Issuu

EIGHTH AUSTRALIAN GEOLOGICAL CONVENTION

1836 SOUTH AUSTRALIA 1986

GEOLOGICAL SOCIETY OF AUSTRALIA

ABSTRACTS NUMBER 15

"EARTH RESOURCES IN TIME AND SPACE" FEBRUARY 16-21,1986 FLINDERS UNIVERSITY, ADELAIDE

Geological Society of Australia

Abstracts NUMBER 15 ISSN 0723-01IX ISBN O S O S S B 9

421

EARTH RESOURCES IN TIME AND SPACE

EIGHTH AUSTRALIAN GEOLOGICAL CONVENTION

FEBRUARY 16-21, 1986 FLINDERS UNIVERSITY, ADELAIDE

Published by the Geological Society of Australia Incorporated Challis House, 10 Martin Place, Sydney PEACOCK PUBLICATIONS, ADELAIDE

Geological Society of Austxalia Incorporated Office Bearers 1985

President Vice-Presidents Hon. Secretary Hon. Treasurer Hon. Editor Hon. Administrative Officer

Prof J.B. Waterhouse Dr M.J. Rickard Prof D.M. Boyd Dr G.W. Hofmann Dr M.K. Herbert Dr R.W. LeMaitre Mr D.H. Probert

Eighth Australian Geological Convention, 1986 Organising Committee CQNygNOR;

Dr Colin D. Branch

SECRETARY:

Mr Lew C. Barnes

SECRETARIAT

FINANCE

Members:

Chairman: Members:

Mr George Kwitko Mr Ted Dubowski Miss Claudia Di Ciano

Dr Jim B. Jago Prof David Boyd Miss Bridget Youngs

SCIENTIFIC/^^^^

PROMOTIONS & PUBLIC RELATIONS

Chairman: Dr Clinton Foster Deputy: Dr Wolfgang Preiss Members: Mr Dave Roberts Prof Chris von der Borch Dr Robin Oliver Mr John Drexel (publications) Mr Stuart Begg (session whip)

Chairman:

Mr Mike Cobb

TRAVEL AND ACCOMMODATION Chairman:

Mrs Elly Main

GEOSCIENTIFIC EXHIBITIONS EXCURSIONS Chairman: Dr A. John Parker Deputy: Mr Bob Dalgarno

SpCIAL PROGRAM Chairman: Dr Andy White Deputy: Mr Terry Crabb

Chairman: Deputy: Members:

Mr Keith Yates Mr John Drexel Dr Tim Hopwood Mr Barry Willis

Geological Society of Australia

Abstract Series

Number 15

CONTENTS

PAGE

LIST OF TITLES Keynote papers Technical papers SGGMP papers Technical posters

4 4 10 11

ABSTRACTS - alphabetically by author Keynote papers Technical Papers SGGMP papers Technical posters

13 22 211 218

AUTHOR INDEX

253

KEYNOTE PAPERS Dr Nicholas Christie-Blick, Lamont-Doherty Geological Observation, Columbia University, USA: "Quantitative Subsidence Analysis and Sequence stratigraphy: New Approaches to the Analysis of Proterozoic and Palaeozoic Tectonics and Sedimentation"• Dr Robert M. Sneider, Robert M. Sneider Exploration Inc, Houston, USA: "The Synergistic Team Aopproach - Its Value to Petroleum Exploration and Development in the Next Decade". Professor Frederick J. Sawkins, University of Minnesota USA: "Ore Generating Systems Through Geologic Time: Uniformitarian and Non-uniformitarian Aspects".

TECHNICAL PAPERS SESSION 1:

ARCHAEAN AND EARLY PROTEROZOIC

1.1

HAYDON, R.C. and McCONACHY, G.W. The stratigraphic setting of the Broken Hill ore bodies - sedimentary or volcanic?

1.2

KINNY, P. Zircon ages from the Narryer Metamorphic Belt.

1.3

ELLIOTT, P.J. The Mount Keith South nickel sulphide deposit - W.A. A geophysical case study.

1.4

MYERS, J.S. The Western Gneiss Terrain of the Yilgarn Block, W.A.

1.5

PIDGEON, R.T. and WILDE, S.A. Geochronology of the Wongan Hills Greenstone Belt, Western Gneiss Terrain, Yilgarn Block, W.A.

1.6

JOHNSON, G.I. and COOPER, J.A. Rb-Sr geochronology of late Archaean syenitic intrusives, eastern Yilgarn Block, W.A.

1.7

FANNING, C.M., COOPER, J.A., OLIVER, R.L. and LUDWIG, K.R. Rb-Sr and U-Pb geochronology of the Carnot Gneisses: complex isotopic systematics for the late Archaean to early Proterozoic Sleaford Complex, southern Eyre Peninsula, S.A.

1.8

MORTIMER, G.E., COOPER, J.A. and OLIVER, R.L. The geochronological and geochemical evolution of the Proterozoic Lincoln Complex, Eyre Peninsula, S.A.

1.9

LAING, W.P. and BARNES, R.G. Metallogeny of the Willyama Supergroup, Broken Hill.

1.10

FANNING, C.M., BLISSETT, A.H., FLINT, R.B., LUDWIG, K.R. and PARKER, A.J. A refined geological history for the southern Gawler Craton through U-Pb zircon dating of acid volcanics, and correlations with northern Australia.

1.11

HIGGINS, M.L. and HELLSTEN, K.J. The Menninnie Dam lead-zinc-silver prospect, S.A.

1.12

MORTIMER, G.E., COOPER, J.A. and OLIVER, R.L. The geochemistry of basaltic dykes in the Proterozoic Lincoln Complex, southeastern Eyre Peninsula, S.A.

1.13

COOPER, J.A., MORTIMER, G.E. and JAMES, P.R. Geochronological studies in the eastern Entia Dome, Arunta Inlier.

1.14

THORNE, A.M. The depositional history of the 2.0 GA Wyloo Group, southern Pilbara, W.A.

1.15

WILDE, A.R., WALL, V.J. and BLOOM, M.S. Wallrock alteration associated with unconformity-related uranium deposits, N.T.

SESSION 2:

MID-PROTEROZOIC

2.1

ROBERTS, D.E. The Olympic Dam Copper-Uranium-Gold

2.2

CARR, G.R. and ANDREW, A.S; . . . Preliminary studies of sulfur isotope variations within sedimentary cycles in the Lady Loretta Zn-Pb-Ag deposit.

2.3

SCOTT, K.M. Geochemistry and mineralogy of metasediments, Mount Dore copper deposit.

2.4

OLIVER, N.H.S., WALL, V.J. and PEARSON, P.J. The Mary Kathleen uranium-REE deposit; a product of regional metamorphism.

2.5

FLINT, D.J. and DUBOWSKI, E.A. Review of diamond drilling of nephrite jade deposits near Cowell, South Australia,

2.6

WELLMAN, P. and PLUMB, K.A. McArthur Basin structure.

2.7

PLUMB, K.A. . . ,, Batten Subgroup, McArthur Basin - modern analogues for an evolving ancient lake.

2.8

SWEET, I.P. Recognition of tidal environments in the Abner and Bessie Creek Sandstones, McArthur Basin, N.T.

2.9

BONE, Y. and YPMA, P. Disulfides and thiospinels from Rum Jungle, N.T.

SESSION 3:

Deposit.

PROTEROZOIC MAGMATISM, METAMORPHISM AND CRUSTAL EVOLUTION

3.1

SIVELL, W. . . u-i K Magmatism, mantle metasomatism and tectonics in a Proterozoic mobile belt, eastern Arunta Block, central Australia.

3.2

LISTER, G.S., THOMAS, A., and DUNN, J. _ . Tectonic processes in the Mount Isa Inlier: the significance of transpressional strike-slip faulting.

3.3

TYLER, I.M. The metamorphic and tectonic development of the southeastern margin of the Pilbara Craton, W.A.: evidence from the Sylvania Inlier.

3.4

OLIVER, R.L. and PURVIS, A.C. High pressure, high temperature metamorphism, northern Eyre Peninsula.

3.5

REINHARDT, J. . , . u Alumino-magnesian clays as precursors of cordierite-anthophyllite rocks in the Rosebud Syncline (Mt. Isa Inlier); geochemical evidence and field relations.

3.6

FODEN, J.D. and BUICK, I.S. Petrology, geochemistry and crustal development, Entia Dome, eastern Arunta Block, N.T.

3.7

DALLMEYER, R.D. Polyphase tectonothermal evolution of the Grenville Front, eastern Canadian Shield.

3.8

OLIVER, N.H.S. and WALL, V.J. Scapolite paragenesis in calc-silicates at Mary Kathleen, N.W. Queensland; implications for metamorphic fluid evolution.

3.9

LAING, W.P. and BEARDSMORE, T.J. ^ , Stratigraphic rationalisation of the eastern Mount Isa Block, recognition of key correlations with Georgetown and Broken Hill Blocks 'in an eastern Australian Proterozoic terrain, and their metallogenic implications.

SESSION 4:

ADELAIDEAN AND CAMBRIAN

4.1

VON DER BORCH, C.C., CHRISTIE-BLICK, N. and GRADY, A.E. Concepts of seismic stratigraphic analysis applied to late Proterozoic Wilpena Group, Adelaide Geosyncline, S.A.

4.2

FANNING, C.M., LUDWIG, K.R., FORBES, B.G, and PREISS, W.V. Single and multiple grain U-Pb zircon analyses for the early Adelaidean Rook Tuff, Willouran Ranges, S.A.

4.3

BELPERIO, A.P. Stratigraphy and sedimentology of the Precambrian Skillogalee Dolomite, northern Flinders Ranges.

4.4

GREY, K. Late Precambrian stromatolite biostratigraphy of the eastern Pilbara.

4.5

PIRAJNO, F. Sn-W metallogeny in the Damara Province, Namibia,

4.6

HAINES, P.W. Late Proterozoic carbonate shelf to shale basin transition, Wonoka Formation, Flinders Ranges, S.A.

4.7

LEMON, N.M. Adelaidean sedimentation adjacent to the Enorama Diapir, central Flinders Ranges, S.A.

4.8

MORRISON, R.S. Early Palaeozoic plutonism in the Peake and Denison Ranges, S.A.

4.9

CREELMAN, R.A. and HLADKY, G. Copper bearing sulphide minerals in colloform textures from the Pernatty Lagoon copper deposits, S.A.

4.10

JENKINS, R.J.F. Ralph Tate's enigma - and the regional significance of thrust faulting in the Mount Lofty Ranges.

4.11

DALGARNO, R. Syndepositional faults, Adelaide Geosyncline.

4.12

EICKHOFF, K.H. The Wonoka Formation in the Fortress Hill area, northern Flinders Ranges.

4.13

GORTER, J.D. Early Ordovician oolitic ironstone, Pacoota Sandstone, Amadeus Basin.

4.14

WHITFORD, D.J., GULSON, B.L., KORSCH, M.J. and SOLOMON, M. Strontium isotopic studies of barites from the Cambrian Mount Read Volcanics, Tasmania.

4.15

SOUTHGATE, P.N. and HENRY, R. Lithofacies and cyclic sequences in an alkaline lake sequence of Lower to Middle Cambrian age from the Officer Basin.

4.16

LAMBERT, I.B., DONNOLLY, T.H., SOUTHGATE, P.N., ETMINAN, H. and WESTE, G. Isotopic and fluid inclusion studies of Cambrian palaeo-environments, eastern Officer Basin.

SESSION 5:

PRECIOUS METALS THROUGH TIME

5.1

BURLINSON, K.G. and MACKIE, A.W. Geology and fluid inclusion decrepitation studies at the Arltunga Gold Field, N.T.

5.2

CARTHEW, S.J. and BRUCE, J.R. The White Range Mineral Province.

5.3

WILSON, P. Potentially economic palaeodrainage systems of central and western Australia using NOAA-AVHRR imagery.

5.4

RANSOM, D.M. and FOTIOS, M.G. Structural control of gold mineralisation in Archaean shear zones.

5.5

STUBLEY, M.P. Megakinks as potential gold targets.

5.6

TUCKER, D.H. and WILSON, P. . . Magnetic and thermal linears: some economic implications for gold mineralization in the Yilgarn Block, W.A.

SESSION 6:

CRUSTAL STRUCTURE AND DEVELOPMENT OF THE LITHOSPHERE

6.1

WAKE-DYSTER, K.D., WRIGHT, C., SEXTON, M.J., JOHNSTONE, D.W., DAY, R.W., MURRAY, C.G., DIXON, 0. and RYNN, J.M.W. First deep seismic profile across Tasman Fold Belt - Cheepie to Beenleigh Queensland•

6.2

DRUMMOND, B.J. and COLLINS, C.D.N. Seismic evidence for underplating of the Australian crust.

6.3

GLEN, R.A. and VANDENBERG, A.H.M. Thrusting and its implications in the Lachlan Fold Belt near Delegate, outheastern Australia.

6.4

LAMBECK, K. and BURGESS, G. . , Seismic evidence for lateral deep crustal structure in central Australia.

6.5

KENNETT, B.L.N. Surface wave tomography as a tool to study crustal heterogeneity.

6.6

GREENHALGH, S.A., TAPLEY, D. and SINGH, R. Crustal structure of South Australia from earthquake and explosion data.

6.7

WRIGHT, C., GOLEBY, B.R., COLLINS, C.D.N, and FINLAYSON, D.M. The central Australian seismic experiment, 1985.

6.8

MCQUEEN, K.G., BROWN, M.C. and TAYLOR, G. Low pressure type metamorphism and tectonic evolution in the southeastern Lachlan Fold Belt.

6.9

HOUSEMAN, G. The geometry of lithosphere extension.

6.10

VOORHOEVE, H. and HOUSEMAN, G. Calculations on a model for lithospheric extension due to a low-angle detachment zone.

6.11

FINLAYSON, D.M. and LEVEN, J.H. Structural styles in central Eromanga Basin seismic data as a guide to crustal and deep lithospheric processes.

6.12

STAGG, H.M.J., RAMSAY, D.C., PIGRAM, C.J., HILL, P.J., DAVIES, H.L., COLWELL, J.B. and COFFIN, M.F. Structure of the Kerguelen Plateau.

6 13 6.14

WRIGHT, C. The interpretation of expanding spread reflection profiles recorded in southern Queensland. LEVEN, J.H. and FINLAYSON, D.M. Basement thrusts in the southern Adavale Basin.

6.15

DASHLOOTY, S.A. and DAVIDSON, G.J. Evolution of the Carpentarian Glyde River Sub-Basin, McArthur Basin, N.T.

6.16

LENNOX, P.G. Quartz veining during folding in greywacke sequences.

6.17

LISTER, G.S., ETHERIDGE, M.A. and SYMONDS, P.A. Detachment faulting and the evolution of passive continental, margins.

SESSION 8:

MID-PALAEOZOIC TO MESOZOIC

8.1

LAWS, R.A. and GRAVESTOCK, D.I. Cambrian oil revisited.

8.2

HUNT, J.W. and SMYTH, M. Permian coals of the intratratonic Cooper Basin.

8.3

MARSHALLSEA, S.J. The thermal history of the southern Bowen Basin: analysis.

an apatite fission track

8.4

LUG, B., WANG, Y., SHEN, P., YANG, X. and ZHENG, G. and MOUNT, T.J. Correlation of terrestrial petroleum in China and Australia.

8.5

PASSMORE, V.L. Basins in the central Eromanga region and their petroleum prospects.

8.6

SAPPAL, K.K. Petrography and depositional environment of Permian coal. Collie Basin, W.A.

8.7

SIVELL, W. and WATERHOUSE, J.B. Petrogenesis of Gympie Group Volcanics and their relation to Early Permian volcanism in eastern Australia and New Zealand.

8.8

MARTIN, D.J. Geochemistry of a suite of teschenitic intrusions from the Sydney-Gunnedah Basin - evidence of crustal tension.

8.9

CAMERON, R.G. Mineralized fractures and lineaments, Macksville region, northern N.S.W.

8.10

ETMINAN, H. and LAMBERT, I.B. Fluid inclusion studies in carbonate hosted lead-zinc mineralisation, Canninq Basin, W.A.

8.11

ALLEN, R.L. Alteration-modified textures in silicic lavas with implications for volcanichosted mineralization.

SESSION 10:

CAINOZOIC ENVIRONMENTS AND RESOURCES

10.1

BELPERIO, A.P. and GOSTIN, V.A. Neotectonics and coastal evolution of the Adelaide region.

10.2

MURRAY-WALLACE, C.V. Quaternary coastal and marine aminostratigraphy, S.A.

10.3

PHILLIPS, S.E. Palaeoenvironmental interpretation of the Ngankipari Sand in the St. Vincent Basin, south of Adelaide, S.A.

10.4

CANN, J.H. and GOSTIN, V.A. Holocene coastal sedimentary facies and foraminiferal biofacies, northeastern Gulf St. Vincent, S.A.

10.5

CHIVAS, A.R., DeDECKKER, P. and SHELLEY, J.M.G. Sr and Mg contents of ostracods used to distinguish past salinities, temperatures and environments of deposition.

10.6

HABERMEHL, M.A. Mound spring deposits of the Great Artesian Basin.

10.7

KREMOR, A. Depositional environments of the Lochiel deposit.

10.8

TOWNER, R. Mineral sands in Australia - a review.

10.9

BENBOW, M.C. A palaeogeographic and palaeoclimatic model for the eastern Eucla Basin in the Early/Middle Miocene.

10.10

ANDREW, A.S. and TAYLOR, G.F. Processes of gossan formation at the Elura Zn-Pb-Ag deposit.

10.11

10.12

10.13

T R U S W E L L , E.M. Oligo-Miocene climates Basin p a l y n o l o g y ,

in s o u t h e a s t e r n

L A B L A C K , K. and B E L P E R I O , A. F o r a m i n i f e r a l b i o f a c i e s in an e v o l v i n g VEEH,

Australia:

Holocene

Murray

.i embayment; Tourville

s e r i e s dating

SILICA

insular

„ Bay,

o K S.A.

IN T H E G E O L O G I C A L

phosphorites.

SESSION

11:

11.1

R O B E R T S O N , R.S. and S C O T T , D.C. P r e c i o u s o p a l and the w e a t h e r e d p r o f i l e at C o o b e r T H I R Y , M. . . . M i n e r a l o g i c a l forms of s i l i c a in s i l c r e t e s .

ENVIRONMENT

11.3

J O N E S , J.B. and F I T Z G E R A L D , M . J . S i l i c a - r i c h layering at B l a n c h e P o i n t ,

11.4

ROBERTSON, A.D. The Marlborough chrysoprase

SESSION

12:

12.1

B A R N E T T , S.R. G r o u n d w a t e r in the M u r r a y

GROUNDWATER:

12.2

GERGES, N.Z. Underground water

12.3

H A B E R M E H L , M.A. The Great Artesian

12.4

H A N C O C K , S. Environmental

12.5

from

H.H.

Uranium

11.2

. . . intimations

S.A.

deposits.

RESOURCE OR ENVIRONMENTAL

Basin

Adelaide

- an

. . . . . i r r i g a t i o n to i r r i t a t i o n .

Metropolitan

- a groundwater

a s p e c t s of p r o p o s e d

KNIGHT, M.J. M a r a l i n g a South A u s t r a l i a

PROBLEM?

B a s i n , S . A . - from

resource,

Pedy.

Area.

resource.

in-situ

. . . leach m i n i n g

ideal d i s p o s a l

site

at B e v e r l e y

for A u s t r a l i a ' s

o . - S.A.

intractable

wastes.

SESSION

14:

EARTHQUAKES AND VOLCANOES

14.1

D E N H A M , D. and M c C A F F R E Y , R. S m. e c h a n i s m s of large A u s t r a l i a n so tu rr ec se ses

14.2

G A U L L , B.A. and M I C H A E L - L E I B A , M . O . N e w e a r t h q u a k e risk m a p s of A u s t r a l i a .

14.3

M c C U E , K. Australia's

14.4

S A M B R I D G E , M . S . and K E N N E T T , B . L . N . D e t e r m i n a t i o n of e a r t h q u a k e h y p o c e n t r e s

14.5

MICHAEL-LEIBA, M.O. V a r i a t i o n s in the s e i s m i c i t y

14.6

D R U M M O N D , B.J. and R O K I A H ESA M O H A M E D . C r u s t a l s t r u c t u r e in the s o u t h w e s t s e i s m i c

14.7

G R E E N H A L G H , S.A., SINGH, R., P A R H A M , R . T . , N A T I O N , South Australian seismicity 1978-1985.

14.8

RYNN, J.M.W. The seismicity

northern

tectonic

of Q u e e n s l a n d

AUSTRALIA

earthquakes

and

. . . regional intra-plate

boundary.

and

at r e g i o n a l

the A u s t r a l i a n

plate,

zone,

northeastern

ranges.

New

0-60®S,

90-165

McDOUGALL,

W.A. R. and

South W a l e s

1866

through

1985.

SESSION 15:

EXTRA-TERRESTRIAL INFLUENCES ON RESOURCES

15.1

ELSTON, W.E. and TWIST, D. Bushveld Complex, South Africa:

15.2

GOSTIN, V.A. and WILLIAMS, G.E. Giant impact structure and widely dispersed ejecta in the Precambrian of South Australia.

15.3

STEWART, A.J. Discovery of shatter cones at the Lawn Hill circular structure, northwestern Queensland.

SESSION 17:

EARTH RESOURCES AND PEOPLE:

Results of Simultaneous Impacts?

HISTORICAL PERSPECTIVES

17.1

CHALMERS, R.O. H.I. Jensen - a neglected pioneer Australian geologist.

17.2

DENMEAD, A.K. and DENMEAD, E.N. Cracow gold rush.

17.3

WEGNER, J.H. Mining the past:

the use of historical records for Queensland mining history.

SGGMP PAPERS FOLEY, S.F., TAYLOR, W.R. and GREEN, D.H. The structural role of fluorine in magmatic systems with application to the genesis of lamproites. GREEN, T.H. and PEARSON, N.J. Experimental investigation of the role of amphibole in the evolution of island arc volcanics, Solomon Islands. KUEHNER, S.M. and GREEN, D.H. High pressure studies of mafic dykes: East Antarctic Shield.

implication for the crustal history of the ^

OLIVER, R.L., FODEN, J.D. and SULLIVAN, S.J. Metamorphism and melting in the eastern Harts Range, N.T. VERNON, R.H. Role of superheated magma in the formation of orbicular granitoids.

TECHNICAL POSTERS ADKINS, J.S. BMR earth-science

research.

BAXTER, J.L., WILDE, S.A., PIDGEON/ R.T. and COLLINS, L.B. A video presentation on the geological setting of ancient detrital zircons within the Jack Hills metamorphic belt, W.A, CIRCOSTA, G., NEW D. and GOSTIN, V.A. Sturtian glaciation, ironstone deposition and penecontemporaneous faulting, northern Yednalue Anticline, central Flinders Ranges, S.A. CLARKE, J.D.A. Depositional tectonics of a synsedimentary graben, Wilkawillina Gorge, S.A, COATS, R.P. and DALGARNO, R. Large scale slumping in the Umberatana Group, Willouran Ranges. COBB, M.A. Groundwater resources of the Barossa Valley, S.A. COOPER, B.J., CORBETT, D.W. and ROGERS, P.A. Johannes Menge (1788-1852) - South Australia's first geologist. DALGARNO, R. Ediacaran extensional faults, Mount Frome. DALY, S.J., PARKER, A.J. and FLINT, R.B. Middle Proterozoic sediments of the Gawler Craton. DiBONA, P.A. and VON DER BORCH, C.C. Wonoka Formation sedimentation, relative sea-level, and basin analysis in the northerr Flinders Ranges, S.A. DREXEL, J.F. and MAJOR, R.B. Mount Painter breccias. DUBOWSKI, E.A. and BARNES, L.C. Gypsum in South Australia. DUNSTER, J.N. Sedimentology of Early Cambrian marine carbonates, northeastern Officer Basin. DYSON, I.A. of the late Precambrian Brachina Subgroup at Hallett Cove, S.A. Geology FERGUSSON, C.L., GRAY, D.R. and CAS, R.A.F. Overthrust terranes in the Lachlan Fold Belt, southeastern Australia. FERMIO, S.J. Petrogenesis of the Cobaw Batholith, central Victoria. FLINT, R.B. Adelaidean sediments of the Peake and Denison Ranges. FORBES, B.G. Geology of the Willouran Ranges. GERGES, N.Z. Underground water resources of the Adelaide Metropolitan Area with the latest understanding of recharge mechanism. GRADY, A.E. Large recumbent folds in Weekeroo schists, Olary District, S.A. GRAVESTOCK, D.I. and JAMES, N.P. Archaeocyathan distribution on the Lower Cambrian carbonate shelf. Flinders Ranges, S.A. GREY, K. and THORNE, A.M. Stromatolites in upward-shallowing

seguences.

HASLETT, P.G. Early Cambrian syndepositional tectonics at Wirrealpa, S.A.

KNIGHT, M.J. Maralinga South Australia - an ideal disposal site for Australia's LAING, W.P. and HAMMOND, R.L. Manual of structural techniques explorationist.

intractable

wastes.

in outcrop and drillcore for the practising

LENNOX, P.G. Microfractures, microfabrics and microstructures developed high pore fluid pressures. McBRIAR, E.M. and JOYCE, E.B. Geological features of scientific

interest

McCALLUM, W.S. Barite deposits of the Oraparinna

Diapir.

in slate rock analogues at

in Australia.

McCALLUM, W.S. and BARNES, L.C. Magnesite in the Adelaide Geosyncline. O'DRISCOLL, E.S.T. The Leigh Creek-Freeling

Heights structural

corridor.

PARKER, A.J., BAILLIE, P.W., BOYD, D.M., FREEMAN, M., McCLENAGHAN, M.P., MURRAY, C., MYERS, J.S., PIETSCH, B.A., RICKWOOD, R.C. and TUCKER, D.M. Mafic dyke swarms of Australia. RICKARD, M.J. and CROOK, K.A.W. The Wagga Wagga to Bateraen's Bay, N.S.W. transect of the Lachlan Orogen: International Geodynamics Project (WG9). TOWNSEND, I.J. Williamstown

kaolin-sillimanite-mica

deposits.

WILTSHIRE, R.G. Basement blocks in Umberatana Group, Olary.

Profile 5,

ABSTRACTS OF KEYNOTE PAPERS

QUANTITATIVE SUBSIDENCE ANALYSIS AND SEQUENCE STRATIGRAPHY: NEW APPROACHES TO THE ANALYSIS OF PROTEROZOIC AND PALAEOZOIC TECTONICS AND SEDIMENTATION Christie-Blick, Nicholas ^ Bond, Gerard C.^ and Kominz, Michelle A.^ ^Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY 10964, U.S.A. We have applied two new approaches, which have been developed for the analysis of modem sedimentary deposits, to the study of tectonics and sedimentation in Proterozoic and Palaeozoic rocks. The first involves the calculation of the tectonic or driving subsidence of a sedimentary basin, and provides quantitative information about both the timing of subsidence and the mechanism of basin formation (Bond and Kominz, 1984). The second approach is to apply seismic stratigraphic concepts to relatively conformable, unfossiliferous Proterozoic and Lower Cambrian successions to identify sequence boundaries (regional unconformities). Sequence stratigraphy permits more precise relative time correlation than has hitherto been possible, and provides a framework with which to interpret large-scale patterns of sedimentation. Tectonic subsidence is calculated by quantitatively removing the effects of sediment compaction, sediment loading, water depth changes and changes in eustatic sea level (if known) from the cumulative thicknesses of basin fill (Steckler and Watts, 1978). Tectonic subsidence curves can then be compared with subsidence curves derived from geophysical models of basin evolution. In the southern Canadian Rocky Mountains, for example, tectonic subsidence calculated from Cambrian and Ordovician shallow-water carbonate rocks indicates that subsidence is a linear function of V time. This result is consistent with models for thermal contraction of the lithosphere following rifting, and confirms the interpretation of lower Palaeozoic rocks in western North America as accumulations on a passive continental margin. The age for the beginning of thermal contraction, and thus the onset of sea-floor spreading, is between 600 and 555 Ma. This age is about 200 m.y. younger than had previously been assigned to the rifting event on the basis of available isotopic ages on volcanic rocks in underlying Proterozoic strata. The discrepancy has provided new impetus for dating and investigating the sedimentological evidence for rifting in Upper Proterozoic rocks thought by some to be passive-margin deposits. Calculations of tectonic subsidence from other lower Palaeozoic successions in both western and eastern North America have also indicated a slight but systematic deviation in the subsidence from a purely thermal form. This result is consistent with a late Cambrian and early Ordovician eustatic high stand indicated by the long-term patterns of transgression and regression on the North American craton (Bond et al., 1983). Preliminary assessment of lower Palaeozoic strata in the Amadeus basin indicates that the subsidence may not be of the simple thermal form observed in western North America. Sequence boundaries are defined by abrupt changes in a stratigraphic section of depositional facies, and in places by onlap, downlap, toplap and erosional truncation, either mapped on aerial photographs or determined indirectly from closely spaced stratigraphic logs. Sequence boundaries are thought to be present in most successions, even Proterozoic ones in which relatively concordant but undated strata are commonly regarded as conformable. Examples have been documented in the Brigham Group of the western United States, the Dalradian Supergroup of Scotland, and the Wilpena Group of the Adelaide Geosyncline (von der Borch et al., 1986, this convention). Four or perhaps five sequence boundaries have been identified in the Brigham Group, which consists of about 5,000 m of fluvial to shallow-marine sandstone, conglomerate, mudstone and subordinate volcanic rocks. The Brigham Group crops out in the Cretaceous and Paleogene Cordilleran thrust and fold belt, and within the Cenozoic Basin and Range extensional province. Owing to these structural complexities, some facies discontinuities are due to the presence of low-angle faults rather than to sequence boundaries, and some stratal terminations are hanging-wall or footwall cutoffs. However, stratigraphic and structural boundaries can generally be distinguished on the basis of contact geometry, regional continuity, and the presence or absence of related deformation. Most identifiable sequence boundaries correspond with formation contacts because sequence boundaries cannot be readily detected where underlying and overlying facies are the same. Other formation contacts, defined by arbitrary cut-off between laterally intertonguing facies, are not sequence boundaries. Most sequences are thought to deepen upward at the base, an observation 14

consistent with a eustatic mechanism for the origin of sequence boundaries, but the presence of several hundred metres of fluvial sandstone over shallow marine shale at one boundary in the Brigham Group suggests an abrupt decrease in the rate of tectonic subsidence and/or an increase in the rate of sediment supply independent of possible eustatic effects. The recognition of depositional sequences in the Brigham Group has permitted the investigation of lateral variations in facies and thickness within time-stratigraphic units, not merely lithostratigraphic ones. Such lateral relations support a new interpretation of the Brigham Group as predominantly a syn-rift deposit. REFERENCES Bond, G.C., and Kominz, M.A., 1984, Construction of tectonic subsidence curves for the early Paleozoic miogeocline, southern Canadian Rocky Mountains: Implications for subsidence mechanisms, age of breakup, and crustal thinning: Geological Society of America Bulletin, v. 95, p. 155-173. Bond, G.C., Kominz, M.A., and Devlin, W.J., 1983, Thermal subsidence and eustasy in the Lower Palaeozoic miogeocline of western North America: Nature, v. 306, p. 775-779. Stickler, M.S., and Watts, A.B., 1978, Subsidence of the Atlantic-type continental margin off New York: Earth and Planetary Science Letters, v. 41, p. 1-13. von der Borch, C.C., Christie-Blick, N., and Grady, A.E., 1986, Concepts of seismic stratigraphic analysis applied to Late Proterozoic Wilpena Group, Adelaide Geosyncline, South Australia: 8th Australian Geological Convention,

