presently under way to measure the composition of He, 3

204 Ne, AT, Kr, and Xe trapped in primitive mafic phenocrysts (olivine and clinopyroxene) extracted from arc lavas. It is hoped that such primitive phenocrysts may have grown and trapped magmatic, mantle-derived noble gases, prior to the host magma exchanging volatiles with the atmosphere. Using this approach the relative roles of the mantle wedge, subducted oceanic lithosphene (and possible sediments), and arc crust, might be evaluated without the complication of gross atmospheric

contamination that arises in hydrothermal fluids. References Baskov et al., 1973. Geocham. Int. 10: 130-138. Hulston et al., 1986. Proc. 8th Geotherm. Workshop, Uni. Auckland Geotherm Inst. Nagao et al., 1981. Earth Plan. Sci. Lett. 53: 175-188. Poreda & Craig., 1989. Nature 388: 473-478. Sano & Wakita., 1985. J. Geophys. Res. 90: 8729-8741 Sano et al., 1987. Geochim. Cosmochim. Acta 51:1855-1860.

A12.3 HELIUM, STRONTIUM AND NEODYMIUM ISOTOPES IN MANTLE XENOLITHS J. Stone, D.R. Porcelli, D. Vance, S.J. Galer and R.K. O'Nions Department of Earth Sciences, University of Cambridge, UK

The isotopic compositions of He, Sr and Nd in ultramafic xenoliths reflect fractionation amongst volatile and lithophile elements in the mantle lithosphere, mediated by melt and fluid transfer. These isotopic systems have been studied in continental and oceanic xenoliths, to place constraints on such "metasomatic" fractionation. Xenolith samples were obtained from young (<1 Ma) continental and ocean island alkali basalt fields worldwide (Porcelli et al., 1986; Vance et al., 1989). They encompass a range of textural and petrographic types, both Cr-diopside- and Al-augite-bearing. Samples were analysed as described in Porcelli et al. (1986) and Vance et al. (1989). Sr and Nd isotopes were measured in hand-picked, acid-leached clinopyroxene separates. He measurements were made on gas liberated from whole-rocks or mineral fractions by crushing or melting in vacuo. Isotopic data are summarised in the figures below. Heavy symbols represent ocean island xenoliths; open symbols, continental xenoliths. Lithophile element (Sr and Nd) isotopic compositions range widely (e.g. continental xenoliths: 0.7027 < 87Sr/ 86 Sr < 0.8360, oceanic xenoliths: 0.7030 < ^Sr/^Sr < 0.7050). Key features of the data are: (1) limited overlap

between xenolith isotopic compositions and those of depleted mantle-derived basalt (MORB): (2) isotopic similarity between Sr and Nd in ocean island xenoliths and lavas forming the islands from which they were obtained: and (3) Sr and Nd isotopic compositions unsupported by their respective parent/daughter ratios. As suggested by previous studies (e.g. Menzies & Hawkesworth, 1987) these features indicate recent "metasomatic" fractionation amongst Rb, Sr, Sm and Nd. In most cases, Sr and Nd acquired in this process cannot have been derived from the depleted (MORBsource) mantle. Helium isotopic compositions record a contrasting metasomatic history. These cover a narrow range (continental samples: 6.2Rt < 3He/*He < 10.6Ra, oceanic samples: 5.2Ra < 3He/4He < 9.5Ra, where Ra denotes the atmospheric ratio 3He/*He = 1 . 4 X 1 0 " 6 ) , coincident with that of MORB basalts (6-10R a ). Neither 3He-rich compositions typical of hotspot ocean islands, nor 4Herich compositions indicative of prolonged radiogenic He accumulation are observed. An important result is that comparative crushing and melting release experiments suggest that sample He resides largely in (C02-rich) fluid inclusions.

36 ' R/Ri

! R/Ra

OHawaii

Iceland

<o Hawaii Iceland

18 MORB

: 17121

18360 ^

C^W . MtSt

17141

17151

17MI 87Sr&6Sr

17171

MORB

05122

•

••Dpgtf

)

1 • . . . . . i . . . 1 . . . 1 . . . 1 . . 05124 05126 05128 05130 0J132 05134 143Nd/144Nd

205

These systematics demand: (1) separate sources for appears unlikely in the 10-30 kbar pressure region where volatile and lithophile components added to the amphibole stability should buffer fluid compositions xenoliths, hence; and (2) a mechanism to account for towards pure C0 . volatile transfer independent of lithophile element These data suggest the existence of freefluidsin the metasomatism. Decoupling of He from Sr and Nd must lithosphere of alkaline volcanic provinces, which may be controlled by addition of a depleted-mantle-derived be sampled by magmas originating there. It is therefore component; essentially a He carrier of high He/Sr and noteworthy that lavas from Iceland (Condomines, 1983). He/Nd. Silicate melt is an unlikely carrier. Production and Hawaii (e.g. Staudigel et al., 1984.) show a wide He of a He-rich, Nd,Sr-poor liquid would require melting isotopic range at essentially constant ^Sr/^Sr. This to a degree less than the bulk distribution coefficient of variation may be driven by addition of Sr-poor fluid, Nd, effectively «1%, and assumes highly incompatible decoupling He and Sr isotopic compositions. If so, it partitioning behaviour for He. Melt separation and flow follows that "component" mixing models which describe would be unlikely at such a melt fraction. Sr and Nd in mantle magmas cannot be extended to Rather, the observed partitioning of He into C0 - determining volatile fractionation histories of mantle rich inclusions suggests that it is carried by a free fluid components. phase, and that decoupling of lithophile and volatile References isotope systems reflects inefficient transport of Sr and Condomines, M. et al., 1983. Earth Planet. Sci. Lett. 66: 125-136. Nd by this fluid. This conclusion is in accord with recent experimental work, indicating low Nd solubility Meen, J.K. et al., 1989. Nature 340: 301-303. in C0 -rich fluid at mantle P and T (Meen, 1989). Porcelli, D.R. et al., 1986. Chem. Geol. 54: 237-249. in Menzies, M.A. & Hawkesworth, C.J. (eds): Mantle These constraints suggest that lithophile metasomatic Refs.Metasomatism. Press, 1987. effects in xenoliths cannot be attributed to interaction Staudigel, H. et al., Academic 1984. Earth Planet. Sci. Lett. 69: 13-29. with C0 -rich fluids. Small-degree partial melts, or Vance, D. et al., 1989. Earth Planet. Sci. Lett., in press. hydrousfluidscapable of dissolving substantial silicate material may instead be responsible, though the latter 2

A12.4 TRACE ELEMENT AND "Sr/^Sr RATIOS IN LAVAS OF SLAMET VOLCANO, JAVA: CONSTRAINTS ON THE MANTLE WEDGE AND "SLAB-DERIVED" COMPONENTS Danilo Vukadinovic and Ian A. Nicholls* Department of Earth Sciences, Monash University

On the basis of stratigraphy and trace element and strontium isotope systematics, Slamet basalt flows can be broadly grouped as the products of high abundance and low abundance magmas (HAM and LAM, respectively). By assuming that Nb and Hf in magma sources within the mantle wedge are unaffected during metasomatism from the subducted lithosphere, a simple model for magma genesis may be developed which requires variable melting of slightly enriched MORB source mantle metasomatised by fluids. This model avoids invoking highly enriched mantle source materials. By examining immobile/mobile incompatible trace element ratios (IMITER; e.g. Nb/U, Zr/K) deductions regarding the nature of the slab-derived metasomatising

agent may be made. Available evidence suggests this agent is a fluid rather than a melt. Fluid compositions were constrained using published mineral-fluid trace element partition coefficients, experimentally determined element mobilities, and many assumptions (e.g. IMITER - 0). The results suggest that HAM sources were metasomatised to a lesser extent, and consequently underwent smaller degrees of partial melting, relative to LAM sources. The IMITER ratio Nb/U has a near-constant value (-47) in oceanic basalts (MORB and OIB). Mixtures of subductionfluidand mantle wedge of diverse Nb content (but uniform Nb/U) can account for the observed range of values for arc basalts. fluid

206

A12.5 SYSTEMATIC COMPOSITIONAL VARIATIONS IN EASTERN SUNDA MAGMATISM: IS SEDIMENT INVOLVEMENT REALLY NECESSARY? R. Varne Geology Department, University of Tasmania

When mafic arc volcanics are compared to non- variation with K/Nb values remaining unchanged while orogenic volcanics with similar K-group element 1^0 contents increase from about 1.0 wt % in basalts to contents, arc volcanics are characteristically poorer in 6.0 wt % in leucitites, whereas La/Nb values rise slightly, Ti-group elements, particularly Nb, and also tend to be Nb/Sr values vary slightly, and K/Rb values decrease. poorer in LREE. These general differences are popularly Increase in K-group element contents correlates well ascribed to the involvement of materialfromsubducted with increasing ^Sr/^Sr and decreasing ^Nd/^Nd sediment and oceanic crust in the sources of arc values. The isotopic and geochemical systematics of eastern volcanics. It has been widely argued that because the mantle Sunda volcanics are compatible with a source composed wedge beneath an arc might be composed of peridotite of mixtures of two principal components, Indian Ocean which has already been through a cycle of partial melting MORB-type mantle and a component enriched in Kand melt extraction, perhaps at a mid-oceanic spreading group elements, which has been variously proposed to centre, then the low concentrations of Ti-group elements, be: (1) subducted sediment and, or, altered ocean crust; particularly Nb and Ta, in arc volcanics might merely (2) subcontinental mantle involved in the arc/continent reflect the low abundances prevailing in the subarc collision zone; (3) a Dupal-like OIB mantle component, mantle. Alternatively, it has been suggested that the either being brought into the arc during subduction, or source materials of arc volcanics may not necessarily already present in the region before subduction initiated be poorer in Ti-group elements than those of non- the eastern Sunda arc in mid-Tertiary times, or (4) orogenic mafic volcanics, but that the source mineralogy metasomatic material from the deep mantle, tapped by might include refractory phases which retain Ti-group the descending slab. The first explanation has been tested by collecting elements during melting. Neither of these explanations of the low Ti-group compositional and isotopic data for sedimentsfromthe element abundances is also capable of accounting for northeast Indian Ocean, south of the Java trench. Sr, the wide variations in K-group element concentrations Nd and Sr isotopic data for the sediments limits to a few per cent at most the possible extent of their involvement displayed by arc volcanicity. Particular difficulties arise if the downgoing slab is in the sources of the eastern Sunda magmatism. composed essentially of ocean-floor basalt poor in K- Geochemical data show that this amount of material is group elements, together with peridotite which, like the insufficient for it to be the source of the LELE-enriched peridotite of the mantle wedge overlying the slab, is component. Analytical data are currently being collected believed to have already undergone melt extraction and for altered oceanic crust Of the other possibilities, (3) is favoured by the depletion in 'incompatible' trace elements. It then becomes necessary to postulate a separate source for recent discovery that early Tertiary alkaline lavas from the K-group element contents of arc volcanics, such as Christmas Island south of Java, geochemically typical continent-derived sediment, altered volcanics in the sub- of OIB, have Dupal-like Sr, Nd and Pb isotopic comducting slab, or a K-rich mantle component, and then to positions which are extremely similar to those of modem invent a satisfactory mechanism for transporting the K-rich lavas from volcanoes in the eastern Sunda arc. It is a tenet of modern isotopic geochemistry that mafic material to the site of the magmatic activity. The eastern Sunda arc of Indonesia is an excellent volcanics with closely similar Sr, Nd and Pb isotopic locality for assessing the extent and importance of the compositions share similar mantle sources. This observinvolvement of sediment and/or altered ocean crust in ation shows that the isotopic signature of eastern Sunda convergent margin magmatism because the young arc magmatism was already present in OIB-type mantle volcanics range from K-poor basalt to K-rich leucitite, from the region before the arc was formed, and is the widest compositional variation known from any therefore highly unlikely to have been generated by the mixing of recently-subducted sediment into subarc active volcanic island arc. When eastern Sunda mafic leucitites are compared mantle. He, Be and Th isotopic data are compatible at similar MgO contents and (Mg/Mg+Fe) values with with this conclusion. eastern Sunda basalts and trachybasalts, the progression Therefore, although plate tectonic theory requires from hy- and Q-normative basalts to ne- and lc-normative that oceanic lithosphere is subducted back into the leucitites is not marked by significant enrichment in mantle, and despite the general acceptance by penoloTi0 and Na^, despite the substantial increase (x 6) in gists and geochemists that material from the subducting K^O contents; the volcanics retain their typically arc- slab is being incorporated in the sources of volcanic like geochemical pattern despite the compositional island arc magmatism, it seems that involvement of 2

207

subducted sediment merely creates "noise" in the distinguish mafic volcanics of the eastern Sunda arc compositional systematics of eastern Sunda magmatism. from non-orogenic igneous rocks of the region must be The causes of the geochemical characteristics which sought elsewhere.

A12.6 EVIDENCE FOR A SUBCONTINENTAL MANTLE SOURCE FOR K-ENRICHMENT IN SUNDA ARC LAVAS FROM *»U SERIES RADIONUCLIDES G.E. Wheller * , R. Varne and A.J. Stolz 1 2

Department of Geology, University of Tasmania CSIRO Division of Exploration Geoscience, North Ryde, NSW 1

Systematic variations among activities of shortlived of the recent U enrichment observed in some island arcs U and Th radionuclides in young volcanic rocks and attributed to the involvement of fluids derived from constitute an unique technique for studying the sources the subducting slab. Indeed, a significant feature of the of modern magmas. This technique is based on the Sunda lavas is their low ( Th/ Th) values compared decay of U to Th and the possibility that radioactive with those of virtually all other volcanic rocks so far disequilibrium between these two nuclides may occur analysed for ^U series radionuclides. In particular, lavas as a result of chemical fractionations between U and from Pacific island arcs have significantly higher (^Th/ Th. Because the half-life of Th is only 75,200 years, Th) values, as do Pacific and Atlantic MORB. fractionations up to about 350,000 years old may be K values, integrated over the age of the earth, were detected. In addition, Th/U values inferred from ( Th/ calculated using a compilation of new and published Pb Th) values (denoted K ^ can be used to characterise isotope data from Sunda arc lavas. Despite representing the sources of basaltic lavas. Important information magmatic compositions ranging from arc tholeiite to about the timing and mechanism of chemical enrichment leucitite, the K values are remarkably uniform, events in the mantle can also be gained by comparing averaging 4.1. This value is closely similar to estimates K^ with time-integrated Th/U values calculated from of the present day Th/U of the bulk earth. Unlike MORB Pb isotope ratios (denoted K^). and OIB and lavas erupted in other island arcs, which To help constrain the origins of K-enrichment among show K^ < K the Sunda arc K^ values cluster closely Sunda arc lavas, we present new U, ^U, Th, Th about the average K^ value. In this respect, Sunda arc and Th activity data measured by isotope dilution lavas are most similar to some continental ultrapotassic alpha spectrometry from historical and other young suites which show K^ > K^. lavas from ten Quaternary volcanoes in the Sunda island The characteristic K^ = K (or K^ < K as in other arc, Indonesia. This arc, together with the Banda arc in island arcs) cannot be explained by incorporation of the east with which it is laterally continuous, is about subducted pelagic sediments. That process would give 4700 km long and possesses the widest spectrum of K^ > K due to the preferential accumulation of U in lava compositions known from any modern orogenic seawater during continental weathering. Instead, we setting, ranging from K-rich leucitite to K-poor arc propose that Th, U and Pb in the Sunda sub-arc mantle tholeiite flows. have behaved congruently for most of the time since the On a ( Th/ Th- U/ Th) correlation diagram the earth formed because of their similar and highly Sunda arc samples generally cluster about the equiline incompatible nature during mantle metasomatism by within two standard deviations, indicating that Th and low-degree silicate melts. Consequently, we suggest U are in secular equilibrium. This relationship indicates that K-enrichment in the Sunda subarc mantle primarily that no Th/U fractionation occurred in the magmas occurred either directly by a process similar to that by these samples represent, as a result of processes such as which subcontinental mantle is formed or indirectly by melting, fractional crystallisation or assimilation, within incorporation of subcontinental mantle material from the last 350,000 years. In particular, there is no evidence beneath nearby Precambrian continents. 230

238

232

230

232

230

232

23S

232

230

228

230

232

238

232

230

238

208

A12.7 ALONG-STRIKE GEOCHEMICAL VARIATION IN THE TAIWAN-LUZON ARC Ulrich Knittel , Marc Defant and Rene Maury 1

Department of Geology, University of Melbourne Department of Geology, University of South Florida, Tampa, USA Laboratoire de Petrologie et URA 699, Universite de Bretagne Occidental, Brest, France 1

The Luzon Arc, as defined by Defant et al. (1989), extends for about 900 km from eastern Taiwan to the island of Mindoro in the central Philippines. In this area late Tertiary to Recent volcanism is considered to be the result of eastward subduction along the Manila Trench. The Luzon arc is composed of five segments, all of which have individual characteristics with respect to tectonic setting and magma chemistry. From north to south these segments are: (1) the Taiwan Segment, comprising the eastern Coast Ranges of Taiwan and the small islands Lanhsu and Lutao; (2) the Babuyan Segment, which includes all the small islands located between Taiwan and Luzon (excluding Lanhsu and Lutao); (3) the Northern Luzon Segment, which extends along the Cordillera Central of northwestern Luzon as far south as the Lingayen-Dingalan Fault; (4) the Bataan Segment; and (5) the Mindoro segment. The latter segment is separated from the Bataan segment by the Macolod Corridor, which Defant et al. (1988) suggest to be a 'pull-apart' zone.Sr-isotopic compositions of Miocene to Recent volcanic rocks erupted in the Taiwan-Luzon arc show remarkable along-strike variation. ^/Sr/^Sr ratios in volcanics from Lutao and Lanshu are 0.7048-0.7052. In the Babuyan Segment ratios are 0.7036-0.7040 for rocks older than 2 Ma and 0.7044-0.7048 for younger lavas. Within the Cordillera Central segment of the arc, ^Sr/^Sr values drop to about 0.7035-0.7037, which makes these rocks indistinguishable from pre-Miocene plutonic rocks with

regard to their Sr isotopic composition. South of the Cordillera, in the Bataan segment, ^Sr/^Sr ratios rise to 0.7042-0.7049 and, further south, in Mindoro and Marinduque to 0.7051-0.7054. Within the Macolod Corridor, which separates the Bataan and Mindoro segments, ratios intermediate between those of these segments are observed (Sr-isotope data from Lan et al., 1986; Knittel & Defant, 1988; Defant et al., 1989 and references therein, plus unpublished data of the authors). Volcanics with the most radiogenic Sr-isotopic compositions also show the highest degrees of LELE and LREE enrichment. The northern and southern parts of the TaiwanLuzon arc, where volcanics have the most radiogenic Sr, are collision zones where the arc collided with continental Asia. We conclude that during collision material from the continental plate has been incorporated into the sources of the arc magmas (either continent-derived sediments or crustal slivers). The change in isotopic composition in the Babuyan Segment may indicate the arrival of such material in the source region. References

Defant, M.J., Jaques, D., Maury, R.C. & De Boer, J.Z., 1989. Bull. Geol. Soc. Am. Knittel, U. & Defant, M.J., 1988. Earth Planet. Sci.Lett.: 87: 87-99. Lan, C.-Y, Shen, J. J.-S. & Lee, T., 1986. Bull. Inst. Earth Sci., Acad. Sinica, 6: 211-226.

A12.8 PARTIAL MELTING OF SUB-ARC LITHOSPHERE: A SOURCE OF POTASSIC MELTS AND GRANULITE RESIDUES? John Foden Department of Geology and Geophysics, University of Adelaide

The eastern Sunda-Banda arc systems of southern 1. Thermal ablation of the lithosphere and lower crust Indonesia are the sites of complex lithospheric plate due to rising geotherms and upwelling asthenosphere. interactions, with the collision of three major plates and This will be the result of melting lower in the mantle the production of many minor ones. This is also the site wedge catalysed by water released from the slab. of voluminous magma production. In this study the 2. Isostatic compensation of lithosphere whose crustal tendency for the lower crust and lithospheric mantle to portion has been rapidly thickened by magmatic respond to various observed or inferred geodynamic addition from the asthenosphere will generate melts processes by partially melting is analysed. The residues in the gradually depressing lower section as of these melting processes can often be considered as subsidence causes migration up the geothermal mafic granulite in a broad sense of the term. gradient. This may be associated with the tendency for the arc to split and begin extension along the axis There are several possible reasons for the generation of the volcanic chain where thickening is greatest of melts in the upper mantle and lower crust:

209

and where maximum thermal "weakening" has taken Several active volcanoes including Tambora and place. Sangeang Api in eastern Sumbawa have a past and 3. Local concentration of more readily fusible composi- present history of eruption of mafic and ultramafic xenoliths. These volcanoes are commonly potassic in tions in the magmatically thickened lithosphere. The important question is then to what extent do composition. Their xenoliths are dunite, olivine these geodynamically controlled processes dictate the pyroxenite, hornblende pyroxenite, phlogopite distinctive geochemical characteristics of arc magmas pyroxenite and hornblende gabbro. Textures range from cumulate to "granulitic" and cataclastic. And in most (eg K-enrichment, Ti-depletion etc ?). The Indonesian arc is undergoing active covergence cases the addition of the mica and /or amphibole is a with the Indian Ocean-Australian plate in a roughly second stage event. Bulk chemistry of these xenoliths north-south direction. At the same time the active suggests they are mixtures of melts and either the volcanic arc is undergoing east-west extension associated cumulates or, partial melt residues of original mafic with which are the development of mainly NNE cross- rocks or magmas. Some of those with granulitic textures arc left-lateral wrench faults. On several of the islands yield temperatures as low as about 900°C. Some groups of the active volcanic arc, these faults are associated of the xenoliths yield Rb-Sr "isochrons" reflecting Late with isostatically controlled, east-west rotation of Mesozoic and Tertiary ages. This is taken to indicate segments, with uplift of the western ends and depression that the xenoliths are fragments of upper mantle or to the east These fault systems, particularly on Flores crustal lithosphere whose present-day composition is a appear to control the occurence of volcano lines. reflection of episodic phases of major arc magmatism Particularly between Sumbawa and Flores, this series and of one or more periods of rifting and back-arc of faults creates a major offset of the active volcano extension. Indications of Miocene ages in the the xenolith line, with opposite senses of younging of volcanoes on data for instance, reflect a period of widespread either side of the fault. These factors suggest that the magmatism throughout the Indonesian arcs and may faults bottom-out on a sub-horizontal detachment reflect changing rates and geometry of subduction surface, which is also a significant magma accumulation following the collision of India and Asia and cessation site (possibly the Moho). These cross arc faults may be of activity at the Ninety-East Ridge. Observed partial melting of the amphibole and/or associated with localised, along-arc peaks in potassium mica components of some of the xenoliths has yielded enrichment in magmas. The arc is bound by normal fault systems, both to very alkaline interstitial glass. On the premise that this the east and south as well as locally northwards. Here is equivalent to the potassic end-member of the mixing the back arc basin is disrupted by south-dipping, back- array that commonly characterises arc magmas, this arc thrust faults. Some seismic data indicates that work promotes the conclusion that the chemistry of arc although the most distal portions of the forearc region magmas is strongly influenced by the re-melting of (with respect to the volcanic arc) closest to the trench, lithophile element-enriched arc lithosphere. The linkage are in compression and are undergoing deformation and between this melting and arc geodynamic characteristics uplift, the forearc basins north of this zone are for the described above is analysed. most part actively sinking. A12 Keynote Address 2 MIGRATION OF FLUID AND GENERATION OF BASALT MAGMAS IN SUBDUCTION ZONES Y. Tatsumi Department of Geology, University of Tasmania

A model for magma genesis in subduction zones is proposed.This attempts to explain the general characteristics of arc magmatism listed below. (1) Width of volcanic-arc and forearc is inversely proportional to the angle of slab subduction. The volcanicfront,the trenchward boundary of a volcanic arc, lies 110 km above the subducted slab, and the backarc side volcanoes occur around 200 km above the slab. This observation suggests that production of arc magmas is governed by pressure-dependent reactions rather than temperature-dependent processes such as slab melting. (2) Alkaline contents in volcanic rocks increase and

number of volcanoes decreases toward the backarc side of a volcanic arc. (3) Boninite or high-Mg andesite magmas may be generated in near-trench regions of some arc-trench systems associated with backarc opening. (4) Subduction-related volcanics are enriched in LIL elements relative to MORB. Decomposition of hydrous phases may control the pressure-dependent magma production. However, slabderived fluid cannot directly trigger melting of wedge peridotites, because the subducted slab is almost anhydrous beneath a volcanic arc. Fluids released from a slab react with mantle wedge materials to form hydrous

210 peridotite beneath the forearc region. The hydrous peridotite must be dragged downward against the slab and dehydrates at three different levels; at around 600°C by decomposition of serpentine, at 3.5 GPa by amphibole and chlorite and at 6-7 GPa by phlogopite. Consequently, three hydrous columns form in the mantle wedge. In subduction zones, the solidus temperature of hydrous peridotite cannot normally be attained in the forearc region, therefore two volcanic chains are produced through pressure-dependent dehydration reactions. On the other hand, under anomalously high temperature conditions beneath the arc and the forearc, which may be caused by the injection of hot asthenosphere into the mantle wedge during the backarc opening, the trenchward hydrous column may melt to produce highMg andesite magmas. The partially molten region expands upwards through the continuous addition of fluid; the degree of partial melting becomes larger at shallower levels within the partially molten column. Mantle diapirs are derived

from the top of the molten column. Larger amounts of fluid may be supplied to the partially molten column beneath the volcanic front, because the total amount of H^O held in amphibole and chlorite is larger than that in phlogopite in down-dragged hydrous peridotite. This may be responsible for the across-arc variaton of number of volcanoes in an volcanic arc. Mantle diapirs stop rising to release magmas at shallower levels beneath the volcanic front, resulting in higher degree of partial melting. This difference in the depth of magma segregation causes across-arc variation of alkaline contents in arc magmas. Selective transportation of LIL elements through dehydration processes produces magma source regions enriched in such elements. High pressure experiments have comfirmed that LIL elements are mobile both through decomposition of hydrous rocks (taking place in the subducted slab and the down-dragged hydrous peridotite layer) and through devolatilisation of hydrous silicate melts (in a partially molten column).

211

A12.9 BONINITE PETROGENESIS AND SHALLOW MANTLE WEDGE FLUIDS Anthony J. Crawford Geology Department, University of Tasmania

Boninitic lavas have been divided into high-Ca (Ca0/Al203>0.75 and <56% Si02) and low-Ca types. High-Ca boninite, exemplified by the Upper Pillow Lavas of the Troodos Ophiolite and suites dredged from the North Tongan forearc, generally contain phenocrystal olivine, and they may also crystallise both high-Ca and low-Ca pyroxenes as phenocrysts and microphenocrysts; their high-CaO nature precludes clinoenstatite crystallisation. Low-Ca boninites show an extensive compositional range, controlled in part by of the source peridotite composition (degree of depletion during earlier melting events, and extent of subsequent metasomatic enrichment in the shallow mantle wedge), and part by the inter-related factors of melting temperature, degree of partial melting and amount of hydrous fluids at the melting site. The most extreme boninite compositions are those which have high Si0 2 (usually >58%) and very low CaO contents (<6%) at Mg/(Mg+Fe2+) values >0.65. Such boninites crystallise very magnesian olivine (to Fo^, exceptionally Cr-rich, Al-poor chromite (Cr/ (Cr+Al) > 0.85 and up to 0.97), and low-Ca pyroxenes, including clinoenstatite. The best examples of this highly refractory variety of boninite magma is the suite from Nepoui in New Caledonia, and they are also recorded from Cambrian greenstone belts in Victoria. The best known boninite suites, including those from the Mariana forearc and Cape Vogel (PNG) are low-Ca boninite that fall compositionally intermediate between the extreme low-Ca endmember described above, and highCa boninite suites. The low Ti0 2 and low HREE levels shown by boninitic magmas demand very refractory source peridotite compositions. A concensus exists that boninites are generated at high temperatures (1150-1350°C) shallower than 50 km depth in the

mantle wedge. Key ingredients for boninite generation are a supply of hydrous fluids into refractory shallow wedge peridotite to lower the solidus temperatures and permit partial melting, and a mechanism for the maintenance of these abnormally high requisite temperatures at such shallow levels in the upper mantle. It is suggested that the spectrum of boninitic magmas from initial high-Ca varieties to increasingly low-Ca types can be generated sequentially from supra-subduction zone peridotite by continued influx of slab-derived hydrous fluids, and partial melting of increasingly refractory but increasingly hydrous harzburgite. Although there is little doubt that the source peridotite of low-Ca boninite magmas is more refractory and depleted than that which yielded high-Ca boninite magmas, increasingly refractory boninite magmas (in terms of low Ti0 2 and HREE contents, and lowest Ca0/Al 2 0 3 values) have increasingly high, not progressively decreasing, contents of LILE, and also of Si02, Na 2 0 and Zr. An obvious source of the anomalous amounts of these elements in refractory boninite is the hydrous fluids derived from subducting ocean crust that fluxed the source peridotite to induce partial melting. These samefluidsare called upon in most models of arc basalt genesis. However, in models of arc basalt pedogenesis, these fluids are not considered to carry Si02, Na 2 0 and especially not Zr. Clearly, at P<10-15 kbar, high temperature fluids derived from the slab can dissolve and transport large amounts of Si0 2 and N a ^ ; it is hypothesised that Zr may complex with Na, also to be effectively transported via these fluids. Finally, tectonic scenarios though to be capable of elevating shallow mantle wedge temperatures sufficiently to generate boninite magmas are discussed, with reference to modern boninite suites in the West Pacific and Lower Palaeozoic boninite lavas in foldbelts.

212

A12.10 FLUIDS AS METASOMATISING AGENTS AND TRIGGERS OF MAGMATISM IN SUBDUCTION ZONES Richard J. Arculus Department of Geology and Geophysics, University of New England

There are however, widely differing opinions held Only within the past twenty years or so have we really begun to appreciate the varieties of magma type as to: (a) whether dehydration/ decarbonation reactions that are produced spatially and temporally within island or dehydration melting occurs in the slab; (b) the exact arc systems, and it is likely that more surprises and nature of anyfluidreleased, distance moved and trajectcomplexity are to come. The marked success in terms ory in the slab and wedge and the nature of the dissolved of recovery of deeply submerged portions of the active element load; (c) the sequence of possible hydrationforearc-arc-backarc systems of the western Pacific, on carbonation reactions in the wedge which coupled with Ocean Drilling Program (and predecessors) Legs 59,60 the dynamics and thermal regime within the wedge are and most recently 125 and 126, is providing critical critical in terms of fluid-solid element partitioning; (d) information as to the nature of volcanismfrominception the details of the processes and depths at which magma of an arc to the present, the distribution of that generation and final melt-residue separation takes place. magmatism in a spatial sense and its relationship to With respect to these problems, direct melting of the other features of tectonic and metamorphic evolution. slab or dehydration is a function of the dT/dP trajectory These terrains are the prime Proterozoic-Phanerozoic experienced during subduction. The predominance of continental crust building blocks, and it is critical that low La/Yb, tholeiitic basalt in modern intraoceanic arcs we understand more of their active construction. suggests that a peridotite source is involved rather than Taken together, some of the most interesting results layers 1 or 2 of the slabs. Under conditions where a from these ODP Legs (Geotimes, July and September higher temperature along the upper slab surface is 1989) are the discovery of: encountered at relatively low pressure (-15-25 kbar, (1) a continuity along the full length of the Izu-Bonin- >750°C), dehydration melting of a garnet amphibolite Marianas arcs of a pulse of boninite and related assemblage and production of high La/Yb trondhjemite magmatism at the Middle Eocene inception of the melts is possible. Such conditions may have prevailed system; in Archean subduction zones. (2) a prolonged tensional regime in the forearcs and Presumably fluid is released semicontinuously from intrusion/extrusion of serpentinite along the crest a subducted slab as prograde reactions occur. Some of of the trench walls of these arcs, with active fluid these fluids released at relatively shallow depths venting; (<30 km) appear to be causing serpentinisation of the (3) forearc magmatism (hot) taking place -100 km to overlying wedges and venting directly in the forearcs of the east of the active arcs and adjacent to the (cold) the Marianas-Bonins (Geotimes, July 1989). With a serpentine extrusions; hotter wedge, melting and boninite generation would (4) a persistence of depleted mantle source involvement occur. The pathways of fluids (s.l.) in an advecting, in the genesis of arc-type magmas, but with an corner-flow regime tend to focus to the maximum rate along-arc variability (Bonin - depleted vs. Marianas of change of the azimuth of the flow (Spiegelman & - relatively fertile) and a dichotomy between arc- McKenzie, 1987). However, hydration (exothermic) backarc geochemistry; dehydration (endothermic) cycles during fluid ingress (5) continuous records via ash layers deposited in the with competing buoyancy-advection flow vectors must forearcs of the explosive history and geochemical render the pathways more complex in detail. evolution of the arc systems from the mid-Eocene Production of the characteristic extreme values of to Recent. alkaline earth/high field strength element ratios in arc In petrogenetic terms, these data are vital for unravell- magmas is clearly not related to saturation (during ing the nature and evolution of the source components melting) with sphene or an Fe-Ti oxide phase. Amphiinvolved, and for providing constraints on the spatial bole may be involved at some stage, but the high eruption distribution and replenishment of these sources in the temperatures of some arc magmas (> 1150°C) preclude manle. Within this broadframework,there are numerous amphibole stability duringfinalmelt-residue separation. lines of petrological and geochemical enquiry that require It is emphasised thatfluidsdeparting the slab are unlikely considerably more analytic and experimental study. For to be saturated with an alkaline earth phase, and are example, a majority of penologists accept as a working capable of considerable further leaching (and exchange) hypothesis that t^O- (C0 -halogen) rich fluids are in of Sr and Ba in the wedge. A lack of correlation of Sr some way involved as selective transporting agents of or Ba/La vs ^Sr/^Sr might be anticipated (Arculus & specific elements within subducted slabs and overlying Johnson, 1981). mantle wedges. Many have also argued that ingress of Finally melting obviously occurs where the local ly) (released from the slab) into a wedge is the prime solidus temperature within the wedge is exceeded. The trigger for the initiation of partial melting in the wedge. correlation of major element geochemistry with depth 2

213

to subduction zone (Plank & Langmuir, 1988) suggests prolonged equilibration with the mantle during magma ascent In fact, the overall downward flow of the advecting wedge implies a greater path length through a compacting matrix for arc magmas than is the case for magmas beneath ocean ridges.

A12.ll

References Arculus, R.J. & Johnson, R.W., 1981. Geochem. J. 15: 109-133. Plank, T. & Langmuir, C.H., 1988. Earth Planet. Sci. Lett. 90: 349-370. Spiegelman, M. & McKenzie, D., 1987. Earth Planet. Sci. Lett. 83: 136-152.

FLUIDS IN SUBDUCTION ZONES: EXPERIMENTAL CONSTRAINTS D.H. Green Department of Geology, University of Tasmania

The understanding of magma genesis in convergent margin regions requires a knowledge of the role of volatiles, particularly in the system C-H-O, in controlling melting, the presence or absence of a fluid phase and element transport in fluid and melt phases. Fluids in the system C-H-0 may be oxidised (C0 2 + H 2 0) or reduced (CH4 + H 2 0) and these more extreme conditions are separated by water-rich fluids in equilibrium with solid phase carbon (graphite, diamond). The modern Earth has a highly oxidised atmosphere/hydrosphere dominated by H 2 0, 0 2 and C0 2 or dissolved (C03=),(HC03"), but the Earth's upper mantle is more reduced with elemental carbon, rare SiC, and /0 2 sensitive spinel and ilmenite solid solutions indicating >0, conditions from FMQ/NNO conditions to IW + 1 log unit. Within the framework of a plate-tectonics paradigm for the modern Earth, the subduction process may be seen as carrying oxidised (fo2 > FMQ) crust and uppermost lithosphere (containing Fe 2 0 3 in solid solutions, carbonates and sulphates) into an upper mantle environment of low Fe203, minor sulphides and graphite or diamond. C-H-O fluids in the subducted lithosphere will be dominated by C0 2 + H p whereas those in the upper mantle will be Hp-rich with minor CH4 + C0 2 and may be CH4-rich from deeper mantle degassing. Redox interactions between mantlefluidsand subducted slab will drive fluids towards Hp-rich compositions with precipitation of graphite or diamond. At pressures of 20-50 kb, i.e. the depth range commonly invoked for partial melting and initiation of diapirism in the mande, variation in f02 of less that two orders of magnitude [from MW or IW + 3 log units to IW + 1 log unit] is sufficient to change fluids in equilibrium with graphite and diamond, from C O ^ H p through H 2 0»C0 2 >CH 4 to H 2 0>CH 4 »C0 2 . Melting relations of peridotite CH-0 are extremely sensitive to small variations in f02 because of the very different interaction of the volatiles CH4, H 2 0 and C0 2 with silicate minerals and silicate melts. Experimental studies in the system peridotite-C-HO may be used to model the possible melting behaviour of peridotite in the convergent margin environment If oxygen fugacity conditions are low such that fluids are

CH4 + H 2 0 then the effect of CH4 is to inhibit melting and, at high pressure, to push the peridotite-C-H-0 solidus closer towards the volatile-free solidus, i.e. the high temperature solidus. Because of the compositional relationships in C-H-O fluids at high pressures, there is a small y0 2 range for which the system peridotite-CH-0 behaves essentially as peridotite-Hp. For these conditions we are concerned with the stability of amphibole as a function of bulk composition and pressure and temperature and also the position in P,T,space of the water-saturated solidus. It has been shown that pargasitic amphibole stability is strongly influenced by the peridotite major element composition. Peridotites which we may call "fertile" or "enriched" may crystallise up to 30% pargasitic amphibole, and this pargasitic amphibile is stable to temperatures of around 1150° at pressures of 15-25 kbar. A peridotite which is less enriched in alkali elements and titanium but which is suitable as a source composition for mid-ocean ridge basalts will crystallise pargasitic amphibole to pressures of 29-30 kbar, but the maximum temperature stability is around 1075° in the 15-25 kbar interval. A more refractory composition which is very depleted in titanium, potassium and sodium (Tinaquillo lherzolite composition) also shows pargasitic amphibole stability to pressures of 29-30 kbar but the upper temperature limit for amphibole stability is 1030°. In considering the convergent margin environment, the release of fluids from the subducted slab may result in amphibole crystallisation within the overlying wedge. It is clear tthat the composition of the lherzolite or harzburgite of the overlying ridge will exercise a major role in determining whether amphibole crystallises, the amount of amphibole, the temperature for the solidus and the presence or absence of an Hp-rich fluid phase. Under more oxidising conditions, fluids may consist of H20-C02and the phase relationships for upper mantle lherzolite become complicated by the presence and role of carbonate. Subsolidus carbonate is stable at pressures >18 kb and temperatures below 930°C. The experimental studies for peridotite-C02-H20 define P,T fields for primary carbonatite magma, including a field overlapping the stabilityfieldof pargasite-bearing lherzolite. Under these conditions (930-1080°C, 21-30 kbar) a

214

sodic, dolomitic, carbonatite liquid may coexist with pargasite lherzolite residue. There will be a distinctive separation of minor and trace elements between those which are partitioned strongly into a sodic, dolomitic, carbonatite liquid and those which are retained in a pargasite-bearing lherzolite. This provides a mechanism for enriching and for decoupling of HFSE and LILE, the former tending to remain in refractory residue while the latter are mobile in the highly enriched carbonatitic melt. Experimental studies lead to models of mantle metasomatism by ephemeral, sodic, dolomitic, carbonatite melts which have an important role at pressures in excess of around 20 kbar but are unstable at lower pressures due to the decarbonation reactions. In the convergent margin environment, redox

interaction and release of water (and C0 ) from the subducted slab suggest that f0 conditions in the lherzolite of the subducted slab and in the wedge overlying the Benioff Zone are likely to be nearer MW than IW + 1. It is hypothesised that ephemeral carbonatitic magmas play a key role in at least some of these areas, accounting for the distinctive trace and minor element characteristics of source regions of island arc magmas, including boninite. The carbonatitic magmas would not normally reach the surface in these areas as they would encounter an overlying wedge of refractory to metasomatised lherzolite containing hydrous silicate melt Reaction of carbonatite melt to release C0 and combine with the silicate melt would enhance the distinctive island arc incompatible element fingerprint of the silicate melts. 2

A12.12 EXPERIMENTAL EVIDENCE FOR AN UNUSALLY HYDROUS MANTLE-DERIVED ANDESITE-DACITE MAGMA, NORTHLAND, NEW ZEALAND T.H. Green* and J. Adam School of Earth Sciences, Macquarie University

High-silica, andesite and dacite lavas from Northland, New Zealand sometimes host abundant igenous and metamorphic xenoliths, and high-pressure phenocrysts. The igneous xenoliths appear to be either cognate to the host, or to closely associated magmas. The mineralogy is widely variable, but the most common xenoliths (and phenocrysts) are pargasitic hornblenderich cumulates, with plagioclase and minor pyroxenes. Rare xenoliths containing garnet, and garnet phenocrysts, (mgoi 20-43; mol. % grossular of 18-25) are a unique feature of this I-type volcanic suite. The most common garnet-bearing xenolith type contains hornblende {mg of 55-63, Al* of0.54-0.76) and plagioclase (An 49-82). Garnet phenocrysts may contain hornblende, clinopyroxene or plagioclase inclusions. A high pressure experimental study of a glass prepared from one of the garnet-bearing dacite samples over the pressure range of 8-20 kbar, 800-1050°C and 3-8% by weight of added water defines overall phase relationships for these conditions. Importantly, amphibole only appears at temperatures <900°C and clinopyroxene at >900°C (with 3% H/)). Orthopyroxene occurs with garnet at lower pressure (-<15 kbar with 3% H 0 and -<11 kbar with 5% H 0). Absence of orthopyroxene from the natural garnet-bearing assemblages indicates pressures above these limits. Plagioclase is markedly suppressed (with respect to temperature) with increasing water content, and for pressures of 10-15 kbar, the maximum water content possible with retention of clinopyroxene and plagioclase together (as evident in xenoliths) is 5-6% by weight 2

Finally the lack of quartz in any of the xenoliths suggests water content higher than 3% (where quartz appears with amphibole at 900°C), since the maximum temperature of stability of quartz decreases with increasing water content, and with decreasing pressure. In experiments with 5% H^O a quartz-freefieldof garnetamphibole-plagioclase occurs between 10 and 15 kbar and temperatures between 850 and 900°C. The results also allow formulation of general compositional trends, which assist further in constraining conditions of formation of the xenoliths and phenocrysts: (1) garnet, amphibole mg increase with increasing temperature; (2) amphibole mg decreases with increasing pressure while Al increases; (3) garnet grossular content increases with increasing pressure; and (4) grossular in garnet and Al* in amphibole are near constant at 850-900°C. These composition trends, together with data for specific experiments with 5% HjO added and run at 10-13.5 kbar and 850-900°C, suggest that the natural assemblages formed at these conditions. This implies that the parental magma for the dacite must have been derived at mantle depths (the Northland crust is <30 km thick), and any basaltic or basaltic andesite precursor must have contained >2-3% wt of water. It also implies that the regional geotherm for Northland in the Miocene is considerably cooler than Eastern Australia and the modern day Taupo Zone. w

215

A12.13 FLUIDS AND MAGMAS IN THE TILBA LAKE VOLCANO, SOUTHEAST NSW A.G. Purvis Department of Geology, University of Adelaide

Abundant ^lO km ) volcanic detritus in sediments weakly ne normative like the later lavas. 125-90 Ma old from southern Papua New Guinea to The felsic dykes include: (1) quartz trachyte to dacite Victoria indicates a mid Cretaceous arc >4000 km long with low Cr; (2) dacite and rhyolite with 50-120 ppm adjacent to eastern Australia. Most of this arc appears Cr increasing with Si0 , trachyte to tephriphonolite to have been submerged during the opening of the with 50-70 ppm Cr and, locally, melanite garnet The Tasman and Coral Seas, but a few volcanoes of this age, Cr may have been derived from Ordovician sediments including those constituting the Mount Dromedary but is not accompanied by Ni as in the Mount Dromedary Monzonite (50-120 ppm Cr; 20-50 ppm Ni). Complex, occur within eastern Australia The Tilba Lake volcano is the best preserved in the The trachyte-phonolite transition involves complex. Early lavas (trachybasalt & shoshonite) were enrichment in Na, K, Ca, Fe, Sr, Ba, CI, F, S0 (or S), intruded by magma chambers (pyroxenite, gabbro & C0 and H 0. In some of the dykes this enrichment monzonite). Closed-system fractionation produced apparently post-dated feldspar phenocryst formation but trachyte-based pyroclastics and resurgent domes (quartz predated complete crystallisation. In other dykes the monzonite & quartz syenite) during caldera collapse. enrichment may have been entirely or largely subsolidus Late dykes (pyroxenite, micromonzonite) and dykes and skarn-like in character, with similar enrichments from rhyolite to tephriphonolite (various ages) are affection nearby lavas. Many of the lavas have similar low Ti, A1 pyroxenes present The early lavas show considerable variation only with Cr-rich cores, in groundmasses of varying alkalinity. partly due tofractionation.An up-sequence increase in This may indicate fluid-magma interactions enriching Na, K, P, Ce, Zr, Sr, Ti and A1 is not accompanied by the magmas in Na, K, Sr, P etc., post-dating pyroxene regularly increasing Si0 , but by a change from hy phenocryst formation, but predating plagioclase phenonormative to ne normative lavas. The monzonites are cryst formation, possibly at or near the base of the crust. 6

A12.14 STRONTIUM AND NEODYMIUM ISOTOPIC STUDIES OF THE MOUNT READ VOLCANICS, TASMANIA David J. Whitford , Anthony J. Crawford , Michael J. Korsch and Stephen J. Craven 1

CSIRO Division of Exploration Geoscience, North Ryde Geology Department, University of Tasmania 2

The Cambrian Mount Read Volcanics of western Tasmania are of considerable interest because of their association with significant base and precious metal mineralisation. This belt forms the eastern margin of the Dundas Trough, and borders the Precambrian Tyennan Block. The Volcanics comprise a basaltandesite-cacite-rhyolite suite with abundant interbedded pyroclastic, epiclastic and shale horizons. Rapid facies changes, complex internal stratigraphy and structure, paucity of fossils, variable alteration, low-grade regional metamorphism and poor exposure have frustrated a detailed understanding of their regional geological and tectonic significance. The Mount Read Volcanics have been subdivided stratigraphically into the Central Volcanic Complex, the Western Sequence and the Tyndall Group (Corbett & Lees, 1987). The Central Volcanic Complex is dominated by intermediate to felsic rocks with abundant ignimbrites. The Western Sequence contains

abundant basalts but also includes a range of intermediate to rhyolitic lavas and large quartz-feldspar "porphyrite" bodies. The Tyndall Group is dominated by felsic tuffs and lavas. Three distinct volcanic suites can be recognised on the basis of major and trace element geochemistry: two high-K calcalkaline suites and an unusual shoshonitic association. The volcanic rocks appear to have been erupted in a series of extensional basins following an arc-continent collision. The Murchison and Darwin Granites are chemically similar to the felsic volcanic rocks; detailed intrusive relationships remain unclear. Tholeiitic basalt and dolerite underlie the Western Volcanic Sequence at Miners Ridge. Tholeiitic rocks comprising the "Henty Dyke Swarm" intrude the Central Volcanic Complex. Basic lavas with tholeiitic affinities also occur in the Henty Fault "Wedge", but their relationships with the other tholeiitic suites remains unknown.

216

Basaltic and andesitic rocks from the Central and Western groups have ^ ( T = 500 Ma) values generally within the range +1 to -2. Relic calcic clinopyroxene phenocrysts have similar e (T = 500 Ma) values of +1 to +2. The high-K calc alkaline and shoshonitic suites cannot be distinguished isotopically. The more felsic rocks have lower e (T = 500 Ma) values to -8.5. Samples with the lowest values are from the quartzfeldspar "porphyrite" bodies of the Western Volcanic Sequence. There is a suggestion that the Western and Central Volcanic belts define distinct trends on plots of versus selected major and trace element abundances. The Tyndall Group and the related granites are isotopically similar to the more felsic volcanic rocks from the Central Volcanic Complex. The felsic rocks have old model ages, relative to a depleted model mantle, of up to 1.8Ga.The low values together with the inverse correlations between and major and trace element abundances reflect the involvement of old continental crust in their formation. The tholeiitic rocks are distinctive with e (T = 500 Ma) values ranging from +5.1 ("Henty Dyke Swarm") to +9.2 (Miners Ridge tholeiite). The isotopic composition of Sr is variable with agecorrected ^Sr/^Sr ratios ranging from <0.7 to >0.72. Both the high and low values probably reflect the Devonian regional metamorphism. There is a clustering of ratios from 0.7065-0.7095 that may reflect primary Nd

values. Isotopic compositions of relic calcic clinopyroxene basalt and andesite samples confirm the relatively high primary ^Sr/^Sr ratios. The unusual isotopic character of the Mount Read Volcanics imposes some constraints on petrogenetic and tectonic models. The felsic rocks are clearly not simple differentiation products of the more mafic rocks and old continental rocks were probably involved in their formation. Some of the quartz-feldspar "porphyrites" along the western margin of the Mount Read Volcanics may represent partial melts of such crust, perhaps formed in response to heat input from coeval mafic magnatism. Many of the intermediate andesite and dacite flows appear to reflect mixing of mantle- and crust-derived components. Whether the basalt and more mafic andesite flows directly reflect mantle compositions or whether they have been contaminated is presently not completely clear. The occurrence of subduction in the Cambrian, inferred from the trace element geochemistry of sampfres from the Mount Read Volcanics, together with other geological evidence, offers a mechanism to enrich the mantle source. In this context it may be significant that the Jurassic dolerite sills, widespread in Tasmania and Antarctica, have similar isotopic characteristics to those observed in the Mount Read Volcanics. Reference Corbett, K.D. & Lees, T.C., 1987. Aust. J. Earth Sci. 34: 45-68.

A12.15 THE SUBDUCTION ZONE AS A GEOCHEMICAL FILTER: IMPLICATIONS FOR MANTLE EVOLUTION AND THE GENESIS OF OIB Jon Woodhead RSES, Australian National University

Although some debate still surrounds the nature of fate of slab residues is poorly understood but fundathe mantle wedge beneath active subduction zones, it is mental problem. widely accepted that components from the subducting Many isotopic studies of ocean island basalt (OIB) slab (altered oceanic crust and sediment) play an suggest that recycled components may be involved in important role in contributing to the characteristic their mantle source. However, trace element data often geochemistry of arc volcanics. appear to be incompatible with the simple (and in many Geochemical data, for intra-oceanic arcs in particular, ways very elegant) concept of wholesale slab recycling. suggest that bulk assimilation of material from the slab This conflict may be due largely to the assumption that is unlikely and that mass transfer is accomplished via slab components sampled at the Earth's surface today either a partial melt, hydrous fluid or some combination (i.e. oceanic crust and sediment) adequately represent of the two. If this is the case, mass transfer is not an their counterparts once subducted into the deep mantle. isochemical process but controlled by either solid-melt After removal of a mobile component to the mantle or solidy-hydrous fluid partitioning behaviour. wedge, as noted above, it is likely that many Consequently, even if transport is not 100% efficient, characteristic trace element ratios in the slab will not there are important implications not only for the arc, but retain their integrity. Hence, it is apparent that some also for the evolution of the deep mantle. The effects of attempt must be made to assess the geochemical addition of a 'metasomatic', slab-derived component to Altering' effect of the subduction zone environment the mantle wedge are readily observed in the The origin of an OIB suite from Pitcairn Island (SE geochemistry of oceanic arc volcanics, but the ultimate Pacific) is briefly reviewed in the light of this model.

217

A12.16 A SUBDUCTION COMPONENT IN KAROO BASALTS RJ. Sweeney Geology Department, University of Tasmania

The Mesozoic flood basalts of the Karoo are dominated by two tholeiitic magma types defined on the basis of their incompatible element content* labelled high Ti-Zr (HTZ) and low Ti-Zr (LTZ) types. There is a geographic association of HTZ types with older Archaean cratons and LTZ types with younger crust peripheral to these cratons. This association is substantiated by geochemical arguments suggesting that HTZ basalts contain a significant incompatible element component from the thick (160-200 km) lithospheric mantle keel underlying these old cratonic regions. In contrast, it is considered likely that the LTZ types derived their incompatible element signature from "other" sources, which include thinner (100-150 km) and less incompatible element enriched mantle peripheral to such keels. If compared with basalts from other environments by plotting on the Ti-Zr-Y tectonic discrimination diagram of Pearce & Cann (1973), the HTZ types plots in the within-platefieldas expected, but the LTZ types plot within the calc-alkaline field. The reliability of these tectonic discrimination diagrams, however, is questionable and it is preferable to compare the abundances of incompatible elements relative to one another for basalts from different tectonic settings (spidergrams). This comparison shows a remarkable similarity between the relative incompatible element content of both the LTZ and HTZ (although elevated in the case of the HTZ) types and examples from island arcs (e.g. Chile and the South Sandwich Islands). The question remains, whether such signatures in the subcontinental lithosphere are produced by an island-arc type component, whether mantle enrichment processes are duplicated in both environments or indeed whether island-arcs derive their signatures from the subcontinental mantle lithosphere. In the case of the Karoo the most likely island-arc candidate is the subduction of the Pacific plate along the Andean margin to the west which has been

continuous since the Devonian. This model would require a convecting mantle to entrain material derived by dehydration and melting of the subducting slab in the region of the mantle wedge, transport it some 2000-3000 km eastward and have the upwelling arm of this large-scale convection cell impinge upon the southern Gondwana lithosphere. This model, however, is inconsistent with Nd- and Sr- isotope data in the Karoo which suggest (at least for the HTZ types) that mantle sources have an average Proterozoic age; considerably older than the Andean subduction zone. This Proterozoic signature in the HTZ basalts of the Karoo more likely reflects the average age of metasomatism of the lithosphere from the Archaean to recent times (i.e. isotopic mixing between an old residual mantle stabilised in the Archaean and metasomatic events through time since then). Furthermore, the model cannot explain the distribution of HTZ and LTZ types in southern Gondwanaland. For instance, mantle sources for the incompatible element component of the HTZ basalts must be three tofivetimes more enriched relative to those producing the LTZ types. It is far more likely that the HTZ derives its incompatible element character from a source which is older and therefore has suffered greater exposure to metasomatic effects. Whether such metasomatism is reflecting the influence of subduction components from island arcs more ancient than the Andean example is more difficult to determine. Alternatively the derivation of island-arc geochemistry from delaminated sub-continental mantle lithosphere is equally difficult to test. Finally, it may be that the similar signatures are merely a result of some "commonality of process" whereby it is dominantly the relative chemical properties of the incompatible elements in mantle metasomatic processes, rather than some common mantle component, that generates the similar relative variations (spidergram patterns) observed.

A12.17 GEOCHEMICAL AND GEODYNAMICAL CONSTRAINTS ON SUBDUCTION ZONE MAGMATISM M.T. McCulloch and J.A. Gamble 1

Research School of Earth Sciences, Australian National University Department of Geology, Victoria University of Wellington, Wellington, New Zealand 1

(see page 316)

218

A15: Geology and Community Education — Initiatives for the 1990s Convenor: G. R. Taylor

A15.1 GEOLOGY IN THE TASMANIAN CURRICULUM R.L. Bugg Education Department, Tasmania

Since the halcyon days of 1976 when 556 students studied Higher School Certificate Geology in Tasmania, the number has steadily declined to 225 in 1989. While the reasons for this worrying trend are numerous and from observations of similar statistics available Australia-wide, are not restricted to Tasmania, the solutions are complex. In Tasmania the decline has occurred in spite of several major changes to the Geology syllabus and the introduction of an extension syllabus— Advanced Geology — in 1988. The existing syllabuses are unitised to allow students or teachers the opportunity to choose courses of study appropriate to their particular circumstances and pursue aspects of geology to a greater depth. The units are: Unit 1, Earth Materials Unit 2, Earth History

Unit 3, Earth Structures Unit 4, Surface Processes Unit 5, Geology and Society Unit 6, The Geology of Tasmania Geology consists of Units 1 and 2 and one from Units 3, 4 or 5. Advanced Geology consists of Unit 6 and two units not studied in the Geology syllabus. While students are satisfied with the existing syllabuses there are obvious shortcomings in terms of the breadth of knowledge of geology a student obtains by studying only Geology. Now Tasmania is witnessing the replacement of existing HSC subjects by those using criterionbased assessment with associated rewriting of syllabuses. What place does Geology have in this restructuring and what can we offer to cater for students' needs?

A15.2 GEOLOGY CURRICULUM DEVELOPMENT FOR THE VICTORIAN CERTIFICATE OF EDUCATION (VCE) Noel Schleiger and Darold Klindworth* Yarra Valley Anglican High School, Ringwood, Victoria

The new Geology study currently being developed in Victoria for year 11 and 12 students is not designed to train geologists. It is designed to interest and inform students of geology. The first semester unit is basic geology in the context of local land use. Unit 2 is

concerned with Earth materials and their relationship to society and technology. Plate tectonics and the Earth's origin are the themes of the third semester. The final unit is a mapping one using the principle of Stratigraphy and information.

219

A 15.3 THE ROLE OF GEOSCIENCE IN WEST AUSTRALIAN SCHOOLS Krishna K. Sappal Curt in University of Technology, Perth

The geoscience education and the minerals industry are of paramount importance to everyone in Western Australia. The industry is one of the most successful and dynamic dollar earner in a highly competitive international market. Western Australia's mineral production has increased in value by 18%, according to the 1987-88 Australian Bureau of Statistics figures. The mineral rich state produced $5.9 billion from mining ventures. This value included $1839 million of gold, $1669 million of iron ore, $248 million of diamonds, $151 million of coal, $115 million of mineral sands and $107 million of salt. The increase in mineral production in WA helped Australia secure its place as the leading producer of zircon (55%), rutile (52%) bauxite (37%), diamonds (31%) and ilmenite (28%). Inspite of all this significance, it is rather tragic that the study of geoscience in primary and secondary schools in Western Australia is in a very weak position in comparison to biology, geography, physics, chemistry and mathematics. The 1983-1987 trends in enrolments show a decline in the number of students studying geology in Year 12. Geoscience study at schools can uniquely introduce students to important concepts like evolution of life forms, time scale in Earth's history, the finiteness of mineral resources and their impact on society. These geological concepts can easily be related to physical, biological and chemical processes active on the surface of the earth. The Beazley (1984) and McGaw (1984) reports made many changes to both the overall organisation and assessment of upper schools subjects in WA schools. Syllabus Committees were asked to divide the two year programmes of upper schools subjects into two single year courses in such a way that Year 11 and Year 12 would be "stand alone" with students able to take a subject at Year 12 without previously studied it at Year 11. Accordingly, the re-organisation of the geology syllabus was completed by the Geology Joint Syllabus Committee for implementation in 1985. The Year 11 syllabus contains broader concepts and principles of geoscience and it is described as "Earthscience". The Year 12 syllabus is more specific and includes crystallography, mineralogy, petrology, palaeontology, stratigraphy and economic geology and it could be described as "Geology". The division of the upper school geology into Year 11 Earthscience and Year 12 Geology was hasty, without adequate consultation with the industry and the resource implications for geology excursions, teaching collection and modification to teacher training were hardly given consideration. With the re-organisation of the syllabus it was anticipated that a number of schools which offer geology at upper

school level would increase and accordingly students completing Year 12 Geology would also increase. Unfortunately a number of schools which offer geology at upper school level in WA is less than 10, and students completing Year 12 Geology has decreased from 164 in 1983 to only 115 in 1986, Dekkers et al. (1986). The low profile of geoscience in general education in schools and tertiary institutions is a repetitive theme at conventions and conferences. The geoscience education and mineral industry is important to everyone in Australia, and we need to continue the struggle to make people aware of this and ensure that public and government attitudes towards geoscience education change. Some of the suggestions which would enhance geoscience education in WA schools are: • The Chamber of Mines and Energy, the Ministry of Education and the Tertiary Institutions support initiatives that would enhance development of curriculum and geological specimens for schools. • Introductory geoscience to be part of the core subjects for all primary and secondary science teacher training programmes. The ministry needs to provide resources for employment of at least one geoscience teacher in every secondary school. The promotion opportunities for geoscience teachers to be equal to those of chemistry, biology and physics teachers. • The minerals industry, in association with the government, undertake an opinion poll of secondary students and community perceptions regarding the significance of minerals industry to WA. • Development of adult education courses in geoscience by Tertiary Institutions in collaboration with the industry, and the overall promotion of geoscience education in major cities and regional centres of the state. • Most geoscience graduates believe that mineral exploration is the only avenue for employment, and tertiary institutions tend to train student for this specific industry. Graduates starting their career should be made aware of the avenues of employment in environmental areas including hydrogeology, engineering geology, soil conservation etc. References Beazley, K.E., 1984. Education in Western Australia: Report of the Committee of Inquiry into Education in Western Australia. Government Printer, Perth. Dekkers, J., De Laeter, J.R. & Malone, J.R., 1986. Upper Secondary School Science and Mathematics Patterns in Western Australia, 1970-1985. WATT 65. McGaw, B., 1984. Assessment in the Upper Secondary School in Western Australia. Government Printer, Perth.

220

A15.4 THE USE OF SECONDARY SCHOOL RESULTS IN PREDICTING TERTIARY STUDY PERFORMANCE IN APPLIED GEOLOGY AT QIT Lloyd H. Hamilton Queensland Institute of Technology, Brisbane

There is some controversy over the use of entrance qualifications (as indicated by TE scores) for predicting academic performance. An evaluation was therefore made of the relationship of Tertiary Entrance (TE) scores to academic performance in the Applied Geology course leading to the Bachelor of Applied Science degree at Queensland Institute (now University) of Technology. The effect on performance of having studied, or not having studied, physics and chemistry at school was also taken into account. It was found that TE score differences are not very significant indicators of performance in the tertiary course. The effect of not having studied physics is rather significant but this is not so for chemistry. The course had 47 fixed subjects: these were not electives. The minimum TE score for entrance was 810. This accounts for the top third of students who achieved TE scores. The maximum possible score is 990. The study compares a group of students with a range of TE scores covering the top 14% of students achieving TE scores in Queensland with a group of low range TE score students covering, from the top, the 32 to 36% bracket. It was found that the students in the low TE score range achieved a success rate of 87% which meant that 7 of the 47 subjects in the course would have had to be repeated for the completion of the course. Those in the high TE score range achieved an average success rate of 91% which meant that four subjects would need repeating for completion of the course. Students who graduated were then considered separately from those who did not (such as those who dropped out, changed courses, cancelled or were

excluded). The low range TE score group of graduates had a success rate of 94% indicating a need to repeat three subjects to graduate. The high range TE score group had a success rate of 96% indicating a need to repeat two subjects. Note that 46% of graduates actually had a success rate of 100%. Graduates who did not study physics at school had an average success rate of 93% (indicating that a low TE score student would have to repeat 6.5 subjects and a high TE score student would have to repeat 5.5 subjects). Graduates who did not study chemistry at school had an average success rate of 99% (3.5 repeats for a low scorer and 2.5 for a high scorer. The course had six physics subjects and seven chemistry subjects. By comparison with graduating students, the success rate of non-graduating students was drastic. This is partly due to the effects of not cancelling subjects by the required dates. Students in the low range TE scores had an average success rate of 35%. Graduation at this rate would require the equivalent of repeating every subject three times. Students in the high range had a success rate of 57% (equivalent to having to repeat every subject nearly twice). Those lacking physics as a school subject had a success rate of 50%. Those lacking chemistry had a success rate of 64%. These figures show that there is a measurable difference between school leavers within the high range of TE scores and those within the low range, but the difference is of low significance, especially for those who completed the course. The effect of physics as a pre-requisite is more significant than the TE score provided the TE score is above 810.

A15.5 GEOSCIENCE EDUCATION IN THE NEXT CENTURY Barry A. Tapp* and Warren A. Peck Royal Melbourne Institute of Technology, Melbourne The past ten years have witnessed significant changes in both secondary and tertiary education in Australia. Educational schemes have been defined to respond to a wide range of policy and political agenda. The essential political thrust has been toward increasing participation and retention rates. Of special significance has been the trend toward increasing graduation rates in 'high-tech' disciplines, and the policy of increasing female enrolments into Engineering and Science coursework. The assumed

linkage is that increasing levels of educational access and participation will inevitably lead to greater economic well-being. Given this framework, many Geoscience educators view the future with some pessimism. Their argument being that Geoscience is a science in decline in terms of relevance and needs revitalising. We refute this argument and contend that Geoscience education should be a core requisite of both secondary and tertiary education. Six policy initiatives are presented.

221

A15.6 A REVIEW OF THE POSITION OF GEOLOGY IN THE K-12 CURRICULA OF THE AUSTRALIAN STATES R J. Stutchbury and R.M. Carter 1

Earth Resources Foundation, University of Sydney Department of Geology, James Cook University, Townsville 1

Decline in interest in geology and the subsequent decline in students opting for courses in the geosciences is of concern throughout Australia. Statistics clearly show the decline in number sitting for the final senior geology examinations in each state over a period of ten years. Nationally, there are no statistics available for the current status of geology in primary or junior secondary school science classes, however, the senior geology statistics possibly reflect the general trend of all geological education. An Australia-wide investigation of the science curricula of all levels of education from kindergarten in primary school to Year 12 in senior secondary school (K-12) indicates that some provision is made for topics in geology to be taught in most Australian states. However, because curricula vary from being totally prescriptive in some states to totally school-based in others there are many variables affecting how much geology is being taught, if at all. Primary school science curricula are more commonly school-based than are the science curricula at other levels of education. Consequently teachers have the opportunity to devise teaching programmes that suit the community lifestyle and developmental stages of the students. This is the case in Victoria. In Queensland on the other hand, the currriculum is prescribed and it is mandatory to teach specified topics from five broad areas, one of which is Earth and Space Science. In all except Year 1 of the seven years of Queensland primary schooling, students do some geological topics. The topics are drawn from the conventional minerals, rocks and erosion with approximately 20% of science lesson time being spent on them. The primary science requirements of the other states varies between these two extremes. Primary school teachers can gain qualifications with as little as fifteen hours science during their training. Almost half of the teacher trainees have not done any science since junior secondary science. Junior Secondary Science curricula also vary from the totally school-based, as in the case of Victoria, to those which prescribe some or all as mandatory. Western Australia has introduced a Unit approach where teachers have the option to teach any six of the 20 Science Units. Five contain some geology and only one is totally devoted to geology. It is possible for students to complete the entire science course without geology. The New South Wales Junior Science Syllabus recommends an integrated approach to the teaching of science; topics are not taught as chemistry, physics, geology or biology, but rather these four major disciplines are used to teach various aspects of a topic. It is recommended that material from all four should be used to teach various

aspects of a topic. It is recommended that material from all four should be used in equal amounts. However just what is taught is at the discretion of the school and again it is possible for a student to pass through all four years of junior secondary school without geological concepts being taught. At senior secondary level geology is offered either as a single science course, as it is in all states, or as a component of a multi-disciplinary course such as the recently introduced 3/4-Unit senior science course in New South Wales. As a single science course the syllabus content varies from the formal academic approach as in New South Wales and South Australia to the approach used in the Victorian Curriculum and Assessment Board's Geology Syllabus for the Victorian Certificate of Education to be introduced into schools in 1991. The course is studied through four units offered over two years and students may enter the course through any one of the first three independent units with only Units 3 and 4 in the final year being designed as sequence. The course is unique in that it emphasises the importance of local geology; Unit 1 is devoted entirely to the Geology of Victoria. Secondary school science teachers are not required to be qualified in all four major scientific disciplines. Many are qualified only in one. Geology and physics are least likely to be included in a science teacher's qualifications. However, all junior secondary school teachers are expected to teach all four disciplines whether or not those disciplines are mandatory. Curricula are never static and currently many of the states' science curricula are under review. The Western Australian junior science curriculum has already changed since this review was undertaken (Leahy, pers.comm., 1989) and all three levels of science are currently under review in New South Wales. One of the most important moves in recent times is move towards a national curriculum. As part of the planning, a national mapping programme of the curricula of each state is being undertaken. The place of geology in the recommendations for the proposed national curriculum is not yet known. To ensure that geology is maintained in all levels of school curricula it is essential to devise a strategic plan supported by all concerned geoscientists and implemented in collaboration with professional educationalists. Before any action can be taken it will be necessary to accurately determine all the factors affecting geological education by a professionally designed survey of primary and secondary science teachers. Further, it is essential that all professional geoscientists work together to ensure that geology is

222

given its rightful place when recommendations for a national curriculum are drawn up. Since a national curriculum will probably have the effect of encouraging all states to heed its recommendations, it would be judicious to seek the support and guidance of the national curriculum organising committee. Geology, however, will never be taught successfully through any curriculum without teachers qualified to

teach it. Consequently it is important for the strategic plan to include recommendations for geology to be part of the minimum scientific qualifications of all primary school teachers and all secondary school science teachers. Reference should be made to the Report of the Discipline Review Committee into Science and Maths Teacher Training in order to determine suitable action.

A15.7 THE "INVOLVEMENT" COMPONENT IN TEACHING EARTH SCIENCES Gabor Markovics Victoria College, Rusden Campus, Clayton

Traditionally the Earth Sciences have been taught with a strong emphasis on "conveying the relevant factual knowledge" as the learning process. With such a method of teaching, a number of important aspects of the learning process are stifled or ignored and it is these aspects that satisfy individual curiosity and encourage scientific inquiry. If the success of teaching can be measured at least in part by the development of inquiring minds able to solve problems, then I feel the following aspects need to be considered for successful teaching: • The teaching of Earth Sciences should be used as a vehicle for developing habits of good scientific approach. This is all the more important at the junior and middle high school levels; • The learning process needs to include challenge, continuity, relevance and accomplishment as necessary ingredients. These ingredients are present in "real

life" situations and need to be built-in by the teachers; • As much as possible teaching needs to be from a "thematic" or "problems" approach where the student can become involved and satisfy curiosity. Again, this is exactly what drives all practising geoscientists; • The teacher needs to break away from the security of the "known" and deliberately place himself in the realm of the uncertain. By doing so, the teacher is a model of science where inquiry and interest is generated by the limited local knowledge. Too often teachers are poor at recreating the excitement and the joy of discovery. It is considered that Earth Sciences taught with a blend of the above will lead to inquiry-based teaching and as a result the response by students is a more positive and meaningful one. In addition, the teacher will alsofindit a satisfying experience by recreating the essence of science.

A15.8 THE SINKING OF THE PORT PHILLIP & WESTERN PORT BAY AREAS, SOUTHERN VICTORIA: A STUDENT SEISMICITY EXERCISE BASED ON THE HEATH HILL FAULT Neville Green * and Gary Gibson 1

Institute of Education, University of Melbourne Seismology Research Centre, Phillip Institute of Technology 1

There is a strong correlation of earthquake frequency within graben type structures that are bounded by four with other geodynamic phenomena: zones of high major NE-SW trending faults. These are the Towsley seismicity generally coincide with plate boundaries, and Selwyn faults and the Tyabb and Heath Hill faults e.g. the San Andreas fault zone. to the west and east of Melbourne and Westernport Bay Australia sits neatly within the Australia-India plate respectively. and should be relatively aseismic. However, there has A total of seven recording stations occur in the area been numerous earthquakes recorded in Australia with covered by the Melbourne 1:1,000,000 topographic magnitudes greater than 5.0. Our largest quakes were sheet. For the exercise a table of P and S arrival times the 1941 Meebenie and 1968 Meckering events with is compiled for each of the stations then the distance magnitudes of about 7.0. In Victoria one of the most between recording station and epicentre is calculated severe events occurred on 7 July 1971. It was felt using a computer generated model which is accurate for throughout the Melbourne area, a major part of west distances up to 210 km. It is also possible to use the Gippsland and reports were received from up to 136 km travel time-distance curves available in various reference away. texts. Port Phillip Bay and adjacent Westernport Bay sit The next step involves drawing circles of radius-

epicentral distance from each of the stations. The intersection or scatter of points is close to the epicentre of the earthquake. A cross-section is constructed using fault depth versus fault angle and direction which in turn enables the student to locate the Heath Hill fault on the various maps being used. The exercise also enables students to calculate origin time and Richter local magnitude using amplitudes recorded on seismic stations; with suitable corrections. The concept of Richter magnitude is stressed and suitable

223

tables and photographs of earthquake devastation are used to stimulate discussion within the group. Excursions to the fault localities are arranged to compliment the laboratory exercise. This seismic study has been successfully used with first year, non-geology major students at the Institute of Education. Versions of this exercise can be used by post Year 9 groups, as it combines map interpretation, simple maths and plotting skills with a legitimate base of real data.

A 15.9 MAKING EARTH SCIENCE TRIPS SUCCESSFUL P.G.L. Harlow Ipswich Grammar School, Ipswich, Queensland

Four years ago there were 26 students in two classes Science trips include at least the following basic in Year 12. In addition, the majority of these students objectives.Is the trip relevant? Is the trip interesting for represented the least able in Year 12 (Earth Science both the teacher and the students? Do the students learn being regarded somewhat as a soft option by many new Earth Science knowledge and do they get the students). Not long after this it was indicated that classes opportunity to apply knowledge they already have? of less than 20 in Year 12 were uneconomic to retain. Does the teacher and the students enjoy the trip? Does Four years later we have an enrolment of 90 in four the trip generate useful publicity for the Earth Science classes in Year 11 and the quality of students have also Department and attract new students to the Department? improved. This year four of last year's students continue Is the trip affordable to every student? on to University to do Geology. One of the reasons for The paper will detail how Ipswich Grammar the increased popularity of the Earth Science course is excursions have been designed to endeavour to fulfil the number of excursions. the above criteria and help with the promotion of the Criteria for defining success or otherwise for Earth Earth Science. A15.10 TEACHING GEOLOGY AS AN EXEMPLAR OF SCIENCE Tim Sprod The Hutchins School, Hobart

Geology in schools, we need to consider who we are educating and to what purpose. Most of our students will not become geologists, many not even scientists; as a descriptive science (at the school level), geology can attract students whose maths is not strong. Clearly our purpose cannot be to prepare "cannon fodder" for University Geology Departments, though we must recognise that some of our students will end up there. Our purpose must be wider. Recent surveys (e.g. the New Scientist survey) have shown a great public interest in science, but a distrust of scientists. So at least one of our aims should be educating students to appreciate the methods and strengths of science, partly for political and social reasons. As all people will see numerous rock outcrops in everyday life, we should also aim at enabling our students to make sense of what they will see — this I would characterise as a recreational aim. I would like to suggest that the best way to achieve these (and other) aims is to try to produce, within the

limitations of their knowledge base, real geologists. By this I mean that as an end point, our students should be able to approach a geological problem in the same way that a professional geologist might. What has all this to do with teaching Geology as an exemplar of science? Science teaching has, in the past (and unfortunately sometimes even now), been merely the presentation of a large swag of facts to be memorised. A more modern approach is to base it on experiments, an approach considered as cookbook science. Neither my mind, is a good model of real science. While an experiment based syllabus is a distinct advance on fact cramming, it leads to the impression that science is merely doing the experiment properly so that the right answer comes out No scientist, of course, knows in advance what answer the experiment is going to give (as opposed to hoping). A scientific problem is an open ended problem. So does Geology have an inherent advantage in teaching real science? I believe it has. For example,

224

each outcrop is an open ended problem, usually plausibly exposures (nor, indeed, only geological problems) which explicable in several ways at many different levels. Of lend themselves to this approach; but I do feel that course, often only one explanation will fit all the possible Geology has an edge, as a science where open ended observations but, to reach this, a large amount of problems are accessible at multiple levels to students hypothesizing and testing is needed, and it is often with a relatively small factual and theoretical base and beyond the ability of a geology class to come to it in the limited mathematical expertise. time available. Nevertheless, all the features of real These and other reflections will be addressed in the science: observing, hypothesising, reflecting, dis- presentation, with an emphasis on practical examples of cussing, predicting, testing and doing it all over again, the views outlined above. are modelled in the attempt to do so. It is not only field A15.ll GEOLOGICAL EDUCATION AND CRITERION BASED ASSESSMENT Roy Pallett Department of Education and the Arts, Tasmania

Most geology courses involve linear programs that require all students to begin at a given point and then proceed lockstep, usually at the group's median ability rate, through an activity, topic or unit. In this scheme, all material presented is redundant to several students in class at any level. The identification of criteria as relatively broad groupings of skills competencies and understandings in geology allows for much moreflexibilityand creativity in learning. Externally imposed linearity is anathema to creativity by disallowing meaningful digressions by individuals. Criterion based course goals must be kept in mind but

that individuals have more autonomy to decide what to study, what sequence, how, when and for how long. Within certain subject limitations relevance is determined by the learner. It is postulated that students emerging from these programs will have more positive attitudes towards geological subjects, have better problem solving abilities and be more creative. The removal of competitive and coercive elements will not lead to reduced student participation but will increase genuinely creative behaviour. Coping with different perceptions of geological education is a major challenge for teachers.

A15.12 RECOMMENDATIONS FROM AN AMIC RESOURCES WORKSHOP: APPLICABILITY TO THE PREPARATION OF GEOLOGICAL RESOURCE MATERIAL Barry Cook and Robyn Stutchbury 1

Geology Department, Salisbury Campus, South Australian CAE Earth Resources Foundation, University of Sydney 2

An Australian Mining Industry Council (AMIC) Education Resources Workshop was held in Ballarat during November, 1989, with the aim of rationalising and making more effective the education resources produced by the mining industry. Many of the recommendations that resulted from the workshop could apply equally to the development and publication of geological education resources and should be considered when material is being developed for release to the public and education establishments. AMIC had already implemented recommendations from a previous workshop having a strong representation from the education sector, particularly those involved in curriculum design. The fifty participants attending the Ballarat workshop were drawn from a wide crosssection of the mining and education communities. Six small workshop groups were formed to discuss and recommend a strategic plan of action. Most of the groups arrived at similar conclusions. These have been adapted

to suit a more geoscientific approach and listed as the following recommendations: 1. To form central organising body, such as a foundation which would be responsible for: • setting standards, establishing aims and seeking funding; • commissioning projects; • developing criteria for projects; • acting as a centre for the gathering of information and as a clearing-house; • ensuring that suitable material was distributed nationally; • undertaking market research to determine what is required; • acting as a reference centre for a network of advisory specialists. 2. To have in the Network, specialists from the following fields: • practising teachers from all levels of education and

225

curriculum consultants; • industry representatives (AMIC, State Chambers of Mines, APEA, etc.); • representatives from relevant professional organisations (GSA, AIG, AusIMM, AGC, etc.); • representatives from organisations involved in school and community education such as State Departments of Mines, Geological Surveys, museums, National Parks and Wildlife, etc.; • representatives of relevant academic departments. 3. To have a national co-ordinator employed to facilitate communication between members of the Network and the Central Foundation. Tasks would also include: • keeping educational resources as a regular agenda item at meetings of relevant organisations; • publishing a 'Diary of Proposed Educational Publications' in an industry newsletter,

• developing better liaison between the various geoscientific professional associations, the Chambers of Mines, the Departments of Education, AMIC, Professional Education Associations, Teacher Training Establishments; • contracting suitable professionals to produce particular projects; • arranging for teacher trainees to assist in information preparation Since there is presently no national body responsible for overseeing ex* co-ordinating geoscientific information being published for release to the public and various educational establishments, it is proposed that a similar approach to that being considered by the mining industry might be adopted by a national, geoscientific association.

A15.13 PROMOTING GEOSCIENCE IN THE SECONDARY EDUCATION ENVIRONMENT Barry A. Tapp* and Warren A. Peck Royal Melbourne Institute of Technology, Melbourne

Secondary school student often have little the vastly more serious and longer term degradation information available to them about Geoscience. This brought on by bad farming practices and tourism. School means that few are aware of the opportunities that a leavers seem to be uninformed on issues such as career in Geoscience offers. Much of the blame for the groundwater. lack of precise information must rest with Tertiary To offset this lack of available information, the academics, few of whom appear to take any interest in Geology Group at RMIT has produced two videos secondary education. depicting some of the aspects of Geoscience. These will The community also is unaware of the contribution be presented along with the rationale and methodology geoscience makes to the quality of life in Australia. For of their distribution and presentation to secondary example much is made of the environmental degradation students and the community. resultant of mining, but little is spoken or written about A15.14 RAISING GEOLOGICAL AWARENESS THROUGH NATURE APPRECIATION Gerhard W. Hofmann Queensland Department of Mines

In response to appalling ignorance in the community of matters geological, an action group of concerned geoscience educators established the Geoscience Awareness Program (GAP) at the 1987 ANZAAS Conference. Its aims are to increase public awareness of, and support for, Australian geoscience. This concern about a neglect of geoscience is not based on selfish professional perception or the importance of mineral development to Australia's economy, but on the recognition that wider geoscience awareness has become the key to survival on our planet. GAP's initial efforts have been directed towards school curricula and science teacher education. More recent projects aim to compile geological road maps and commentaries for airline magazines to arouse the

interest of the travelling public. There is considerable community interest, particularly among professionals in kindred disciplines such as town planning and engineering, in the history of formation of eye- catching landmarks. That interest is not met because of a lack of easily accessible and understandable literature. Geological texts are absentfromtourist information centres or bookshops. In general, little information is available on the landscapes of National Parks, although Rangers continually receive enquiries about unusual rock formations. The availability of geological tourist information depends very much on personal initiatives by members of the profession; for instance, a few typed pages on Mount Warning could be found at the Murwillumbah Tourist Information Centre thanks to

226

the effort of Dr P.J. Solomon, who completed a Masters Thesis on that volcano. A list of Australian geological excursion guidebooks, compiled by Kidd (1989) in 1986, includes 314 tides. Of these, about 43 make some allowance for amateur naturalists, science teachers, and students of earth science at tertiary and senior secondary level. Some of these are inventories of geological monuments and sites of significance. Another 38 guidebooks address special recreation groups such as cavers and tourist prospectors. All but one or two guidebooks have no glossary to help the reader to track down their point of sale as few have been commercially published. Bookshops are caught in a vicious circle: because there is not enough demand, geological guides are not prominently displayed and because of this, shoppers do not know of their existence. It appears unlikely that all these excellent guidebooks will attract the attention of the casual seeker of geological information. The reasons why the hundreds of Australian guidebooks have not found popular use appear to be remote points of sale, lack of advertising outside the newsletters of geoscience associations, assumption of familiarity with geological principles and terminology, and a generally doctrinarian approach to presentation. Attempts to present the systems, schemes and classifications of earth science before giving information of interest to the reader have the same effect as teaching the rules of grammar before conversational phrases to travellers to foreign countries: they make the subject appear boring. As advancing technology and reducing working hours produce more leisure time, people are pursuing more educational or recreational goals. Adult education, the movement of The University of the Third Age, greater awareness of health and fitness, and prominence of the environment provide great opportunities for the dissemination of popular geoscience. Already, the Earth Sciences Department at La Trobe University has mature age students who enrolled after they brought up their families or retired. The potential for marketing geological information to the leisure industry has not been fully defined yet. The Geological Society through its Divisions, some State Geological Surveys and other Government Departments, and individuals have published geological guides to National Parks, but it was left to Reader's Digest to produce an illustrated book of the scenic wonders of Australia (Reader's Digest Services Pty

Ltd, 1976). Geologists need to provide easily understood information to the travelling public whose interest is aroused by unusual landmarks and spectacular landscapes. The planned annotations to Royal Automobile Club road maps are a step in that direction. We need to produce information on Australia's natural attractions to tourists. Tourism is the country's fastest-growing industry and provides a growing market for popular geological literature including guidebooks. Local Authorities and tourist operators in Queensland have shown interest in the promotion of natural attractions. The Cairns City Council contributed to the field expenses of the academic co-author of a recent guidebook for the Cairns district (Willmott & Stephenson, 1989). A booklet on the formation and assets of the Gold Coast was published by its City Council (Herbert & Stevens, 1983). The Chillagoe Tourist Association was instrumental in the production of a guidebook of its district (Willmott & Trezise, 1989). Special interest travel is growing worldwide as people seek unusual travel methods and off-beat locations. Geological information can round off that nature appreciation holiday, and some safari-type travel operators are now actively seeking geological expertise. The geological profession must take these opportunities for wider promotion of geological awareness and support the recent initiatives of GAP and of the Education Subcommittees of the Geological Society of Australia. Those who have been put off by an overly didactic and theroretical teaching of geology at school must be won back. What better way is there for raising geological awareness than through building on the interest of a captive audience, in a learning for leisure setting without pressure, which is seeking to enhance the aesthetic value of nature appreciation. References Herbert, H.W. & Stevens, V.C., 1983. The ancient history of the Gold Coast. Gold Coast City Council, Gold Coast Kidd, C.M., 1989.BMR Report 293. Reader's Digest Services Pty Ltd, 1976. Scenic wonders of Australia. Reader's Digest, Sydney. Willmott, W.F. & Stephenson, P.J., 1989. Rocks and landscapes ofthe Cairns district. Queensland Department of Mines, Brisbane. Willmott, W.F. & Trtezise, D.L. 1989. Rocks and landscapes ofthe Chillagoe district. Queensland Department of Mines, Brisbane.

A15.15 EARTH SCIENCE IN OTHER DISCIPLINES

227

Ian Hawkins The University of Melbourne, Melbourne

While Earth Science should be available to senior secondary students, I contend that it is even more important to ensure appropriate and adequate teaching within junior science. Teachers of Science, and especially those in tertiary institutions, have a highly specialised training. This is the major reason for their tendency to choose traditional and sometimes rather artificial, examples to

illustrate the concepts being taught. Those of us involved with the Earth Sciences should endeavour to make our colleagues in other disciplines aware of the value of Earth Science as one of the best vehicles for integrating scientific disciplines, and as a source of concrete, worthwhile and novel illustrations for abstract concepts in physics, chemistry and biology.

A15.16 SCIENCE EDUCATION REQUIREMENTS FOR AN EXPANDING WORLD POPULATION E. Brennan Brennan and Associates, Cleveland, Queensland

Science education for the future, which must include geosciences, must be based on the need to educate students in understanding the resource requirements of man in a rapidly expanding world population. Unless our future generation understands the resource requirements of mankind in order to survive on earth, then there will be decisions made that will lead to unnecessary shortages of food, clean water and clean air. The world faces a minimum population of the order of 13 billion in the year 2050. Without changes in the

world's attitude to population control, this figure could be as high as 21 billion. There will need to be massive increases in the use of energy to provide water for irrigating the food necessary and massive increases in the use of mined fertilisers to assist in that food production. Transportation systems will need to increase to maintain supply of foodstuffs to cities. Without proper understanding of the resource requirements and usage, the people of the earth can be lead into many disastrous decisions.

A15.17 A TERTIARY PERSPECTIVE ON SECONDARY GEOSCIENCE EDUCATION Geoffrey R Taylor University of New South Wales, Sydney

Universities teach geology to three groups of students; those who intend to become professional geoscientists, those who need some geology to support their chosen discipline and those from an unrelated discipline who merely wish to broaden their knowledge. Geology departments entrance requirements have little impact on the second group but can have significant impact on the student intake for the first and last groups. At a time of apparently falling numbers and calibre of geoscience students at both secondary and tertiary levels (Taylor, 1988), there is a strong tendency for each section of educators to blame the other for the current situation.This paper analyses the causes of low geology numbers and attempts to arrive at solutions to the problem. Current Federal and State education policies are leading toward the privatisation of education at all levels.

University geologists are increasingly having to look to the major employers, mostly the mining industry, for funds for research, facilities and student support. This gives the industry lobby a strong lever in university decision making and the industry" catchcry" for several years has been to rationalise, or close, geology departments. Whilst such moves will save both government and industry money, is it either fair or commercially sensible? There are approximately 4500 geoscientists employed in Australia today; 3000 are geologists and 1000 are geophysicists. Roughly 400 retired from the profession last year but only 200 new geoscientists graduated (D. Emerson, pers. comm.). Can we be confident in a less-prosperous Australia that we will always be able to import the best overseas geologists or is industry going to have make do with the dross. We need to train more, not less, professional geologists and

228

it is unlikely that a "rationalised" tertiary sector would be able to satisfy the demand. Therefore the current low enrolments in tertiary geology courses should be of concern to government and industry alike. Starting salaries for new geology graduates last year were third behind only dentistry and medicine (The Australian, 19 July 1989), showing that market forces are beginning to take effect on the employers, but there has been no marked turnaround in the demand for tertiary geological training from the students. The reasons for the low demand must lie elsewhere. The schools have long complained that an overemphasis on the need for HSC level physics and chemistry has precluded students without these subjects from entering university geology courses. This was a valid criticism but in the main it has been put right by changes to university entrance requirements and the provision of introductory courses in these subjects. A greater problem in some states, particularly in New South Wales, has been the existence of irrational restrictions on the amount of science a student can take at the HSC level, supposedly in the interests of a balanced education, while no equivalent restriction is imposed upon humanities students. This has lead to the slow death of the HSC geology course as students attempt to keep their options open and maximise their tertiary entrance scores. This has serious implications for the university geology intake for, as a recent GAP survey has shown (Stuchbury and Carter, 1989), exposure to aspects of earth science is a major factor in influencing a students choice to take geology. Nevertheless, if Year 10 students were interested in geology they can develop this interest by taking HSC geology or, if geology is not offered at their school, they can take other science subjects and still move into

geology at the university level. In looking for an explanation for the lack of interest in geology we must therefore look toward the influence of community attitudes in general and to the presentation of earth science in the Years K to 10 curriculum. The mining industry must shoulder much of the blame for adverse community attitudes to matters geological. In a time of perceived environmental crisis, much of the industry still acts irresponsibly or puts too little effort into defending the necessity of its operations or the rationale for its actions. It's low level of financial support for community education initiatives (such as the Society's Geology Road Map Project), is witness to this. However, the greatest influences on our school-age children are the attitudes expressed to them by their primary and secondary teachers. In the main these teachers come from a life-sciences background and have had no training in geology and so their treatment of the geology content is minimal. Their own lack of interest in the subject is often transferred to their students. The remedy is to ensure that the teachers themselves receive a broad education that includes some of each of the basic sciences and applies these to the study of not only the biosphere but also the lithosphere. To maintain the health of the profession we must therefore pressure the various State and Federal government education bodies to review teacher training programs and where necessary modify these to ensure that there is an adequate exposure to the earth sciences. References Stuchbury, R. & Carter, R. M., 1989. Contributions of the Economic Geology Research Unit, James Cook University 33: 31pp. Taylor, G. R., 1988. The Australian Geologist 69: 29-31.

229

A16: Magmas, Gemstones and the Mantle Convenors: M.J. Rubenach and B. Franklin

A16.1 EARLY BIMODAL MAGMATISM AND MAGMA MIXING FROM KANGAROO ISLAND IN THE SOUTHERN ADELAIDE FOLDBELT, SOUTH AUSTRALIA Songfa Liu*, Peter D. Fleming and Chris M. Gray Department of Geology, La Trobe University

Dykes intruded the coastal outcrops west of Cape and their tectonic setting in the southern end of the Hart, Kangaroo Island, South Australia, at the beginning Adelaide FoldbelL of the Delamerian Orogeny. They have a wide range of Field evidence, microstructural evidence including compositions from mafic (Si0 = 49%) to transitional disequilibrium textures, and geochemistry all suggest (Si0 = 57-66%) to granitic (Si0 = 72%). Field and that the hybrid dykes originated from mixing of mafic geochemical evidence suggests that the mafic and and granitic magmas. Results of least square mixing granitic dykes are products of bimodal magmatism and calculations for major elements are consistent with this that the transitional (hybrid) dykes result from model. magma mixing. Field and microstructural eviThe bimodal magmatism and observed magma dence indicates that these dykes intruded the Kanmantoo mixing on Kangaroo Island have important petrogenetic Group metasediments pre- to syn-Dj (Liu & Fleming, and tectonic implications. The magma mixing indicated by the hybrid dykes implies a similar origin for some 1989a). The mafic dykes are derived from a fractionated other granitic rocks in southeast South Australia. The early bimodal magmatism suggests a high MORB-like magma as is the general case for similar dykes in the southern Adelaide Fold Belt (Liu & Fleming, geothermal gradient at the beginning of the Delamerian 1989b). Chemical analyses of these dykes show that Orogeny in the studied area. This is consistent with they resemble oceanic tholeiites in many aspects previous evidence of the early onset thermal activity in including flat REE patterns. Two high-Al examples the southern Adelaide Fold Belt (see Liu & Fleming, (A1 0 = 18-20%) are interpreted to be products of 1989a), including metamorphic crystallisation of porphyroblasts, partial melting in migmatites, fluid plagioclase accumulation. The relationships between the three types of dykes circulation, and early granite intrusion (prior to the are well demonstrated in a " A l ^ vs Ti0 " diagram and climax of the Delamerian Orogeny documented by "CaO, MgO & ICp vs FeO*" diagrams. Analyses of Milnes et al 1977). The heat source for the early (premafic dykes from Kangaroo Island fall along the fraction- to syn-Dj) thermal activity would be best interpreted as ation/accumulation trends of the mafic dykes in the heatingfrommafic dyke intrusion and upwelling mantle. southern Adelaide Fold Belt The granitic dykes plot far The occurrence of bimodal magmatism also supports away from the mafic dykes in the southern Adelaide the tectonic model proposed previously by Liu & Fold Belt, i.e., they do not fit into the fractionation Fleming (1989a, b), who suggested that the Kanmantoo scheme deduced for the mafic dykes. The third group of Group sediments were most probably deposited in a dykes, termed here "hybrid dykes", is transitional in region of crustal stretching/thinning, i.e. in a marginal chemical composition between the mafic and granitic basin/back-arc basin. References dykes. Examination of the geochemical (Milnes, 1973 and Gray, C.M., 1990. Aust. J. Earth Sci. (in press). this study) and Sr isotope data (Gray, 1990) for the Liu, S.F. & Fleming, P.D., 1989a. Geological Society of Australia Abstracts No. 24: 89-90. Kanmantoo Group metasedimentary rocks suggests that S.F. & Fleming, P.D., 1989b. Geological Society of the granitic dykes on Kangaroo Island and the granites Liu,Australia Division), September, 1989. in the Encounter Bay area are most likely to be derived Milnes, A.R., (Victorian 1973. Unpubl. Ph.D thesis, Univ. Adelaide. from partial melting of the Kanmantoo Group meta- Milnes, A.R., Compston, W. & Daily, B., 1977. J. Geol. sediments. Current and future work is directed towards Soc. Aust., 24: 87-106. further assessing the pedogenesis of these granitic rocks 2

230

A16.2 MID-PROTEROZOIC IGNEOUS ROCK SUITES OF THE CENTRAL AND EASTERN GAWLER CRATON, SOUTH AUSTRALIA J. M. Scheffler and M. J. Abbott School of Earth Sciences, Flinders University

The central and eastern Gawler Craton encompasses several discontinuous mid-Proterozoic igneous sequences. Spatially, the Gawler Range Volcanics dominate the regime, overshadowing several smaller, lesser-known sites. While including the Gawler Range Volcanics (GRV) this study emphasises the smaller suites: namely, the Myola Volcanics, McGregor Volcanics, Moonta Porphyry, Cultana Porphyry (unofficial name), and related intrusives (Hiltaba Granites, Beda Volcanic-equivalent basic dykes). Geochronological control, taken from Fanning et al. (1988), is good throughout the study subjects with the exception of the Cultana Porphyry. The Cultana Porphyry is considered a "wild card", as there are no comparative published geochemical data on the unit. Comparisons of the geochemistry of the abovementioned sequences show several interesting trends. The Moonta Porphyry, in keeping with its lithological homogeneity (away from mineralised zones), exhibits the most uniform behaviour by showing very little scatter in most variation diagrams. The Moonta samples tend to plot within the McGregor Volcanic fields, as, for example, in the Ti0 -Si0 diagram. Related petrogenetic origins are indicated for these complexes, supported by their virtually identical ages (ca. 1740 Ma). The Myola Volcanics, while older (ca. 1791 Ma) than the Moonta and McGregor rocks, seem apparently more fractionated (i.e. higher Na 0+K 0, lower Ti0 at comparable Si0 values). The GRV demonstrate 2

internally consistent variation characteristics which occasionally overlap the fields of the other complexes but usually display discrete patterns. The Cultana Porphyry is the most chemically variable suite, despite its generally consistent quartz phenocrystrich appearance. The porphyry's trends show wide ranges which at least partially overlap the fields of the other suites. The basic dyke rocks, assumed to be of Beda Volcanic age, are markedly younger than the balance of the study subjects; as such, their chemistry is studied separately from the felsic rocks. They are considered to be products of a separate, although possibly related, stage of magma genesis. This study concurs with recent theories that the GRV may have been derived from basic underplating of a stationary continent, although the spatially related Hiltaba granites probably reflect more crustal involvement. However, the fact that, according to available data, almost 150 my elapsed between the GRV and the Moonta-McGregor event cannot be ignored. The penological differences in the older suites coupled with the age gap to GRV extrusion suggest that different mechanisms may have been acting on the older igneous rocks. Reference

Fanning, C.M., Flint, R.B., Parker, A.J., Ludwig, K.R., & Blissett, A.H., 1988. Precambrian Research 40/41: 363-386

A16.3 ORIGIN OF THE PROTEROZOIC GRAPHITE DEPOSITS OF THE SOUTHERN EYRE PENINSULA, SOUTH AUSTRALIA — CONSTRAINTS FROM STABLE ISOTOPE GEOCHEMISTRY W.R. Taylor and R.F. Berry 1,2

1 2

Key Centre for Strategic Mineral Deposits, University of Western Australia Key Centre for Ore Deposit and Exploration Studies, University of Tasmania

Crystalline flake graphite is a commodity that is currently in short supply on a world-wide basis as reflected by prices of more than $US1000 per ton for the highest grade ores. Accordingly, there has been general exploration interest in graphite deposits in Australia. However, there is little modern published information on the mode of occurrence or genesis of Australian graphite deposits, many of which have been sporadically mined in the past. Some of the best known crystalline graphite occurrences, including the Uley graphite mine (with historical production of -700 tons and current production

underway), are hosted in deformed upper amphibolite grade metasediments of the Proterozoic (1.9 Ga) Hutchison Group of the southern Eyre Peninsula. The graphite deposits occur along more than 75km of strike length, immediately to the west of the major Kalinjala mylonite zone which separates the Hutchison Group from Lincoln Complex granitoids to the east. Features of the deposits, including the presence of late vein graphite and zoned garnets with cores or halos containing abundant graphite inclusions, indicate a level of carbon mobility that is atypical for a syngenetic (i.e. organic sedimentary derived) style of mineralisation. These

231

features are, however, similar to some epigenetic deposits, such as those of New Hampshire, which originate by graphite precipitation from fluids of mixed metasedimentary/igneous origins. The spatial association of graphite with the Kalinjala mylonite zone is suggestive of a genetic model for the deposits in which the shear zone may have acted as a major channel for migration of deep-seated (lower crustal or mantle) carbonic fluids that subsequently interacted with crustal fluids causing graphite deposition in the host sediments. To test this model, carbon isotope studies of graphite samples from the Uley mine, Koppio mine and other localities along 40km of strike length were undertaken to determine the relative genetic importance of externally derived fluids versus those of metamorphic derivation. The results show that 813C values for graphites from metapelites, quartzites, weathered ores and veins cluster near -25%o(Fig. 1), a value typical of organically derived carbon. Associated marbles show no imprint of light 813C fluids (Fig. 2) while magnesite associated with some of the deposits has a 813C signature identical to Tertiary calcrete indicating that its origin is unrelated to Proterozoic graphite deposition. The results therefore do not suggest involvement of external fluids of isotopically heavier carbon content. Furthermore, the effects of fluid migration appear to have been significant only on a local scale (-1-2 m or less) and to have been controlled largely by the relative permeability of rock

units during metamorphism. Thus graphites hosted in schists immediately adjacent to completely decarbonated diopside-quartz calcsilicates show an isotopic shift of ~+8%o due to interaction with devolatilised C0 2 from the calcsilicate protolith. At distances greater than a few metres no isotopic shift is observed. Since late vein graphite is isotopically indistinguishable from that in the metasediments, vein carbon must have originated from metamorphic fluid activity. It is probable that late stage carbon mobility was the result of local heating accompanying pegmatite intrusion. Since C-O-H fluids generated at >600°C, - 5 kbar can carry significant amounts of both C0 2 and CH4, cooling such a fluid will result in graphite precipitation with no significant isotopicfractionationfromthe original carbon source. The high carbon contents of some rocks (such as graphite gneisses) may have resulted from concentration of carbon during earlier episodes of carbon mobility. In conclusion we find that the graphite deposits of the southern Eyre Peninsula were derived from an original organic sedimentary source. The unique mixed syngenetic and epigenetic features of the deposits are due to metamorphic processes that have mobilised and locally concentrated the graphite mineralisation. The role of the Kalinjala mylonite zone in the genesis of the graphite deposits, if any, is not related to deep-seated fluid movements.

8 E3 Gneiss, Schist, Quartzlte, Graphite Veins £3 Calcsilicate, Pegmatite

-15

-25

-35

S,3C

-5

(per mil)

Figure 1 — Carbon isotope composition of crystalline graphite from gneiss, schist, quartzite, calcsilicate, pegmatite and graphite veins from Hutchison Group metasediments 8 •

Marble

Magnesite

£3 Tertiary calcrete

•35

-25

-15

-5

5 1 3 C (per mil) Figure 2 — Carbon isotopic composition of carbonate samples from Hutchison Group marble, magnesite associated with graphite and Tertiary calcrete.

232

A16-4 THE GEOLOGICAL CONTEXT OF SAPPHIRE OCCURRENCES IN THE ANAKIE REGION, CENTRAL QUEENSLAND P.J. Stephenson

Geology Department, James Cook University Abundant alluvial sapphires are associated with the Hoy volcanic province, which is characterised by alkaline basaltic volcanic plugs. Over 70 plugs occur within a nearly circular area 50 km in diameter, centred 12 km NE of Mt Hoy, here referred to as the Hoy Basaltic Plug Province. Outside this circular area there are outlying, more isolated intrusions, volcanoes andflowswhich show some differences and appear not to have sapphire associations. It is difficult to confirm how many of the plugs in the province contain sapphires but various similarities imply that perhaps most are carriers. The Hoy plug province is located across the faulted east margin of the Devonian-Carboniferous Drummond Basin against the older Retreat Granite (Devonian). Nearly two-thirds of the plugs form a NNE-trending zone 10 km wide near the centre of the field, suggesting a deep structural control. The plugs typically form hills varying in prominence. The contacts of the plugs can rarely be observed, but in the majority of cases appear to be simple near-vertical contacts offinegrained basalt with the country rock. A few breccia bodies have been reported near Rubyvale and at some plug margin. The intrusive character of the plugs is indicated by their oval to circular outcrop, and a generally clear-cut outline on aerial photographs. The plugs range in size up to 400 m in diameter. The Anakie province plugs are consistent in character petrologically. Surprisingly, age determinations have confirmed four distinct periods of activity. Policeman's Knob — 56.2-55.5 Ma 1 plug — 31.3 Ma 3 plugs — 28.2-25.6 Ma 3 plugs — 19.2-17.9 Ma The basaltic rocks are nearly all fine grained, regardless of location within a plug, but several consist of fine dolerite. The basalts invariably have microphenocrysts or larger crystals of olivine. Except for the dolerites the other plugs contain a mixture of plagioclase, olivine, clinopyroxene, anorthoclase, spinel and other oxide xenocrysts, up to megacryst size. Chemically, basanite accounts for nearly half of 52 analysed rocks from 44 centres. Alkali basalt (19%), nepheline hawaiite (15%) and hawaiite (12%) have significant occurrence, but only single specimens of nephelinite, transitional hawaiite and transitional basalt were encountered. Potassic rocks comprise 46% of the total and of these, 70% are basanites. The plug province appears to be the source for the regional alluvial gem deposits which yield abundant sapphires and zircons, and rare diamonds. The first two have been found as rare xenocrysts in two plugs (Mts.

Leura and Pleasant). The question of sapphire and zircon paragenesis, whether semi-cognate or completely accidental xenocrystic, has not been resolved. The sapphires show igneous-like details, especially growth zoning details. The sapphires in this province, and in other basalt occurrences in north Queensland, are found in some variety of host basaltic rocks, making it very unlikely they are some peculiar high pressure phenocryst formed in certain basaltic compositions. Rarely, the sapphires contain C02 fluid inclusions. One example, from an anorthoclase-sapphire xenolith at Mt Leura, has been measured for fluid densities (freezing stage). Extrapolation to magmatic temperatures (assuming 1000°C) indicated pressures around 10 kbar. The fluid inclusions are pseudo-secondary with details indicating necking-down after expansion fracturing around an originally larger fluid inclusion. The estimate of pressure is therefore a minimum, indicating a mantle-depth origin. Irving (1986) confirmed the presence of a second type of fluid inclusion in some sapphires from the district which are multiphase and composed of subequal amounts of two phase C0 and high salinity aqueous fluid containing chloride and other daughter minerals. Irving proposed that the sapphires (and the rare zircon xenocrysts) crystallised originally from phonolitic melts. The sapphires found in their basalt host have narrow pleonaste reaction rims. Green (1978) demonstrated this reaction experimentally, and confirmed the relative speed with which corundum resorbs in basanite melts. The source association of the sapphires remains uncertain. The anorthoclase-sapphire xenolith mentioned is perhaps still unique for the Hoy province, but a parallel is known from Scotland (Upton et al., 1983). Two hypotheses have been proposed to account for the relative abundance of sapphires in alluvials: a) Residual minerals from a formerly extensive basalt flow province. The rare xenocrysts were concentrated residually from a relatively large volume source; b) Residual minerals from pyroclastic deposits associated with the basaltic volcanism, containing higher concentrations of sapphires. However, pyroclastic vents appear to be uncommon, and surface wash 'spec' sapphires are reported across much of the region near the present plugs. It is noteworthy that the sapphire-bearing plugs are implied to range over 25 Ma in age. A preferred interpretation envisages a mantle source region of limited extent which contains older, fractionated sapphire-bearing source rocks, periodically sampled on an accidental basis by mafic volcanism. Transportation of sapphires to the surface required a direct ascent, and pauses would have 2

233

resulted in sapphire, other megacryst and xenolith interpreted on the Rubyvale sheet as younger than the fractionation. The original age of the sapphires and basalts. The Mt Tabletop highlands to the west of the zircons themselves remains unknown, even fission track province rise to 500 m above the level of the plugs and study showing only a basaltic reset age. must have been uplifted in the late Cainozoic; they now The alluvial distributions also present problems, since form a dissected tableland region with radial drainage, some gem-bearing deposits occur over 25 km from the the plug province being on its north east flank. Some plug province (eg the Willows gemfield). The sources sapphire-bearing alluvials occur on the western side of must have been much more extensive, and apparently this tableland divide, presumably isolated by the uplift there were drainage changes during the later Cainozoic. The Hoy province has experienced considerable erosion The present gem gravels are located in alluvialfieldsup to remove its extrusives, and a complex history of to 2 km across,which underlie plains adjacent to some successive reworking with drainage modifications of the present drainage systems (Geol. Surv. Qld., concentrating the residual alluvials. Rubyvale, Geological Sheet 8451). They are older References sediments deposited by drainage broadly similar to that Green, T.H. et al., 1978. Abs Prog., 3rd Aust. Geol. Conv. of today. The geomorphic context and evolution of the (Townsville): 34. region awaits special study, including relationships to Irving, A.J., 1986. Geol. Soc. Aust. Abs 16: 263-264. the lateritic residuals east of the plug province, Upton, B.GJ.etal., 1983. J. Geol. Soc. London 140:105-121. A16.5 BASEMENT MORPHOLOGY, PERMIAN AND JURASSIC TESCHENITES OF THE GUNNEDAH BASIN DJ. Martin * and N.Z. Tadros 1

School of Earth Sciences, Macquarie University NSW Dept of Minerals and Energy, Coal Geology Branch 1

The Sydney-Gunnedah-Bowen Basin has been Gunnedah Basin, the Meandarra Gravity Ridge (Fig. 1) affected by three rifting episodes accompanied by consists of a series of gravity highs, which Qureshi igneous activity. These were: Late Carboniferous/Early (1984) and Murray et al. (1989) suggest are due to large Permian basin formation; a Jurassic tensional regime, high-density utramafic bodies in the upper crust during Gondwanaland break-up; and a tensional event Compression, which started during the mid Permian during the Tertiary opening of the Tasman Sea. and culminated during the mid to late Triassic produced Scheibner (1974) proposed a rift origin for the basin, reverse overthrusting of fault wedges in the basin comand Gibbs (1984) and Lister et al. (1986) outlined the partments along buried ramps and detachment faults features and mechanisms of the crustal extension model. and caused, at least in part, an offset between the gravity Extension occurs by means of tilted half-grabens anomalies and the troughs in the basement (Tadros, developed on lystric faults and detachment ramps. 1988). Transfer/transverse faults alow discontinuous slip of The tensional tectonic regime caused by the Jurassic individual compartments. Thermal subsidence and break-up of Gondwanaland reactivated the deep crustal thining of the lithospheric plate at the basin margin fractures formed in the Permian, providing pathway follow. for the lavas and intrusions. The concentration of Late Carboniferous to the Early Permian volcanic intrusions on the eastern side of the basin may eruptions followed detachment and transverse faults imply a source associated with the Meandarra Gravity andflooredthe developing Gunnedah Basin with basalts, Ridge. while acidic volcanics developed parallel to the basin Jurassic complexes of the basaltic Garrawilla margins. Structure contours on these basement volcanics Volcanics have K-Ar dates of220-170 Ma in the north, outline three NNW-trending sub-basins lying between centre and south of the basin, with 150-145 Ma for latebasement highs (Tadros 1988). The Boggabri Ridge stage phonolites. One of the numerous basaltic intrusions separates the eastern Maules Creek Sub-Basin from the has given a Jurassic age similar to the eruptives, which central West Gunnedah Sub-Basin which is separated many of them underlie. from the western Gilgandra Sub-Basin by the Rocky Geochemically, the intrusions are similar to the Glen Ridge. The sub-basins are sub-divided into a series Garrawilla Volcanics. On a plot of Normative Anorthite of smaller troughs by transverse basement highs (Tadros vs Differentiation Index, a Permian basalt, the intrusions 1988). and the eruptives form a single trendfromnear-primary The West-Gunnedah Sub-Basin corresponds to the basanite to phonolite. Their trace element signatures are Meandarra Gravity Ridge (Tadros 1988) which extends similar. In particular, their Ti/V ratio label them as from the southern end of the Sydney Basin to about intra-plate basalts, which are characteristic of incipient 24°S in the Bowen Basin (Murray et al., 1989). In the and small-scale rifting.

234

151

149

Bouguer anomaly contour Basement structural contour — 30°

Drillhole with intrusion Tertiary Volcanics Garrawilla Volcanics Permian Volcanics

— 31

COONABARABRAN . Warrumbungle Complex

^W *

rU\

• t-3 <• / •

l Range Complex

&M(n\ <J x1 IS C

« L* V^r \)

0 I L

SINGLETON

Figure 1 — Total Bouguer gravity map superimposed on structural contours on the surface on the Permian basement volcanics (after Tadros, 1988).

The Tertiary opening of the Tasman Sea was accompanied by large Tertiary volcanic complexes (Nandewar and Liverpool), however, these volcanic centres have not been part of this study. References Dulhunty, J.A., Middlemost, E.A.K. & Beck, R.W., 1987.7. Proc.. Roy. Soc. NSW 120: 71-90. Gibbs, A.D., 1984. J. Geol. Soc. London. 141: 609-620.

Lister, G.S., Etheridge, M.A. & Symmonds, P.A., 1986. Geology 14: 246-250. Murray, C.G., Scheibner, E. & Walker, R.N., (in Press) J. Geophys. Res. Qureshi, I.R., 1984. Aust. J. Earth. Sci 31: 293-302. Scheibner, E., 1974. Geol. Soc. Aust. J. 20: 405-426. Tadros, N.Z. 1988. NS.W. Geol. Surv. Quart. Notes 73:1-20.