" T H E SYNERGISTIC T E A M A P P R O A C H — I T S V A L U E TO PETROLEUM EXPLORATION A N D D E V E L O P M E N T IN THE N E X T D E C A D E " Robert M . S n e i d e r Robert N . S n e i d e r E x p l o r a t i o n , I n c . , H o u s t o n , T e x a s T e c h n o l o g i c a l and b u s i n e s s changes in the p e t r o l e u m i n d u s t r y demand that the t e c h n i c a l / m a n a g e m e n t team find and d e v e l o p fields in m o r e efficient and c o s t - e f f e c t i v e w a y s . U n p r e c e d e n t e d c h a l l e n g e s face our industry in e v e r y facet of the p e t r o l e u m e x p l o r a t i o n and d e v e l o p m e n t b u s i n e s s . M u c h of the search for o i l and gas is in h i g h - c o s t , f r o n t i e r and h o s t i l e e n v i ronments - in d e e p e r w a t e r , in the arctic and in remote d e s e r t t e r r a i n . Economic pressures require that fields be b r o u g h t on stream quicker and that r e c o v e r y be i n c r e a s e d , w h i l e at the same time n e w fields are often s m a l l e r and m o r e c o m p l e x - m a n y are m a r g i n a l and e c o n o m i c a l l y sensitive, b e c a u s e of their g e o g r a p h i c l o c a t i o n . In a d d i t i o n , one sees rapid to e x p l o s i v e advances in n e w e x p l o r a t i o n and d e v e l o p m e n t t e c h n o l o g y , w i t h increased e m p h a s i s on e n g i n e e r i n g . P r i m a r y and e n h a n c e d r e c o v e r y p r o cesses r e q u i r e m o r e a c c u r a t e and c o m p l e t e r e s e r v o i r d e s c r i p t i o n to r e d u c e engineering risks w i t h i n tolerable b o u n d s . A l l the above factors are c o u p l e d w i t h the fact that the world demand for p e t r o l e u m c o n t i n u e s to increase (Figure 1 ) , and the i n d u s t r y c o n t i n u a l l y fails to r e p l a c e r e s e r v e s c o n s u m e d . G i v e n the recent w o r l d w i d e d e c l i n e in e x p l o r a t i o n and d r i l l i n g , the world r e s e r v e s of p e t r o l e u m w i l l c o n t i n u e their downward trend w i t h the p r o s p e c t s for a s u p p l y crisis in the 1 9 9 0 ' s . A n o r g a n i z a t i o n a l a p p r o a c h to help solve some of the p r o b l e m s and c h a l lenges that lie ahead in the n e x t d e c a d e is the u s e of a v a r i e t y of g e n e r a l i s t s and s p e c i a l i s t s in a t a s k - f o r c e a p p r o a c h - a s y n e r g i s t i c tea m! The d i c t i o n a r y d e f i n e s s y n e r g y - "as the c o o p e r a t i v e a c t i o n of d i s c r e e t agencies so that the t o t a l effect is g r e a t e r than the sum of the effects taken i n d e p e n d e n t l y . " In the c o n t e x t of the p e t r o l e u m b u s i n e s s , s y n e r g y means - geologists, geophysicists, engineers work together more effect i v e l y and e f f i c i e n t l y as a team or task force than w o r k i n g as a " g r o u p " of i n d i v i d u a l s . The s y n e r g i s t i c a p p r o a c h among m e m b e r s from d i f f e r e n t d i s c i p l i n e s yield b e t t e r r e s u l t s , and q u i c k e r than can i n d i v i d u a l members working alone. The a d v a n t a g e s and d i s a d v a n t a g e s of the s y n e r g i s t i c team a p p r o a c h are e x a m i n e d t h r o u g h four case h i s t o r i e s : (1) d i s c o v e r y of the g i a n t E l m w o r t h d e e p b a s i n gas f i e l d , w e s t e r n C a n a d a ; (2) d i s c o v e r y and a p p r a i s a l of the g i a n t B a l d e r f i e l d . N o r t h Sea; (3) a b i l l i o n b a r r e l e x t e n s i o n to the older E l M o r g a n f i e l d , o f f s h o r e Red S e a , E g y p t ; and (4) e v a l u a t i o n of r e m a i n i n g hydrocarbon potential for s u p p l e m e n t a l r e c o v e r y in o l d e r f i e l d s . W e s t Texas-New Mexico, USA. F o r s y n e r g i s m to b e a s i g n i f i c a n t force in an o r g a n i z a t i o n and for it to b e t r u l y e f f e c t i v e in e x p l o r a t i o n and p r o d u c t i o n , a l l p a r t i c i p a n t s m u s t b e o p e n to n e w ideas and both the individuals and m a n a g e m e n t m u s t w a n t and b e able to w o r k and s h a r e . T h e r e are b a r r i e r s to s y n e r g i s m and the syne r g i s t i c t e a m . Some p a r t i c p a n t s have a p e r s o n a l i t y that is not s u i t a b l e , o t h e r s lack good c o m m u n i c a t i o n skills - an e s s e n t i a l i n g r e d i e n t , and some fear that b y w o r k i n g in a team that their c o n t r i b u t i o n s w i l l not be f u l l y r e c o g n i z e d . Some m a n a g e r s lack the u n d e r s t a n d i n g of the c o n c e p t or do n o t

wish to share the glory, defeats or problems with other managers. Some organizational structures inhibit synergistic decision-making. What is the proper organizational structure/management style to make the synergistic team most effective? The answer is not known as yet. Several major companies, and small and large independent companies are experimenting. One major oil company is setting up a small, "independent division that will operate as a separate company and compete in the same geographical areas as its "regular" operating divisions. Management will compare results in five years. The use of synergistic teams is for both large and small companies. It has an easier chance for success in the smaller organizations. In larger companies, synergism is and can be employed, although the bureaucracy and size often tend to dilute and hinder it, sometimes to the point of ineffectiveness . Whatever problems there are with the synergistic team concept, in the next decade economics, the compelling need for future petroleum reserves, and the explosion in technology demand that the petroleum industry develop interdisciplinary, cooperative teams. Optimum exploration and reservoir management requires teamwork and close coordination among geologists, geophysicists, engineers, scientists and managers through all stages of the life of a reservoir. Figures 2 and 3 illustrate the interrelationship of functions and data of a synergistic team.

WORLD OIL DEMAND i UNITED STATES

WESTERN EUROPE

I OTHER DEVELOPED NATIONS

1973

1985

1990

2000

{ i l l JAPAN

DEVELOPING NATIONS

1985

1990

RMS8S1130 Figure 1.

Historical and forecast of world oil demand. 17

2000

ROLE OF THE SYNERGISTIC TEAM IN HYDROCARBON EXPLORATION & PRODUCTION FAIRWAY D E T E R M I N A T I O N • LOCATE TRAPS • DETERMINE PROBABLE RESERVOIRS, SEALS AND SOURCE ROCKS PROSPECT D E L I N E A T I O N • LOCATE EXPLORATORY WELL(S) RMS84878

Sneider/1977

SEISMIC L I N ^ S ^

APPRAISAL PHASE

• CONFIRM TRAP AND HYDROCARBONS +—+ —

+•

• EVALUATE RESERVOIR QUALITY AND THICKNESS

• CALIBRATE SEISMIC +

^ ^

—

RMS84879

After Dikkers 1964/Sneider

1977

F U L L APPRAISAL • ESTABLISH MINIMUM HYDROCARBON VOLUME TO JUSTIFY DEVELOPMENT • EVALUATE AQUIFER, BARRIERS AND PRODUCTIVITY • GENERATE DEVELOPMENT PLANS RMS84880

Figure 2A.

After Dikkers, 1964/Sneider

Interrelated roles of the synergistic team. 18

1977

ROLE OF THE SYNERGISTIC TEAM IN HYDROCARBON EXPLORATION & PRODUCTION PRIMARY DEVELOPMENT

RESERVE ESTIMATE

RMS84881

A f t e r Sneider 1 9 7 7 / D i k k e r s 1 9 6 4

GAS INJECTION WELLS

ENHANCED RECOVERY • NET PAY ROCK TYPES VS. HYDROC. SATURATION • NET PAY CONTINUITY • BARRIERS TO FLUID FLOW • kh AND MAPS • ADAPT GEOLOGICAL GEOPHYSICAL MODEL INTO RESERVOIR SIMULATION MODEL

RMS84882

Figure 2B.

After Sneider 1 9 7 7 / D i k k e r s

Interrelated roles of the synergistic team.

SYNERGISTIC STUDIES

rigure 3.

Flow of data and results of a synergistic study. 19

1964

ORE GENERATING SYSTEMS THROUGH GEOLOGIC TIME- UNIFORMITARIAN AND NON-UNIFORMITARIAN ASPECTS

Frederick J. Sawkins

Department of Geology and Geophysics, University of Minnesota Minneapolis MN USA

Ore deposits of various types are the result of specific chemical transport systems that operate in specific lithotectonic settings. It follows that plate tectonic analysis of a wide spectrum of metal deposit types can provide insights into their distribution in time and space. The purely physico-chemical parameters that control metal transport and deposition, and the generation of potential ore fluids clearly have not changed with geologic time. Thus, the causes for a disparate distribution of major metal deposit types in the geologic record must be sought in either secular changes that have accompanied earth evolution, and/or in isostatic constraints. This latter factor is of major import in terms of the erosion or unroofing of various metal deposit types. Important evolutionary factors that impact on the distribution of major metal deposit types are primarily related to atmospheric evolution and the related development of life forms, decay of the major heat-producing elements K, U and Th, and the evolution of continents and the tectonic style engendered thereby. Most metal deposit types that are dependent on the evolution and crystallization of magmatic systems have not changed fundamentally with time, although isostatic and related erosional factors impact markedly on their preservation. This applies particularly to the porphyry, and related skarn, replacement and vein base and precious metal ores of continental margin and island arc subduction systems. The largest porphyry copper systems, the most important of this group of ore deposits, contain approximately 107 tons Cu (equivalent to 1 billion tons 1% copper ore). Given ore fluids that contain an average 1,000 p.p.m. copper, a fluid volume of ^lOkm^ is required for metal transport and deposition. This in turn equates with a source magma volume of 200km^ that is capable of generating 2 wt % H2O (approx. 5 volume %) during crystallization. Thus, magmatic systems of dimensions of 4 x 7 x 7km are capable of generating the largest porphyry copper deposits The postmagmatic ores of subducted-related arcs are emplaced in the upper, erosionally fragile carapace of such arcs and preservation of older examples requires special conditions. Within this general category of magmatic deposits I would include volcanic-hosted massive sulfide deposits of Kuroko-type, despite the general lack of consensus regarding the precise

role of magmas in their genesis. However, the intra-arc rifting that provides the setting for their formation causes crustal thinning and thus allows the preservation of much older examples. Several ypes of important sediment-hosted lead-zinc and copper deposits are essentially restricted to Proterozoic and Paleozoic terrains. The largest sediment-hosted lead-zinc deposits contain approximately 2 x 107 tons Pb + Zn. Given 500 p.p.m. of Pb + Zn in saline ore fluids generated by dewatering of rift sediments would require 40km^ of fluid. If 5 vol % hydrous fluid can be generated by such processes, a source sediment volume of ^ 8 0 0 km^ is indicated. This in turn equates with a sediment package of 4 X 10 X 20km. It follows that initiation of suitable generative mechanisms for these deposits could not occur until the establishment of widespread continental regimes in which rifting could occur. The paucity of major examples of this group of deposit types in post-Paleozoic terrains presumably reflects their burial in as yet untectonized basins and along current passive continental margins. In addition, a combination of widespread rifting events and atmospheric evolution capable of producing oxidized groundwaters probably accounts for the strong concentration of stratiform copper deposits in late Proterozoic time. Major lode gold deposits exhibit a strong concentration in Archean greenstone belts. The largest of these deposits contain approximately 5 x lO^gm Au. Assuming 500 p.p.b. Au in the ore fluids involved their generation requires 5km^ of fluid. We still have much to learn regarding the genesis of such deposits, but metamorphic dewatering of volcanic and volcaniclastic sequences remains a distinct possibility, that is supported by the high CO2 content of such fluids as indicated by fluid inclusion studies. Given a 1 volume % generation of fluids by metamorphism of the source rocks would require a source volume in the underlying metamorphic terrains of ^ 500 km^, equating with a litho block of 5 x 10 x 10km. The high heatflow that must have characterized Archean volcanic and sedimentary terrains may have been a critical factor in the formation of such deposits and their distinctive alteration features. The closest Phanerozoic analogues to these Archean gold deposits are those along the Mesozoic Mother Lode belt in California. The somewhat speculative generative models outlined above for certain major ore deposit types conform to available data bases and provide acceptable volumes in terms of source rocks. These latter indications have interesting exploration implications in terms of minimum spatial intervals required between major deposits of certain types.

ABSTRACTS OF TECHNICAL PAPERS

ALTERATION-MODIFIED TEXTURES IN SILICIC LAVAS WITH IMPLICATIONS FOR VOLCANIC-HOSTED MINERALIZATION Rod L. Allen Department of Earth Sciences, Monash University, Melbourne Ancient volcanic terrains and in particular those which host hydrothermal m i n e r a l i z a t i o n are in m a n y c a s e s a l t e r e d to an extent that makes recognition of original volcanic lithologies difficult. With increasing a w a r e n e s s of the r e l a t i o n s h i p b e t w e e n mineralization and volcanic stratigraphy there is increasing need to interpret these altered volcanic r o c k s and i d e n t i f y o r i g i n a l lithologies and emplacement processes. Detailed mapping, core logging, and petrography of extensive altered rhyolitic units in the Cowombat Rift near Benambra southeastern Australia reveal the nature of some alteration processes that not only d i g u i s e primary rock textures but also modify them over large areas to produce remarkably misleading apparent or pseudo lithologies. Extensive rhyolite porphyry units which appear to contain abundant pyroclastic and other volcaniclastic r o c k s i n c l u d i n g w e l d e d i g n i m b r i t e are s h o w n to be predominantly altered rhyolite lavas and associated autoclastic facies. The Cowombat Rift is a deformed volcanic and sedimentary sequence which hosts two Zn-Cu-Pb massive sulphide deposits and is one of a number of rift basins of Silurian age in southeastern Australia which contain massive sulphides. Silicic volcanics of the Cowombat Rift were emplaced in a terrestrial to marine environment. Examples of altered rhyolites from three areas are discussed. Depositional environments for two of these areas are defined by intercalated sediments as shallow to moderately deep marine, and deep marine respectively. Sediments are rare in the third area and those present suggest either a marine or non-marine subaqueous environment. On the b a s i s of relict primary textures and contact relationships documented from drill core and outcrop three main component facies are recognized within the altered rhyolites: (1) massive coherent; (2) flowbanded; (3) hyaloclastite and autobreccia. Pyroclastic and significantly reworked clastic volcanic rocks are relatively minor in abundance. However, in situ hyaloclastite is common, occurs as massive units of angular relict glassy fragments, and may easily be misidentified as pyroclastic material. Alteration of the rhyolites encompasses syn-volcanic devitrification of glassy material to finely crystalline quartzofeldspathic aggregates, and local and regional polyphase phyllosilicate-rich (sericite, pyrophyllite, chlorite) and quartz-rich hydrothermal alteration. The development and effect of alteration on original lithology has been documented by studying a wide range of samples from relatively unaltered through to intensely altered. Relict perlitic and devitrification textures are common in all three of the major rhyolite facies types and indicate that much of the rhyolite was originally glassy. This feature is important as volcanic glass is highly susceptible to alteration, and perlitic f r a c t u r i n g , devitrification, and hydrothermal alteration of glassy rhyolite are the fundamental processes which have led to the development of a range of r e m a r k a b l y d e c e i v i n g p s e u d o c l a s t i c textures in the rhyolite. In particular, polyphase effects such as the modification of devitrification features by hydrothermal alteration have resulted in a number of peculiar and misleading textures. Quartzofeldspathic devitrification and hydrothermal alteration of perlitic fractured rhyolite may result in a distinctly fragmental appearance. In addition, alteration to micaceous phyllosilicate assemblages produces a 23

mechanically weakened rock which p r e f e r e n t i a l l y takes up a h i g h d e g r e e o f t e c t o n i c strain. Resultant f o l i a t i o n in p h e n o c r y s t - r i c h r h y o l i t e imparts a granular appearance t o the r o c k ; high s t r a i n in p e r l i t i c r h y o l i t e t e n d s t o d i s l o c a t e t h e a r c u a t e p e r l i t i c f r a c t u r e network i n t o an aggregate o f arcuate a l t e r e d g l a s s fragments l o c a l l y with the appearance o f p y r o c l a s t i c shards. S y n - v o l c a n i c q u a r t z o f e l d s p a t h i c d e v i t r i f i c a t i o n and s i l i c i f i c a t i o n were both widespread, and on hand specimen t o o u t c r o p s c a l e l e d t o the formation o f p a l e c o l o u r e d s i l i c e o u s a l t e r e d z o n e s w i t h i n o t h e r w i s e dark c o l o u r e d p h y l l o s i l i c a t e altered rhyolite. Both d e v i t r i f i c a t i o n and s i l i c i f i c a t i o n t e n d t o o b l i t e r a t e c r y s t a l and g l a s s t e x t u r e s w h e r e a s p h y l l o s i l i c a t e a l t e r a t i o n o f moderate i n t e n s i t y may p r e s e r v e and even enhance the o r i g i n a l distribution of c r y s t a l s . T h i s has i n many c a s e s l e d t o a p p a r e n t d i s c r e p a n c i e s between c r y s t a l c o n t e n t s in s i l i c e o u s versus p h y l l o s i l i c a t e zones. D e v i t r i f i c a t i o n p r o d u c e d s o l i t a r y s p h e r u l i t e s , l i t h o p h y s a e nodules, and more e x t e n s i v e i n t e r c o n n e c t e d zones. These t e x t u r e s are b e s t o b s e r v e d i n hand s p e c i m e n s o r d r i l l c o r e which have been c u t , ground smooth and lacquered. The d e v i t r i f i c a t i o n s t r u c t u r e s have s h a r p l y d e f i n e d , b u l b o u s c a u l i f l o w e r shaped m a r g i n s i n d i c a t i n g t h a t d e v i t r i f i c a t i o n propagated through the o r i g i n a l g l a s s y r h y o l i t e as an expanding d e v i t r i f i c a t i o n f r o n t . Those a r e a s of g l a s s y r h y o l i t e not d e v i t r i f i e d to c r y s t a l l i n e q u a r t z o f e l d s p a h t i c aggregates were subsequently a l t e r e d t o p h y l l o s i l i c a t e c o m p o s i t i o n s at lower temperatures. Where d e v i t r i f i c a t i o n spread out evenly a s h o r t d i s t a n c e from f l o w l a y e r s , r o c k s o f a p p a r e n t t h i n b e d d e d c h a r a c t e r were produced. More e x t e n s i v e d e v i t r i f i c a t i o n r e s u l t e d in rocks w i t h an i n t e r c o n n e c t e d c o n t i n u o u s s i l i c e o u s p h a s e and i s o l a t e d r e l i c k e r n e l s o f dark c o l o u r e d p h y l l o s i l i c a t e m a t e r i a l . These dark patches are g e n e r a l l y s h a r p l y d e f i n e d and i r r e g u l a r i n shape and may r e a d i l y b e m i s i n t e r p r e t e d as a l t e r e d pumice c l a s t s in a p y r o c l a s t i c r o c k . In many o f the r h y o l i t e u n i t s , where ( 1 ) d e v i t r i f i c a t i o n was p a r t l y c o n t r o l l e d by planar flow fabrics; o r ( 2 ) t h e r o c k has a s t r o n g f o l i a t i o n , t h e s e p s e u d o c l a s t s are l e n t i c u l a r i n shape g i v i n g t h e r o c k t h e a p p e a r a n c e o f welded i g n i m b r i t e . S i l i c e o u s h y d r o t h e r m a l a l t e r a t i o n superimposed on p h y l l o s i l i c a t e a l t e r a t i o n has i n many c a s e s p r o d u c e d m o d i f i e d t e x t u r e s almost i d e n t i c a l t o those o f e x t e n s i v e d e v i t r i f i c a t i o n d e s c r i b e d above, and s h o u l d be c o n s i d e r e d e q u a l l y i m p o r t a n t i n m o d i f y i n g l i t h o l o g i c a l appearance. Subsequent t o these f i n d i n g s a t Benambra p s e u d o c l a s t i c and h y a l o c l a s t i c t e x t u r e s i n l a v a u n i t s p r e v i o u s l y t h o u g h t t o be p y r o c l a s t i c have been i d e n t i f i e d i n two o t h e r A u s t r a l i a n m a s s i v e s u l p h i d e d i s t r i c t s ; in a n d e s i t e s i n t h e Cambrian Mt. Read V o l c a n i c s o f northwest Tasmania and in d a c i t e in the Ordovician Mt. Windsor V o l c a n i c s o f n o r t h Q u e e n s l a n d . The n e e d t o r e c o g n i z e l a v a f l o w s and domes and d i s t i n g u i s h them f r o m p y r o c l a s t i c r o c k s i s o f t e n i m p o r t a n t i n e x p l o r a t i o n as l a v a s g e n e r a l l y i n d i c a t e p r o x i m i t y t o c e n t r e s o f v o l c a n i s m and m i n e r a l i z a t i o n whereas p y r o c l a s t i c s may be d i s t a l . In e x p l o r a t i o n f o r m a s s i v e s u l p h i d e s an a b u n d a n c e o f s i l i c i c l a v a s r e l a t i v e t o p y r o c l a s t i c s may a l s o r e f l e c t p o t e n t i a l l y p r o s p e c t i v e submarine e n v i r o n m e n t s as o p p o s e d t o s u b a e r i a l conditions. A r e i n v e s t i g a t i o n o f host rocks appears warranted at those m a s s i v e s u l p h i d e d e p o s i t s w h e r e s u b a q u e o us i g n i m b r i t e s and o t h e r p y r o c l a s t i c b r e c c i a s are documented.

10.10

PROCESSES OF GOSSAN FORMATION AT THE ELURA Zn-Pb-Ag DEPOSIT A.S. Andrew and G.F. Taylor

CSIRO Division of Mineral Physics and Mineralogy, North Ryde, NSW The 100m deep gossan profile developed on the southern apophysis of the Elura Zn-Pb-Ag orebody has many of the features of a 'classical' mature gossan, A 6m thick, complex, supergene enrichment zone occurs between the ironstone gossan and underlying massive sulfide. Within the ironstone gossan, goethite, hematite and quartz are the main minerals with an increasing secondary sulfate and arsenate mineral content towards the base. No diagnostic boxworks or significant primary ore structures are preserved. There has been leaching of some ore minerals over an additional 20m below the supergene zone. Initial oxidation of the ore resulted in the destruction of all sulfides. Zinc, Tl, Bi and Cd were lost from the gossan and Zn now occurs in anomalous concentrations deep in the weathered profile for some distance around the orebody. Whereas some Fe oxides formed essentially in-situ by oxidation of the Fe sulfides to form direct gossan, the remaining Fe was taken into solution to be precipitated in the immediate wall-rocks or as botryoidal, mamillary on stalactitic growths of Fe oxides in solution-deposited gossan. The direct gossan has significantly higher concentrations of the least soluble elements. Silver, Ba, Cu and Hg were also partially depleted in the gossan but Pb, As, Sb and Sn were largely retained. Lead and As occur as the highly stable secondary mineral beudantite, PbFe^OO^)(AsO^)(OH)^, which by analogy with other minerals of the alunite-jarosite series may incorporate Sb as an anion and Ag, Ba, Cu and Hg substituting for Pb. The S content of the ore (>20%) has been reduced to some 2% in the ironstone. The S isotope values for solutiondeposited and direct gossans = 8.6 to 9.1 permil CDT) are similar to values measured on pyrite from the unweathered ore zone (mostly 8 to 10 permil). The 6m thick supergene zone consists of approximately equal thicknesses of sulfide and overlying oxidate minerals. The predominant minerals in the sulfide zone are primary pyrite and galena, supergene pyrite, galena, chalcopyrite, marcasite, digenite and bornite and secondary anglesite (PbSO,) and beudantite. Whereas many of the supergene sulfides are depleted in ^^S relative to the ore = 5.8 to 6.7 permil) the supergene galena is enriched with one sample having a value of 19.1 permil. Oxidation of the ore is not presently occurring except for slight leaching below 106m, so that the waters within the gossan are no longer acidic. Over a long period waters flowing through the gossan have become slightly alkaline (pH 7.7-8) due to leaching of bicarbonate from the surrounding rocks and saline (2.5% TDS). Under these conditions beudantite is less stable and elements initially trapped in the ironstone are in part mobilized. After percolating through the gossan these waters reached the standing water table where changes in pH and ionic composition caused sequential precipitation. There now exists a complex supergene oxidate zone with transition from arsenate-rich (mimetite, Pb^(AsO^)^Cl) through sulfate-rich (beudantite then anglesite) to supergene galena. Native silver containing up to 20% Hg, cerussite PbCO^, nadorite PbSb02Cl, lanarkite and barite are minor phases in this zone. At the margin of the gossan values (6.7 to 10.3 permil, 0.6 to 3.8% S) have a range greater than but overlapping values measured on samples from fresh ore and gossan.

These data suggest that initial oxidation of the orebody resulted in the formation of an ironstone gossan with some ferruginization of the immediate wall rocks. Less mobile elements were retained within the gossan as beudantite, some elements were concentrated at the water table as supergene sulfides or native elements (Ag), and the remaining elements were essentially lost to the country rocks. Later changes in groundwater chemistry resulted in some of the elements trapped in beudantite being remobilized to form complex secondary minerals at the water table. Modification of the gossan profile is a continuing process. Enriched ^^S values in supergene galena and in an oxidate sample containing barite and lanarkite (17.2 permil) suggests that, in the later stages of gossan formation when oxidation of the ore has ceased addition of sulfate dissolved in groundwaters may be important. A sample of groundwater from the C.S.A. Siltstone 200m east of the orebody at 370m depth has 1550 ug/1 S and value of 13.8 permil. Sources of S enriched in ^^s are plentiful in the geological record. In arid terrains evaporites are a possible source. This mechanism is supported by the chemistry of groundwaters which, in the vicinity of the supergene zone, are rich in Pb and Zn compared with waters in the ore and country rocks. To our knowledge this is the first time that multi-phase gossan formation has been demonstrated. It has far-reaching implications for the interpretation of the geochemistry of gossans in mineral exploration.

12.1

GROUNDWATER IN THE MDRRAY BASIN, S.A. FROM IRRIGATION TO IRRITATION S. Barnett Geological Survey of South Australia

The Murray Basin is a broad, flat, saucer-shaped intracratonic basin extending over 320 000 km^ from the Snowy Mountains westward to the Mount Lofty Ranges in South Australia, Extensive Ter iary sedimentation attains a maximum thickness of about 600m in the eastern part of South Australia, with units being generally flat-lying and uniform in thickness. A long term study of the Murray Basin has been undertaken by the BMR in collaboration with State Geological Surveys and Water Authorities with the primary aim of obtaining an understanding of the geology and hydrogeology of the basin as a whole, unencumbered by State boundaries. Sedimentation began in the Eocene with deposition of the Renmark Beds which consist of thick fluvial quartz sands grading upwards to a fluviallacustrine sequence of interbedded carbonaceous sands, clays and lignites. A major marine transgression in the mid-Oligocene led to the deposition of glauconitic marls of the Ettrick Formation and, as sea level rose and shallow marine platform conditions prevailed, the fossiliferous limestones of the Murray Group were laid down. Towards the margins of the basin to the north and east, these limestones grade laterally into marls of the Winnambool Formation which in turn grade into the black carbonaceous Geera Clay. This transition reflects the change in depositional environment from shallow-marine platform through restricted marine and lagoonal areas, to marginal marine tidal flat, swamp and deltaic environments towards the basin margin. Another marine transgression of limited duration occurred in the early Pliocene and deposited the fossiliferous marine clays of the Bookpurnong Beds. The subsequent regression saw the laying down of an extensive sand sheet of the prograding strandplain called the Pliocene Sands (also known as the Loxton and Parilla Sands). A minor late Pliocene rise in sea level deposited oyster beds known as the Norwest Bend Formation in the ancestral River Murray estuary. In South Australia, a thin veneer of Pleistocene sediments overlie the Tertiary. The Blanchetown Clay, deposited in a fluvial-lacustrine environment in the moist late Plio-Pleistocene, is blanketed by aeolian dunefields and carbonate palaeosols which reflect the onset of arid conditions in the Quaternary. The Renmark Beds, Murray Group and Pliocene sands form the three main aquifer systems in SA. The confined aquifer of the Renmark Beds is not widely developed because of its depth and unreliable yield except on the basin margins where it is shallower and often contains better quality groundwater than the overlying unconfined aquifer. The water table aquifer of the Murray Group limestones is widely utilized for stock, domestic, irrigation and town water supplies. In the Mallee region, an estimated 5300 Ml was withdrawn last year, predominantly for irrigation. However, the salinity increases downgradient due to the accretion of meteoric salt to over 20 000 mg/L where it discharges to the River Murray. The overlying Pliocene sands aquifer contains even higher salinity groundwaters which also discharge to the river upstream of Overland Corner. Total groundwater inflows contribute an estimated 150 000 tonnes of salt per year to the River Murray. An interception scheme between Waikerie and Overland Corner, where groundwater inflows are concentrated, is currently being evaluated. The CSIRO is investigating 27

the effects of clearing of the natural vegetation which increases recharge from less than Imm per year to about 10mm per year. Due to the concentration of salt in the root zone of the mallee, this increased recharge is saline. The resultant effect of increased saline recharge and hence increased groundwater inflows on the salinity of the river in the future will be disasterous.

4.3

STRATIGRAPHY AND SEDIMENTOLOGY OF THE PRECAMBRIAN SKILLOGALEE DOLOMITE, NORTHERN FLINDERS RANGES A.P. Belperio Geological Survey of South Australia

Detailed stratigraphic measurements and sedimentological observations of the Late Proterozoic Skillogalee Dolomite were made during regional mapping in the Willouran Ranges. The Skillogalee is a distinctive siliciclastic - stromatolitic dolomite intraclastic magnesite sedimentary succession characterised by rhythmic vertical facies changes on the scale of decimetres, and imperceptible lateral facies changes. Sheet deposition occurred in a widespread, shallow and fluctuating epeiric sea. An arid, evaporative climate, and progressive subsidence resulted in deposition of up to 3700 m of shallow marine, paralic and playa sediments. Correlation was possible across a series of northwest trending sub-basins that were most likely a continuous depositional entity. Organic-rich cryptalgal laminites with stromatolites and dolomite grainstones were deposited in shallow subtidal to intertidal environments. Dessication features (mudcracks, tepees, rip up crusts, halite hoppers) and magnesite mud pellet conglomerates were associated with periodic shoreline migration,and thin siliciclastic sheets resulted from seasonal sheet flooding. Individual magnesite conglomerate beds are up to 1 m thick with composite beds up to 3.5 m thick and typically grading to 80-98% magnesite. Specific marine and non-marine depositional environments are now largely indistinguishable. The oxygen isotope composition of dolomite is compatible with a marine origin and indicates some evaporative enrichment of 0^^ (-8 to +3 PDB). Magnesite crusts and conglomerates show C^^ enrichment relative to dolomites and, together with physical features, such as intermixing of dolomite and magnesite intraclasts, indicate a primary origin for the carbonates. Strontium isotopes and carbon to pyritic sulphur ratios also indicate interbedded marine and terrigenous deposits. Organic carbon content of the cryptalgal laminities typically varies from 0.5 to 1.5% by weight and the thick sequence in the Willouran Ranges probably had significant hydrocarbon generation potential. The sequence is now most likely thermally post-mature, although hydrocarbon indications have been recorded from equivalent Late Proterozoic units around Australia. Differential subsidence resulted from progressive rifting and extensional block faulting, resulting in over 7000 m of Burra Group deposition in the Willouran Ranges. Penecontemporaneous deformation of underlying Callanna Group sediments accompanied Skillogalee deposition in the Rischbeith area, but at no stage generated major positive relief and thus had little effect on sediment lithology.

10.1

NEOTECTONICS AND COASTAL EVOLUTION OF THE ADELAIDE REGION A.P. Belperio^ and V.A. Gostin^

^Geological Survey of South Australia ^Dept of Geology, University of Adelaide A variety of evidence, including relevelling of benchmarks, tidal records and geomorphic change, has been used to deduce a recent sea level rise and/or subsidence of the Port Adelaide region. Estimates of this short-term (lO^-lO^ years) relative sea level rise have been as high as 8 mm per year. Causative factors advocated have also varied widely, from global eustatic changes to localised tectonics, soil compaction and groundwater extraction. The alarmingly high estimates appear largely unjustified in view of the accuracy of the records on which they are based, and the time scale of the measurements. Longer term average subsidence rates (10^-10^ years) can be derived from stratigraphic and chronologic studies of Holocene and Pleistocene coastal deposits. Differences in shoreline elevations along the Gulf St Vincent coast are used to deduce neotectonic warping. These indicate real, but much lower rates of subsidence, in the range 0.04-0.06 mm per year. Subsidence is centred on the Port Adelaide region and has clearly influenced coastal stratigraphic development. At the head of Gulf St Vincent, calcreted Late Pleistocene (c.110,000 yr B.P.) coastal sediments (Glanville formation) extend into the modern supratidal zone. Limited Holocene sediment accumulation has occurred over this calcrete pavement, and the present day intertidal and supratidal morphology is largely inherited. In the Port Adelaide region, the calcreted Glanville Formation extends only up to Low Water Datum, some 5-6 m below its highest occurrence at the head of the Gulf. This lowering of "local basement" has allowed greater accumulation of Holocene coastal sediments to take place, and a well-developed intertidal zonation of prograding environments and facies is evident. Skeletal carbonate detritus from epiphytic and epibenthic organisms (principally foraminifera, gastropods, molluscs and calcareous algae) associated with subtidal and intertidal seagrass communities are a major source of sediment. Fluvially-derived terrigenous- clastics are significant only in the immediate vicinity of ephemeral stream mouths. Mangrove, samphire and cyanobacterial vegetation colonise the prograding tidal flats, and further stabilise and modify the sediments. Within the Port Adelaide estuary, Holocene sediment thickness increases westward from St Kilda to 7 m beneath Port Adelaide, 9 m beneath Torrens Island, and 11 m beneath Outer Harbor. Over the past 150 years, extensive reclamation has occurred of the intertidal and supratidal margins of this tidal basin for a variety of industrial and residential purposes. The Holocene sediments built upon are unconsolidated, saturated and highly permeable, and have very poor foundation characteristics. South of Torrens Island, widespread decalcification of these sediments has occurred, possibly as a result of long term toxic liquid waste disposal into the groundwater system. These anthropogenic activities may have contributed to sediment compaction and subsidence. Tide gauge data from throughout the world that purport to indicate a global sea level rise may similarly be biased by anthropogenic effects.

10,9 A PALAEOGEOGRAPHIC AND PALAEOCLIMATIC MODEL FOR THE EASTERN EUCLA BASIN IN THE EARLY/MIDDLE MIOCENE M.C. Benbow Geological Survey of South Australia The Eucla Basin is a very broad, shallow basin of Cretaceous to Tertiary age, located on the southern margin of Australia. It is a supra-cratonic extension of the Great Australian Bight Basin, both having formed in response to the separation of Australia and Antarctica. During the late Early/early Middle Miocene, a time of transgression, the Eucla Basin may be regarded as a high energy distally steepened carbonate ramp or shelf. Massive algal and foraminiferal limestone (Nullarbor Limestone) up to 30 m thick, was deposited over much of the shelf. Echinoids, bryozoa and mollusca form a smaller component. Lithology is more variable and sandy on the northeast margin. Unfossiliferous (dominantly) intraclastic limestone in the Lake Ifould area was deposited in lagoonal to supra-tidal environments, attesting to limited regression at least, before maximum transgression in the early Middle Miocene. On the northwest part of the shelf there is a broad region of higher energy calcareous sandstone and calcarenite (Colville Sandstone) with a narrow belt of near-shore siliclastic sand facies (Plumridge Beds). The Colville Sandstone is barely represented on the northeast part of the shelf, with the Nullarbor Limestone quickly passing into the Plumridge Beds. Here a high energy strandline with a series of beaches, bays and headlands can be delineated. Landward of the strandline was a coastal dune, the Ooldea Range, which may have formed during the Late Eocene or Early/Middle Miocene. Aeolian deposition is likely to have been reactivated at least during the Early/Middle Miocene transgression. The Ooldea Range lay along 425 km of this part of the Miocene coast, standing 150-180 m above the flat to gently dipping Nullarbor Limestone; average width is 15 km. Aeolian siliciclastic sands were probably deposited along the entire north and east margin of the shelf, for they also blanket elevated crystalline basement southeast of the Ooldea Range. Shallow water clay and carbonate (equivalent to the Garford Formation) up to 20 m thick, intraclastic, oolitic and stromatolitic in part, were deposited in a number of extensive isolated lakes. This occurred between the Ooldea Range and a second significant and parallel dune, the Barton Range, which had its origin in the Late Eocene. Arcuate structures east of Wyola Lakes are suggestive of former regressive lake margins. These undated lacustrine sediments may be correlated with the palynologically dated Early/Middle Miocene lacustrine carbonates in the basins focused on Lake Eyre and Lake Frome. Northeast of the Barton Range similar lacustrine carbonates were deposited within the more confined Tallaringa and Garford palaeochannels, with more extensive deposition focused in the Wilkinson Lakes area and behind the Barton Range. There is limited evidence for earlier fluvial deposition in the Tallaringa and Garford palaeochannels. Much more extensive fluvial deposition (up to 80 m thick) occurred on the southeast margin of the basin, beyond the limits of the Ooldea Range and Barton Range. Such sediments occur in the Narlaby Palaeochannel, for example, south of the Gawler Ranges. However, deposition of these sediments may have occurred some time after the Nullarbor Limestone.

A regional topo-contour map of western South Australia (scale 1:500 000) based on the integration and generalisation of detailed height data (scale 1:50 000, contour interval 10 m) has been a very useful aid in delineating various components of this preliminary palaeogeographic model. The palaeoclimatic significance of the Ooldea Range and Barton Range lies in both their formation and their preservation in the landscape today. Their orientation and the lack of similar features along the western margin of the Eucla Basin suggest that prevailing wind direction at the time of their formation had a marked westerly component (i.e. shoreward directed) as is the case for the region today. They have a similar orientation to the coastal dunes of Pliocene to Holocene age in the southeast part of the State. The formation of such coastal dunes indicate a wave-dominated strandline and relatively high energy wind conditions. The preservation of these aeolian sand ranges since at least the early Middle Miocene (14 my ago) indicate relatively arid conditions since that time, supporting earlier conclusions of other workers, based on the lack of karst development and incision of the Nullarbor Limestone. If these ranges in fact formed earlier, during the Late Eocene, their preservation is the more remarkable. The lack of carbonaceous sediments of the same age as the Nullarbor Limestone landward of the Ooldea Range and the presence of extensive carbonates indicate a relative degree of aridity during the Early/Middle Miocene, in contrast with the Middle to Late Eocene for the region.

2.9

DISULFIDES AND THIOSPINELS FROM RUM JUNGLE, N.T. Y. Bone and P. Ypma

Department of Geology and Geophysics, University of Adelaide Pyrite is the most common of the disulfides at Rum Jungle and formed in a variety of environments. Diagenetic pyrite occurs throughout the Namoona and the Mount Partridge Groups. S isotope data indicate that this pyrite formed from S with biogenic characteristics. In contrast, much of the pyrite which occurs within the polymetallic sulfide deposits shows complexity in textures and geochemistry. S isotope data indicate that this pyrite could have formed from S from the remobilization of the diagenetic pyrite. However, an additional source of metals to form the polymetallic sulfides is required apart from that of the sedimentary environment. It is suggested that hydrothermal fluids were that source. The presence of extensive tourmalinites provides evidence for these fluids. Pyrite alone does not impose tight limits on formation parameters such as temperature, S fugacity, C02 partial pressure, etc., although the presence of its polymorph, marcasite, suggests low temperature and pH. At Rum Jungle, the presence of a number of disulfides and thiospinels, e . g . the disulfides of the villamaninite-bravoite series, does allow tighter limitation to be placed on the ranges of the above parameters. This is apparently the first report of villamaninite in Australia. Villamaninite (Cu,Co,xNi,Co)S2 has the same space group (Pa3) as, and forms a solid solution series with, pyrite. It is an extremely rare mineral, being the only known ternary compound in the Cu-Ni-S disulfide system. This rarity is due to the preferential formation of sulfides such as chalcopyrite and pentlandite when Fe is also available, as Ni + Cu sulfides are metastable in the presence of Fe except under unusual geochemical conditions, e . g . temperature<200®C. , when chalcopyrite and pentlandite do not form because of sluggish reaction rates. Considerable chemical variability occurs within the limits of the series formulae, i.e. the Cu:Ni:Co:Fe ratios are not constant, and leads to the formation of zoned crystals - a feature which is also seen at the only other localities where villamaninite has been found (Villamanin, Karniowice, and Lubin). Similarly, at each locality the host rock is a carbonate. The disulfide bravoite ((Fe,Ni)S2) is a common alteration product, usually being cupriferous. The idiomorphic form of the villamaninite (+ Cu-bravoite) crystals with chalcopyrite infilled fractures suggests villamaninite formed early in the mineralisation sequence, thus implying high S fugacities and low temperature, with the latter supported by fluid inclusion data. Similarly, the association with hematite implies that the CO2 partial pressure was low whereas, when CO2 partial pressure was high, hematite would not have formed and S activity would have been lower and thiospinels would have formed. Indeed, thiospinels are present at Rum Jungle: in particular, the isomorphous series seigenite ((Co,Ni)3S4) and the very rare fletcherite (Cu(Ni,Co)2S4), with the former the more common. Seigenite can also form from the breakdown of villamaninite, but at Rum Jungle some crystals exhibit seigenite cores and villamaninite rims, suggesting primary seigenite. 33

Alteration of these Cu-rich disulfides and thiospinels results in bornite, marcasite, regular bravoite and chalcopyrite - an assemblage which is common at Rum Jungle. Thus, the discovery of these minerals has furthered the understanding of the genesis of the polymetallic sulfides at Rum Jungle.

5.1

GEOLOGY & FLUID INCLUSION DECREPITATION STUDIES AT THE ARLTUNGA GOLDFIED, N.T. K.G. Burlinson^ and A.W. Mackie^

^Burlinson Geochemical Services Pty. Ltd., Darwin ^Northern Territory Geological Survey, Alice Springs The Arltunga Historical Reserve, located 110 km east of Alice Springs, includes many old gold workings. Most gold production came from the White Range area on the eastern margin of the reserve but many smaller workings occur in the vicinity of the old battery. These latter small workings were the focus of this study to investigate the useage of decrepitation analyses in discriminating between mineralised and barren quartz veins in a structurally and metamorphically complex area. The host rocks in the study area are the Cavenagh Metamorphics which are part of the Lower Proterozoic Arunta Block. They are comprised predominantly of feldspathic schist with lesser amounts of quartz rich metasediment, marble. Banded Iron Formation, amphibolite, quartzite and granitic gneiss. This unit was regionally metamorphosed to amphibolite grade at 1719 m.y. (Rb-Sr dating by Stewart), subjected to overthrusting and local metamorphism at 500-300 m.y. and retrogressively metamorphosed to greenschist facies at about 335-310 m.y. The gold mineralisation occurs in pyritic quartz veins infilling tension gash structures and aligned fractures which formed during the intense folding events. It is suggested that the gold was hydrothermally leached from older volcanic assemblages of the basement complex by fluids associated with the greenschist facies retrograde metamorphic event at 335-310 m.y. (Alice Springs Orogeny) and deposited in these structurally favourable sites. Twenty quartz samples from 13 veins, both mineralized and barren, were collected and analysed by decrepitation using the BGS 04 decrepitometer. The results show that there are 4 distinct types of quartz veins. The most obvious difference between the decrepigrams is the presence or absence of a decrepitation peak at a low temperature, near 250®C. Microscope observations of thin sections of 9 of the samples show that this decrepitation peak is caused by the presence of a population of CO2 rich fluid inclusions. In these CO2 i^lch inclusions the internal pressure of the inclusion rises very rapidly during heating, eventually exceeding the strength of the host mineral and causing decrepitation at temperatures which may often be lower than the homogenisation temperature. The 9 samples showing such a low temperature peak also have intense additional peaks at 450°C and 580°C, the latter peak being related to the alpha - beta phase inversion of the quartz structure. These samples are all closely spatially related to the old gold workings. A second group (comprised of 6 samples) has similar decrepigrams to the above group, but lacks the peak at 250°C. These samples came from small old workings which were probably sub-economic. The third group (comprised of 4 samples) has decrepigrams with only very few counts overall and which are bimodal with peaks at about 500®C and 580°C. These samples are remote from old workings and are probably barren. The remaining sample shows only a single decrepitation peak at the quartz inversion temperature of 580®C. This sample was also from a barren background area, remote from old workings. 35

The decrepitation results clearly discriminate between different types of quartz which cannot easily be distinguished by hand specimen examination. As there are vast numbers of fluid inclusions per gram of rock (often some 10^) the method is more reliable than the analysis of trace constituents, which may be inhomogenously distributed, in distinguishing between quartz types. In addition, the technique is not affected by leaching during weathering and is little influenced by sampling error problems, in distinct contrast to the problems of using chemical analyses of trace constituents. The Arltunga results show a close relationship between the presence of low temperature decrepitation peaks caused by CO2 rich fluid inclusions and the occurrence of gold mineralisation. This relationship has also been reported from the Pine Creek Area, N.T. and is also known to occur in the Kalgoorlie area, W . A . and the Timmins district, Ontario, Canada. It is of considerable interest that the technique is applicable in this structurally and metamorphically complex region although it must be remembered that the gold mineralisation is probably one of the latest events in the area.

8.9

MINERALIZED FRACTURES AND LINEAMENTS, MACKSVILLE REGION, NORTHERN NSW Roger G. Cameron

G e o l o g i c a l Survey of New South Wales, Sydney In the Macksville region, between Kempsey and Coffs Harbour on the NSW north coast, there are 120 known metalliferous mineral deposits, lying within the eastern part of the fault-bounded Nambucca Block of the New England Orogen. Most deposits are fracture-controlled vein deposits with internal breccia textures. These deposits occur in four areal groups each with its characteristic mineralization: 1 2 3 4

Metasediment hosted quartz-antimony breccia veins at Taylors Arm Metasediment hosted quartz-antimony breccia veins at Munga Creek Granite and hornfels - hosted molybdenum, silver, base metal and goldbearing quartz veins at Yarrahapinni Granite, hornfels and metasediment hosted molybdenum, silver, base metal, antimony and gold-bearing quartz veins in the Valla-Nambucca area

The metasedimentary host rocks are the Early Permian Nambucca Beds, a multiply deformed sequence of metasandstone, metamudstone, schist and phyllite with sporadically interbedded basic meta-volcanics. Along the eastern coastal margin are stocks of Triassic-Jurassic granitoids (the Valla and Yarrahapinni Adamellites). The Taylors Arm and Munga Creek groups of deposits are each distributed along major lineaments, both of which have previously been interpreted as major faults -the Taylors Arm Fault and Willawarrin Fault respectively. In the Taylors Arm area deposits also occur along arcuate east-west lineaments that cut the northwesttrending Taylors Arm Fault. The discrete breccia veins at Taylors Arm and Munga Creek generally strike northeast and are located in fracture zones, those in the Munga Creek group being late-stage strike-slip faults. Field textures indicate that (hydrothermal hydraulic ?) fracturing and brecciation accompanied sulphide deposition. Evidence of grinding is absent in the Taylors Arm fractures: angular fragments of host rock and quartz are wholly supported by stibnite, there is no discernable offset between hanging and footwall rocks and slickensides are absent. Two morphological end members exist for the mineralized fractures at Taylors Arm; differences lie in their internal morphologies. Their names are derived from representative mines. The 'Bradleys' type dips from 55® to 75®, east or west, and internally has chaotically reoriented cleavage; mineralization is usually located at the footwall. The 'Purgatory' type dips from 80® to vertical and consists of relatively undeformed numerous fault slices of cleaved metasediment where the cleavage in each slice is of slightly different orientation from the adjacent slices; mineralization is preferentially located on one side. Some mineralized fracture zones in the Munga Creek area show evidence of a two or three stage formation process (cf. one for Taylors Arm). An early 'milling' e f f e c t in the fracture produced rounded host rock fragments, some being exotic with respect to the host walls. These fragments have cockade quartz overgrowths and are in turn wholly supported within a stibnite or quartz matrix. Late stage quartz veins show abundant slickensides. The Yarrahapinni area is characterized by a grid-like meshwork of lineaments confined to the granite and hornfels (a Lineament Complex); deposits occur very close to these lineaments. In the Valla-Nambucca area few lineaments cut the granite or hornfels and only half of the fracture controlled deposits are close to lineaments. The simple, compound and brecciated veins of the Yarrahapinni and Valla-Nambucca groups of deposits are peripherally distributed around the Yarrahapinni and Valla

Adamellites. In both cases there is an outward zoning of deposits from molybdenum occurrences in the granitoids, through a silver-lead zone, to a gold-silver-arsenic zone (Valla-Nambucca) or a silver-arsenic zone (Yarrahapinni), to an outermost amtimony zone surrounding only the Valla pluton. The Valla Adamellite is more unroofed than the Yarrahapinni pluton. This is evidenced by differing hornf els/granite areal ratios, differing geophysical signatures and lineament morphology. Deposits peripheral to the Yarrahapinni Adamellite lie within northstriking fracture systems that are spatially related to the lineament complex. Computer plots of deposits in the Dorrigo-Coffs Harbour 1:250 000 Metallogenic Mapping Project (of which this study is a part), show that many ore deposits lie in lineament-like corridors and strike obliquely to the corridor's alignment. Likewise for this study area, interpretation of Landsat imagery at 1:100 000 scale indicates that most of these deposits occur either as clusters or individually, along or near lineaments. This study area has three dominamt lineament orientations: an east-west trend that is often convex southwards, an east-northeastern trend and a northwestern trend. The grid-like lineament complex occurring at Yarrahapinni generally has a north-south/east-west orientation. The proximity of these obliquely oriented mineralized fractures to lineaments shows a very strong correlation in the Munga Creek and Yarrahapinni areas with 66% and 80% of deposits respectively being within 200 m of a Landsat lineament. Likewise, 96% and 93% of deposits are within 400 m of a lineament. The Taylors Arm and Valla-Nambucca areas show a moderate correlation: for deposits within 200 m of a lineament the respective percentages are 45% and 48%, and within 400 m, 71% and 61%. The proximity and the obliquely angular relationships between the mineralized fractures and the east-west lineaments, suggest that these lineaments are l e f t lateral, strike-slip fault zones. Additionally, this is consistent with the veins, having been initiated as extension fractures, subsequently favouring hydro fracturing by channelling fluid flow. Analysis of the geometric relationships between mineralized fractures and three east-west lineament systems in the Taylors Arm and Munga Creek areas suggests that there is a near-uniform northeast trending maximum compressive stress direction (ie Oi) on a regional scale. Similar analysis of the coastal granitoids is equivocal. The Valla-Nambucca veins are either conjugate shears with a northeast Oi if they occurred at the same time, or they are separate extensional fracture systems if they occurred at different times. For the Yarrahapinni area, 0^ trends from the north. The antimony veins probably formed at temperatures similsir to the filling temperatures (ranging from 100° C to 250° C) in the veins of the Hillgrove antimony field near Armidale. This range is within the temperature range of metamorphic waters and it is suggested that these veins probably formed from metamorphic waters rather than from igneously derived hydrothermal fluids. No plutons occur in the Taylors Arm and Munga Creek areas. Metal ratios for the antimony veins show relatively higher abundances of silver and base metals in the western part of the Taylors Arm district, possibly suggesting formation at slightly higher temperatures than those further east and southeast - but there are no fluid inclusion data to support this idea. The lineaments over the Yarrahapinni granitoid are intrinsically different from the longer inland lineaments. The lineament complex exhibited by this pluton and its contact aureole is probably the result of extension above the granite during its diapiric emplacement, thus providing depositional sites for mineralization. By contrast, the longer lineaments in the Taylors Arm and Munga Creek districts are presumed to reflect deep fundamental fractures acting as conduits to concentrate hydrothermal fluids being remobilized from deep crustal stratabound antimony deposits. (Published with permission of the Secretary, Department of Mineral Resources, New South Wales)

10.4

HOLOCENE COASTAL SEDIMENTARY FACIES AND FORAMINIFERAL BIOFACIES, NORTH EASTERN GULF ST. VINCENT, SOUTH AUSTRALIA J.H, Cann^ and V.A. Gostin^ ^S.A. College of Advanced Education (Salisbury), Adelaide ^University of Adelaide

The sedimentary environments described in this paper are located on the eastern coast of Gulf St. Vincent, about 30 km north of Adelaide. The area includes the site of the now disused Port Gawler, on the estuary of the Gawler River. The eastern coast of northern Gulf St. Vincent is normally subject to a low energy wave regime. This fact, together with northward longshore drift, ensures that it is an area of active sedimentation. Low topographic relief of the coastal areas, and extensively developed sand and mud flats, result in broad intertidal areas. High tides associated with storm surges cause inundation of normally supratidal environments. Port Gawler is an area where marginal marine sediments are accumulating under the baffling, trapping and binding actions of seagrasses, cya'nobacterial mats, mangroves, samphires and saltbush. These plant communities occur in essentially discrete zones, successively adjacent and parallel to the tidal shore lines. Their influence causes progradation and aggradation of bioclastic carbonate-quartz sands and muds. Bivalves, gastropods and foraminifera are abundant and contribute significantly to the carbonate component of sediment. Distribution of molluscs is closely related to plant communities, sediment type and period of tidal inundation. Thus a series of laterally adjacent sedimentary facies are recognised to be associated with the following sub environments: subtidal and intertidal seagrass meadows; mangrove woodland; tidal distributaries; cyanobacterial mats of the inner sand flat; high tide beach; supratidal lagoons or sabkhas, dunes and storm ridges; and finally the estuary and associated backwaters of the Gawler River. In subtidal seagrass meadows Fosidonia austvalis predominates. These plants baffle, trap and bind the poorly sorted, coarse, shelly sand. Sediment accumulates to low tide level and the outer margin of the sand bank progrades seawards. The lower energy midtidal area is colonised by Zostera muelleri and other species able to tolerate significant periods of low tide emergence. Organic rich, sulphide-carbonate mud is partly oxidised and homogenised by bioturbation. Intertidal seagrass meadows may be colonised by juvenile mangroves. The pneumatophores of these plants further facilitate aggradation of the sand flat until the period of high tide inundation is insufficient to sustain adequate watering. Alternatively, continued aggradation of sediment in the intertidal seagrass meadow may lead to the formation of an essentially bare inner sand flat, colonised by a cyanobacterial mat. These mats are delimited by grazing gastropods seawards and inadequate high tide watering shorewards. Cyanobacterial mats also occur on the floors of mangrove woodland and in some supratidal areas. They efficiently bind sediment wherever they develop. Storm ridges of poorly sorted shell debris represent episodes of high tide s t o m surge deposition. Sediment of this kind has largely been removed from the study area by mining, but ridges are well developed immediately 39

to the north. Low energy high tide waves rework the seaward sides to form beaches. Finer, lighter sediment may be preferentially wind transported further landwards resulting in low dune forms. Beyond the mangroves and storm ridges, normally supratidal areas may receive sea water at times of exceptionally high tide. They may also be watered by seasonal or groundwater supply. Low lying supratidal lagoons accumulate fine aeolian and evaporitic sediment. Sedimentation is in part effected by samphire plants and cyanobacteria. Throughout this coastal system, intertidal water movement is facilitated via a meandering and reticulating system of channels. These also serve, in part, to maintain boundaries between some of the identifiably different sedimentary environments. Tidal distributary sediments vary greatly in texture and composition. The Gawler River estuary is effectively a large tidal distributary. F o r most of the year there is little or no fresh water flow along the river. Minor amounts of terrigenous sediment are transported to the area at times of winter rainfall. Rarely, flooding may occur following summer thunder storms. Backwaters of the estuary, have been sites of accumulation of peat. Foraminiferal biofacies, based on selected sieved fractions of denseliquid flotation concentrates, are established for the various sedimentary environments. F o r example, the subtidal Fosidonia seagrass meadows are characterised by Disoovbis dimidiatus} Tvochccmmina inflata is abundant in the estuary; and maximum species diversity occurs in the intertidal Zostera seagrass meadows. These foraminiferal biofacies are identifiable in Holocene sediments recovered by vibrocoring. Vibrocores VC 136 recovered sediment from the open inner sand flat, and VC 138 from mangrove woodland adjacent to the estuary. Both cores penetrated about 3 metres. Palaeoenvironmental interpretation of the cores, based on both lithology and assemblages of foraminifera, shows that the earliest sediments of the Holocene transgression were essentially samphire muds and shelly storm debris. Later sedimentation, after sea level stabilised, occurred through processes of progradation and aggradation, inextricably related to communities of seagrasses, mangroves, cyanobacterial mats and samphires. In VC 138, the abundance of Troohammina inflata at a depth of 25-50 cm marks the time of migration of the Gawler River estuary to its present site. Prior to that event, its sites of debouchment were 2 km and 2.5 km to the south-east. Within Gulf St. Vincent and surrounding coastal lands, a l l Holocene sediments deposited under marine influence are referred to the S t . Kilda Formation. This usage includes those sediments, subtidal, intertidal and supratidal, forming at present. The Germein Bay Formation of Spencer Gulf is equivalent.

2.2

PRELIMINARY STUDIES OF SULFUR ISOTOPE VARIATIONS WITHIN SEDIMENTARY CYCLES IN THE LADY LORETTA ZINC-LEAD-SILVER DEPOSIT G.R. Carr and A.S. Andrew CSIRO Division of Mineral Physics and Mineralogy, North Ryde

Sulfur isotope values of pyrites from fine-grained hanging-wall sediments of the Lady Loretta Zn-Pb-Ag deposit have been studied to test some environmental models for sediment-hosted base metal deposits. The environment of deposition of Mount Isa-McArthur HYC type shale-hosted base metal deposits remains controversial despite almost 50 years of study. The most widely accepted theories envisage a shallow water depositional environment with associated evaporite and stromatolite colonies. A significant variation on this model for the Mount Isa area, recently proposed, involves deposition in a playa lake environment with periodic emergence. This new model suggests sulfides were deposited during diagenesis/epigenesis by replacement of sulfate mineral in coarse grained turbidity current deposits. The interbedded sulfide-free sediments were considered to represent deposition from the water column. The Lady Loretta deposit, like Mount Isa and HYC is stratiform within pyritic, dolomitic and carbonaceous fine-grained sedimentary rocks. All three deposits are considered to be stratigraphically equivalent and all have similar model Pb ages. At Lady Loretta, hangingwall sediments are cyclic with sequences of fine-grained pyrite-rich beds overlain by dolomite or siderite-rich fine sands or silt-shales and shales. The contact between the pyrite-rich beds and the underlying shale is sharp, whereas the upper contact is gradational. These sequences are considered to have resulted from the interplay of current deposition, water column deposition and chemical sedimentation. Cycle thicknesses vary from about 150 to 400mm through the hangingwall succession with pyrite beds varying from 1 to lOOmm. Sulfur isotope studies at Lady Loretta have shown that pyrites from the ore horizons range from 4 to 22 permil CDT. In this study seven pyrite-rich beds (3 to 30mm) were chosen for analysis. Pyrite occurs as fine euhedral grains 2 to 20|im in diameter, showing some growth zoning. Two to six sub-samples representing 2 to 4mm of stratigraphy have been taken from each bed. There is significant variation in 6 8 values within individual beds with up to 10 permil variation in less than 10 mm. Two patterns emerge: (a) (b)

values decrease up through the pyrite bed, 6 3 values increase or remain constant, commonly significant decrease in the uppermost subsample.

with

These fine-scale variations in values place constraints on proposed genetic models. The Mount Isa diagenetic replacement model requires replacement of a porous, current deposited sulfate-rich bed by later sulfide minerals. As these sulfide minerals would have inherited the sulfur isotopic composition of the precursor sulfate, the systematic variation observed must also have been present in these sulfates. We consider it unlikely that sulfate minerals would segregate in a turbidity current according to their isotopic makeup. Alternative replacement models using non-sulfate precursors and introduced sulfur cannot, in our opinion, explain the systematic variation through a current bed. A biogenic reduction model whereby sulfides are deposited with the sediments in a closed basin environment predicts a progressive enrichment in 6 S in the residual basin waters. Pyrite deposition from this fluid is reflected 41

in enrichment up through the bed. Evidence for deposition in a restricted circulation basin can be found elsewhere, but such a model can explain only some of the sulfur isotope trends. To explain the overall variation it is necessary to propose a model involving the mixing within the basin waters of sulfur from two sources, with a possible overprint of closed basin biogenic reduction. Further sulfur and lead isotopic studies are planned.

5.2

THE WHITE RANGE MINERAL PROVINCE S.J. Carthew^ and J.R. Bruce^

^Rocks Prospecting and Associates, Brighton, S.A. ^Manager, White Range Gold Mine, Alice Springs, N.T.

The White Range mineral province is found 110km east- northeast of Alice Springs and is centred on the past gold mining township of Arltunga. In this area numerous gold and silver + base metal occurrences have been found, of v^^ich White Range and Bluey's Silver Prospect are the more important. The regional controls of precious metal mineralization are 1) lineaments and structural intersections of lineaments 2) the proximity to greenschist zones of retrograde metamorphism 3) the distribution of the chrome bearing mica ? fuchsite 4) the association of the effects from high heat flow and water 5) the presence of rocks suffering'brittle deformation 6) mineral deposition in the up-thrust zone of thrust nappe development during its southward translation 7) partial association with carbon rich sediments (shale and silty sandstone). The precious metal mineralization is a hydrothermal quartz-lode type , structurally emplaced during the waning phases of the tectono-thermal thrust nappe development of the Carboniferous Alice Springs Orogeny. These hydrothermal fluids infilled anticlinal tension gashes, shear zones, joints, and favourable bedding planes in either basement, the Heavitree Quartzite (White Range) or the Bitter Springs Formation (Bluey's Silver Prospect). There are two principal mineralizing events. Many of the auriferous reefs postdate the schistosity and form conjugate sets of veins subparallel to the general east-west (070° or 110°) axial planes of the late stage kink folds. The second (younger) phase is associated with northeast (025°) plunging anticlines and associated narrow shears that host high grade gold mineralization at Round Hill, Wipeout and Mount Chapman. This second phase resulted in brecciation, cockade effects and gold enrichment on the quartz lodes at Ishite Range. This mineralization is found adjacent to an intense deformation zone of the Woolanga Lineament, in the Weldon Tectonic Zone, where heat and high water flow have retrogressed basement rocks to greenschist metamorphism and produced mesoscopic folds, reclined folds verging south, necked conglomerates and mineral elongation (in the southward direction of thrusting) by ductile elongation in essentially the lower two units of the Heavitree Quartzite. These effects are found in the northern (Mt. Harding) and western portions of White Range. On the east side of Wliite Range v^ere the more massive, resistant, upper quartzite crops out, brittle deformation and fracturing dominates. Mapping has shown that the major thrust nappe deformation was by thrusting and that the thrust sheets are generally upright recurring at different levels in the basement and in the early Adelaidean stratigraphy sequence. In this general area, alteration consists of silicification, chlorite, epidote, tourmaline, feldspar, and fuchsite.