235

A16.6 THE GEOLOGY, GEOCHEMISTRY AND TECTONIC IMPLICATIONS OF METAMORPHOSED MAFIC IGNEOUS ROCKS IN THE WONOMINTA BLOCK, NSW Bo Zhou & KJ. Mills Department of Geology &. Geophysics, University of Sydney

The Wonominta Block is located in far-western Geochemicall, the mafic rocks may be divided into New South Wales, about 200km north-east of Broken two spatially distinct groups, An Eastern Volcanic Hill (Fig. 1). The Wonominta Beds form the foundation Sequence (EVS) has a tholeiitic character, while a of the Block, and are composed of composite sequences Western Volcanic Sequence (WVS) is of alkaline basalt of low to medium grade metamorphic mafic igneous composition. Variations within these sequences are also The Wonominta Block is located in far western New observed. Within the EVS, an increase in Th and a South Wales, about 200 km northeast of Broken Hill decrease in Nb (Ta) occurs from Wilandra to Boshy (Fig. 1). The Wonominta Beds form the foundation of Tank to Ponto Mine; a change that could be explained the Block, and are composed of composite sequences of by increasing crustal contamination of the primary low to medium grade metamorphic mafic igneous and magma. In the WVS, the bimodal character of the Mt sedimentary rocks. The largely unfossiliferous Wright Volcanics contrast with the unimodal pillow Wonominta Beds, containing the various mafic units, lavas and alkaline basalts of the Packsaddle-Nundora are subdivided on the basis of lithology into elongate region; otherwise, all three sampled localities in the strike belts. One of the authors (KJM) has tentatively WVS are chemically indistinguishable. The geochemical assigned ages to these various belts as equivalent to the data support the structural interpretation that the EVS is Willyama Supergroup and the Adelaidean sequence on equivalent to the Willyama Supergroup, and that the the adjacent Broken Hill Block, and to a lower Cambrian WVS is of both Adelaidean (Nundora, Packsaddle) and succession analogous to the Kanmantoo Group in South Lower Cambrian age (Mt Wright). Australia (Fig. 1), and in the Altas of New South Wales The Wonominta Block has been assigned alternately (Harriman & Clifford, 1987). West of Cymbric Vale to a portion of the Precambrian basement. The and west of Mount Arrowsmith richly fossiliferous Early Wonominta Block has been assigned alternatively to a Cambrian limestones are closely associated with other portion of the Precambrian basement over which the western margin of the Tasman Phanerozoic belt mafic volcanic units. The mafic units of the Wonominta Block occur as developed (Veevers, 1984; Powell et al., 1989), or to an tuffs, massive flows, pillow lavas, hypabyssal doleritic equivalent to the Early Cambrian Kanmantoo Fold Belt and dioritic intrusions and gabbroic plutons. All of (Leitch & Scheibner, 1987). In the current study, the these rocks have experienced a degree of penetrative concept of a Tasman Transitional Zone is proposed to deformation and metamorphic reconstitution The mafic describe the transition from the Precambrian craton to units of the Wonominta Block occur as tuffs, massive, the Phanerozoic Tasman Fold Belt in the context of flows, pillow lavas, hypabyssal doleritic and dioritic progressive maturity of continental margins. When intrusions and gabbroic plutons. All of these rocks have continental margins are initially mobilised the volcanic experienced a degree of penetrative deformation and activity is of tholeiitic character; such a character is metamorphic reconstitution that has obscured their shown by the EVS and the basic gneisses in the Willyama relationship to the sedimentary successions in which Supergroup. A later phase of magmatic activity is they occur. In the northern part of the Block the characterised by alkaline basalts, such as can be observed metamorphism is of greenschist facies, while in the WVS and in the Adelaidean Poolamacca Group pumpellyite facies assemblages are found at Comarto on the Broken Hill Block. and at Mt Wright in the south. Within the greenschist References terrain, an upper greenschist facies actinolite-epidote- Harriman, R.J. & Clifford, E.S. (eds.), 1987. Atlas of New South Wales. Central Mapping Authority. Department of chlorite assemblage is observed in the east around Ponto Lands, N.S.W. Mine and Boshy Tank; while a lower greenschist facies Kenny, E.J., 1934. Geological Survey of NSW: Mineral assemblage of chlorite-albite-epidote is found in the Resources No. 36. west on Nundora and Packsaddle. A more complicated Leitch, E.C. & Scheibner, E., 1987. In Leitch, E.C. & metamorphic sequence is revealed at Wilandra, in the Scheibner, E. (eds.): Terrane Accretion and Orogenic south, where a weak pumpellyite assemblage overprints Belts. American Geophysical Union. Geodynamics Series components of the upper greenschist facies. And at 19: 1-19. Cymbric Vale, a Willyama-like sequence of medium Powell, C. McA., Li, Z-X, Thrupp, G.A. & Schmidt, P.W., grade mica schists and amphibolite is observed. This 1989. J. Struct. Geol. (in press). unit may be allochthonous, considering the sharp contrast Veevers, J.J. (ed.), 1984. Phanerozoic Earth History of Australia. Clarendon Press, Oxford: 418 pp. in metamorphic and structural style with the surrounding sequences. t

LEGENO Granitic Rocks Ordovician — Middle Devonian Late Cambrian— Ordovician Early—Middle Cambrian Shelf Facies Early—Middle Cambrian Slope Facies Late fVoterozoic (Adelaidean) with pillow lavas Middle Proterozoic (Willyama Supergroup)

Packsaddle

White Cliffs #

Mt. Wright

/Wilandra

Comarto WILCANNIA

New South Wales 50km

FIG. 1

Figure 1 — Map showing a tentative stratigraphic subdivision of exposures older than Late Devonian in far-western New South Wales.

237

A16.7 THE LOWER CRUST BENEATH THE EASTERN MARGIN OF THE AUSTRALIAN CRATON: XENOLITH EVIDENCE FOR THE GABBRO TO ECLOGITE TRANSITION. NJ. Pearson , S.Y. O'Reilly and W L Griffin 1

School of Earth Sciences, Macquarie University Division of Exp Loration Geosciences, CSIRO, North Ryde 1

Heat flow studies, theoretical modelling and the and across coronas, as well as compositional differences geothermobarometry of xenolith suites all indicate that between the different generations of minerals. young fold belts andriftzones have elevated geotherms The microstructural and mineralogical changes relative to cratonic areas. The large lateral variations in shown by the EMAC xenolith suites are interpreted as temperature at the base of the crust (depending on tec- reflecting progressive cooling, following episodes of tonic environment) are significant to the relative stab- igneous intrusion in the lower crust and upper mantle. ility of eclogite and granulite mineral assemblages. The P-T data obtained from the mafic xenoliths define a restriction of lower-crustal eclogite suites to cratons or curve stretching from ~600°C, 7 kbar to 1000°C, craton-margins, and their apparent absence from younger 21 kbar. This curve lies some 150°C below the Southteiranes, is a consequence of this temperature difference. East Australian geotherm, indicating a distinct thermal Xenolith suites collected from kimberlites and basalt- regime. ic rocks near the eastern margin of the Australian craton The stability of eclogite in the lower crust may be (EMAC) are dominated by mafic lithologies with understood by modelling the thermal evolution of a subordinate spinel and garnet lherzolites and quartzo- lithospheric column which has undergone magmatic feldspathic granulites (qtz+kfs+plag+grt+ky+ rut ± bio). underplating. The advective transfer produces a high, The mafic xenoliths can be divided into four groups: strongly convex geotherm comparable to that established (1) garnet websterites (cpx+opx+grt±sp); (2) mafic for south-east Australia. When underplating ceases, granulites (cpx+opx+plag±grt±amph±qtz±scap); thermal relaxation will move the ambient temperature (3) kyanite-bearing mafic granulites (grt+cpx+ky +plag at depth toward the steady state conductive geotherm. +qtz±opx±scap±rut); (4) eclogites (cpx+grt±amph±rut). As this cooling proceeds, an increasing proportion of Microstructural evidence can be used to identify three mafic compositions will enter the eclogite stability field. transitional stages in the transformation of igneous The proportions of granulite to eclogite will depend not assemblages and microstructures to granoblastic only on P-T constraints but on bulk composition and granulites and eclogites. kinetic factors, as evidenced by the arrested reactions Chemical evidence for the granulite to eclogite seen in many of the EMAC xenoliths. transition includes zoning both within discrete grains A16.8 TRACE ELEMENT RESIDENCE SITES IN METASOMATISED MANTLE: IMPLICATIONS FOR BASALT CONTAMINATION Suzanne Y. O'Reilly and W.L. Griffin 1

School of Earth Sciences, Macquarie University Division of Exploration Geoscience, CSIRO, North Ryde 1

To assess the geochemical characteristics of continental lithospheric mantle, a comprehensive study on a suite of spinel lherzolite xenoliths from western Victoria, southeastern Australia, has been carried out These xenoliths are samples of the lithospheric mande, showing varying degrees of metasomatism. Forty carefully selected whole rock spinel lherzolites have been analysed (using a variety of techniques) for major and trace elements and over thirty for Nd and Sr isotopic composition. Twelve of these were chosen for protonmicroprobe analysis to establish the distribution of trace elements in coexisting phases of cryptically and modally metasomatised lherzolites. To assess mass balance, the modes of the spinel lherzolites analysed by protonmicroprobe were calculated using the whole-rock

composition and electron-microprobe analyses of constituent minerals. Mass balance was established within error limits for all rocks where interstitial glass was not present. This indicates that few of the LIL elements reside in "interstitial sites" or as grain-boundary coatings, but are contained in metasomatic phases. 1. This work demonstrates that the trace-element abudances and patterns of mantle rocks are controlled primarily by the crystal chemistry of metasomatic phases (cryptically-metasomatised clinopyroxenei amphibole, mica, apatite). The variable distribution of these volatile-bearing phases in space and time results in a decoupling of major, minor and trace elements during metasomatism, mainly reflecting crystal/fluid partitioning. For example, Nb is

238

restricted to amphibole or mica-bearing rocks and Sr, REE, Pb, U and Th are most enriched in rocks containing modal apatite. 2. Open-system crystallisation has taken place, with the mode of the rock determining the bulk K between rock<andfluid.If no volatile-bearing phases are formed (amphibole, mica or apatite), uptake of LIL and HFS elements is limited by the capacity of clinopyroxene to accept these elements. 3. The trace element characteristics of some of the mande rocks are similar to those typically ascribed to "crustal contamination" of melts. This applies to LREE, Zr and Ba abundances, and even isotopic signatures, for the western Victorian mantle. These results have important implications for contamination of small-volume, apparently primitive or D

primary magmas, including most continental alkali basaltic types. The LIL and HFSE trace elements in modally metasomatised mantle rocks are concentrated in phases which are less refractory and more easily broken down in heating events (e.g. along magma conduits). When these phases break down, the trace elements cannot be accepted into the residual lherzolite phases and are therefore partitioned into melt, providing very easy contamination of ascending or infiltrating magmas. Therefore, the heterogeneity observed in the trace element patterns of sequences of continental basaltic rocks does not necessarily reflect source heterogeneity. It may merely be the cumulative imprint of varying degrees of contamination by different types of metasomatised lithospheric mantle.

A16.9 A STUDY OF MANTLE XENOLITHS FROM MT GOWRIE, SOUTHEASTERN QUEENSLAND Y. D. Chen and S.Y. O'Reilly School of Earth Sciences, Macquarie University

A suite of upper mantle xenoliths from Mt Gowrie, located within the Main Range Basalt Province in southeastern Queensland is being investigated to determine geochemical (including isotopic) characteristics of the upper mantle, the thermal state of the mantle and the nature of crust-mantle boundary beneath this region. The xenoliths are dominantly spinel lherzolite with less common spinel websterite and wehrlite, and very rare garnet websterite (with altered garnet). The fabrics of these mantle rocks include coarse, porphyroclasitic and mosaic-porphyroclasitic varieties (Harte, 1977). This confirms that the generalised lower crust-upper mantle lithologies are similar to those in other regions beneath the eastern Australian basaltic provinces. Temperatures calculated from these xenoliths range from 900 to 1050°C at the time of entrapment. The uppermost limit of observed spinel lherzolite occurrence corresponding to the lowest T value (i.e. 900°C) defines the maximum depth to the crust-mantle boundary. By reference to xenolith-derived eastern Australian geotherm (O'Reilly & Griffin, 1985), this depth is 30 km.

The Eromanga-Brisbane seismic traverse (Finlayson et al., in press) shows a package of reflectors above and below this depth. The xenolith data suggest the base of the crust is located within this zone of reflectors. No hydrous minerals such as mica and amphibole, indicative of modal metasomatism, have been found in these xenoliths. Since some xenoliths contain abundant fluid inclusions, cryptic metasomatic effects are expected to show in trace element abundances and isotopic compositions currently being analysed. The oxygen fugacites of the upper mantle beneath this region, as calculated from the xenoliths studied by using the olivine-orthopyroxene-spinel oxygen geobarometer (O'Neill & Wall, 1987), lie between the Quartz-FayaliteMagnetite and Wustite-Magnetite buffers. References

Finlayson, D.M., Collins, C.D.N. & Wright, C., in press. BMR Bull. 232. Harte, B., 1977. J. Geol. 85: 279-288. O'Neill, H.St.C. & Wall, V.J., 1987. J. Petrol 28:1169-1191. O'Reilly, S.Y. & Griffin, W.L., 1985. Tectonophysics 111: 41-63.

239

Ba PARTITIONING AND ANORTHOCLASE MEGACRYST GENESIS

A16.10

J.F.Guo, T.H.Green and S.Y.O'Reilly School of Earth Sciences, Macquarie University

Alkali feldspar was crystallised from a natural trachyte composition over a temperature range of 900-1100°C and a pressure range of 10-25 kbar under both hydrous and anhydrous conditions. The orthoclase contents of these synthesised alkali feldspars vary from 38 to 61 (sanidine). Partition coefficients (D ) for Ba between alkali feldspar and the coexisting silicate liquid determined from the experiments varied, from 1.5 to 8.7, depending on pressure, temperature, the composition of alkali feldspar and the presence of water. This encompasses all Ba partitioning data obtained through natural phenocryst/groundmass pairs in volcanic rocks. Evaluation of the effect of feldspar composition on D allows a reasonable extrapolation to give a basis for the interpretation of the origin of anorthoclase megacrysts. The proper choice of experimentally determined D values at different P-T conditions can be used to calculate the Ba content of the parent magma from which anorthoclase crystallised. Partition coefficients determined at 10-15 kbar and 1000-1050°C best approach the condition of anorthoclase formation, with a small amount of water permitted. An intermediate D value (4.2) could be estimated for alkali feldspars (Or ) crystallised under similar conditions in a trachyte composition. Since D decreases with decreasing the K/Na ratio of alkali feldspar, the natural anorthoclase megacrysts of lower K/Na ratios must have a lower D . Extrapolation of D values (1.4) for alkali feldspar of Or and the above intermediate D value (4.2) for alkali feldspars of Or results in an approximation of 2.6 for anorthoclase of Or^, equivalent to the average composition of anorthoclase megacrysts. Anorthoclase could not be produced near the liquidus of alkali basalt composition under any P-T conditions.They probably represent phases precipitated at a late stage from highly evolved derivative of basaltic magma after a large degree of crystallisation of mafic minerals such as olivine, clinopyroxene and orthopyroxene. However, the nature of this evolved magma has long been enigmatic.The D value (2.6) estimated Ba

47J

for alkali feldspar of Or^ is therefore used to calculate the Ba concentration of the parent magma for anorthoclase megacrysts. The majority of anorthoclase megacrysts have BaO (wt)% of about 0.12-0.36%, with all lying in the range of 0.01-0.55 (wt)%. The anticipated silicate melt in equilibrium with anorthoclase would contain Ba in the range 410-1240 ppm. Ba content increases from basalt through hawaiite and mugearite to benmoreite, and decreases towards trachyte. The derivative benmoreite magma contains an amount of Ba close to the whole range of Ba anticipated for magma from which anorthoclase megacrysts crystallised. The measured specific gravities of a series of volcanic rocks and anorthoclase megacrysts also support this result The anorthoclase phenocrysts of trachyte are typically strongly zoned with variable K/ Na ratios in contrast to the homogeneous nature of anorthoclase megacrysts, implying low-pressure dynamic crystallisation, and is unlikely to be the parent magma of anorthoclase megacrysts. High pressure tends to favour the crystallisation of alkali feldspar of higher Or content in trachyte and is also unable to produce the anorthoclase. Other evidence for the origin of anorthoclase megacrysts comesfromisotopic studies. In some cases, the ^Sr/^Sr ratios for anorthoclase megacrysts and the host are far too different for a cogenetic link. In other cases, the Sr^ Sr ratios are not significantly different or overlap each other, suggesting a probable cogenetic relationship. Some new and more accurate isotopic analyses of eastern Australian materials, however, clearly demonstrate that within a given volcanic province, although individual anorthoclase/host pairs exhibit isotopic disequilibrium, the variations in the '"Sr/^Sr ratio for anorthoclase are in the range of ^Sr/^Sr ratio of basalt in that province. This strongly supports that anorthoclase megacrysts in basaltic rocks crystallised from a highly evolved derivative of basaltic magma within that province. 87

240 A16.ll TIME SCALES OF HEATING, METASOMATISM AND DEFORMATION IN THE UPPER MANTLE

W.L. Griffin , D. Smith , S.Y. O'Reilly and C.G. Ryan 1

Division of Exploration Geoscience, CSIRO, North Ryde Dept of Geological Sciences, University of Texas, Austin, Texas School of Earth Sciences, Macquarie University 1

Proton microprobe analyses of trace elements have entrained in their host magma, and on a shorter timescale been combined with diffusion modelling of zoning than the main heating event. Intense shearing in some high-T xenoliths juxtaposes profiles, to determine the time scales of processes that have modified the composition and microstructures of domains of differing composition. Intergrain variations garnet peridotite xenoliths from kimberlites and minettes. in trace element contents and zoning patterns show that The partitioning of Ni between garnet and olivine in these domains have different histories of heating and peridotite xenoliths is strongly temperature(T)-dependent metasomatism. This suggests that they were separated .(Griffin et al., 1989a). The Ni content of olivine is much by some significant distance prior to shearing. A traverse higher than that of garnet, and relatively constant. The across the foliation of PHN1611 (equilibration T Ni content of the garnet therefore gives a direct estimate >1400°C) shows sympathetic variations in the Fe and of equilibration T, and zoning of Ni in such garnets Zn contents of olivine; Ni varies inversely with Zn in records heating (or cooling) events in the mantle. The part of the traverse, and sympathetically in other parts. shape of the rim-to-core concentration gradient is a Boundaries between domains vary from sharp to diffuse. function of the absolute T attained (which controls the Modelling of compositional gradients across these diffusion rate) and the rate of heating. Modelling of Ni boundaries indicates that shearing and transposition of zoning in a sample from Arizona indicates that heating domains occurred on timescales ranging from 10 to 300 from 900°C to 1160°C occurred over a period of ca. days prior to eruption. Domain boundaries formed by 13,000 years (0.02°C/yr). Other samples, with shearing prior to this have been obscured by subsequent equilibration T in the range 1100-1300°C, may record diffusion. The cores of high-T zoned garnets are generally T increases of 50-150°C, while few samples with equilibration T >1300°C show marked Ni zoning. The depleted in Zr and Y compared to garnets from typical implied rates of heating are greater by several orders of low-T peridotite xenoliths, indicating that the protoliths magnitude than conductive heating rates, and imply were depleted, rather than "fertile". The infiltration of that heat was transported into the vicinity of the protolith melt has caused substantial changes in the bulk by the movement of magma, well in advance of eruption. composition of the sheared high-T xenoliths, and many The garnets of high-T peridotite xenoliths, and lower-T, less-sheared ones as well. Since these changes especially those of sheared xenoliths, are commonly shortly predate eruption, the composition of such zoned, with rims enriched in Zr, Ti, Y and Ga relative xenoliths should not be taken as representative of any to the cores (Griffin et al., 1989b). The S-shaped zoning long-lived mantle reservoir. The core-to rim changes of profiles measured in such grains can best be modelled Ti/Zr and Zr/Y in these zoned garnets suggest that the in terms of an overgrowth, followed by partial diffusive infiltrating melts were similar to OIB, and may be equilibration between the core and the overgrowth. related to the parental magma of the low-Cr megacryst Zoning profiles in garnets from African kimberlites and suite. This would place significant timing constraints Colorado Plateau minettes yield diffusion times in the on the possible relationship between the megacryst suite range 50-1000 years prior to eruption. The overgrowths and the host kimberlite or minette. are attributed to infiltration of melt into the xenoliths, References introducing Ca, Fe, Al, Ti, Zr, Y, etc. This infiltration Griffin et al., 1989a. Contr. Mineral. Petrol. 103: 199-202. clearly occurred shortly before the xenoliths were Griffin et al., 1989b. Geochim. Cosmochem. Acta 53:561-567.

241

A16.12 ION PROBE U/Pb ISOTOPIC AGES OF GEMMY ZIRCONS FROM EASTERN AUSTRALIA, INCLUDING TASMANIA F.L. Sutherland and P.D. Kinny 1

Division of Earth Sciences, The Australian Museum, Sydney Research School of Earth Sciences, Australian National University 1

Eastern Australia is noted for abundant Cainozoiczirconfissiontrack (13.9 ± 0.7 Ma) and K-Ar (14.2 ± Mesozoic basaltic rocks and Mesozoic-Palaeozoic 0.1 Ma) dates from host anorthoclase megacrystgranitic rocks. Equally notable are widespread, scattered bearing ne hawaiites and ne mugearites. The evidence occurrences of large and gemmy zircons up to a few cm favour crystallisation of these zircons as late "lower" in size along this magmatic belt (Hollis & Sutherland, temperatures phases from alkaline fractionates of 1985). such host rocks. Many of the zircon finds are alluvial, but some shed 3. In other cases ion probe ages significandy exceed directly from basaltic sources. The zircons show a wide fission track ages (Weld River-Branxholm, NE range in colour, crystal habit and U contents, suggesting Tasmania; Rylstone, NSW; Rubyvale, Qld). These a variety of original sources. Fission track ages on zircons formed from magmas unrelated to those that zircons range up to 190 Ma and mostly represent ages erupted them. reset by heating associated with the Cainozoic-Mesozoic NE Tasmanian zircons particularly show this, as volcanism. Some euhedral zircons associated with felsic the ion probe gave common Permo-Triassic hosts are probably cognate, but larger, corroded crystals (206-290 ± 9-25 Ma), minor Jurassic (146 ± 13 Ma) from basaltic hosts are xenocrysts. They come from and mid-Cretaceous (98-99 ± 5-7 Ma) ages, comsub-volcanic sources of undisclosed age and depth. pared withfissiontrack ages on the zircons between Some potential sources for these zircons include: (a) 42-71 Ma. Three groups in the ion probe ages plutonic felsic bodies related to the Cainozoic-Mesozoic (206-216,223-247,290 Ma) correspond surprisingly volcanism; (b) pegmatitic bodies associated with well with three ages of limited volcanism known in Mesozoic-Palaeozoic granitoids; and (c) metamorphic eastern Tasmania Permian-Triassic sedimentary basement rocks of Palaeozoic-Precambrian ages. sequences, viz. 214 Ma rhyolitic pyroclastics, 233 At several places zircons occur with other gem Ma fractionated alkali basalts and early Permian minerals, such as sapphire, ruby and diamond, some in metabentonites (Clarke & Forsyth, 1989). commercial quantities. To help resolve the origins of 4. Small zircon inclusions in sapphires from the New these gemstones, over forty types of zircons from thirty England gemfield gave ion probe ages around localities from Tasmania to north Queensland were 33-36 ± 3 Ma (Coenraads et al., 1989). This suggests examined by U/Pb isotopic analysis (SHRIMP ion probe growth of sapphires during Tertiary basaltic activity. technique). Similarly, a result on zircon included in a spinel The results showed that: megacryst from the Walcha province suggests that 1. Most of the zircons (>80%) are geologically "young", these minerals crystallised together around 55 ± 5 Ma ranging from Quaternary to late Cretaceous in age. ago. This required special treatment of the data, the 5. Some groups of zircons showed low U contents measured ^Pb being used to estimate the proportion (<35 ppm), comparable to many "kimberlitic" zircons of non-radiogenic Pb present in the zircons. The (Gundagai, Shooters Hill, Bald Knob, NSW; resultant ^Pb/^U ages, quoted here with 2-sigma Rubyvale, McLeanfield,Qld). These proved capable uncertainties (0.0 ± 0.4 Ma to 76.1 ± 5.6 Ma), strongly of yielding measurable ion probe ages, but with link much zircon formation to the post-Tasman rift relatively larger uncertainties (± 10 -56 Ma). Ages basaltic activity. The youngest age (Bullenmerri ranged from 0 to 102 Ma, suggesting that zircons maar, Western Victoria) shows that magmas continue formed and erupted with deep-seated highly to crystallise zircons up to recent times. undersaturated melts at intervals throughout 2. In many cases ion probe ages are within error or Cainozoic-Mesozoic time. Some of these may be close to the zirconfissiontrack ages (Boat Harbour, linked to diamond sources in eastern Australia Tasmania; Trentham, Point Leo, Tolmie, Victoria; (Sutherland et al., 1986). Tumbarumba, Gundagai, Oberon, Barrington, 6. None of the zircon ion probe ages corresponds to Nundle, Uralla, Oban, New England gemfield,New ages of local granitic bodies. This precludes South Wales; Rubyvale, Mt Moffat, Eungella, pegmatites from such bodies as likely sources. The McLean basalt field, Queensland). Thus, these zircons revealed no evidence of significantly older zircons have close affinities to the magmas of the cores. The maximum difference in apparent isotopic related volcanic episodes. age observed within any one grain was 40 Ma (194-234 ± 14-19 Ma, Rubyvale), which may be The Boat Harbour, NW Tasmania, results the result of minor Pb loss. No ages older than (12-20 ± 2-5 Ma) mostly lie within error of both

242

Permian were observed (290 ± 25 Ma, Branxholm), so that older metamorphic basement sources are also unlikely. In synthesis, most zircons probably formed from fractionated basaltic melts related to CainozoicMesozoic magmatism. Sapphires formed with some zircons in the Tertiary (New England), but associations with Permo-Triassic zircons (NE Tasmania and central Queensland) raises the possibility of Mesozoic sapphire sources below these areas. Some low-U zircons formed with highly undersaturated melts and erupted between

more active periods of magmatism and volcanism. References

Clarke, M.J. & Forsyth, S.M. 1989. In Burrett, C.F. & Martin, E.L. (eds): Geology and Mineral Resources of Tasmania. Spec. Publ. Geol. Soc. Aust. 15, 293-338. Coenraads, R.R., Sutherland, F.L. & Kinny, P.D., 1989. MineralMag. in press. Hollis, J.D. & Sutherland, F.L., 1985. Rec. Aust. Mus. 36: 299-311. Sutherland, F.L., Raynor, L.R., Hollis, J.D. & Temby, P.A., 1986. Geol. Soc. Aust. Abst. Ser. 16: 484-486. y

A16.13 DIAMONDIFEROUS LAMPROITES OF WESTERN AUSTRALIA A.L. Jaques Bureau of Mineral Resources, Canberra

Diamondiferous olivine lamproite suites of Proterozoic (1180 Ma) age at Argyle (AK1 pipe) in the East Kimberley and Miocene (20 Ma) age at Ellendale in the West Kimberley, lie within Proterozoic mobile belts surrounding the Precambrian Kimberley block of Western Australia. The Argyle lamproite occurs as a single pipe whereas the Ellendale lamproite forms part of the West Kimberley lamproite province which comprises more than 100 intrusions, occurring as pipes, dykes, plugs, and sills. These are all of Miocene age and range in composition from olivine-rich (29% MgO) to leucite-rich (< 5% MgO, 12% K^O) lamproite (Jaques et al., 1986). The Argyle and Ellendale lamproite suites have high MgO, Ni and Cr contents and are highly enriched in Rb, Sr, Ba, Th, U, K, Nb, Ta, LREE, P, Zr, Hf, and Ti but depleted in Al, Fe, Ca, Na, Sc, and HRE (Jaques et al., 1989). Tertiary lamproites are more strongly enriched in Rb, Ba, Th and LREE, and have higher Ba/Rb, Ba/ La and La/Nb, and lower K/R, and K/Ba. Both suites have strongly radiogenic Sr and unradiogenic Nd isotopic compositions (Jaques et al., 1989a; Nelson et al., 1986). The calculated Sm-Nd trajectories for thee source regions of the Miocene lamproites at 1180 Ma overlap those of the Argyle pipe suggesting derivation from similar enriched sources. Both lamproite suites have crustal Pb isotopic signatures; the Argyle initial Pb isotopic compositions (at 1180 Ma) fall on the average crustal growth curve (Jaques et al., 1989a), whereas the Ellendale lamproites have very high initial PX>P *¥b (Nelson et al., 1986). The lamproites are interpreted as small-volume melts of ancient, formerly refractory subcontinental lithosphere which has undergone long-term (r 2 Ga) geochemical enrichment (high Rb/Sr, low Sm/ Nd). Involvement of recycled early Proterozoic crust in the pedogenesis of the lamproites may explain the very high Ba, the high Ba/La and Pb/La, the low K/Ba and Sr/Nd, the negative Eu anomalies, and the high U/Pb required in the early evolution history by Pb isotopes (Nelson et al., 1986). 7ffr

Rare xenoliths and xenocrysts and inclusions in diamond confirm that the Argyle and Ellendale lamproites are underlain by refractory peridotite, mostly lherzolite. The Argyle peridotite xenoliths, some of which are diamondiferous, equilibrated under reducing conditions (f0 - IW + 2 log units) at 1140-1290°C and 150-180 km depth (i.e. within the diamond stability field) and then retrogressed to spinel peridotite. The PT field defined for the Argyle peridotites xenoliths lies slightly above a 40 mW/m (conductive) geotherm and is similar to that defined for diamondiferous lherzolites from kimberlite. The Ellendale lamproite sampled mantle peridotite with varying degrees of depletion over a range of depths (spinel to garnet peridotite) within the lithosphere. The peridotites are highly refractory, characterised by high Mg/(Mg+Fe) and depletion of lithophile elements and HFS cations. The abundance of lherzolite xenoliths (Argyle), the calcic nature (i.e. lherzolite facies) of Cr-pyrope xenocrysts and inclusions in diamond, and the abundance of clinopyroxene inclusions in Ellendale diamonds indicate less extreme mantle depletion than observed in the roots of Archaean cratons such as the Kaapvaal craton (Boyd & Gurney, 1986). The Argyle peridotites record evidence of metasomatic enrichment in incompatible elements. Diopsides from both heavy mineral concentrate and the xenoliths, including diamondiferous peridotites, have high K^O contents (up to 1%) indicating LIL element enrichment near the base of the lithosphere. Rare Cr-titanate resembling Cr-armalcolite but containing significant K, Ca, Sr, Ba and Zr indicates enrichment of Ti and other HFS cations as well as LIL elements. Most of the Argyle and nearly half the Ellendale inclusion-bearing diamonds are of eclogitic paragenesis. The eclogitic suite includes garnet, omphacite, coesite, kyanite, rutile (Zr and Nb-rich), sulphide, ilmenite and moissanite (SiC). Argyle omphacites are enriched in K 0 (up to 1.3%), Sr and Zr, and Argyle eclogitic garnets have a wide range in Ca-Mg-Fe and many have 2

243

high Ti, Na, P, Y and Zr contents (Jaques et al., 1989b; eclogite apparently by fluid phase, possibly associated Griffin et al., 1988). The eclogitic inclusions, dated at with carbon introduction. Isotopic and trace element 1580 Ma (Richardson, 1986), formed at high tempera- data for the lamproite samples show that the mantle tures (~1250°C) near the base of the lithosphere enrichment is heterogeneous, even on a small scale. (5-7 GPa). The eclogitic suite diamonds from Argyle Part of the enrichment appears to be associated with are strongly depleted in C (-5 to -16%o PDB) and recycling of early Proterozoic crust Peridotitic suite are interpreted as formed from recycled crust (Jaques et diamonds formed from mantle carbon whereas the al., 1989b). The peridotitic suite diamonds have small strongly C-depleted eclogitic diamonds from Argyle negative d C values similar to peridotitic diamonds are inferred to have formed from recycled crustal carbon. elsewhere. Lamproite magmatism in the mid-Proterozoic and midDiamondiferous lamproite and kimberlite sample Miocene resultedfromreactivation of this geochemically mantle at similar depths. Early cratonisation (1.8 Ga) of enriched lithosphere. the mobile belts has resulted in craton-type (up to 220 References km) lithospheric thicknesses beneath the Proterozoic Boyd, F.R & Gurney, J.J., 1986. Science 232: 472^76. mobile belts of the Kimberley region of Western Griffin, W.L., Jaques, A.L., Sie, S.H., Ryan, C.G., Cousens, D.R. & Suter, G.F., 1988. Cora. Mineral. Petrol. 99:143Australia. Both eclogite and peridotite, mostly lherzolite, 158. were stabilised within the diamond stability field in the source regions of the West Australian lamproite suites Jaques, A.L., Lewis, J.D. & Smith, C.B., 1986. Bull. Geol. Surv. WA. 132: 268 pp. prior to the early-mid Proterozoic (> 1.5 Ga). The A.L., Sun, S-S. & Chappell, B.W., 1989a. Geological reduced formerly refractory peridotite may be the Jaques, Society Special Publication No. 14:168-186. residual from early Precambrian tholeiitic basalt Jaques, A.L.,ofAustralia Hall, A.E., Sheraton, J.W., Smith, C.B., Sun, Smagmatism whereas the eclogite is thought to be recycled S., Drew, R.M., Foudoulis, C. & Ellingsen, K., 1989b. oceanic lithosphere. Compared to Archaean cratons such Geological Society of Australia Special Publication No. 14: as the Kaapvaal, this (younger?) subcontinental mantle 976-999. is less refractory but, in common with the roots to such Nelson, D.R., McCulloch, M.T & Sun, S-S, 1986. Geochim. cratons, has undergone long-term geochemical Cosmochim. Acta 50: 231-245. enrichment Both the peridotite and eclogite have been Richardson, S.H., 1986. Nature 322: 623-626. geochemically enriched but to varying extents, the 13

A16.14 COMPARATIVE GARNET, PYROXENE, CHROMITE AND Mg-ILMENITE XENOCRYST COMPOSITIONS IN SELECTED KIMBERLITIC SOURCES AND THEIR RELEVANCE TO DIAMOND EXPLORATION R. R. Ramsay and N. M. S. Rock 1

1 Ballarat CAE, Victoria Key Centre for Strategic Mineral Deposits, University of Western Australia

Diamond is the high pressure polymorph of carbon, (c) Discrete nodules: coarse grained minerals (commonly which is transported to the Earth's surface in alkaline Mg-ilmenite, CPX, garnet, OPX, olivine and zircon), ultramafic melts as a mantle xenocryst (Phillips et al., regarded as the crystallisation products of asthenospheric 1989). At the depths of diamond stability (>150 kms), melts within the upper mantle. Mineral compositions mantle minerals include olivine, Mg-ilmenite, Mg-rich from discrete nodules generally indicate relatively high garnet, clinopyroxene (CPX), orthopyroxene (OPX) and temperatures of equilibration under oxidising conditions Mg-chromite. However, these minerals are also stable which are regarded as a hostile environment for diamond at levels above the diamond stability field and thus crystallisation. occur in a variety of non-diamondiferous rock-types The most appropriate material for assessing the which have transported mantle xenocrysts to the Earth's diamond potential of a source which contains mantle surface. xenocrysts is through the detailed study of polymineralic At depths approaching the diamond stability field, mantle nodules. These provide evidence of mantle minerals from the mantle can be subdivided into three geotherms, mineral compositions which co-exist with diamond, and other processes which may have effected dominant parageneses. (a) Peridotitic: in equilibrium with olivine. The most the diamond content of the mantle (Nixon, 1987). common rock-type is lherzolite (olivine+OPX+ However, mantle nodules from diamondiferous rocks CPX+garnet) but wehrlite (olivine+CPX+garnet) and in Australia have rarely been recovered, possibly because harzburgite (olivine+OPX+garnet) also occur. sampling has generally been restricted to the exposed (b) Eclogitic: not in equilibrium with olivine. The rocks surface of the pipes (where weathering may have consist mostly of omphacitic CPX and calcic almandine- destroyed most of the fragile minerals in the nodules), or because there is an intrinsic rarity of nodules in the pyrope but may also contain rutile and kyanite.

244

host diatremes. Generally, the most abundant mantlederived material available from Australian sources is heavy mineral concentrate. In order to compare minerals from selected Australian heavy mineral concentrates with mande rocktypes as described from nodules, a computer database of published poly-mineralic nodule and discrete nodule mineral analyses has been compiled. A study of mande-derived garnets from Western Australian heavy mineral concentrates indicated that peridotitic and discrete nodule paragenesis garnets were the most abundant in the selected samples (Lucas et al., 1989). A survey of the garnet compositions from peridotitic nodules in the database shows that in addition to distinct pyroxene assemblages, lherzolite, harzburgite and wehrlite carry distinctive garnet compositions (Fig. 1). Evidence of a mineralogical transition between the peridotitic mantle rock-types is represented by several nodules containing subcalcic (harzburgitic) garnet also containing CPX. Peridotitic garnet analyses and other phases co-existing as inclusions within single diamonds suggest that diamond crystallisation has occurred within all these (lherzolitic, harburgitic and wehrlitic) rocktypes. An initial survey of the garnet and pyroxene compositions described as discrete nodules indicates that some have compositions similar to those in peridotitic nodules. This suggests that either some coarse-grained mantle material has become disaggregated during transport to the surface, or the asthenospheric melts had a composition which

crystallised nodules with a similar composition to those in the sub-continental mantle. Reflected light examination of analysed discrete nodule Mg-ilmenite shows that post-crystallisation deformation occurs in grains from many occurrences; multiple growth phases (demonstrated by grain rims in a different optical orientation) may also occur. This suggests that discrete nodules did not crystallise from the host kimberlite magma. It presently appears that heavy mineral concentrate garnets reflect the rock-types traversed during the ascent of a mantle derived magma. Work is continuing to determine whether CPX, OPX and chromite compositions are also indicative of individual mantle rock-types or the processes which resulted in diamond crystallisation. The authors wish to thank the management of CRA Exploration Pty Limited for their support and assistance with the project and the permission to publish this article. References

Lucas, H. L., Ramsay, R. R., Hall, A. E., Smith, C. B. & Sobolev, N. V. 1989. In Ross, J. R. (ed.): Kimberlites and related rocks. Vol. 2. Geol. Soc. AusL Special Publication 14: 809-819. Nixon, P. H. 1987. (ed). Mantle Xenoliths. Wiley, New York: 844 pp. Phillips, D., Onstott, T. C. & Harris, J. W. 1989. Nature 340:460-462. Sobolev N. V., Lavrent'ev, Yu. G., Pokhilenko, N. P. & Usova, L. V.1973. Contrib. Mineral Petrol 40:39-52.

Figure 1 — Garnet compositionsfromperidotitic nodules.

245

A16.15 CONSTRAINTS ON THE COMPOSITION OF THE CONTINENTAL LITHOSPHERIC MANTLE W. F. McDonough and Klaus Peter Jochum * 1

Research School Earth Sciences, Australian National University Max-Planck-Inst it id fur Chemie, Mainz, F. R. Germany 2

Major and trace element data for 374 spinel lherzolite and harzburgites xenoliths from continental basalts have been compiled to obtain an estimate of the composition of the continental lithospheric mantle. For the major elements and compatible trace elements (e.g., Ni, Co) average and median values agree, whereas median values of the incompatible trace elements are systematically lower than average values. Several element ratios (e.g., Zr/Hf, Y/Ho, Nb/Ta (particularly for high quality data)) show limited or no deviation from primitive mantle values, suggesting that these element pairs are not significantlyfractionatedby partial melting. The average and median REE compositions show a smooth LREEenriched pattern, with LREE at about ten and three times CI chondrite, respectively, and a flat HREE pattern at about two times CI chondrite. Relative to primitive mantle the highly incompatible elements in the average and median are enriched, whereas the mildly incompatible elements (e.g., the middle and heavy REE, Hf, Ti, V, Sc, etc.) are depleted. The incompatible patterns in the average and median are similar to that found in ocean island basalts (OIB), although there is a slight Ti depletion (relative to Eu) in the peridotites. For elements that show a strong degree of skewedness in their data it is proposed that the median composition provides a more reasonable estimate of the bulk composition of the continental lithospheric mantle (CLM). Individual peridotite xenoliths with LREE-depleted

compositions have bulk rock incompatible element patterns similar to MORB residues, with increasing depletion from Ce through La to Nb and Ta. In contrast, those with LREE-enriched compositions have bulk rock incompatible element patterns similar to OIB melts, however the degree of enrichment is generally an order of magnitude less. When restricted to only the highest quality data, there is no evidence for Nb-anomalies in the incompatible element patterns of these peridotites, which is inconsistent with these peridotites being sources or residua from convergent-margin (i.e., island arc) magmatism. It is proposed that the CLM grows by underplating refractory peridotitic diapirs onto its base. Incompatible element ratios show that these diapirs are produced dominantly as a result of intraplate and divergent margin magmatism. Absence of a convergent margin chemical signature argues against growth of CLM in such tectonic settings, although this signature may be carried in refractory peridotite in the down-going slab, not in the diapirs which underplate the continents. The average incompatible element character of the bulk CLM does not possess the necessary source composition for continental flood basalts, therefore alternative crustal and or mantle sources need to be considered for these incompatible element-enriched large volume melts. The calculated proportional mass contribution of the CLM to the primitive mantle is about 5% or less for the most incompatible elements.

A16.16 THE Pb ISOTOPIC COMPOSITION OF THE LOWER CRUST: THE ROLE OF MAGMATIC UNDERPLATING R.L. Rudnick * and S.L. Goldstein 12

Research School of Earth Sciences, Australian National University Max-Planck-Inst. fur Chemie, Mainz, West Germany 2

The "Pb paradox" (Allfcgre, 1969) is the apparent contradiction that both the upper crust and depleted mantle have Pb isotopic compositions more radiogenic than the bulk earth, as defined by the geochron. That is, they are not complementary to one another and require the presence of a third, unradiogenic Pb reservoir in order to bring the bulk earth Pb isotopic composition onto the geochron. Because the lower crust is widely believed to be composed of U-depleted, low U/Pb granulite facies lithologies, it is considered a likely candidate to resolve the Pb paradox. To shed light on this problem, we have measured

the Pb isotopic compositions of three suites of well characterised granulite facies xenoliths. Unlike granulite facies terrains, xenoliths carried by Recent volcanic rocks represent samples of the present-day lower crust The xenoliths investigated here are derived from diverse crustal settings: the McBride and Chudleigh volcanic provinces, north Queensland, Australia; and the Eifel volcanics, West Germany; comprising a Proterozoic inlier, Paleozoic fold belt and Cenozoic rift, respectively. The three suites are dominated by mafic lithologies, most of which formed as a result of underplating of the crust by mafic magmas, but felsic and intermediate

246

composition granulite xenoliths also occur in the McBride suite. All three suites have radiogenic Pb isotopic compositions and plot to the right of the geochron, near the fields of modern basalts. Correlations between Sr, Nd and Pb isotopes in the three suites point to an origin by mixing of mantle-derived basaltic magmas with lower crust at the time of basaltic underplating (i.e., <100 Ma for Chudleigh, -300 Ma for McBride and -450 Ma for Eifel). Because the Pb concentration of the continental crust is much greater that that of mantle-derived basaltic magmas, the Pb isotopic composition of the underplated magmas are shifted dramatically by the mixing, allowing delineation of the isotopic characteristics of the surrounding lower crust In all three cases, the lower crust had radiogenic Pb (and Sr) isotopic compositions and unradiogenic Nd isotopic compositions, yielding Proterozoic Nd model ages. Such radiogenic lower crust contrasts markedly with the Pb isotopic characteristics of most Precambrian granulite facies terrains. Whereas the Nd isotopes reflect the average age of crust formation,

the Pb isotopic characteristics of the lower crust appear to be a function of the tectonothermal age of the crust: unradiogenic Pb can only develop in regions which have remained stable for long time periods (e.g., cratons). In areas where orogenies have occurred subsequent to initial crust formation, the Pb isotopic composition of the lower crust is "rejuvenated" through mixing with radiogenic Pb from the upper crust and underplated, mantle-derived magmas. On the basis of this proposed age correlation, we estimate the Pb isotopic composition of the lower crust using the data for granulite xenoliths and granulite terrains. This, combined with an upper crust defined by modern sediments and ores, yields a model total crust composition which falls significantly to the right of the 4.57 Ga geochron. Thus the lower crust does not appear to be sufficiently unradiogenic to balance the radiogenic upper crust and depleted mantle Pb reservoirs and thereby resolve the Pb paradox. Reference Allfcgre, C.J., 1969. Earth Planet. Sci. Lett. 5: 261-269.

247

A17: Antarctic Geology Convenor: I. Buick

A17.1 SUSPECT TERRANES OF THE CENTRAL TRANS ANTARCTIC MOBILE BELT AJ. Rowell * and Margaret N. Rees 1

Department of Geology, University of Kansas, Lawrence, USA Department of Geoscience, University of Nevada, Las Vegas, USA 1

Within the central Transantarctic Mountains, two Their lithofacies demonstrates that they accumulated in subparallel belts of Cambrian rocks record geological tectonically active basins, many of which were walled histories sufficiently distinctive and different from each by Shackleton Limestone whose debris is conspicuous other that it is unlikely that they were deposited in the in the coarse alluvial-fan and braid-plain deposits. Their relative positions that they now occupy. We conclude present distribution, provenance and fades are consistent that minimally two tectonostratigraphic terranes were with deposition in one or more strike-slip basins added to the Greater Antarctic craton in the interval associated with transcurrent faulting. between the late Proterozoic and latest Cambrian. The The outboard belt of Cambrian rocks forms the Liv inboard terrane accreted subsequent to c. 750 Ma (Borg Group of the Queen Maud Mountains and we regard & DePaulo, 1989) but before the deposition of the this block as a suspect terrane (Fig. 1), the Queen Maud Shackleton Limestone in middle Early Cambrian time. Terrane. Its inboard boundary closely approximates an The outboard belt includes Middle Cambrian rocks that isotopic boundary between two crustal provinces, and we consider allochthonous (Rowell & Rees, 1989) and the terrane reportedly lacks Precambrian crystalline it docked probably prior to emplacement of the 500 Ma basement (Borg & DePaulo, 1989). Its Cambrian stratiRoss granites, which crop out in both terranes. graphic sequence is not well known, but it is The isotopic composition of the upper Proterozoic characterised by abundant silicic volcanic rocks together Goldie Formation on the Beardmore Glacier Block (Fig. with limestones. Poorly preserved Middle Cambrian 1) suggests it is part of a terrane that is allochthonous trilobites have been reported from the carbonates but relative to the Antarctic craton as exposed in the Miller the age of the oldest beds is debatable. Range (Borg, DePaulo & Smith, in press). Although the The marked difference in facies of broadly correlative overlying Cambrian succession, the Byrd Group, does strata in the two belts of Cambrian rocks together with not crop out in the Miller Range, the distribution of conspicuous differences in tectonic and volcanic lithofacies within it suggests that it previously extended histories, suggests that these two areas were farther on to the craton forming an overlap assemblage. apart in Middle Cambrian time than they are now. We This assemblage constitutes the inboard belt of consider that present geographic relationships are Cambrian rocks. It consists of a Lower Cambrian car- probably the consequence of accretion of the Queen bonate platform represented by the Shackleton Maud Terrane to the continent prior to intrusion of the Limestone (Rees, Pratt & Rowell, 1989) and a terminal Ross granites. conglomeratic and siliciclastic succession, which References includes the Douglas Conglomerate that rests with Borg, S.G. & DePaulo, D.J. 1980. Abstracts, 28thInternational Geological Congress 1: 173. angular unconformity on underlying strata. Compositional analysis of the unfossiliferous Douglas Borg, S.G., DePaulo, D.P. & Smith, B.M., in press. J. Geophys. Res.. Conglomerate and associated siliciclastic beds indicates Pratt, B.R. & Rowell, A.J., 1989. Sedimentology a recycled orogen provenance: the implication is that Rees,36:M.N., these strata were deposited prior to extensive volcanism Rowell, 341-361. A.J. & Rees, M.N., 1989. Antarctic Science 1(3): associated with the ca. 500 Ma Ross magmatic arc and 249-260. are probably of Middle and perhaps Late Cambrian age.

248

z^Byrd

Gl.

LEGEND

Churchill 0

Cambrian overlap assemblage

Mountains

Cambrian of Queen Maud Terrane _Nimrod

Gl.

Beardmore Glacier Block

M i l l e r _J eardmore Range \ / V v i ^ v ^ B Be

\ v.^r-r s

Gl.

Inboard margin of Beardmore Gl. Block Inboard margin of Queen Maud Terrane

I i , Queen

/170 W

Outcrop of Cambrian sedimentary and volcanic rocks shown in black

v/•

-V

•f

: Maud

-f 130 W 84 S

-f X 88S 89 S

Inboard margin of Beardmore Glacier Block from Borg, DePaulo, & Smith, in press.

249

A17.2 HISTORY OF GEOLOGICAL INVESTIGATIONS IN MACROBERTSON LAND AND ADJOINING PORTIONS OF THE AUSTRALIAN ANTARCTIC TERRITORY P.W. Crohn 1 Durham Road, Surrey Hills, Victoria

Geological investigations in MacRobertson Land and adjoining portions of the Australian Antarctic Territory fall into three major phases. Early investigations, such as those of the BANZARE Expedition of 1929-31 consisted largely of penological studies of collections from isolated coastal landings, combined with ship-board observations of coastal and near-coastal features. Following the establishment of Mawson and Davis Stations in 1954 and 1957 respectively, there was a relatively brief period of regional reconnaissance mapping, which went hand in hand with the exploration of major inland features, notably the Prince Charles Mountains and the Lambert Glacier system, including much new information from aerial observation and air photography. This phase in turn was followed from about 1965 by systematic mapping at 1:250,000 and larger scales, supplemented by more specialised studies with increasing emphasis on laboratory aspects, such as radiometric dating, geochemical studies aimed at elucidating the genesis of various magma types, and ever more refined studies of the sequence of metamorphic and deformational events affecting some

of the major rock units. This paper will deal predominantly with the second of these three phases, which is of interest because it resulted in a series of major discoveries within a relatively short period. Apart from the general mapping and description of the igneous, medium- and highgrade metamorphic rocks which make up most of this area, these included the first accounts of the Permian sedimentary rocks of the Beaver Lake area and the major dolerite dyke swarms of the Vestfold Hills area. There was also the recognition of the dual nature of the charnockitic rocks of MacRobertson Land, including phases with both igneous and metamorphic characteristics, as well as the recognition of major block faulting in the northern Prince Charles Mountains and the recognition of raised beaches on the coastline between Mawson and King Edward VIII Gulf. Concurrent with this work were the first systematic observations on ablation, accumulation, surface temperature and coastward movement of the ice sheet, leading to the first calculations on the mass and energy balances of this ice sheet.

A173 VERTEBRATE FOSSILS FROM MARINE PLAIN, VESTFOLD HILLS, ANTARCTICA P.G. Quilty , R.E. Fordyce , C. Jones and N. Schroeder 1

Australian Antarctic Division, Kingston, Tasmania Geology Department, University of Otago, New Zealand Department of Earth Sciences, Monash University 1

Marine Plain, 10 km south of the Australian station Davis in the Vestfold Hills, is yielding abundant, reasonably well preserved cetaceans from Early Pliocene marine sediments in a section up to 8 m thick. These fossils are the only vertebrate remains so far recovered from Antarctica since the late Eocene and thus are the only vertebrate fossils known from the modern Antarctic ecosystem. So far, six skulls have been returned for study and another remains in the field, still attached to its backbone. Several large fragments of vertebral columns, to about

5 m, remain in thefield.Some 18 closely-spaced fossils localities have so far been identified, all from surface indications. Other vertebrate fossils include a few fish, possible pengion bones but so far no seals. The accompanying invertebrate fauna includes bivalves, ophiuroids, asteroids, and bryozoans, all awaiting expressions of interest in their study. At the time, Antarctica seems to have been significantly warmer than at present and the ice sheet much smaller.

250

A17 Keynote Address ANTARCTIC CENOZOIC GLACIAL HISTORY P.J. Barrett Antarctic Research Centre, Victoria University, NZ.

Antarctic glacial history has until recently been for the most part determined indirectlyfromthe lithologies of deep-sea sediments and oxygen isotope measurements on the benthic and planktic foraminifera they contain. This history has been presented as a progression through a series of thresholds,firstlywith late Eocence cooling followed by widespread glaciation, and subsequent midMiocene formation of the Antarctic ice cap, which has remained a semipermanent feature to the present day (Kennett 1977). Geological evidence from the Antarctic now suggests ice sheet formation in the Oligocene. Early Oligocene glacial sediment has been cored both in McMurdo Sound and Prydz Bay 3000 km apart (Figure 1), supporting the continental scale of early glaciation (Barrett, 1989; Hambrey et al., 1989). However in both areas the glacialpreglacial transition has yet to be sampled and may be somewhat older. The McMurdo core also shows, for the late Oligocene, covariation in ice extent and sea level change consistent with ice volume changes of many tens of metres. This suggests that the Oligocene third order cycles (1-3 m.y.frequency,-100m amplitude) in the Haq et al. (1987) sea level curves are of glacial origin. The younger history of the ice sheet is also being reexamined following the discovery of Pliocene marine diatoms in glacial beds in the Transantarctic Mountains and Prydz Bay (Webb et al., 1984; Pickard et al., 1989). The diatoms indicate the presence of seas in the interior of East Antarctica, and hence the absence of a continental ice sheet, as recently as 3-4 m.y. ago. This implies much larger variations in ice volume than had previously

been considered for the post-Miocene ice cap. Fossil beech leaves and pollen have been found in both Oligocene and Pliocene strata of the Ross Sea region, presenting special problems in explaining the survival of this vegetation through a series of continental glaciations over most of the last 40 Ma. The present cold ice sheet may have therefore been a relatively recent development. References Barrett, PJ. (ed.), 1989. DSIR Bulletin, 245: 251 pp. Hambrey, M.J. et al., 1989. Polar Record 25: 99-106. Haq, B.U. et al 1987. Science 235: 1156-1167. Kennet, J.P. 1977. J. Geophysical Research 82: 3843-3860. Pickard, J. et al 1988. Geology 16: 158-161. Webb, P.N. et al 1984. Geology 12: 287-291.

ROSS SEA

180- CIROSFigure 1 — Map of Antarctica showing the location of the CIROS-1 drillhole and Prydz Bay, the site of recent drilling by the Ocean Drilling Program.

A 17.4 DANIELS RANGE, ANTARCTICA: THE ANSWER TO KANMANTOO CONUNDRUMS R.L. Oliver University of Adelaide

In the Daniels Range, pelitic to semi-pelitic metasedimentary rockss have been deformed by at least three folding events and intruded by an abundance of predominantly felsic igneous material. The mineralogy of the metasedimentary rocks is monotonously biotitequartz-feldspar. The presence in some localities, of garnet and elsewhere of fibrolitic sillimanite and/or cordierite and/or anthophyllite together with plagioclase of intermediate composition indicates a metamorphic grade corresponding to the amphibolite facies. Some of the garnets are zoned, with peripheral compositions reflecting a cooling of crystallisation temperatures, verified by the application of garnet-biotite geo-

thermometry. Disequilibrium compositional variation within some garnet grains is a function of proximity to adjacent retrograde green biotite. Much granodioritic intrusive is closely admixed (migmatitic) with metasedimentary rocks and is known as the Wilson Plutonic Complex. Somewhat more discrete (later?) felsic igneous bodies are classified as the Granite Harbour intrusives. The U-Pb age of the Wilson Plutonic Complex is ca 640 m.y. and the Rb-Sr age of the Granite harbour intrusives is 450-500 m.y. (Sheraton et al., 1987). Several Gondwana reconstructions have been proposed by a number of writers. Some of these suggest

251

correlation of the Wilson terrain in Antarctica with the Glenelg River Complex, in the southwestern corner of the Victoria, and the Kanmantoo in South Australia. There is some support for this, viz.: (i) the lithological similarity (metagreywacke with intermittent pelitic horizons displaying greenschist to upper amphibolite fades metamorphism, the grade being a function of proximity to granitoid intrusions); (ii) ca 500 Ma age of granitoids in both the Daniels Range and the Australian localities mentioned above (plus Tasmania); and (iii) subdivision of the granitoids intruding the Kanmantoo and Glenelg River Complex

into post-tectonic and syn-tectonic intrusives and the subdivision of the Daniels Range granitoids into the Granite Harbour intrusives and those of the Wilson Group. Tectonically, there are suggested similarities between the postulated allochthonous Cambrian and younger Palaeozoic terrain east of the Daniels Range in northern Victoria Land thrust westward against the Antarctic craton and postulated thrusting of the Kanmantoo against Proterozoic Adelaidian and possibly similar histories in the Glenelg River area and in Tasmania.

A17.5 ZIRCON AGES AND THE DISTRIBUTION OF ARCHAEAN AND PROTEROZOIC ROCKS IN THE RAUER ISLANDS P.D. Kinny1* and L.P. Black2 1

Research School of Earth Sciences, Australian National University 2 Bureau of Mineral Resources, Canberra

The Rauer Islands of Prydz Bay in the Australian Antarctic Territory form part of the late-Proterozoic high-grade metamorphic belt of East Antarctica, which also includes the Rayner Complex of Enderby Land and the northern Prince Charles Mountains. They have generally been considered to comprise rocks of predominantly Proterozoic age, with a minor component of reworked Archaean crust, the latter identified by SmNd model ages (Sheraton et al, 1984). Harley (1987) estimated the proportion of Archaean crust to be as low as 10%. However, the results of new fieldwoik (January, 1988) and subsequent laboratory age determinations (Ion probe zircon U-Pb technique) indicates that Archaean rocks may indeed account for up to 50% of exposures in the island group. Archaean rocks occur in a continuous belt on the eastern side of the archipelago, adjacent to the mainland icesheet, including Torkler Island in the south (see Figure 1). The dominant lithology is a homogeneous tonalitic orthogneiss, shown by zircon-dating to be composed of at least two age components, an older 3300 Ma component and a younger 2830 Ma component, which were otherwise indistinguishable in the field. Both components have Sm-Nd rTCHUR model ages between 150 and 300 Ma older than the indicated crystallisation ages, implying limited crustal prehistories for the respective igneous protoliths. These rocks are also associated with minor mafic gneisses and paragneisses. One such paragneiss on Torkler Island was found to contain two principal populations of detrital zircons with ages corresponding exactly to the two identified ages of orthogneiss, implying local derivation. Proterozoic rocks are most prevalent on Filla and Hop Islands, and include a variety of rock-types. Two of the principal lithologies on Filla Island, a K-feldspar

porphyritic granite gneiss (KGG — Harley, 1987) and an equigranular granitic leucogneiss (LCG), have yielded zircon crystallization ages of 1030 Ma and 1000 Ma, respectively. These compare closely with mid-late Proterozoic Sm-Nd model ages obtained previously by Sheraton et al. (1984) from Filla Island and other localities. All zircon populations analysed thus far, whether Archaean of Proterozoic, form linear discordance arrays, each projecting to a lower concordia intercept age of ca 500 Ma. This age is well-known as a time of widespread resetting of mineral isotopic systems over East Antarctica, and also of minor injection of granites, such as at Landing Bluff, some 200 km southwest of the Rauer Group. Evidently, the majority of Archaean zircons were unaffected by the earlier high-grade thermal events which accompanied the formation of the ca 1000 Ma Proterozoic crust Despite the present juxtaposition of the Rauer Group and the Vestfold Hills (Fig. 1), and despite numerous similarities between the rock-types of both terranes, it is now clear that the Rauer Islands do not represent a Proterozoic reworking of the ca. 2500 Ma-old Vestfold Hills crust (see Black et al, this volume). Rather, the Rauer Group must now be considered as a transitional belt comprising a hitherto unrecognised block of Archaean crust up to 3300 Ma in age (and hence clearly older than the Vestfold Block), that was redeformed in late Proterozoic times and mixed with substantial new crust generated at that time. References Black, L.P., Kinny, P.D. & Sheraton, J.W., 1990. This volume. Harley, S.L., 1987. Aust. J. Earth Sci. 34: 175-207. Sheraton, J.W., Black, L.P. & McCulloch, M.T., 1984. Precambrian Res. 26: 169-198.

252

RAUER

ISLANDS

PRYDZ

ANTARCTICA

FILLA IS

LONG Pt

2830 3300 1

TORKLER\lS

28301 VESTFOLD HILLS x

DRAGON CAPE

DAVIS

RAUER GROUP

^ ^

S0RSDAL GLACIER

Figure 1 — Sketch map of the Rauer Islands (after Harley, 1987) showing zircon sample localities and the proposed western limit to Archaean crust Note that rocks of Proterozoic age are not restricted to the western side of this boundary. For example, Harley (1987) mapped the KGG gneisses on Filla Island as far to the east as Efremova Island, and Sheraton et al. (1984) reported Proterozoic model ages for rocks at Dragon Caope in the south.

253

A17.6 A REVISED CHRONOLOGY FOR THE VESTFOLD BLOCK BASED ON ION-PROBE ZIRCON AGES L.P. Black *, P.D. Kinny , J.W. Sheraton 1

Bureau of Mineral Resources, Canberra Research School of Earth Sciences, Australian National University 1

U-Pb zircon ion-probe data and new field studies form the basis of a revised chronostratigraphy for the Vestfold Block of East Antarctica. Previously reported ages do not unequivocally record episodes of igneous crystallisation because they were exclusively based upon Rb-Sr and Sm-Nd systems, both of which are relatively susceptible to isotopic resetting during granulite fades tectonothermal events. The rocks of the Vestfold Block were classified by Collerson et al. (1983) into four units. Two of these have not been dated here, for neither the metasedimentary rocks of the Chelnok supracrustal assemblage nor the mafic/ultramafic rocks of the Tryne metavolcanic rocks are likely to contain syn-depositional zircon. In addition, it is doubtful that the Tryne metavolcanics are sufficiently contiguous and voluminous to warrant classification as a discrete unit. In spite of this conclusion, the rocks of the region are still best considered in terms of four units, because the previously designated Crooked Lake gneiss is retained for the older and generally more mafic (mostly diorite, quartz diorite, monzodiorite and quartz monzonite, and rare gabbro) phases of the originally defined unit, such as those occurring at Crooked Lake itself. Gneiss of dominantly granodioritic composition, occurring as irregular sheets and pods within earlier gneiss, and cropping out sporadically in the north of the Block, is informally named the Grace Lake gneiss. Evidently, this gneiss was emplaced during D -M ^ the last major tectonothermal event to have affected the region, and thus the older Crooked Lake gneisses as defined here would appear to predate D -M . The dominantly tonalitic Mossel gneiss predates D J - M J and D -M , both of which involved intense deformation at granulite facies. The new isotopic data show that Collerson et al. (1983) produced an erroneously expanded chronostratigraphy for the felsic orthogneisses of the Vestfold Block, basically through injudicious sample selection from this complex terrane. Emplacement of all these rocks occurred over an approximately 50 Ma interval at the end of the Archaean. Earliest emplacement (of the 2

Mossel gneiss) was at about 2526 Ma, but neither the Mossel nor Crooked lake gneisses were derived from completely isochronous igneous precursors, both units appearing to show a progressive northwards decrease in age. Emplacement of the Grace Lake gneiss at about 2490 Ma was followed by the intrusion of post-granulitefacies felsic dykes 2480 Ma ago. The major tectonothermal events are separated not by several hundred million years (as claimed by Collerson et al., 1983), but by less than 20 Ma. Most of the analysed rocks contain no isotopic evidence of significantly older crustal pre-history. However, Mossel and Grace Lake gneisses at two localities in the northern part of the Vestfold Block contain, in addition to their -2500 Ma syn-emplacement zircons, components up to 2800 Ma in age, indicating that crust of comparable antiquity underlies these areas. This interpretation is supported by limited Sm-Nd model work. Although these unexposed basement rocks are of similar age to some of the oldest exposed rocks in the adjacent Rauer Islands area, the results of Kinny & Black (1990) show that it is unlikely that the two terranes were juxtaposed in Late Archaean times. The intense -2500 magmatic activity in the Vestfold Block was broadly synchronous with the emplacement of some granites in the Napier Complex (Black et al. 1986), another geochronologically well-documented Archaean craton in East Antarctica. However, the two major tectonothermal events in the Vestfold Hills do not correlate temporally with any of the three major events recognised in the Napier Complex, occurring closest to, but significantly before, the 2456 Ma D^Mj event in that terrane (Black et al., 1983). References

Black, L.P., James, P.R. & Harley, S.L., 1983. J. Met. Geol. 1: 277-303. Black, L.P., Sheraton, J.W. & James, P.R., 1986. Precambrian Research 32: 343-368. Collerson, K.D., Reid, B. & McCulloch, M.T., 1983. Antarctic Earth Science. Australian Academy of Science, Canberra: 77-84. Kinny, P.D. & Black, L.P., 1990. This volume.

254

A17.7 EQUILIBRIA IN GRANULITE FACIES CALCSILICATES: IMPLICATIONS FOR GRANULITE FACIES METAMORPHISM IN EAST ANTARCTICA. I.S. Buick *, S.L. Harley & I.C.W. Fitzsimons 1

IASOS/Department of Geology, University of Tasmania Grant Institute of Geology, University of Edinburgh 2

The role of volatile species in the lower (granulitic) crust is contentious. Models for granulite genesis that invoke pervasive infiltration of C0 -rich fluids to account for the stabilisation of anhydrous granulite assemblages are currently receiving much attention. The use of devolatilisation equilibria to constrain activities of C0 provides a critical test for these models. In this study data from calcsilicates from the Rauer Group and the late Proterozoic North Prince Charles Mountains (NPCM) are presented and the role of volatile species in the stabilisation of the East Antarctic granulite provinces is discussed. Calcsilicates in the Rauer Group experienced late Proterozoic granlite facies metamorphism under (P,T) conditions of (6-8 kbar, 800-850°C). These calcsilicates contain the assemblage wollastonite + grossular garnet (Grs ) + scapolite (EqAn^^) + clinopyroxeneiquartz ±calcite±plagioclase (An ), and occur as concentrically zoned boudins hosted within metapelitic granulites. Concentric compositional and assemblage zonation reflects diffusional metasomatic profiles developed in response to chemical potential gradients in CaO, MgO and to a lesser extent, Si0 and C0 , between boudin cores and the surrounding metapelites. The preservation of wollastonite + grossular garnet + scapolite in boudin cores allows maximum a to be constrained via the decarbonation equilibria: calcite + quartz = wollastonite + C0 1, and 5calcite + meionite + 3quartz = 3grossular + 6C0 2. These equilibria imply maximum a of 0.35-0.5. Low a (<0.2-0.4) is implied by anhydrous assemblages in the surrounding metapelites and felsic gneisses. Similarly, in the NPCM wollastonite-bearing 2

7(V90

95 9 g

CQ2

H2Q

calcsilicates are interlayered with opx-bearing (low a ) felsic gneisses on a metre scale. A for these wollastonite + scapolite + clinopyroxene calcsilicates is constrained to be <0.3-0.4 given (P,T) estimates of (5-6 kbar, c. 750°C) (Fitzsimons, unpubl. data). The widespread distribution of low a calcsilicates throughout the Rauer Group and the NPCM argues against pervasive infiltration of C0 -rich fluids (i.e. X > 0.8) during Proterozoic granulite facies metamorphism in East Antarctica. The a ^ data obtained from calcsilicates alone in both terrains are consistent with two alternate models for fluid processes: (1) a free (essentially binary Hp-C0 )fluidwas present, in which case felsic gneisses and metapelites must have equilibrated with C0 -rich fluids whereas calcsilicates equilibrated with H^O-rich fluids; or (2) vapour absence, such that a and a varied independently, being buffered by the relevent solid phase assemblages. The latter model is favoured because of the lack of fluid inclusions observed in any lithology. Preliminary ( 0, C) stable isotope data indicate that the calcsilicates and marbles have, however, experienced widespread O, and variable C depletion from sedimentary values. This indicates that significant fluid-rock interaction has occurred at some time during their history. On the basis of the available data the fluid involved is likely to have been meteoric water, which flushed through the rocks before granulite facies metamorphism. Such a pre-metamorphic regional isotopic depletion has also been invoked for granulite provinces in Australia (Arunta Complex & Fraser Range Province; Wilson & Baski., 1983). Reference H2Q

CQ2

H2Q

Wilson, A.F. & Baski, A.K., 1983. Precamb. Res. 23:33-56.

A17.8 HIGH-GRADE METAPELITIC MIGMATITES FROM PRYDZ BAY, EAST ANTARCTICA: PROTEROZOIC METAMORPHISM AND MELTING. I.C.W. Fitzsimons* & S.L. Harley Grant Institute of Geology, University of Edinburgh, U.K. Well-layered to migmatitic pelitic and semipelitic lithologies from Brattstrand Bluffs, 80km south-west of Davis Station, East Antarctica record maximum granulite conditions of 6 kbar, 850°C. There is abundant field evidence for the generation and extraction of leucocratic melts in equilibrium with garnet and cordierite from the metapelitic gneisses during and immediately after peak metamorphism:

(a) Migmatitic textures are frequently developed. Pelitic schlieren and pods in a leucocratic garnet- and cordieritebearing matrix are interpreted as restite and recrystallised partial melt respectively. (b) Leucogneiss sheets and lenses, often containing minor garnet and cordierite, cut accross layered and migmatitic units and are interpreted as recrystallised bodies of extracted melt

255

The equlibria involved in this melting event are best preserved in the layered pelites, where apparent melt a late shear fabric, accompanied the input of a fluid production was low, and products and reactants are phase: crd+kfs+Hp=bt(II)+sil+qtz (6) retained in close proximity. grt+kfs+Hp=bt(II)+sil+qtz (7) An early grt(I)-sil-bt(I)-qtz assemblage is preserved (8) in garnet cores, but is replaced in the matrix by an Reaction (8)spl+qtz+kfs+H^ObtaiHsil may form post-shear biotite-sillimanite assemblage characterised by absence of biotite and coronas on spinel. presence of cordierite and second generation garnet on spinel include: Other late, corona-forming reactions (grtfll)-crd(I)-sil-qtz-kfs). Garnet(II) and cordierite(I) spl+qtz=crd (9) spl+qtz=grt+sil (10) are correlated with garnet and cordierite present in the Reactions (6) to (10) depend on very local bulk comleucogneiss material and interpreted as solid products position factors and are interpreted as the result of of incongruent vapour-absent melting of biotite cooling to a stable geotherm following earlier (Thompson, 1982; Grant, 1985; Waters, 1988) by decompression. reactions such as The clockwise decompression-cooling path defined bt(I)+sil+qtz=grt(II)+kfs+L (1) above is consistent with other Proterozoic outcrops of bt(I)+sil+qtz=grt(II)+crd(I)+kfs+L (2) Prydz Bay (Harley, 1988; Stuwe & Powell, 1989), Mineral assemblages continued to evolve during and although in contrast to pelites from the Larsemann Hills after partial melt extraction and crystallisation. The (Stuwe & Powell, 1989), spinel-bearing assemblages appearance of spinel, and then a second generation of formed during decompression and not at the metacordierite, followed reactions such as morphic peak. The decompression was synchronous grt+sil=spl+qtz (3) with progressive extensional deformation culminating grt+sil=spl+qtz+crd(II) (4) in shear zone development (Fitzsimons & Harley, in and produced sil-spl-grt-qtz-kfs, sil-spl-crd(II)-qtz and prep.) which may have accompanied the input of an grt-spl-crd(II)-qtz assemblages. Thermobarometry of aqueous fluid permitting rehydration reactions as the crd-spl-sil-qtz assemblages implies conditions of -5 kbar terrane began to cool. at >800°C, assuming low XH 0 in cordierite, i.e. References decompression of ca. 1 kbar from peak conditions. Grant, J.A., 1985. Am. J. Sci. 285: 409-435. Minor plagioclase growth by the reaction Harley, S.L., 1988. J. Petrol. 29: 1059-1095. Stuwe, K. & Powell, R., 1989. J. Met. Geol. 7: 465-483. grt+qtz+sil=pl (5) suggests further decompression to -3.5 kbar. Re- Thompson, A.B., 1982. Am. J. Sci. 282: 1567-1595. appearance of biotite, particularly in areas affected by Waters, D.J., 1988. J. Met. Geol. 6: 387-404. 2

A17.9 TWO STAGE DECOMPRESSION IN MAFIC GARNET-BEARING GRANULITES FROM SOSTRENE ISLAND, PRYDZ BAY, EAST ANTARCTICA D.E. Thost *, B J. Hensen & Y. Motoyoshi 1

University of New South Wales, Sydney National Institute of Polar Research, Tokyo 1

Preferred P-T estimates for the garnet core Mafic garnet-bearing granulites from Sostrene Island, 150km south-west of Davis Station on the coast of (Py Alm Gr Sp ) are -10 kbar at 980°C. The finePrydz Bay, East Antarctica, exhibit two stage sym- grained symplectite formed at -7.7 kbar and 850°C. plectitic coronas on garnet An outer corona of opx The enclosing felsic gneisses yield P estimates of (Mg^ + plag (An^ 7) + minor hbl mantles a finer between 5 and 7 kbar, which compares with conditions grained inner corona of opx (Mg^) + plag (An ^ + spl of ~6 kbar and 775°C in the nearby Bolingen Islands (Mg ). Both symplectites contain minor ilmenite- (Thost et al., 1988). These lower P-T estimates are magnetite intergrowths. The finer grained symplectite considered to be representative of the widespread 1100 Ma metamorphic event recognised in outcrops also occurs along a fracture cleavage in the garnet. The outer corona originated as a result of decom- along the Prydz Bay Coast. The mode of occurrence of these garnet-bearing pression by the reaction gt + Si0 = opx + plag (1), whereas the inner corona formed later in response to mafic rocks, as relict zones in deformed concordant further uplift and deformation, resulting in the fracture layers within their felsic host, suggests that they are not cleavage in the garnet, according to the reaction allochthonous tectonic inclusions, but that both mafic gt = opx + plag + spl (2). The grossular content of the rock and host have shared a common history. Whether garnet (X^O.168) is almost exactly that which is these mafic layers were in fact once mafic dykes cannot required for the stoichiometric breakdown by reaction be ascertained due to intense deformation, and the mafic bodies may equally well represent flows of sills within (2) (calculated X^O.167). 40

256

the metamorphic protolith. The outcrop may represent a late Archean or early Proterozoic remnant, which has been overprinted by a late Proterozoic metamorphic event, which effectively obliterated evidence of its earlier history, with the exception of the rare relict zones which

have escaped complete re-equilibration to lower P-T conditions. Reference Thost, D.E., Motoyoshi, Y. & Hensen, B.J., 1988. Terra Cognita 8: 247.

257

A18: Granites and Associated Mineralisation Convenor: V. Guthrie

A18 Keynote Address 1 A COMPARATIVE PETROGENETIC STUDY OF GRANITES FROM THE LACHLAN FOLD BELT AND THE CANADIAN APPALACHIANS Ki.Currie * and J.B.Whalen 1

' Bureau of Mineral Resources, Canberra Geological Survey of Canada, Ottawa, Canada

In the Lachlan Fold Belt and the Canadian Appalachians about 20-25% of surface exposures consist of granitoid rocks dominated by relatively silicic varieties (Si0 > 62%) of Silurian to mid-Devonian age. In both cases igneous rocks were emplaced mainly into flyschoid sedimentary sequences of Ordovician and older age. However the granitoid rocks differ in overall composition, in processes of chemical variation, and in magmatic history. Relative to the Lachlan Fold Belt, restite-controlled chemical variation and S-type granites are rare in the Appalachians, and the S-types that are present are richer in Na, indicating derivation from a less mature protolith. Appalachian granites include a higher proportion of A-types, which are both more alkaline and more diverse in age and tectonic style than those of the Lachlan belt Appalachian complexes tend to be distinctly bimodal, in some cases including a mafic component of clear mantle affinities. Appalachian granites exhibit an almost continuous spectrum of (U-Pb zircon) ages from 450 to 340 Ma, with episodes of possibly related magmatism dating back to 800 Ma, compared to a relatively sharp magmatic pulse in the Lachlan belt of late Silurian to midDevonian age, with hints of a late Precambrian episode. These data imply differing plutonic styles for the Canadian Appalachians and the Lachlan Fold Belt with greater recycling of deep continental crust and 2

participation of upper mantle in protoliths of Appalachian granites. The Lachlan Fold Belt and Canadian Appalachians represent near end-members of a continuum of plutonic styles. The Lachlan Fold Belt was dominated by thermal input, producing deformation and concomitant sedimentation as secondary effects, and a brief magmatic pulse appropriate to thermal upwelling and crustal underplating. Terranes in this belt are older than the magmatism, although they affect the availability of protoliths. The Canadian Appalachians, by contrast, were dominated by mechanical input, involving both subduction/obduction and major transcurrent motion. Quiet conditions appropriate for development of restitecontrolled suites rarely occurred, but long-continued evolution of the Iapetan/Atlantic ocean margin gave repeated opportunities for the uprise of mantle material and its interaction with deep crust of varying character. Neither the Lachlan Fold Belt nor the Canadian Appalachians gave rise to typical calc-alkaline sequences, although such sequences (of differing ages) are spatially associated with them. At least three distinct environments of granite (sensu lato) generation must therefore exist The examples of the Lachlan Fold Belt and Canadian Appalachians suggest that such environments will be characterised by distinctive patterns of sedimentation and structure, as well as granitoid plutons.

258

A18.1 THE MARULAN BATHOLITH: AN ATYPICAL GROUP OF EARLY DEVONIAN GRANITOIDS FROM THE EASTERN LACHLAN FOLD BELT Paul F. Carr* , Brian G. Jones and Bruce W. Chappell 1

Department of Geology, University ofWollongong Department of Geology, Australian National University 1

The Marulan Batholith is a composite, subvolcanic intrusive complex comprising at least 14 plutons which are elongate meridionally and occur at the easternmost limit of exposures in the Lachlan Fold Belt (LFB). The Marulan granites are very closely related to lavas, pyroclastics and volcaniclastics of the Bindook Volcanic Complex and the relatively small size of the batholith (approximately 220 km ) is, in part, a consequence of its having been only partly unroofed. K-Ar data for biotites together with Rb-Sr biotite-whole rock ages indicate emplacement between 406 and 391 Ma and an initial Sr isotopic ratio of approximately 0.7060. These isotopic data accord with biostratigraphic data which indicate a minimum age of Late Devonian and a maximum age of Late Silurian to earliest Devonian. The granites of the Marulan complex are therefore of the same general age as most of those elsewhere in the LFB and are part of the world-wide, Caledonian-age magmatic event at close to 400 Ma. Granites of the Marulan Batholith are I-type and despite the close similarity in age, their chemical character differs in detail from most other granites of that type in the fold belt First, their overall compositional patterns are unusual in showing a continuous and fairly evenly distributed range of compositions from 55% to 73% silica. The largest body, Arthursleigh (40 km ), itself covers a large proportion of that range (56.6% to 70.0% silica) and has a well developed, unusual zonation from a relatively mafic core to a more felsic rim. In contrast, the second largest unit (Glenrock, 25 km ) is exceptionally homogeneous with four widely 2

separated samples having indistinguishable major and trace element contents. Second, trends for Ba and Zr which both increase with increasing silica are unusual for the LFB. Third, the Marulan granites contain high concentrations of As, Sb and Mo relative to other I-type granites of the LFB. Plutons of the Marulan Batholith are located only a few kilometres from the northernmost outcrops of the Bega Batholith which is of similar or slightly younger age. This latter batholith is extensive and can be traced from Bass Strait to north of Braidwood over a distance of some 250 km. The distinctive features of the Marulan granites terminate abruptly at the northern end of the Bega Batholith close to latitude 35° south and have led to the recognition of a fundamental boundary between the Bega and Bathurst Basement Terranes. The Marulan granites are also fairly closely associated in the field with some of the Carboniferous granites that extend along the western margin of the Sydney Basin and share the Bathurst Basement Terrane with these younger plutons. This close association of granites of different ages is unusual and may reflect the intersection of two separate magmatic arcs of different ages. In fact, the Marulan granites may be the only plutonic arc (in the sense in which the term is applied to younger Cordilleran granites) in the LFB, and this may account for its distinctive features relative to the rest of that belt It is this possibility, and its location in the easternmost part of the exposed LFB, that lends considerable interest to its study.

A18.2 THE HIGHLANDS IGNEOUS COMPLEX, ARMIDALE: A HIGH K 0 AND HIGH MgO SYENITIC TO GRANITIC SUITE, THE PROBLEM OF ITS SOURCE MAGMAS. 2

Jonathan A. Kilpatrick Department of Geology, Australian National University

The Permian Highlands Igneous Complex (HIC), northeast of Armidale N.S.W., consists of a central adamellite, arcuate quartz monzonite and marginal quartz syenites and granites emplaced at a shallow level. Two small areas, the Sugarloaf and Puddledock Associations, contain a number of intrusions. The inferred sequence of intrusion has the quartz syenites earliest, followed by quartz monzonites, adamellites, and then the granites. Mineralogical variation was investigated. Olivine (Mg/(Mg+Fe ) = 72-75) occurs in the quartz syenites (Mg/(Mg+Fe ) = 75). Clinopyroxene (diopside, salite and augite) and orthopyroxene (Mg/(Mg+Fe ) = tot 2+

tot

87.4-54.3) record crystallisation from high pressure (10-12 kbar)/high temperature (>1100°C), low pressure/ high temperature, to low pressure/low temperature. High pressure pyroxenes contained in the HIC have Ca:Mg:Fe+Mn ratios (26.2:53.5:20.3) identical to the Sugarloaf and Puddledock Syenites. Biotite shows decreasing Mg/(Mg+Fe ) through the sequence and its early crystallization was promoted by the highly potassic chemistry of the HIC magmas. Plagioclase cores show decreasing An content through the sequence. Chromian magnetite was an early phase in the primary HIC magmas. It is suggested that crystallisation has occurred tot

259

since magma separation from the source, and that early parental magma composition. Due to its high temperature precipitated minerals (clinopyroxene, orthopyroxene and (>1200°C) it most likely represents a liquid. chromian magnetite) have been retained in the ascending The generation of the primary magmas for the HIC magma. is problematical. The kinked geochemical trends and The cognate enclaves of the HIC are petrographically the evidence of fractionationfromthe deep crust discount identical, although more mafic, to the host intrusions magma mixing. Fractional crystallization requires a and contain identical mineralogical trends. It appears starting basalt composition with particularly high KÔ that these cognate enclaves are related to the HIC which undergoes an extremely complex array of magmas, and formed as a result of flow fractionation. crystallizing phases and extreme degrees of fractionation This process calls for the coating of conduit walls with and is thus considered unlikely. Partial melting of the chills from a series of co-genetic magmas that rise lower crust could not generate the HIC magmas. It is through the crust, and the incorporation of these coatings, proposed that the least evolved HIC magmas were as cognate enclaves, in the rising magmas. generated by a localised partial melting episode within The HIC is characterised by high Kfi, MgO, Cr, a gradational heterogeneous phlogopite bearing crust/ and Ni abundances.The Mg/(Mg+Fe *) values range mantle wedge interface at a depth greater than 40 km from 74.9 to 45.7. Distinct geochemical trends are with temperatures in excess of 1200°C. Due to the apparent with the least evolved magmas being the complex tectonic history of the New England such a Sugarloaf and Puddledock Syenites. These syenites source is likely beneath the HIC. Two subduction although spacially isolated have nearly identical periods have enriched and diffused the crust/mantle chemistry. The Sugarloaf Syenite's analysis is listed wedge interface. Similarites between the HIC and other below. New England intrusions suggests that this source is This composition is regarded as being close to the extensive. Si0 Ti0 A1A Fe 0 FeO MnO MgO CaO NaÔ KÔ P 0 LOI (wt%) 2

6036 1.19 11.87 0.69 4.48 0.10

7.49 4.08 2.09 6 30 034 0.5

Cr 349

Zr 308

Ni Zn 252 76

Rb 282

Sr 465

Ba 825

Nb 16

Y 21

Th 22

La 43

Cc 98

U 5

Ga Pb 18 18

V 94

Cu (ppm) 35

A183 HORNFELS AND MIGMATITE ABUTTING THE BOGONG GRANITE: INFILTRATION AT WORK MJ. Drummond, Brenda J. Franklin * and Brian Marshall Department of Applied Geology, University of Technology, Sydney

Mafic hornfelses (ol-amph-cpx-opx-plag), stromatic biotite-migmatites, and cordierite-biotite and calc-silicate rocks occur as two large roof pendants within the highlevel, I-type, Early Devonian Bogong Granite, east of Tumut in southern NSW. Several other occurrences of high-grade hornfelses border the Bogong Granite, but the greater part of the aureole is of much lower grade. Previous work on the high grade hornfelses invoked special metasomatic processes because of supposed unusual rock compositions, whereas the migmatites and associated rocks were said to be a fragment of exotic (Ordovician?) basement brought up with the Bogong Granite, or emplaced with the Early Silurian Coolac Serpentinite. The present work opposes these views. Mapping shows a transition over a few hundred metres from low-grade metabasalt of the Tumut Trough sequence, through medium-grade hornfels, into highgrade, olivine-bearing mafic hornfels. Analytical data also strongly suggests that the migmatites, cordieritebiotite and calc-silicate hornfelses all have low-grade correlates within the Silurian Tumut Trough sequence. The above correlation is supported by mesoscale structural analysis to the extent that similar deformational

histories characterise the low-grade rocks and highgrade mafic hornfelses, and microstructural studies confirm that the high-grade mineralogy has numerically enhanced the Late Silurian fabric elements. Structural analysis of the stromatic migmatites is less compelling; it demonstrates that a more complex deformational history is associated with zones of more intense leucosome production. Although such locally developed complexity is consistent with rheological changes induced by partial melting, a simplistic correlation with the Late Silurian fabric elements is not possible. The preponderance of evidence suggests that the high-grade hornfels assemblages may be ascribed to a heat source involving the Bogong Granite and minor (earlier?) dioritic phases. The Bogong Granite and dioritic phases are capable of producing the high grade assemblages (up to 700°C close to the granite) provided that heat transfer is efficient. Infiltration processes involving a reactive aqueous fluid could have achieved efficient heat transfer, and are invoked to explain the variability of the Bogong Granite aureole. The infiltration mechanism could also have facilitated localised migmatite production.

260

A18.4 THE STRUCTURE, PETROLOGY AND GEOCHEMISTRY OF THE SCOTTSDALE BATHOLITH, NE TASMANIA M.P. McClenaghan Department of Resources and Energy, Hobart

The granitoids of the composite Scottsdale Batholith intruded the folded Ordovician to Lower Devonian (Banks & Smith, 1968; Rickards & Banks, 1979) Mathinna Beds, which consist of interbedded quartzwacke, siltstone, and slate, regionally metamorphosed to low grade. The intrusions produced narrow contact metamorphic aureoles, and took place shortly after regional folding of the country rocks, which is correlated with the Tabberabberan deformation of eastern Australia. K/Ar biotite dating has given ages of 370-395 m.y. (McDougall & Leggo, 1965; unpublished data). Individual plutons are generally steep sided and elongated north-south in the general fold-trend. On the basis of petrographic and geochemical characteristics the batholith can be divided into three suites: the Diddleum, which consists entirely of granodiorite; the Russells Road, which ranges in composition from granodiorite to granite; and the Mt Stronach, which ranges in composition from adamellite to alkali-feldspar granite. The Diddleum pluton forms the more basic part of the Diddleum suite and lies on the western side of the batholith being the only pluton to the west of a north-south screen of Mathinna Beds. The other plutons in the suite are those of the Tulendeena and Porcupine Creek. The Russells Road suite consists of the large Russells Road pluton which has not yet been completely mapped and may be a composite. The Mt. Stronach suite consists of the Mt Stronach pluton near Scottsdale and the granitoids of the Ben Lomond area. The mineralogy of the granitoid bodies is very similar consisting generally of plagioclase (An 70-20), quartz, K-feldspar, amphibole and biotite with accessory apatite, zircon, ilmenite, sphene and allanite. Rare augite is present in some specimens from the Diddleum pluton. Amphibole ranges in composition from actinolite to hornblende, and amphibole and biotite often occur together in composite patches. Plagioclase frequently

shows distinct core regions of approximately uniform composition (An 50-70) rich in fine sericite and surrounded by clear oscillatory zoned rims (An 50-20). The granites and some of the adamellites have biotite as the only mafic mineral. The Mt Stronach suite granitoids have annite instead of biotite and the plagioclase is albite. Plots of chemical components against total iron as FeO show straight line relationships for the Diddleum and the Russells Road suites. The trends for some components (eg MgO, CaO, Na^, Sr, Zr, V, Cr, Ni) are clearly different between the two suites while others are very similar. The trends for the Mt Stronach suite appear continuous with those of the Russells Road but show marked enrichments in some components (eg Rb, Th, Y) compared to the Russells Road suite. The straight line relationship shown by the chemical plots and the presence of plagioclase crystals with uniform core compositions suggest that the variation in the granitoid composition may have been produced by the unmixing of restite and granitic melt. Using the criteria of the restite model (Chappell et al., 1987) to estimate melt, restite and source rock composition, the Diddleum and Russells Road suite source rocks are considered to have been andesite and dacite. The strong enrichment in Rb, Th, and Y for the Mt Stronach suite granitoids suggest that they were produced by crystal fractionation of the Russells Road suite magma that was free of restite material. Metasomatic alteration may also have affected the composition of the Mt Stronach and some of the Russells Road suite rocks. References Banks, M.R. & Smith, A., 1968. Aust. ScL 31: 118-119. Chappell, B.W., White, A.J.R. & Wyborn, D., 1987. J. Petrology 28: 111-1138. McDougall, I. & Leggo, P.J., 1965. J. Geol. Soc. Aust. 12: 295-332. Rickards, R.B. & Banks, M.R., 1979. Alcheringa 3:307-311.

A18.5 GRANITES AND TECTONICS B.W. Chappell Department of Geology, Australian National University

Many granites, particularly those that are more mafic, form images of their sources, so that their chemical composition reflects of that of their source rocks. Hence, for example, those granites derived from older igneous rocks are generally metaluminous, and those that came from metasedimentary sources are peraluminous. Within these broad I-type and S-type groups (White & Chappell,

1988; Chappell & Stephens, 1988) there is much finestructure, recognizable as suites, corresponding to more subtle differences in source-rock composition. Granites are produced when 'fertile' rocks within the crust are heated to a temperature sufficient to cause melting of those fertile or essential components — quartz, feldspars, and water that is either free or the

261

result of breakdown of hydrous phases. The occurrence and unique insight into the tectonics of the belt, since of granites is therefore related to the development of they also enable us to look at even older events that sufficiently high temperatures in fertile crust. These shaped the character of their source rocks. Data obtained temperatures may resultfromthe intrusion of hot mantle from the granites of that belt indicate that those rocks material, or continental collision, CM* from processes not formed in a tectonic environment unlike any known at yet fully understood. The occurrence of granites per se the present time. Observations relevant to this conclusion does not therefore have any necessary tectonic include the much greater width of the LFB relative to significance, although it is observed that they are many younger belts, the widespread development of Sconcentrated at what were active continental margins; type granites that make up slightly more than half the that occurrence could however result from the presence exposures of granite in the belt, distinct differences in of fertile rocks in such regions, as much as resulting composition of most of the I-type granites (e.g. lower from any particular tectonic regime, or heat source. Na/K and generally granodioritic rather than tonalitic Young volcanic rocks are used to discuss what are compositions) and the more evolved isotopic virtually current tectonic regimes. However, even in compositions of those granites (Chappell & Stephens, this simplest case, volcanoes may have compositions 1988). that do not conform to their present tectonic setting in Studies of granites in the Lachlan Fold Beit have led terms of conventional models. Johnson (1987) has to a tectonic model involving the earlier assembly of pointed this out in the case of western Melanesia and microcontinents at a time near the Proterozoicsuggested that it results from the mantle source regions Palaeozoic boundary. These separate terranes were having been produced during earlier subduction. This is covered by extensive Ordovician flysch deposits and analogous to the situation for granites. Further problems are now evident only from the provincial characteristics arise with volcanic rocks since those remnants not of the granites derived from those different terranes, destroyed by erosion are generally highly altered and which have been termed basement terranes by Chappell modified in chemical composition. In contrast, old et al. (1988). There is no evidence for subduction or granites are generally pristine, and even when mineral- plate movements at the time of granite magmatism, but ogically altered can generally be shown to have preserved there is evidence for such processes having taken place their chemical integrity. If petrology is to be used in at an earlier stage in the development of the fold belt, analyzing old tectonic environments, it is therefore and these may have been a necessary precursor to the necessary to develop a fuller understanding of granite intense episode of magmatic activity that occurred at around 400 Ma. genesis. Because the compositions of most granites reflect, References to some extent at least, those of their sources, care must Chappell, B.W. & Stephens, W.E. 1988. Trans. R. Soc. Edinburgh: Earth Sciences 79: 71-86. be taken in using granite compositions directly in tectonic reconstructions. The composition of granites might Chappell, B.W., White, A.J.R. & and Hine, R. 1988. Aust. J. Earth Sci. 35: 505-521. reflect tectonic events that produced the source rocks Johnson, R.W., 1987. Pacific Rim Congress 87: 211-214. and which could have occurred long before the formation White, A.J.R. & Chappell, B.W. 1988. Trans. R. Soc. of the granite. However, studies of granites in the Lachlan Edinburgh, Earth Sciences 79: 169-181. Fold Belt have shown that they do provide a powerful A18.6 PETROLOGY OF PROTEROZOIC IGNEOUS CHARNOCKITES FROM MAWSON, ANTARCTICA: HIGH-TEMPERATURE, SYN-OROGENIC GRANITOIDS PRODUCED BY ANATEXIS IN THICKENED CRUST D.N. Young* and D J. Ellis Geology Department, Australian National University

Charnockites of the Mawson Coast, Antarctica, are a series of granitoid plutons of intermediate composition (mainly 55-65 % Si0 ) which intrude granulite-facies supracrustal gneiss. Intrusive field relationships and an igneous origin for the charnockites werefirstrecognised by Crohn (1959). The composite batholith outcrops over an area of at least 3100 km and perhaps as much as 5000 km . The basement gneisses preserve evidence for two folding events which predate the charnockites. A third deformation folded the gneisses again, and produced a foliation within the charnockites as well as 2

flattening of enclaves and xenoliths. The mafic mineralogy of charnockites is dominated by orthopyroxene with lesser biotite and ilmenite. Many of the more mafic charnockites have orthopyroxene plus clinopyroxene and felsic rocks may have metamorphic garnet, but amphibole is absentfrommost rocks. If the near-anhydrous mineral assemblage was formed by igneous crystallisation rather than a metamorphic overprint of mica or amphibole bearing granites, then high magmatic temperatures and low water contents are required. Despite the metamorphic overprint

262

which recrystallised all charnockites to varying degrees, evidence can be found for high magmatic temperatures in the form of remnant phenocrysts of antiperthite with strongly ternary bulk compositions (high Or contents), complexly exsolved pyroxenes (albeit largely disturbed and recrystallised by deformation), and extensive partial melting of granulite-facies gneissic xenoliths. A number of suites are recognised from regional petrographic and whole-rock geochemical study of the batholith. These can be grouped into low- and high-Ti categories. Compared to typical crustal granites and calcalkaline rocks, both types of charnockite have unusual geochemical compositions such as high An and Or but low Ab, and an enriched component of FeO plus MgO compared to the other components on an ACF ternary plot. This, along with their near-anhydrous mineralogy and the evidence for very high magmatic temperatures, reflects their unusual pedogenesis. They are thought to have formed by high degrees of melting of intermediate to mafic crust under conditions of very high temperature and low water contents, the low-Ti magmas forming at a deeper level in the crust than the high-Ti magmas. The high-Ti charnockites were previously described by Sheraton (1982) and have some affinity to A-type granites, with high contents of elements such as Nb, Y, Zr and Ga. The distinction is that the high-Ti Mawson charnockites were emplaced during rather than after orogeny, and have considerably lower Si0 and higher P 0 and Ti0 than typical A-type granites. The low-Ti charnockites were also intruded after D but before D 2

and were not recognised as a separate group before this study. They are distinguished from high-Ti rocks by lower P 0 , Ti0 and Zr and higher Ni, Cu and Mg/ Mg+Fe for a given silica content. The most mafic lowTi rocks are weakly metaluminous or weakly peraluminous, whereas the most mafic high-Ti rocks are more strongly metaluminous. Many low-Ti rocks are strongly depleted in heavy rare earth elements, a feature not found in any high-Ti rocks. This requires garnet in the residue from partial melting, indicating pressures >15 kbar and providing evidence for an overthickened crust during magma genesis approximately 960 Ma ago. An ion-probe U-Pb study of zircons (Young & Black, in prep.) has found igneous ages of 954 ±12 and 985 ±29 Ma for samples of low-Ti and high-Ti charnockites respectively. These are consistent with a reinterpnetation of existing Rb-Sr data (Arriens, pers. comm.) to give an age of 959 ±58 Ma rather than the reported age of 1070 ±30 Ma. Zircon rims in a xenolith of gneiss give a U-Pb age of 921 ±19 Ma, thought to record the post-chamockite deformation D . Most zircon cores in this gneiss sample are 1.6-2.0 Ga old, providing evidence against an Archaean age for the basement sequence at Mawson. 2

References Crohn, P.W., 1959. BMR Bull. 52. Sheraton, J.W., 1982. In Craddock, C. (ed.): Antarctic Geoscience. Young, D.N. & Black, L.P., in prep.

A18.7 POST DELAMERIAN MAGMATISM — IS LITHOSPHERIC THINNING GUILTY? Simon Turner* and John Foden Department of Geology and Geophysics, University of Adelaide

The Cambro-Ordovician Delamerian Orogeny brought the sedimentological history of the Adelaide Geosyncline to a close producing a Buchan-style metamorphic fold belt The eastern edge of outcrop in the fold belt is composed of the Cambrian Kanmantoo Group in which deformation is often quite intense and metamorphic grades reach middle to upper amphibolite facies. Associated with this high grade region are various, predominantly granitic, intrusives which can be divided on structural, petrographic and geochemical grounds into syn- and post-tectonic suites. The Early Ordovician post-tectonic magmatism was bimodal, producing both mafic dykes and plutons as well as high silica granites and rhyolites. These posttectonic granites and associated volcanics extend across the south east of South Australia and into western Victoria where a similar syn- and post-tectonic magmatic history in the Glenelg River Complex supports other evidence that this area is part of the Kanmantoo terrain. The post-tectonic granites evolved as relatively dry,

high temperature magmas as indicated by their low modal content of hydrous phases, the occurrence of pyroxene and even olivine and their evolution to onefeldspar mineralogy (hypersolvus granites). The presence of granophyric intergrowths, beta quartz and the association with volcanics indicates that they were intruded fairly rapidly to high crustal levels. These rocks evolved via extensive low pressure fractionation at the granite minimum and their initial Sr and Nd isotope ratios suggest a relatively primitive source. They show low A1 and Ca contents and are enriched in LREE, Nb, Y, Zr, Ga and F. These features are characteristic of A-type granites which are typically associated with anorogenic or extensional tectonic settings. The mafic component of this post tectonic suite provides additional evidence for a post Delamerian thermal pulse and is manifested by the Black Hill intrusive complex and basaltic dykes. The Mannum Granite, for example, lies close to one of the Black Hill plutons, is cut by a dolerite dyke, and shows convincing

263

evidence of mingling with mafic magmas. The Black assimilation. Mineralogical trends such as the presence Hill intrusive complex also contains numerous felsic of an olivine hiatus and a pigeonite zone are reminiscent veins or dykes with compositions like those of the post- of the Skaergaard intrusion. Plots of incompatible tectonic granite suite. These are very much like the late elements, rare earths and Nd isotopes show that the stage granophyresfromsimilar layered mafic intrusives original tholeiitic magma was not simply a depleted and in places these, too, show evidence of mixing and asthenospheric (MORB-like) melt but contained a hybridization with the mafic magmas. from an incompatible element enriched The undeformed Early Ordovician Black Hill component However this component did not have high Rb, intrusive complex, which lies just to the east of the Late Ksource. or initial Sr ratios, which suggests that it may have Proterozoic and Early Palaeozoic sequences in the Mt been enriched, sub-continental lithospheric mantle rather Lofty Ranges, is evidence of major mantle derived than continental crust. mafic activity following the Delamerian Orogeny. The The current data suggests that post-Delamerian complex comprises at least three large, layered plutons magmatism may have involved melting of an with a continental tholeiitic nature which are not unlike incompatible element-enriched lithospheric mantle or those of the extensional Tertiary igneous province of lower crust The melts produced were intruded rapidly the north-east Atlantic. At Black Hill the range of into the upper crust where the granites and rhyolites lithologies extends from peridotites and troctolites may have evolved as highly differentiated silicic caps through olivine gabbros and norites to gabbronorites, to the mafic plutons. In particular, the required thermal pyroxene monzonites and granites. Geochemistry pulse, coupled with the cessation of deformation, could indicates that the Black Hill magma evolved via be explained by thinning of the lower lithosphere in plagioclase + olivine and plagioclase + pyroxene frac- response to Delamerian crustal thickening. tionation combined with concommitant crustal A18.8 THE PRECURSORS OF RAPAKIVI OVOIDS John Elliston Elliston Research Associates Pty Ltd, Castlecrag, NSW

Detailed studies of rapakivi granites (Sederholm, observed features should be used as an indication of 1928; Backlund, 1938; Dawes, 1966; Elders, 1968; how they may have formed in the same way as the Marmo, 1971; Vorma, 1971; Stull, 1978; Key & Wright, observation of a fossil is used as an indication of the age 1982; Elliston, 1985) have determined that this texture of a rock. could not be the direct result of melt cooling. A The writer (Elliston, 1985) has listed 46 detailed metasomatic or auto-metasomatic origin is usually observations relating to rapakivi granites which are proposed. consistent with their crystallisation from re-mobilized However, many geologists have believed that granite or intruded sedimentary materials containing accretions. fluidity can only be due to melting and therefore assume The review of oibicules (Elliston, 1984) established that the metasomatic changes which giveriseto rapakivi that 28 positively documented features of these texture must apply to materialfirstproduced by fusion spectacular spheroidal structures which occur in granites, of sediments. This committed attitude unfortunately certain basic rocks, and sediments equate precisely with makes it appear unnecessary to investigate the those of colloidal concretions. If due regard is given to considerable work now established on the rheology of the detailed evidence of rapakivi and orbicular textures, non-Newtonian fluids and macromolecular interactions there is no doubt that granitoids which display them in sediment particle systems. have crystallised directly from pre-ordered hydrous Restriction to the view that it is only possible to sedimentary precursors. liquefy or crystallise sedimentary materials by fusion The important sedimentary precursor principle has has also led to circuitous illogical reasoning whereby it also been pioneered at Broken Hill by Stanton (1972, is assumed that when such things as rapakivi ovoids, 1976, 1982, 1983 a & b), who has demonstrated the granitic orbicules, kelephites, oscillatory zoning, crystallisation of a range of metamorphic and granitoid myrmekites, microveinlets through phenocrysts, minerals directly from pre-existing precursor materials micrographic intergrowths, basic dykes re-broken after of sedimentary-diagenetic origin. The most important, emplacement, inclusions of porphyroblasts in xenoliths of course, are the simple rock-forming quartz, feldspars, and wall rocks, fossils or coaly plant fragments enclosed and biotite/amphibole/muscovite/chlorite which were in feldspars, biogenetic amines in fluid inclusions, derived by precursor dehydration reactions. These are ptygmatic aplite veins, colloform muscovite, and comb the major constituents of the schists, gneisses, miglayering are observed in granites, then these features matites, and granites of the Broken Hill district. must characterise congealed melts. Logically the Accretion is the highly significant process by which

264

sediments are retextured to nodular or mottled clays References when remobilized as semi-consolidated pastes. Backlund, H.G., 1938. J. Geol. 46: 339-396. Subsequent crystallisation gives rise to igneous-looking Dawes, P.R., 1966. Nature 209: 569-571. rocks such as porphyroids, nodular limestones, Elders, W.A., 1968. /. Geol. 76: 37-49. migmatites, and certain granitic rocks. It is responsible Elliston, J.N., 1968. Proceedings 23rd Int. Geol. Congress 8: 85-104. for "close packing" the heterogeneous sediment con- Elliston, J.N., 1984. Earth Sci. Rev. 20: 265-344. stituents so that they are more readily able to crystallise Elliston, J.N., Earth Sci. Rev. 22: 1-92. and generate the heat associated with granitic and Frisch, H.L. & 1985. Simha, R., 1957. In Eirich, F.R. (Ed.): Rheology, metamorphic rock masses. Theory and Applications I. Academic Press, New York: "Accretion" means the growing together of particles 525-613.. by external addition. It is an intrinsic property of any Healy, T.W., 1975. Australian Mineral Foundation Inc. Course concentrated particle system which contains or is Notes, November 1975: 103 pp. comprised of colloids and it occurs when such a system Key, R.M. & Wright, E.P., 1982. Geol. Soc. Lond. 139: 109-126. flows or intrudes as a slurry or paste (Frisch & Simha, 1957; Elliston, 1968; Healy, 1972; Yariv & Cross, 1979). Marmo, V., 1971. Developments in Petrology 2. Elsevier, Amsterdam: 244 pp. The most compelling evidence that rapakivi granite R.L., 1972. Econ. Geol. 67: 1128-1145. ovoids have crystallised from precursor illitic accretions Stanton, R.L., 1976. Parts 1, 2, 3 and 4. Trans. Inst. Min. is the variation between adjoining ovoids. They vary in Stanton, Metall. 85: B33^6, B118-31, B132-41, B221-33. composition, shape, size, rimming patterns, abundance Stanton, R.L., 1982. Proc. Aust. Inst. Min. Metall. 282:11-32. of inclusions, and crystal structures. They do not show Stanton, R.L., 1983a. Aus IMM. Conf., Broken Hill: 11-28. any uniformity commensurate with crystallisation from Stanton, R.L., 1983b. Econ. Geol 78: 422-437. the same melt. Stull, R.J., Lithos 11: 243-249. An outline of the theoretical and experimental work Sederholm, J.J., 1928. Bull. Comm. Giologique Finland No. in colloid chemistry (see also Elliston, 1985, pp. 44-77) 83: 83-105. and some examples of accretionary textures in re- Vorma, A., 1971. Bull. Comm. Giologique Finlande. No. 246. mobilised sediments such as turbidites, intrusive sandstones, limestones, and claystones are given in the Yariv, S. & Cross, H., 1979. Geochemistry of Colloidal Systems. Springer, Berlin-New York: 450 pp. paper. A18 Keynote Address 2 GRANITE EMPLACEMENT AND TEMPORALLY RELATED GOLD MINERALISATION V J. Wall* and J.R. Taylor Department of Earth Sciences, Monash University

Economic geologists have recognised the association of tin and tungsten mineralisation with granites and suggested links between granitoid plutonism and the formation of some gold deposits. This paper examines the nature and evolution of thermal aureole gold, a distinctive association of ore deposits which occur in the thermal aureoles of granite batholiths and which can be related to processes operating during pluton emplacement and contact metamorphism. Before discussing the occurrence of and controls on the localisation of some thermal aureole gold deposits, we begin by examining some simple spatial, kinematic and mass balance relations to set the broader context. The heat released by the crystallisation and cooling of a granitoid pluton creates a transient thermal anomaly in the pluton's country rocks, the volume of the resulting thermal aureole depending primarily on the size of the intrusion and the ambient temperature of the country rocks (Jaeger, 1969). Devolatilisation of these rocks and the crystallisation of the hydrous granitic magma liberates substantial quantities of hydrothermal fluids which must vent through the aureole. For example, the

emplacement into sedimentary country rocks of a model granitic pluton of typical sill-laccolithic form (20 km in diameter, 5 km thick and with an H 0 content of 2-3 wt %; Burnham, 1979), produces between 170 and 240 km of fluid. If this fluid contains 10 ppb gold then between 1200 and 1700 tonnes of gold are mobilised. Even larger masses of fluid may be circulated if hydrothermal convection is possible in and around the pluton (Cathles, 1981). The focussing of relatively small portions of the hydrothermal fluids through small volumes of rock in the thermal aureole in which the physicochemical conditions are appropriate for gold precipitation could result in the formation of significant gold deposits. Deformation of and displacements in the rock mass surrounding the pluton, associated with its emplacement and required to accommodate the magma volume, may provide the structurally-controlled fluid pathways. Given this simple conceptual framework, it may be evident that thermal aureoles are potentially ore-forming systems. The Pine Creek (NT) region represents the most intensely explored terrain which exhibits thermal aureole 2

265

gold deposits (e.g. Enterprise, Cosmo Howley, Goodall, their country rocks appear to have a greater bearing on Woolwonga, Moline, Mt Todd, Mt Bonnie, Tom's Gully, the occurrence of gold deposits. The thermal aureole etc), aggregating over 250 tonnes of gold. These deposits gold association is transitional to epithermal styles and occur in a range of lithological and stratigraphic settings also to the regional metamorphic gold association. and exemplify a variety of mineralisation styles, A key factor in the localisation of thermal aureole including stockworks and sheeted veins, disseminations deposits is the evolution of local and broader scale, and replacements. However, on the basis of their gold structurally-controlled plumbing systems. The structures structural histories and alteration parageneses, the that channel fluids may be reactivated pre-existing mineralisation may be related to deformation and contact structures or those that formed at the time of magma metamorphism accompanying the emplacement of the emplacement In either of these cases, such structures Cullen Batholith into the Early Proterozoic meta- are systematically related to the pluton emplacement sedimentary succession. mechanism and the contemporaneous regional The Cullen Plutons, of I-type affinities (Stuart-Smith defomation field. In particular, pluton emplacement & Needham, 1984), exhibit a range of emplacement largely by roof lifting and pluton lateral expansion styles and depths, and were apparently intruded during contribute to the development of brittle and brittlea protracted regional shortening. Emplacement of the ductile structures with which the gold deposits are Burnside, Shoebridge and Fenton granites was associated. accompanied by penetrative deformation of their The thermal aureole gold association includes a aureoles, whereas contacts of other plutons (e.g. in the wide range of deposit types and it is unlikely that any southern and eastern parts of the batholith) are strongly single chemical mechanism may be responsible for ore discordant and their aureoles show only localised brittle formation. However gold is spatially and paragenetically deformation. The very broad thermal aureoles developed related to sulphides (arsenopyrites, pyrites/pyrrhotite) (e.g. Stuart-Smith & Needham, 1984), indicate that precipitated in veins or as replacements in reactive much of the Pine Creek region is underlain by granite lithologies, implying either that sulphide deposition at shallow depth and constitutes the roof zone of the effects gold deposition or that similar mechanisms are Cullen Batholith. Most of the gold deposits are situated responsible for contemporaneous sulphide and gold in these roof zones, but some occur in pluton sidewall deposition. Such mechanisms include sulphidation of environments, and are localised in a limited variety of iron rich and/or calcareous host rocks, and in the case structural settings. The Pine Creek region exhibits gold of some vein/stockwork systems, fluid mixing and also deposits formed at high temperatures in inner aureoles simple cooling of the fluid. Much remains to be learnt and also low temperature mineralisation in outer about the provenance, path history and chemical aureoles, the nature of associated alteration and mineral evolution of the ore-forming fluids. parageneses reflecting the host rock lithology and also The evolution of thermal aureole gold is a subset of the metamorphic grade. more fundamental problems involving the mechanics Other major gold deposits (e.g. Telfer, Goellnicht et and kinematics of granitoid intrusion as well as al., 1988; The Granites) of thermal aureole association deformation and fluid migration in the Earth's upper occur in several provinces of the Australian Proterozoic. crust. These are fertile fields for research which will However, the association is not restricted to this era and require multi-faceted approaches integrating field examples are given of significant deposits in the observations and geochemical data with modelling of Palaeozoic of Victoria and NSW. These deposits occur coupled thermal, mechanical and fluid transport systems. around S-, I- and ?M-type granitoids and their occurrence This work will also lead to a better understanding of is not readily correlated with the state of differentiation thermal aureole gold, an ore association which is both of the associated plutons. However the depth and prospective and underprospected. mechanisms of pluton emplacement and the nature of A18.9 THE GABBRO-QUARTZ MONZODIORITE-ALKALI GRANITE ASSOCIATION IN SOUTHERN NSW: IMPLICATIONS FOR INTRUSIVE RELATED GOLD MINERALISATION RJ.Wormald* and R.C.Price LaTrobe University, Melbourne

Exploration in southern NSW has shown that the late Ordovician to early Silurian intrusives and cogenetic volcanics to the east of the Gilmore Suture form a major gold province which was part of either a back arc or intra-continental rift during the early Silurian. The style of sulphur-rich gold mineralisation associated

with these intrusives is critically dependent on the oxygen fugacity (f0 ) of the magmas. At Mt Adrah, an "ilmenite series" suite of intrusives composed of plagioclase-rich gabbroic cumulates and a quartz monzodiorite/alkali granite stock has intruded Cambro-Ordovician metasediments and metabasic rocks. 2

266

A sub-economic resource of Au mineralisation estimated body estimated at 5.6 million tonnes with a grade of 2.5 at 13 million tonnes at 1.3 g/t Au (Suppel et al., 1986) g/t Au (Suppel, 1986). This ore body is hosted by occurs within the propylitically altered quartz volcanics co-genetic with a "magnetite series" suite of monzodiorite and the alkali granite which shows albite, propylitically altered high-K monzodiorites and quartz sericite and carbonate alteration. The dominance of monzodiorites. The latter contain early crystallising ilmenite over magnetite suggests conditions below the euhedral sphene, magnetite and hornblende. Sphene QFM buffer at magmatic temperatures, probably crystallised directly from the magma, with magnetite, approaching the f0 of S-type magmas (Fig. 1). Sulphur, suggesting f0 conditions above the SMQHIL buffer, being relatively insoluble in reduced melts would have possibly approaching the magnetite-haematite (MH) partitioned mainly into an immiscible sulphide melt buffer (Fig. 1). Under these highly oxidised conditions phase (Carrol & Rutherford, 1985). Sulphur precipi- sulphur will occur predominantly as S0 with SOftLfi tated as arsenopyrite and pyrite as the result of second perhaps equal to 1000 or more (Burnham and Ohmoto, boiling and the loss of a C0 rich fluid phase, with Au 1980) and during second boiling most of the sulphur strongly partitioned into the arsenopyrite. The low f0 will partition into the fluid phase taking with it any conditions of precipitation were similar to those gold. No immiscible sulphide phase will form at the prevailing in porphyry Sn deposits (Fig. 1). magmatic stage to scavenge the gold. Escape of this Similar intrusives to the Mt Adrah Suite, near oxidized sulphur-rich fluid phase into the overlying Wallundry, show evidence of crystallisation at a higher volcanic pile and mixing with meteoric waters caused f0 , represented by the SMQHIL buffer (hornblende + gold precipitation in zones of advanced argillic alteration. ilmenite —» sphene + magnetite + quartz); typical of The common occurence of highly oxidized I-type more oxidized I-type magmas (Fig. 1). These will be intrusives with gold mineralisation elsewhere in SE less likely to produce immiscible sulphide melts because Australia and world wide (e.g. Canadian Archaean, the solubility of S is higher in oxidized magmas (Carroll Hattori, 1987) suggests that high f0 coupled with high and Rutherford, 1985). Some sulphur from the magma sulphur is an essential prerequisite for intrusive related will be partitioned intofluidsas S O ^ S O ^ S = 0.1-10, gold mineralisation. Bumham and Ohmoto 1980) with the gold probably References transported as chloride complexes at these high Burnham & Ohmoto, 1980. Mining Geology Special Issue No. 8: 1-11. temperatures and high f0 . These intrusives are associated with auriferous quartz veins in the surrounding Carroll & Rutherford, 1985. J. Geophys. Res. 90, suppl.: C601-C612 metasedimentary country rocks. K., 1987. Geology 15: 1107-1111. At Gidgingbung, near Temora, economic acid Hattori, Suppel, D.W., 1986. New South Wales — Quarterly notes sulphate style Au mineralisation has formed an ore 64: 1-23. 2

-5 -10

en -15

-Jo

-20

-25 -30

Temperature °C

Figure 1 — Oxygen fugacity vs temperature diagram showing the approximate fields for various magma types and mineral deposits associated with them ( modified from Burnham and Ohmoto 1980). Oxygen buffer curves shown are MW (magnetite + wurstite), NNO (nickel + nickel oxide), QFM ( quartz + fayalite + magnetite), SMQHIL (sphene + magnetite + quartz + hornblende + ilmenite), and MH ( magnetite + haematite), all at 1000 bars. The boundary for SOj/H^ = unit activity at pH = 4 is shown by the dotted line.