The White Range gold Icxies are found on the eastern flank of a broad anticlinal structure where brittle fracturing and deformation of unit three in the Heavitree Quartzite is paramount. This is a general area of structural crossroads vdiere strong east-west structures from the Winnecke Corridor (120®) interact with northeasterly structures of the Cattle Highway trend (060°). Significantly, both trends are mineralized. Mineralogy is simple, in the sulphide zone, being domonantly quartz and pyrite with subordinate chalcopyrite, chalcocite, and native silver. Liberated grains of native gold, ranging in size from 10 to 350 um, occur on the edge of, or in cracks in, the pyrite. Some gold is also locked in the pyrite and non opaques. Free gold is often observed in the ironstone gossan found in the lower portion of the hill. Higher up the hill, leaching by acid sulphate waters intensifies, replacing the iron oxides with kaolin. At Luces, panning the kaolin yields gold. The lodes, named Excelsior, Extended, Luces, and Great Western, are structurally controlled shear zones showing cockade brecciation and enechelon arrangement on the 070® trend. Ihey have a step like appearance following shear zones, dominant joint patterns and bedding planes , making for complex irregular and variable mineralized bodies. In some cases,e.g. Luces,\Ahere the dominant joints parallel the main fissures, comparatively regular reefs, up to 400m long, with simpler surface configurations have formed. Elsev^here (e.g. Excelsior),the often 120' is at a shallow angle to the dominant shear trend, (either 70' or 90') resulting in the en echelon arrangement of short (up to 100m in length), thick quartz lodes and numerous secondary quartz veins. Inevitably, there is local arching of the quartzite and more intense jointing often quartz filled, adjacent to the shear zones. The east-west trending (often 070®) shear zones are parallel to and on the flanks of small anticlinal structures. Here , the near vertical trace of the axial plane fractures intersect the dominant southward dipping joint systems e.g. at Great Western, Luces, and Extended. Hossfeld [1937], estimates from past battery crushings up to 1920 and his sampling programme, that the Im to 2m wide lodes averaged 28.5g/t Au. Benching has revealed numerous steeply dipping fracture controlled narrow quartz veins trending either 070® or 110® magnetic, and some bedding controlled veins that together can form a stockwork pattern e.g. at Wests to the side of the main lodes. Bulk sampling over 20 metres at Excelsior has recovered 5g/t Au by gravity jig methods. The Black Devil project is within the thrusted shear zone between the Heavitree (Quartzite and the overthrusted Atnarpa Igneous Complex on the east side of l>hite Range. This thrust surface has in part silicified and tourmalinised the quartzite. Within the retrogressed chlorite zone, gypsum and tourmaline possibly indicate volcanic exhalation. Tourmaline hosts the gold mineralization. Bluey's Silver Prospect is found in the transition zone between high energy clastics and low energy shales and carbonates on the Cattle Highway Lineament at the south end of White Range. There, the mineralization is found in the basal portions of the Bitter Springs Formation, in a synclinal structure that plunges northeast. This silver-dominant mineralization was structurally emplaced with siliceous solutions that reacted favourably with bedded pyrite. At this prospect the soils average 29g/t Ag using a 5g/t cut off. Native silver petrologically occurs as fine grains and as very late stage veinlets, crosscutting all other components and former patches of iron, copper and lead sulphides. Basement rocks have been thrusted at a shallow angle (approximately 20') on to the mineralised zone. 44

17.1

H.I. JENSEN - A NEGLECTED PIONEER AUSTRALIAN GEOLOGIST Oliver Chalmers

Honorary Research Associate - The Australian Museum, Sydney Harald Ingemann Jensen (1879-1966), born in Denmark, was brought to Queensland by his parents in 1885. The progressive view of his two grandfathers influenced his firm belief in socialism throughout his entire lifetime. The family settled in Caboolture, 45 km north of Brisbane. Jensen won a scholarship at Brisbane Grammar School that took him to The University of Sydney in 1898 where he majored in Geology under the magic spell of Professor T.W. Edgeworth David. His academic career was brilliant. He was the first D.Sc. medallist. His thesis dealt with the alkaline intrusives of Eastern Australia. He was the first to be awarded a Macleay Fellowship by the Linnaean Society of N.S.W. (1905-1908). In 1912 Jensen was appointed first Director of Mines in the Northern Territory. Dr. J. A. Gilruth, a veterinary scientist, was the Administrator. Gilruth was an arch conservative, arrogant and dictatorial. In contravention of strict guidelines laid down by the government, a representative of a mining syndicate, of which Gilruth and the Chief Justice were under cover members, asked Jensen to grant them the lease of the Daly River Copper Mines. Jensen, strictly honest and incorruptible, refused. Gilruth dismissed him. Jensen, in 1917, was appointed to the Geological Survey of Queensland. He began geological exploration for oil which remained one of his chief interests. Jensen was soon confronted by what he considered to be dishonest practices on the part of E.G. Theodore, Premier of Queensland. He conducted investigations in the mineral rich Chillagoe-Mungana region. Theodore and William McCormack,who succeeded Theodore as Premier, were under cover members of a mining syndicate. Jensen was offered a share in the syndicate if he would recommend a Government grant to dewater the Girofla Mine. Jensen refused. Having incurred the enmity of Theodore, Jensen resigned from the Survey in 1922. Between 1917 and 1928 he stood unsuccessfully for election to the Queensland parliament and the Senate, three times as a Labor candidate and once as an independent after Theodore engineered his expulsion from the Australian Labor Party in 1926. From 1938 to 1940 he was a senior geologist in the Queensland section of the Aerial, Geological and Geophysical Survey of North Queensland. From 1943 to 1944 he made an official survey of the mica deposits of the Harts Range and the Plenty River, Northern Territory. He made his living from 1922 on mainly as a consulting geologist. He spent the greatest amount of time and effort in oil exploration in the Roma-Maranoa-Sprinsure region of Queensland. Jensen paid dearly for having progressive political views about socialism which he never tried to conceal. This did not endear him to the establishment. He estimated that he applied for about 40 academic or official Government positions with no success. He had a wife, two sons and three daughters to maintain. He was never well off financially. He tried to make a living poultry farming at Caboolture at which he was not very successful. Rheumatism prevented him from continuing as a geological consultant. Eventually his rheumatism became so bad that he had to discontinue his poultry farming in 1957 when he was 78. The year before he had managed to get the inadequate old age pension. Jensen himself estimated that in the course of 40 years he had written over 400 scientific papers in the journals of learned societies and in Government publications. Compilation of his complete bibliography is 45

going to be a time consuming task. Jensen had scientific interests other than geology, such as the periodicity of volcanic and seismic activity. He had visited Savaii in the Samoan Islands in 1906 to study the effects of an enormous eruption. He was interested in meteorology, long range weather forecasting and the influence of sun spots on weather. Jensen had an extremely active and questing mind. In the thirty years from 1898 to 1928 Jensen estimates that he wrote on the average two political articles a week for Labor papers. These articles on political and economic subjects appeared in the "Australian Worker", in the "Queensland Worker" and in various other newspapers. Dr. Jenner, a prominent character in the section of Frank Hardy's "Power without Glory" that deals with the Theodore-McCormack period in Queensland politics and the Royal Commission into the Mungana scandals is, of course, Jensen. Theodore and McCormack were exonerated by the Commission. Jensen*s impressive and varied contributions to the geology, not only of Australia, but also of a number of countries in the South Pacific, should surely qualify him to be regarded as a most important figure. There has been a conspiracy of silence, one suspects, mainly because of his antiestablishment view frequently expressed vigorously, and his involvement in left wing politics. Except for one brief biographical note by A.B. Walkom, published by the Linnaean Society of New South Wales in 1925, there is a complete absence of detailed biographies or obituaries of Jensen in any Australian publication. This is in marked contrast to the detailed biographies with photographs and complete bibliographies of Linnaean Macleay Fellows, especially the older ones, that the Linnaean Society of N.S.W. publishes as a memorial series after their death, from time to time. A biography in Danish was published in a Danish scientific journal in 1967. In a published account of the Federal Government's involvement in geology, written some years ago by a very senior member of the staff of the Bureau of Mineral Resources, now deceased, there is no mention of the Northern Territory Mines Department nor of Jensen. One would strongly infer that this is a manifestation of the conspiracy of silence.

10.5 Sr A N D M g CONTENTS OF OSTRACODS USED TO D I S T I N G U I S H PAST S A L I N I T I E S , TEMPERATURES A N D ENVIRONMENTS OF D E P O S I T I O N

A.R. Chivasl, P. De Deckker^ and J.M.G. Shelley^ ^Research School of Earth Sciences, 2 Australian National University, Canberra Department of Geography, Monash University, Melbourne The molar Sr/Ca and Mg/Ca ratios of the calcitic values of non-marine ostracods are a function of these ratios in the organisms' host waters. The Mg/Ca ratio is also affected by temperature, but the Sr/Ca is largely independent of temperature. We have determined the CaCO^/H^O distribution coefficients (KD) for Sr and Mg for several species of the genera Mytilocypris, Australocypris, and Cyprideis. The determinations are based on both laboratory culture-experiments and from ostracods and waters gleaned from natural settings, principally a series of crater lakes in western Victoria. The distribution coefficients are strongly genus-dependent, e.g. KoCSr/Ca) for Mytilocypris and Australocypris is 0.082, whereas for Cyprideis the KD(Sr/Ca) is 0.488. These results have been applied to fossil ostracod shells to infer palaeosalinities for several lacustrine systems that span the last - 100,000 years. In the case of Ca^+ and HC03--rich lakes, the buffering effect of Ca^+ in both water and shell, appears to simplify the uncertainties in interpretation, so that the palaeo-Sr-content of the lake (a function of salinity in a closed lake basin) is directly proportional to the Sr content of the ostracod. Our palaeosalinity curve for the last 10,000 years in Lake Keilambete is in striking agreement to that obtained by an independent sediment grain-size technique. Results from the Gulf of Carpentaria, indicate that 40,000 yr ago a perched Lake Carpentaria was composed of freshwater with Mg/Ca - 1. Mg/Ca ratios of ostracods indicate a series of salinity increases and decreases since 40,000 yr. At some time the lake was connected to the sea as indicated by the calculated Sr/Ca and Mg/Ca of the water and the presence of open ocean Foraminifera and Ostracoda. Periods of shallow water are indicated by constant Sr/Ca but variable Mg/Ca caused by temperature fluctuations. ^^^^^^^^ ostracod The salinity and temperature estimates based on Sr and Mg contents of ostracods in a 9,300-yr-old swamp and lacustrine sequence from the Great Western Erg in the northern Sahara are in good agreement with estimates based on assemblages of diatoms and ostracods and 6'®0 of authigenic CaCOg.

1.13

GEOCHRONOLOGICAL STUDIES IN THE EASTERN ENTIA DOME, ARUNTA INLIER J.A. Cooper, G.E. Mortimer and P.R. James Dept Geology and Geophysics, University of Adelaide

A continuing geochronological programme is being carried out in the Harts Range region of the eastern Arunta Inlier. Unlike some of the northern Australian Proterozoic blocks, the grade of metamorphism and degree of deformation are such that a stratigraphy has not been unravelled and available mapping is essentially lithologically based. Hence, dating targets are primarily lithological types in structurally-defined domains. We discuss zircon U-Pb isotopic analyses on four gneissic units exposed in the bed of Huckitta Creek 0-5 km N.W. of Quartz Hill. The site is part of the Division II grouping of B.M.R. interpretation, just within the Entia Dome of the east Harts Range, but in the underlying '^basement*' infrastructure to regional overthrusting movements recognized by University of Adelaide studies. The gneissic sequence in Huckitta Creek largely reflects the lithological variation observed throughout the Entia Dome. The sequence consists of variably-interlayed compositional units of quartz-feldspar gneisses and biotite-quartz-feldspar gneiss, with garnet as a frequent accessory phase, minor calc-silicate horizons and thick interlayed to pod-shaped amphibolites. This essentially supracrustal sequence, which elsewhere in the Dome contains prominent marker horizons of (politic) kyanite-phogopite schist, is strongly foliated and lineated with a recumbent and often isoclinally folded layer parallel tectonothermal fabric. More homogeneous graniodiorite-tonalitic coarse megacrystic gneisses are considered to reflect granitoid melts injected into the supracrustal pile during the early stages of the recumbent-tectonic activity, and vary from 0-5 - 1 M. thick interleaved subconcordant sheets to the very thick layers which form the lopolithic scale Huckitta and Inkamulla Granitoid bodies. The granitoids are also foliated and lineated and isoclinally folded but, although they appear to have suffered as intense a deformation as the supracrustals, there is less complexity of superimposed deformation events. Three of the samples come from closely spaced units of a layered gneissic sequence just north of the road crossing. A gneissic granitoid sheet produces a perfect fit discordia with upper intercept of 1762 ^ 2 Ma which is within error of the excellent age determination (in prep.) of the abundant Entia Leucogranite Gneiss of the Entia Dome immediately to the west. It is slightly but significantly older than the Bruna Granodiorite Gneiss which encircles the Entia Dome and was apparently emplaced during intense thrusting. A two feldspar layered gneiss sampled specifically to detect inherited zircons gives a poor discordia fit and a short discordia line. Its upper intercept is slightly older but, due to poor line definition, cannot be distinguished from the gneissic granitoid sheet upper intercept. Thus, in this instance, no record of an ancient provenance is detected. A micaceous gneiss, of presumably more politic origin, was sampled to indicate the time of high-grade metamorphic recrystallization. In contrast to the previous two feldspar gneiss, it shows clear signs of inherited zircons; however, the excess age indication is only a few tens of millions'of years older than the age of the gneissic granitoid sheet. Hence, again no ancient crustal material has been detected. 48

A migmatitic amphibolite, which contains minor interlayers and schlieren of deformed tonalitic melt, crops out through surficial cover lOOin to the south of the creek crossing. Zircons, presumably extracted mainly from the tonalite component of the bulk rock, yield a perfect fit discordia upper intercept of 1,730 t 1 Ma. This is considered to be a crystallization age. The high precision of the dates on the granitoid sheet and tonalite allow us to place severe constraints on the geological evolution of the Entia Dome. The supracrustal sequence, containing the amphibolite, two feldspar gneiss and micaceous gneiss, must be older than 1,760 Ma , the age of the granitoid sheet. In addition, we suggest there have been at least two main phases of igneous activity at 1,760 Ma and 1,730 Ma in the supracrustal sequence. Intriguingly, the evidence of intense tectonic activity (high-grade LS fabrics and recumbent isoclinal folds) occurring at or later than 1,730 Ma (the tonalite age) conflicts with the suggestion that the 1,750 Ma Bruna Gneiss intruded synchronously with major thrusting along the basement and cover contact significantly after the deformation of the basement. More detailed work is required to resolve this inconsistency. The two perfect-fit discordia lines give young lower intercept ages of Cretaceous and Tertiary respectively, indicating the times at which episodic lead loss occurred. No unambiguous zircon response to a Palaeozoic disturbance such as the Alice Springs Orogeny is detectible at this site.

4.9

COPPER BEARING SULFIDE MINERALS IN COLLOFORM TEXTURES FROM THE PERNATTY lAGOON COPPER DEPOSITS, SOUTH AUSTRALIA Robert A, Creelman^ and George Hladky^

^School of Earth Sciences, Macquarie University, North Ryde NSW ^CSIRO Division of Mineral Physics and Mineralogy, North Ryde NSW

Upper P r o t e r o z o i c metal deposits at P e r n a t t y L a g o o n on the Stuart Shelf are hosted by the A d e l a i d e a n P a n d u r r a F o r m a t i o n , Whyalla Sandstone, and Tapley H i l l F o r m a t i o n ; all flat lying unmetamorphosed sediments. The Willouran P a n d u r r a F o r m a t i o n is a red hematite bearing quartzarenite and f o r m s the local " b a s e m e n t " of a structural high called the P e r n a t t y C u l m i n a t i o n . The W h y a l l a Sandstone is a white to buff coloured quartzarenite and the Tapley Hill F o r m a t i o n a s t r o m a t o l i t e and oolite bearing dolomite unit on the P e r n a t t y C u l m i n a t i o n , but an o r g a n o - d o l o m i t i c black shale off the C u l m i n a tion. S e d i m e n t a t i o n of the M a r i n o a n Whyalla Sandstone and Tapley Hill F o r m a t i o n was controlled by the P e r n a t t y C u l m i n a t i o n and the units frill graben and half g r a v e n structures as they onlap the paleostructural high. It would appear that the Tapley H i l l F o r m a t i o n did not cover the C u l m i n a t i o n , consequently in the mine environs the Whyalla Sandstone lies disconformably on the P a n d u r r a F o r m a t i o n . E c o n o m i c ore bodies have been found only in the arenites, but there is significant m i n e r a l i z a t i o n hosted by the Tapley Hill F o r m a t i o n in the vicinity of the orenitic-hosted ores. The largest ore body, the C a t t l e Grid, is predominantly in the Pandurra in a v a r i e t y of breccias thought to be of periglacial origin. The smaller E a s t and West L a g o o n bodies were in both the W h y a l l a Sandstone and the P a n d u r r a F o r m a t i o n . A l l are mineralogically complex a c c u m u l a t i o n s of copper with substantial quantities of zinc and lead. C o b a l t and bismuth are important a c c e s s o r y metals and there are smaller quantities of nickel, arsenic and silver. The smaller ore bodies were copper sulfide dominated with minor copper iron and lead/zinc sulfides. The C a t t l e G r i d contains the most complete and diverse m i n e r a l o g i c a l record. The body contains substantial quantities of lead and zinc sulfides that in c e r t a i n zones exceed the copper-bearing minerals. Important a c c e s s o r y minerals are pyrite, m a r c a s i t e , arseniferous pyrite, carrollite, wittichenite, and native bismuth. Silver o c c u r s within the copper sulfides. A feature of the sulfide a s s e m b l a g e s is evidence of dissolution and redeposition cycles that obliterate most of the early a s s e m b l a g e s and textures, however s y s t e m a t i c s a m p l i n g of the ore bodies allows a r e c o n s t r u c t i o n of the m a i n episodes that lead to the present mineralogy. A significant, and possibly unique set of textures are c o l l o f o r m bandings of copper-bearing sulfides. The bands consist of chalcopyrite, bornite, digenite, a v a r i e t y of covellites and there ore rare examples of galena, wittichenite and native bismuth bands. The bornite in the bands has been shown by electron microprobe analysis to be s u l f u r - r i c h bornite, a low temperature phase considered to be unstable above 75°C. In detail there are a number of features that suggest the texture is more than a simple coating of minerals on a substrate. C o a t i n g s of c o l l o f o r m copper minerals contain f r a g m e n t s of substrate sulfide, and in m a n y instances, the substrate and included copper sulfides ore replaced by s u l f u r - r i c h bornite and chalcopyrite. Syneresis-like

c r a c k i n g is c o m m o n , especially in samples f r o m the E a s t and West L a g o o n ore bodies. Thicker c o l l o f o r m bands have copper sulfide crystals on the outer edge of the sulfides. The largest c r y s t a l found was 50 m m long and 30 m m wide. Earlier interpretations of c o l l o f o r m textures held as a tacit assumption the involvement of colloids in their genesis. M o r e recent interpretations attribute their origin to a high degree of supersaturation which results in many points of nucleation and rapid c r y s t a l l i z a t i o n . Supersaturation is easy to achieve in solutions of substances with low solubilities, and the balance between conditions that produce fine banding versus crystals is delicate. S t a g n a t i o n of solution versus flow would be a major control, suggesting the c o l l o f o r m minerals are the product of local isolation of groundwaters in the ore bodies and subsequent g e o c h e m i c a l change of the waters. B a s e d on available t h e r m o c h e m i c a l data and studies of water g e o c h e m i s t r y in the area, a t h e r m o c h e m i c a l model involving Eh, pH, total sulfur, the a c t i v i t y of iron and copper in solution has been erected. A total of 37 equations can be written to fully define the system, but these can be reduced to 11 in the face of geological realities. The presentation of the multidimensional data is difficult, and a s y s t e m of e l e c t r o n a c t i v i t y versus copper a c t i v i t y plots for various conditions of total iron, total sulfur and p H has been adopted. The model demonstrates that prime influence changing the phase being deposited is electron a c t i v i t y . The s y s t e m is relatively insensitive to total iron and total sulfur, and the influence of p H slight. It is postulated that changes in E h are brought about by blooms and decays in the a c t i v i t y of sulfate reducing bacteria. In stagnant conditions reduction can become intense enough to reverse the loss of iron f r o m copper iron sulfides, hence the replacement reactions on the copper sulfides coated by the c o l l o f o r m textures. B a c t e r i a l reduction in stagnant conditions is self limiting as nutrient and sulfate is consumed. A flushing of the s y s t e m would appear to be necessary to reset the depositional cycle. The c o l l o f o r m textures are therefore a s s o c i a t e d with present and near present-day groundwater g e o c h e m i s t r y and configurations.

4ai

SYNDEPOSITIONAL FAULTS, ADELAIDE GEOSYNCLINE R. Dalgarno Consulting Geologist, Adelaide

The Flinders Ranges in South Australia offer exposures of Late Adelaidean to Mid Cambrian sediments with contemporaneous extensional faults showing small scale but similar geometries to high extension basins such as the North Sea. Small scale listric faults with associated rider faults^ antithetic structures and rollover anticlinal features are observed in the Heysen Super Group and Hawker Group on the PARACHILNA 1:250 000 Sheet Area. The sole faults are listric in character and appear to be detachments at the contacts with diapiric anticlines of Callanna Group sediments which are apparently folded disharmonically with respect to the Late Adelaidean and Cambrian. Major northerly and NNW trending longitudinal faults of probable deep basement origin are inferred from surface structures and from the disposition of diapiric bodies. These may represent very large scale crustal ramp structures with horsetailing synthetic fans as has been interpreted from deep seismic studies of the North Sea Basin^ e.g. trend through Baratta^ Oraparinna to Blinman. Cross-trends with ENE orientation are frequently syndepositional structures and may be transfer faults^ e.g. the 080 belt from Parachilna through Blinman and Wirrealpa Diapirs to Mt. Chambers. This appears as a right-stepping en echelon pattern^ of which some elements were significant growth faults and hinge zones during Early Cambrian deposition. This trend may be interpreted as one of a system of transfer faults resulting from differential subsidence rates within the Adelaide Rift. Wirrealpa and Frome/Mt. John diapiric trends were eroded as "bald cap" structures during the Early Cambrian. The resultant hard pans and conglomerates illustrate several cycles of reworking. Oversteepening on the flanks of the diapirs has resulted in local angular unconformities. Re-intrusion of diapirs has resulted in stoped blocks of the Early Cambrian sequence which have foundered into the plastic diapiric cores and remain enveloped by breccia derived from the Early Adelaidean Callanna Beds. High level diapiric bodies in the Cambrian sequence tend to show quite different features to deep structural sections such as the Worumba Anticline. Thus at Wirrealpa, uplift during the Cambrian was apparently focused on marginal faults while the crestal area was being eroded. At nearby Mt. John a gently folded, lozengeshaped syncline of Cambrian limestones rests with apparent unconformity on the crown of the diapiric body. It is hoped that in the next decade deep seismic information will lead to better interpretation of the crustal structure beneath the Adelaide Geosyncline. At this time, inferences must be drawn from the patterns of deformation of the Adelaidean and Cambrian sequences.

3.7

POLYPHASE TECTONOTHERMAL EVOLUTION OF THE GRENVILLE FRONT, EASTERN CANADIAN SHIELD

R.D. Dallmeyer Department of Geology, University of Georgia, Athens, Georgia, USA P r e c a m b r i a n r o c k s of the C a n a d i a n S h i e l d may be divided into s e v e r a l lithot e c t o n i c p r o v i n c e s which are g e n e r a l l y b o r d e r e d by r e l a t i v e l y narrow z o n e s of s t r u c t u r a l t r a n s i t i o n . The G r e n v i l l e Front (GF) r e p r e s e n t s one of the most c o n t i n u o u s of these z o n e s , e x t e n d i n g c . 1900 km from t h e Labrador C o a s t to the n o r t h e r n shore of Lake H u r o n . It s e p a r a t e s the G r e n v i l l e P r o v i n c e from v a r i o u s older t e r r a n e s to the n o r t h w e s t . The GF d i s p l a y s marked v a r i a t i o n s in c h a r a c t e r along its l e n g t h . In p l a c e s it is c h a r a c t e r i z e d by d e v e l o p m e n t of ductile shear z o n e s with p e n e t r a t i v e l y d e v e l o p e d L S - f a b r i c s s u g g e s t i n g large scale t h r u s t i n g . E l s e w h e r e the GF is only r e p r e s e n t e d by a steep m e t a m o r p h i c g r a d i e n t . The c h r o n o l o g y and r e g i o n a l e x t e n t of Grenville age t h e r m a l and d e f o r m a t i o n a l e v e n t s along the GF has p r e v i o u s l y been poorly c o n s t r a i n e d . H o w e v e r , recent c o l l a b o r a t i v e field and ^ ^ A r / ^ ^ A r i n c r e m e n t a l - r e l e a s e g e o c h r o n o l o g i c a l s t u d i e s have been c o m p l e t e d at four l o c a t i o n s along the G F . R e s u l t s of t h e s e integrated p r o g r a m s have revealed a c o m p l e x , p o l y g e n e t i c e v o l u t i o n for the G F . The GF has been examined at l o c a t i o n s w h e r e it s e p a r a t e s the s o u t h e a s t e r n G r e n v i l l e P r o v i n c e from: 1) A r c h e a n r o c k s of the S u p e r i o r P r o v i n c e near C h i b o g a m a u , Q u e b e c ; 2) A p h e b i a n r o c k s within the L a b r a d o r T r o u g h near Labrador C i t y ; 3) 1800 Ma c r y s t a l l i n e s e q u e n c e s within the C h u r c h i l l P r o v i n c e near C h u r c h i l l F a l l s ; a n d , 4 ) 1 5 0 0 - 1 8 0 0 Ma rocks w i t h i n the M a k k o v i k P r o v i n c e in the Smokey a r c h i p e l a g o on the e a s t e r n Labrador C o a s t . R e s u l t s from these four widely s c a t t e r e d a r e a s show very c o n s i s t e n t t r e n d s . H o r n b l e n d e within kyanite and h i g h e r grade rocks r e c o r d w e l l - d e f i n e d p l a t e a u a g e s which range b e t w e e n c . 900 and 1000 M a . These s y s t e m a t i c a l l y d e c r e a s e from kyanite into higher grade a r e a s and are i n t e r p r e t e d to date d i a c h r o n o u s p o s t - G r e n v i l l e m e t a m o r p h i c c o o l i n g t h r o u g h t e m p e r a t u r e s a p p r o p r i a t e for i n t r a c r y s t a l l i n e retention of '^^Ar (c. 500°C). M u s c o v i t e from these a r e a s y i e l d s c . 20-25 Ma younger d a t e s than c o e x i s t i n g h o r n b l e n d e , p r e s u m a b l y r e f l e c t i n g the s l i g h t l y lower t e m p e r a t u r e s required for i n t r a c r y t a l l i n e argon r e t e n t i o n (c. 425°C). In marked c o n t r a s t , b i o t i t e plateau a g e s range from c . 8 3 5 Ma to 2100 Ma s u g g e s t i n g w i d e s p r e a d i n t r a c r y s t a l l i n e c o n t a m i n a t i o n with e x t r a n e o u s argon c o m p o n e n t s . H o r n b l e n d e within g a r n e t g r a d e t e r r a n e s d i s p l a y s i n t e r n a l l y d i s c o r d a n t r e l e a s e spectra i n d i c a t i v e of l o w - t e m p e r a t u r e e x p e r i m e n t a l e v o l u t i o n of e x t r a n e o u s argon c o m p o n e n t s . H i g h - t e m p e r a t u r e p l a t e a u x are locally def i n e d , and y i e l d ages of c . 1 2 5 0 - 1 3 0 0 M a . These are i n t e r p r e t e d to date cooling f o l l o w i n g an early [Mj^) p h a s e of G r e n v i l l e t e c t o n o t h e r m a l a c t i v i t y . M u s c o v i t e from garnet g r a d e rocks r e c o r d s plateau d a t e s of 9 5 0 - 1 1 2 5 M a . These are i n t e r p r e t e d to date c o o l i n g f o l l o w i n g the l a t e r , Mg t h e r m a l e v e n t . B i o t i t e from g a r n e t grade rocks again d i s p l a y s widely v a r y i n g plateau d a t e s s u g g e s t i v e of w i d e s p r e a d e x t r a n e o u s argon c o n t a m i n a t i o n .

6.15

EVOLUTION OF THE CARPENTARIAN CLYDE R I V E R S U B - B A S I N , McARTHUR BASIN, N.T. S . A . Dashlooty^ and G . J . D a v i d s o n ^ ^Billiton A u s t r a l i a , A d e l a i d e ^University of T a s m a n i a , Hobart

T h e C l y d e R i v e r s u b - b a s i n e x i s t e d as a discrete sedimentological identity during the deposition of the Barney Creek Formation ( B C F ) w i t h i n the B a t t e n T r o u g h of t h e C a r p e n t a r i a n M c A r t h u r B a s i n , N . T ^ S e d i m e n t a t i o n was contemporaneous with the deposition of base metal s u l p h i d e s in the B u l b u r r a Depression to the north along the Emu F a u l t . The area of interest is overlain b y u p to 80 m of C a m b r i a n B u k a l a r a S a n d s t o n e . P h o t o - l i n e a m e n t s t u d i e s , gravity m e a s u r e m e n t s , input E . M . and diamond drilling undertaken d u r i n g b a s e m e t a l s e x p l o r a t i o n , p r o v i d e d d a t a for a s t u d y of the s e d i m e n t a t i o n a n d tectonic development of the carbonaceous B C F , outlining a 20 x 7 km t r o u g h . Shallow to sub-aerial hypersaline dolomites of the C o x c o D o l o m i t e p r e c e d e d the d e p o s i t i o n of the B C F . T h e two u n i t s a r e s e p a r a t e d by the dolomitic p o t a s s i c W - F o l d Shale m e m b e r , c o n t a i n i n g a b u n d a n t a i r f a l l and d e t r i t a l v o l c a n i c m a t e r i a l . The transition to euxinic basin conditions favourable for t h e d e p o s i t i o n of the H Y C P y r i t i c S h a l e m e m b e r o c c u r r e d w i t h i n 10 m o f sequence and is preserved in a "transition zone" (local n a m e ) containing both W-Fold and HYC m a t e r i a l . A tectonic origin for the rapid environmental change is favoured because it corresponds to: 1) a peak in volcanic d e t r i t u s , 2) dip d i s t u r b a n c e s a c r o s s the b a s i n , and 3) b e c a u s e s h a l l o w w a t e r c o m p o n e n t s c o n t i n u e d to s h e d f r o m the n e a r b y t r o u g h m a r g i n , i . e . l i t t l e r e l a t i v e s h o r e l i n e m o v e m e n t . Subsidence occurred along NNE and NNW faults (presently traceable as lineaments on the B u k a l a r a S a n d s t o n e s u r f a c e ) c r e a t i n g s e v e n s e p a r a t e b l o c k s . Initially this deepening occurred in the north and south of the sub-basin with a horst in b e t w e e n . Subsequently, c a r b o n a c e o u s H Y C s h a l e d e p o s i t i o n b e c a m e w i d e s p r e a d a t t a i n i n g a m a x i m u m t h i c k n e s s of 70 metres against the Emu Fault (this compares with 500 m in the Bulbarra D e p r e s s i o n ) . A t e c t o n i c e v e n t r e s u l t i n g in r e g i o n a l change of block attitudes may have b e e n r e s p o n s i b l e for a r e t u r n to c l a s t i c d o m i n a t e d p r o c e s s e s , p o s s i b l y t h r o u g h r e n e w e d b a s i n c i r c u l a t i o n . In t h e northern part of the s u b - b a s i n , several subsidence events caused local folding and d e c o l l e m e n t c l o s e to t h e Emu F a u l t . The early-developing horst continued to separate s e d i m e n t a t i o n , as e v i d e n c e d by a c o a r s e n i n g - u p w a r d turbidite sequence that developed only in t h e n o r t h . To the s o u t h , the a c i d t u f f c o m p o n e n t i n c r e a s e d s o u t h w a r d s m a r k e d l y u n t i l 17% of the t o t a l s e q u e n c e is v o l c a n o c l a s t i c , i m p l y i n g a proximal volcanic s o u r c e . The potential of the BCF as a h y d r o c a r b o n s o u r c e w a s h i g h l i g h t e d by a g a s b l o w - o u t d u r i n g e a r l y A m o c o b a s e m e t a l s d r i l l i n g . W i t h i n the B C F , hydrocarbons notably occur as bitumen within p o r o u s a r e n i t e s a n d as f l e c k s within the upper layers of t u f f . T h e a r e a w a s e r o d e d b a c k to B C F l e v e l p r i o r to the C a m b r i a n by regional reverse movements on the Emu and s e c o n d a r y f a u l t s o u t l i n e d in t h i s s t u d y . E r o s i o n w a s d i f f e r e n t i a l w i t h greatest vertical movements occurring in the south and w e s t . T h i c k n e s s d i f f e r e n c e s in the B u k a l a r a S a n d s t o n e s u g g e s t differential tectonic uplift has continued to the present d a y .