267

A18.10 LASER RAMAN MICROPROBE STUDIES OF FLUID INCLUSIONS AND THE DEFINITION OF THE "COPPER WINDOWS" IN PORPHYRY COPPER-GOLD MINERALISATION (TIRAD PROSPECT, NW LUZON, PHILIPPINES) Alfonso G. Trudu *, Terrence P. Mernagh and Mark S. Bloom 2

Department of Earth Sciences, Monash University Bureau of Mineral Resources, Canberra 2

The Tirad porphyry copper-gold prospect is located Trudu & Bloom (1988) between fluid inclusion along the Central Cordillera of northern Luzon populations (i.e. assemblages of fluid inclusions) and (Philippines). The geological setting is described in alteration types. We suggest that population 1 (types A detail by Sillitoe & Angeles (1985) and by Trudu (in and any type of Q is associated with K-silicate alteration prep.); it consists of a deep-seated biotite schist, of and population 2 (types A and B) with the sericitic probable Cretaceous-Paleogene age, overlain by the stage. This interpretation is based upon the abundance Balili Sequence, a relict andesitic stratovolcano of mid- of inclusion populations in veins correlated to specific Miocene age. The Tirad stock, a porphyrytic quartz alteration types. It is still uncertain which of these two diorite, intruded this stratigraphy during the Pliocene populations is related to the chloritic alteration. and was responsible for the emplacement of the Our laser Raman spectrometry (LRS) investigation mineralisation and consanguineous alteration. An early shows that amongst the most common polyatomic gases K-silicate stage of alteration/mineralisation (biotite- (S0 , C0 , CO, J^S, CH , N2, NH,, and HJ possibly magnetite-bornite-chalcopyrite-chalcocite-gold) is present in addition to the solvent HjO, only C0 is overprinted by a chlorite-dominated assemblage (the sporadically present in quantities just above the detection SCC alteration of Sillitoe and Gappe, 1984) of limit (0.1-0.2 mole %). By comparing the LRS spectra chlorite±epidote-hematite-chalcopyrite-gold. obtained from daughter crystals with those of solid Subsequently, sericitic alteration took place with the standards, we have identified anhedral chalcopyrite, development of sericite-clays-pyrite-chalcopyrite- which could not be unambiguously identified by optical bornite±gold. An advanced argillic stage of clays-pyrite- methods. This identification forms the basis for the enargite-covellite-chalcocite formed at shallow levels, distinction between type C-l and C-4, and C-2 and Cpossibly contemporaneous with the sericitic alteration 3 fluid inclusions. Other daughter crystals determined at depth (Sillitoe & Angeles, 1985). Although all altera- by LRS are antarcticite (CaClj.61^0) and carnallite tion types pervasively altered the host rocks, (KCl-MgCl^HjO). Combined SEM-EDS analysis on mineralisation is predominantly confined to veins in all daughter crystals confirms the presence of sylvite, alteration types, except for the advanced argillic stage. chalcocite, anhydrite and K-mica. Through LRS analysis Three fluid inclusion types are recognised: vapour on inclusions frozen at -160°C, the occurrence of rich (type A), liquid-rich (type B) and halite-saturated additional constituents is ascertained through the (type C). The latter type is further subdivided into formation of their chloride hydrates at such low subtypes according to daughter crystal assemblages as temperature. In type A, hydrohalite (NaCl.21^0) is follows: C-l (halite-hematite-liquid-vapour); C-2 (halite- found; in type B, mainly hydrohalite and minor hematite-other transparent phases-liquid-vapour); C-3 antarcticite; and in type C, hydrohalite, antarcticite, (halite-hematite-other transparent phases-Cu-bearing bischofite (MgCl^Hp), MgC^. 12H 0 and carnallite, phase-liquid-vapour); and C-4 (halite-hematite- with rare kalinite KAKSO^.l 1H 0 and hydromolysite chalcopyrite-liquid-vapour). Heating and freezing (FeClyGUp). The presence of H^S in the vapour or microthermometry indicates that homogenisation (S0 )^ in the liquid was not detected in any of the temperatures (Th in °C) and salinities (Sal. in wt. % inclusions examined. However, due to the rather high NaCl equiv.) have the following ranges: type A (Th = Raman detection limit for l^S (0.2 mol. %), this solute 320-480, m = 385; Sal = 1.2-21.3, m = 8.3); type B (Th would not be identified in concentrations below roughly = 210-490, m = 388; Sal = 0.2-22.2, m = 5.3); type C- 0.1 molal, which is generally considered to be the upper 1 (Th = 230-600, m = 301; Sal = 27-64, m = 38.3); type limit of H S contents typical of porphyry copper C-2 (Th = 250-600, m = 417, Sal = 27-64, m = 45); C- environments (Barnes, 1979). It is noteworthy to mention 3 (Th = 380-575, m = 462; Sal = 47-56, m = 51); and that the high concentration of Ca * in these fluids C-4 (Th = 340-380, m = 360; Sal = 35-42, m = 37). In decreases the maximum concentration of (S0 ) ' (i.e. in type C inclusions, partial homogenisation takes place equilibrium with anhydrite) to values below the detection by either halite dissolution or vapour disappearance. limit of the LRS. Our data emphasise that at the Tirad porphyry copperBoth types B and C coexist with type A inclusions, indicating that phase separation took place during their gold prospect, early, high temperature and high salinity fluids (type C inclusions) are not only rich in Na, Fe, entrapment From coexisting type A and C inclusions, a pressure range of 180 to 550 bars is estimated. On a Cu, S, CI and H p (Trudu & Bloom, 1988), but also in deposit scale, we revise the correlation proposed by Ca, Mg and K. Fluids trapped as type C inclusions span 2

268

a broad range of temperatures (230-600°C) and salinities respect to chalcopyrite could also be a function of the (27 to 64 wt. % NaCl equiv.). However, we are able to Fe and S content of the fluid and of the oxidation state bracket specific temperature and salinity intervals (the of such elements in the ore-bearing solutions. Gold, "copper windows") during which the solutions were the which is texturally related to copper-bearing phases in most effective in transporting copper. Such "copper veins, shows a direct correlation in grade with copper windows" are represented by inclusion types C-3 (mean and was transported by the same solutions as copper, Th = 462°C, mean salinity = 51 equiv. wt. % NaCl) and although the two metals may not have precipitated due C-4 (mean Th = 360°C; mean salinity = 37 equiv. wt. % to the same mechanism. NaCl), both of which contain a copper-bearing daughter References crystal. The chemical complexity of the richest copper- Barnes, H.L., 1979. In Barnes, H.L. (ed): The Geochemistry of Hydrothermal Ore Deposits. Wileybearing fluids does not appear to be an important Interscience, New York: 404—460. parameter in characterising such solutions, as daughter crystal assemblages can range from multi-phase (up to Sillitoe, R.H. & Angeles, C.C. Jr., 1985. Asian Mining '85. Inst Min. Metall. London: 15-26. 7) in type C-3 inclusions, down to tri-phase (haliteR.H. & Gappe, I.M. Jr., 1984. Comm. hematite-chalcopyrite) in type C-4 inclusions. In Sillitoe, Coordination Joint Prosp. Resources (CCOP) Tech.Pub. addition, not all inclusions with high salinity and 14: 1-89. temperature contain Cu-bearing daughter minerals, Trudu, A.G., in prep.. Unpubl. PhD thesis, Monash suggesting that the Cu content of the fluids fluctuated University. with time, and with episodic influx of high-Cu solutions Trudu, A.G. & Bloom, M.S., 1988. Geol. Soc. Aust. Abs responsible for mineralisation. However, saturation with 22: 211-216. A18.ll FOLEY'S ZONE, CLEVELAND Sn-Cu-W-Mo-Bi DEPOSIT, TASMANIA. P.G. Jackson *, A. Changakoti , J. Gray , H.R. Krouse and T.A.P Kwak . 1

Department of Geology, LaTrobe University Department of Physics, University of Alberta, Edmonton, Canada Department of Physics, University of Calgary, Calgary, Canada 1

2 3

Foley's Zone within the Cleveland Mine, northwestern Tasmania, consists of a profoundly altered, quartz-porphyry dyke and an associated vein halo. The alteration sequence in the dyke can be correlated with vein mineralisation and mineralisation of the carbonate replacement bodies. Hydrothermal alteration in the dyke can be divided into four overlapping stages, characterised by the formation of four principal alteration facies: (1) sericitised feldspar facies (2) quartz-muscovite greisen (3) quartz-topaz greisen (4) quartz ultragreisen The distribution of these facies in the dyke, defines a zonation with quartz-muscovite greisen forming along the flanks of the dyke, enveloping a quartz-topaz greisen core which intercalates with quartz ultragreisen at depth. Relict, sericitised feldspars are seen in irregular patches in the quartz-muscovite greisen zone. Fourteen distinct vein generations are recognized from vein crosscutting relationships. Vein filling can also be divided temporally into four arbitrary stages: Stage 1 (Quartz Stage) — quartz dominant veins

Stage 2 (Greisen Stage 1) — phyllosilicate and adularia bearing veins Stage 3 (Greisen Stage 2) — topaz, fluorite and sulphide-rich veins. Stage 4 (Carbonate Stage) — carbonate-fluorite veins Geochemical trends with progressive alteration in the dyke suggest that the composition of the vein fluids and element distribution in the dyke, veins and vein margins was controlled both by wall-rock reactions along the margin of fluid pathways andfractionationat the fluid source. Fluid inclusion studies of vein minerals show that the hydrothermal fluids responsible for vein mineralisation, were low to moderately saline (3-14 wL % equiv. NaCl) and contained up to 22 mole % CO Fluid filling temperatures increased through Stage 1 to a maximum of 420°C and began to decrease towards the end of Stage 3. Oxygen, hydrogen and sulphur isotopic data indicate that the hydrothermal fluids involved in vein filling have narrow compositional ranges (8 0 +10.8 to +15.3%o and 5 S +1.7 to +4.1%o) and were probably of magmatic origin. r

269

A18.12 GEOCHEMISTRY OF PROGRESSIVE HYDROTHERMAL ALTERATION ASSOCIATED WITH W-Mo-Bi MINERALISATION IN THE I-TYPE BAMFORD GRANITE, NORTH QUEENSLAND P. L. Blevin Department of Geology, James Cook University of North Queensland

The Carboniferous Bamford Granite is located 5 km A1 which remain the same. Tungsten, Mo, Bi and Sn are N of Petford, north Qld. It is a fractionated, I-type not significantly enriched during exocontact alteration. granite and intrudes a rhyodacitic ignimbrite and a SREE decreases, Eu/Eu* remains the same, despite the monzodiorite to subvolcanic levels. The granite is complete loss of feldspars, and La/Yb increases. Within dominantly medium-grained and contains quartz, alkali the granite W-Mo mineralisation occurs during Stage II and sodic feldspars, minor Fe-Al biotite and accessory in the range 400-360°C with Bi being deposited at allanite, magnetite, xenotime and zircon. Porphyritic, lower temperatures (<300°C). fine-grained and aplitic variants are also present mainly The behaviour of F and CI have been monitored by along the SE contact zone. mica chemistry. From Stage I (~500°C) to Stage III Hydrothermal alteration is distributed over the S (300°C) logCfH.O/ZHF) and logCfH/y/HCl) rise from and E apical and contact zones of the pluton 4 to 4.5 and 2 to 5 respectively. The increase in log(/HF/ (endocontact) and into the adjacent rhyodacite and /HC1) is most probably due to both the lack of a CI monzodiorite (exocontact). The sequence of differing reservoir within the early formed micas and the alteration types can be organised into three alteration increasing role of unexchanged waters with time. stages common to both the endo/exo subsystems: Stage LogCft^O/ZHF) values are between those of porphyry I (>500°C; feldspars stable); Stage II (~500-300°C; Mo and Cu systems, and below those of "specialised" feldspars unstable) and Stage III (<300°C; chlorite, Sn granites. This result is consistent with the presence sericite, base metal sulphides, Ca-phases). Within the of andalusite rather than topaz within the exogreisens. exocontact subsystem cordierite formed during early Endocontact alteration was caused by a cooling, low Stage II with andalusite forming later, both at the expense to moderate salinity aqueous fluid. Boiling occurred of feldspar. Both alteration systems are crosscut by during Stage II at ~400°C and -300 bars causing intense Stage HI alteration types, with alteration styles changing local greisenisation and precipitation of quartz, from disseminated to more fracture controlled and wolframite and molybdenite. The Bamford Hill greisen focussed with time. system is distinct from those of "Sn granite" systems During endocontact greisenisation there is an increase and other specialised granitoids by virtue of its low in Li, F, Th, U, Th/U, Cs, Y, Sn, W, Mo and Bi; a content of Li (<50ppm), Be (<10ppm), B (<10ppm), decrease in Ba, Sr and all majors except Si, Fe and Mn. F (<1500ppm), Rb (<500ppm) and Ta (clOppm). The Other elements are variable (eg. Zn, Ga, Rb, Pb) or different REE behaviour, lack of W-Mo mineralisation, show no change (eg. Zr, Ti, Nb). Both SREE and La/Yb higher aW and /0 and (locally) lower aSi within the increase while Eu/Eu* decreases. During exocontact exocontact subsystem during Stage II suggests the greisenisation there is an increase in Li, F, Rb, Th, Th/ involvement of different, mixed or (exchanged fluids U, Nb, Cs, Sn, Mo; a decrease in Ba, Sr, Pb, U, Y, Zn to those present within the granite. and all majors other than Si which increases, and Ti and 2

A18.13 CHEMICAL CONTROLS ON HYDROTHERMAL TUNGSTEN TRANSPORT IN SOME MAGMATIC SYSTEMS AND THE PRECIPITATION OF FERBERITE AND SCHEELITE S. Jaireth *, C.A. Heinrich and M. Solomon 1

Department of Geology, James Cook University of North Queensland Bureau of Mineral Resources, Canberra 2

Deposits discussed here include (1) cassiteritewolframite-quartz veins and greisens (2) wolframitequartz veins and (3) wolframite-molybdenite greisens and pipes. Granitoids associated with group 1 (tin granites) are highly fractionated, ilmenite-bearing alkali feldspar granites, those with group 2 are highly fractionated, magnetite-bearing leucogranites. Oreforming fluids at magmatic temperatures (> 650°C)

exsolve in equilibrium with silicate melt and during subsequent transport and cooling react with crystalline granite. W precipitation in greisens generally occurs below 450°C while ore deposition in veins is mainly between 400 and 250°C. Scheelite occurs only at a late stage. Fluid inclusion and stable isotope data indicate that boiling, cooling by mixing with groundwater and wall-rock reactions may be important processes.

270 For most chlorine-bearing, low-sulphur, tungsten- with falling temperature because of the dissociation of bearing ore solutions at high temperature the buffers HC1 and the constant high concentration of Hj. The HC1/C1" and i y H 2 0 probably control the pH and effect of decreasing pH is counteracted by the oxidation state. On cooling in contact with excess granite temperature effect and 99 per cent of W is precipitated (i.e. rock-buffered) HC1 and Hj in the fluid are consumed between 350 and 300°C. Cooling by conduction is of by feldspar hydrolysis and reduction of Fe3+ in silicate limited significance either in veins or greisens and is and oxide minerals. The oxidation potential is buffered more likely a result of mixing with groundwater. by the biotite+magnetite in the granitoids of group 2 Adiabatic boiling of similar tungsten-undersaturated and biotite+ilmenite in group 1. With increased fluid- fluid at 400°C causes precipitation of wolframates but rock ratio and the development of an aureole of altered even in this case the associated drop in temperature is rock subsequent fluid batches are fluid-buffered and the dominant driving force. likely to follow a chemical evolution dominated by At equal concentrations of iron and calcium the equilibria among solute species and precipitated minerals solubility of scheelite at temperatures > 400°C is an (fluid-buffered). order of magnitude lower than that of ferberite. The Experimental studies on the solubility of ferberite commonly observed replacement of early wolframite and hubnerite (Kolonin et al., 1975; Burovski, 1981), by late scheelite in veins indicates that for the early ore tungstic oxide (Eugster & Wilson 1985) and scheelite fluids Fe»Ca. This might be the result of the early, (Foster, 1976; Krumhansl, 1976; Wesolowski, 1984) in high temperature precipitation of calcic phases such as conjunction with potentiometric studies of tungstate apatite and fluorite. Mixing with meteoric water may solutions (Wesolowski et al., 1984), show that at not only cause cooling but could also add calcium to the temperatures > 250°C mononuclear species of tungsten primary ore fluid, a likely effect in the later stages of a (H2W04, HW04 and W04"2) are the dominant aqueous magmatic system. Introduction of Ca by wall-rock species (Wesolowski et al., 1984). Thermodynamic reaction is probably the reason for the abundance of calculations for W species are based on data derived by scheelite in tungsten skarns. Wesolowski et al. (1984). In chloride-bearing fluids at References temperatures > 250°C dissolution of ferberite, scheelite Burkovsky, S.I., 1981. Izv. A.N. KazSSR, Seria Geologia 3: 43-46. and hubnerite can be expressed by the following Eugster, H.P. & Wilson, G.A., 1985. Mining Geology reactions, (where M = Fe/Ca/Mn): SpecJssue 8, 1-11. MW04 + H* + CI" = MCI' + HWO/ Foster, R.P., 1977. Trans. Inst. Min. Metall. 87-98. MW04 + H+ + 2C1- = MC12 + HWO4Kolonin, G.R., Shironsova, G.P. & Laptev, Yu.V., 1975. If thefluidis buffered by feldspar+muscovite+quartz ForschMiner. 52: 161-167. and annite+magnetite assemblages the solubility of Krumhansl, J.L., 1976. Ph.D. thesis, Stanford Univ. ferberite drops from > 1000 ppm at 400°C to < 10 ppm Weselowski, D., 1984. Ph.D. thesis, Pennsylvania State Univ. at 300°C, a reduction of 99% of total dissolved W. Weselowski, D., Drummond, S.E., Mesmer, R.E. & Ohmoto, Underfluid-bufferedconditions a W-Cl-S-F-Na-K-CaH., 1984. Inorg.Chem. 23: 1120-1132. Fe-Al-Si-0 H fluid becomes more acidic and reducing

A18.14 THE BEHAVIOUR OF TIN IN MAGMATIC-HYDROTHERMAL SYSTEMS J.R.Taylor* and VJ. Wall Earth Science Department, Monash University Melbourne

Despite exhaustive research, the association between tin mineralisation and felsic magmatism remains poorly understood. Experimental investigations into the behaviour of tin in melts and supercritical fluids have been undertaken to evaluate the physical and chemical constraints on the mobilisation of tin from magmas, and its subsequent transport in a magmatic-hydrothermal aqueous phase. The solubility of Sn02 in chloride (and/or fluoride) bearingfluidsin equilibrium with a number of crystalline silicate acid-buffers has been examined at 2.0 kbar, as a function of temperature (700 to 800°C), oxygen fugacity (QFM to QFM + 1 . 5 log units) and fluid chemistry. Hundreds to tens of thousands parts per million tin can be dissolved in dilute, chloride-bearing

acid solutions. Fluoride was found to have no significant influence on the solubility of Sn02 at 700°C. In alkalichloride bearing, acid solutions, across the entire range of oxygen fugacities examined, tin transport is effected by a complex series of stannous, chloride-bearing species. These include simple chloride, mixed ligand (hydroxy-chloride) and alkali-bearing chloride and hydroxy-chloride stannous complexes (SnOHCl, SnCl2, KSnOHCl2, KSnCl3, and NaSnOHCl2, NaSnCl3 etc.). Apparent equilibrium constants for the predicted aqueous tin species have been calculated, and species abundances determined for different solution compositions. The solubility of Sn02 is favoured by increasingfluidacidity (M(HC1)), elevated alkali/acid ratios and reducing conditions, but is largely unaffected by temperature

271

changes in the 500 to 750°C range. The concentrations of tin in equilibrium with Sn02saturated, synthetic, peraluminous granitoid melts, has been examined as a function of temperature (700 to 800°C), pressure (2.0 to 3.0 kbar), oxygen fugacity (QFM to QFM +1.5 log units) and granitoid composition (haplogranite, k'spar-quartz and albite-quartz). Melts were found to contain between 400 and 2500 ppm tin. The solvent capacity of the granitoid melts is increased by decreasing oxygen fugacity, and increasing ALK/ AL (i.e. Kf> + Na 2 0/K 2 0 + NA 2 0 + AL2OJ and Na/ K of a melt. Nuclear gamma resonance analysis of haplogranite glasses indicate that both Sn2+(MELT) and Sn4+(MELT) will be stable over the range of oxygen fugacities typically attending the crystallisation of granitoids. The coordination and likely speciation of Sn2+ and Sn4* has been examined. The basicity of a melt (activity of free oxygens -a02-), which is often strongly correlated with the ALK/AL of a melt, is likely to be very important in influencing the acidity of a coexisting fluid phase. Peraluminous melts (low ALK/AL) will generally display low melt basicities, and therefore buffer the acidity of a magmatic aqueous phase at far higher values than peralkaline melts. Tin can be efficiently extracted from peraluminous melts by supercritical chloride-bearing fluids. Fluid/

melt partition coefficients for tin (KdSn) are directly proportional to the square of the aqueous chloride concentration. Across the entire range of temperatures, pressures and oxygen fugacities examined, tin is preferentially partitioned from any peraluminous melt, into a coexisting aqueous phase (KdSn > 1) if the aqueous chloride concentrations exceed ~1M. Tin is most efficiently extracted from high temperature, potassiumrich peraluminous magmas, under relatively reducing conditions. However, it will be effectively removed from various melt compositions over a wide range of physical and chemical conditions as a result of the dependence of KdSn on (M(Cl)aq)2. Fluid/melt partition coefficients for chlorine (Kda) were also examined in the Sn02-bearing systems. Kda increases with increasing chlorine concentrations within a melt Over the range of variables examined, subde increases in K C I Q can also be effected by decreasing temperature, increasing pressure and increases in K(MELT)/Na(MELT). This experimental study has confirmed the genetic link between magmatism and tin mineralisation, and provided a quantitative analysis of the extraction, transport and depositional processes, over a comprehensive array of magmatic-hydrothermal physicochemical conditions.

A18.15 FRACTIONATION OF THE LOTTAH GRANITE, BLUE TIER BATHOLITH, NE TASMANIA: IMPLICATIONS FOR THE ORIGIN OF TIN-ENRICHED PERALUMINOUS GRANITE. D.E. Mackenzie Bureau of Mineral Resources, Canberra

Alkali-feldspar granites, especially the so-called LiF granites, and equivalent pegmatites have received a great deal of scientific attention in recent years. This is partly because of their common association with Sn and rare metal mineralisation, and partly because the origin of their unusual mineralogical and chemical characteristics is contentious. There are currently two principal genetic models for Li-F granites: (1) they are essentially of magmatic origin; (2) they are products of intense, pervasive metasomatism and/or hydrothermal alteration of differentiated, but otherwise "normal" granites. The latter model has been applied to the alkalifeldspar granites of NE Tasmania in general, and to the Lottah Granite in particular (e.g. Higgins et al., 1985; Higgins, this volume). The Lottah Granite is a stocklike body, about 9 km in diameter, of highly felsic alkali-feldspar granite with sharp intrusive contacts against the surrounding Poimena Granite, a major (70 x 25 km) component of the Blue Tier Batholith. Details of the geology, petrography and mineralogy of the Lottah and Poimena Granites are presented by McClenaghan et al. (1982), McClenaghan & Williams

(1982) and Mackenzie et al. (1988). The Poimena Granite is a felsic, slightly peraluminous, feldspar-phyric biotite granite of I-type character. It contains restite minerals (plagioclase, rare hornblende), and shows limited, generally straight-line differentiation trends consistent with restite separation. The most differentiated rocks are preserved mainly in the topographically highest parts of the pluton, near its southern end. The Lottah Granite as exposed consists mainly of fine topazzinnwaldite sub-solvus granite which passes downward into coarser, porphyritic, Li-rich (muscovite-)biotite granite containing accessory topaz. Both types contain accessory cassiterite and, commonly, fluorite. Much of the Lottah Granite is slightly to moderately weathered and/or hydrothermally altered to white mica and clay(s), which have replaced feldspars, primary micas and topaz along grain boundaries, cracks, and cleavages. Hydrothermal alteration, and probably metasomatism, are strongly developed only in restricted areas, principally in the Anchor mine area where they have affected both the Lottah and Poimena Granites, and accompany SnW base metal mineralisation. The Lottah Granite differs from the Poimena Granite

272

in many respects. It is more peraluminous, much richer in Na 0, Li, Rb, Nb, Sn, W, Ga, and F, and poorer in Ti0 , MgO, CaO, Ba, Sr, Pb, Zr, and LREE than the Poimena Granite. It shows geochemical trends of increasing Na 0, Li, Rb, Sn and F with decreasing Si0 , Ba and Sr: these trends are generally narrow, curvilinear, and separate from and non-convergent with those of the Poimena Granite. They may be modelled as the results of fractional crystallisation of quartz, plagioclase (oligoclase, then albite), K-feldspar, and minor biotite. The Poimena Granite shows no enrichment in Li, Sn, or F, and only minor enrichment in Rb. REE data (Mackenzie et al., 1988) show that all REE decrease with differentiation in the Poimena Granite, and yet the most mafic Lottah Granite sample analysed is richer in HREE than any Poimena Granite sample. However, the Lottah Granite does not differ significantly from the Poimena Granite in heat generating capacity, as is required by the metasomatic-hydrothermal model of Higgins (in press): HGU (heat-generation units — 0.19KP + .062U + 0.17Th) for the Lottah Granite is 11.815.8 compared to 9.3±2.2 for the Poimena Granite, and most of the Lottah Granite samples with relatively high heat-generation capacity, due to elevated U contents, are altered and mineralised rocks from the Anchor mine area. Rb-Sr isotopic data (Mackenzie et al.,1988) show that the Lottah Granite was emplaced about 10 Ma after the Poimena Granite and has a significantly higher initial ^Sr^Sr ratio (0.715±0.002, cf. 0.7093±0.0008). Sm-Nd isotopic data (Mackenzie et al., 1988; Sun et al., in prep.) also show that the Lottah Granite is higher in eNd (about -2.5) than the Poimena Granite (about -6). The initial ratios reflect differences in source compositions rather than differences in post-magma generation history. For example, there is no correlation between the degree of hydrothermal alteration in the Lottah Granite and either initial ^Sr/^Sr or eNd; the difference in eNd between Lottah and Poimena Granites cannot be generated by any known metasomatic or hydrothermal process. Finally, the mineralogical and chemical characteristics of the Lottah Granite are fully in accord with the behaviour of F-rich peraluminous granite melts as shown in many experimental studies, including those of Kovalenko (1973), Manning (1981), London (1987), Webster et al., (1987), Weidner & Martin (1987), and 2

London et al. (1989). These studies have shown, for example, that increasing F contents in peraluminous LiF granitic melts displace the minimum towards the Ab corner in the Q-Ab-Or system, that quartz and two alkali feldspars may be on or near the liquidi, and that minerals such as topaz and zinnwaldite crystallise from such melts. In conclusion, the bulk of the geological, mineralogical, geochemical and isotopic data summarised above are consistent with fractional crystallisation of a Li-Frich magma genetically unrelated to the surrounding Poimena Granite. Metasomatism and hydrothermal alteration were generally very limited, and have had significant impacts in a few restricted areas, principally the Anchor mine area, and the Lottah Granite there is not typical of the Lottah Granite as a whole. It is highly unlikely, in the light of the data referred to previously, and given the lack of contrast between them in heat generating capacity, that the Lottah Granite was comprehensively modified, both chemically and mineralogically, by intense hydrothermal alteration up to 10 Ma after emplacement (Higgins, in press) while the adjacent Poimena Granite remained largely unaffected. These conclusions have important implications for the origin of other Li-F granites interpreted as products of intense metasomatism and/or hydrothermal alteration. References Higgins N.C. in press. Geochim. Cosmochim. Acta. Higgins N.C., Solomon M. & Varne R., 1985. Lithos 18: 129-149. Kovalenko V.I., 1973. Geochimiya 1: 57-66. London D., 1987. Geochim. Cosmochim Acta 51: 403-420. London D., Hervig R.H. & Morgan G.B. VI, 1989. Contrib. Mineral. Petrol. 102: 1-17. Mackenzie D.E., Black L.P. & Sun S.-S., 1988. Geochim. Cosmochim. Acta 52: 2507-2524. Manning D.A.C., 1981. Contrib. Mineral. Petrol 76:206-215. McClenaghan M.P., Turner N.J., Baillie P.W., Brown A.V., Williams P.R. & Moore W.R., 1982. Tasm. Dep. Mines Geol. Surv. Bull. 61: 198 pp. McClenaghan, M.P. & Williams, P.R., 1982. Tasm. Dep. Mines Geol. Surv. Pap. 4, 32p. Sun S.-S., Higgins N.C. & McCulloch M.T. in prep. Webster J.D., Holloway J.R. & Hervig R.L., 1987. Geochim. Cosmochim. Acta 51: 389-402. Weidner J.R. & Martin R.F., 1987. Geochim. Cosmochim. Acta 51: 1591-1597.

A18.16 ORIGIN OF ALKALI FELDSPAR GRANITES: THE LOTTAH GRANITE REVISITED

273

N.C. Higgins Bureau of Mineral Resources, Canberra

A recent study of the origin of the Lottah alkali sericitisation of both granite types occurred (Type III feldspar granite (Mackenzie et al., 1988) concluded that alteration), and resulted in the introduction of sericite, its mineralogy, chemical variation, isotopic variation, muscovite, fluorite, carbonates and sulphides, and finally and much of the associated mineralisation at the Anchor kaolinite (Groves 1973; Ross, 1983; Higgins mine, were primarily magmatic in character. They also et al., 1985). The& Taylor, non-porphyritic AFG composed concluded that the Lottah granite is genetically unrelated principally of quartz+K-feldspar (remnant)+albite+ to the Poimena granite, which it intrudes. Earlier workers zinnwaldite+accessories represent extensively reinterpreted the Lottah granite and associated Mt Paris equilibrated and recrystallised granite. The main ore granite as metasomatised differentiates of the Poimena zones are flat-lying, cassiterite-bearing 'sheets', granite. A re-examination of the available data suggests and occur near the upper contact of thegreisen Lottah granite. that the conclusions of Mackenzie et al. (1988) are et al. (1988) showed that the Poimena incompatible with certain criticalfieldrelations, regional and Mackenzie granites have separate chemical trends and comparisons, and mineralogical, chemical, and isotopic that Lottah there is a compositional break between the two. data. These relationships and data show that the Lottah However, published analyses fill this compositional granite has had an extensive sub-solidus history of fluid- gap indicating that there is a continuum in composition rock interaction. These processes gave rise to changes between granites. The Mt Paris samples occupy a in mineralogy and chemical composition, and chemicallytheintermediate position between the Poimena perturbations of isotopic systems. and Lottah granites, although there is overlap between The Lottah alkali feldspar granite (AFG) is part of all units. Type I alteration of the AFG, through the the Devonian Blue Tier Batholith (BTB) and consists of replacement of magmatic minerals and infilling of two texturally distinct phases; a quartz-feldspar phyric cavities, resulted in chemical gains in Ga, Na and Al phase with a fine grained groundmass, and a medium to (albite), F (green mica+topaz+fluorite), Li and Rb (mica), coarse grained equigranular phase containing abundant P20 (apatite), Y (fluorite), Sn (cassiterite), W, Nb pegmatitic segregations. At the Anchor mine the (wolframite), and losses in Eu, Sr, Ca (plagioclase porphyritic phase overlies the equigranular phase and replacement), Ti, Mg, Fe (biotite replacement), K (Itmay represent a chilled margin to the AFG magma feldspar replacement), and possibly Si (quartz leaching). Both gradational and sharp contacts exist between the Type II alteration of the Lottah AFG (and some Poimena two AFG phases (McClenaghan & Williams, 1982; samples) resulted in the addition of K, Rb and Li (mica), Ross, 1983). The AFG phases intrude K-feldspar F (topaz and fluorite), P 0 (apatite), Sn (cassiterite), megacrystic, biotite granite of the Poimena granite. The W, Nb (wolframite), Fe, Zn, and Cu (sulphides+siderite), fine grained and most differentiated phases of the latter and decreases in Na (through replacement of albite). are spatially associated with the AFG. Type III alteration of the AFG (and some Poimena The occurrence of abundant miarolitic cavities, now samples) did not markedly effect their chemical filled by hydrothermal minerals, indicate the Lottah composition except to raise Rb, and Li (mica), F granite magma crystallised in the presence of a vapour. (fluorite), and Fe, Zn and Cu through the addition of The AFG also equilibrated with this and other sulphides and carbonates. hydrothermal fluids after complete consolidation. The Oxygen isotope data from the Lottah granite reveal phyric AFG may be the least affected by the early sub- a complex history of re-equilibration of magmatic and solidus alteration, with phenocrysts of oligoclase and post-magmatic minerals. Quartz in albitised granite K-feldspar preserved along with red-brown biotite. uniformly exhibits high 6 O values (range 11.5 to However, areas of phyric AFG to the south of the mine 13.2%o) with slightly lower values obtained for are extensively sericitised. In the least sericitised rocks greisenised granite (range 11.7 to 10.0%o). In both the primary magmatic minerals are overprinted by patchy equigranular and phyric AFG there are reversals of the but widespread development of secondary albite and usual granite trend of decreasing O composition and the alteration of the brown mica to a light green (feldspar > muscovite > biotite), and A O(q-f) greater mica (Type I alteration assemblage). This alteration is than the normal range of 1.5 to 2%o observed in granite widespread and not localised on the Anchor mine. Type without hydrothermal alteration. This suggests that I alteration may have been accompanied by the formation minerals other than quartz have equilibrated with later of minor zinnwaldite, topaz, fluorite, and cassiterite. fluids down to quite low temperatures. A number of Subsequent hydrothermal overprinting involved quartz-mineral fractionations from the Type I altered widespread replacement of feldspars and mica by topaz, granite suggest final temperatures of equilibration for zinnwaldite, cassiterite and fluorite (Type II alteration- mica and feldspar probably occurred between 430° and " greisenisation")- Later pervasive but generally weak 350°C. Assuming a reasonable range of temperature for 3

smow

274

this alteration event (> 400°C), the O composition of Lottah granite, is exposed some 3 km NE of the Anchor the fluid in equilibrium with the quartz is « 7 ± 1 %o, mine. Its Rb/Sr biotite and muscovite ages are 383 and within the range normally regarded as magmatic. The 382 Ma, respectively. This can be taken to represent the 0 data for quartz, albite and zinnwaldite from Type II true intrusive age of the Lottah granite. Poimena granite alteration assemblages suggest the minerals are not in biotites located closer to the main Lottah granite than isotopic equilibrium. Assuming a temperature of « 350°C the leucogranite dyke have reset K-Ar ages between for greisen formation, albite and zinnwaldite have 368-373 Ma. Biotites located further away have K-Ar equilibrated with low O fluids generally less than 0%o. ages of ~381Ma (Higgins unpublished data). The 10 These data and a few D/H determinations on greisen Ma age gap is interpreted to reflect the episodic history micas suggest that the fluid responsible for type II of mineralisation under the influence of a long-lived alteration was an externally derived, non-magmatic fluid, thermal anomaly associated with the Lottah granite, with low 5 O . Sm/Nd and Sr isotope data of which on the basis of contents of radiogenic elements hydrothermal minerals also support this interpretation. (K,U,Th), is a high heat producing granite analogous to Mackenzie et al. (1988) measured the ages of the the SW England granites. Poimena and Lottah granites at 380 ± 2 Ma and 370 Ma, References respectively, and used this as confirmation of their Cocker, J.D., 1982. J. Geol. Soc. Aust. 29: 139-157. independent origins. However, greisenised Poimena Groves D J. & Taylor R.G.,1973. Trans. Inst. Min. Metall. 82: B135-B146. granite has a reset age of 370 Ma, thus allowing the possibility that the measured age for the Lottah granite Higgins N.C., Solomon M. & Varne R., 1985. Lithos 18: 129-149. reflects closure (around 350°C) of the isotopic system Mackenzie D.E., Black L.P. & Sun S-s., 1988. Geochim . after the greisen-forming event. In fact open system Cosmochim 52: 2507-2524. behaviour is characteristic of all AFG in the BTB Ross A.F., 1983.. Acta MSc thesis, James Cook Univ of Nth (Cocker, 1982). A non-greisenised leucogranite dyke Queensland. (the biotite-muscovite leucogranite of Mackenzie et al., McClenaghan M.P. & Williams P.R., 1982. Tasm. Dep. Mines 1988), chemically and mineralogical similar to the main Geol. Surv. Pap. 4: 32 pp. ls

smow

A18.7 THE MT BISCHOFF Sn DEPOSIT TASMANIA J.H. Wright * and T.A.P. Kwak 1

CI- Mt Kersey Mining, Perth Department of Geology, La Trobe University 1

The large Mt Bischoff tin deposit lies within an The greisens were overprinted by assemblages inlier of Precambrian sedimentary rocks which are containing phyllosilicates (chlorite, talc, phlogopite, surrounded by lower Paleozoic and Tertiary sedimentary serpentine, phengite), carbonates (magnesite, siderite, and volcanic rocks. Tin is contained within a network of dolomite), abundant sulphides (pyrrhotiteipyrite, altered Devonian porphyry dykes (endogreisens), sphalerite), late fluorite and later argillic alteration. Only replaced dolomite (exogreisen), mineralised hydro- limited quantities of dolomite were replaced by these breccias, thin veins and alluvials. Greisen style alteration later post-greisen assemblages. of porphyry dykes is zoned inward and downward from Fluid inclusion temperatures range from 90°C to nearly fresh p-form quartz+K-feldspar dyke material near 500°C with the greisen being produced between through muscovite+fluoriteitourmaline greisen and approximately 340°C to 420°C or higher and the later topaz+quartz greisen to quartz greisen. Tin grades of up overprints at progressively lower temperatures. Salinities to 2 wt % occur within endogreisen with higher grades at the greisen stage of mineralisation were high, with occurring near to the topaz+quartz/quartz greisen measurements commonly between 30% to 40% total dissolved salts. Ca, Na, K, Fe, Mn chlorides, possible contact Replacement of dolomite by greisen assemblages fluorides, C0 and hydrocarbons(?) were deduced as and greisen style alteration of porphyry dykes was components of the fluid phase. Lesser amounts of cogenetic. The mineral assemblages and the spatial dissolved Si, Al, B and rare Ti, Ca, La, P, Sn, Cu, S distribution of greisen minerals, such as mica, topaz, were inferred from daughter mineral identifications. Composition of the dykes changed with time from tourmaline, in endo- and exogreisens are similar. Topaz, quartz, fluorides, cassiterite and tourmaline occur in boron bearing quartz feldspar-biotite porphyry to high endo- and exogreisen although the proportion of fluoride fluorine bearing quartz and feldspar porphyry. The latter minerals (sellaite and fluorite) is high where dolomite dykes are associated with the bulk of tin mineralisation is replaced. Within exogreisens the highest grades of Sn and were the source of highly saline, tin bearing occur at the contact between topaz-bearing endo- and solutions. It is postulated that late, volatile charged, exogreisens, grades obtained from drill core range up to ongonitic style melts separated into two immiscible phases upon injection into the shallow seated dyke 22 wt % (1.3 m intersection). 2

275

zones at Mt Bischoff. Immiscible phases were hot (600°C), saline (50-70%), hydrothermal solutions and topaz-quartz forming melts. The deposit was formed from acidic and relatively reduced hydrothermal solutions in subvolcanic conditions, at a depth of approximately one kilometre, above a leucogranite pluton (granite inclusions exist in the dykes). High level brecciation was cogenetic with porphyry dyke intrusion and both occurred in several separate intrusion/mineralisation cycles.

Migration of magmatic fluids into pre-existing, Bbearing, porphyry dykes led to greisen transformations. The agent of greisenisation in the Pig Rat area of the White Face dyke was saline (30-50 wt %), moderately hot (350-420°C), boiling, hydrothermal solutions derived from crystallisation of fluorine- rich melts and modified by reaction with dyke conduits. Greisen brine composition progressively change with time from Fe, Mn, Na, K-rich aqueous solutions to dominantly Carich brines upon interaction with dolomite.

A18.18 GEOCHEMISTRY OF LATE PALAEOZOIC, FELSIC I- AND S-TYPE TIN GRANITES OF THE MT SURPRISE-COOKTOWN AREA, NORTH QUEENSLAND David C. Champion1', Robert J. Bultitude2 and Bruce W. Chappell1 2

1 Department of Geology, The Australian National University Geological Mapping Subprogram, Queensland Department of Mines

The Carboniferous-Permian I-type granites and the Permian S-type granites of the Mount Surprise Cooktown area form part of the extensive late Palaeozoic igneous province of North Queensland. They intrude the Middle Proterozoic supracrustal Georgetown and Dargalong Inliers, and the Ordovician to Devonian Hodgkinson Basin (HB). Both I- and S-type granites have felsic compositions (average Si0 2 = 74%) with high Nap, Kfi, Fe/Mg, LILE, and low Ti0 2 , MgO, CaO, ferromagnesian and feldspar compatible trace elements. Dominant lithologies are adamellite and granite with lesser amounts of granodiorite. The I-type granites comprise more than 120 plutons covering an area of more than 7000 km2 that are posttectonic, epizonal, and associated with extensive volcanic rocks. Eight suites have been identified belonging to three supersuites, the Almaden, Ootann and O'Briens Creek supersuites (AS, OS, OBS, respectively). The AS forms a minor portion of the I-type granites (500 km2) and is the most mafic (Si0 2 = 63-70%). Compared to similar rocks elsewhere, e.g. from the Lachlan Fold Belt and the Peninsular Ranges Batholith, the AS is more fractionated in trace elements and contains higher CaO, MgO, K 2 0, Rb/Sr, Th, U, and lower Na20, Ba and Sr. The OS contains four suites with a total area of over 4000 km2 and is the dominant group within the I-type granites. In this suite (Si02 70-78%), I^O, Rb, Th, U increase, and Ti0 2 , FeO*, MgO, CaO, P205, Ba, Sr, Eu and V decrease with fractionation. Compared with the AS, the OS has higher Ti0 2 , P 2 0 5 , Ba and Zr and slightly lower U and Th. Chemical trends argue against any major restite component and the variation can be most easily modelled by crystal fractionation. The OBS (over 3000 km2, Si0 2 72-78%), is strongly enriched in F, Rb, Th, U, Y, HREE ((La/Yb)N < 1-8), and strongly depleted in MgO, Ba, Sr and Eu (Eu*/Eu

£ 0.01-0.25). Differences between the OS and OBS are minimal, the most notable being the HFSE and HREE which rapidly increase with fractionation in the OBS possibly reflecting the role of F in decreasing compatibility of those elements. The OBS contains the great majority of granites associated with Sn mineralization. Published data show initial ^Sr/ w Sr ratios around 0.71, requiring an extensive, isotopically homogeneous and long-lived crustal protolith (possibly produced via crustal underplating) for all supersuites. In addition, geochemical similarities between suites (especially the OS and OBS) suggest that an extensive, homogeneous, andesitic to dacitic source has given rise to at least most of the suites. If this is the case, then the OC and especially the OBS are products of both low degrees of partial melting (<20%) and high-level fractionation. The S-type granites (Si0 2 = 68-77%) outcrop over 3000 km2 in the HB occurring as epizonal to mesozonal plutons. Five major suites are known, belonging to two supersuites, Cooktown (CS) and Whypalla (WS), with the supersuites outcropping as separate linear belts parallel to the coast (CS to the east). The CS comprises the granites of the Cooktown tin field. The CS differs from the WS in having higher Ti02, P205, Rb and U, and lower CaO, Ba, and Sr. These Stype granites had a supracrustal source but do not appear to be related to known sediments of the HB. Compared to the WS, the CS requires either a greater component of pelitic material and/or a more mature source. Differences between the I- and S-type granites (especially in the OBS and CS) include the presence of tourmaline, high peraluminosity and P 2 0 5 , and decreasing REE, Zr, Y, Th, Th/U (with fractionation) in the S-type granites. Tourmaline within the granite and in some cases, high P205, are diagnostic of those granites in North Queensland.

276

Poster Session

Convenor: DL. Huston GOLD DEPOSITS IN THE NORTHERN TERRITORY Masood Ahmad and Andrew S. Wygralak Northern Territory Geological Survey, Darwin

Total estimated gold reserves in the Northern Territory stand at 136,028 kg with average grades varying between 1 and 22 g/t. Total gold production in 1988 was 10,426 kg. The bulk of this production came from 13 major operaing mines; eight in the Pine Creek Geosyncline, three in the Tennant Creek Block and two in the Granites-Tanami Block. The major goldfields of the Northern Territory are located within the Early Proterozoic metasedimentary sequences of the Pine Creek Geosyncline, Tennant Creek and Granites-Tanami Blocks. The Pine Creek Geosyncline hosts a large number of small to medium size (0.5-10 mt), low grade (3-8 g/t Au) gold deposits. Mineralisation types include stratiform deposits within iron formations, gold-quartz veins in the form of shear fillings, saddle reefs and stockworks, and unconformity-related uranium-goldplatinum-palladium deposits. The stratiform deposits, considered to be synsedimentary in origin, are contained within iron formations and carbonaceous mudstone with some exhalative component. Minor remobilised transgressive gold-quartz veins are present within some stratiform deposits. The gold-quartz vein deposits appear

to be closely associated with granitoid intrusion and accompanying thermal metamorphism. The origins of gold, platinum and palladium in unconformity related uranium deposits are not clearly established. In the Tennant Creek Block gold is present alone or in combination with copper and bismuth and is contained within hematite-magnetite-chlorite-quartz bodies. The deposits are small to medium size (0.3-7 mt) but are high grade (3-54 g/t Au). The mineralisation is considered to be related to volcanic activity associated with granitoid intrusions. In the Granites-Tanami Block gold is associated with thinly bedded limonitic chert (iron formation) as high grade (7-30 g/t Au) transgressive quartz veins and as low grade (3-10 g/t Au) stratiform disseminations. The mineralisation is considered to be essentially syngeneic within iron formations with local remobilisation into high grade transgressive quartz veins. Minor gold-quartz veins are present within the rocks of the Arunta Block. They are considered to be epigenetie and are related to remobilisation of gold during metamorphic and tectonic events.

HOLOCENE EVOLUTION OF PRIMARY DOLOMITE FORMING ENVIRONMENT IN THE KINGSTON LAKE, SOUTHEAST SOUTH AUSTRALIA: A STATISTICAL APPROACH R. Ahmad Australian National University, Canberra

The Kingston Lake is located at Lat. 36°40.4'S and Long. 139°50.6'E near Kingston in southeast South Australia. It is 6 km long and 2 km wide, and has deposited about 2 m thick Holocene carbonate sequence which is dolomitic in its top 35 cm. Surface sediment distribution in the lake was mapped. Sediments of the Holocene sequence were cored from the central part of the lake. The surface and core samples were analysed by XRD for bulk mineralogy. Diagenetic mineralogy and texture were

investigated in the core samples by SEM, and the bulk sediments of selected samples were analysed for major and trace elements by XRF and Atomic Absorption Spectroscopy. TOC was determined using a Leco Induction Furnace. Water samples from the surface as well as from different depth levels below the sediment/ water interface were collected during different seasons over a number of years and were analyzed for pH and TC0 by Ion Selective Electrodes, Na, K, Mg and Ca by Atomic Absorption Spectroscopy, CI and S0 by Ion 2

277

Chromatography, and the density by density measuring the surface waters ranges from about 4 to 9, S0 content bottle. Aerosol and sea-spray fallouts were collected at varies from about 1000 to 5000 ppm. The subsurface two sites on lake shore in 1986 and were analyzed for water Mg/Ca ratio varies from 2.3 to 6 and the S0 ion Ca, Mg, CI and S0 by Ion Chromatography. concentration ranges from 1300 to 15000 ppm. The Based on the bulk XRD-mineralogy and physical Mg/Ca ratio, S0 ion concentration and the total examination of the surface sediments, four microfacies dissolved solids (TDS) are all generally lower in the were recognized at the lake surface. These are: subsurface of the lake marginal flat relative to that of Microfacies A, dolomitic wackestone of the lake centre; the lake centre. Microfacies B, nearshore protodolomitic wackestone; Mineralogy, texture, structure, and fossil content Microfacies C, organic-rich carbonate mudstone of the indicate that the sediments of Facies I were probably marginal flat; and Microfacies D, skeletal carbonate deposited in a pre-lagoon pond fresh/brakish water grainstone of the lake shore. Minerals present in the environment formed in the Pleistocene interdunal surface microfacies are generally quartz, protodolomite, corridor at the site of the lake. Microfacies IIA containing dolomite, low and high Mg-calcites, and aragonite. abundant bivalve molluscs represent open lagoon Mole% MgC0 in protodolomite/dolomite ranges from enviroment formed as a result of post-Pleistocene rise 38 to 45 and are disordered to partly ordered. Weight in sea level. Microfacies IIB indicates a restricted lagoon percent total carbonate content (TCC) varies from <50 environment formed due to restricted circulation caused by the building up of modern barrier dunes of to >90, and is highest in the Microfacies D. Statistical evaluation of a large number of Younghusband Peninsula. Microfacies IIC possibly physicochemical variables (mineral contents (wt.%); indicates hyper-restricted/ephemeral lagoon environporosity percent (POR); saturated bulk density (SBD); ment which was followed by complete isolation fron grain density (GD); moisture content (MC); acid the Southern Ocean to form the lacustrine environment insoluble residue (AIR); total carbonate content (TCC); wherin deposited the dolomitic carbonates of Facies HI. loss on ignition (LOI); and the geochemical (major and The Mg/Ca ratio in the lake water is gradually increasing trace element) data of the cored sediments) by cluster with time, thereby giving rise to the incorporation of analysis technique as well as physical examination of progressively higher mole% MgC0 in the protothe sediments and their fossil content shows the presence dolomite/dolomite and Mg-calcite. Presently, the lake of three fades in the Holocene sequence. These are: is probably behaving as a closed/semi-closed basin in Facies I, carbonate wackestone that lie at the bottom; which well developed rhomb, clusters of incompletely Facies II, carbonate packstone-grainstone lying in the grown rhombs, and globular forms of dolomite/ middle; and Facies in, dolomitic carbonate wackestone protodolomite, aragonite, and low Mg-calcite are lying at the top of the sequence. Facies II has three forming by direct inorganic precipitation from the lake microfacies (Microfacies IIA, IIB and HQ. Cementation water. Sources of the lake water are the direct rainfall of detrital quartz grains by calcite, and replacement of within the lake, surface runoff and shallow ground water calcite by authigenic chert are conspicuous diagenetic seepage from the local catchment Sources of Mg and Ca ions are the aerosol and sea-spray, rain water, features in the Facies I and II. of the exposed Pleistocene calcrete in and Chemical composition of the surface and subsurface dissolution the lake basin, and the wind blown carbonate waters in the lake vary conspicuously depending on the around time of sampling during the yearly seasonal cycle as allochems from the modern barrier dunes of the well as on the location of samples. The Mg/Ca ratio of Younghusband Peninsula. 4

PHYSICO-CHEMICAL PROPERTIES AND HYDROGEOCHEMISTRY OF THE HOLOCENE DOLOMITIC CARBONATE SEDIMENTS OF THE PELLET LAKE, COORONG REGION, SOUTH AUSTRALIA: A MODEL FOR PRIMARY DOLOMITE FORMATION R. Ahmad and P. B. Hostetler * 1

Australian National Univesity, Canberra Macquarie University, Sydney 2

Pellet Lake is located at Lat. 36°9.6' and Long. 139°39.6' near Salt Creek in the Coorong Region, South Australia. It has deposited up to 5 m of a Holocene carbonate sequence comprising dolomite, hydromagnesite, magnesite, low and high Mg-calcite, aragonite, gypsum etc. Core samples obtained from the deeper part of the lake were examined with naked eye and under binocular

microscope; samples from different depth levels within the cores were analyzed for bulk mineralogy by XRD; major and trace elements were determined by XRF; SEM investigations were carried out for texture and mineralogy. Surface as well as subsurface water samples were collected from the lake in different seasons over a number of years and were analyzed for pH, TC0 , density, Na, K, Mg, Ca, CI, S0 etc. 2

278

Based on statistical (cluster analysis) evaluation of the physicochemical data obtained from the solids as well as by physical examination of the sediment samples, three facies were recognised within the Holocene sequence. Facies I lies at the bottom of the sequence and is represente4 by carbonate wackestone that consists of dolomite, quartz and occasional gypsum. Facies II overlies Facies I and is represented by carbonate packstone that consists of dolomite, quartz, aragonite, magnesite, hydromagnesite, low and high Mg-calcite and rare gypsum. Facies in lies at the top of the Holocene sequence and is represented by carbonate mudstone that consists of dolomite, quartz, aragonite, hydromagnesite and low Mg-calcite. The dolomites occur in globular and poorly developed rhombic forms. Mole% MgC0 in the dolomites ranges from 43 to 46 and their ordering varies from disordered to partly ordered to ordered. The carbonates of Facies II and III have undergone extensive replacement by chert Facies II and III each contain two microfacies. Fossil content and mineralogy suggest that carbonate sediments of Facies I were deposited in prelagoon fresh to brakish water pond environment that 3

existed at the site of the lake; sediments of Facies II were deposited in restricted to ephemeral lagoon environments; and those of Facies III were deposited in lake environment which is persisting still today. The Mg/Ca weight ratio of the surface and subsurface lake water is very high that ranges from about 5 to >50. The S0 content of the water ranges from 800 ppm to >10000 ppm depending on the sampling season as well as site. The Mg/Ca ratio in the bulk sediments represented mainly by carbonates generally show an upward increasing trend in the sequence. Based on the mineralogy of the dolomite and the hydrogeochemistry of the lake, we infer that the dolomites were formed as a result of inorganic precipitation from the lake/restricted lagoon waters that had high Mg/Ca ratio and S0 content. Dolomites of the lake facies (Facies III) were formed in a closed basin environment in the lake. The source of the lake water is the surface run off from the local catchment and local ground water. Aerosol and sea-spray monitoring in the area suggest that these are the major sources of the Mg ions for the dolomite, hydromagnesite and magnesite. 4

THE STRUCTURE OF THE ROSEBERY MINE SEQUENCE, WESTERN TASMANIA R.F. Berry Geology Department, University of Tasmania

The structure of Rosebery has been the subject of a large number of interpretations over the last 40 years (Brathwaite, 1972,1974; Green et al., 1981; Aerden, 1989). This study was designed to compare these interpretations with the mesoscopic structure in the southern ore zone. Throughout the host sequence there is a strong cleavage with associated strong down-dip stretching lineation. This structure was also evident in the barite lenses and in silicate-rich layers within the high grade ore. Strong reverse faulting occurs throughout the mine and is closely associated with quartz-carbonate veining. The cleavage is strongly developed near the faults and the veins are folded and boudinaged indicating that the cleavage, faulting and veining were synchronous. The style of folding and faulting was characteristic of brittleductile deformation. The cleavage is at 2-20° to bedding. The bedding cleavage intersections and fold axes have shallow plunges. Ninety percent of the host sequence is east facing and zones of west facing are restricted to minor fold hinges. This structure conflicts with the evidence of a wide range of fold plunges and cleavage parallel to layering reported by Brathwaite (1972). The repetition of the ore horizon is due to steep reverse faults. There is widespread evidence that the ore and its alteration halo is pre-cleavage. All the alteration assemblages are cleaved. The silicic alteration has a spaced cleavage. The chlorite-pyrite alteration and quartz

sericitic alteration are very strongly cleaved with a downdip stretching lineation. In the carbonate alteration zone, the cleavage wraps around the carbonate pods. There was no evidence from within the alteration zones of post cleavage alteration and no obvious evidence for syn-cleavage alteration. The barite ore is cleaved with a strong down-dip stretching lineation. Within the ore, there are strongly cleaved and folded quartz chlorite lenses that are probable relicts of primary banding. Pyrite chalcopyrite bands are tightly folded with the cleavage as axial plane. Most of the ore is banded on a scale of 10-50 mm. This banding shows gradational contacts to banding in the alteration zone below the ore and is conformable with layered sedimentary rocks above the ore. The overall geometry of the ore lenses is conformable with local stratigraphy. The layering within banded ore shows the same relation to the cleavage as bedding in the least altered host rocks. It is not parallel to the cleavage. The banding is strongly deformed around boudinage structures and syntectonic veins. The tectonic foliation in banded ore varies from a dimensional preferred orientation of chlorite to a 0.1-2 mm scale layering. It is at a low angle to the centimetre scale banding. The foliation is mylonitic in galena-rich ore. Chalcopyrite and galena have been mobilized into extensional fractures during the boudin formation. Pyrite porphyroblasts has asymmetric tails and there are shear bands with a consistent reverse sense of movement. The textures in the ore support a strong rotational deformation

279

overprinting a pre-kinematic coarse pyritic layering. Syn-tectonic quartz carbonate veins are closely related to reverse faults oblique to the cleavage. The vein systems indicate syn-tectonic mobility of silica and carbonate and may suggest immobilisation of the ore. However these veins are variably mineralised depending on there environment indicating a local source for the sulphides. In barren host sequence they have no sulphide phase. In banded sphalerite ore they contain galena and sphalerite. In pyritic ore, they contain chalcopyrite. The structural evidence supports a three stage process for the evolution of the Rosebery ore: the development a conformable massive sulphide deposit with widespread subhorizontal pyrite bands, intense rotational deformation of the ore with the development of a fine mylonitic

foliation, and a patchy post-kinematic recrystallisation. The dominant structure within the mine are the reverse faults. The faults parallel to the cleavage do not produce intense deformation of the fault walls and are usually very narrow zones of strong cleavage development These faults are part of a duplex structure within the Rosebery Mine sequence with major repetitions of the ore lenses in the southern section of the mine. References

Aerden, D., 1989. Geol. Soc. Aust. Abstracts 24: 1-2. Brathwaite, R.L., 1972. Proc. Aust. Inst. Min. Metall. 241: 1-13. Brathwaite, R.L., 1974. Econ. Geol. 69: 1086-1111. Green, G.R., Solomon, M. & Walsh, J.L., 1981. Econ. Geol. 76: 304-338.

ORTHOPYROXENE-RICH ULTRAMAFIC-MAFIC ROCKS FROM WESTERN TASMANIA AND THEIR PGE CONTENTS A. V. Brown Geological Survey of Tasmania, Tasmania

Within western Tasmania there are fifteen separate areas of ultramafic-mafic rocks. The areas are usually fault-bounded, and consist of rocks belonging to one or more of three different ultramafic successions, which can be distinguished by field criteria and/or mineral chemical data (Brown, 1986). The three associations are: a Layered Dunite-Harzburgite (LDH) succession; a Layered Pyroxenite-Dunite (LPD) succession; and a Layered Pyroxenite-Peridotite and Associated Gabbro (LPG) succession. Whole-rock samples from all three of these successions have detectable PGE concentrations. Chondrite-normalised PGE diagrams, containing plots of whole-rock samples, show that the Tasmanian samples have a similar slope to samples from 'ophiolite' bodies, but, in comparison to these, the Tasmanian samples has anomalously high Pt values. Work to date on Tasmanian samples indicates that PGE values are higher in rocks with a high chromespinel content. So far only Os-Ir-Ru alloy and laurite grains have been observed in polished sections. No PtPd-Rh sulphide or alloy minerals have been observed, indicating that these elements are probably dispersed throughout the silicate phases and were not concentrated by a sulphide phase. The LDH succession consists of interlayered dunite, orthopyroxene-bearing dunite, and harzburgite. The rock types depend on the amount of orthopyroxene within any specific layer. Rocks within this succession contain olivine, enstatite and chrome spinel. The chemical range of these minerals is very restricted, Olivine grains have the composition of Fo to Fo^; enstatite, En and En , with a calcium content of less than 0.5 wt%, indicating an original clinoenstatite composition. Chrome spinel grains have a 100 x Cr/Cr+Al ratio (Cr*) 93

of 87-93 (Brown, 1986). Late-stage, coarse-grained orthopyroxenite contains enstatite crystals of E n ^ , chrome spinel grains with a Cr* of 92 to 94, and minor olivine of Fo ^ . This succession is considered to have been formed at high temperatures and low pressures as the magma chamber product of a boninitic magma (Brown & Jenner, 1989). Os-Ir-Ru alloy grains have been mined from alluvial and eluvial deposits associated with all areas of LDH succession. Records show that between 1880 and 1980, just over 880 kg of 'osmiridiunT had been recovered from the four main areas: Adamsfield; Heazlewood; Mt Stewart; and Wilson River. Smaller amounts are known to have been obtained from the Boyes River and Rocky Boat Harbour (Botrill, 1989; Cabri and Harris, 1975; Ford, 1981; Reid, 1921). The LPD succession consists of thin (<5 mm200 mm), uniform layers of orthopyroxenite, olivine orthopyroxenite and dunite. Harzburgite layers have not been found in this succession. Orthopyroxene grains range in composition from En^ to En^, and have calcium contents varying between 0.6 and 2.0 wt%. Olivine grains vary between Fo^ and Fo . Minor chromediopside has a very limited composition with an average Ca:Mg:Fe = 47:49:4. Chrome-spinel grains have an average Cr* of 64 (Brown, 1986). Although values for PGE elements have been recorded for samples from the LPD succession, so far, no PGE minerals have been recorded from this association. The LPG succession is a multi-phase ultramaficmafic succession. It consists of two ultramafic phases and a later intrusive phase of gabbro. At Serpentine Hill (Brown et al., 1988) the LPG succession consists of fault-disrupted blocks of what 9

280 was originally a layered, plagioclase-bearing, orthopyroxene-rich sequence of pyroxenite, olivine pyroxenite, harzburgite and dunite, with numerous sedimentary-like structures. This sequence was later intruded and dismembered by a second magma which formed a layered, plagioclase-bearing, olivine-rich succession, which incorporated blocks of the first sequence and contains zones rich in chrome-spinel. Both of the ultramafic sequences were later intruded by a magma phase, or phases, which formed a massive, two-pyroxene gabbro. Although the mineral chemistry of the constituent mineral phases of the LPG succession have a similar chemical range to those in the LPD succession, the LPG succession contains plagioclase as a pervasive post-cumulus phase, and contains different rock types and layering characteristics to the LPD and LDH successions. The LPG succession is considered to have formed as the magma chamber product of a lowtitanium, tholeiite magma. The study of Brown et al., (1988) over Serpentine Hill showed that all the different parts of the succession contained whole-rock PGE values, all with anomalously high Pt values. However, individual grains of PGE alloys or PGMs were rare in polished thin-sections. Because of the results from the above study, a follow-up programme of six, 100 m drill holes was undertaken on the different lithologies of the LPG succession at Serpentine Hill. The purpose of the holes was to obtain the distribution of PGE values across a stratigraphic distance in areas where PGE values had previously been obtained. Five samples were taken from each drill hole, at approximately 20 m spacings, for analysis. Three holes were placed in the olivine-rich layered sequence to intersect a chromite-rich zone which had been defined by surface mapping. The dominant rock type intersected was serpentinite, after pyroxene-bearing dunite, which has a high percentage of disseminated chrome-spinel which has a Cr* around 66. Chromitite lenses were not intersected by any of the drill holes, indicating that the zone defined by surface mapping is discontinuous. The fourth hole was drilled at Melba Flats, at the site of the sample which gave the Pt reading of 1244 ppb in the earlier study. The resultant core was dominantly sheared serpentinite with disseminated

chrome spinel. The top ten metres of the drill hole, and the zone between 75 and 85 m, contained chromespinel with a Cr* around 84, whereas the rest of the core contained chrome-spinel with a Cr* around 60. The fifth hole was drilled into the associated two-pyroxene gabbro, and the sixth into the low-titanium basalt. The analyses obtained from the drill hole samples do not reflect the results obtained from grab samples of surface outcrop. At Melba Hats, with the exception of one 50 mm thick chromitite band encountered at 18 m, none of the samples gave PGE values greater then detection limit values. Samples from the three holes drilled into the olivinerich sequence Pt, Pd, Os and Ru were all below detection limits. Ir values ranged between 0.9 and 5.0 ppb, with an average of 2.3 ppb for ten samples. However, the fifteen samples from the three holes averaged 25 ppb gold, and small Ni-Fe-Co sulphide grains were ubiquitous throughout the samples. All PGE were below detection limits for the samples of two-pyroxene gabbro. Gold averaged 28 ppb across the section. The low-titanium basalt recorded an average of 21.5 ppb for Pd, and below detection limits for the other PGE. Gold averaged 86 ppb across the 100 m section. This follow-up study to Brown et al. (1988), shows that: (1) Platinum Group Elements and/or minerals are not distributed evenly over stratigraphic depth; (2) The chromitite zone defined by surface mapping is discontinuous at depth as well as along strike; (3) Grab samples from surface outcrop are not necessarily representative of a continuous section through a specific unit; and (4) When dealing with low level PGE contents, different parts of a specific whole-rock sample do not necessarily give repeatable PGE values. References

Bottrill, R. S., 1989. Huntley. Explan. Rep. geol. Surv. Tasm. Brown, A.V., 1986. Bull geol. Surv. Tasm. 62. Brown, A.V., Page, N.J. & Love, A.H., 1988. Can. Mineral 26:161-175. Brown, A.V., Jenner, G.A., 1989. In Crawford, A.J. (ed). Boninites and related rocks. Allen & Unwin : London. Cabri, L.J. & Harris, D.C., 1975. Can. Mineral 13:266-274. Ford, R.J., 1981. Econ. Geol 76:408-186. Reid, A.M. Bull geol Surv. Tasm. 32.

281

FLUID INCLUSION AND ISOTOPIC CHARACTERISTICS OF THE BENDIGO-BALLARAT VERSUS MELBOURNE TROUGH GOLD DEPOSITS A. Changkakoti1, Z. Gao1, T.A.P. Kwak1, J.Gray2 and H.R. Krouse3 2

1 Department of Geology, La Trobe University Department of Physics, University of Alberta, Edmonton, Canada Department of Physics, University of Calgary, Calgary, Canada

The Bendigo-Ballarat (BB) and Melbourne Trough (MT) metallogenic provinces of central Victoria contain major turbidite-hosted gold- and antimony-quartz veins. In the BB province, Au-mineralisation occurs as veins in association with quartz and carbonate. Arsenopyrite, pyrite, galena and sphalerite also occur in these veins. The host rocks for the mineralisation are regionally metamorphosed Lower to Middle Ordovician turbidite sediments. At Maldon, auriferous quartz veins occur within the contact aureole of the Harcourt granodiorite. In the MT province, Au and Sb-Au mineralisation occur in quartz veins hosted by Silurian-Devonian sediments. Dyke and granitoid affiliated mineralisation also occur. Gold in these deposits occurs in the native form, as well as, in sulphides. Stibnite is the dominant sulphide mineral in the Sb-Au association. Other sulphides encountered in the veins include pyrite, chalcopyrite and arsenopyrite. Fluid inclusion studies have revealed aqueous and C02-bearing fluids during Au- and Sb-Au mineralisation in both provinces. Homogenisation temperatures (Th) of fluid inclusions in quartz from the BB deposits range from 271 to 327 in Maldon, 265 to 309 in Wattle Gully, 220 to 302 in New Cambrian and 189 to 238°C in Maxwell. Salinities of the aqueous fluids range from 8.4 to 10.3 in Maldon, 8.4 to 10.7 in Wattle Gully, 6.1

to 7.8 in New Cambrian and 7.0 to 10.4 wL% NaCl equivalent in Maxwell. Homogenisation temperatures of fluid inclusions in Sb-bearing quartz from both provinces range between 150 and 220"C. A higher Th was recorded in Nagambie. Salinities of the fluids range from 3.5 to 10.2 wt.% NaCl equivalent. Laser Raman microprobe analyses have revealed the presence of CH4, N2 and Hfi from a number of deposits. The 8MS values of stibnites range from -4.9 to 1.2 and -0.2 to 11.2%o (CDT) in the BB and MT provinces respectively. The sulphur isotope data suggest variable sources of sulphur for the BB and the MT deposits. An igneous source is postulated for the BB and some of the MT deposits. Contribution from a MS-enriched source, possibly marine sulphate, is suggested for the other MT deposits. The 8 18 0 values of Sb-bearing quartz range from 19.3 to 22.9%o (SMOW). 8D values of fluid inclusion waters in quartz range from -70 to -100%*? (SMOW). Measured 8D and calculated 8 ls O values suggest an evolved meteoric water origin for the Sb-bearing orefluids. In contrast, the measured 8D and calculated 8 ls O values suggest a metamorphic origin for the fluids responsible for depositing the Au-bearing quartz in the BB region.

MAGNETOTELLURIC DEEP-SOUNDING IN THE CLARENCE MORETON BASIN I. J. Chant and L.M. Hastie Physics Department, University of Queensland, Brisbane

A magnetotelluric deep-sounding was carried out at a site 10 km south of Ipswich, Queensland in the Clarence Basin (Chant, 1986). Data was analysed by the conventional rotated tensor method and inverted jointly using the Singular Value Decomposition method (Jupp & Vozoff, 1975) and the Molochnov-Bostick method (Dekker, 1983). The inversion was carried out by using the Molochnov-Bostik (M-B ) resistance-depth profile to determine an initial starting model for the inversion. Inversion profiles were chosen for reasonable agreement with the M-B profiles. This resulted in a faster path to a solution and in a more complex model than would otherwise be expected. The profiles for the TE and TM polarisations (figs 1 and 2) show increasing resistivity with depth, ranging from 1-10 Wm in the shallow Moreton Basin structures,

to 10-40 Wm in the Ipswich Basin rock types and 30-100 Wm in the underlying pre-Carboniferous basement structures. This is in accordance with expected geology as we pass from the Jurassic sediments of the Moreton Basin, through the metamorphosed Triassic sediments of the Ipswich Basin, to the Carboniferous Basement formed by the Yarol Shelf. The models for the two polarisations show good agreement in their basic layered structure. The variation in resistivities between the models may be attributed to the dominantly north-south faulting of the geology. This also results in some shift in the layer depths, indicating that the profile in figure 1 might be more accurate in this case. The large error bars between 300 m and 1000 m in the M-B profiles are attributable to the apparent threedimensional nature of the electromagnetic scatterers at

282

these depths, and are consistent with the existence of highly folded and faulted structures corresponding to the Marburg Formation and Helidon Sandstone strata. Divergence of the two polarisations at depths greater than 10 km is typical of deep elongated bodies in sedimentary basins (Rokityanskii 1982 ). Here the phenomenon may be associated with the West Ipswich fault-Moreton Anticline, approximately 10 km northeast of the sounding site.

References Chant, I.J., 1986. Unpubl. MSc thesis. University of Queensland. Dekker, D.L., 1983. Unpubl. PhD thesis, University of Queensland. Jupp, D. & K. Vozoff, 1975. Geophys. J. R. Astr. Soc. 42: 957. Rokityanskii, I J., 1982. Geoelectromagnetic Investigation of the Earth's Crust and Mantle. Springer-Verlag, New York:

219.

o eo >

W H

fc: o N o •

o c

Ou cu 3 r- T3

IE _o o o cu .g •£ o •3 •s

r* a> u 3

(sejjauj) H±d3Q

c c o o o c •a a -a cs 2 vo •O C 8 d u r ° r ° ce/D (X, PU botoo— v-r v-r c 3 30 O o •P - !2 c3 "75 £

H _ fc 2 E• §o a >

£ h-

3s c c > -c3 « PsQa PQ aPQ C/) C3 co c c c a) 15 5/5 O oo o5 oo uu x & V- In o UoI o DC 8 2 PQ • 1 1 s s o>

c JB

—

J £.a -»* £ E "o s i

(sejjew) H l d 3 Q

au-> 3 60

283

TEMPORAL GEOCHEMICAL VARIATIONS IN UNGARAN VOLCANO, CENTRAL JAVA.

Richard Clapnoth and Paul Carr Indonesian Volcanological Survey, Bandung, Indonesia Department of Geology, University ofWollongong, Wollongong 1

The development of Java Island which forms part of the Sunda Arc is due to subduction of the northwardmoving Indian-Australian Plate beneath the Eurasian Plate. Ungaran volcano, Central Java, is situated 200 km above the Benioff Zone dipping at 55°, and forms part of the second of three cycles of volcanism recognized on Java Island. The volcano, which was active between the Late Pliocene and Late Pleistocene, is characterised by three stages of growth, interrupted by two episodes of cone collapse, and the products of eruption can be grouped into four major units comprising Oldest Ungaran, Old Ungaran, Parasitic Cones and Young Ungaran. Detailed mapping has enabled the subdivision of these major units and has provided the basis for the collection of 57 stratigraphically controlled samples which form the basis of the present investigation. Lavas from Ungaran comprise basalt, basaltic andesite and andesite which exhibit strong similarities in mineralogy and texture. Phenocrysts comprise plagioclase, clinopyroxene, Fe-Ti oxide and amphibole. Biotite is a subordinate phase in some samples of basaltic andesite and andesite, whereas olivine occurs in a few samples of basalt and basaltic andesite. The groundmass of holocrystalline samples consists of feldspar, Fe-Ti oxide and accessory apatite. Lavas from Ungaran are dominated by mafic to intermediate compositions with a mean silica content of 54.00% and a range between 48.95 and 60.80%. Compared with most rocks with similar silica contents, the lavas from Ungaran are

characterized by high contents of alumina, total alkalies and incompatible elements, high ferric/ferrous iron ratios, and low MgO contents. Basalt samples from Ungaran range from Ne-normative to Q-normative depending on the ferric/ferrous iron ratio used in the calculation. Low Mg-numbers (maximum value = 0.55) indicate that these basalts crystallized from derivative melts and do not represent primary, mantle-derived magmas. Although the lavas have been erupted over a relatively short time, significant differences exist in the Sr isotopic ratios which have a mean value of 0.70497 and a range between 0.70467 and 0.70534. Most basalt samples are shoshonitic whereas most of the basaltic andesite and all andesite samples have affinities with the high-K calcalkaline series. Shoshonitic rocks dominated the early stages of magmatic activity whereas high-K calcalkaline rocks were produced during later stages. The most striking feature of plots of elemental abundances against stratigraphic order is the lack of any simple trend either within any of the four major stratigraphic units or between these units. Similarly, Sr isotopic ratios for the 21 samples analysed have no simple relationship with time, and the variation negates the possibility of a simple fractionation model for linking all lavas.The range in Sr isotopic ratios spans much of the spectrum of compositions for lavas from Java and suggests that within volcano variations within island arc settings may be significant

REMOTE SENSING TECHNIQUES FOR IRON ORE EXPLORATION IN THE OPHTHALMIA RANGE AREA, HAMERSLEY BASIN, W.A. TJ.Cudahy , A.R. Gabell , & M. Pal 1

CSIRO, Division of Exploration Geoscience, Perth CRA Services Limited, Iron Ore Division, Perth

Remote sensing techniques were applied successfully to Landsat Thematic Mapper (TM) data for the discrimination of iron ore in the Ophthalmia Range area, Hamersley basin, Western Australia. The approach comprised integration of laboratory spectral data with strategic image enhancement. Seventy-five laboratory reflectance spectra (within the 0.4 to 2.5 |im wavelength regions) were measured from fresh and weathered surfaces of representative rocks and soils collected from the region. These spectra showed that weathered, exposed iron ore could generally be discriminated from other iron-rich materials by it's very low average reflectance. Spectral complications

arose with the addition of surface coatings of clay or limonite. Image enhancement of the Landsat Thematic Mapper (TM) focussed on the low albedo of the iron ore. Four separate image processing techniques were assessed/ developed, including: (i) principal components (PC); (ii) log residuals; (iii) discriminant statistics; and (iv) a brightness filter function. The invertedfirstPC discriminated low albedo areas as bright, and provided useful geographical information. The log residual technique effectively removed the albedo and topographic illumination. A ratio of the log residual TM bands 3 and 5 (0.66/1.65 irn) highlighted

284

the iron-rich surface materials, including the Marra Mamba and Brockman Iron Formations. A part of the statistical analysis involved a "Mahalanobis Distance" function which weighted all pixels to a single training area centred over a mapped, exposed iron ore-body. The resultant image was similar to the "brightness filter function". This brightness filter isolated all those pixels

which fell within a specific range of digital numbers, as determined from canonical variate statistics and the laboratory spectral data. The resultant colour composite images of these particular image enhancements highlighted areas that correlated well with areas of mapped iron ore.

CHLORITE GEOTHERMOMETRY IN LOW-TEMPERATURE (DIAGENETIC) INVESTIGATIONS P. de Caritat and J. L. Walshe Department of Geology, Australian National University

Frey et al. (1988) have reported three average chlorite Chemical composition of the mineral chlorite, obtained by microprobe analysis, can be used to constrain chemical analysis from low-grade metasediments of the physico-chemical conditions of formation of Trinidad. The computed temperatures range from 221 hydrothermal and geothermal chlorites (Walshe, 1986). to 282°C, with again the lowest one obtained for the More specifically, the temperature of mineral formation most potassium-rich (K 0 = 0.05%) and aluminum-poor can be calculated, and paleogeothermal information (A1 0 = 13.86%) sample. These results are in good obtained. Here, we report on our preliminary experience agreement with other paleogeothermometers reported in applying the chlorite geothermometry tool to in Frey et al. (1988). These include calcite-dolomite geothermometers, giving a range of 300 to 344°C, and low-temperature diagenetic environments. In a first step, we have calculated chlorite formation muscovite-paragonite, giving a temperature "definitively temperatures from microprobe analyses reported in the below 350°C". No relative timing of appearance of the literature. In the case of the Cretaceous Cassipore Basin, various minerals (chlorite, muscovite, paragonite, calcite, offshore Brazil (Chang et al., 1986, Table 3), a good dolomite) is given. A study of low-grade metmorphic mudstone to slate agreement is obtained between the computed chlorite (chlorite/saponite, or C/S) temperatures and present day assemblagesfromPennsylvania, is presented by Lee et al. (1984). Six average chlorite compositions yield downhole temperature measurements. The present day geothermal gradient is 22°C/km, and the sediments are computed formation temperatures between 199 and presently at their maximum temperature. The calculated 292°C for the slates. An average composition for shales chlorite temperatures vary between 40 and 140°C. At results in calculated temperature of formation of 178°C. low burial depth (<2500 m), calculated temperatures This time the analysis with the highest IÔ (0.3%) and are lower than present-day temperatures by 20-40°C. the lowest A1 0 (20.9%) does not give the lowest At greater depths, three analyses fall within 10°C of calculated temperature, but instead gives a value of present-day temperatures, whereas three others give 250°C. Here again interstratification is reported, this higher temperatures. C/S reportedly order at about 60°C time chlorite/illite and chlorite/vermiculite. Chlorites derived from a weathered basalt where (or 1500 m) in sandstones , and 70°C (or 2000 m) in analysed by TEM at the ANU (R.A. Eggleton, pers. shales of the Cassipore Basin. Chlorite formation temperatures computed from four comm.), and computed temperature from average comauthigenic chlorite compositions given in Kantorowicz position yields a value of 36°C. This is in good agree(1984, Table 3) give a range of 172-202°C. Those ment with the assumed weathering temperature of 25°C. An investigation into the diagenetic history of chlorites (chamosites) come from the Middle Jurassic Ravenscar Group in Yorkshire, and occur as radial pore Permian clastic sediments (Caritat et al., 1990) from the linings with poorly developed honeycomb or rosette Denison Trough, Queensland, has identified the presence morphology. The lowest temperature (172°C) is obtained of chlorite, both authigenic and (?re-equilibrated) for a sample containing 0.14% KÔ (the highest value detrital. Approximately 150 analysis with IÔ < 0.5% of the group of analysis) and 13.86% A1 0 (the lowest were recorded. The temperature frequency distribution value). Potassium contamination is interpreted as being shows a range from 50 to 300°C, with a clear peak the result of both interstratification of chlorite with between 100 and 125°C. More than 50% of the computed vermiculite and interlayer cation (K ) invasion. There temperatures fall within the interval 75-150°C. The appears to be a fundamental contradiction between the presence of a tail on the high temperature side of the interpretation of authigenic chlorite being formed by histogram indicates probably the presence of detrital early freshwater diagenesis of non-marine sandstones chlorite, inherited from a hydrothermal source terrain, (Kantorowicz, 1984) and the results of the chlorite- which contributed sediments to the Denison Trough in the Permian. These chlorites may have undergone partial model calculations. 2

285

re-equilibration to the lower temperature diagenetic environment, and partially conserved a high-temperature compositional character. Petrographic investigation of these high-temperature chlorites confirms that detrital appearence, with a typical squeezed-rock-fragment or detrital mica morphology. Other parameters that may be responsible for the wide range of temperatures obtained include: non-ideal behaviour and variable silica activity in the diagenetic realm with time. Chlorite formation temperature as

calculated by the model appears to be strongly related to, and negatively correlated with, the abundance of tetrahedral silica in the mineral structure.

References

Caritat, P. de et al., 1990. This volume. Chang, H.K. ct al., 1986. CI. CI. Min. 34: 407^*23. Frey, M. et al., 1988. J. Geol. Soc. Lond. 145: 563-575. Kantorowicz, J., 1984. CI. Minerals 19: 359-375. Lee, J.H. et al., 1984. Contrib. Mineral. Petrol. 88: 372-385. Walshe, J.L., 1986. Econ. Geol. 81: 681-703.

TECTONOSTRATIGRAPHIC TERRANES OF THE LACHLAN FOLD BELT, SOUTHEASTERN AUSTRALIA Christopher L. Fergusson University ofWollongong, Wollongong.

Following the recognition of far-travelled allochthonous terranes in the Cordillera of western North America terrane analysis has been applied to other orogenic belts including the Tasmanides. Several different terrane nomenclatures have been applied to the Lachlan Fold Belt although a popular concept is that most of the belt constitutes one large tectonostratigraphic terrane. Within the eastern Lachlan Fold Belt palaeogeographic relationships remain uncertain for the extensive Ordovician quartz turbidite succession (Hotham Group and equivalents in Victoria, Triangle Group and equivalents in New South Wales) and the Ordovician island arc succession. No stratigraphic relationship has been unequivocally demonstrated between these units and a faulted contact (a thrust and/ or strike-slip fault) remains a possibility. The monotony of the Ordovician quartz turbidite successions east and west of the intervening Ordovician island arc succession supports the hypothesis that the island arc is allochthonous and unrelated to the Ordovician quartzose turbidite wedge. Therefore Ordovician andesitic volcanic and volcaniclastic rocks in the eastern Lachlan Fold Belt are grouped into the Parkes terrane and the

Ordovician quartz turbidite successions are part of the Benambra terrane ofFergussonetal (1986). The Parkes terrane was probably overthrustfromthe northeast above the Ordovician continental margin deep-marine turbidite wedge during the Early to Middle Silurian Benambran Orogeny. Deformation associated with this event produced east-west trending upright folds, related thrusts, and NW-trending dextral strike-slip faults which occur mainly in the Omeo-Wagga Metamorphic Belt and adjoining regions (Powell 1984, Fergusson 1987). During the Late Silurian to Middle Devonian interval these terranes have been overlapped by widespread volcanics and sedimentary rocks, intruded by abundant granitoids and extensively deformed (Powell 1984).

References

Fergusson, C.L., 1987. American Geophysical Union, Geodynamic Series 19: 39-56. Fergusson, C.L., Gray, D.R. & Cas, R.A.F., 1986. Geology 14: 519-522. Powell, C.McA., 1984. In Veevers, J.J. (ed.) Phanerozoic Earth History of Australia. Oxford University Press: 290-340.

286

REGIONAL STRUCTURAL CONTROLS ON GOLD MINERALISATION IN THE MURCfflSON PROVINCE, YILGARN BLOCK, WESTERN AUSTRALIA M.W. Grigson , J.R. Vearncombe and D.I. Groves 1

Homestake Australia Ltd, Western Australia, Key Centre for Strategic Minerals, University of Western Australia 1

The Archaean Murchison Province is the second largest gold-producing greenstone terrane within the Yilgarn Craton of Western Australia, and presendy contains 13 major mining operations with a forecast 1988-89 production of 21 t Au. The two largest gold deposits, the BIF-hosted Hill 50 mine and the enigmatic Big Bell mine, were a focus of pioneering research on the genesis of Archaean gold mineralisation in Western Australia. All rock types (including both 2.7 and 2.95 Ga greenstones), at regional metamorphic grades ranging from sub-greenschist to upper amphibolite, host mineralisation. Structural control on gold mineralisation is obvious in all mines within the Murchison Province. Both granitoids and greenstones within the Murchison Province are deformed by a system of major NNE-trending dextral, ductile strike-slip shear zones (up to 2 km wide) separating broader domains of discrete, linked oblique- and dip-slip (reverse) shear zones and faults with inferred NE- and SW-up motion. Gold mineralisation is mostly restricted to the oblique- and dip-slip domains. The deformation styles within these

domains are dependent on the regional metamorphic grade, with greenschist- to amphibolite-facies domains characterised by broader, ductile-brittle shear zones (up to 300 m wide) hosting mostly lode-type (disseminated) mineralisation. In contrast, sub-greenschist-facies domains are characterised by narrower shear zones (< 20 m) and brittle faults hosting vein-type mineralisation. Ore shoots are localised within bends in shear zones, at the intersection of shear zones or faults, in zones of non-cylindrical folding in BEF, and in fracture sets in competent lithologies. In general, ore shoots are parallel to stretching and intersection lineations and the plunge of folds. Regional oblique compression is implicated by the above data. In fact, the tectonic controls on gold mineralisation appear strikingly similar to those in the highly mineralised Norseman-Wiluna Belt to the east. The apparent synchroneity of gold mineralisation in both terranes suggests a common cause, probably oblique basin closure at a convergent plate margin.

UNUSUAL VERY HIGH P-T METAPELITES FROM THE RAUER GROUP: RELICS OF A 1000°C ARCHAEAN GRANULITE METAMORPHISM? Simon L Harley Grant Institute of Geology, Edinburgh, Scotland

Unusual magnesian metapelites with distinctive compositions, and assemblages indicative of anomalously high Pressure-Temperature (P-T) conditions of metamorphism occur as rafts and discontinuous layers within felsic orthogneisses in the east and northeast parts of the Rauer Group. Preliminary Sm-Nd isotopic data (Sheraton et al., 1984) and recent zircon U-Pb dating (Kinny & Black, 1990) indicate Archaean magmatic ages for the host orthogneisses, and it is suggested that the high P-T metapelites are relics of an Archaean granulite event. A domain of reworked Archaean basement can be defined in the Rauer Group on the basis of these metapelites and their occurrence in association with abundant deformed metabasite dykes, forsterite-diopside-spinel marbles, and garnet-bearing pyroxene granulites. Quartz-bearing metapelites are migmatitic and contain simple garnet (Grt)-sillimanite (Sil) assemblages (Kfs, Plag, Rut). However, garnet porphyroblasts are

highly magnesian (Mg where Mg = (100 Mg/Mg + Fe)) and overlap in composition with the most magnesian garnets from Napier Complex metapelites. Pressures (Grt-Sil-Qtz-Plag) 10-11 kbar at 900-1000°C are calculated for these assemblages. A quartz-deficient, migmatitic, metapelite contains antiperthite-mesoperthite + orthopyroxene (Opx) (Mg^ veinlets produced by in-situ anatexis. The gneissose paleosome contains the initial assemblage Grt-Opx-SilKfs. Garnet porphyroblasts containing inclusions of Opx, Sil and rare Bt are the most pyrope-rich garnets so far reported from granulite facies metapelites: Mg . Coexisting Opx contains up to 9.5 wt % A1 0 . Calculated P-T conditions are extreme; 11-12.5 kbar and 1000-1050°C, but are consistent with the Qtzbearing rocks noted above. Decompressional P-T histories at high T are recorded in the development of Opx + cordierite (Crd), Crd + Spl, and Bt + Crd + Spl intergrowths and symplectites. 57 5 9

287

However, an early very-high T decompression phase is recorded in the Qtz-deficient rocks in the formation of Sapphirine + Opx + Crd intergrowths after Mg^ garnet, and Sapphirine + Opx + Sil intergrowths after Mg garnet A two-stage decompressional history involving initial decompression from -12 kbar to 8 kbar at temperatures in excess of 900°C and further decompression from 7-8 kbar to 5 kbar at somewhat lower T (700-800°C) is inferred. This history is similar to that recorded by granulites from Forefinger Point, near the Napier Complex, and may also correlate with the two-stage decompression reported by Thost et al. 70

(1989) at Sostrene Island, some 100 km south. Whilst the second stage of the decompression correlates well with the late-Proterozoic P-T history recorded in "normal" pelites of the Rauer Group, the initial decompressional phase is suggested to be related to a pre-dykes, probably Archaean, metamorphic evolution. References

Kinny, P.D. & Black, L.P., 1990. This volume. Sheraton, J.W., Black, L.P. & McCulloch, M.T., 1984. PreCambrian Research 26: 169-198. Thost, D.E., Hensen, B.J. & Motoyoshi, Y., 1989. Abstracts, 1GCP 235 conference, UNSW 1989.

LOW SINUOSITY CHANNEL PATTERNS IN THE MIDDLE TRIASSIC HAWKESBURY SANDSTONE, SYDNEY BASIN B.G. Jones , A. Griffith and B.R. Rust 1

Department of Geology, University ofWollongong Dept. of Geology, University of Ottawa, Ontario, Canada 1

The Hawkesbury Sandstone represents the deposit of a large, sand dominated, braided to low sinuosity river system which flowed northeastwards across the Sydney Basin. The Bundeena and Kurnall areas represent the middle reaches of this river system and are characterised by the deposition of sheet-like channel sandstone complexes. Three lithofacies dominate the Hawkesbury Sandstone in the Bundeena-Kurnell area; planar crossstratified sandstone, massive or structureless sandstone and trough cross-stratified sandstone in decreasing order of abundance. Laterally extensive cosets of planar crossstrata represent the deposits of straight to slightly sinuous bars, larger sandwaves or re-entrant secondary channels that migrated under floodstage conditions. Individual planar cross-beds sets (up to 4 m thick) can be traced laterally for up to 500 m and down-current for up to 250 m indicating that they represent major bars extending at least half way across the channel. Numerous reactivation surfaces within these deposits and the presence of secondary flow structures on a few laterally accreting bar margins suggests intermittent, probably seasonal, fluvial flood cycles separated by periods with low flow activity. Massive sandstone is common at the base of channel sequences and represents the product of sand bank or bar collapse during falling water stage of a flood cycle. Medium-scale cosets of trough cross-beds also occur in the basal part of many channel sequences. Individual troughs in these cosets are generally less than 1 m in width and they represent the movement of small dunes under deep water flood stage conditions. Modification of mid channel bar tops during falling water stage also lead to the development of thin cosets of trough cross-stratified sandstone. Mudrocks form a

minor constituent within the unit. Individual preserved channel sequences in the sheetlike channel sandstone complexes have high width to depth ratios (usually » 50:1) and low palaeocurrent variance (<500). Channel widths are generally in the order of 1 km and preserved channel depths range from 4 m to 15 m. Channel margins within the cohesionless sands are typically gently sloping but can be defined by continuous erosion surfaces, with intraclast and pebble lenses, that separate the successive channel sequences. In a few areas sand channel margins are steeper and can be readily identified by a discordant erosional contact. Cliff top and wave-cut platforms provide excellent plan views of the channel contacts as well as providing an indication of the lateral extent of the main channel bedforms. The mean palaeocurrent trend for the KurnellBundeena area is parallel to the regional pattern but a very significant feature shown by the superimposed channel sequences in this mid braidplain situation is the variation in flow direction between successive channels (commonly up to 90°, rarely up to 180°) compared with the very low within channel flow variance. Some of this variation can be attributed to divergent and convergent flow around braid islands but the resultant channel orientations rarely differ by more than 60°. Alternatively, within this part of the braidplain the main channel may have subdivided to produce an anastomosing pattern of distributary channels or, more probably, the channel combed across the lower gradient portion of the floodplain with lateral channel movement mainly occurring by avulsion.

288

CAMBRIAN SEQUENCE ANALYSIS FROM OUTCROPS, WELL LOGS AND SEISMIC, AMADEUS BASIN: A NEW LOOK AT BASIN HISTORY J.M. Kennard and J.F. Lindsay Bureau of Mineral Resources, Canberra

The Amadeus Basin is a broad intracratonic depression that contains shallow marine and terrestrial sediments of Late Proterozoic to mid Palaeozoic age. Tectonic subsidence curves (Lindsay & Korsch, in press) suggest three main stages of basin evolution; an initial episode of crustal extension at about 900 Ma; a second episode of crustal extension at about 600 Ma; and an episode of crustal shortening (Alice Springs Orogeny) that terminated deposition at about 360 Ma. To establish a sequence stratigraphy for the basin, all available data have been integrated; seismic, outcrop and wells. Five major depositional sequences developed in latest Proterozoic-Cambrian time during the second extensional episode. Throughout much of this time, clastic sediments were shed from tectonic highlands at the southern margin of the basin (Petermann Ranges), and subsidence was greatest in a series of sub-basins and a connecting trough near the present erosion-defined northern margin (Lindsay & Korsch, in press). A low east-westridge(the Central Ridge) separated these subbasins from a large platform area to the south where Cambrian sediments are much thinner. Although sediment dispersal and accumulation were primarily controlled by basin dynamics, eustatic overprints are clearly recognized by changes in the pattern of successive depositional systems. Sequence 1 (latest Proterozoic) is a coarsening basinal-deltaic-fluvial clastic succession (lower Arumbera Sandstone) that extends throughout the subbasins and connecting trough in the north. This sequence onlaps successively older units on the flanks of the Central Ridge and records a relative sea-level highstand. Basal transgressive sands at the margin of the eastern Ooraminna sub-basin form the reservoir at the Dingo Gas Field. Sequence 2 (Early Cambrian) is restricted to the sub-basins. It consists of a lowstand basinal-deltaic succession (upper Arumbera Sandstone), overlain in the eastern Ooraminna sub-basin by tidalflatcarbonates, retrogradational oolitic barrier bars and highstand archaeocyathan buildups (Todd River Dolomite). Equivalent carbonates are poorly developed in the central Carmichael sub-basin (Namatjira Formation). Sequence 3 (?Early to Mid Cambrian) records a major eustatic cycle and maximum areal expansion of

Cambrian marine sediments within the basin. During the initial sea-level lowstand, thick halite deposits, red shale and carbonate (Chandler Formation) formed in a deep desiccated sub-basin; subsidence of this evaporite basin may have been controlled by salt flowage in the deeply buried Bitter Springs Formation. As sea-level rose, glauconitic sands (Dingo Member of the Giles Creek Dolomite and basal Tempe Formation) onlapped lowstand evaporites and older sequences. The overlying highstand deposits record the establishment of a carbonate ramp in the east (middle and upper units of the Giles Creek Dolomite), and the widespread deposition of locally organic-rich marine muds (Tempe Formation and lower Hugh River Shale) in the west Sequence 4 (Mid to Late Cambrian) comprises a complex west-to-east facies mosaic. During the sealevel lowstand, fluvial sands prograded eastward into the'B', Carmichael sub-basin (Illara Sandstone), and cyclical shallow-marine shale and intertidal stromatolitic carbonate (lower Shannon Formation) accumulated on the ramp in the east. As sea-level rose, cyclical shale and subtidal thrombolitic carbonate (upper Shannon Formation and Jay Creek Limestone) spread westward across the ramp, and marine muds extended into the Carmichael sub-basin. During the highstand, gravelly fluvial sands (Cleland Sandstone) accumulated within the western Idirriki sub-basin, and a thick fluvial-deltaic unit (Petermann Sandstone and Deception Formation) prograded eastward into the Carmichael sub-basin. As the supply of clastic sediments waned, deltaic and fluvial facies were onlapped by late highstand peritidal sands and oolitic carbonate (lower Goyder Formation). Sequence 5 (Late Cambrian) consists of mid to outer shelf calcareous sands (upper Goyder Formation). Its widespread distribution indicates relatively uniform subsidence across the basin at this time, and heralds the onset of a period of protracted slow thermal subsidence that continued throughout Ordovician and Silurian time. Analysis of eustatically controlled sequences enables refined correlation between eastern carbonate-rich and western clastic units, and helps predict the distribution of reservoir and potential source rocks. References Lindsay, J.F., & Korsch, R.J., 1989. Basin Research 2, in press.

MINERALOGY, ORE METAL DISTRIBUTION AND ZONATTON AT BAWDWIN MINE, NORTHERN SHAN STATE, MYANMAR (BURMA): AN Ag-RICH VOLCANIC-HOSTED, POLYMETALLIC MASSIVE SULPHIDE DEPOSIT

289

Khin Zaw Key Centre for Ore Deposit and Exploration Studies, University of Tasmania

The Bawdwin Mine is one of the largest volcanic- level of the mine and Pb-Zn-Ag and barite minerals hosted polymetallic (Pb-Zn-Cu-Co-Ni-Ag-Ba) deposits confined to the upper part of the orebodies. Gangue in the world before World War II. It is located at 23°?^ minerals are quartz, carbonate, sericite, chlorite and and 97°18'E in Namtu township, northern Shan State, alkali feldspars. Both chloritic and sericitic alteration is Burma. Three ore lenses, from south to north, Meingtha, noted but no pipe-like stringer mineralisation nor Chinaman, and Shan, occur as almost upright pipes in footwall chloritic alteration zone underlies the ore bodies. a rhyolitic volcaniclastic pile of Cambrian age. The The ore lenses are conspicuously zoned: Co-Ni±Cu orebodies were localised along a NW-SE fracture zone concentrated at the footwall and Pb-Zn-Ag and Ba in which dome-shaped rhyolite intrusions were also increasing to the top of the ore lenses. More than 10% emplaced. The total ore reserves reported in 1938 for Cu values were recorded as Cu spines within the the above three ore lenses was 10,803,244 tons of ore Meingtha and Shan lens and Co-NidbCu is concentrated with 22.8% Pb, 13.9% Zn, 1.05% Cu and its unusually at the lower levels of the Meingtha ore lens reflecting high Ag content (671.3 g/t Ag) is three times higher a possible hydrothermal venting position. Thus, the than the silver content of the Rosebery deposit, western present geological and mineralogical characteristics and Tasmania (187 g/t Ag) (Lees et al., 1990) and seven ore metal distribution and zonation of the Bawdwin times higher than that of the Brunswick No. 12 deposit deposit are comparable with that of the Kuroko-style of the Bathurst district, Canada (97.7 g.t Ag)(Luff, mineralisation. References 1986). Major ore mineral assemblages are galena, sphalerite, Lees, T., Khin Zaw, Huston, D.L. & Large, R.R., 1990. Australas. Inst. Min. Met Spec. Vol. 13, in press. chalcopyrite, pyrite with subordinate amounts of sulphosalts and Co-Ni minerals (e.g. gersdorffite). Co- Luff, W.M., 1986. Min. Engineering 38(10): 963-968. Ni±Cu sulphosalt minerals are concentrated at the lower

290

A FLUID INCLUSION STUDY OF TENNANT CREEK IRONSTONES: IMPLICATIONS FOR ORE GENESIS AND EXPLORATION Khin Zaw , D. L. Huston , T. Mernagh and C. Hoffman 1

Key Centre for Ore Deposit and Exploration Studies, University of Tasmania Bureau of Mineral Resources, Canberra 2

Mineralisation in the Tennant Creek goldfield of the 14 to 45 NaCl equivalent wt % with a mode of 25 NaCl Northern Territory occurs in pipelike, ellipsoidal, or equivalent wt %, whereas preliminary results from lensoidal lodes which are localised in anticlinal flexures Type I inclusions from barren portions of ironstone within turbiditic sediments of Proterozoic age (Large, bodies yielded a lower salinity 2-5 NaCl equiv. wt %. 1975). A fluid inclusion study has been undertaken to These studies indicate that Tennant Creek ironstones estimate the temperature, salinity and gaseous were formed from low temperature and salinity fluids, composition of the ore fluids and to evaluate the potential and subsequent Cu-Au mineralisation was formed from of fluid inclusion studies for use in exploration. Three later higher temperature and salinity fluids. different inclusion types can be distinguished in quartz Preliminary gas analysis by Laser Raman Spectroassociated with mineralised and barren ironstones at scopy using the DILOR MICRODIL-28® at the Bureau Tennant Creek: (1) liquid-vapour inclusions with low of Mineral Resources indicated the presence of N and vapour/liquid ratios (Type I); (2) liquid-vapour C0 , with very minor CH and no detectable H^S and inclusions with high vapor/liquid ratios and characteristic NI^ in Type II inclusions from the selected Tennant dark bubbles (Type II); and (3) liquid-vapor-halite Creek orebodies. inclusion with low vapour/liquid ratios (Type III). Type This study demonstrates that ore depositingfluidsin I inclusions are present in barren ironstones and in the Tennant Creek goldfield were high temperature weakly mineralised portions of fertile ironstones, but (>320°C) and high salinity (to 45 NaCl equiv. wt %). Type II and III inclusions are recognised only in fertile The high temperatures and salinities of the Tennant ironstones. Trains of Type II and in inclusions cut Creek ore fluids is consistent with upward migrating trains of Type I inclusions. Type I and III inclusions connate brines, but a magmatic contribution cannot be homogenised to liquid, but Type II inclusions ruled out. Hetrogeneous trapping in the mineralised homogenised both to liquid and vapour. ironstones (e.g. Juno, TC8 and Gecko K-44) is observed Fluid inclusion studies of mineralised zones in the as Type II inclusions in the same healed micro-fractures Juno, TC8, Eldorado and Gecko YLA4 deposits indicate homogenise both to liquid (Type Ha) and vapour (Type homogenisation temperatures for Type II and III lib). This heterogeneous trapping is interpreted to be inclusions of 230-440°C with a mode at 320°C. At the due to unmixing (exsolution) of a gas-rich (e.g. N , TC8 deposit, homogenisation temperatures of Type II C0 ) fluid during the upward migration of the metal and III inclusions are 270-440°C with a mode of 320°C bearing brine. in the Au-rich core and 230-360°C with a mode of Fluid inclusion data has important implications 320°C in the overlying Cu-rich zone. In contrast, Type regarding the deposition of gold in the ironstones, and I fluid inclusions in barren parts of the ironstone bodies may have application in discriminating fertile from yielded homogenisation temperaturesof 200-250°C with barren ironstones. a mode at 220°C. Salinity measurements on Type II and References III inclusions from mineralised deposits vary between Large, R.R., 1975. Econ. Geol. 70: 1387-1413. 2

FIELD ASPECTS OF THE ARDERY CHARNOCKITIC INTRUSIONS, WINDMILL ISLANDS, ANTARCTICA. A DYNAMIC MAGMA CHAMBER Jonathan A. Kilpatrick, David N. Young and David J. Ellis Department of Geology, Australian National University

Detailed field investigation of the Ardery Charnockitic Intrusions, Windmill Islands, Antarctica (66°16'S, 110°32'E), were undertaken in 1988. It was found that the Ardery Charnockitic Intrusions were considerably more complex than previously described, and showed complex interrelationships between cogenetic intrusions. The charnockitic bodies are distinctly intrusive and being relatively pristine present an excellent

opportunity to investigate the pedogenesis of charnockitic magmas. A number of fractionation processes are evident from the field studies. These processes include: 1. Marginal mafic cumulates consisting of orthopyroxene and clinopyroxene with intercumulate quartz+plagioclase+biotite, occurring on both the northern and southern contacts.

291

2. Magmatic layering. The evidence of a magmatic in scattered locations in the Ardery Charnockitic origin of the layering is unequivocal. The rhythmical Intrusions. Amphibole megacrysts are commonly graded layers show truncation of layering by other associated with mafic schlieren. Alkali feldspar layers, graded bedding, erosional lower contacts and megacrysts are also associated with schlieren and cumulate pyroxenes. also occur as nearly monomineralic aggregates. 3. Mafic and felsic schlieren. Schlieren are dispersed 6. Combinations of 2 to 5 have been observed. throughout the Ardery Charnockitic Intrusions and Preliminary geochemistry indicates a composite consist of petrographically identical mineralogy as pluton consisting of a wide range of co-genetic the host Ardery Charnockitic Intrusions and are compositions (Si0 : 55.8-76.2%) showing systematic recognisable through subtle modal variations. fractionation trends. 4. Cognate enclaves. These occur singularly or in This work on the Ardery Charnockitic Intrusions is swarms and are generally finer grained, more mafic, part of ongoing research at the Australian National rounded, lack reaction rims, and show variation in University into felsic magmatism in the lower crust mode and petrography that mimics that of the Ardery with related studies continuing into rocks from Mawson, Charnockitic Intrusions. Antarctica, and the Musgrave Ranges, Central Australia. 5. Megacrysts of amphibole and alkali feldspar occur 2

STRUCTURAL AND MICROSTRUCTURAL TIMING CRITERIA USED TO IDENTIFY SYNTECTONIC BASE METAL DEPOSITS: THE ELURA AND WOODCUTTERS DEPOSITS AS EXAMPLES K. C. Lawrie James Cook University of North Queensland, Townsville

Recent structural and microstructural studies have demonstrated that a number of Australian ore deposits, which have been regarded as syngenetic or controversial in origin, were in fact formed by hydrothermal replacement during regional deformation. Examples include the Mt. Isa Cu orebodies (Perkins, 1984; Swager, 1985; Bell et al., 1988), Cu mineralisation at Hilton (Valenta, 1988), and deposits in the Cobar Trough (de Roo, 1989; Hinman, 1989; Lawrie, 1990a). In deformed terrains a major point of controversy is often the possibility and/ or extent of syntectonic remobilisation of pre-existing sulphide orebodies. To resolve these problems a multiple-scale approach to structural studies is always required, with examination of host rock, alteration and sulphide textural inter-relationships essential. Structural studies on the macro-, meso- and microscopic scale on the Pb-Zn-Ag massive sulphide orebodies at Elura (de Roo, 1989; Lawrie, 1990a) and Woodcutters (Lawrie, 1990b), have demonstrated a syntectonic origin for both these deposits. The epigenetic,replaciveorigin of both deposits is demonstrated by the marked discordance to bedding of both the mineralisation and alteration, with the Elura orebodies and the main 'bonanza' pods at Woodcutters occuring in the cores of doubly-plunging fold hinges. Several criteria indicate a syntectonic origin for these deposits. These include preservation of regional cleavages within superimposed massive alteration in both the margins of the deposits and host rock fragments within the sulphide orebodies, as well as within a zone underlying one of the Elura pods. Other criteria include vein and cleavage controlled clustering of porphyroblasts in fold axial planes and porphyroblast overgrowths over cleavage (de Roo, 1989), lack of thickness changes across replacement fronts within deformed beds, and

rotation and silicification of cleavage in host rock fragments within breccia veins. Extensive development of syntectonic crack-seal veins with sulphide growth in pressure fringes indicates that there was syntectonic subvertical extension during mineralisation. The parallelism of the subvertical axis of the largest Elura orebody with the dominant extension lineation combined with the consistency of extension lineation data around the orebodies indicates a lack of a pre-existing anisotropy. The dissolution of porphyroblasts at the margins of the deposits against later cleavages places an upper limit on the timing of alteration associated with mineralisation and is particularly important at Elura where later deformation is generally partitioned around rather than through the orebodies (Lawrie, 1990a). In the massive sulphide orebodies, textural studies of orientated, doubly-polished thin sections prior to and after etching, allows correlation of the deformation affecting the sulphides with cleavages in the host rocks as well as the extent of ductile or solution remobilisation to be assessed. Studies of sphalerite reveal tectonic cleavages and deformation twins observed in both deposits, constraining the upper time limit for emplacement of mineralisation. References Bell, T.H., Perkins, W.G. & Swager, C.P., 1988. Econ. Geol. 83: 69-85. Hinman, M.C., 1989. Abstract. In Proc. Geol Soc. Aust. SGTSG Conf. 24: 71-72. Lawrie, K.C., 1990a. Econ. Geol. submitted. Lawrie, K.C., 1990b. Unpubl. report to Nicron Resources Ltd. Perkins, W.G., 1984. Econ. Geol. 79: 601-637. de Roo, J.A., 1989. Econ. Geol. 84: 256-278. Swager, C.P., 1985. Econ. Geol. 80: 107-125. Valenta, R., 1989. Tectonophysics 158: 191-208.

292 HORNBLENDES FROM THE MUSGRAVE RANGES, CENTRAL AUSTRALIA: METAMORPHIC CONDITIONS, MICROSTRUCTURES AND ^Ar-^Ar AGES M.A.H. Maboko , J. D. Fitz Gerald , D J. Ellis ,1. McDougall and P.K. Zeitler u

' Department of Geology, Princeton University, USA; RSES, Australian National University Department of Geology, Australian National University; Geological Sciences, Lehigh University, USA. 2

- - M1 Rims

- 1 2 0 0 Ma)

1.2

L J M2 Rims

y U

M4 Coronas

0.8 l

<D CL

M4 Rims 0.4

( - 1 2 0 0 Ma)

-^VI5 Shears ( - 5 4 0 Ma)

400

1200

800 T °C

Figure 1 — PTt path inferred from the mafic granulite suite. The M3 event (not shown) may overlap with M2. 1200 t

o <O)

1150

c 5

coa <0

a. <

-w

1100 -

85-242 Hornblende

1050 0.0

0.2 Fraction

0.4 39

0.6

0.8

1.0

Ar Released

Figure 2 — ^Ar-^Ar age spectrum from the highly exsolved hornblendes characteristic of rock 85-242.