14.1 SOURCE MECHANISMS OF LARGE AUSTRALIAN EARTHQUAKES AND REGIONAL INTRA-PLATE STRESSES David Denham^ and Robert McCaffrey^ ^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^MIT, Cambridge, Massachusetts, USA

We have determined the source mechanisms and depths of the six largest intra-plate earthquakes from the Australian region during the last 25 years by inversion of long period P and SH waves. All these events occurred in the western half of the Australian continent, apart from the 1983 Tasman Sea earthquake which took place within oceanic lithosphere. All the source mechanisms require a P axis close to horizontal and, although there is some scatter, the average direction of the axes is close to eastwest. These mechanisms are therefore consistent with the results obtained from smaller earthquakes in south and southeastern Australia and with in-situ stress measurements and borehole deformation observations. It therefore appears that the eastern part of the Australian Plate is being compressed in an east-west direction. The earthquakes examined varied in size from Nm (1972 Simpson Desert) to 19^^Nm (1968 Meckering) and each earthquake mechanism was satisfactorily modelled by a point source with time functions varying from 1 to 10s in duration. The four onshore earthquakes were all shallow (^10 km) and were associated with almost pure thrust faulting. The Meckering and 1979 Cadoux events generated fault scarps consistent with the focal mechanisms, and the long duration of the Meckering event (10s) implies a rupturing of the crust to a depth of almost 15 km. The 1970 Canning Basin and 1972 Simpson Desert events took place in remote areas covered by unconsolidated Quaternary sand and no fault scarps have been observed. The offshore earthquakes - 1979 Broome and 1983 Tasman Sea - were deeper (29 and 25 km respectively) and the Broome earthquake was associated with strike slip faulting along a northwest trending fault.

17.2

THE CRACOW GOLD RUSH

A.K. Denmead and E.N. Denmead Christian Street, Clayfield, Queensland During the Great Depression which began in 1929 the Queensland Government encouraged unemployed men to go prospecting by giving them a weekly allowance of $3 ($4, if they were marked) and equipping them with a pick and shovel. Most of the men who accepted the offer had had no previous experience in prospecting, and, not unexpectedly, the Mines Departmnt was inundated with claims of gold discoveries, one of which was near the Cracow Cattle Station, some 400 kilometres north-west of Brisbane. The current geological map showed Cracow to lie within the Great Artesian Basin, an impossible location for gold occurrence, so, in April 1931, they sent their least experienced geologist, A.K. Denmead, to inspect the prospect. On arrival at the site I learned that our map was inaccurate, though there were some outliers of basal Artesian Basin sediments. There were numerous quartz veins in Permian andesite and by sampling I was able to confirm the existence of a small shoot of quite high-grade ore. Following an announcement by the Minister for Mines dozens of prospectors (some with their families) converged on the discovery site - this was the start of the rush. On my second visit a few months later only one further discovery (and that of low grade) had been made. The new arrivals included two unemployed men from Taroom - John Mohr and George Ryan. Finding that all known quartz outcrops had been pegged, they applied for a prospecting area covering a sandstone-capped plateau west of the original discovery. On inspection I found a few angular quartz chips, some of which, on crushing and panning, showed colours of gold. At my suggestion they dug a trench and revealed a body of quartz over 60 feet wide. Sampling revealed a section 12 feet wide asaying 4 V2 pennyweights of gold per ton, much higher than V2 pennyweight average revealed by my sampling of outcrops away from known enrichments. Thus was discovered the only really worth-while mine on the Cracow field - THE GOLDEN PLATEAU. On my next visit to Cracow, in June 1932, I learned that several new discoveries had been made, including one rich intersection in Mohr and Ryan's leases. By now mineral—seeking organizations were starting to take an interest in the field. They included Carpentaria Prospectors Ltd., founded by Randolf Bedford, M.L.A., and an influential Collins House (Melbourne) group whose field officer, Tom Victor, happened to be in Queensland at the time. Bedford acquired Lambert's original find, and floated a new company. Golden Mile N.L., to develop this and other prospects. Tom Victor and Randolf Bedford fought a running battle to acquire Mohr and Ryan' s leases. Tom Victor eventually won and a new company. Golden Plateau N.L., was floated. I returned to Cracow four months later with instructions to make a geological map of the entire area covered by prospecting and mining titles, and to sample all known prospects, which by this time measured some 25 to 30 square miles. Obviously a topographic map was needed on which to plot the geology. My request to hire a chainman was refused. A few months earlier I had married a graduate geologist and she agreed to come to Cracow with me. When we arrived there I conned her into acting as my chainman... — OVER TO E.N. DENMEAD. 56

The following comments concern what might be called the "domestic" side of our visits to Cracow i.e. the experiences of a city dweller living for a time in a totally different environment. On approaching Cracow, one passed through "suburbs" of tents and shacks, set up in a haphazard manner amongst the trees. The main street was easily identifiable by the fact that the buildings, in the main, were in line. They were almost entirely iron and canvas structures, all but a few having only dirt floors and none with glass windows. One of these "buildings" was a small, iron hut, measuring 10ft x 12ft. One feature which distinguished it from most others was a wooden floor, and a door that could be locked. Luckily we were able to rent it and so it became our home for 6 weeks. By this time the town had a number of shops, cafes, grocery stores and a milkman (2 rounds a day). A doctor, a dentist and a chemist were all resident. One of the hardest things to adjust to was the shortage of water which had to be carted 10 miles from the Dawson River. One result was great difficulty in having baths or showers. Our private "shower room" was canvas rigged round a small clump of saplings. The water was hauled up in a tin with a rose at the bottom, and operated by pulling a string which released the water. By careful usage, we managed to have a shower on a dipperful of water. All sorts of conditions of vehicles were to be seen, mostly so rusty and frail, it was amazing that they ever reached their destination. Some had come a long way e.g. from Port Lincoln and Beechworth. I visited Cracow 3 times with Alan, twice in midsummer and once in midwinter. On the 2nd visit, we stayed at Biddle's Boarding House, where conditions were extremely primitive e.g. there were no bathing facilities whatever. We were advised to use the public baths, which consisted of shower cubicles with no doors or curtains, so we traded on the kindness of several friends we had made for their generous gesture of friendship "Come and have a bath". On the 3rd occasion, we were fortunate enough to hire a 2-roomed cottage which, however, was infested with hordes of fleas and cockroaches. However, it did have the advantage of water laid on (a tank) and we could bathe as often as we liked, in a round tub on the back verandah. While there, I acted as Alan's chainman for several weeks and found the experience of doing some original geological mapping quite exciting.. — BACK TO ALLAN. By 1937 the rush had just about fizzled out, though a few small mines, notably Rose's Pride, were still producing. The Golden Plateau mine, under the highly efficient management of Howard Miller, continued to expand. When it finally closed down in 1976 the Golden Plateau mine had produced 592 440 fine ounces of gold and 654 000 ounces of silver from 1 466 122 tons of ore treated, and the Company had paid $4 298 301 in dividends on a paid up capital of $228 500, a return of over 18 times the money invested. I think it will be agreed that the discovery that led to Australia's most recent rush was well worthwhile.

6.2

SEISMIC EVIDENCE FOR DNDERPLATING OF THE AUSTRALIAN CRUST B.J. Drummond and C.D.N. Collins

Bureau of Mineral Resources, Geology and Geophysics, Canberra Seismic velocity/depth models from eight geological provinces in Australia and Papua New Guinea were adjusted for the effects of pressure and temperature so that their average crustal velocities could be compared. The velocities show a positive correlation with crustal thickness - thick crust has a higher average velocity than thin crust. For example, crust 30 km thick has an average seismic velocity of 6.5 km/s, and crust 50 km thick has an average velocity of 6.7-6.8 km/s. The upper 30 km of the crust in all provinces has an average velocity of 6.4-6.5 km/s (neglecting sediments), so that the increase in average crustal velocity with increasing crustal thickness must occur in the lower crust. The velocity in the lower crust is highest in areas with thick crust. For example, in areas where the crust is 30 km thick, lower crustal velocities seldom exceed 7.0 km/s, but in crust 50 km thick they can exceed 7.5 km/s. Note that all of these velocities are for the conditions of 1 GPa and 25°C. With the exception of Archaean provinces, the average seismic velocity is also seen to increase with the age of the province. Proterozoic and lower Palaeozoic provinces have thicker crust with higher seismic velocities than Mesozoic provinces. Seismic velocities in young provinces are consistent with crust which is mainly felsic in nature; those in old crust imply a more mafic character and higher metamorphic grade. The implication is that the crust thickens and becomes dense with time. This can be achieved by crustal shortening accompanied by metamorphism and partial melting in the lower crust, with the lighter melt fraction removed upwards and eroded. However, after allowing for the effects of isostatic rebound and erosion, the amount of shortening required to produce the observed effects is extreme. For example, shortening by a factor greater than 5 is necessary to produce crust 50 km thick from crust that was originally 25 km thick. This is an extremely large amount of shortening to invoke in all areas where thick, dense old crust might have been produced by shortening of thin young crust. It would lead to high metamorphic grades being observed everywhere at the surface. In addition, the seismic velocities that can be achieved by the extreme metamorphism of thin felsic crust are not high enough to account for the observed velocities at the base of thick crust. Considerable amounts of mafic material need to be added to the lower crust to achieve the high A7"elocities. The addition of mafic material to the lower crust is accomplished mainly by underplating of mafic magmas. Old provinces with thick crust are therefore more underplated than young provinces with thin crust. Pristine Archaean blocks seem to be an exception to this trend; they do not seem to be underplated. Rather, post-Archaean tectonism affected the mobile belts around their margins, but just as it did not affect their upper crustal geology, it did not underplate their lower crust.

14.6

CRUSTAL STRUCTDRE IN THE SOUTHWEST SEISMIC Z O N E , WESTERN AUSTRALIA 1 2 B . J . Drummond^ and Rokiah Esa Mohamed

^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Western Australian Institute of Technology, Perth The Southwest Seismic Zone is defined by a belt of earthquake epicentres approximately 200 km wide and 500 km long trending in a north-northwest/south-southeast direction across the southwestern portion of the Archaean Yilgarn Block. It is oblique to the Darling Fault and no causal relationship seems to exist between the Darling Fault (and therefore the origin of the Perth Basin), the margin of the continent (and therefore the breakup of Gondwana), and the contemporary stresses evidenced by the seismicity. The proximity of the Southwest Seismic Zone to metropolitan Perth means that the risk of damage caused by large earthquakes in the Southwest Seismic Zone is considerable. The region has therefore become an area of increasing study. As part of that study, the Bureau of Mineral Resources recently conducted a seismic refraction survey in the region. The aims of the seismic survey were to develop crustal models of the region as an aid to accurate earthquake location, to study structures in the crust to see if they had any link with the earthquakes, and to develop structural models of the Archaean crust as part of a study of Archaean crustal evolution. Two seismic profiles were recorded. One trended northwest/southeast, approximately parallel and adjacent to the boundary between the Western Gneiss Belt and the Southern Cross geological provinces. This line also transected the region where most of the recent seismic activity has occurred. It was bisected by the second line which trended northeast/southwest and was oriented approximately at right angles to the regional geological strike. The crust in the southern Yilgarn Block to the east of the Southwest Seismic Zone is two-layered. Seismic velocities in the upper crustal layer range from 6.1 km/s at the top of the layer to 6.25 km/s at the bottom of the layer at 10-12 km depth. In the lower crustal layer, velocities increase from 6.35-6.55 km/s at the top to about 6.8 km/s at the base of the crust. The boundary between the crustal layers is transitional over several kilometres, and the crust/mantle boundary is transitional over 5-10 k m . The crust is 30-35 km thick. The current data set is insufficient to define in detail the structures in the crust to the southwest of the Southwest Seismic Zone, but the crustal velocities in the southwest appear to be higher. The lateral change in crustal velocities occurs over a narrow zone. In the velocity range 6-8 km/s, velocity is approximately proportional to density, so that regional gravity data can be used to trace the position of the lateral velocity change to the north and south of the seismic line. In the south, it corresponds to the southern part of the Southwest Seismic Zone and it lies along the western margin of the most recent seismicity in the n o r t h . It is approximately parallel to and lies just to the west

of the b o u n d a r y between the Western Gneiss Belt and the Southern Cross Province. In p l a c e s , the lateral velocity (density) change is quite a b r u p t , occurring over only a few kilometres; in other places it is more gradual, and may occur across a zone 20-30 km w i d e . The W e s t e r n Gneiss Belt contal-is outcropping granulites w h i c h w e r e u p l i f t e d from mid-crustal levels in the A r c h a e a n , and the geomorphology suggests that uplift may also have also occurred in recent times. The earthquake belt' therefore corresponds broadly to a lateral change in crustal seismic velocities which in turn correlates with the b o u n d a r y of an ancient zone of u p l i f t . The seismicity therefore seems related to recent u p l i f t along an ancient crustal feature.

4.12

THE WONOKA FORMATION IN TBE FORTRESS HILL AREA, NORTHERN FLINDERS RANGES

K. H. Eickhoff Flinders University, Adelaide In the Flinders Ranges, South Australia, there are four adjacent major erosional incisions which were cut during the late Proterozoic and are part of the Adelaidean Geosyncline. The incisions are filled by a series of debris flows and turbidites which were deposited during Wonoka Formation time. More than 1000 metres of Siltstone and fine Sandstone of the Brachina and Bunyeroo Formation have been eroded during the process of canyon cutting. The sedimentary record for each of the incisions is almost identical while palaeocurrents taken from flute casts at the bases of turbiditic sandstone beds alternate by close to 180° from one incision to the next. All four incisions are interpreted to be part of the same ancient meandering submarine canyon system. The final Thalweg channel is invariably located next to the north wall of each east-west oriented incision and is outlined by locally outcropping lenticular and channelised sandstone beds with sharp bases. These are interbedded with submarine debris flows showing a variety of siliciclastic and calcareous clasts. Some of the calcareous clast types show evidence of syntransportational ductile deformation. A striking example is provided by armoured mudballs which occur at the base of the oldest debris flow. They have a dolomitic matrix and are armoured with pebbles of mature quartzose sandstone. Sediments in the final Thalweg channel may show slumps up to several hundred metres wide which occasionally incorporate Brachina clasts derived by slumping from the adjacent canyon walls. These Brachina clasts show a well developed jointing system which predated the slumping. Palaeocurrent directions obtained from current ripples are scattered more widely and have a more northerly trend than correlated flute casts. The slower currents depositing the current rippled sands show more deflection down the north-dipping palaeoslope than the purely erosive and more rapid currents that scoured the flutes. In general the channels are filled by an upwards fining sequence of siliciclastic sands which contain isolated pelloidal calcareous beds in the upper part. The channel levee zone is a broad, often hardly identifiable feature that is commonly breached by channel sands. It shows abundant low angle climbing ripples and small (cm scale) slumps. The central part of the incisions is filled by parallel laminated silts and fine sands which show small scale water escape structures and varying palaeocurrent directions obtained from crossbedding in isolated sandy beds. This facies is interpreted to represent the fine overbank material related to the coarser sands in the final Thalweg. Interfingering with this facies is a submature to mature mica-rich sand deposited next to the southern walls. This sand typically shows a wavy parallel,lamination and current ripples. Current directions obtained from the ripples show a wide scatter with a maximum for currents flowing to the northeast. This sequence is interpreted as a canyon wall overspill sand. Large slumps of Brachina Formation material commonly separate these sands from the incision wall composed of Brachina Formation. The canyon covering sequence is composed of a series of fine sands to coarse silts. Interbedded calcareous layers become more frequent and the sequence eventually passes into a turbidite series showing rythmitic interbedding of fine arenitic calcareous pelloid layers and siliciclastic silts. 61

Palaeocurrents obtained from current ripples show palaeodirections to the north east in the lowermost canyon cover while for the rythmitic series a current pattern with a maximum to the north west is evident. The rythmitic sequence is covered by a series of parallel laminated siltstones which have a calcareous cement and build the uppermost part of the Wonoka Formation in the Fortress H i l l area. The contact with the overlying Billy Springs Beds is usually gradational and marked by abundant slump rolls of several metres in height. The Billy Springs Beds are dominated by fine sandstones and coarse siltstones w h i c h , in contrast to the underlying Wonoka Formation siltstones do not have a calcareous cement component.

1.3

THE MT. KEITH SOUTH NICKEL SULPHIDE DEPOSIT - W.A. A ffiOPHYSICAL CASE STUDY P.J. Elliott Billiton Australia, Adelaide

The Mt. Keith South Prospect is located approximately 500 kms north of Kalgoorlie and 80 km north of Agnew, Western Australia. The prospect is covered by a Joint Venture between Cliffs International Inc (51%), Charterhall Mining Exploration Pty Ltd (25.95%), Hartogen Energy Limited (13.07%) and Greenbushes Tin Limited (9.98%). Cliffs International are the present operators. The ore-zone consists of massive pyrrhotite-pentlandite (NiS) which, together with its host rock, is weathered to an average depth of 80 metres. It varies in thickness from less than 0.3 m to about 10 metres. It has a shallow plunge to the south and occurs as large interconnected lenses which thicken and thin along strike. Sulphides have been intersected at depths over 500 metres. The mineralization is over a kilometre in strike length with an approximate north-south trend. It is flanked on the west by an ultramafic rock suite which has undergone hydrothermal alteration, including carbonitization, talc-magnesite alteration and chloritization. To the east is a sequence of altered meta-sediments and meta-volcanics. Three currently favoured TEM (Transient Electo-Magnetic) systems were tested over the Mt Keith South nickel sulphide ore-deposit during 1982 and 1983. The systems used were: Sirotem Mkll (CSIRO); EM37 (Geonics); and UTEM III (University of Toronto Electro-Magnetic System, Lamontagne). The Sirotem surveys incorporated both moving loop and fixed loop modes. All loop configurations used clearly resolved the ore-zone. The 200 m separated loop mode produced a particularly strong response over the mineralization. The fixed loop mode enabled many of the physical parameters of the ore-zone to be resolved. The EM37 fixed loop surveys clearly resolved the ore-zone using a 2.5 hertz basic pulse frequency. However the 25 hertz basic pulse frequency did not allow measurements to be taken to late enough times to fully define the electro-magnetic response of the ore. The UTEM survey also defined the ore-zone and showed that the mineralization could be clearly detected with a transmitter loop over 300 m away. The UTEM system appears to be able to detect the electro-magnetic response of the ore-zone at earlier times than the other two systems with the transmitter loop well back from the target. All three TEM systems used in this program produced a well defined anomaly over the Mt Keith South ore-zone. The Sirotem system in particular could be used in areas where the overburden conductivity and/or thickness are much larger. The UTEM system also is well suited to exploration for this type of target.

15.1

BUSHVELD COMPLEX, SOUTH AFRICA:

RESULT OF SIMULTANEOUS IMPACTS?

W.E. Elston^ and D. Twist^ ^University of New Mexico, Albuquerque, NM 87131, USA ^University of Pretoria, Hillcrest, Pretoria, South Africa

Since 1957, U.S. and Soviet space programmes have stimulated awareness of Earth as an astronomical, as well as a geological, object. The search for scars ("astroblemes") from collisions between Earth and asteroids or comets has been successful; the Sudbury and Bushveld Complexes are among proposed (but controversial) Proterozoic astroblemes with great mineral potential. Questions remain: Is the impact hypothesis reasonable for the cluster of Bushveld basins, collectively 400 km in diameter? What are the criteria for identifying the largest astroblemes? Like Moon, Mars, and Mercury, Earth presumably underwent a high rate of post-accretional bombardment and impact cratering, which had decayed exponentially by the early Archaean, about 3500 Ma ago. Since then, the rate of collisions probably approached steady state. Astrogeologists like E.M. Shoemaker have calculated that the number of Earth-crossing asteroids lost by collision with planets approximately balances the number created through inter-asteroidal collisions and propelled into Earth-crossing orbits by gravitational perturbations of Jupiter and Saturn. The smaller number of Earth-crossing comets lost by collision or ejection from the solar system is probably replenished by comets ejected from the Oort cloud by gravitational effects of passing stars. Astrogeologists have applied uniformitarian principles by telescopically observing orbits of Earth-crossing objects (49 discovered by 1983), calculating probabilities of collisions, and comparing results with the cratering records of Moon, Earth, and other planets. In the words of G.W. Weatherill and E.M. Shoemaker, "during ... Archean, Proterozoic, and Phanerozoic times, impacts of extraterrestrial bodies up to tens of kilometers in diameter should be thought of as ordinary and inevitable phenomena of a well-behaved modern solar system." Astroblemes formed explosively from the kinetic energy of such collisions have diameters many times greater than the impacting bodies. They can be identified by characteristic geological structures and by evidence of shock metamorphism and heat- or shock-induced melting of situ and ejected target rocks. In the largest, deep fracturing can be expected to induce mantle-derived gabbroic magmatism. The rate at which craters ^ 10 km in diameter form on continents is about 3.4 ± 1.7 per 10^ km^ in 10^ years. Three craters ^100 km are known and several more are suspected. By extrapolation, Bushveld-sized features probably exist but are rare; their identification is more problematic. The Bushveld cluster and Vredefort dome of South Africa are probably related, because they are unique, coeval (2100 Ma, within uncertainties of radiometric dating), in close proximity, and represent regions of intense episodic deformation in an otherwise stable craton. As at Sudbury, the impact origin of the Vredefort dome, proposed by R.A. Daly in 1947, is supported by shock metamorphism, shatter cones, and pseudotachylite dykes in the basement. The shallower Bushveld basins lack

these features; nevertheless, their impact origin was proposed by Robert Dietz, Warren Hamilton, and R.C. Rhodes. Clustering of Bushveld-Vredefort astroblemes could be explained by impact of a weakly coherent body, e.g. an asteroid fragmented by earlier inter-asteroid collisions. Such an object would be disrupted by Earth's gravity prior to impact. According to the impact hypothesis, the Bushveld following stages:

complex formed

in the

Sedimentation in stable intracratonic basin (Pretoria Group).

Multiple impacts: Excavation of Bushveld-Vredefort basins, extreme local deformation, ejection of shock - and heat - induced crustal melts (lower Rooiberg Felsite), elastic rebound forms central uplifts.

Deep fracturing, multiple gabbro pulses from partial mantle melts, differentiation during resumed tectonic quiescence (cumulate layers continuous over hundreds of km); metamorphism (to 900®C, 4-5 kb) obliterates shock metamorphism.

Crustal anatexis, water-magma interactions: Explosive volcanism (multiple upper Rooiberg rheoignimbrites with accretionary lapilli zones, megabreccias, clastic interbeds); intrusion of Bushveld granite.

Basin filling by Loskop Group.

Interpretations hinge critically Rooiberg Felsite (or impactite?):

the characteristics

the

dacitic

Volume >300,000 km3 no known source(s).

(the largest siliceous volcanic mass on Earth),

Temperature unusually high, viscosity low: Quench textures (acicular crystals of plagioclase, opx, quartz pseudomorph after tridymite); amygdaloidal zones; gradations from ignimbrite to lava ("rheoignimbrite").

Chemistry resembles subgreywacke more than felsite (high Fe*; high but variable MgO; low alkalies, esp. Na20), several magma types but no simple differentiation trends or links to gabbro.

Textural gradations from sandstone xenoliths to felsite; disequilibrium crystallization (normative plagioclase more calcic than sparse phenocrysts).

High-energy environment: Quartzite xenoliths to 50 m, especially abundant above intensely disturbed (tightly folded, boudinaged, shattered) basement in central uplifts (elastic rebound?).

Impact deserves to be included among multiple working hypotheses for the origin of the enigmatic Bushveld Complex.

8.10

FLUID INCLUSION STUDIES IN CARBONATE HOSTED LEAD-ZINC MINERALIZATION, CANNING BASIN, W.A. H. Etminan and I.B. Lambert Baas Becking Geobiological Laboratory, Bureau of Mineral Resources, Canberra

Fluid inclusion studies are being conducted as part of a wide-ranging research program aimed at elucidating the diagenetic and mineralizing processes in Devonian carbonate complexes of the northemCanning Basin. The Pb-Zn mineralization of Wagon Pass is concentrated in a semi-massive lens in fore-reef facies of the Frasnian and Famenian Virgin Hills Formation. The country rocks are dolomitized and chloritized and contain late calcite spar. At Pillara Pb-Zn mineralization occurs in calcite rich veins and breccia cements in the ca. 600m thick platform facies Pillara Limestone; there is no dolomitization associated with this mineralization. Fluid inclusion studies on Wagon Pass samples reveal that brines at temperatures between 70 and 110°C were involved in the formation of dolomite closely associated with mineralization. Freezing temperature measurements were complicated by the formation of waxy phases but limited measurements indicate high salinities. At Pillara the freezing temperatures in sphalerite vary between -15 and -32°C; the freezing temperatures below the eutectic of NaCl system (-20.8°C) imply the presence of major concentrations of Ca and/or Mg in solution. The temperature of homogenization in sphalerite varies between 70 and 110°C. The wide range of freezing and homogenization temperatures in early and late veins at Wagon Pass and at Pillara are consistent with variable degrees of mixing of ascending basinal brines with relatively low salinity waters of marine and meteoric derivation. This interpretation is in accord with isotopic data for early and late cements. Further evidence for the role of basinal brines comes from the presence of hydrocarbons in fluid inclusions. Application of Laser Raman microprobe and Fourier transform Infrared micro-spectrometry has permitted the detection of aliphatic and aromatic hydrocarbons in fluid inclusions in sphalerite and in dolomite at varying distances from mineralization. The purplish colour zoning in crystalline sphalerite from Pillara has been found to be caused by aromatic and aliphatic hydrocarbons which have comparable infra-red absorption bands to the hydrocarbon bearing fluid inclusions in clear zones of Pillara sphalerite, and to the Blina crude oil (Canning Basin). Abundant oil inclusions have been found in late barite associated with saddle dolomite and Pb-Zn sulphides elsewhere in the basin. The association of hydrocarbons with Pb-Zn mineralization and later barite provides further information on timing and genesis of petroleum and metalbearing fluids in this region.

I AO

A REFINED (XOLOGICAL HISTORY FOR THE SOUTHERN GAWLER CRATON THROUGH U-Pb ZIRCON DATING OF ACID VOLCANICS, AND CORRELATIONS WITH NORTHERN AUSTRALIA

C.M. Fanning^, A.H. Blissett^, R.B. Flint^, K.R. Ludwig^ and A.J. Parker^ ^Australian Mineral Development Laboratories, Adelaide ^Geological Survey of South Australia ^Branch of Isotope Geology, U.S. Geological Survey, Denver, U.S.A. Tectonic development of the Gawier Craton, South Australia, spans a time interval greater than 1000 million years from ca 2700 Ma to ca 1400 Ma. Late Archaean and Early Proterozoic sediments, volcanics and intrusive granites were deformed and metamorphosed during two principal orogenic cycles, the Sleafordian Orogeny ca 2500-2300 Ma and the Kimban Orogeny ca 1850-1600 Ma. The Late Archaean to Early Proterozoic Sleaford Complex in southern Eyre Peninsula comprises: granulite facies grade Carnot Gneisses which record complex Rb—Sr and U-Pb zircon ages in the range 2650-2300 Ma; amphibolite facies grade Wangary Gneisses; and high crustal level granitoids of the Dutton Suite, which have regional Rb-Sr isochron ages ca 2500-2300 Ma. These gneisses and granitoids are basement to the Early Proterozoic Hutchison Group and thus define a maximum age for the deposition of that unit in southern Eyre Peninsula. In central Eyre Peninsula the basal unit of the Hutchison Group, the Warrow Quartzite, overlies a migmatitic, grey, quartz-feldspar-biotite gneiss (Miltalie Gneiss) which has a Rb-Sr total rock isochron age of 1697 ± 65 Ma with an IR of 0.7340 ± 0.0048. This age and exceptionally high IR have been interpreted as reflecting partial to complete resetting of the Rb-Sr systematics during the Kimban Orogeny, and this gneiss is seen to form part of the older basement. Conventional U-Pb zircon analyses for a sample collected from one of the Rb-Sr sites, yields moderately discordant results with an upper concordia intercept at 2014 ± 28 Ma and lower intercept of 664 ± 150 Ma (MSWD = 5.94). The zircons are round to elongate, generally colourless, and are considered to at least have been modified by metamorphic processes. Analyses of the clearest least magnetic zircons from the coarsest and intermediate size fractions by the single grain technique indicate that complex U-Pb systematics are present in this gneiss, and a near concordant analysis for -92+72 ^m zircons at ca 1950 Ma is considered to give the best estimate for the formation of zircon in the gneiss. This places a new constraint on the maximum age for the Warrow Quartzite and Hutchison Group in central Eyre Peninsula. There is no direct evidence that the Hutchison Group has been intruded by the I-type, Donington Granitoid Suite near Port Lincoln (1818 ± 13 Ma, Rb-Sr total rock isochron), but in northeastern Eyre Peninsula similar granitoids contain rafts of lithologies identical to the Warrow Quartzite and Hutchison Group calcsilicates. Therefore it is inferred that the age of the granitoids at ca 1850-1820 Ma gives a minimum time of deposition. Acid volcanics have not been identified within the Hutchison Group in the Gawier Craton. However, immediately east of Iron Baron there are some strongly lineated, locally metamorphosed acid volcanics, the Myola Volcanics, which appear to to have been deformed by D^ of the Kimban Orogeny, but not necessarily by Dj. A combination of five conventional and one "single" grain U-Pb analyses of zircons from the Myola Volcanics yields a well defined upper concordia intercept of 1791 ± 4 Ma (MSWD = 1.88) which records the age of crystallisation of the zircons in these volcanics. 67

Nearby, at Moonabie, the McGregor Volcanics can be seen to be less deformed than the Myola Volcanics and folded only by the Dg Kimban event. U-Pb zircon analyses for the McGregor Volcanics yield an upper intercept of 1755 ± 12 Ma (MSWD = 23.9) for six conventional and one single grain measurements. This poorly fitted chord is controlled by significantly discordant fractions, thus the nearly concordant analyses would be expected to give a better indication of the true age of the zircon. The three most concordant analyses, including the only slightly discordant "single" grain analysis indicate a younger age of ca 1740 Ma for the McGregor Volcanics. The D2 Kimban Event is therefore constrained by the period 1791-1740 Ma. Timing of the D3 event is not as well known,although the age of the Moonta Porphyry on Yorke Peninsula (1737 ± 5 Ma, U-Pb zircon), the McGregor Volcanics, and an undeformed pegmatite intruding the Kalinjala Mylonite Zone on the east coast of Eyre Peninsula (ca 1710 Ma, Rb-Sr total rock-biotite) suggest an age of 1737-1710 Ma for the Dg event. Total rock Rb-Sr isochron ages for the essentially post-tectonic Gawler Range Volcanics (GRV) are generally not well defined and range from 1510-1565 Ma. Conventional and single grain U-Pb analyses of zircons from two units within the GRV yield remarkably concordant results for the low U (20-100 ppm), euhedral zircons. An older GRV unit at Toondulya Bluff (equivalent of Waganny Dacite) records an age of 1591 ± 3 Ma (MSWD =1.35), and in the younger GRV sequence the black Yardea Dacite yields 1592 ± 3 Ma (MSWD = 2.27). These two ages are essentially the same and a pooled regression gives a near perfect fit chord (MSWD =1.91) with an age of 1592 ± 2 Ma which most likely records the timing of this massive extrusion of acid volcanics. Similar U-Pb zircon ages have also been found for acid volcanics well to the east of the Gawler Craton. On the Benagerie Ridge in the Curnamona Nucleus east of the Adelaide Geosycline, a rhyolite from xMudguard nl yields a poorly defined upper concordia intercept at 1599 ± 40 Ma (MSWD = 20), and within the Mount Painter Province a deformed acid volcanic at the Gunsight Prospect also gives a poorly defined zircon date of 1575 ± 14 Ma (MSWD = 33). Further analyses of the single grain type will refine these dates, nevertheless they do confirm earlier correlations of post-tectonic acid volcanism on the Gawler Craton with similar volcanism now cropping out as inliers within and to the east of the Adelaide Geosyncline. Although, the intra-cratonic sediments of the Corunna Conglomerate on northern Eyre Peninsula have not been precisely dated they are believed to be coeval with the Gawler Range Volcanics. This is confirmed by a U-Pb zircon study of the Charleston Granite which intrudes equivalent sediments in Moonabie Range. Conventional U-Pb zircon measurements are significantly discordant, recording a poorly defined upper intercept of 1645 ± 46 Ma (MSWD = 10.5), however carefully selected clear euhedral zircons from the least magnetic -92+72 jum fraction yield a near concordant result and suggest that a more realistic estimate for the Charleston Granite is 1583 T 26 Ma. Once again this date will be refined by further "single" grain style analyses as it is clear that there are possible inheritance problems when larger zircon aliquots are analysed by conventional techniques. Conclusions drawn from the above results are extremely significant with respect to the tectonic evolution of Australia, in particular correlations with northern Australia, the Pine Creek Geosyncline and Mt Isa Inlier. Numerous comparisons have been made in the past, however with the above results it is now possible to make specific comparisons based on both similarity of sequences and timing of formation, as the same and very reliable dating technique forms a strong basis for correlations. 68

1.7 Rb-Sr AND U-Pb ffiOCHRONOLOGY OF THE CARNOT OTEISSES: COMPLEX ISOTOPIC SYSTEMATICS FOR THE LATE ARCHAEAN TO EARLY PROTEROZOIC SLEAFORD COMPLEX, SOUTHERN ETRE PENINSULA, SOUTH AUSTRALIA C.M. Fanning^, J.A. Cooper^, R.L. Oliver^ and K.R. Ludwig^ ^Australian Mineral Development Laboratories, Adelaide ^Department of Geology and Geophysics, University of Adelaide ^Branch of Isotope Geology, U.S. Geological Survey, Denver, U.S.A.