Mineral assemblages and reaction textures in mafic granulites from the Amata area in the Musgrave Ranges, Central Australia, record a complex metamorphic history characterised by a sigmoidal PTt path (Fig. 1). The oldest recognisable metamorphic assemblage, Ml, in the granulites is represented by two-pyroxene-bearing mineral assemblages which equilibrated at PT conditions of -12 kbar and ~860°C. Subsequent to the Ml event,

the granulites experienced an episode of near isobaric cooling which resulted in the formation of M2 garnetbearing mineral assemblages at the expense of Ml orthopyroxene and plagioclase. Gamet-clinopyroxene thermobarometry indicates PT conditions of about 11 kbar and ~720°C for the M2 event. Isothermal and, possibly, isobaric hydration of the granulites during M3 resulted in the formation of hornblende- and biotitebearing mineral assemblages. Petrographic (optical, EPMA, SEM and TEM) studies reveal a complex microstructure for the M3 hornblendes. Of hornblende populations investigated in eight rocks from different localities, two have simple microstructures with virtually no exsolution and relatively low dislocation densities. Two other rocks have hornblendes with no exsolution; however the grains have experienced varying degrees of deformation as shown by the presence of healed cracks and/or shear bands. Hornblendes from the remaining rocks all show different degrees of deformation, exsolution and formation of reaction coronas consisting of a combination of clinopyroxene, blue-green amphibole, ilmenite and, sometimes, garnet and biotite. Exsolved textures include dense bands or isolated blebs of ilmenite and/or ilmenite+clinopyroxene aligned in up to four orientations per grain. In two of the rocks with exsolved hornblendes, products of exsolution are dominated by non-pleochroic (amphibole?) lamellae with only minor ilmenite and clinopyroxene. In the other rocks, lamellar exsolution is the minor component and is generally restricted to the edges of a few hornblende grains. Despite the complex microstructure, Âr-Âr release spectra from hornblendes separated from these eight rocks are each remarkably simple (Fig. 2) and release patterns for Âr/Âr, an isotope ratio linearly related to Ca/K, are relatively constant (max/min usually much less than 3). However, the hornblendes yield scattered Ar- Ar ages which vary by up to 760 Ma even within a single outcrop. This age scatter, which shows no correlation with the chemical composition and/or microstructural complexity of the hornblendes from the group of samples, is interpreted as reflecting variable excess argon contamination during slow cooling. Thus, the youngest bulk age, -930 Ma, from a rock with microstructurally simple hornblendes, sets an older limit to the time at which hornblende from the granulites became closed to argon diffusion. Similarly, a coexisting biotite which yields a bulk age of -690 Ma sets an older limit to the time when these granulites cooled below the argon diffusion cutoff temperatures for the biotite. 40

CONTRASTING MAGMA TYPES, ERUPTIVE STYLES AND MINERAL DEPOSITS OF THE PERMO-CARBONIFEROUS FEATHERBED VOLCANICS, NORTHEASTERN QUEENSLAND

293

D.E. Mackenzie Bureau of Mineral Resources, Canberra

The Featherbed Volcanics constitute the largest caldera-related volcanic field in the 1100 km long, 300 km wide Coastal Ranges Igneous Province of northeastern Queensland. They are mostly confined to a composite volcano-tectonic depression about 100 km long and 30 km wide, made up of eight subsidence structures. The volcanic rocks and associated granitoids fall into two distinct groups: I-type rocks of late Carboniferous age (315-300 Ma); and A-type rocks of early Permian age (290-280 Ma). Late Carboniferous I-type rocks are preserved mainly in the southeast, where two overlapping volcanic sequences form a basin-like sag structure intruded by high-level, porphyritic granite, but lacking other features of a cauldron. Other late Carboniferous volcanics occur in eroded caldera and outflow sheet remnants flanking, and generally partly overprinted by, the early Permian composite cauldron. Total remaining volume is about 500 km. Each volcanic succession ranges from rhyolitic to less abundant, mostly younger, dacitic to andesitic ignimbrites (in the SE sag); lava is rare. The granitoids have a parallel compositional range, but granodiorite is relatively more voluminous than dacite. The late Carboniferous rocks commonly contain pyroxene and/ or hornblende, are oxidised, metaluminous to mildly peralkaline, and are moderately high in MgO, CaO, Sr, Ni and Cr, and low in Pb relative to typical I-type rocks. Mineralisation associated with the I-types is widespread and abundant; it includes W-Sn, W-Mo-Bi, base metals, and Au. In contrast, Sn-dominant mineralisation of the Herberton tinfield to the southeast is associated with late Carboniferous A-type granites. The early Permian A-type volcanics comprise rhyolitic ignimbrite, relatively minor rhyolite lava, and rare andesite and dacite preserved in four nested calderas; total preserved volume is about 2000 km. The composite cauldron has been intruded by resurgent-style granitoid

plutons and microgranite ring dykes, and rhyolite domes, plugs and flows are concentrated around its margin. However, other classic features of the conventional caldera-collapse model, such as caldera-wall collapse breccias and moat sediments are absent, there is little evidence of outflow deposits, and resurgent structures are incipient at best. Relative to the late Carboniferous I-type rocks, the early Permian rocks are low in MgO, CaO, V and Cr, and high in Ba, Pb, Zr, Nb, Y, REE, Zn, Sn, W, Ga, Ga/Al, and F. Associated mineralisation is sparse, and includes Pb-Zn-Ag, Sn, Au, U, and F; it is distributed almost entirely in and near the margins of the cauldron. The data summarised above, together with preliminary isotopic data, indicate that intermediate Itype magma was derived from an old (Precambrian?), hydrous igneous source, and felsic A-type magma was derivedfromrelatively dry, F-rich, probably high-grade metamorphic rocks of Precambrian age. Caldera collapse did not occur in the SE sag structure because the I-type magma was relatively dense, hydrous, small in volume, and/or deeply emplaced. A-type magma, due to its composition, and relatively low density and large volume, was emplaced at very shallow levels, resulting in large-volume eruptions and caldera collapse; infilling kept pace with subsidence, impeding caldera-wall collapse. The great thicknesses (cooling units 1 km thick) within the composite cauldron, and the apparently poor development of outflow deposits may be explained by combinations of erosion, caldera topography, vent geometry, eruption column assymetry, low column height, and high eruption rates. Contrasts in type and style of mineralisation may be related to extended fractionation of oxidised, hydrous, I-type magma on one hand, and shallow emplacement, more complete magma chamber evacuation, and limited fractionation of relatively reduced, F-rich magma on the other.

THE FOOTWALL PRECIOUS METAL ZONE AT QUE RIVER: TRANSITION FROM A VMS TO AN EPITHERMAL MINERAL ASSEMBLAGE P. J. McGoldrick , R.R. Large and G.W. Jenkins 1

GSWA, Kalgoorlie Regional Office Key Centre for Ore Deposit and Exploration Studies, University of Tasmania 1

Que River deposit (PQ and P-north lens system) is the richest volcanogenic massive sulphide (VMS) in terms of gold, silver and base metals in Australia. The massive sulphide body (now folded into a tight to

isoclinal north-plunging upright syncline) is enveloped by hydrothermally altered (sericite-silica-pyrite + base metal sulphides) andesitic lavas and volcaniclastics. The most intense alteration (corresponding to the focus

294

of mineralising fluid input) is developed beneath the central part of the ore lenses and carries minor amounts of base metals. The remainder of the alteration zone is mostly barren of base and precious metals with the exception of a broad zone to the east of the northern third of PQ lena. This precious metal zone (PMZ) is anomalous in gold and silver (about 0.5 to 2 ppm and 10 to 80 ppm respectively). The PMZ is best developed in a coarse pyritic polymictic andesitic volcaniclastic and has some features characteristic of epithermal gold deposits. For instance, patches of 'white alteration' (commonly visible in hand specimen) comprise white mica-sphalerite aggregates which appear to post-date an early phase of K-feldspar (adularia?) + sphalerite alteration. Furthermore, as well as elevated gold contents, many samples from the PMZ have anomalous arsenic, antimony and barium. There appears to be a crude zonation from a copper-bearing core beneath the central part of the orebody to lead-zinc

Massive polymetallic Massive

sulphide

pyrite

bearing stringer to a distal lead-zinc-silver-gold-arsenicantimony stringer in the PMZ. Fluid inclusions in samples from the PMZ formed over a rangefromabout 110° to 340°C. Sulphur in pyrite from the PMZ is distinctive isotopically with a general decrease in 8 s values from PMZ to 'normal* stringer to massive ore. Based on these observations we interpret the PMZ to be a more distal part of the alteration zone developed in andesitic volcaniclastic footwall rocks during formation of the PQ-Pnorth massive sulphide orebody (Fig. 1). Best development of the distal gold-bearing stringer occurs in more porous and permeable volcaniclastics. Gold remained in solution in the more proximal parts of the stringer because physico-chemical conditions in the hydrothermal fluids forming the stringer favoured gold transport as bisulphide complexes. Gold did not precipitate until thefluidswere oxidised or dramatically cooled in the outer parts of the stringer zone. s4

Andesitic

volcaniclastics

C o a r s e g r a i n e d units

S t r i n g e r and d i s s e m i n a t e d p y r i t e In altered andesitic

volcaniclastics

Figure 1 — Palinspastic north-south section through Que River massive ore and footwall stringer depicting the position of the precious metal zone (PMZ) and metal zonation in the stringer.

THE RAMAN MICROPROBE: A TOOL FOR ANALYZING GEOLOGICAL SAMPLES FROM THE SURFACE TO THE MANTLE T.P. Mernagh Bureau of Mineral Resources, Canberra

Laser Raman microprobe spectroscopy is a nondestructive technique that can be used both for laboratory-based studies and as a geochemical exploration tool. It can be used to analyse a range of samples including those from the regolith and stream sediments to inclusions in diamonds as outlined below. Heavy mineral concentrate from stream sediment surveys is a useful indicator of source rock and possible mineral deposits. Often the fine grain-size of the concentrate does not permit a mineralogical identification but the Raman microprobe can rapidly identify minerals as small as a few microns in diameter. Raman spectra of some heavy minerals are presented and used

to show how various polymorphs can be readily distinguished. A method for determining the total salinity of aqueous fluids without prior knowledge of the fluid composition is also demonstrated (Mernagh & Wilde, 1989). Rapid salinity estimates can be done on inclusions containing fluids with greater than 1 equiv. wt % NaCl. The method is shown to be useful for sorting out genetic ore types as, in some cases, salinity is the only way of distinguishing between them (e.g. magmatic from meteoric fluids). Salinity measurements on inclusions in detrital quartz scattered amongst the regolith may also be used to indicate sub-surface fluid flow or mineral

295

deposits. The laser Raman microprobe provides rapid record near-atmospheric pressures within the analysis of these fluids compared with the slower and experimental uncertainty (Liu et al., 1990). Data on the more expensive methods of isotopic and thermometric compressibility and thermal expansivity of both diamond techniques. and garnet were used to define a P-T curve for the The laser Raman microprobe is also an important entrapment of garnet in diamond. A window within the tool for the analysis of the vapour, liquid and daughter range 47 kbar at 1100°C (150 km) to 93 kbar at 1500°C minerals in fluid inclusions. Results are presented from (280 km) for the formation of syngenetic, eclogitic Raman studies offluidinclusions form the unconformity- garnet inclusions in diamond is defined by the related uranium deposits of Koongarra, Nabarlek and intersection of the continental geotherm with the Jabiluka, Northern Territory, Australia (Wilde & diamond-graphite boundary and the entrapment curve Memagh,1989). Vapour bubbles in these inclusions determined in the present study. Most eclogitic contain C0 , CH and minor N . Thefluidis hypersaline paragenesis diamonds from Argyle are estimated to and is calcium-rich (typically 35 equiv. wt % CaCy have formed at a depth of less than 250 km, if the and is saturated with as many as eleven solid phases, temperature estimate from petrological studies was used. References including hematite, dolomite and iron-rich chlorite. The Raman microprobe has also been used to Liu, L, Mernagh, T.P. & Jaques, A.L., 1990. Contrib. Mineral. Petrol, (submitted). determine the Raman shift of an eclogitic garnet (extracted from an Argyle diamond) as a function of Mernagh, T.P. & Wilde, A.R., 1989. Geochim. Cosmochim. Acta 53: 765-771. pressure in a diamond-anvil cell. On the basis of these data, we further found that the in situ garnet inclusions Wilde, A.R. & Mernagh, T.P., 1989. Econ. Geol. (in press). 2

ISOTOPIC EVIDENCE FOR THE MIXING OF MAGMATIC AND SEDIMENTARY COMPONENTS IN THE FORMATION OF THE DACHANG TIN-REPLACEMENT DEPOSITS, CHINA Minlu Fu , A.Changkakoti , J. Gray , H.R. Krouse and T.A.P. Kwak 1

Department of Geology, La Trobe University Department of Physics, University of Alberta, Edmonton, Canada Department of Physics, University of Calgary, Calgary, Canada 1

2 3

The Dachang tin field, located in the Guangxi to 22.8 respectively. The 8 O values of the Longxianggei granite (whole province of southern China, is one of the largest tin fields in the world. It comprises the Lamo skarn Cu-Zn rock) range from 10.7 to 12.8. Most granite samples deposit and five carbonate replacement tin deposits, of have similar 8*S values from -0.1 to -4.7, mainly from which the Changpo deposit is the largest, and accounts 0.0 to -1.0. The 8D values of water of inclusion fluids for more than half of the total resources of tin in the are between -47 and -94 (SMOW). The 8 O values of ore skarns (whole rock and Dachang ore field. The Lamo skam deposit occurring at the contact of garnet) range from 8.6 to 10.9. The ore skarns have the the Longxianggei granite is distinguished by the ore similar 8 S values to granite, form -3.9 to 0.4. The mineral assemblage of chalcopyrite, sphalerite, calculated 8 0 values of water in equilibrium with galena, pyrite, pyrrhotite and arsenopyrite. Gangue these garnet based on thefluidinclusion study are from minerals are dominated by skarn minerals, such as 10.5 to 12.8. The 8D values of water in inclusion fluids wollastonite, garnet, vesuvianite, plagioclase, quartz and are between -67 to -117. Four garnet samples collected from the ore skarn have 8 O values between 8.6 and fluorite. The Changpo carbonate replacement tin deposit 10.9. Metamorphic skarns characterised by layered consisting mainly of two very large stratabound orebodies (No. 91 and No. 92) is located 1 to 1.5 km structure have 8 O values ranging from 17.2 to 21.6, horizontally to the north of the Lamo skarn with a showing similar 8 0 values with argillaceous mineral assemblage of pyrite, pyrrhotite, cassiterite and limestones. The §"S values of metamorphic skarns are sphalerite. Gangue minerals are dominated by carbonate in a large range, from -0.7 to 18.9. The host rock argillaceous limestones in the Changpo and quartz. Zinc mineralisation continues between the deposit have 8 C, 8*S and 8 0 values of 1.6 to 2.0, Lamo and the Changpo deposits. -5.9 to -7.6 and 17.9 to 19.0 respectively. The black Unmineralised Middle Devonian fossil-bearing calcareous shales collected 1-2 km awayfromthe granite shales have 8 O values of 15.3 to 18 and 8*S values of contact have 8 O values between 22:2 and 24.6 -9.3 to-11.2. Six cassiterite grains have 8 0 values between 5.21 (SMOW), and 8*S values from 15.6 to 27.1(CDT). Marbles of the Upper Devonian limestone at Lamo and 6.31. The homogenisation temperatures of fluid have 8 C and 8 O values of 1.6 to 2.0 (PDB) and 22.2 inclusions in these cassiterite grains are between ls

296

325-380°C and with a peak at 365°C. The 8D values of water in inclusionfluidsof cassiterite are between -115 and -120. The calculated 8 0 values of water in equilibrium with these cassiterite are between 9.3 and 10.8. Five quartz samples associated with cassiterite have homogenisation temperatures similar to cassiterite. The calculated 8 0 values of water in equilibrium with these quartz are in a large range, from 10.6 to 17.9. Seventeen calcite samples collected from orebodies and adjacent limestones from the Changpo replacement tin deposits have 8 0 values between 12.6 and 17.0. Homogenisation temperatures of fluid inclusions in the calcite range from 320 to 370°C with a peak at 340°C. The calculated 8 O values of water in equilibrium with these calcites are between 9.5 to 10.7. Hydrogen isotopic ratios of fluid inclusions in vein calcites are between -72 to -95, which are similar to the ratios of water in granite. Twenty-two samples of skarn were collected from a long geological profile at the 535 level of the Lamo skarn to investigate the mechanism of the formation of the skarn deposit based on O, S, C and H isotope studies. Unmineralised Middle Devonian fossil-bearing calcareous shales collected 1-2 km awayfromthe Lamo granite have 8 O values of carbonates between 22.2 and 22.8. Towards the granite, their 8 0 values decrease gradually to about 15.6 near the skarn, suggesting 18

isotopic exchange between the magmatic water and the marine limestones. Sulphide isotopes show the similar feature, from 17.7 to 27.1 in the black shale to -0.1 to +2.0 in the ore skarns and also in granite. In the Changpo area, the sulphur-rich black shales (main host rocks of orebodies) have 8*S values from -9.3 to -11.2 whereas the limestones have 8 S values in the range of 2 to 10. Sulphur isotopic ratios of sulphide minerals in the Changpo are between -6 to -9, suggesting a possible originfromthe black shales. The 8 S values of trace sulphur isotopes increase from -6 in orebodies to +4 in altered limestones, suggesting mixing of sulphur between these two sources. The 8 S values of the early stage sphalerite with a mineral assemblage of arsenopyrite, pyrrhotite and cassiterite rangefrom-6.8 to -8.3; the 8*S values of the sphalerite with a mineral assemblage of pyrite and pyrrhotite range from 0.2 to -7.7,whereas the 8"S values of the late stage sphalerite with a mineral assemblage of marcosite, jamesonite and stannite range from 1.1 to 3.6. Increasing 8"S values of sulphides from the early to late stage suggest mixing of heavier sulphur leached from the limestones with the lighter sulphur from the blackshale located at the bottom of the deposit during migration of the hydrothermal fluids. The oxygen and hydrogen isotope data suggest mixing between magmatic and meteoric waters. s4

THE GEOLOGY, PETROLOGY AND ALTERATION GEOCHEMISTRY OF THE MAGPIE VOLCANOGENIC MASSIVE SULFIDE DEPOSIT, NORTH QUEENSLAND, AUSTRALIA Ian R. Mulholland Otter Exploration NL, Perth, WA

The Magpie volcanogenic massive sulfide deposit is hosted within the Mt Windsor Volcanics of the Seventy Mile Range Group. An original chlorite±quartz± sericite±sulphide alteration zone is interpreted to have formed as a result of Mg-Fe-Al-K metasomatism (hydrothermal alteration) associated with the formation of the deposit. This assemblage has undergone low pressure, medium to high temperature metamorphism by the nearby Ravenswood Granodiorite. Conditions of metamorphism were deduced as P = 1-2 kbar, T = 500-575°C, log f0 = -16 bars, and log fS = -1 to -2 bars. The stratigraphic footwall to the deposit consists of highly altered rhyodacitic tuff and mafic volcanics. The host horizon is interpreted as being originally a siliceous, Mg-chlorite-rich hydrothermal exhalite interlayered with massive sulphide, primarily sphalerite and pyrite with minor chalcopyrite and galena. The stratigraphic hanging wall is a series of rhyodacitic and rhyolitic tuffs. 2

Structural analysis indicates that the deposit is essentially undeformed. Alteration in the stratigraphic hanging wall of the deposit is manifest by breakdown of feldspar to sericite, and widespread silicification. Alteration effects are stronger in the mineralized horizon, where gross enrichment of Mg, Al, Fe, K base metals and S has occurred. Widespread enrichment of these elements also occurs in the stratigraphic footwall, decreasing with distance away from the mineralised horizon. Geochemical trends show that within the hangingwall sequence depletion in N a p and addition of Fe has occurred. Marked enrichment of Fe, MgO, and basemetals, and depletion of CaO and N a ^ has taken place within the host mineralised horizon. The footwall alteration zone shows strong depletions of CaO and Na 0, and enrichment of Fe, MgO, MnO, and basemetals. 2

297

MODERN FLOODPLAIN SEDIMENTATION IN THE MURRAY BASIN — BARMAH LAKES AND THE GREAT CUMBUNG SWAMP P.E. O'Brien, R.V. Burne and G. Bickford Bureau of Mineral Resources, Canberra

Floodplain sediments have recieved little attention Lachlan flowing south, a line of Pleistocene lunettes in the development offluvialfades models. This poster forms a topographic barrier to the west and the Lachlan covers two contrasting areas offloodplainsedimentation, water is lost from the swamp so that it does not develop the Barmah Lakes on the Murray River and the "inland into a lake which might then overflow and cut a channel delta" of the Lachlan River, the Great Cumbung Swamp. through the barriers. The water is lost by evapoThe Barmah Lakes formed when the Cadell Fault transpiration and into underlying aquifers. diverted the Murray River. Initially the Murray flowed Upstream from the swamp, the Lachlan River is north around the fault scarp but then abandoned this sinuous and 20 m wide and up to 2 m deep. Its banks are course andflowedsouth into a groundwater lake basin. lined with stands of cumbungi (Typha orientalis). It has The Murray channel divides the lake basin in two. Low levees about 30 cm high. The channel bed consists of natural levees flank the Murray channel. They are dark grey clay with thin beds of sandy clay probably breached towards the upstream end of the lakes by represent deposition by largefloods.As itflowsinto the inflowing channels and by outflow breaches at the swamp, the river channel becomes wider, up to 70 m, downstream end. Inflow breaches are constructing and less sinuous. Cumbungi is replaced by reeds crevasse splays. The lake shores away from the Murray (Pragmites australis). It then becomes narrower and channel are also prograding into the lake because more sinuous again, eventually becomeing a shallow, sediment is brought in by overbank sheet flow and sinuous, ephemeral channel that peters out before reaching the Murrumbidgee. creeks fed by upstream flooding. The lake basins away from active inflow breaches The bulk of the swamp area is Phragmites marsh areflooredwith grey silty clay in open water areas and with standing water less than about 20 cm deep. Different in the extensive beds of rushes (Juncus ingens). Crevasse areas are subject to varying degrees of desiccation splays start as a simple breach wide in the Murray levee depending on slight differences in elevation. Open water that feeds water into the lake and deposits sand in a areas within the marsh are semi-permanent whereas the sheet. With time, the splays develop levees, colonised edges of the Phragmites marsh and the channel levees by rushes, eucalypts and grasses. The channels have are frequently exposed. All environments in the Great sandy beds whereas the levees consist of silts and clays Cumbung Swamp, including the channel, deposit black overlying sand. Where active crevasse channels enter clay. In Phragmites and cumbungi beds, the clays are the lakes, they build a mouth bar of rippled fine sand. covered by a layer of rotting vegetable matter and are As the splay channels lengthen, they receive less flow full of roots. In the river channel, the clay is very soft so they are filled by grey to black clay. The Barmah on the surface but becomes stiffer with depth. When Lakes depositional system is depositing a sequence 2 m dried out, deep cracks form in the clays. Thus, the final thick consisting of floodbasin clays passing up into depositional product of the Great Cumbung Swamp would be a blanket of black clay showing varying mouth bar sand overlain by levee clays and silts. The Lachlan River ends in marshlands known as the degrees of bioturbation by roots and subtle textural Great Cumbung Swamp. The Murrumbidgee River has variations caused by pedogenesis. constructed a low alluvial ridge which prevent the STRATIGRAPHY, PALAEOGEOGRAPHY AND MINERALISATION IN THE LOWER DEVONIAN SNOWY RIVER VOLCANICS, EASTERN VICTORIA Karin Orth and Rodney Nott Geological Survey of Victoria

The Lower Devonian Snowy River Volcanics occur in a long north-south trending belt in eastern Victoria and consist of a complex sequence of essentially subaerial acid volcanics, minor intermediate and lesser basaltic rocks, and sedimentary units. They rest unconformably on strongly folded Ordovician and Silurian sedimentary rocks and Upper Silurian granitoids. The

sequence is in part conformably overlain by shallow marine carbonates and mudstone of the Lower Devonian Buchan Group. On most previously published maps the Snowy River Volcanics have remained undifferentiated, despite concentrated work in scattered areas. The central portion of the sequence, extending from Black Mountain Station in the north to Buchan in

298 DISTRIBUTION OF SUBGROUPS WITHIN THE SNOWY RIVER VOLCANICS 1: Marroo Subgroup 2: Timbarra Subgroup 3: Wombargo Subgroup 4: Berrmarr Subgroup 5: White Monkey Subgroup 6: Tulloch Ard Ignimbrite 7: Devils Den Conglomerate 8: Little River Subgroup 9: Holy Hell Subgroup 10: Mt. Dawson Subgroup ^ g Rhyolite intrusives U: Undifferentiated Snowy River Volcanics

SEQUENCE: Q

Buchan Group Snowy River Volcanics (see above) I I I Sedimentary and granitic EM bedrock ^ N Fault

the south, has recently been mapped by the Victorian Geological Survey (Murrindal 1:100,000 mapsheet). The often disjointed stratigraphy of the Snowy River Volcanics is a reflection of various eruption styles from a number of centres over time, attendant complex sedimentation and erosion, Middle Devonian deformation, and movement on older faults. Many faults appear to have been active during volcanism, commonly moving blocks of volcanic and older rocks, and causing ponding of ignimbrites and sediments. This confining of units often makes large scale regional correlation difficult. Eight major subgroups were recognised in the Snowy River Volcanics. They are the result of several phases of large scale volcanic activity. The oldest widely distributed unit is a sequence of quartz-rich ignimbrites named the Marroo Subgroup. This marks the onset of volcanism in the central and northern map area. In the southwest, the base of the Snowy River Volcanics is represented by the Timbarra Subgroup, which is composed of breccias, ignimbrite, lava lenses and some fine grained marine sediments. In this area volcanism accompanied by subsidence and sedimentary infilling had been going on before the eruption of at least part of the Marroo Subgroup. This is indicated by the intercalation of the basal unit of the Marroo Subgroup with units of the Timbarra Subgroup. In the northwest, the southern end of what is probably an extensive unit occurs. This is the Wombargo Subgroup which is faulted against bedrock to the west and is unconformably overlain by the Berrmarr Subgroup in the area north of Wulgulmerang. The Berrmarr Subgroup contains a megabreccia composed of greater than house size blocks of ignimbrite and lesser basement rock. It probably marks a fault scarp active during

volcanism along the northeastern margin of the Snowy River Volcanics. On Mt Gelantipy east of the Snowy River the White Monkey Subgroup is separated from the main body of the Snowy River Volcanics by numerous fault blocks, along which rhyolite lava has been intruded. The Subgroup consists of a basal breccia, welded ash and numerous ignimbrites unique to this area. At Tulloch Ard an upfaulted block appears to form the divide between two units of limited extent In the east is an ignimbrite unit rich in sedimentary lithics and in the west is a sequence of conglomerate. Both these are unconformably overlain by the Little River Subgroup. These features suggest fault movement in this area, followed by minor volcanism with subsequent erosion in the north and deposition in a fault controlled depression around Tulloch Ard. The Little River Subgroup marks a new phase of volcanic activity. It is an extensive and varied unit, containing the products of several major volcanic centres. It includes vitric sediment, feldspathic and quartz-rich ignimbrite, and rhyolite lava bodies. The most extensive unit in this Subgroup is the Gelantipy Ignimbrite, which appears to have emanated from the Little River Gorge area where it is up to 500 m thick. This unit probably filled a caldera and poured out southwards. Other restricted units are the Holy Hell Subgroup and the Mt Dawson Subgroup. They may have formed in confined depressions prior to the outpouring of the Little River Subgroup. The Holy Hell Subgroup is found in and west of the Buchan River valley. It is composed largely of terrestrial sediment, minor intermediate lava, volcanogenic breccia and ignimbrite. The Mt Dawson Subgroup is restricted to the area east of the Buchan

299

River and is composed mainly of feldspar rich ignimbrite. Mineralisation within the Snowy River Volcanics is concentrated in the upper part (Little River Subgroup) and with the transition from these essentially subaerial acid volcanics and sediments to transgressive marine shelf sedimentation (Buchan Group). The mineralisation is both diverse and widespread but as yet no major mineral deposits have been discovered. Four main types of mineralisation have been recognised in the map area: 1. Epithermal precious metal vein deposits within acid volcanics (e.g. W-Tree and Pyramid Mountain gold prospects, Glen Schiel silver lode).

2. Syngenetic base metal sulphide mineralisation associated with epiclastic deposits (e.g. Shaws Creek and Running Creek prospects). 3. Stratabound lead-zinc sulphide deposits, of probable exhalative origin, in basal dolomites of the Buchan Group (Hume Park and Pyramids deposits). 4. Epigenetic base metal vein deposits within acid volcanics (e.g. Halls Peninsula copper prospect), and as fracture controlled mineralisation within bedrock where the veins are spatially associated with dykes of the Snowy River Volcanics (e.g. Campbells Knob and Deddick mineral fields).

DELTAS: THE MISSING GRAIN SIZE COMPONENT G J. Orton and H.G. Reading 1

Department of Earth Sciences, Monash University Department of Earth Sciences, University of Oxford, England 1

The study of deltas, largely owing to their agricultural and economic importance, has probably advanced further than in any other environment, with the first facies models developed more than a century ago. During the first 75 years of study emphasis was upon grain size and the use of sedimentary structures as indicators of palaeocurent directions and geographical basin analysis. This changed in the 1960s and 1970s with stress being increasingly placed on the complex relationship between fluvial input and the reworking ability of basinal processes. Variability in delta form and process was largely explained by the relative strength of hydraulic parameters such as river discharge, discharge variability, wave energy flux and tidal range. None of these values, however, can be directly determined in ancient successions. We illustrate that, not only is grain size an essential measureable quantity, but that the amount, mode of transport and grain size of the sediment load delivered to a delta front will have a considerable effect on the facies, formative physical processes, related depositional environments, and morphology of the deltaic depositional system. It will be shown how the type of modern delta that forms is directly dependent on available grain sizes which influences (1) the gradient and channel pattern of the fluvial system on the delta plain, (2) the mixing behaviour of sediment as it discharges into the ambient basin waters at the river mouth, (3) the type of shoreline, whether reflective or dissipative, and its response to both wave energy and tidal regime, (4) the frequency, magnitude and organisation of deformation and resedimentation processes on the subaqueous delta front.

For each of these components of a deltaic system, the ramification of a given grain size on the effectiveness of alluvial discharge in coping with basin energy is identical. Fine-grained depositional systems, for instance, will always have the greatest potential for seaward progradation through: (1) establishment of permanent distributary positions on a low-gradient alluvial plain, (2) dispersal of suspended sediment load well beyond the immediate confines of a river mouth within buoyancy dominated effluent patterns to form low-gradient delta front profiles, (3) attenuation of wave energy on dissipative coastlines, (4) nearshore trapping of sediment in estuaries and bays by flood tidal currents, and 5) infrequent and isolated redistribution of delta front sedimentation by gravity mass movements. At the other end of the spectrum, coarse alluvial systems will have great difficulty coping with any basin energy. Furthermore, as the size and scale of each of the deltaic sub-environments is inversely related to its gradient, and hence grain size, fine and coarse grained deltaic systems will also differ in their response to changes in hinterland characteristics (climate, tectonism) or base level controls. Small, coarse-grained deltas will be most sensitive to changes in sediment supply, whereas the depositional response of large, fine-grained systems will usually lag considerably behind the actual climatic or tectonic event. The magnitude of this hysterisis between cause and effect is discussed with reference to correlation and integration of deltaic, coastal, and submarine fan depositional systems along continental margins.

300

MAFIC DYKE SWARMS OF SOUTHERN AUSTRALIA A. John Parker South Australian Geological Survey

Mafic dyke swarms are common in many Precambrian and Palaeozoic provinces of Australia. They often form long linear zones of consistent orientation across entire provinces and, as such, contain vital information not only on the tectonic evolution of that province but also on relationships with adjoining provinces. Within southern Australia, each of the principal Precambrian crustal blocks has specific dyke signatures which record the post-orogenic extensional tectonic history of the blocks and also act as "finger prints" identifying each block. The large and very long Widgiemooltha Suite dykes of the Yilgarn Block are a prime example because they transgress the entire block from east to west and are apparently unique to that block. No other Precambrian blocks of southern Australia contain dykes of similar size and orientation but magnetic anomalies on the northern margin of the Coompana Block may represent dykes of similar magnitude but different orientation. If the latter could be demonstrated to be of the same age and similar chemistry to the Widgiemooltha Suite then the Coompana Block could be considered as a

fragment of the Yilgarn Block albeit dislodged and rotated. Most of the Archaean rocks of the Gawler Craton were extensively deformed during the ca. 1850-1700 Ma Kimban Orogeny. Therefore, there is little chance of major ca. 2420 Ma dykes being preserved as long linear province-wide features. Nevertheless, there are no known deformed dykes of similar size and continuity to the Widgiemooltha Suite so the Archaean of the Gawler Craton may not have been contiguous with the Yilgarn Block. Similar analysis and comparison of younger dyke swarms can be undertaken between the various Precambrian provinces of southern Australia. This suggests that much of southwestern Australia has been relatively stable and coherent since the Middle Proterozoic. With improved aeromagnetic data supported by extensive drilling, mapping, geochemistry and geochronology, mafic dyke swarms offer a valuable means of reconstructing crustal plates not only between continents (Australia and Antarctica) but also within continents.

MIDDLE CAMBRIAN SEQUENCE STRATIGRAPHY, GEORGINA BASIN: IMPLICATIONS FOR PHOSPHATE AND OIL J. H. Shergold & P. N. Southgate Bureau of Mineral Resources, Canberra

New and revised data relating to Middle Cambrian phosphogenesis in the Georgina Basin has accrued over the past four years from the activities of International Geological Correlation Programme Project 156 (Phosphorites) and the Australian Bureau of Mineral Resources phosphate research project (PHOSREP). In synthesizing this material it has become necessary to re-assess the concepts and correlation of lithostratigraphic units throughout the Basin, re-evaluate the currently available biochronology, and revise palaeogeographic models. Even though only a limited amount of modern seismic stratigraphic data is available for the Georgina Basin, we have attempted here to apply recent sequence stratigraphical concepts to model Middle Cambrian phosphogenetic events. As a result, we have been able to integrate a large array of sedimentological observations with corehole analyses to provide a more highly resolved lithostratigraphy than has been hitherto available. Revision of the biostratigraphy at critical intervals has allowed a more refined biochronological

framework for predicting times of subsidence and stability. The sequence stratigraphic approach to phosphogenesis has also allowed us to correct some earlier interpretations of the stratigraphy, subsidence history, internal basin structure, and timing of phosphogenetic events. For example, the Georgina Basin can no longer be regarded as a simple down-warped epeiric shelf structure. Instead it comprises at least three, perhaps four, geophysically defined and geographically delineated structural domains, each having a slightly different sedimentological history during the Middle Cambrian. Additionally, we have recognised that phosphogenesis is associated with systems tracts of more than one age superseding the initial notion that all of the 18 documented phosphate deposits were contemporaneous. Sequence #2 (documented below) gives the first indications, from its composition and limited distribution, for subsidence of the Mount Isa Block, an event having considerable significance for stratigraphical interpretation and correlation.

301

Three stratigraphic sequences of Middle Cambrian sediments are recognised in the Georgina Basin. In these, phosphorites and their associated organic-matter rich shales are restricted to retrogradational parasequence sets in the transgressive systems tract of each stratigraphic sequence. Each transgressive systems tract comprises a suite of lithofacies whose relationships depend on relative sea level position, palaeogeography and tectonics. Economic phosphorite deposits, and their associated phosphatic limestones and organic-matter rich shales, which contain both benthonic and planktonic faunas, accumulated in shallow water environments of rapidly fluctuating oxygen tensions close to the boundary of the oxygen minimum zone and the sea floor. Continuing transgression drowned any palaeotopographic highs and promoted a shoreward migration of the oxygen minimum zone. This produced anaerobic bottom water conditions which led to the deposition of organic-matter rich shales containing elements of a planktonic fauna, but lacking benthos. As transgression progressed and water depths increased, organic-matter was degraded in the water column eventually resulting in aerobic bottom waters and the deposition of platform and ramp carbonates of the highstand systems tract. Phosphate deposits in stratigraphic sequence #1 occur along the northern and northeastern margins of the Georgina Basin during the Oidian/early Templetonian. Although sequence #1 is of basin wide occurrence, only the small phosphate deposits associated with the Border

Waterhole Formation between Highland Plains and Riversleigh accumulated at this time. Sequence #2 is of local occurrence between Thorntonia and May Downs, and is related to an early phase of subsidence of the Mount Isa Block during the latest Qrdian/early Templetonian interval. The phosphate deposits of the Lady Annie, D Tree and Yelvertoft areas are associated with sequence #2 which was terminated by a rapid fall in relative sea level. Sequence #3 retrogradational parasequence sets have been recognised in the Burke River Structural Belt, and at Ardmore, Elkedra, Wonarah and Alexandria during the late Templetonian/Floran, and in the late Floran Euagnostus opimus Zone south of Thorntonia. This sequence contains the phosphate deposits of the Monastery Creek Phosphorite Formation at Phosphate Hill (Duchess deposit) and Ardmore, the Wonarah Beds (Wonarah deposit), and the Gowers Formation (at Thorntonia). Organic-matter rich shales are intimately associated with the phosphogenetic events. In sequence #1 they form a condensed interval in the Lower Hay River Formation and its correlatives along the southern margin of the Georgina Basin; and in sequence #3 they occur in retrogradational parasequence sets containing the Monastery Creek Formation, Inca Formation, Blazan Shale, Arthur Creek Formation and Currant Bush Limestone.

PERMIAN PALAEOGEOGRAPHY OF AUSTRALIA: BMR-APIRA PALAEOGEOGRAPHIC MAPS PROJECT J. M. Totterdell & A.T. Brakel Bureau of Mineral Resources, Canberra

Seven interpretive palaeogeographic maps, compiled as part of the BMR-APIRA Palaeogeographic Maps Project, illustrate the development of Australia during the Permian. Each map represents not a snapshot, but a generalisation of the palaeogeography during a given period of time. Permian 1 (Asselian; palynological Stage 2) shows Australia during the Late Palaeozoic glacial maximum. Ice sheets were centred over the Pilbara and Yilgarn Blocks, and central and South Australia. Lobes from an ice sheet centred over Victoria Land covered most of Victoria and Tasmania. Much glacigenic sediment was deposited as both marine and terrestrial fades in adjacent basins. Alpine glaciers were present in highlands along the east coast and a transient ice cap may have formed over the Kimberley Block. Rift-related volcanism occurred along a belt from the Sydney Basin to the Bowen Basin. Permian 2 (early middle Sakmarian; Stage 3a) shows an essentially deglaciated continent with extensive shallow marine and terrestrial areas. Post-glacial sea

level rise is evident in many basins, but in central Australia the influx of reworked glacial detritus caused progradation of the shoreline. On the east coast, convergent margin (in the north) and rift-related volcanism continued; marine deposition occurred on a narrow continental shelf and in deeper waters beyond. During Permian 3 (late Sakmarian-middle Artinskian; Stage 3b), there was a minor drop in relative sea level. Shallow marine conditions prevailed in the western basins with the deposition of nearshore clastic and shallow marine carbonate sequences. Later, a large delta formed in the Carnarvon Basin. Extensive coal measures accumulated in central and eastern Australia, and in the southern Perth Basin. Permian 4 (middle Artinskian-Kungurian; Stage 4) marks a cessation of deposition in all central Australian basins except the Cooper. Early in Permian 4, uplift in the New England Orogen led to the development of a prograding shoreline in the northern Sydney Basin. This was followed by a period of renewed marine trans-

302

gression in many basins, including the Sydney, Bowen and Tasmania Basins. Permian 5 (early Kazanian) a transgression in eastern Australia established a seaway along the Sydney-Bowen trough, and marine conditions in Tasmania. In the Canning Basin, however, deposition became confined to the Fitzroy Trough. Permian 6 (middle Kazanian-middle Tatarian) was characterized by the development of large progradational complexes on both sides of the continent and the establishment of widespread coal measure deposition in eastern Australia; in Western Australia a large delta

developed in the Bonaparte Basin. Marine conditions persisted for some time in the Bowen Basin. Oogenic activity in eastern Australia coincided with extensive silicic cauldron-centred volcanism in New England. Permian 7 (late Tatarian) continues the trend to dominantly continental sedimentation in eastern Australia, with coal measures deposited in large fluviodeltaic systems in the Sydney and Bowen Basins. Orogenesis and silicic volcanism continued in eastern Australia. In the Carnarvon Basin, deposition became confined to the north, while marginal marine to terrestrial environments prevailed in the Canning Basin.

VOLCANIC ROCKS FROM THE WESTERN WOODLARK BASIN, PAPUA NEW GUINEA, A RE-ACTIVATED SUBDUCTION ENVIRONMENT? G-E.Wheller , R.A.Binns , DJ.Whitford , R.L.Chase and PJ.Michael 1

CSIRO Division of Exploration Geoscience, North Ryde Department of Geological Sciences, University of British Columbia, Canada 1

Submarine volcanism in the western Woodlark Basin lavas they contain slightly higher abundances of is associated with active propagation of the east-west incompatible elements (Ba/Y 0.6-3.3) and are slightly Woodlark seafloor spreading axis into Mesozoic-Tertiary LREE enriched (Ce /Sm 1.0-1.2). They also have continental crust (see accompanying abstract by Binns slightly higher Sr/ Sr values (0.7028-0.7032). et al.). Near the tip of the spreading zone, new Tholeiitic andesites dredged from depths of 2200 m bathymetric data and seafloor photography indicate the from Cheshire Seamount, north of the Woodlark presence of a series of small en echelon basins together spreading axis, are further enriched in LREE (Ce^Sn^ with three volcanic seamounts. The seamounts vary in 1.4-1.7) and incompatible elements (Ba/Y 1.0-13). Their tectonic setting from ridge axis (Franklin) and off-ridge ^Sr^Sr values (0.7035-0.7039) are also higher than (Cheshire) in the east, to ridge tip (Dobu) in the west those from Franklin Seamount. However, one group of They consist of basaltic and andesitic breccias, basalts from Cheshire is depleted in LREE (Ce^/Sn^ hyaloclastites and lobate and pillow lava flows. Basaltic 0.8) and has low ^Sr^Sr (0.7030-0.7031). The widest spectrum of lava compositions was lavas also form the floor of one of the basins (East Basin). Despite high regional sedimentation rates, little dredged from depths of 350-1540 m from Dobu sediment covers the volcanics, indicating they are very Seamount at the ridge tip. These include unusual Mgrich transitional andesites together with peralkaline young. Overall, Sr/* Sr values and K, Rb, Ba and LREE rhyolite blocks. The Mg-andesites contain 7.5-8.5 % abundances in mafic lavas increase progressively from MgO and 55-53 % Si0 and have high Mg# (66-68). ridge axis rift, to ridge axis seamount, to off-axis They also contain the highest enrichments in inseamount, to rift-tip seamount. Ti0 concentrations compatible elements (Ba/Y 12.2-17.9) and LREE (CcJ decrease in parallel. In East Basin, 3200 m deep, four Sm 2.3) and highest "ST^'ST values (0.7039-0.7040). distinct glassy transitional basalt and andesite Less Mg-rich high-AI alkali andesites from Dobu are (Wilkinson, 1986) lavas were sampled. The two basaltic similarly enriched (Ba/Y 8-18, Ce^Sm,, 2.2, ^Sr^Sr lavas are relatively primitive and similar to N-MORB, 0.7039). The rhyolites are compositionally similar to having high Mg# (66.4, 67.6) and low Si0 (50.5, subaerial peralkaline rhyolites on adjacent Fergusson 49.4%), together with low abundances of incompatible and Sanaroa Islands (Smith & Johnson, 1981) but have elements (Ba/Y 0.3-1.0), LREE depletion (Ce /Sm significantly higher "SrP'Sr values (0.7066-0.7068) 0.7) and low ^Sr/^Sr (0.7028-0.7030). The two andesitic which, from preliminary Nd and Pb isotope data, are lavas are markedly more Ti- and Fe-rich but their Sr/ probably due to seawater contamination. Sr values (0.7029) and degree of LREE depletion Many of the characteristics of the mafic lavas are (Ce /Sm 0.75) are similar to those of the basalts, similar to those found in back-arc basalts. Moreover, indicating an origin involving more marked fractional the distinctive pattern of increasing Sr/ Sr and crystallisation. enrichments of K-group incompatible elements and Lavas dredged from depths of 2300-2500 m from LREE, and decreasing Ti0 contents, in conjunction Franklin Seamount, which lies on the 'normal* Woodlark with moderate Nb depletions, represents a systematic spreading axis, are mainly tholeiitic andesites with increase in "island-arc" character from ridge-axis to moderately high Mg# (55). Compared to East Basin ridge-tip. However, the the Woodlark Basin is not a N

303 typical back-arc basin because of the absence of an associated active subduction zone. The compositional variation shown by the ridge-tip volcanics may, however, be ultimately due to subduction-related mantle enrichment processes that may have occurred during the Neogene as parf of the Trobriand subduction zone. In addition, the presence of high-Mg andesites at Dobu

Seamount suggests that melting is also occurring at relatively shallow levels within the mantle. References Smith, I.E.M. & Johnson, R.W., 1981. J. Geophys. Res. 86:10257-10272. Wilkinson, J.F.G., 1986. J. Petrol 27: 31-62.

GEOCHEMISTRY OF THE HOST ROCKS TO THE SCUDDLES VOLCANOGENIC MASSIVE SULPHIDE DEPOSIT, WESTERN AUSTRALIA: POTENTIAL OF LITHOGEOCHEMISTRY IN EXPLORATION David J. Whitford1 and Paul M. Ashley2 1

CSIRO Division of Exploration Geoscience, North Ryde 2 University of New England, Armidale

Major and trace element abundances, including REE, have been determined in a representative suite of the host rocks to the Scuddles volcanogenic massive sulphide deposit in the Archaean Yilgarn Block of Western Australia. The aim has been to evaluate the usefulness of lithogeochemistry in exploration for this style of ore deposit, and to test the applicability of exploration techniques based on studies of similar deposits from Canada (Lesher et al., 1986). In particular, we have attempted to test the "fertility" concept whereby it is claimed that volcanic sequences hosting volcanogenic mineralisation have a distinctive primary geochemical signature. The geochemical characteristics of the host rocks from Scuddles appear to reflect the superimposed effects of hydrothermal alteration on a range of primary compositions from andesite to rhyolite. Rocks from the footwall are geochemically the most uniform in elements traditionally regarded as both immobile and mobile. Rocks from the hanging wall are geochemically diverse and those from the mineralised horizon exhibit great variability. The distribution of elements such as Ti, Zr, Y, V and REE (excluding Eu), reflects primary lithologic variation across the Scuddles stratigraphic sequence and can be used for local correlation of units (Ashley et al., 1988). A lithogeochemical alteration halo defined by mobile elements on a scale of hundreds of metres about the orebody extends into the footwall as a semi-conformable zone for several kilometres along strike to the adjacent Gossan Hill volcanogenic massive sulphide deposit, and beyond. Alteration zoning around ore grade Cu, Zn sulphides is best defined by enrichments of base and precious metals, Mn, Mg, S, C02 and H^O, and depletion of Na, Ca, Sr, and K. Whereas some of the geochemical parameters in the less altered rocks match those observed in mineralised volcanic sequences in Canada, other characteristics could be interpreted to indicate little mineralisation potential.

In terms of Zr/Y and La/Yb ratios, footwall rocks have the most "fertile" signatures, whereas the hanging wall is more "barren". However both footwall and hanging wall have only marginally "fertile" signatures in terms of their Eu anomalies. The mineralised horizon, including samples of massive and stringer ore, are geochemically variable but include samples with the most "fertile" Zr/Y and La/Yb signatures. On the other hand, the mineralised horizon is characterised by widespread Eu enrichment, a common feature of many massive sulphide deposits where Eu enrichment is apparently matched by a complementary depletion in the surrounding alteration halo (Whitford et al., 1988). If the behaviour of Eu in the mineralised horizon reflects hydrothermal alteration related to mineralisation, the "fertile" Zr/Y and La/Yb signatures noted in the same horizon might also reflect alteration. On this basis it is possible that at least some "fertile" geochemical signatures could be reflecting secondary alteration. The "fertility" concept relies on the recognition of primary geochemical characteristics that remain unmodified by subsequent hydrothermal alteration and regional metamorphism. Accordingly, Eu anomalies are ascribed to fractional crystallisation of plagioclase. On the other hand there is good evidence for Eu mobility during hydrothermal alteration which suggests that "fertile" volcanic signatures could reflect both primary and secondary processes. The muted Eu anomalies observed in the host rocks at Scuddles may be reflecting relatively oxidising alteration conditions during mineralisation, additional evidence for which is the widespread occurrence of magnetite in the ores. The application of lithogeochemistry to identify prospective horizons for massive base-metal sulphide deposits requires an understanding of the penological and geochemical evolution of individual volcanic belts. Applying exploration techniques and models developed in a particular province may not be

304

necessarily relevant in other geological domains. References Ashley, P.M., Dudley, RJ., Lesh, R.H., Marr, J.M. & Ryall, A.W., 1988. Econ. Geol 83: 918-951.

Lesher, CM., Goodwin, A.M., Campbell, I.H. & Gorton, M.P., 1986. Can. J. Earth Sci. 23: 222-237. Whitford, DJ., Korsch, M.J., Porritt, P.M. & Craven, S.J., 1988. Chem. Geol. 68: 105-119.

ALKALI GRANITES FROM THE MID AND LATE PALAEOZOIC OF SOUTHERN NEW SOUTH WALES: PETROGENETIC AND TECTONIC IMPLICATIONS RJ.Wormald, R.C.Price and C.M.Gray La Trobe University , Melbourne

Early Silurian alkali granitic rocks at Wallundry, 20 km northeast ofTemora, are associated with plagioclase rich cumulate gabbro, diorite and quartz monzodiorite. The most evolved member of the suite is a hypersolvus muscovite-biotite granite. Mineral chemistry and geochemical trends suggest the suite evolved by plagioclase, olivine, clinopyroxene and ilmenite fractionation from a basaltic andesite parental magma. The trend toward hypersolvus peraluminous granite also involved amphibolefractionationwith the peraluminous condition attained by the peritectic reaction amphibole + melt -> biotite. An age of 431 ± 4 Ma with an initial ratio of 0.7039 ± 0.0001 (MSWD = 0.51), obtained from Rb-Sr dating of fresh whole rock samples, is a similar age to that obtained on the high-K basaltic andesite near Temora (Perkins et al., 1989). Similar intrusives to the Wallundry Suite occur near Bethungra, Adelong and Mt Adrah. Late Devonian biotite, hornblende - biotite, and aegirine- arfvedsonite granitic rocks, which crop out north ofTemora (Wormald & Price, 1988), are associated with rare alkali dolerite and quartz syenite intrusions. In contrast to the early Silurian suites, the association is bimodal with no intrusives representing compositions between 50 and 66 wt% SiOr Whole rock Rb-Sr dating on the metaluminous-peraluminous granite samples yields an age of 364 ± 2 Ma and an initial ratio of 0.7042 ± 0.0002 (MSWD = 2.81). The least evolved peralkaline granite (Sr = 24 ppm) plots on the same isochron but the more evolved rocks have been

isotopically disturbed. Penological, geochemical and isotopic evidence indicates that the most plausible petrogenetic model to produce the observed range in rock types involves partial melting of high-K gabbroic to dioritic rocks during injection of alkali basalt into the lower crust. This produced a range of melt compositions from quartz syenite to granite, which being relatively dry, were able to move to high levels in the crust and fractionate during transit Evolved peraluminous granites formed by crystal fractionation involving amphibole, and peralkaline granite formed by plagioclase crystal fractionation from quartz syenite compositions ; the peritectic reaction ferro-hedenbergite + sodic plagioclase + melt —> arfvedsonite was important in attaining the hypersolvus and peralkaline condition. The early Silurian granites formed in an extensional tectonic regime associated with sinistral transtension along the Gilmore Suture and the opening of the Tumut Trough. The late Devonian granite suites post-date deformation of the early Silurian granitic and sedimentary rocks with magmatism probably associated with thrust relaxation on major faults inferred to pass through the Temora region. References Perkins, C., McDougall, I., Claoue-Long, J. & Heithersay, P., 1989. Second Australian Conference on Geochronology (Abstracts). Wormald, R.J. & Price, R.C., 1988. Aust. J. Earth Sci. 35: 209-221.

305

R. L. Stanton Symposium — New Frontiers in Ore Deposit and Exploration Studies Convenor: R. R. Large THE ROLE OF RESEARCH AND DEVELOPMENT IN SUCCESSFUL EXPLORATION S. M. Richards Aberfoyle Limited, Melbourne

Exploration is research. The whole process equates to classic scientific research method, and exploration management has recognised that the activity proceeds more effectively if most individual explorers are imbued with a research attitude. New technology has made available abundant and relatively cheap physical and chemical data, and given explorers the means to create prospecting maps to utilise their favoured search concepts, drawing on multiple data sets. The inter-relationship of exploration and the outcome of research which is itself a product of exploration is illustrated by examining the case history of Aberfoyle's base metal search programme in western Tasmania: • Beginning with essentially unexplored ground, the first exploration stage was pragmatic, drawing only on research elsewhere which had established the characteristics of a particular ore style, and the general distribution of the Mt Read volcanics. • The second stage, after the discovery of Que River, involved considerable data gathering by mine geologists, explorers and detached research groups. Search recipes were developed, and experimentation with geophysics was the main preoccupation. • The third stage, after the discovery of Hellyer, resulted in a clear understanding of the whole ore environment and the enclosing volcanic pile. The research conducted in this stage underpins the most difficult

stage of exploration where the search is focussed entirely on orebodies which may be several hundred metres below the surface. This stage brings together all of the accumulated geological and geochemical knowledge and provides a much more reliable context for the interpretation of surface and downhole geophysics. It also provides an invaluable reference point for search in the rest of the Mt Read volcanics. The case history is generalised by reflecting on the search for stratiform base metal deposits in metamorphosed terrains. The problem is to confidently interpret what is observed. Of his many contributions to the exploration of stratiform ores, Professor Stanton's research on the understanding of metamorphosed chemical sediments and alteration mineral assemblages is highlighted. He has shown that we should be able to recognise feeders, stringer zones, exhalites and plumes rather than regard the indicative mineral assemblages as unknowable metamorphic aberrations. To encourage and maintain a research attitude within exploration teams, it is necessary to set aside considerable funds for further study, including postgraduate work; for state-of-the-art technology and learning how to use it effectively; for collaborative research to develop information networks; and for drilling, even when the limits of interpretation are being stretched.

THE SEARCH FOR NEW WITWATERSRAND GOLDFIELDS D.A. Pretorius EGRU, University of the Witwatersrand, Johannesburg, South Africa

In 1905, nineteen years after the discovery of the Witwatersrand auriferous conglomerates, South Africa overtook the United States as the world's leading gold producer and has remained unchallenged in this position up to the present day. In the 102 years since the first Witwatersrand gold was won in 1887, it is estimated that South Africa's production of 42,075 tonnes accounted for approximately 50% of the total world output No less than 98% of the South African figure

has had its origin in the Witwatersrand sediments. The concepts on which exploration was based up to the early 1980s held that the Witwatersrand depository was an intracratonic, extensional, oval-shaped basin, with its long exis trending northeastwards. In crosssection, it was an asymmetrical syncline, with the steeper limb along the northwestern margin. Majority opinion subscribed to a paleoplacer origin for the apparently detrital gold and uraninite hosted in quartz-pebble

306

conglomerates, pyritic sands, and kerogen-rich bands, considered to have once been algal mats. Each of the seven goldfields was interpreted as a fluvial fan developed where a major drainage system discharged into a shallow lake, these fans prograded preferentially along the northwestern shoreline of the depository and attained their optumum development in the upper quarter of the stratigraphic pile. The source of the gold was thought to be in the lode-gold orebodies typical of Archean greenstone belts, while the uraninite was derived from the granitic component The age of the depository was favoured to be anomalously early Proterozoic with a lower limit of 2800 Ma and an upper limit of 2500 Ma. Although such an age range fell within late Archean times, the postulated intracratonic setting and the dominantly clastic lithologies were put forwad as evidence of a Proterozoic affinity. In the past seven years, the results of considerably expanded research have compelled significant and wideranging changes in the basin-model which is to be employed in refining search strategies. As the 1980s draw to a close, new exploration thinking has to take into consideration the following: • The basin, as preserved, has an arrow-head shape, with the shafts being a NE-trending anticline, flanked by two longitudinal synclines which narrow in width towards the northeast. The shape indicates a regional plunge to the northeast, from a transverse, NWtrending anteclise in the southwest towards a transverse syneclise in the northeast The southwestern limit of the depository rests with an erosional contact against the flank of the anteclise, while the northeastern extremith plunges beneath a lobe of the Bushveld Complex disposed along the syneclise. • Between the anteclise and the syneclise are several transverse anticlines and synclines which trend northwestwards. The interaction between the longitudinal and transverse folds has resulted in an interference pattern consisting of structural culminations (domes) and depressions (basins). Structural cells are constituted by a central depression contained between four cornerculminations or -domes. The cells can be delineated by gravimetric and magnetic means. Too small a volume of vibroseismic information is available for it to have contributed definitively, as yet, to the deciphering of the geometric pattern of the basin. • Each one of the seven goldfields can be shown to occupy a separate structural cell. The field is located down the slope from a corner-dome, towards the central depression of the cell. Rather than a single source region to the northwest for all of the gold in the basin, it would seem that local sources, tied-in to some of the domes, also contributed to the development of the auriferous strata. Six additional cells have been defined within the presently known limits of the basin, and, in one, a minor goldfield is located. • The style of basin evolution can be accommodated better in a compressional, rather than an extensional

model. The interference fold pattern points to two directions of compression. A depository forming under compression, with one side marked by thrusting, coarse-clastic sedimentation, regressive reworking, cannibalisation of earlier strata, repeated unconformities, and general conditions of offlap, is more typical of a cratonic foreland than of an intracratonic environment. In such a setting, the depository could well have taken the form of a backarc or retro-arc basin associated with a zone of backthrusting on the cratonic side of a magmatic arc or orogen. • Recent zircon dating requires the Witwatersrand to be moved father back in time, to between 3100 and 2750 Ma. Only an Archean stamp can be placed on such ages, and the early Proterozoic classification has to be abandoned. The new dates have a degree of overlap with those of the range 2800-2500 Ma, which marks the time of extensive and substantial lode-gold mineralisation in Archean greenstone belts around the world. At 2800 Ma, younger granites, accompanied by pervasive, hydrothermal alteration intruded the older, typical, Archean-basement granites and gneisses. The Central Rand Group, representing the uppermost quarter of the stratigraphic column, is bracketed between 2900 and 2750 Ma. Thus, granite emplacement and hydrothermal activity now would seem to be coeval with the laying-down of the Central Rand clastics, which have acted as host to 97% of the gold and uranium won from the Witwatersrand Bain. A new model of Witwatersrand mineralisation envisages a rapid succession of: (a) emplacement of primary, epigenetic gold and uranium at about 2800 Ma by granites and hydrothemal activity in the magmatic arc and bordering terrane; (b) uplifting of such a provenance by thrusting and granite diapirism; (c) degradation and erosion of dominantly siliceous material; (d) fluvial transport of erosional debris and detrital gold and uranium; (e) deposition of placer heavy minerals in conglomerates and other horizons on 2900-2750 Ma-old braid-plains and -deltas; and (0 reconstitution of detrital gold and uranium, sulphides, and matrix material by metamorphic fluids emanating from contemporary and subsequent events in the magmatic arc. To the placer component of the total amount of gold and uranium present in the reefs must be added contributions from hydrothermal solutions related to the 2800 Ma-old granites and, possibly, from surface waters, containing dissolved metals, which transported the detrial heavy minerals, as well as the sand and silt from which the Central Rand sediment swere formed. It is now postulated that all three of the longstanding theories for the genesis of Witwatersrand mineralisation — modified placer, hydrothermal, and precipitation — should be integrated in order to offer a generally acceptable explanation of how the gold and uranium were introduced into an Archean back-arc basin during the closing stages of its sedimentary infilling.