The Late Archaean to Early Proterozoic Sleaford Complex in Southern Eyre Peninsula, South Australia consists of two distinct sub-complexes which represent markedly different levels of crustal exposure. A western sub-complex comprises low-mid amphibolite facies, Wangary Gneisses which are intruded by the Dutton suite of granitoids. This relatively higher crustal level sub-complex is generally restricted to south-west Eyre Peninsula, and its relationship to a south central sub-complex of granulite facies rocks, the Carnot Gneisses is veiled by Cainozoic cover. The Carnot Gneisses comprise a wide range of lithologies which form an extensive layered sequence. A supracrustal origin is proposed, based on the corundum normative nature of the voluminous garnetiferous quartzo-felspathic gneisses, and less abundant cordierite garnet and biotite garnet gneisses. The latter are clearly derived from a pelitic precursor. Augen gneisses and massive medium grained granitic gneisses are thought to represent partial melts in the sequence formed at or about the time of granulite facies metamorphism. The ubiquitous basic granulites preserve for the most part a prograde assemblage and hence the original quartz tholeiite dykes or sills were intruded pre- to early in that event. Hypersthene gneisses form prominent macro layers in the sequence, in places disrupting the basic granulite bodies and are thought to be late stage developments, possibly intruded contemporaneously with granulite metamorphism. Rb-Sr total rock measurements were made for each of nine gneissic compositions, including the basic granulites. Sampling for a specific composition was made over an area of 1-2 square meters in order to minimise IR heterogeneities and all samples were collected at or within 1-2 km of Cape Carnot. A cordierite garnet gneiss yields the only near perfect fit isochron with a calculated age of 2412 ± 72 Ma and IR of 0.7060 ± 0.0008. An intrusive hypersthene gneiss records a slightly, but not significantly younger total rock age of 2300 ± 131 Ma with an IR of 0.7092 ± 0.0017. A "grand" regression for 44 of 55 samples analysed at Cape Carnot, excluding the hypersthene gneiss, approximates the well fitted cordierite garnet gneiss isochron, with an age of 2416 ± 49 Ma and IR of 0.7065 ± 0.0011. It has previously been proposed that this age reflects the time of granulite facies metamorphism during the Sleafordian Orogeny. The other 11 samples plot above this "regional" isochron and it is inferred that this is due to either the presence of older crust not reset at the time of the granulite event, or the presence heterogeneous IR's, or both. In detail individual isochron plots for a granitic gneiss, intimately interlayered gneisses, and biotite garnet gneisses show sub-parallel younger cross trends, probably a consequence of partial resetting during the Middle Proterozoic Kimban Orogeny. This resetting is more apparent on the mineral scale where total rock - potassium feldspar joins yield a range in ages from 1500-1720 Ma and biotites record ages of ca 1515 Ma. A high IR of between 0.7060 - 0.7066 for the majority of the Carnot Gneisses is supported by analyses of the basic granulites. 69

U-Pb zircon studies were carried out on four gneissic compositions previously analysed by Rb -Sr. For the augen gneiss, layered garnet gneiss and cordierite garnet gneiss, six of eight conventional U-Pb measurements comprise the same size fraction subdivided into six magnetic fractions. The augen gneiss zircons are generally colourless, round grains with relatively simple elongate to ovoid morphologies. The six magnetic fractions (ca -75+55ium) define a moderately fitted chord (MSWD = v^.35) with intercepts at 2305 ± 9 and 286 ± 24 M a . The clearest least magnetic zircons from the ca -250+150 ium and -95+75 ^m were also analysed, but these scatter widely from any chord of best fit. Zircons from the layered garnet gneiss are similar to those in the augen gneiss and the six magnetic splits of -75+55iLtm fraction define a poorly fitted chord (MSWD = 38) with intercepts at 2261 ± 27 and 214 ± 87 M a . The ca -250+150 and -95+75 mm clearest least magnetic grains lie in close proximity to this chord. Zircons from the cordierite garnet gneiss show complex morphologies in the coarser fractions, with multi-lobed round grains. Proportions of this type of grain decreases in the successively finer size fractions and simpler bi-lobed and oval shaped grains are dominant. As above the magnetic splits of the -95+75jLtm fraction define a moderately fitted chord (MSWD = 7.31) with intercepts at 2550 ± 12 and 352 ± 43 M a , whereas the two other non-magnetic fractions (ca -75+55 & -250+150 am) scatter about that chord. Analyses were also made on a single grain with complex multi-lobed morphology and another of a simple oval shape. The complex grain analysis shows considerable lead loss and lies well below any chord of best fit, whereas the simple oval shaped grain is more concordant than any of the analyses. Its age indicates a ca 2450 Ma rather than 2550 Ma upper intercept. Markedly different U-Pb systematics are seen in the hypersthene gneiss. The zircons are hyacinth in colour, have oval morphologies with some bi-lobed grains. Six different size and magnetic fractions were analysed and five lie along a moderately well fitted chord (MSWD = 3.16) with intercepts at 2613 ± 150 and 1720 ± 240 M a . A l l fractions show a tight grouping along this chord with ca 50^ lead loss. The non-magnetic -160+92 /um lies significantly above this chord of best fit. Single grain analysis of a clear non-magnetic zircon from this anomalous fraction also plots above the chord and the two analyses appear to define a line extending to an upper concordia intercept at ca 2400-2450 M a , similar to that seen in single grain analysis for the cordierite garnet gneiss. A clear non-magnetic grain from the -92+72 urn fraction is almost concordant and significantly enhances the overall regression to give intercepts at 2637 ± 21 and 1756 ± 40 Ma (MSWD = 2.41). The augen gneiss and layered garnet gneiss appear to define a crude two stage chord and it may be interpreted that zircon crystallisation in these gneisses occured at ca 2300 Ma. Similarly the cordierite garnet gneiss analyses may indicate crystallisation at ca 2550 Ma. However these chords are characterised by scatter much in excess of experimental error which suggests a more complex lead loss history. The hypersthene gneiss discordia indicates zircon crystallisation at ca 2640 Ma and lead loss at ca 1756 Ma during the Kimban Orogeny. This chord is appropriately aligned above the most concordant points of the other three gneisses and may provide the key to the complex U-Pb systematics. It is argued that the augen, layered garnet, and cordierite garnet gneisses are also about 2640 Ma in age, and that their zircons lost ca 60^, 60% and 15% respectively of their original lead during the Kimban Orogeny. These zircons then remained closed until the Late Palaeozoic when further episodic lead loss occured giving rise to their present alignment on the concordia diagram. The hypersthene gneiss zircons do not appear to record this younger event. 70

4.2 SINGLE AND MDLTIPLE GRAIN U-Pb ZIRCON ANALYSES FOR THE EARLY ADELAIDEAN ROOK TUFF, WILLOURAN RANGES, SOUTH AUSTRALIA C.M. Fanning^, K.R, Ludwig^, B.C. Forbes^ and W.V. Preiss^ ^Australian Mineral Development Laboratories, Adelaide ^Branch of Isotope Geology, U.S. Geological Survey, Denver, USA ^Geological Survey of South Australia The Rook Tuff is a thin, partly volcanogenic silty formation within the early Adelaidean Callanna Group. This tuffaceous unit is confined to the northern part of the Willouran Ranges which represent a northwesterly extension of the Flinders Ranges. During early Adelaidean time this region was occupied by a relatively narrow, northwest trending fault-controlled trough that accumulated a thick (up to 11km) sequence of clastics, carbonates, and evaporites (subsequently replaced). During the late Adelaidean this trough was overlapped by thick glacial and interglacial sediments and post-glacial transgressive sequences that thinned onto the adjacent cratonic Stuart Shelf. Pre-Adelaidean basement is not exposed in the Willouran Ranges and the oldest rocks of the Callanna Group are structurally disrupted limestone marbles (Black Knob Marble) and the basic Noranda Volcanics. The more or less intact younger sequence (Curdimurka Subgroup of the Callanna Group) commences with the ca 1500m thick Dome Sandstone, followed conformably by dark coloured siltstones of the Rook Tuff. The relationship between the Noranda Volcanics and Dome Sandstone is possibly preserved in one diapiric xenoclast near Chintapanna Dam, where the contact appears to be conformable. In the northeastern part of the Willouran Ranges the Rook Tuff contains a thin (2~3m), regionally lenticular interbed of porphyritic dacite with gradational upper and lower contacts. It is suggested that this interbed represents a welded tuff formed contemporaneously with early Adelaidean sedimentation. Previous attempts to place constraints on the timing of early Adelaidean basic volcanism in the Flinders Ranges have had limited success as both K-Ar and Rb~Sr systematics have either been reset during the Delamerian Orogeny or affected by alteration. Dating of the base of the Adelaidean has largely relied on correlation of basic lavas in the Geosyncline with those on the adjacent, essentially undeformed Stuart Shelf, in particular the Beda Volcanics which have a minimum Rb-Sr total rock age of 1070 Ma. The Rook Tuff is one of the rare contemporaneous acid extrusives in the Adelaidean likely to contain primary igneous zircons suitable for U-Pb dating. On the basis of the rock relationships discussed above, an age determination for the Rook Tuff is considered likely to yield a date only slightly younger than that for the mafic lavas near the base of the Adelaidean. An initial sample (A) of the welded tuff was collected ca 0.5km northwest of the eastern Rook workings. Following standard Wilfley table, heavy liquid and para-magnetic techniques, insufficient zircon was separated for conventional U-Pb dating which generally involves 5 to 15 milligrams of zircon per analysis, i.e. hundreds to thousands of grains. A second sample (B) was collected approximately 250m along strike from the first, within the same tuffaceous horizon. An estimated total of 30-60 grains of pale yellow coloured euhedral zircons were obtained from these two samples; still clearly insufficient zircon to attempt conventional U-Pb dating techniques, hence "single" grain analyses were carried out, three from each sampling. Clear pale yellow euhedral zircons were carefully hand selected, 71

with one (weighing 27.9 Mgm), four (19.4juigm) and three (ISagm) grain analyses from sample A , and one (23.9Mgm), two (15.7agm) and two (10.4/ugm) grain analyses from sample B. Measurements were made with a Micromass 54E mass-spectrometer utilizing both a Daly multiplier and Faraday cup. The total blank for the procedure is well constrained at 28-30 picograms of common lead, with measured ratios in the range 350 to 1300. Estimates for the uncertainty in ^^^Pb/^^^Pb ratios are 0.2% (955fe confidence limits) and 0.5^ for the U/Pb ratios. Five of the six analyses are coincident and concordant to within experimental errors at 795-808 Ma. The sixth analysis records a slightly younger, barely discordant age, but the error ellipse overlaps the other five points on a standard concordia plot. Given the general concordant nature of the data a weighted average of the ages gives an age of 802 ± 10 Ma at 95^ confidence limits with an MSWD of 2.4. The concordance of points and agreement to within analytical uncertainty indicates conclusively that the age of crystallisation of the zircon and the Rook Tuff occurred close to 800 Ma. Sample B also contains some highly coloured, elongate grains with round terminations which appear to have metamorphic affinities. Although not sized, these grains are clearly finer (say 50-75Mm ) than the euhedral, presumed igneous zircons seen to be concordant. A hand selected fraction comprising six fine highly coloured grains (lO.l/ugm) was also analysed, yielding a concordant result with a ^^^Pb/^'^^Ph age of 1640 ± 19 Ma. It is possible that these zircons represent contamination during the mineral separation procedures, however analyses of "single" grain fractions of other samples processed through the same mineral separation procedures, at or about the same time did not reveal concordant zircons of this age. It is therefore more probable that these fine highly coloured zircons represent crustal contamination of the Rook Tuff. The well constrained 802 ± 10 Ma U-Pb zircon date casts uncertainty on other geochronological data obtained for Adelaidean rocks using Rb-Sr systematics. In particular there is doubt as to the lithological and stratigraphic correlation of the early Willouran mafic lavas of the Adelaide Geosyncline with the Beda Volcanics of the Stuart Shelf (ca 1070 Ma). If the Rb-Sr total rock isochron age of 750 ± 53 Ma is accepted for shales from the late Sturtian Tapley Hill Formation on the Stuart Shelf, then at most ca 50 Ma is available for Willouran and Torrensian sedimentation, early Sturtian tectonism and two phases of Sturtian glaciation. This is compared to an estimated 150-200 Ma for the deposition of the essentially conformable remaining Adelaidean sediments. It is noteworthy that the age for the Rook Tuff is in general agreement with an Rb-Sr total rock minimum age of ca 830 Ma for the Wooltana Volcanics, extrusives of Willouran age in the Mount Painter area. Attempts were also made to date the base of the Adelaidean in the Mount Lofty Ranges. A "conventional" (0.1-1.0 milligram fractions) U-Pb zircon study of two gneissic rocks from the Houghton Inlier suggests that these gneisses have sedimentary precursors, as the points scatter widely on a concordia plot with ^o7p^/zoGPb ages in the range 1550-1700 Ma^ This study failed to define an age for basement to the Adelaidean in this area, however the data for the Rook Tuff is not in conflict with a preferred 850 ± 32 Ma total rock isochron age for paragneisses at Inglewood. It is hoped that further U-Pb studies on well constrained stratigraphic horizons will clarify some of the anomalies raised by the Rook Tuff age.

6.11

STRUCTURAL STYLES IN CENTRAL EROMANGA BASIN SEISMIC DATA AS A GUIDE TO CRUSTAL AND DEEP LITHOSPHERIC PROCESSES D . M . Finlayson and J . H . Leven

Bureau of Mineral Resources, Geology and Geophysics, Canberra The central Eromanga Basin region of Queensland has a geological record which includes four major tectonic cycles contributing to the complex lithospheric structure of the region. The Late Proterozoic - Early Palaeozoic cycle involved the evolution of an epicontinental shallow sea behind a convergent margin/island arc with the palaeo-Pacific p l a t e . The subsequent mid-Silurian orogeny formed the basement rocks of the region, the Thomson Fold Belt. The second major cycle involved the formation of the Devonian Adavale Basin and its associated troughs which were subsequently deformed in the mid-Carboniferous (Kanimblan) orogeny. The third major cycle involved the formation of the Permo-Triassic Cooper and Galilee Basins. The fourth cycle involved the widespread deposition of Jurassic-Cretaceous Eromanga Basin sequences and their subsequent deformation in the mid-Tertiary. These four tectonic cycles can be recognised in the extensive seismic reflection profiling data from the region. Within pre-Devonian basement there are very few structural features recognised, indicating that there must be extensive folding, faulting and shearing within basement rocks; basement is essentially transparent. In the Devonian and subsequent sedimentary sequences there is ev:'.dence that a number of transpressional tectonic regimes affected the region. The structures seen in the sediments are interpreted as resulting from lithospheric reactivation of basement faults. Basement structural features and basins form linked systems and it is the recognition of the links which is essential for the tectonic reconstruction of the region. Faults which may be normal early in a region's history may be reactivated as thrust or shear faults in later orogenic episodes. In the central Eromanga region the Devonian sequences were deformed by a transpressional event in the mid-Carboniferous, with the compressional axis being NW-SE and a left-lateral shear component across the area. The post-Devonian sequences were deformed by a transpressional event in the mid-Tertiary, with NE-SW compression and a right-lateral shear component. Can such major tectonic episodes be recognized deeper in the lithosphere? Deep seismic information is available from 1400 km of 20s reflection profiling data and 700 km of seismic refraction profiling. The upper crustal basement appears to be transparent from the reflection profiling in the two-way time range 2.5 to 8 s. Between 8 and 13 s there are numerous reflecting segments emphasizing the difference in seismic character between the upper and lower crust. The process involved in the formation of the lower crustal reflections seems, on the evidence, to be different to that affecting the upper crust. The change in reflection characteristics at mid-crustal levels corresponds with a prominent mid-crustal velocity increase determined from refraction profiling. The velocity increase from about 6.3-6.4 km/s in the upper crust to 6.9-7.0 km/s in the lower crust, further highlights the difference between the upper and lower crust. The reflections in the lower crust are not featureless. Some of the structural features seen in the sedimentary sequences of the upper crust 73

are u n d e r l a i n by strong mid-crustal reflecting horizons and it seems likely that such structures are restricted to the upper crust, e . g . the Gumbardo structure in the Adavale Basin. Other features of the lower crustal reflections, h o w e v e r , do seem to correlate well with near-surface features. A prominent cut-off in the lower crustal reflections at the Warrego F a u l t , for instance, seems to indicate a process which cuts through the upper and lower crust and into the upper m a n t l e . The Warrego Fault is interpreted as a shear zone from the near-surface structural data and there does, therefore, seem to be some evidence that the shear zone is truely lithospheric in sccile. There is also evidence that the features of reflections at m i d - c r u s t a l depths correlate with upper crustal structures such as the Quilpie Trough and the arch southeast of the Warrego Fault. At the M o h o , h o w e v e r , (depth 36-41 km) tliere is little evidence of major depth fluctuations from the seismic refraction and gravity data. There are few reflections from w i t h i n the upper m a n t l e . This suggests that the crust/mantle b o u n d a r y is a young feature and it is perhaps only in areas of current tectonic activity that substantial Moho topography can be sustained. H e n c e , in the central Eromanga region we seem to have identified three different processes; those which are limited to the upper crust, those w h i c h are limited to the lower crust, and those that traverse through the upper and lower crust and into the upper mantle and can therefore be described as truely lithospheric in scale. Such processes are identified in seismic data from other parts of the w o r l d . In the Basin and Range province of western USA we see examples of structural features which are limited to the upper crust by a detatchment surface in a zone of ductile material. In other areas of the same province we also have examples of structural features going to m i d - c r u s t a l and to upper mantle depths. From the Keeweenawan Rift r e g i o n , K a n s a s , U S A , we have examples of a 'transparent' upper crustal b a s e m e n t overlying multiple lower crustal reflections. From the M i s s i s s i p p i Embayment we see a prominent mid-crustal velocity increase like that seen in the central ii'omanga region. There is, h o w e v e r , no certainty about hoA^ the process-..^: involved at various levels in the lithosphere will appear in seismic data. If we are to gain a greater understanding of these processes it is important to refine seismic methods and apply tomographic techniques to examining deep structures w h i c h , at the m o m e n t , we only see as tantalizing features on single profiles.

2.5

REVIEW OF DIAMOND DRILLING OF NEPHRITE JADE DEPOSITS NEAR COWELL, SOUTH AUSTRALIA D.J. Flint and E.A. Dubowski Geological Survey of South Australia

Jade was first discovered 20 km north of Cowell in 1965 by a local farmer, H.A. Schiller. To date, 113 outcrops have been identified. Investigations by private companies and the South Australian Department of Mines and Energy (SADME) have established general geological details of the deposits. Jade at Cowell is nephrite formed within the migmatised Early Proterozoic Minbrie Gneiss Complex, with host rocks of dolomitic marble and banded calc-silicates. Alteration zones around nephrite lenses are common and consist of chlorite, tremolite, talc, epidote and clinozoisite. Three diamond-drilling programs have been undertaken to determine controls, origin and subsurface extent of jade. In 1970, Analytical Exploration Pty Ltd drilled six holes, but were successful in intersecting nephrite in only one. In 1973, Centamin N.L. drilled a further seven holes including holes on outcrops previously drilled and, again, were successful in intersecting nephrite in only one. SADME drilled four holes in 1983, with nephrite intersected in three. Of the 113 outcrops known, at least 36 have been quarried or have some form of a prospecting pit with at least eleven considered mined out. Many of the remaining outcrops are either small, with surface dimensions of several metres or less, or tend to consist of coarse-grained nephritic tremolite which will not polish. Only four outcrops have yielded intersections of nephrite following testing by seventeen drillholes on nine outcrops. Anomalously, one of the outcrops drilled unsuccessfully in 1983 is outcrop 35 which has yielded about 400 tonnes, of nearly 40% of the total estimated production of the Cowell jade province. All successful drillholes have two things in common: - they are located on outcrops with large surface dimensions, ranging up to 60 m strike length - they are inclined to the southeast and northeast. Eight of the first 13 holes were drilled inclined to the west, the same direction as the regional layering. Combined with a preference for drilling small outcrops with greatest surface dimension rarely exceeding 5 m, it is not surprising that so many of the early holes failed to intersect nephrite. Even two vertical holes either side and only 1.3 m away from a 9x2 m outcrop were unsuccessful. Geological interpretation of early diamond drilling is hampered by a lack of core: DDK 1-6; core available but nephrite intersections of the only successful hole (DDH6) are partly missing. DDR 7-13; core missing - includes successful holes DDK 7 and 8. Evaluation of present and reinterpretation of past drilling data has, however, provided new information as to controls of nephrite formation. A distinctive chloritised feldspar rock intruding dolomitic marble is often associated with nephrite. The intrusion promotes alteration of dolomitic marble producing contact alteration zones containing tremolite, chlorite, talc, epidote/clinozoisite as well as nephrite. The altered intrusive now consists of coarse-grained microcline, with lesser 75

plagioclase, and disseminated aggregates of fine-grained green to black chlorite or clusters of tremolite or actinolite. Contact alteration zones are in part mylonitised, suggesting intrusion and nephrite formation as no later than the D3 phase of the Kiraban Orogeny, Nephrite lenses are also found where dolomitic marble is faulted and offset by late-Kiraban D4 fractures. Diffusion along the fractures promotes alteration of dolomitic marble and formation of nephrite by addition of Si02 and removal of CaC03. Uralitization of diopside is a third method of nephrite formation. Coarse-grained, brecciated diopside retrogresses sporadically to nephrite and nephritic schist. However, jade formed this way is often in very small bodies with occurrences in drillholes down to only a few centimetres across, is often schistose and is impure with abundant inclusions of relict diopside. Commercial production of 1 055 tonnes of nephrite from 1967 to 1985 has been from both of the main types of lenses.

3.6

PETROLOGY, GEOCHEMISTRY AND CRUSTAL DEVELOPMENT, ENTIA DOME, EASTERN ARDNTA BLOCK, NORTHERN TERRITORY J.D, Foden^ and I.S. Buick^

^Department of Geology and Geophysics, University of Adelaide ^Department of Earth Sciences, Cambridge University The Entia Dome is part of the eastern Arunta Complex, N.T. and is composed of a range of para- and ortho-gneissic rocks referred to as the Entia Gneiss complex. These rocks may be subdivided into supracrustal and intrusive series. The supracrustal protoliths were mafic and possibly felsic volcanic rocks interlayered with sedimentary rocks including quartz-rich, clay-rich and calcareous varieties. These now form an association of folded ortho- and para-amphibolite, quartzite, layered biotite gneiss, knotted phlogopitekyanite schist, kyanite-biotite-garnet-muscovite gneiss, calc-silicate gneiss and migmatite. The supracrustal rocks were complexly folded and buried to depths equivalent to 7-8 kb pressure (20-25 km) before they were intruded by the orthogneissic rocks. The orthogneiss association is composed of four major intrusive suites. These are referred to as the Huckitta granodioritic gneiss, the Inkamulla granitic gneiss, the Huckitta tonalitic gneiss and the Entia leuco granite gneiss. These are largely conformable, or near-conformable with the host supracrustal gneisses and must have been intruded as sills. Following the intrusion of the orthogneisses, the Entia Gneiss Complex was deformed in several stages. The earliest of these events produced two phases of large-scale recumbent isoclinal folds (nappes) with horizontal axial planes. The first of these folding events produced the major regional mineral fabric. Peak preserved metamorphic mineral assemblages, characteristic of upper amphibolite facies, appear to have developed soon after this fabric but are refolded by the second recumbent event. Metamorphism was associated with the production of abundant migmatite throughout the period of the two recumbent deformations. This metamorphic event is widespread throughout the east Arunta Block and has been responsible for wholesale isotopic re-equilibration at about 1760 ma. After the two post-orthogneiss recumbent fold events, several upright, tight to open style fold sequences occurred. These do not form any fabric, but the domal form of the Entia Gneiss Complex results from the interference between the two latest of these events. Pegmatites are widespread throughout the Entia Dome and appear to span a lengthy period of the geological history of the area. Some are relatively undeformed and postdate the intrusion of the orthogneisses. Others are highly deformed, are often more sodic in composition and predate the granitoid intrusion. The four orthogneiss suites are distinguished on geological, geochemical and petrographic grounds 1. The Huckitta Granodioritic Gneiss. This is a multiphase gneiss with several components. The major host phase is a mesocratic gneiss of granodioritic composition composed of plagioclase, hornblende, quartz, biotite and potassium feldspar with minor apatite, sphene and allanite. This has about 4.0% CaO, 1.0% MgO, 1.5% K2O and an Na20/K20 ratio of about 3. Cutting this are thin sills and dykes of a leucocratic granodiorite with prominent clots of hornblende as well as a network of aplitic granite veins. 77

This suite has broadly I-type geochemical affinities with <1.0% normative C or with very minor normative diopside. It commonly hosts xenolithic inclusions of amphibolite or mafic diorite, 2. The Inkamulla Granitic Gneiss. This is a massive, relatively homogeneous sill-like body. It is pink coloured, true granite composed of quartz, plagioclase, potassium feldspar and biotite with very scarce amphibole. 3. The Huckitta Tonalitic Gneiss. This is a much more mafic suite than the other orthogneisses and ranges in composition from gabbro to granodiorite. The rocks are composed of plagioclase, quartz, hornblende and minor clinopyroxene. They are diopside normative rocks with MgO contents which range from about 12.0% down to less than 2.0%. Their alkali and LREE contents are quite high (Sr 400-600 ppm, Rb 40-80 ppm, Ba 600-2500 ppm, Ce 50-60 ppm, Y 12-16 ppm) yet incompatible element levels are also significantly high (Ni 40-120 ppm). These factors suggest that the source of this suite must include a significant component of mafic or ultra-mafic material. This could either be the upper mantle or perhaps lower crustal cumulate or underplate material (?). 4. The Entia Leuco Granite Gneiss. This is a very widespread orthogneiss phase and ranges in composition from potassium-rich granite to trondhjemite. It is a quartz-plagioclase-potassium feldspar rock with minor biotite occuring thinly interleaved with the supracrustal association. The general model for lower Proterozoic crustal development presented by the situation in the east Arunta Block is on of crustal thickening, first by tectonic processes and then by crustal melting combined with the involvement and addition of a juvenile, subcrustal component.

14.2

NEW EARTHQUAKE RISK MAPS OF AUSTRALIA

Brian A. Gaull^ and Marion 0. Michael-Leiba^ ^Bureau of Mineral Resources, Mundaring, Western Australia ^Bureau of Mineral Resources, Canberra New earthquake risk maps of the Australian continent, including continental margins have been prepared. The risk is depicted as contours of peak ground velocity, acceleration and ground intensity with a 10% probability of being exceeded in 50 years. These maps have been based on the Cornell-McGuire methodology. This method requires the definition of earthquake source zones, each one having its own magnitude-frequency recurrence relationship and of the ground motion attenuation. The maximum likelihood procedure was used to determine the magnitude recurrence relations in the zones for which sufficient data were available. Completeness of the earthquake catalogue was tested for each magnitude interval in each of the zones. The earthquake process was assumed to be Poissonian and consequently foreshocks and aftershocks were eliminated. The maximum magnitude in each zone was assumed to be half a magnitude unit greater than the largest recorded Richter magnitude for that zone. Background seismicity levels (outside the source zones) were derived and normalised and used in the risk estimates. Separate attenuations of ground intensity with hypocentral distance were obtained for Western and Eastern Australia by determination of mean isoseismal radii from Australian isoseismal maps. Conversion of ground intensity (I, on the Modified Mercalli scale) to peak ground acceleration (A, in ms"2) and velocity (v, in mms"^) was carried out using the empirical relations 2^=7v/5 and log A = 1/3.1 - 2.3. Recordings of peak ground acceleration were used where available to check the attenuation. The- regions subject to the greatest earthquake risk are situated in southeast Canning Basin, the Simpson Desert, the Yilgarn Block to the of Perth and the Dalton-Gunning region to the north of Canberra. There 10% probability that in a 50 year period the following ground motions be exceeded in these areas respectively 2.4 (or 240 mms~^), 2.0 (or 200 mms"^), 1.6 ms"^ (or 160 mms"^) and 1.2 ms"^ (or 120 mms"^).

the east is a will ms"^

It is stressed that offshore areas have been treated the same as onshore. Because there is no strong motion data available for the sea floor the contours have been dotted. We also emphasize that the analysis does not include the effects on long period structures of longer period seismic waves generated by great earthquakes in the Indonesian (or possibly New Guinea) regions. We recommend expansion of strong motion instrumentation to improve our knowledge of attenuation which was the largest single contributer to risk estimate errors. Also microzonation studies near to Australia's capital cities are recommended in order to quantify variations of risk due to geology.