307 TIN GRANITES: THEIR EVOLUTION FROM FERTILE SEDIMENTS BY PARTIAL MELTING AND FRACTIONAL CRYSTALLISATION B.W. Chappell1 and A.J.R. White2 1

Department of Geology, Australian National University 2 Department of Geology, La Trobe University

Primary tin mineralisation is so intimately associated with granites that a genetic relationship is not in doubt All rich and economically important tin deposits occur within granites or close to their contacts, and at or near the roof zones of the granite plutons. Apart from being felsic or very felsic, those granites associated with tin deposits are indistinguishable in the field from barren granites. Hence many criteria, chiefly geochemical, have been suggested to characterise "tin granites". These criteria are somewhat empirical and have not been accepted by the exploration industry, presumably because they are unreliable and in some cases too complicated. With others, we have been involved in detailed studies of the geochemistry of granites from two major areas of tin production, Cornwall (Chappell & Hine, 1990) and southeast Asia (Liew, 1983). In addition, in our regional studies of the granites of the Lachlan Fold Belt (LFB), we have found in the Wagga Basement Terrane (WBT), a complete sequence of granites whose compositions can be traced progressively from very mafic S-type granites, through all intermediate stages to highly fractionated rocks with compositions including some that match the Cornubian granites very closely. We have thus been able to trace and study the complete evolution of very felsic tin granites through from their mafic precursors. The Cornubian Batholith (CNB) comprises six major bodies of dominantly two-mica granite in southwestern England. The granites have a close spatial association with the rich mineralisation of the Cornubian Peninsula, which has yielded 2,500,000 tonnes of Sn and significant amounts of other elements such as Cu and W (Hawkes, 1984). Isotopic data for Sr, Nd, Pb and O all support an origin of the granites dominantly or exclusively from metasedimentary source rocks. The two-mica granites of the CNB are felsic, strongly peraluminous, and have a high total alkali content and a distinctive low Na:K ratio. Minor elements that are relatively very abundant are Li, B, Cs and U, while Rb, Ga and Sn are quite high. A comparison with experimental data shows that the rocks attained their major element composition under conditions of crystal-liquid equilibrium at magmatic temperatures and at low pressures (-50 MPa). The tight cluster of these granites when their compositions are projected onto the Q-ab-or plane argues against any significant role for late-stage movement of Na and K during the formation of these rocks. In contrast, most workers on the CNB have placed much importance on petrographic observations of alteration of the primary feldspars and biotite, and have as a result ascribed an

important role to post-magmatic alteration resulting from the movement of fluids. In our view, the chemical evidence indicates very limited subsolidus change in bulk composition. We do not dispute the petrographic evidence for the movement and redistribution of the chemical compositions, but such a process must have been effectively restricted to a scale less than the size of the samples used for chemical analyses. The Cornubian granites can therefore be seen as an excellent example of the resilience of the primary chemical composition of granites to the many mineralogical changes that can accompany slow cooling. It has also been suggested (e.g. Charoy, 1986) that some trace elements were introduced at a late stage and were redistributed through widespread circulation of postmagmatic fluids. However, there are good correlations between the abundances of these and other elements (except for B, Cu and U), which implies a mineralogical control through fractional crystallisation. Peraluminous granites are abundant in southeast Asia, where they are also related to very substantial mineralisation. Liew (1983) studied the geochemistry of granites from two parts of the Main Range Province of peninsular Malaysia. His data show many similarties with the Cornubian granites, but there are some significant differences, such as higher Ca, lower A1 and much less normative C. Trace element differences include Li, B, Rb and Cs which are less abundant in the Malaysian rocks, and Sr, Ba, Pb and Th which are present at higher levels. Sn contants are very similar with average values of 16 ppm in both areas. The Malaysian compositions are again those of granite liquids produced by fractional crystallisation, despite the differences in compositions with the CNB. Chappell et al. (1988) subdivided the LFB into nine basement terranes, in each of which the granites have a distinctive character. S-type granites occur in five of these terranes, and in the most easterly of these, the Kosciusko Basement Terrane (KBT), the variation in composition of the S-type granites has been ascribed to varying degrees of separation of solid material residual from partial melting, or restite, from a felsic melt, with the melt itself mot having undergone any significant changes in composition by fractional crystallisation (Chappell et al., 1987; White & Chappell, 1988). In the Melbourne Basement Terrane there has been very limited fractionation of the felsic melts (White & Chappell, 1988). In the other three terrances, Wagga, Bassian and Taswegia, the granites include fractionated types similar in composition to those of the CNB. in the WBT, extending from central New South Wales into northeast

308

Victoria, the S-type granites show a continuous range in composition, from mafic rocks containing abundant cordierite and sometimes almandine, through to very felsic two-mica granites, so that the complete process of fractionation can be studied. The most mafic S-type granites of the WBT are similar in composition to those of the KBT and the compositional trends are similar at those compositions as variation in both cases is dominated by the separation of restite from melt. However, in the more felsic rocks, the contents of many elements diverge due to fractional crystallisation in the WBT. As a result, the concentration of elements such as P, F, Li, Rb, Cs, Nb, Ga and Sn rise to much higher levels, while Mg, Ca, Sr, Ba, Zr, Y, REE, and the trace transition metals fall to lower levels than are ever found in S-type granites of the KBT. Such compositions have many similarities with the Cornubian granites, and some samples from the WBT are extremely close to those of the CNB in composition. The Ardlethan Granite within the WBT is associated with a major Sn deposit and is the most fractionated granite in the LFB. Analysed samples have compositions generally close to the thermal minimum in the Q-ab-or system, with evidence for slight post-magmatic alteration. The rocks are extremely felsic, and the trace element features indicate prolonged feldspar fractionation. There is a compositional gap between these and the other granites of the WBT but they probably represent a more advanced stage of fractionation from those compositions. The Sandy Cape Granite is a small body of strongly peraluminous felsic granite outcropping on the western coast of Tasmania (Wyborn et al., 1990). It is of particular interest because it exhibits compositional changes that result from the extreme fractionation of a felsic granite melt. The least felsic granite in this unit has a composition remarkably close to that of the average granite of the CNB and the more felsic granites of the WBT. It carries those already felsic compositions to

even more evolved levels, and shows in an unequivocal way the nature of the chemical changes that occur as a result of extended crystal fractionation from a strongly peraluminous liquid (3.0 to 4.0% normative Q . Analysed samples of this granite show a Si0 2 range from 72.2% to 74.5% and with fractionation the following changes in composition are observed: Rb (from 424 to 950 ppm), Sr (57 15), Ba (170 -» 2), Nb (27 -> 47), Sn (21 46) and W (12 -> 25). S-type granites are derived from fertile sediments that contain enough of the components Q, ab, or and H p to produce a magma (White & Chappell, 1988). Initially, at least, this magma will be rich in restite and the removal of that solid material must dominate the early stages of fractionation, as in the KBT and the more mafic granites of the WBT. More felsic granites result from the fractional crystallisation of that melt and, using simple geochemical criteria, tin granites can be shown toresultfromthe prolonged operation of that mechanism. Apart from a fertile character, specialised or unusual source rocks are not needed. Hence, all of the chemical features of the tin granites can be explained on the basis of partial melting of rocks such as feldspathic greywackes in the crust, followed by restite removal and fractional crystallisation. References Chappell, B.W. & Hine, R., 1990. In prep. Chappell, B.W., White, A.J.R. & Hine, R., 1988. Aust. J. Earth Sci. 35: 505-521. Chappell, B.W., White, A.J.R. & Wyborn, D., 1987. Petrol 28: 1111-1138. Charoy, B., 1986. J. Petrol 27: 571-604. Hawkes, J.R., 1984. In Xu Keqin & Tu Guangchi (eds): Geology of Granites and their Metallogenic Relations. Science Press, Beijing: 571-593. Liew, T.C., 1983. Unpubl. PhD thesis. Australian National University. White, A.J.R. & Chappell, B.W., 1988. Trans. R. Soc. Edin. Earth Sci. 79: 169-181. Wyborn, D., Chappell, B.W. & Sawka, W.N., 1990. In prep.

PORPHYRY COPPER DEPOSITS G.H. Brimhall* and K. Danti Department of Geology and Geophysics, University of California, Berkeley, California, USA

Briefly, porphyry copper deposits are large, lowgradefracture-controlledveinlet networks resulting from aqueous fluid/wall rock interaction within immense convective hydrothermal systems developed in and around granitoid intrusions. Parent intrusives of Mesozoic and younger ages are emplaced at relatively shallow crustal depths within active orogenic areas such as the pacific Rim characterised by deformation, faulting and rapid uplift (Titley & Beane, 1981). Older belts of porphyry copper deposits, such as the Tasman, Tethyan, and Caledonian are related to porphyritic intrusives as old as Palaeozoic. Even Precambrian examples are

known, making the apparent temporal distribution of these deposits extensive but sporadic, perhaps because of their occurrence within the upper levels of orogenic zones where they have been subject to rapid erosion and may hence be incompletely preserved in the geological record. Sulphide mineralisation is of two types: spatiallyzoned primary sulphide veins, veinlets and dissemination involving one or more of the minerals chalcopyrite, bomite, enargite, tennantite, pyrite, pyrrhotite, magnetite. Silicate alteration patterns include early potassic assemblages with hydrothermal alkali feldspar and biotite formed largely from metasomatic reactions

309

involving magmatic water and later phyllic and argillic assemblages of sericite, quartz and clays superimposed upon the early alteration by collapse of the hydrothermal convection system during retreat of the thermal anomaly (Norton & Knight, 1977) with incursion of meteoric water (Taylor, 1974). Of critical importance to the profitable mining of these deposits is the fact that upon erosion and exposure of primary sulphides ores to oxidative weathering, leaching of copper above the ground water table occurs, and with downward ground water flow, remobilises copper with redeposition below under water-saturated reducing conditions in a highly enriched form (Locke, 1926) of a chalcocite enrichment blanket which is sub-parallel to the present surface topography. The economic and societal importance that these widespread deposits have had a major and lasting impact on mineral exploration research as well as on the technology of open pit and high capacity underground mining and extractive metallurgy. Consequently, the scientific and engineering literature is immense and can only be highlighted here in the references. Several genetic models have been widely used with much success in exploration (Lowell & Guilbert, 1970; Gustafson & Hunt, 1975; Hollister, 1978). Global and regional tectonic associations were also recognised (Sillitoe, 1972; Dickinson & Payne, 1981) and put to much use in identifying analogous regions at destructuve plate margins with present or past subduction such as the southwestern Pacific islands and Australia (Gustafson & Titley, 1978) where tectonic patterns indicated possible ore-forming environments. Recent summaries of major advances in the last five to ten years in understanding porphyry copper deposits (Titley & Beane, 1981; Titley, 1981) include the history of many geochemical advances: experimental phase equilibria relevant to alteration processes (Hemley et al., 1969, 1980), remobilisation of copper from the potassic assemblage protores into late veins by hypogene leaching (Brimhall, 1983), and distinction of porphyry copper deposits on the basis of associated molybdenum or gold (Kelser, 1973) depending on magma generation from lithophile-rich continental crust or more primitive oceanic crust. Distinction of magma types into Magnetite- versus Ilmenite-type (Ishihara, 1977) and Iversus S-types (Chappell & White, 1974) helped illuminate the redox and trace metal histories of magmas. Reconstruction and interpretation of the intrusive environment including the existence of a volcanic edifice (Sillitoe, 1972; Lipman, 1981) provided insight into the nature of regional faulting and widespread fluid circulation from which associated epithermal deposits may form. Practical application of sulphide zoning has been made in targeting drill holes using relict sulphide patterns (Hunt, 1980). Since understanding of porphyry copper genesis is already in an advanced stage of development, stimulating research frontiers exist and present challenging possibilities for expanding their utility to the earth

sciences community at large and technology transfer to exploration for less-well studied or more complex hydrochemical systems, such as laterite-covered deeply weathered terrains. Of great utility is the realisation that porphyry copper deposits are geochemically very reactive sulphide hydrochemical systems. As such, they can be viewed as natural petrochemical sensors for surficial conditions of the past Because of this response to imposed conditions and its record in the leached capping, porphyry copper deposits offer many advantages over normal rocks. Copper which behaves as a conservative trace element upon redox remobilisation, can be used in mass balance analysis and used to reconstruct the surficial conditions at the start of oxidative weathering (Brimhall et al., 1985). From these results, the amount of erosion can be calciulated, and with dating of supergene alunite, rates of erosion emerge (Alpers & Brimhall, 1988). These techniques provide a basis for recognising the direction and stage of palaeohydrological evolution during climatic change (Alpers & Brimhall, 1989). Trends towards increasing climatic aridity optimise supergene enrichment and because of consequent decreasing erosion rates, guarantee preservation of the most highlyenriched supergene systems in the geological record. Kinetics and ground water flow in supergene hydrochemical processes can be simulated to firstprinciple numerical computer calculations (Ague & Brimhall, 1989). Recent hydrothermal experiments of chalcocite replacement of pyrite indicate that it is possible to extrapolate results to lower temperatures in order to infer time intervals necessary for supergene enrichment and response time to climatic evolution from copper diffusion rates into pyrite. Applications to exploration include improving understanding of the fate of invisible gold in pyrite during its oxidation, coarsening of gold by transport and nugget formation mechanisms, both having utility in laterite gold investigations. Regional petrology of granitic batholiths can reveal the depth of exposure and geochemical character of associated plutons (Ague & Brimhall, 1988a,b; Brimhall & Ague, 1989), offering insights into the most likely magmatic hydrothermal system. References Ague, J.J. & Brimhall, G.H., 1988a. Geol. Soc. Am. 100: 891-911. Ague, J.J. & Brimhall, G.H., 1988b. Geol. Soc. Am. 100: 912-927. Alpers, C.N. & BBrimhall, G.H., 1988. Econ. Geol. 100: 1640-1656. Alpers, C.N. & Brimhall, G.H., 1989. Econ. Geol. 84:229-255. Ague, J.J. & Brimhall, G.H., 1989. Econ. Geol. 84:506-528. Brimhall, G.H., Alpers, C.N. & Cunningham, A.B., 1985. Econ. Geol. 80: 1227-1256. Brimhall, G.H. & Ague, J.J., in press. In Barnes, H.L. & Ohmoto, H. (Eds): Granitic Systems. Reidel Publ. Brimhall, G.H. & Ghiorso, M.S., 1983. Econ. Geol. 78:73-90. Chappell, B.W. & White, A.J.R., 1974. Pac. Geol. 8:173-174. Dickinson, W.R. & Payne, W.D., 1981. Ariz. Geol Soc. Digest XIV: 288 pp.

310 Gustafson, L.B. & Hunt, J.P., 1975. Econ. Geol., 70:857-912. Gustafson, L.B. & Titley, S.R. (eds), 1978. Econ. Geol. Spec. Issue 73: 597-986. Hemley, J.J., Hostetler, P.B., Gude, A.J. & Mountjoy, W.T., 1969. Econ. Geol. 64: 599-612. Hemley, J.J., Montpya, J.W., Marinenko, J.W. & Luce, R.W., 1980. Econ. Geol. 75: 210-228. Hollister, V.F., 1978. Geology of the Porphyry Copper Deposits of the Western Hemisphere. NY Soc. Min. Eng. AIME: 219 pp. Hunt, J.P., 1980. 50th Cong, of Mining Porphyry Copper Deposits. Inst. Min. Eng., Santiago, Chile. Ishihara, S., 1977. Mining Geol. 27: 293-305.

Kesler, S.E., 1973. Econ. Geol. 68: 106-112. Lipman, P.W., 1981. Ariz. Geol. Soc. Digest XIV: 199-214. Locke, A., 1926. Leached Outcrops as Guides to Ore. Williams Wilkins Co., Baltimore: 166 pp. Lowell, J.D. & Guilbert, J.M., 1970. Econ. Geol. 65:373-408. Norton, D. & Knight, J.E., 1977. Am. J. Sci. 277: 937-981. Taylor, H.P., 1974. Econ. Geol. 69: 843-883. Sillitoe, R.H., 1972. Econ. Geol. 67: 184-197. Titley, S.R., 1981. Advances in the Geology of Porphyry Copper Deposits of Southwestern North America. Univ. Ariz. Press, Tucson: 93-116. Titley, S.R. & Beane, R.E., 1981. Econ. Geol., 75th Anniv. Vol.: 214-269.

UNCONVENTIONAL THINKING AND EARTH SCIENCE R.W. Hutchinson Colorado School of Mines, Golden, Colorado, USA

The natural sciences are intrinsically inexact; less amenable to quantification than the physical sciences, and the earth sciences perhaps, are the least so of all. Whereas all natural sciences deal fundamentally with the parameters that are only partially or imprecisely measurable, the earth sciences are vitally concerned with time as a fourth dimension and the changes that have accompanied its passage through four billion years of earth's history. These conditions demand particular attention by earth scientists to scientific method, attention that is too often overlooked. First, unique answers, exact and comprehensive explanations, particularly for genetic problems are extremely complex. Too many inter-dependent and qualitative factors that vary alike in the three spatial and fourth temporal dimensions render them elusive at best, impossible at worst, but always interpretative. Inherently therefore they can be only partly correct. Nevertheless and rightly, earth scientists seek definitive solutions. The danger lies not in doing so, but in an inability or unwillingness to recognise and accept their inevitable failings. The arduously sought solutions may be advanced and ultimately accepted, not as hypotheses to be questioned, challenged, tested, but as proven. They become conventional. Research may become introverted and seek to demonstrate their validity, unversality and applicability to comparable, albeit new examples, rather than to question the extent of their validity and challenge their basic assumptions in seeking to improve, or if necessary, replace them. Second, because the causal relationships in earth science are so numerous, complex in their interdependency and variable in four dimensions it is important to question critically the fundamental significance of data. Are they definitive of one unique set of circumstances, or could they be explained by two, more, or some combination thereof? Definitive evidence

is rare in earth science. Commonly, data sets may be explained adequately by multiple hypotheses. The best of these must then be preferred — that which explains most leaving least unexplained, but without totally abandoning the others, while seeking further evidence. Disagreement over which is the best hypothesis is inherent, forces reconsideration thereby elucidating all, hence must not be stifled in favour of one. Multiple hypotheses are thus essential, conventionalism must be avoided and an unwillingness to accept the favoured or established view as uniquely and totally correct is imperative. Alternatives must be retained, reconsidered and all possibilities re-evaluated in the light of new evidence. None of this is new, only simple adherence to scientific method which is vital to all science. The enduring needs are to critically evaluate the fundamental significance of data, to question the basic assumptions, to challenge interpretive hypotheses and not to accept, totally or blindly, the conventional view. Truly fundamental advances in sciences depend on development of new, basic insights. These are sometimes surprisingly simple and arise mainly not from agreement amongst scientists and acquiescence with accepted views, but rather from disagreement and challenging them. Wegener's hypothesis provides an example. But conventional thinking too often comes at a high price. New ideas, like Wegener's, have been less than welcome in earth science. Challenging hypothesis are sometimes taken not as challenges in search of scientific advance, but as troublesome challenges in orthodoxy, to the accepted view directed personally against those who advocate them. The ultimate argument in defense of conventional thinking is numerical; that because the challenging alternative is unpopular, perhaps advocated by one alone or only a few, it is incorrect and worse, unacceptable. This complete abandonment of scientific

311 method constitutes an ultimate disservice to science itself and a sad injustice to proponents of the challenge. Unconventional thinking then, is an integral part of scientific method and remains vital in all science. The scientist we honour at this symposium, R.L. Stanton, is an unconventional thinker. He was amongst the earliest to recognise from his work in the New England fold belt of New South Wales, and from it to offer the new insight that certain massive base metal sulphide deposits might be of synsedimentary-volcanogenic origin. The hypothesis was widely denounced by those entrenched in the conventional epigenetic replacement theory for a decade until it was finally substantiated by direct sea floor observation of the process in action. His recent advocacy of the importance of intensive early

hydrothermal metasomatism in rocks near these deposits during subsequent regional metamorphism offers a challenging new insight for metamorphic petrology. When integrated with this field it promises new understanding of both metamorphic rocks and ore deposits. Our science is the attempt to explain, as accurately and completely as possible, the phenomena that have affected the earth as we know it It is immensely broad, embodying many of its own specialisations as well as those of the other sciences. It must therefore be ever open to questioning and re-evaluation to accommodate new information and concepts. Conventional thinking serves this purpose inadequately. An unconventional approach is essential.

EPITHERMAL DEPOSITS: STYLES, ANALOGUES AND EXPLORATION R.W. Henley Consultant Geologist, 4 Juad Place, Aranda, ACT

Following epithermal discoveries such as McLaughlin (California) and the growth of understanding of the relation between gold deposits and high level magnatism based on studies of active geothermal fields recent epothermal exploration has focused on young volcanic terranes. This has resulted in a number of major new discoveries (e.g. Lihir Island, Papua New Guinea, Hishikari, Japan and Sleeper, Nevada), as well as progressive exploration of earlier developed prospects (e.g. Kelian, Indonesia and Porgera, Papua New Guinea) and redevelopment of epithermal deposits mined earlier this century (e.g. Waihi and Golden Cross, New Zealand). Discoveries of epithermal gold deposits have also been made through systematic exploration of older volcanic terranes (e.g. Pajingo and Wirralie). Epithermal precious metal deposits form in the upper few hundred metres of large hydrothermal convection systems in volcanic terrane. Most commonly vein and disseminated deposits are hosted by rocks altered to sericite/clay-K-feldspar assemblages. Gold deposition is frequently the result of boiling and is accompanied by abundant silica and adularia. A second common association is between gold-copper (often as enargite) ores and zones of high temperature advanced argillic alteration. What differs between epithermal deposit styles relates largely to the relative depth of the degassing parent magma. Thus the alunite-kaolinite style deposits are initiated by high level magmas (e.g. rhyolite domes) with mineralisation developed by later hydrothermal activity driven by the larger, deeper magma system. Adularia-sericite style systems relate to deeper magma

bodies (4-8 km) degassing into and dispersed by an overlying convecting groundwater system. In high permeability systems dispersion may be so strong as to prevent the formation of an ore deposit even though a billion grams of gold may be disseminated at low grade in the upper few hundred metres of the system. In lower permeability host rocks, major structures control groundwater flow and focus fluids to high level deposition sites (e.g. Hishikari). In between are stratabound styles where mineralisation has been constrained by the intrinsic competence or permeability of specific host rocks. Much recent attention has been given to establishing the role of magmas in the formation of epithermal deposits. Stable isotope techniques have seldom been definitive but the growing body of field evidence from active geothermal areas and from active volcanoes is unequivocal; high level magmas are the source of gold and in most cases the sulphur required for gold transport These data also emphasise the continuum through to the deeper subvolcanic porphyry gold style deposits (e.g. Bougainville). Other metals such as lead and zinc may be host-rock derived but their transport dependent on the salinity of the groundwater system. In regional exploration the recognition of the structural environment of epithermal mineral formation may be as, if not more important than the determination of chemical parameters. Modern airborne survey techniques are particularly valuable in the recognition of characteristic signatures and structural plays at an early stage and in guiding follow-up geological or geochemical programs.

312

ARCHAEAN GOLD DEPOSITS David I. Groves Key Centre for Strategic Mineral Deposits, University of Western Australia

Woodall (1988) emphasized the bimodal temporal with high Fe/(Fe + Mg) ratios (e.g. basalt, dolerite, BIF) distribution of gold mineralisation, with the late- at mid- to upper-greenschist facies in structures of the Archaean Witwatersrand and greenstone-hosted lode brittle-ductile transition: minor felsic intrusions are deposits dominating production from Precambrian important host rocks in Canada and Zimbabwe. Gold terranes, and Mesozoic to Quaternary porphyry- to mineralization, with or without quartz veins, is associated epithermal-style deposits and associated placers with laterally zoned, wallrock alteration haloes that providing the majority of younger gold. Lode gold involved volume increase due to metasomatic addition deposits are, in fact, one of the most characteristic of S i0 + K p + C0 + H p + S + Au ± Na ± Ca: alterfeatures of most major Archaean cratonic areas (e.g. ation assemblages vary with the inferred depth of Australia, Brazil, Canada, India, South Africa, formation and the metamorphic grade of host rocks. Zimbabwe). These deposits, which range in production The deposits are normally "gold-only", the Au being from <11 Au to >10001 Au (e.g. Porcupine, Timmins; associated with As, Ag, W Sb, Te and B, and variable, Golden Mile, Kalgoorlie) at grades between 2 to 50 g but commonly low, concentrations of Cu, Pb, Zn and Au/t, have accounted for a high proportion of world Mo. production, and contain a significant proportion of Archaean granitoid-greenstone terranes show a known world gold resources. characteristic heterogeneous distribution of gold mineralisation, with the best mineralised terranes being Prior to the 1960s, an epigenetic model that stressed the structural control of gold mineralisation was in those with a high density of anastomosing, strike-, vogue (see Boyle, 1979, for summary): granitic reverse- or oblique-slip transcraton shear zones. Such intrusions were viewed as the heat and/or fluid source. zones are thought to have focused fluid flow in However, the successful application of syngenetic greenstone belts that otherwise exhibit low strain on a models to several ore-deposit classes (e.g. VMS deposits) regional scale. There is commonly a conjunction of previously considered to be epigenetic replacement these structures, regional carbonation, emplacement of deposits (see Stanton, 1972, for an excellent review of swarms of minor intrusions comprising intermediate to his and other researchers' contributions), led in the felsic porphyries and lamprophyres (at least at high 1970s to syngenetic models for BIF-hosted gold deposits structural levels), and gold mineralisation. Available and lateral-secretion ("remobilization") models for lode- timing constraints suggest that mineralisation in type deposits. There were proponents of epigenetic greenschist facies terranes occurred late in greenstonemodels at this time (notably R. Kerrich and co-workers), belt deformation and metamorphism, just prior to but it was not until the 1980s that the importance of cratonisation, but, enigmatically, most deposits in such models was generally re-established (e.g. papers amphibolite facies terranes appear broadly synchronous in Foster, 1984; Macdonald, 1986; Ho & Groves, 1987; with peak metamorphism. Absolute age determinations Colvine et al., 1988). Now, there is almost universal in Canada and Western Australia show that gold mineralacceptance of an epigenetic model for all but a few isation significantly postdated greenstone-belt controversial BIF-hosted deposits, but the information volcanism, and emplacement of most minor felsic explosion triggered by the 1980s gold "boom" has in intrusions, and was generally post-2.7 Ga. turn raised heated debate on the source of hydrothermal Gold mineralisation is interpreted to have resulted fluids and ore components, the mechanisms of fluid from highly focused flow of low salinity, H 0-C0 infiltration and gold deposition, and the timing and fluids along ductile transcraton shear zones with transient tectonic setting of gold mineralisation. Most of the fluid flow into adjacent brittle-ductile structures. research data are from gold deposits in greenschist Extensional structures focused fluid flow as a result of facies terranes, as these are the dominant group, and the lowering pressure, whereas compressional structures summary below, based on the last 5-10 years research, are mineralized due to rock failure caused by high fluid reflects this bias. pressures. Reactivation of pre-existing faults was The Archaean gold deposits are consistently sited in important (e.g. Sibson et al., 1988), as was the presence greenstone belts or immediately adjacent granitoids in of anisotropics, such as contacts between lithologies second- or third-order structures related to transcraton with different rock strengths or massive granitoid bodies shear zones. They display a wide range of structural in layered sequences. Gold precipitation, normally at styles, varying from shear zones to extensional veins 300 ± 50°C and 1 to 3 kbar in greenschist facies domains, and breccias, and ore shoots show a corresponding but under higher P-T conditions in amphibolite facies range of structural controls. Although mineralisation domains, commonly occurred as a result of wallrock occurs in a variety of host rocks at variable metamorphic sulphidation or phase separation within thefluids.Stable grade, many of the large to giant deposits occur in rocks isotope data support either metamorphic or magmatic 2

313

fluid sources, but available data appear to exclude in Kambalda area of Western Australia). Exploration significant meteoric water influx. Limited radiogenic successes similar to those generated by definition of isotope data suggest that fluids interacted with lower better exploration models, via superior genetic models, granitic crust or intrusions derived from it. Collectively, such as those developed for VMS and SEDEX deposits these data, combined with timing constraints, appear to by R.L. Stanton and other researchers, will be required exclude fluids derived exclusively by exsolution from in the future. Innovative research into such aspects as: exposed felsic intrusions or from metamorphic i) the exact nature of structural controls of deposits in devolatilisation of greenstone lithologies. anisotropic terranes, ii) precise depositional mechanisms Evidence is accumulating that the late-Archaean and their recognition via geochemical or isotopic gold deposits may be equivalents of the mesothermal signatures, iii) the penological, geochemical and deposits found in Mesozoic to Quaternary settings geophysical signatures of wallrock alteration envelopes around the Pacific Rim; that is, they may be related to at different metamorphic grades, iv) the importance or closure of ca. 2.7 Ga volcanic arc/marginal basin otherwise of commonly associated minor felsic to complexes in convergent margin settings: this potentially lamprophyric intrusions, and v) the precise timing of explains the bimodal temporal distribution of gold the mineralisation event or events, are all future deposits. If so, mineralization is best viewed as the challenges. Interactive computer databases and result of high lithospheric fluid and heat flux, with Geographic Information Systems are likely to allow fluids and ore components being added from a variety the development of more sophisticated predictive models of lower crustal (or even mantle) sources and being for the location of "blind" Archaean gold deposits, modified by upper crustal pathways. Given such models, given the quantity and quality of research data being it is critical that more attention is given to the gold collected on the deposits. deposits in amphibolite to granulite-facies terranes, as References these should be deposited from less-modified Boyle, R.W., 1979. Geol. Surv. Can. Bull. 280: 584. hydrothermal fluids that better reflect their source than Colvine, A.C. et al., 1988. Ontario Geol. Surv. Open File those from the better studied depositsfromhigher crustal Rept. 5524: 98. levels. It is also vital to better constrain the tectonic Foster, R.P. (ed.), 1984. Gold'82: The Geology, Geochemistry and Genesis ofGold Deposits. A. A. Balkema, Rotterdam: setting and history of the granitoid-greenstone terranes, 753 pp. such that the gold deposits can be better placed in a Ho, S.E. & Groves, D.I., 1987. Geol. Dep. &. University regional framework. Exptension, Univ. West. Aust. 11: 368 pp. Most currently mined deposits have been discovered Macdonald, A.J. (ed.), 1986. Gold '86: An International by re-assessment (normally by drilling) of previously Symposium on the Geology ofGold Deposits. Konsult Int. known mineralization, by surface or near-surface Inc., Ontario: 517 pp. geochemistry, or by geophysical techniques. However, Sibson, R.H., Robert, F. & Poulsen, K.H., 1988. Geology 16: 551-555. genetic- and deposit-models for Archaean gold deposits will become increasingly important for development of Stanton, R.L., 1972. Ore Petrology. McGraw-Hill Book Co., New York: 713 pp. exploration philosophy and exploration models as near- Woodall, R., 1988. Geol.Soc.Austr. Inc. Abstracts 22:1-12. surface deposits are exhausted: such models have already been successful in limited areas (e.g. WMC exploration SEDIMENT-HOSTED STRATIFORM LEAD-ZINC DEPOSITS Neil Williams Carpentaria Exploration Company Pty Ltd, Brisbane

Sediment-hosted stratiform lead-zinc deposits are a major source of the world's lead and zinc. Deposits which were, are, or may become economically important include Mount Isa (lead-zinc), Hilton, and HYC in Australia; Sullivan and Howards Pass in Canada; Red Dog in Alaska; Rammelsburg and Meggen in West Germany; and Tara, Tynagh and Silvermines in Ireland. Features common to these deposits are their association with sediments, their tabular morphology, their concordant orientation and their simple sulphide mineralogy dominated by sphalerite, galena and either pyrite or pyrrhotite. Beyond these similarities there are

fundamental differences between deposits. These are greatest between examples from different geological provinces and include diferences in the depositional environment of the host sediments, structural setting, styles of sulphide textures, metal ratios, stable isotope geochemistry, association with discordant mineralisation, and association with volcanic rocks. Given the diversity among deposits, it is not surprising that many important aspects of their formation remain unclear. Whereas the genesis of many ore-types involves variation only on one or two themes, it seems likely that the genesis of sediment-hosted stratiform

314

lead-zinc deposits involves variations on many different and spread through the unconsolidated sediments themes related to the evolution of the sedimentary basins beneath the HYC lake or lagoon. The uppermost fluid, in which they occur. The elucidation of these themes is which sits beneath sulphate- and nutrient-rich surface difficult because the mineralisation is generally fine waters, is a reduced fluid rich in iron and sulphur. It grained and texturally complex and quite unsuitable for deposits iron monosulphides in the sediments. The the kinds of studies, such as fluid inclusion work, that deeper and denser fluid isrichin lead and zinc, but poor have proved so helpful in the case of other ore types. in reduced sulphur. When previously formed iron To overcome these problems, genetic studies must monosulphides encounter this fluid they are dissolved be focussed on the best preserved deposits and must and galena and sphalerite are precipitated. Ferrous iron utilise as many approaches as possible to constrain released by these reactions moves upwards and is either interpretations. Among the giant Carpentarian-age reprecipitated as iron monosulphides in the domain of deposits of northern Australia, the HYC deposit is the upper hydrothermal fluid or as pyrite in the surface particularly well suited to such research. Recent water domain by way of microbial sulphate reduction. approaches that have proved particularly useful include As sedimentation continues, pyrite is buried but behaves sedimentological studies, in situ sulphur isotope inertly in the domains of both hydrothermal fluids. analyses, detailed mineragraphic studies, and metal ratio The most dramatic illustration of the model is studies. The research has demonstrated that the provided by 2 orebody, the lowest ore zone in the HYC mineralised sediments were deposited in a saline lake deposit, and by the overlying 2/3 interore zone. Whereas or lagoon, rather than in a deep marine environment as 2 orebody is unusually enriched in lead and zinc and previously assumed. Isotopically, all the pyrite in the depleted in iron relative to the other orebodies in the deposit is quite different from galena and sphalerite. deposit, the overlying poorly mineralised but pyriteThe pyrite has a variable biogenic sulphur isotope rich 2/3 beds contain just the right amount of iron to character, whereas the galena and sphalerite have a give the combined 2 and 2/3 intervals an overall (Pb+Zn)/ more constant hydrothermal character. There are also Fe ratio of unity like the rest of the deposit. Because the textural differences between pyrite and the galena and 2/3 beds are composed predominantly of sabkha-facies sphalerite. Pyrite tends to occur as well-defined laminae nodular dolomite, this remarkable separation of iron whereas galena and sphalerite are more diffuse in their from lead and zinc is attributed to the lower than normal distribution. However despite these differences, the ratio levels of the water table at the time that would have of Pb+Zn to Fe throughout the deposit indicates that greatly depressed the levels of the underlying pyrite is in some way related to galena and sphalerite hydrothermal fluids relative to the sediment surface. because it is remarkably constant and approaches unity The applicability of the permeation model to other on an atomic basis. sediment-hosted stratiform lead-zinc deposits cannot These findings place important new constraints on be determined at this stage because critical data are ore formation. They cannot be easily explained by a lacking in most instances. The most serious deficiencies simple exhalative model and a new genetic model is concern details of the iron geochemistry of deposits, the herein proposed to account for thefindings.It is termed in situ sulphur isotope geochemistry of deposits, and the permeation model because it involves two distinct the depositional environments of the sediments hosting and spatially separated mineralising fluids that pass into the deposits. MASSIVE SULPHIDE DEPOSITS OF THE MODERN OCEAN FLOOR Steven D. Scott University of Toronto

Japanese geologists recognised in the early part of this century that their Kuroko volcanogenic massive sulphide deposits (msd) had formed by hotspring activity on the seafloor. However, it wasn't until mid-century that the concept independently took hold in the west. An early proponent of the volcanogenic theory for msd was a young Richard L. Stanton who, at a 1959 symposium on The Genesis of Massive Sulphide Deposits" organised by J.E. Gill in Montreal for the Canadian Institute of Mining and Metallurgy, extended his earlier ideas from studies in Australia and made what, at the time, was an almost heretical proposition that msd of the Bathurst, New Brunswick, district in Canada were "simply sulphide 4

metamorphic rocks" that had volcanic-exhalative and biological origins. The correctness of this assertion can be seen from modern studies of massive sulphide deformationfromhydrothermal systems on the presentday ocean floor. The modern ocean floor is an exceptional natural laboratory for studying processes and environments responsible for the formation of ancient massive zinccopper-lead-silver-gold sulphide ores that are now being mined on land. Twenty-five years of sea bottom exploration, primarily in the Pacific during the past ten years, has resulted in the discovery of >100 sites of former or present hydrothermal activity. Several of these

315

sites contain sulphide deposits that are within the size range of those being mined on land. Seafloor msd are now recognised on sedimented (Middle Valley, Escanaba Trough) and sediment-starved mid-ocean ridges (Galapagos at 86°W, East Pacific Rise, Juan de Fuca Ridge, Explorer Ridge, TAG, Central Indian ridge) that are both fast and slow spreading, seamounts, (Axial, 13°NEPR), island arcs (Ogasawara), back-arcs (Okinawa Trough, Mariana, Lau basin), marginal basins (N Fiji Basin), and rifted continental margins in both early (Western Woodlark basin) and late (Guaymas Basin, Red Sea) stages of development. Collectively, these span the recognisable tectonic settings of ancient msd as well. Sulphide deposition in the modern oceanic environment is the result of complex inter-relationships among tectonics, volcanism, sedimentation and heat flow, processes that must also be important for ancient ores. Although most modern seafloor analogues of basalthosted msd are small, some are in the range of several million tons (Galapagos at 86°W, seamount at 13°N EPR, Explorer Ridge). These deposits formed during a hiatus in volcanism as evidenced by the relative age of the basalts. Such hiatuses are commonly marked around ancient volcanogenic msd by distinctive iron and silicarich cherty sediments on the ore horizons such as the ochres of Cyprus or the " tetsusekiei" ofJapanese Kuroko deposits. Similar chemical sediments on the modern ocean floor have a dominant biogenic component and may be produced by fallout from hydrothermal plumes or by surficial oxidation of sulphides exposed to ambient seawater. The manner in which the larger modern deposits and, especially, ancient massive sulphide orebodies have formed on the seafloor is puzzling in the light of the very low efficiency of precipitation on sulphides from black smokers (about 5%). The efficiency can be greatly increased by a sedimentary trap. The largest known deposits by far on the present-day seafloor, containing in the order of 10's of millions of tons of hydrothermal

precipitates, are all in heavily sedimented active or failed spreading centres (Atlantis II Deep, Middle Valley, Escanaba Trough). The Guaymas Basin deposits in the Gulf of California, although small in comparison with others, offer insights into the "ore"-forming processes in sedimented settings. Vent fluids from Guaymas Basin are highly modified by reaction with sediments which have buried the spreading axis. The sediment has stripped metals from the fluid resulting in a higher efficiency of precipitation and lends credence to the concept that ancient massive sulphide ores in clastic sediments may have formed just below and not on the sea floor. This stripping has resulted in mounds and chimneys at Guaymas Basin being depleted in total metal sulphides (average 1 % Zn, 0.2% Cu, 0.4% Pb, 69 ppm Ag, 0.15 ppm Au) relative to basalt-hosted sites (e.g. average 5.4% Zn, 4.8% Cu, 0.05% Pb, 105 ppm Ag, 0.8 ppm Au at Explorer Ridge). However, the Guaymas Basin deposits have a higher ratio of Pb/(Zn+Cu), about two orders of magnitude greater than the Explorer Ridge, and they are enriched in barite, calcite and anhydrite reflecting a source of Ba, Ca and Pb in the underlying sediments. Extremely young liquid hydrocarbons (three oil samples dated at 4600 ± 200 yBP by C in the Guaymas Basin fluids, produced by rapid thermal maturation of organic matter in the sediment, saturate the mounds and may be an explanation for carbon haloes that are characteristic of many ancient sediment-hosted msd. The Canning Basin of Western Australia, which contains both hydrocarbon and base metal deposits, and Besshi-type deposits of Japan and Nam ibia have geologic settings that are perhaps not unlike that of Guaymas basin. Analyses of random samples and the sizes of known deposits, together with the expectation that additional large deposits are yet to be found in the more favourable tectonic environments, suggest that seafloor sulphides may possibly become a resource in the future. For now, though, seafloor hydrothermal deposits are of greater value for their scientific knowledge than for the metals they contain. 14)

316

A12.17 GEOCHEMICAL AND GEODYNAMICAL CONSTRAINTS ON SUBDUCTION ZONE MAGMATISM M.T. McCulloch and J.A. Gamble 1

Research School of Earth Sciences, Australian National University Department of Geology, Victoria University of Wellington, Wellington, New Zealand 1

Subduction zones provide a critical link for components will therefore be dependent on the balance interaction between the continental crust and mantle, between magma output and asthenospheric input, with and to understand more fully the changing role of the latter depending on the rate of back-arc spreading subduction zone processes it is necessary to consider axis then the volume of the mantle wedge (per km) is both geodynamical and geochemical constraints. The given by ( l ^ L ^ a n 0 where 8 is the dip of the slab and following questions are considered. What are the major L the distance between the back-arc spreading axis and processes controlling mass transfer from subduction the fore-arc trench. Qualitatively, the more extreme zones into the continental crust? Are residual phases depletions of both HFS and LIL elements observed in such as rutile important in the geochemistry of IAB? Is the Kermadecs relative to other Pacific rim oceanic arcs there any relationship between the geochemistry of may well be a result of the relatively small wedge erupted basalts and dynamical parameters such as volume and slower subduction rate. subduction rates, back-arc spreading and the dip of The simple multi-stage melting model of the mantle subducting slabs? wedge that has been developed assumes replenishment From quantitative models for slab-wedge and wedge- of the wedge with both asthenospheric and slab derived arc crust mass transfer we have examined the components. This enriched wedge is then partially melted implications of the hypothesis that the apparent depletion to produce the island-arc magmas. In a steady state of HFS elements, particulary Nb, is not due to retention melting regime the slab is required to supply the in residual phases such as rutile, but is an inherent dominant proportion of LIL elements such as Rb, Cs, feature of the mantle wedge. It is assumed that IAB are Ba, Sr, Pb, U, Th and LREE (e.g. La, Ce). The exception produced by partial melting of the mantle wedge which is the incompatible but immobile element Nb (and Ta). has been modified by additions of relatively mobile LIL Nb and other immobile (but more compatible) elements elementsfromthe subducting slab. Amongst the essential such as Zr and Ti are derived predominantly from the features of this model are the recycling and possible asthenosphere. From quantative modelling of mass remixing of mantle depleted by both melt extraction at transfer from the slab to the wedge via fluid-rock mantle wedge as well as in the back arc basin melting interactions we have been able to make predications of axis, yet enriched by slab fluxing processes. How of the relative magnitude of fluid-rock distribution asthenospheric material into the wedge region from coefficients. Clearly further experimental work is upwelling mantle passing through the back-arc basin required to quantify these distribution coefficients. The and will necessarily be less fertile with regard to basaltic lack of reliable data is currently the major impediment components and consequently it will be depleted in to a more detailed understanding of subduction zone incompatible elements relative to MORB. The extent of magmatism depletion of the mantle wedge for non-slab derived

Author Index

Abbott MJ 230 Adam J 214 Aerden DGAM 174 Ahmad M 157 276 Ahmad R 76 276 277 AhmatA 115 Allen RL 3134 Alley NF 56 66 Anderson G 145 Anderson JA 121 Andrew AS 23 135 Andrew RL 119 Angus J 116 ArakelAV 68 Archer DL 104 Arculus RJ 212 Arnold GO 108 Ashley PM 23 303 Ballhaus C 135 140 Barley ME 157 Barnes SJ 123 Barnicoat AC 144 Barrett PJ 250 Barron LM 132 Barton CE 92 Baxter JL 118 165 Beaumont C 81 Beams SD 37 Beard T 145 BeesonR 113 Bernecker T 69 Berry RF 106 230 278 Bickford G 297 Bills RT 22 Binns RA 14 302 Black LP 36 251 253 Blevin P 269 Bloom MS 267 Bo Zhou 235 BoneY 72 BottrillRS 28 Boudreau A 131 141 Bourman RP 66 Bowler JM 1 Bowman JR 96 98 Boyd D 99 Brabham GR 150 Brakel AT 63 301 Braun J 81 Brennan E 227 Brill B A 133 Brimhall GH 308

Brock GA 50 Brown AV 279 Brown MC 74 Brown RW 200 Bryce N 162 BuggRL 218 BuickIS 254 Bultitude RJ 275 BuineRV 297 BurrettCF 40 43 Callen RA 56 Calver CR 59 Camacho A 185 Cameron RG 102 Cameron W 126 CarrPF 258 283 Carrasco P 116 Carter L 58 Carter RM 57 58 221 Cas RAF 29 31 34 Cazzulo-Klepzig M 51 Cevallos C 86 Chamalaun FH 86 Champion DC 275 Changkakoti A 169 268 281 295 Chant IJ 86 89 281 Chappell BW 258 260 275 307 Chase RL 302 Chen XY 92 Chen YD 238 ChoyG 98 Claoue-Long J 17 Claproth R 283 Clarke DS 19 170 Clifford BA 29 Cohen A 68 Collins PLF 29 Connors K 175 Cook B 224 Cooper BJ 192 Corbett KD 30 Cornelius M 137 Correa da Silva ZC 51 Cowley W 110 Cox S 180 Cox SF 168 Coxhell S 150 Craven SJ 215 Crawford AJ 211215 Crohn PW 249 Crookes RA 159 CudahyTJ 149 283

Cummins P 96 CuirieKL 257 Dahlhaus P 121 DantiK 308 Davidson G 163 de Caritat P 77 284 DefantM 208 Dent VF 99 Dewey J 98 Douglas JG 54 Dowling SE 123 Downs RC 8 176 Draper JJ 52 Drown CG 8 176 Drummond MJ 259 Dunlop AC 135 Durney DW 186 Eggins S 203 Eggleton RA 74 76 77 Eisenlohr BN 152 EldridgeCS 25 Elliott CG 193 Ellis DJ 261 290 292 Ell is ton J 263 Engelbretson MJ 50 Etheridge MA 143 Evans PR 195 Ewers GR 36 Fanning M 110 Fergusson CL 194 200 285 Fielding CR 196 Finlayson DM 196 Fitzgerald JD 183 292 Fitzsimons ICW 254 Fleming BS 137 Fleming MJ 109 Fleming PD 229 FodenJ 208 262 Fordham BG 48 FordyceRE 249 Frakes LA 55 Francis JE 55 Franklin BJ 259 Franklin JM 11 Frost KM 124 Fulthorpe CS 57 58 GabellAR 283 Gadsby MR 170 GalerSJ 204 Gallagher K 81 Gamble JA 316 Gammons CH 25

318 GaoZ 281 Gatehouse CG 192 Gemmell JB 8 106 Gibson G 98 222 Giles AD 177 GliksonAY 140 GoedeA 67 Golding SD 120 153 Goldstein SL 245 GoleMJ 123 Goscombe B 188 Gostin VA 66 Gravestock DI 192 Gray CM 229 304 Gray DR 193 194 Gray J 169 268 281 295 Green DH 135 213 Green GR 15 16 Green N 169 222 Green TH 214 239 Greenwood D 56 Griffin WL 237 240 Griffith A 287 Grigson MW 286 Groves DI 120 124 147 151 153 159 286 312 Guerra- Sommer M 51 GuoJF 239 Haggert K 180 Haines PW 60 Hamilton L 145 Hamilton LH 220 Hamlyn PR 127 128 129 Hand SJ 65 HarleySL 254 286 Harlow PGL 223 Hartley JS 37 HastieLM 86 89 281 Hawkins I 227 Heinrich CA 6 269 Heinson GS 88 Heithersay P 17 Henderson RA 200 Henley RW 311 Hensen BJ 135 255 Hermanto R 85 Hewson N 165 HigginsNC 273 Hill RET 123 HillisR 82 Hinman M 174 HirdesW 152 Ho SE 145 147 Hoatson DM 130 Hobbs BE 184 Hoffman C 290 Hofmann GW 225 Hongjun T 68 Hostetler PB 277 Hronsky JMA 147 Hudson DR 125 Hunns S 10 Huston DL 28 106 178 290 Hutchinson RW 310

Ireland T 162 Ishikawa Y 34 Jack DJ 7 Jackson I 96 Jackson PG 268 Jacobs C 91 JagoJB 42 192 JairethS 269 James PR 185 Jaques AL 242 Jenkins GW 293 Jessell M 180 JochumKP 245 JohanZ 132 Jones BG 58 258 287 Jones C 249 Jones T 98 Keays RR 66 127 128 129 130 133 138 Kellett RL 88 KennardJM 288 Kennett BLN 96 98 Khin Zaw 10 106 289 290 KilpatrickJA 258 290 Kinny PD 241 251 253 Klindworth D 218 KlootwykCT 39 KnittellU 208 KorschMJ 215 Krouse HR 169 268 281 295 Kwak TAP 169 268 274 281 295 Laing WP 18 Lambeck K 79 Large RR 8 10 26 106 161 293 LawS 36 Lawrie KC 179 291 Leach JHJ 121 Leaman DE 100 LeitchEC 200 Li ZX 93 Lilley FEM 88 Lindsay JF 288 LiuS 229 MabokoMAH 292 Mackenzie DE 36 271 293 Mackenzie DH 116 Main JV 113 Markovics G 222 MaroneCJ 181 Marques-Toigo M 51 Marshall B 177 259 Martin DJ 233 Maury R 208 Mawson R 50 Mayer W 91 McArthur GJ 8 108 McClenaghan M 260 McConchie D 68 McCulloch MT 316 McDonough WF 245 McDougall I 17 292 McGoldrick PJ 115 293 McKay WJ 5 McKibbenMA 25

McLaren AC 84 McNaughton NJ 120 153 159 McPhieJ 36 McQueen H 104 McQueen KG 74 166 McWatters R 68 Memagh TP 267 290 294 Michael PJ 302 Michael-Lieba MO 97 Middleton MF 82 Mills KJ 235 MinluFu 295 MorandVJ 194 Motoyoshi Y 255 Mulholland IR 296 Murray-Wallace CV 67 Nansen GC 58 Nelson CS 58 70 Nicholls IA 205 NicollRS 46 NottR 297 O'Brien PE 61 297 O'Nions RK 204 O'Reilly SY 237 238 239 240 O'Shea A 150 Ohguchi T 34 Ohnenstetter M 132 Oliver NHS 171 Oliver RL 250 OrdA 185 OrthK 297 OrtonGJ 35 299 Oversby BS 36 PACLARKTeam 14 Page RW 18 Pailles C 68 Pal M 283 Pallett R 224 Palmieri V 52 53 Panozzo-Heilbronner R 182 Parfirey SM 52 Parker AJ 300 Parkinson WD 85 Parson L 91 Partington GA 155 Paterson MS 96 Patterson D 203 Paul J 187 Pearson NJ 237 Peck DC 138 Peck WA 220 225 PerfitM 202 Perkins C 17 Perriam RPA 147 Pharaoh TC 140 Phillips GN 148 Porcelli DR 204 Powell CMcA 93 190 Pretorius DA 305 Price GD 119 Price RC 265 304 Purvis AG 215 QuiltyPG 249 Qureshi IR 102

319 Ramos VA 199 Ramsay RR 243 Rand SW 9 Rao CP 72 73 Rattenbury MS 160 Reading HG 299 Rees MN 247 Reeves SJ 128 129 Remus DA 195 Richards SM 305 Richardson RG 100 Rickard MJ 166 RigbyJF 52 Roach M 75 Roberts RH 109 RockNMS 243 Ross AF 150 Ross MI 39 RothE 145 RowellAJ 247 Rudnick RL 245 Rust BR 287 RuxtonP 115 Ryan CG 240 SaintyRA 38 S appal KK 219 Scheffler JM 230 Scheibner E 102 Schleiger N 218 Schmidt PW 93 Schofield N 117 Schroeder N 249 Scott SD 14 314 Scowan PAH 127 Senapati N 58 Shaohua Zhou 83 Sheppard S 159 SheppardWA 113 Sheraton JW 253 ShergoldJH 43 300 Simpson PG 159 SivellWJ 197 Slansky E 132 Smith D 240 Smith IEM 19 Smith JV 186 Solomon M 6 269 Southgate PN 300 Spencer R 102 Sporli KB 170 SprodT 223 StaggHJ 39 Staley R 145 Stephenson PJ 232 Stockley JL 20 Stocksiek CM 197 Stolz AJ 26 106 207 Stone J 204 StumpflEF 137 Stunitz H 183 Stutchbury RJ 221 224 Sumpter DJ 148 Sun SS 130 Suppel D 132

Sutherland FL 241 S wager CP 115 Sweeney RJ 217 Symons P 145 TadrosNZ 233 Taheri J 16 Talent JA 39 50 TappBA 220 225 Tatsumi Y 209 Taylor G 74 Taylor GR 227 Taylor JR 264 270 Taylor WR 230 ThostDE 255 ThruppG 95 Thurlow JG 14 Tipper JC 76 77 Totterdell JM 46 63 301 TruduAG 267 Truswell EM 74 Turner S 48 Turner SP 262 Tyne ED 102 Valenta RK 171 173 van Moort JC 112 Vance D 204 Varne R 206 207 Vearncombe JR 147 151 286 Vernon RH 185 Vukadinovic D 205 Wall VJ 171 264 270 Wallace DA 130 Wallace MW 66 WalsheJL 5 6 76 77 284 Walter MR 3 Wang Shi-Tao 48 Waterhouse JB 197 Waters JC 32 Webb JA 69 Webby BD 45 Wedekind R 161 Wells AT 61 WhalenJB 257 Whellams J 86 WhellerGE 207 302 White A 88 White AJR 307 Whitford DJ 215 302 303 Willcox JB 39 Williams N 313 WindhJ 157 WinsorCN 167 Wojtal S 187 Wongwanich T 43 73 WoodheadJ 216 Woodward NB 187 193 Wormald RJ 265 304 Wright J 274 Wybom D 36 126 Wygralak AS 157 276 Yamagishi H 34 Yates MG 118 Young DN 261 290 Young GC 39 41

Zadins Z 187 ZeiderPK 292