12.2

UNDERGROUND WATER RESOURCE, ADELAIDE METROPOLITAN AREA Nabil Z. Gerges Geological Survey of South Australia

The Adelaide Metropolitan Area is part of St. Vincent Basin. Groundwater occurs in both pressure and water table aquifers within Tertiary to Recent sediments. The bulk of abstracted groundwater is obtained from Tertiary sediments comprising three main confined aquifers designated A, B & C in order of increasing depth. On six occasions since 1915, it has been necessary to augment the Metropolitan Water Supply from groundwater sources. The latest occasion was in the 1967-1968 summer, where 9 500 - 10 700 ML-1 was pumped into the distribution system during a 7 month period. Water quality in the shallow Quaternary aquifers is dominated by surface drainage. Water table contours indicate that groundwater flows rather uniformly northwest across the area. As there is practically no pumping from this aquifer, the water is not diverted to any point of withdrawal. The movement of water appears to be controlled chiefly by topography, surface drainage and major recharge from bedrock through the Eden-Burnside Fault system. The present day flow pattern should be similar to the historic flow regime. The Quaternary aquifers show variable salinity which is the result of upward or downward leakage, lateral flow, fault lines and various mechanism of natural recharge. In aquifers A and B salinity is generally less than 1000 mgL"^, and large quantities of groundwater are being pumped for use in industry for schools and recreation grounds. Aquifer A is divided into subaquifer Al; the Hallett Cove Sandstone-Dry Creek Sand and subaquifer A2; Upper Port Willunga Limestone. The two subaquifers are separated by the Croydon Facies which acts as a semi-confining bed. Subaquifer A2 has the potential for large yields of sand free water. Its transmissivity has been estimated to be between 80-100 m^day~^m""^ with a storage coefficient of 2.0x10""^. Extraction for 1984 from aquifer A has been estimated at 6000-8000 ML. Aquifer C is generally too saline for use except in an area where it is shallow and close to the recharge zone. Water quality in the Aquifer A is dominated by the historic flow pattern, particulary in an area between Brownhill and Sturt Creeks. This historic flow pattern is severely modified by present day pumping. Salinity stratification in the Para Fault Block area in particular indicates some change in vertical hydraulic conductivity. The potentiometric surface of Aquifer A is modified by present day pumping and shows three steep cones of depression increasing to their maximum extent in February-March and decreasing to their minimum in AugustSeptember of each year. However the direction of groundwater movement and the size of the cones during every summer and winter are identical and indicate that the system has reached a new equilibrium. Preliminary investigations indicate that up to 9000 ML year"^ are available for extraction without causing great stress to the system. This assumes that available information on hydraulic parameters is accurate. However steepening of the gradient near the recharge zone for example could increase the amount of natural recharge, thereby increasing the "safe yield" of the system.

The confining bed between aquifer A and B is the Munno Para Clay, which consists of dark grey ^ clay with thin layers of limestone* Laboratory permeability tests indicate that leakage across the clay could be 20 m^day~^km""2. In an area west of the Para Fault, groundwater extraction from Aquifer B is insignificant* The present day potentiometric surface of Aquifer B indicates flow is to the north-northwest which is similar to the Quaternary and bedrock flow patterns. The reason for this slight shift to the north is heavy extraction from the Northern Adelaide Plains market gardening area. However this is almost identical to the Quaternary and bedrock flow pattern and similar to the first Tertiary aquifer historic flow pattern across the Para Fault Block. This historic flow pattern was reconstructed from limited water level data together with input from a conceptual model. This supports the theory that all aquifers are in equilibrium and flow continues to be toward the northwest. Information obtained from recent drilling together with re-interpretation of old data have provided a better understanding of the recharge mechanism. Major recharge occurs from bedrock to deeply buried thick clastic sediments deposited along the Eden-Burnside fault zone as a Tertiary marginal marine to non-marine facies. This recharge mechanism differs from the previous theory that surface water contributes significantly to recharge of the Tertiary confined aquifer. Evidence from salinity distribution, stratification in Quaternary and Tertiary aquifers, the pre-pumping potentiometric surface and isotope studies support the new recharge theory.

6.3 THRUSTING AND ITS IKPLICATIONS IN THE lACHLAN FOLD BELT NEAR DELEGATE, SOUTHEASTERN AUSTRALIA R.A. Glenl and A.H.M. VandenBerg^ ^Geological Survey of New South Wales ^Geological Survey of Victoria A small thrust belt of probable middle Silurian age has been recently mapped in northeastern Victoria and southeastern New South Wales. Lying within the Snowy Mountains Structural Zone, the eastern edge of the belt, marked by frontal thrusts, coincides with the I-S Line southwest and west of Delegate. The extent of the belt to the west and south is uncertain. Two main stratigraphic units occur within the thrust belt. The oldest is a thin-bedded black shale horizon called the Warbisco Shale. It contains Eastonian (but in places Gisbornian to Bolindian) graptolites. The main unit is the Yalny Group (c. 2 km thick) which has an age around the middle Llandovery. The Yalmy Group consists of lenticular shallow-water deposits at the base which pass up into turbidites at the top. Thin units of the Ordovician Adaminaby beds occur within the belt near its eastern margin. The thrust belt is unconformably overlain by the Early Devonian Snowy River Volcanics. The key to reinterpretation of geology in this part of the Lachlan Fold Belt was the discovery of Late Ordovician black shale horizons within a sequence containing rare Early Silurian fossils. These black shale horizons are structurally incompetent. They are marked by repetition of and gaps in, the faunal succession. In outcrop, the horizons contain thrust faults and crumpled and contorted bedding. We think that these horizons are fault repetitions of the one stratigraphic unit, the Warbisco Shale. We thus suggest that the Warbisco Shale acted as a decollement or sliding horizon which localized the development of listric thrusts, each thrust sheet passing up from Warbisco Shale at the base into the overlying Yalny Group. Regional mapping suggests that west of Delegate these thrusts have an eastwards transport direction. Further south the transport direction has swung to the south. We think that the mapped thrusts are listric on a sole thrust which is south to east directed. Duplex structures probably occur in the McLaughlan Creek Fault Zone which marks the sole thrust west of Delegate. Further south, the sole thrust is the Yalny Fault. The thrust belt west of the I-S Line contrasts with a more orogenic style of deformation to the east. We infer that crust below the thrust belt is more rigid than that to the east: this is consistent with previous ideas based on granitoid genesis. We further suggest that the I-S Line corresponds to a ?Cambra-Ordovician hinge line between normal thickness crust to the west and thinned ± oceanic crust to the east which was anderplated at the end of the Ordovician. (Published with the permission of the Secretary, Department Resources).

of Mineral

4.13

EARLY ORDOVICIAN OOLITIC IRONSTONE, PACOOTA SANDSTONE, AMADEUS BASIN John D. Gorter Pancontinental Petroleum Limited, Sydney

Early Ordovician oolitic ironstone was first reported from the Amadeus Basin in Tempe Vale No. 1 well and subsequently found in Tent Hill No. 1 well. Density log correlations and cuttings descriptions showed the presence of similar ironstone in other wells in the central western part of the Basin. Several measured sections also indicate that the oolitic ironstone facies is widespread in this part of the Amadeus Basin. All occurrences are in the upper part of the Pacoota Sandstone, a latest Cambrian to Early Ordovician marine sequence deposited extensively in the Amadeus Basin. The wide spread of the oolitic ironstone facies and its consistent presence in the upper Pacoota Sandstone suggest that it could be a useful stratigraphic marker in a formation renown for its lack of dateable body fossils, particularly in the western part of the Basin. The Pacoota Sandstone oolitic ironstones were deposited upon shoaling barrier bars in a low energy, shallow water intracratonic sea during the Arenig Stage of the Ordovician. The shoaling sequence is superimposed upon the major Late Cambrian to Early Ordovician transgression which culminated in the deposition of the Horn Valley Siltstone.

15.2

GIANT IMPACT STRUCTURE AND WIDELY DISPERSED EJECTA IN THE PRECAMBRIAN OF SOUTH AUSTRALIA Victor A . Gostin^ and George E . Williams^

^Department of Geology and Geophysics, University of Adelaide ^BHP, Exploration Department, Adelaide

A giant hypervelocity impact structure occurs in middle Proterozoic (c.1590 Ma) volcanics in the Gawler Ranges, central South Australia. The central depressed area of 30 km diameter contains the Lake Acraman salina with raised outcrops near its centre. Satellite imagery also shows an intermediate depression or ring c.90 km in diameter, and a possible outer annulus up to c. 160 km in diameter. Outcrops of dacite within the central area are intensely shattered, and contain shatter cones. In thin section quartz grains exhibit multiple sets of shock-induced deformation lamellae including those parallel to the basal plane. The Acraman Lake structure is the largest known impact structure in Australia and is among the dozen largest such structures in the world. Some 300 km east of Lake Acraman, within late Proterozoic (c.600 Ma) marine shales of the Bunyeroo Formation of the Adelaide Geosyncline, a unique thin layer of shattered crustal rock fragments has been traced over a north-south distance of 260 km. These sand to boulder-size fragments consist entirely of acid to intermediate volcanics (c.l560 Ma), displaying shattered mineral grains, shock lamellae in quartz, and micro shatter cones. The larger fragments weigh up to 6.4 kg and show evidence of vertical fall emplacement. All evidence points to their derivation from a major projectile impact into preexisting acid volcanics west of the Adelaide Geosyncline. The large fragments are overlain by fine grained sediments probably redeposited from suspension following disturbance of the seafloor by impact shock waves. A final layer of crossbedded and rippled sand usually less than 10 cm thick was probably formed by current and wave reworking of proximal impact debris during later storm activity. Correlation of the impact ejecta with the Lake Acraman impact site is supported by the similarity of lithologies and shock-metamorphic structures, as well as by the size, location and estimated ages of the respective features. This is the first instance of a major terrestrial impact structure being linked with widely dispersed crustal ejecta.

14.7

SOUTH A U S T R A L I A N SEISMICITY

1978-1985

S.A. Greenhalgh^, R. Singh^, R.T. Parham^, R. Nation^ and R . McDougall^ ^School of Earth Sciences, Flinders University of South Australia ^Department of Mineral Resources, Suva, Fiji department of Physics, South Australian College of Advanced Education ^Department of Physics, University of Adelaide South Australia has long been recognised as one of the most earthquake prone regions in Australia. Seismic activity is only minor in a global context, but occasionally damaging earthquakes do occur. In the last 100 years South Australia has experienced twelve earthquakes of Richter magnitude 5 or greater, the most recent being in 1972. The largest earthquake to have occurred in the State's history was the Beachport-Kingston earthquake of 1897 (Ml 6%). The population centre of Adelaide has the highest earthquake risk of any capital city in Australia. The 1954 (Ml 5%) Adelaide earthquake involved 30,000 insurance claims totalling $6 million. The South Australian seismic network has grown steadily from its original three stations in 1963 to twelve in 1979. The network now extends to the South-East of the State (to monitor seismic activity possibly associated with volcanism in the region), and the density of the network has been increased around the northern tip of Spencer Gulf (to study the microseismicity of the seismically active Adelaide Geosyncline adjacent to this important industrial area). At the present time approximately 300 earthquakes per year are sufficiently well-recorded on the network to enable a location to be made. From 1978 onwards nearly all events of Ml greater than 2 in the known seismic regions of South Australia should have been mapped. The main limitations in the present coverage are in the far-north and west of the State. A plot of epicentres for the period 1978-1985 reiterates, in greater detail, the earlier established pattern,with most of the activity occurring in the Adelaide Geosyncline zone from Leigh Creek in the north to Kangaroo Island in the South. The greatest concentration occurs in the Flinders Ranges adjacent to the northern tip of Spencer Gulf and Lake Torrens. These epicentres appear to be associated with a north-south chain of about 40 diapirs, which have been interpreted as intrusive features which occur at the intersection of deep faults and shears in the basement system. They are zones of structural weakness which may promote crustal stress relief. The other two major zones of seismic activity are also clearly identified in the post-1978 data; namely Eyre Peninsula (particularly the eastern side) and the South-East corner of the State. Several new features are evident on the epicentral map which were not defined in previous studies. A branching of the Flinders Ranges Zone occurs near Quorn, where a line of epicentres runs north-east along the Olary Province towards Broken Hill. Another area of seismicity is at latitude 30® near the New South Wales border, which is geologically associated with the Frome Embayment. The focal depths for most South Australian earthquakes are quite shallow, being confined to the upper crust. Average magnitudes show a general increase with the focal depth. The total energy release per 5 km depth interval is essentially constant throughout the crust, implying a balance between the size and number of events, and a continuation of faulting to

considerable depth. Recurrence relationships have been established for the various seismic zones. The least squares fit for the whole of the State is: log N = 3.89 (± 0.18) - 0.99 (± 0.05) ML which yields an average return period of 112 years for an earthquake of magnitude 6 or greater. Focal mechanisms of three post-1978 earthquakes occurring in the central portion of the Geosyncline have been determined from the distribution of P wave first motion. The sense of motion is strike-slip on steeply dipping faults. The predominant principal stress direction is NE-SE compression.

6,6

CRUSTAL STRUCTURE OF SOUTH AUSTRALIA FROM EARTHQUAKE AND EXPLOSION DATA S.A. Greenhalgh^, D. Tapley^ and R. Singh^

^School of Earth Sciences, Flinders University of South Australia ^Esso Australia Ltd, New South Wales ^Department of Mineral Resources, Suva, Fiji The atomic weapons test at Maralinga during the 1950's provided the first opportunity to record long-range crustal seismic refraction profiles in South Australia. The resulting crustal model of average P wave velocity 6.3 km/s and thickness 39 km, has been subsequently revised through systematic analysis of local and regional earthquakes recorded on the South Australian seismic network. A Pj^ velocity of 7.98 km/s to 8.27 km/s has been established from various explosion seismic experiments (e.g. Ord River, Mt. Fitton) conducted by the Bureau of Mineral Resources (BMR) during the 1970's. The first positive indication of a mid-crustal (Conrad) discontinuity in South Australia was provided by Shackleford and Sutton (1981), who recorded two reversed refraction profiles from Leigh Creek and Iron Baron quarry blasts. They observed both refracted and wide-angle reflected arrivals originating from a layer of velocity 6.42 km/s underlying an upper crust of velocity 5.94 km/s. We have re-interpreted this data using an iterative ray-tracing technique to yield depth variations to the Moho and the Conrad beneath each profile. As a follow-up investigation, we recently recorded high quality, digital waveforms of quarry blasts from Leigh Creek on a 21-element synthetic aperture seismic array, located on Shackleford and Sutton's profile in the vicinity of Hawker. Digital array processing of the data has disclosed the presence of several intracrustal and sub-Moho reflections. An average crustal velocity function for South Australia has been obtained by Wiechert-Herglotz inversion of slowness measurements made for microearthquakes and various explosions recorded on the permanent and portable stations of the State seismograph network. The P wave velocity increases from 5.1 km/s at the surface (neglecting the sedimentary section) to 7.1 km/s at a depth of 35 km. Lateral velocity variations in the upper crust have been mapped using the relatively new technique of geotomography. The input data comprised P- travel times from 1340 local earthquakes recorded during the period 1978-1983. To reduce errors in the earthquake hypocentres and origin times, we used an averaging technique of dividing the area under consideration into a series of overlapping cylindrical cells. All earthquakes and seismic stations were assigned to the centre of the nearest cell. Travel times between all centres were computed by averaging over all relevant cell pairs, after first scaling all travel times by a factor equal to the distance between cell centres divided by the actual ray path length. Starting with a first guess model (based on the total energy dwell time for all rays intersecting each cell) the travel times were calculated along all ray paths and compared with the observed values. The errors (or residuals) were back-projected for each ray over all the chords along the ray and the average cell slowness adjusted in an iterative sense until the computed and observed travel times agreed within a specified error tolerance. The area with poor coverage (lack of earthquakes and/or stations) could not be imaged. Results indicate significant lateral inhomogeneity of the upper 20 km of the crust. The velocities, which range from 5.9 to 6.5 km/s, 87

correlate remarkably with time terms computed in a separate study of Moho refracted (P^) arrivals. Low time terms correspond to high crustal velocity and high time terms reflect anomalously low crustal velocity. A small component of the P time terms can be explained in terms of variations in depth to the Moho of the order of 5 km. The area covered lies mostly within the Adelaide Geosyncline, where there is believed to be a thick sequence of Proterozoic sediments. The prominent features of the tomographic velocity map (and the P time term contour map) are a north-south trending velocity low at the head of Spencer Gulf, a velocity high near Clare at the eastern edge of the Geosyncline, and a ridge of high velocity running north-east of Port Pirie, along the Flinders Ranges. The velocity low can be associated with a region of low gravity aligned parallel to the Torrens Hinge Zone. The northern end of the high velocity ridge coincides with a region of positive Bouguer gravity. However, no gravity correlation can be made for the velocity high near Clare. Teleseismic P wave residuals, computed for 200 earthquakes and 11 nuclear explosions, suggest significant lateral variability of the crust and upper mantle .beneath South Australia. Results are consistent with an upper mantle velocity anomaly of + 0.15 km/s extending to a depth range of 300 km.

4.4

LATE PRECAMBRIAN STROMATOLITE BIOSTRATIGRAPHY OF THE EASTERN PILBARA

Kathleen Grey Geological Survey of Western Australia R e c e n t G S W A g e o l o g i c a l m a p p i n g of late P r e c a m b r l a n s e q u e n c e s in the e a s t e r n P i l b a r a ( p a r t i c u l a r l y the BALFOUR D O W N S and R O B E R T S O N 1:250 000 S h e e t a r e a s ) indicates that the M a n g a n e s e G r o u p is o v e r l a i n (rather than u n d e r l a i n ) u n c o n f o r m a b l y by the Y e n e e n a G r o u p . The Y e n e e n a G r o u p is in turn u n c o n f o r m a b l y o v e r l a i n by the " C a l y i e / M c F a d d e n s e q u e n c e " ; no longer c o n s i d e r e d to be a t i m e - e q u i v a l e n t of the 1.1 Ga B a n g e m a l l G r o u p , b u t p a r t of the later P r e c a m b r l a n . R e c e n t u n p u b l i s h e d P b / P b d a t e s i n d i c a t e an age of a p p r o x i m a t e l y 8 5 0 - 9 5 0 Ma for the Y e n e e n a G r o u p . The i m p l i c a t i o n s of this revised s t r a t i g r a p h y are s t i l l b e i n g a s s e s s e d , but the d i s t r i b u t i o n of s t r o m a t o l i t e taxa is c o n s i s t e n t w i t h the n e w s c h e m e . Conophyton garganicum a u s t r a l e W a l t e r 1972 and other (as y e t u n d e s c r i b e d ) forms occur in both the M a n g a n e s e and B a n g e m a l l G r o u p s . Baicalia capricornia W a l t e r 1972 is found in both the B a n g e m a l l and the U a r o o G r o u p s , but n o t in the M a n g a n e s e G r o u p . R e c e n t studies of the B a n g e m a l l G r o u p by G . Chuck s u g g e s t that lower B a n g e m a l l c a r b o n a t e s e q u e n c e s can be d i v i d e d into two s u b g r o u p s and that B. capr icornia is r e s t r i c t e d to the upper s u b g r o u p . The M a n g a n e s e G r o u p is a p r o b a b l e time e q u i v a l e n t of the lowermost B a n g e m a l l s u b g r o u p . The Y e n e e n a G r o u p (with s t r o m a t o l i t e l o c a l i t i e s on the B A L F O U R D O W N S , R U N T O N and R U D A L L S h e e t a r e a s ) h a s taxa in common with the W a l t h a W o o r a F o r m a t i o n (which o u t c r o p s on the N U L L A G I X E Sheet a r e a ) , s u p p o r t i n g a p r e v i o u s l y p r o p o s e d c o r r e l a t i o n of these two u n i t s . Baicalia hurra P r e i s s 1972 o c c u r s at severaJ l o c a l i t i e s in P r e c a m b r l a n rocks u n d e r l y i n g the O f f i c e r B a s i n , s u g g e s t i n g a c o r r e l a t i o n with the Burra Group of the A d e l a i d e Geosyncline of South Australia. A s t r o m a t o l i t e o c c u r i n g near the b a s e of the M c F a d d e n S a n d s t o n e is very similar to Acaciella australica Waiter 1972 from the B i t t e r Springs Formation.

10.6

MOUND SPRING DEPOSITS OF THE GREAT ARTESIAN BASIN M.A. Habermehl

Bureau of Mineral Resources, Geology and Geophysics, Canberra

Groundwater from active springs, most of which are thermal mineral water springs, and carbonate rocks from fossil spring deposits in the Queensland and South Australian parts of the Great Artesian Basin have been sampled, in order to provide further information on geological and hydrogeological processes in the discharge margins, and to assist in the interpretation of the paleohydrology of the basin. Springs and seepages are abundant in the marginal areas of the Great Artesian Basin, and more than 600 springs are concentrated in eleven groups. Artesian springs are generally associated with (1) faults along which the water flows upwards, (2) the abutment of aquifers in the sedimentary Jurassic and Cretaceous sequence against impervious bedrock, and (3) confined water breaking through thin confining beds near the discharge margins of the Basin. Spring discharges are generally small, and the water reflects the regional hydrochemistry of the basin. Many artesian springs have built up conical mounds of several to tens of metres in diameter and several metres high. The mounds consist of particles brought up from the confined aquifers, the confining beds and accumulated aeolian material, and carbonate deposits. The latter are dominated by tuffs, travertine and very fine-grained or crystalline limestone, and probably originated from the combined chemical precipitation of calcium carbonate out of the artesian groundwater, and precipitation by algae and bacteria. Terraced mounds and waterfall or cascade deposits produced by algae are common, though most accumulations consist of steeply sloping mounds. Artesian springs and their deposits in the Lake Eyre region show a range from topographically high springs to younger, topographically low springs as a result of the lowering by erosion of the land surface and spring outlet levels in Quaternary times. Fossil spring deposits of very finegrained or crystalline limestone, containing abundant reed casts, gastropod shells and algal structures, rise several tens of metres above the present land surface where the present active springs occur. These older limestones of possible Pleistocene age, cap circular mesas and hills and overlie pedestals of Cretaceous mudstones. Twenty-three fully cored shallow holes (up to 30 metres deep) have been drilled with the BMR Gemco drilling rig, to provide stratigraphic information and material for sedimentological and dating studies. These holes penetrate several of the fossil spring deposits in the CurdimurkaCoward Springs-Strangways area (SA). Water sampling for hydrochemistry, environmental isotopes, and hydrocarbon and noble gases, and rock sampling has been carried out on springs in the area, and a detailed topographic and levelling survey by the Australia Survey Office, in five of the spring areas studied, was initiated.

12.3

THE GREAT ARTESIAN BASIN - A GROUNDWATER RESOURCE M.A. Habermehl

Bureau of mineral Resources, Geology and Geophysics, Canberra

The Great Artesian Basin is a multi-layered, confined groundwater basin, comprising aquifers in continental quartzose sandstones and confining beds of siltstone, mudstone and marine argillaceous sediments of Middle Triassic to Late Cretaceous age. The Great Artesian Basin occupies about one-fifth of Australia, and underlies arid and semi-arid regions. Discovery of the Basin's groundwater resources around 1880, partly as a result of the first application of scientific research to Australian water resources problems, enabled settlement and the establishment of an economically important pastoral industry. Pastoral, domestic and town water supplies are largely dependent on artesian groundwater, and groundwater from the aquifers in the Lower Cretaceous-Jurassic sequence is of good quality, with generally about 500 to 1500 mg/L total dissolved solids, dominated by sodium bicarbonate. The high temperature up to 100°C, is a minor inconvenience, and necessitates cooling. Recharge areas and regional groundwater movement patterns are characterised by different isotope ratios and hydrochemical fades. Continuing recharge from geological to modern times has been confirmed. Residence times of the groundwater have been determined with the environmental isotopes ^ C and CI, and the isotope derived isochrones are in good agreement with ages obtained from hydrodynamic data. In the southwestern part of the Basin two regional groundwater flows meet and mix , and are directed towards the main natural discharge area near the Basin margin. The westwards flowing groundwater originating from the Basin's main eastern recharge zone is of Na-HCO^-Cl type, and the eastwards flowing water, derived from the smaller recharge area in the western margin, is of the Na-Cl-SO type; salinity values increase towards the discharge area. ^ Exploration and development activities reached a high during the first half of this century, and major changes occurred in the discharges and pressures of flowing artesian waterwells. Hydraulic conditions stabilised during the last three decades, and though heavy development caused considerable drawdowns of hydraulic heads in some areas, the potentiometric surfaces of most aquifers are still above groundlevel in nearly all parts of the Basin. The majority of the flowing and pumped artesian waterwells are located in the near-marginal areas of the Basin, and free flows exceeding 100 L/s have been recorded. Further groundwater abstraction in the Basin will produce only minor changes according to model predictions. Much larger development is possible, though this will affect the potentiometric surface and free flows. Reconditioning of the wells, upgrading the wellheads and installing piped distribution systems would require the abstraction of less groundwater, and lead to a significant recovery of the artesian pressures. More efficient use of the Basin's groundwater could facilitate the expansion of groundwater abstraction without the creation of substantial withdrawal effects. An effective management program is required, guided and accompanied by modelling, together with adequate monitoring and control.

4.6

LATE PROTEROZOIC CARBONATE SHELF TO SHALE BASIN TRANSITION, WONOKA FORMATION, FLINDERS RANGES, S.A. Peter W. Haines

Department of Geology and Geophysics, University of Adelaide The Ediacaran Wonoka Formation comprises part of the upper of two major coarsening and shallowing upward (regressive) tectono-sedimentary cycles above the last (Marinoan) glacial interval, within tlte late Proterozoic sequence of the Adelaide Geosyncline. In the Central Flinders Zone, the recently-redefined Wonoka Formation consists of a carbonate-dominated sequence largely of shelf origin. Within this area, a basal siliciclastic turbidite and mudstone unit is overlain by distal carbonate turbidites which grade up into a thick interval of storm-resedimented carbonates. The sequence is capped by intertidal to supratidal carbonates and siliciclastics. The contact with the overlying red siliciclastics of the Bonney Sandstone is diachronous and involves multiple intertonguing related to repeated phases of progradation of deltaic and continental facies. At Bunyeroo Gorge and localities further south and west, most of the interval of storm carbonates has a proximal aspect characterized by thick bedding and abundant hummocky cross statification, with very little interbedded shale. In successive sections to the north and east, an increasing proportion of this interval grades into a distal storm-bed facies, characterised by thin bedded carbonates with sharp bases and wave rippled tops, interbedded with shale. This facies subsequently grades into thin bedded carbonate turbidites in some places. These facies changes and palaeocurrent data indicate a gradual deepening to the northeast. Along the northern margin of the Central Flinders Zone, the equivalent sequence is largely comprised of slope rhythmites (thin planar and rhythmically bedded limestone and shale) and thin bedded turbidites, interbedded with units of massive green calcareous mudstone. In the Northern Flinders Zone, lateral equivalents are comprised of massive green calcareous mudstone and minor carbonates of basinal aspect. Precise correlation of equivalent sequences is hampered by the general lack of fossils; however, a very thin basal dolomite and a thin but widespread "glauconitic" purple mudstone and limestone band in the upper part of the formation are considered to be regional time markers. The close association of the latter with a thin but widespread trace fossil horizon supports this view. The shelf to basin transition took the form of a distally steepened carbonate ramp. The position of the shelf-edge was apparently largely fault controlled, as its position coincides with the Norwest Fault and other related faults in the Warraweena area. Regional down-faulting of the northe m area was initiated during early Wonoka time, and basal units have been locally removed by slumping and erosion along this hinge line. In adjacent areas, the same units show palaeocurrent evidence of rapidly changing palaeoslope direction, uncharacteristic of the Wonoka Formation in more stable areas. There is marked evidence of increased slope in the lower Wonoka Formation along this hinge line, including thick debris flows, slope generated slumps and intraformational truncation surfaces of varying scale. The latter often consist of mudstone overlying mudstone with little or no associated coarse debris and thus probably represent slope-generated slump scars rather than erosional surfaces. Some, however, do contain preferential accumulations of turbidites and debris flows and may represent the shallowly-incised tributaries of several larger-scale and more deeply-cut turbidite-filled erosional channels in this area and further north.

Syndepositional salt diapirism occurred in a number of areas within the Adelaide Geosyncl ine, but active diapirs were concentrated along the shelfedge during late Wonok^ time, probably in response to the active faulting in this area. Contemporaneous diapirism is associated with marked local facies changes, indicative of local shoaling, and associated with stromatolitic carbonate build-ups in the Warraweena area. Abundant reworked material of obvious diapiric derivation indicates periodic exposure of the diapir core. Well-preserved sequences adjacent to other diapirs display local progressive angular unconformities related to successive phases of upturning of marginal sediments, followed by erosion before sedimentation resumed. The stratigraphic position marking the first evidence of diapiric influence in the Warraweena area coincides with a vertical facies change from proximal to more distal storm-dominated sedimentation further south. This suggests that the diapiric islands were beginning to form partially effective wave barriers at this time. Diapiric islands formed very effective barriers during latest Wonoka time. The island chain separated relatively quiet lagoon, tidal flat and supratidal environments in the south from deeper,open marine storm-influenced to tidal siliciclastic deposition in the north. Subsidence was much more rapid north of the hinge line, where a considerable thickness of relatively-mature sands are the lateral time equivalents of relatively-thin ooid, peloidal and stromatolitic limestones, and interbedded immature siliciclastics, in the south. The black organic-rich nature of the lagoonal limestones indicate restricted circulation behind the barrier. Recent workers have suggested that the Proterozoic-Cambrian Adelaide Geosyncline is in part a passive continental margin sequence with pre-rift, rift and post-breakup sedimentary packages preserved. Evidence from the Wonoka Formation suggests deposition in the post break-up phase marginal to a relatively deep ocean to the north and southeast (not discussed here) of its present outcrop.

12.4 ENVIRONMENTAL ASPECTS OF PROPOSED IN-SITU LEACH MINING AT BEVERLEY - SOUTH AUSTRALIA Stephen Hancock Australian Groundwater Consultants Pty. Ltd, Melbourne

The Beverley Uranium Project is based upon a sedimentary (roll front?) uranium deposit in fluvio-lacustrine sediments of the Miocene Namba Fm. on the margins of the Lake Frome Basin adjacent to the North Flinders Ranges. The mineralization is mainly in the form of coffinite with some uraninite. Sulphides are minor. A significant proportion of the mineralization occurs in permeable confined aquifers which exist amongst clayey deposits at depths between 100 and l^lO m below land surface. These mineralized deposits are hydrologically confined by a dense^ low permeability^ illitic clay bed which varies in thickness from 5.5 to 27 m. This clay is defineable across the extent of the ore body by the characteristic trace it produces on geophysical log curves. The overburden includes alluvial fan deposits of the Upper Tertiary Willawortina Fm._, a semi confined to unconfined aquifer, which is used for stock water supply. Underlying the ore body is a thick sequence of shales and dolomite which form the major thickness of the Namba Fm.. These separate the orebody from the other main aquifers of the region, the Palaeocene - Eyre Fm. and the Lower Cretaceous Cadna - Owie Fm.. Hydrogeological relationships in the area are complicated by structural deformation with faulting providing vertical hydrological interconnection between the deeper aquifers. Similarly, the shallower aquifer zones show complex recharge and hydrochemical relationships due to variation in recharge opportunity, discharge and flow patterns. These have created recharge mounds beneath creeks which have prevented water exchange in the mineralized sands. All these aquifers are highly valued and the proponents have had to address the problems of aquifer protection during and beyond mining life very carefully. Conventional mining would have required major dewatering of the Willawortina Fm. aquifers and would have resulted in direct connection between that aquifer and the radio-active waters in the Namba Fm. upon cessation of mining. For these reasons it is proposed that the uranium resources be mined by in-situ leach techniques, otherwise known as solution mining. The detailed evaluations of the site involved detailed drilling and coring with geological definition being obtained from geophysical log suites calibrated against detailed core logs and evaluations. Evaluations of the hydrology were undertaken by pump and slug testing. Careful examination of hydrogeochemistry and consideration of the in-situ leach chemistry as it would operate on the minerals in the aquifer and confining beds during and beyond mine life were carried out. Computer modelling has been undertaken to test in situ hydrological stress development during mining and careful core collection and handling was undertaken to preserve in situ conditions so that detailed laboratory examinations and evaluation of hydrological properties could be performed. 94

The results indicate that the mineralization has been protected from dissolution by the orientation of the mineralized sands, by structural controls and by the recharge mounds deriving from the streams at either end of the ore b o d y . In-situ leach mining technique proposed will use sulphuric acid with hydrogen peroxide or dissolved oxygen as the lixivient. Pre-treatment of the aquifer will be necessary to remove excess calcium carbonate, and the system will employ a slightly overpumped output of fluid through the wellfield to reduce the risk of excursions of mining solutions. The input and output patterns will also be varied to take account of the hydro-geological conditions such as confining bed thickness and permeability. Much study has been directed towards the post mining condition of the ore zone and the threat it may pose to the water resources of the region. This has involved consideration of the realistic value of water and the aquifers in the area and detailed consideration of the physical and chemical reactions consequent on mine cessation and the monitoring and management of these. The mineralogy of the clays and sands include exchangeable and soluble calcium and carbonate which will act to both neutralize acids and cause gypsum precipitation when a spent sulphuric acid leachate moves into unconditioned material. These, plus the presence of continuous, regionally extensive, dense, clay layers both overlying and underlying the mineralized zone preclude significant chance of leachate escape. Problems of improperly sealed exploration drill holes remain to be resolved and operational optimization of both mining and rehabilitation techniques awaits the pilot testing stage. The proponents and their consultants are confident that in-situ leaching at Beverley can be conducted without the many problems which have been encountered or attributed to the originators of this mining technique in North A m e r i c a .

1.1

THE STRATIGRAPHIC SETTING OF THE BROKEN HILL OREBODIES - SEDIMENTARY OR VOLCANIC?

Robert C. Haydon and Geoffery W.

McConachy

The Zinc Corporation Limited, Broken Hill

Ihe Broken Hill lead-zinc-silver lodes and three other less significant zones of lead-zinc-silver mineralisation namely White Leeds/Rising Sun^ the Western Mineralisation and the Centenary Mineralisation are hosted by rocks of the Broken Hill Group which is part of the Willyama Supergroup. Data from correlation of 100,000 metres of diamond drill core from the mine environs, show that while the compositional and distributional character of the lower units within the group are similar in each zone; within the upper portions the lithologies and their distribution are unique in each zone and characterised by rapid facies changes. The sequence is interpreted as a major upward-coarsening cycle of sedimentation representing a prograding clastic wedge deposited in a deltaic environment. Garnetiferous quartzofeldspathic gneisses known locally as Potosi gneiss, characteristic of the upper formation of the Broken Hill Group and previously regarded by some workers as an acid volcanic, are considered here to constitute immature clastic sediments. The significant Pb-Zn mineralisation at Broken Hill is also associated with the upper formations of this group as well as the basal formation of the overlying group and is hosted in lithologies interpreted to have been originally coarse-grained. Electric log (gamma, neutron-neutron, density, resistivity) signatures are shown to highlight the sedimentological character of the succession and show the value of the tool for both stratigraphic correlation and sedimentological interpretation in the Broken Hill area where an extensive number of drill holes is available. Similar gamma and neutron-neutron electric log signatures and trends for barren psammites and lode mineralisation serve to support the view that the orebodies at Broken Hill are psammite-hosted and enclosed within pelitic and psammopelitic metasediments. No lithologies capable of being interpreted as meta-acid volcanics can be found associated with the orebodies in the upper formations of the Broken Hill Group. A volcanic exhalative origin for the Orebodies and mineralisation at Broken Hill is thus rejected in favour of a sedimentary exhalative-type origin.

1.11

THE MENNINNIE DAM LEAD-ZINC-SILVER PROSPECT, SOUTH AUSTRALIA M.L. Higgins and K.J. Hellsten Billiton Australia Australia, Adelaide

The Menninnie Dam Prospect represents a discovery of "blind" lead-zinc-silver sulphides within a Lower Proterozoic supracrustal sequence on northern Eyre Peninsula, South Australia. The sequence is one of mixed pelitic and psammitic clastics and chemical sediments with likely volcanic input, which represents a particularly carbonate-rich variant of the Hutchison Group. The area has undergone Upper Amphibolite facies prograde regional metamorphism with peak conditi ons of 650-700 C and 3-4 kbars as inferred from the calcsilicate assemblages. At least one retrogressive metamorphic event is identified. Exploration by RAB, percussion and diamond drilling, sirotem and magnetic surveying has outlined a 4 x 2 km inlier of Hutchison Group lithologies, unconformably overlain by Middle Proterozoic Gawler Range Volcanics. The latter crop out extensively in hills surrounding the prospect. Exposure of Hutchison Group lighologies is limited to one small outcrop of silicified carbonate, originally mapped as Warrow Quartzite. The mineralised sequence is completely concealed by between 20-60 metres of calcrete, silcrete and kaolin-quartz material. The Hutchison Group sequence is subvertical and is represented by basal Warrow Quartzite of unknown thickness. Overlying this is SUITE 1 ankeritic dolomites, graphitic and quartz-biotite schists, SUITE 2 psammopelitic schists with rare amphibolites, SUITE 3 calcite marbles with clusters of thin massive sulphide bands, and an uppermost SUITE 4 of mixed ophicalcite, ankeritic dolomite, calcsilicates, BIFs, graphitic schists, cherts and clusters of thin massive sulphide bands. The ophicalcite is a dominant lithology in Suite 4 and is a mottled magnetite-forsterite marble, thought to represent the metamorphic product of a reaction between two main original rock constituents, namely impure dolomite and chert. The main massive Pb-Zn-Ag sulphide mineralisation occurs within SUITE 4 and is hosted by finely laminated graphitic ankeritic dolomite, bounded stratigraphically above and below by amphibolitic magnetic-BIF horizons and pyritic graphitic schists. This mineralisation occurs as clusters of bands comprising massive pyrite-sphalerite-galena with minor chalcopyrite. These bands are individually in the range of 0.5 m wide, but can contain between 10 and 50% Pb + Zn, some examples including: 0.65 m @ 5.8% Pb, 14.0% Zn, 60 g/t Ag; 0.6 m @ 23.0% Pb, 12.4% Zn, 108 g/t Ag; 1.92 m @ 1.5% Pb, 3.65% Zn, 44 g/t Ag; 0.55 m @ 13.3% Pb, 38.5% Zn, 180 g/t Ag. Host rocks between the massive sulphide bands may be barren or weakly mineralised, thus broader intersections bulk out to best widths of, for example, 2.3 m @ 5.4% Pb, 14.0% Zn, 117 g/t Ag and 3.4 m (? 7.3% Pb, 5.2% Zn, 67 g/t Ag. The gangue is talc, carbonate, quartz and tremolite with the commonest being talc. The massive sulphides display millimetric banding due to grainsize and sulphide-gangue compositional variations. They reveal ample evidence of deformation and metamorphic recrystallisation including. In places, coarse anhedral, polygonal textures.

The stratigraphic footwall sequence contains base metal sulphides over at least 170 metres and, together with the main mineralised zone, is characterised by unusually high Mn levels which average in excess of 1% MnO. Included in the footwall is a 25 m wide zone of pyritic dolomites which average 9.3% MnO, and an underlying 20 m wide zone of chert-dolomite"sulphide breccia. Both zones contain about 1.5% Pb + Zn throughout and are intimately spatially related to best massive Pn-Zn-Ag sulphide development. These may represent "stringer" type hydrothermal mineralisation. The base m e t a l sulphides are thought to have been deposited within a localised, restricted pool or sub-basin in a sub-tidal or upper shelf facies carbonate-mudstone sequence. They were localised by organic detritus and correlate with a stratigraphic interval identified by increased iron and manganese chemical precipitation and potassic tuffaceous sedimentation. Hydrothermal activity underlying this pool vented base m e t a l - b e a r i n g solutions upwards to surface, forming siliceous and pyritic breccia and disseminating sulphides throughout the footwall sequence.

6.9

THE GEOMETRY OF LITHOSPHERE EXTENSION Gregory Houseman

Research School of Earth Sciences, Australian National University, Canberra In recent years, lithospheric extension models such as that proposed by McKenzie have been used increasingly to explain the tectonic subsidence, thinned crust, and high heat flow found on Atlantic type continental margins and in many sedimentary basins. More recently, attempts have been made to explain the dynamics of extensional tectonics in relation to the stresses generated by convective flow in the mantle. Hot thermal plumes or sheets in the convecting mantle cause uplift of the overlying lithosphere, and thus introduce a horizontal extensional stress regime. Previous calculations have shown that current rheological models of the continental lithosphere, in which a brittle upper layer overlies a temperature activated ductile layer, permit large extensional strains for the lithosperic stress field induced by uplift of approximately one kilometre.In this mechanism, uplift is the precursor to subsidence of the surface and sediment deposition, which follows if there is significant extension. In the calculations referred to above, the geometry of extension was required to be two-dimensional, as if the uplift resulted from a long linear thermal sheet in the mantle. However, the geometry of convective flow in the mantle is not well constrained. It is considered unlikely that hot upwelling regions in the mantle are only found beneath mid-ocean ridges. On the contrary, it is quite probable that at least some of the upward flow is in the form of axial type plumes which may be associated with hot-spot activity and which appear to be scattered around the globe. The Hawaiian hotspot is probably the most clearcut example of this type. The numerical experiments presented here aim to investigate the extensional response of a continental lithospheric sheet which is uplifted by an axial type plume, and to describe in horizontal planform the distribution of crustal thinning with respect to the axial plume. The lithosphere is modelled as a thin viscous sheet with non-Newtonian rheology, and the stress balance equations are solved using the finite element method. This method allows the lithospheric response to be studied for different effective lithosphere rheologies, including cases where there are pre-existing strength heterogeneities in the lithospheric sheet. If the effective rheology is strongly non-Newtonian, necking of the lithosphere is predicted to develop from initial minor perturbations to a horizontally uniform lithosphere thickness. For plane-strain extension the magnitude of an initial crustal thickness perturbation can increase by a factor of approximately where B is the average extension factor (the ratio of the area of the extended lithosphere to its original area) and n is the stress-strain rate exponent. The numerical experiments permit the extensional response of the lithosphere to the axial stress field to be investigated for randomly distributed initial perturbations. The results will be compared in general terms with the observed structure of extensional type basins such as the Bass basin and the North Sea basin.

8.2

PERMIAN COALS OF THE INTRACRATONIC COOPER BASIN J.W. Hunt! and Michelle Smyth^ ^Delhi Petroleum Pty Ltd, Adelaide ^CSIRO Division of Fossil Fuels, Sydney

The Australian Permian cratonic basins contain an abundance of coals which are characterised by their high inertinite contents. The inertinite contains a large proportion of inertodetrinite compared with Permian foreland basin coals and coals of other ages. The inertinite macerals micrinite and macrinite, which are otherwise rare, are also common in the cratonic basin coals. The Cooper Basin, which is the main basin discussed, contains within the Patchawarra Trough an unusually thick (>30m), inertinite-rich coal, here named the Malabine Coal, with low sulfur (<0.55 wt%) and mineral matter (6 vol%) contents. Petrographically and chemically similar but aerially restricted thick coals are also found in several smaller Permian cratonic basins, - the Wolfang, Blair Athol and Oaklands Basins. The cratonic basin coals, in particular the Malabine Coal, are associated with mainly fluvial sediments. In the Cooper Basin lacustrine delta-fill sequences of the Epsilon Formation and Daralingie beds contain thinner coals with higher vitrinite and sulfur contents. Sulfur content is low (<0.55%) in the fluvial coals, and medium in the deltaic coals (0.55% to 1%). High sulfur coals (>1%) are rare, confirming the absence of marine sulfate during deposition of the coals. There is no evidence that in the Permian the type of flora and/or the climate were significantly different in the cratonic basins, compared with the nearby foreland basins. Therefore we do not consider these factors to be the main control of coal type in the cratonic basins. In the absence of modern petrographic analogues, subsidence rate and associated water table fluctuations are assumed to be the factors that controlled coal type. Variation of petrographic composition in coals from the Patchawarra Trough parallels variation of subsidence rates inferred from sediment thickness. Vitrinite content in coals of the Toolachee Formation increases and the amount of inertodetrinite decreases from the edge to the centre of the Patchawarra Trough. Under conditions of slow subsidence, water table variations were probably extreme. Shallow flooding and drying of the peat resulted in sub-aerial and sub-aqueous oxidation off the accumulating organic matter, to give the inertinite-rich coal type found today. Deposition of the thick coals in deep lakes is considered unlikely, because of their extent, thickness and low mineral matter contents. A modern morphological analogue for these coals is most probably the high-latitude blanket mire.

100

4.10

RALPH TATE'S ENIQIA - AND THE REGIONAL SIGNIFICANCE OF THRUST FAULTING IN THE MT. LOFTY RANGES Richard J.F. Jenkins

Department of Geology and Geophysics, University of Adelaide In a Presidential Address to the Royal Society of South Australia over a century ago, Tate (1879) commented on the huge cumulative thickness of sediments present in the Mt. Lofty Ranges and noted that "it is remarkable that the apparently less metamorphosed strata occupy the lowest position". Geophysics does not substantiate a deep crustal root and some of the oldest sediments show the greatest degree of defomation: hence the enigma. Known faults in the central and southern Ranges are of two main categories. Major faults indicated by zones of strongly-foliated rock, commonly with markedly deformed breccias or zones of small attenuated isoclinal folds, and rotated bedding-cleavage lineation, are apparently of Palaeozoic origin and primarily comprise thrusts dipping eastwards between 20®and 45®. At the eastern margin of the Ranges, a thrust dips westwards. In the central Ranges, the thrust faults form an imbricate pattern. Sub-horizontal, raft-like masses of the Stonyfell Quartzite enveloped by phyllites and arranged in an en-echelon manner characteristically show leading anticlines and trailing synclines; overturned limbs are broken or attenuated. The rafts clearly define an antiformal thrust culmination. The oblique intersection between the axis of this culmination and the pattern of thrusts suggests that the inliers of basement gneiss and entire sedimentary prism represent an allochthon. It is suggested that this has been emplaced over a decollement. Faults represented by lines of brittle fracture (undeformed breccias) and tending to be associated with some of the major Palaeozoic thrusts mark south-eastern boundaries of Tertiary sub-basins. They reflect the reactivation of older zones of weakness at low confining temperatures and pressures. The major thrust masses mantling the basement cores and para-autochthonous Torrensian window are each clearly characterised by their strato-tectonic components, style of folding, and metamorphic grade. The gross sequence shows a geosynclinal cyclicity with flyschoid and molassic sequences especially well developed during the Cambrian. Towards the east, adjacent thrust elements show upward increase in metamorphic grade, suggesting emplacement of 'hot slabs' at the time of thrusting. The metamorphosed, but commonly rather broadly deformed, mainly flyshoid sediments of the Cambrian Kanmantoo Group constitute the major internal zone, but a small synformal gneiss belt enveloped by migmatites possibly represents the earliest emplaced and highest thrust element/s. Mylonites, mafic bodies and granitic plutons occur in the most internal region, which is evidently intersected by a major, NNW-SSE trending, dextral, transform shear zone in the underlying basement. Magmatism occurred along this zone, which extends towards the Tertiary rift between Tasmania and Antarctica. The major thrusts within the Ranges likely propagated from Riedels. A molassic basin floors much of the present Gulf St. Vincent: on the north coast of Kangaroo Island, this molasse evidences overthrusting and progressive unroofing of the tectogene. In scale and intensity of deformation, the southern Mt. Lofty Ranges show similarities to parts of major ensialic mountain belts such as the Pyrenees, Alps and Appalachians. 101

1.6

Rb-Sr GEOCHRONOLOGY OF LATE ARCHAEAN SYENITIC INTRUSIVES, EASTERN YILGARN BLOCK, W.A.

G.I. Johnson and J.A. Cooper Department of Geology and Geophysics, University of Adelaide Mildly alkaline syenitic intrusives of the eastern Yilgarn Block crop out sporadically from Fitzgerald Peaks, 100 km south-west of Norseman, to Lake league, 100 km north-east of Wiluma. Members of the suite are characterised by the presence of one or more of the following : alkali pyroxene, alkali amphibole, mesoperthitic alkali feldspar, and low modal quartz contents. Rb-Sr isotopic studies are being carried out on several discrete, widely separated syenitic bodies. Data obtained to date substantiate the conclusions of previous workers that alkaline activity was a late Archaean phenomon in the Yilgarn Block, but suggest that such activity was more complex than was previously thought. Gilgarna Rock, lying 103 km east-north-east of Kalgoorlie, is a small circular body of approximately 1 square km in area. It is composed of two syenitic phases, referred to here as the medium grained phase and the coarse grained phase. Both phases consist of coarse euhedral to subhedral zoned alkali feldspar, aegirine-augite and quartz, with accessory sphene, fluorite, calcite, zircon, apatite, opaques and occasional phlogopitic biotite. Magnesioriebeckitic amphibole is occasionally present as a replacement product of aegirine-augite. The coarse phase contains xenoliths of the medium phase, indicating it to be younger in age. In addition, the coarse phase is cut by a narrow dyke of finer grained quartz syenite. The two main phases are mineralogically similar in hand specimen, but are distinguishable on standard Marker and ternary elemental variation diagrams. The medium phase has higher Fe203, MgO and Ti02 contents than the coarse phase, but lower AI2O3 and K2O contents. Both phases have high Na20 values (7-8%); evidence of internal sodic metasomatism during the latter stages of crystallisation of both phases includes the following:- alkali feldspars exhibit albite rimming, aegirineaugites have aegirine-rich rims, and magnesioriebeckite replaces and rims aegirine-augite in places. The quartz syenite dyke crosscutting the coarse phase shows no evidence of such metasomatism. New whole-rock Rb-Sr isotopic data yield perfect-fit Model 1 ages for both the medium and coarse phases. The medium phase gives an age of 2642 ± 57 Ma with an initial 87Sr/86Sr ratio of .70064 ± .00039 and the coarse phase gives an age of 2542 ± 17 Ma with an initial ratio of .70142 ± .00013. Both ages are interpreted as crystallisation ages, and are consistent with observed field relations. Our Gilgarna Rock data show that initiation of alkaline activity was earlier than previous Rb-Sr data suggested, although a poor Pb-Pb isochron from the Red Hill area, 400 km north of Kalgoorlie, indicated this possibility (Stuckless et al, 1981). In addition, we have shown that a second intrusion of similar but geochemically distinguishable syenitic magma occurred some 100 Ma after the first episode. The very low initial 87Sr/86Sr ratios suggest sources for these rocks of depleted mantle type, e (Sr) values for the medium and coarse phases are -11.09 ± 3.9 and -1.76 ± 1.3 respectively. If we assume a multi-stage derivation of the syenitic magma from a source similar to CHUR, it can be suggested that the source region for the medium grained phase would have been strongly depleted earlier than 3320 Ma. Partial melting of this material at 2642 Ma could have given rise to the medium grained phase. Further tapping of the medium grained mkgma type 100 Ma later could have led to the intrusion of the coarse grained phase. Geochemical indications are that the quartz syenite dyke crosscutting the coarse phase is probably a differentiate 102

of the coarse phase, and analysed dyke samples lie on the perfect fit isochron for the coarse phase. Only one of the previously determined Rb-Sr ages for the syenitic rocks of the eastern Yilgarn Block is useful for such petrogenetic investigations. This is the Twelve-Mile Well body, situated 106 km north-east of Kalgoorlie, which yielded an age of 2489 ± 82 Ma and an initial ratio of .7012 ± .0003 (Libby and DeLaeter, 1980). This gives an s (Sr) value of -4. ± 3., which supports the idea of depleted mantle type sources for these rocks.

103

11.3

SILICA-RICH lAYERING AT BLANCHE POINT, SOUTH AUSTRALIA J.B. Jones and M.J. Fitzgerald

Department of Geology and Geophysics, University of Adelaide The late Eocene Blanche Point Formation at Blanche Point begins with about two and a half metres of glauconitic clay with a rich shelly fossil fauna constituting the Tuketja Member. This is succeeded by some twelve and a half metres of the Gull Rock Member, also a similarly fossiliferous calcareous mudstone but characterised by the presence of several tens of silicified layers of variable thickness. Siliceous sponge spicules are common within the Member but are abundant enough to oharactevize the ten and a half metres of the overlying Perkana Member. The Formation is completed by the Tuit Member of about seven metres of calcareous mudstones, again characterized by the presence of several silicified nodular bands. Calcite - and to a lesser extent aragonite - are abundant throughout the section and are probably entirely biogenic detritus. Opal-A as sponge spicules reaches a peak in the Perkana Member but occurs throughout. The primary clay mineral throughout the section appears to have been an essentially pure smectite, although much of this is altered to kaolinite within the Perkana Member. Clinoptilolite and glauconite occur throughout the Formation with the latter conspicuous in the Tuketja Member but the clinoptilolite much reduced in the Perkana Member. Angular quartz grains of average diameter less than lOOym are present to the extent of a few per cent: they are clear and show sharp extinction under the polarising microscope. The silica of the silicified layers is opal-CT occurring both as a featureless cement but also as lepispheres in open cavities and coating sponge spicules ; it also occurs as disseminated lepispheres in the less lithified portions of the Formation. Most workers in the field regard pure or nearly pure smectite as the dominant mineral in the less than 2ym fraction of a sediment as conclusive evidence of altered fine volcanic detritus. Add to this the presence of significant clinoptilolite, glauconite and opal-CT; all characteristic, although not uniquely so, of volcanic sequences; and the conclusion that the non-biogenic portion of the sequence is of volcanic origin seems inescapable. Although there is no direct evidence of shallow water (such as the presence of ripple marks), the presence of glauconite, the average Mn0/Ti02 ratio of 0.02 - 0.25, the neritic character of the fauna and the pervasive bioturbation all indicate shallow water conditions. Again, although there is no direct evidence of its presence, it seems certain that there would have been extensive bottom flora of algae and the like. The presence of silicified layers in approximate coincidence with bedding and of reasonably constant thickness over tens of metres in the Gull Rock Member and again in the Tuit Member suggests to us a process of repeated silicification which in turn suggests a repetitive causal mechanism. Since the sequence is volcanic, the obvious possibility is repetitive volcanism. A model is developed whereby, following the death of algae and other organisms as a result of a sufficiently rapid and large ashfall, organic chelating ligands are released which react with silicon to form anionic octahedral complexes. These complexes are a couple of orders of magnitude more soluble in water than silica, and the resultant solution moves downwards until physical conditions of lower pH or raised oxygen 104

tension causes it to decompose releasing silica. With cessation of significant ash deposition, a flourishing bottom biota would again re-establish, allowing the process to be repeated by the next eruption. The details of the model are complex and probably not fully understood but it does appear to fit the observed chemical trends and is capable of explaining the variability of layer thickness.

105

6.5

SURFACE WAVE TOMOGRAPHY AS A TOOL TO STUDY CRUSTAL HETEROGENEITY B.L.N. Kennett Research School of Earth Sciences, Australian National University, Canberra

For most earthquake sources a large fraction of the seismic energy travels as relatively slowly moving surface waves in which the energy is concentrated near the Earth»s surface. At high frequencies, 0.4 Hz or greater, these surface waves are sensitive to the presence of changes in the structure along their propagation. In particular, the phase Lg with a group velocity around 3.5 km/s is strongly affected by horizontal gradients of velocity within the crust. The Lg train can be viewed as the superposition of multiply reflected S waves trapped in the waveguide formed by the low S velocities lying between the surface and the Mohorovicic discontinuity. When the crustal structure varies horizontally the regular pattern of intracrustal reflections is disrupted and the character of the Lg train is altered. Thus the presence or absence of sediments along portions of the propagation or changes in crustal thickness will have an effect on the appearance of the recorded Lg wave train. With a good areal coverage of a region by surface wave propagation paths between sources and receivers, the differing character of the Lg phase can be used to map heterogeneity in crustal structure. Such a technique enables the oil-bearing graben structures in the North-Sea basin of Northwestern Europe to be delineated using remote observations. Propagation across the graben almost extinguishes the Lg wave energy and so the character of the Lg waves can provide a strong constraint on the location of these major crustal features. Theoretical calculations show that the severe attenuation of the short period surface waves can arise from crustal thinning beneath the main sediment accumulations in the graben structures. In other areas the effects can be more subtle and good path coverage can be difficult to achieve. In the Bass Strait the character of the Lg train appears to correlate well with the sediment distribution in the Gippsland and Bass basins, without involving more complex structures.

106

1.2

ZIRCON AGES FROM THE NARRYER METAMORPHIC BELT Peter Kinny Research School of Earth Sciences, Australian National University, Canberra

Zircons are strongly resistant to isotopic disturbances, and comparison of the dual decay schemes of and allows a determination of the systematics of radiogenic Pb loss to be made. Thus, U-Pb isotopic studies of zircon can provide reliable rock crystallization ages in many geological environments. However, in Archaean terrains interpretation of zircon data is complicated by the preservation of multiple stages of zircon growth in a single rock. This problem has been overcome by the development of a high resolution ion-microprobe capable of precise U-Pb isotopic analyses on a 25]xm scale. This technique permits the characterization of individual pre-magmatic, magmatic and metamorphic components in meta-igneous rocks and individual detrital components of metasediments. Using the ANU ion probe we have investigated at a reconnaissance level the zircon geochronology of major lithologies in the Narryer Metamorphic Belt in the northern part of the Western Gneiss Terrain, Yilgarn Block, Western Australia. The results record a complex episodic history of crustal formation, deformation and metamorphism that extended over the entire duration of the Archaean period. Original magmatic zircon from samples of banded biotite adamellite gneiss representative of an extensive but poorly-outcropping series of orthogneisses (termed the Meeberrie Gneiss) indicate ~ 3670 Ma as the time of formation. These gneisses were intruded at - 3^20 Ma by hornblende-bearing leucogneiss (the Dugel Gneiss) which contains enclaves of an older suite of layered meta-anorthosite, gabbro and ultramafic rocks (the Manfred Complex). Zircons which crystallized in the anorthositic components indicate an age of 3750 Ma; these rocks are therefore the oldest known in Australia. However, they were clearly not the first crustal rocks to have formed in the region: detrital zircon in quartzites at Mount Narryer and in the Jack Hills have been found with ages up to 4300 Ma old. Other components in the orthogneiss terrain give a protolith age of 3500 Ma. There are many examples from the orthogneisses and from the anorthositic inclusions of zircons which have metamorphic overgrowths, and some cases of whole grains which grew in response to metamorphism. The ion probe U-Pb data for this material give an age of - 3320 Ma which agrees with a whole rock Rb-Sr isochron for the Meeberrie Gneiss and is interpreted as the time of major deformation which resulted in gneiss formation. Less-deformed porphyritic granite intruded soon after at 3300 Ma. The youngest detrital zircons in the sillimanite-bearing Mount Narryer quartzite and in a sequence of paragneisses which occurs west of Mount Dugel give U-Pb ages of - 3100 Ma; these provide a maximum limit for the time of deposition and demonstrate that the high-grade metasediments belong to younger sequences than the orthogneisses with which they are in tectonic contact. Zircon from one sample of hypersthene-garnet-cordierite paragneiss have overgrowths dated by the ion probe at - 2780 Ma. We believe that this age represents a second major regional tectonothermal episode which was responsible for the high-grade mineral assemblages preserved in the metasediments, and this interpretation is in agreement with the results of ""^Ar-^^Ar data on hornblendes from these rocks. 107

The last major addition to the crust in this area was the intrusion of granites between 2600-2700 Ma which are similar to those granites which intrude the greenstone belts to the east. One sample from near Mount Narryer, which has given an Sm-Nd ^.^del age of 3120 Ma, contains magmatic zircons 2630 Ma) with older cores apparently inherited from the source rocks which melted to form the pluton. Some cores are only 50 m.y. older but most have an age of ^ 2920 Ma which doesn't correspond to the known age of any exposed crust in the region. Summary of Archaean Geological History 4300-4100 Ma 3750 Ma 3700-3400 Ma 3350-3300 Ma 3100-2800 Ma 2780 Ma 2700-2600 Ma

Formation of earliest known crust Formation of gabbro-anorthosite complex Intrusion of orthogneiss protoliths Major deformation and metamorphism followed by felsic magmatism Sedimentation of quartzite and paragneiss protoliths (following uplift) High-grade metamorphism and deformation Felsic magmatism

The relationship between the Western Gneiss Terrain and adjacent granite-greenstone terrains of the Yilgarn Block is unclear, and numerous models have been proposed. There is mounting evidence from recent zircon studies that the WGT may represent part of an extensive sialic basement on which the greenstone sequences formed, such as the discovery of zircon xenocrysts in the Hangingwall Basalt at Kambalda with U-Pb ages in excess of 3400 Ma, and the occurrence of detrital zircon in a quartzite from the Maynard Hills in the central Yilgarn with U-Pb ages up to 3750 Ma. It would therefore be worthwhile to investigate whether inherited zircon cores exist in the granites of the central and eastern Yilgarn similar to those which occur in the young granites at Mount Narryer.

108

12.5

MARALINGA SOUTH AUSTRALIA - AN IDEAL DISPOSAL SITE FOR AUSTRALIA'S INTRACTABLE WASTES

Michael J . Knight School of Applied Geology, University of N.S.W. Australia has quantities of intractable waste containing stable compounds that are difficult to dispose of currently in an environmentally acceptable manner. The main wastes in this category are industrial organo-chlorine compounds (e.g. PCB, Dioxin) and some radioactive wastes. There are about 8000 tonnes of organo-chlorine compounds stored in Australia and the generation rate is nearly 950 tonnes/year. Low to Intermediate level radioactive wastes from institutional (e.g.' medicine) and industrial sources are being generated at about 2 cm^/person/year. In aggregate, the stored and generated radioactive waste to 1995 could be placed in a single trench 5m deep, 5m wide and 40m long. There have been strong moves in the last five years to dispose of the intractable chemical wastes by high temperature (~1200°C) incineration. However, the three attempts to site a burner (Sydney, Melbourne and Broken Hill) have met severe opposition and have, to date, failed to establish a facility. Maralinga is an ideal site to locate the National Facility required for both chemical and radioactive wastes. There are two possible disposal strategy options; Option A consists of an incinerator and two secure landfills and Option B; one deep (550m) geological repository excavated in Archean diorite which is overlain by 300 m of Chocolate Shale. Groundwater at Maralinga is mainly located in sandstone and limestone units of the upper 150 m zone above the shale. Groundwater salinities are generally high and in some areas approach sea water (30,000 ppm or more). The area is remote, seismically stable, pastorally very poor and has very low rainfall and nearby there are soils rich in lime that could neutralize any acidic fallout if needed. The site has been contaminated in part by the past British Atomic Weapons testing program. One advantage of the deep geological repository option is the possibility of providing a safer site for long-life elements that need to be cleaned up from the contaminated ground surface. Royalties could be paid to South Australia by states that use the facility and there would be some employment generated. The site is well serviced by a sealed road from a siding on the Trans-Australia railway. Rail transport from most significant generation points in Australia is safe and inexpensive (about $50/tonne). The capital cost for either option will be $10 - 20 m . Unit disposal costs amount to about $300 /tonne for a mined cavity isolation and $500 1000/tonne for incineration (chemical wastes). A secure landfill would cost $150,000 - 300,000 (capital cost) with a disposal cost of $1030/tonne. There is an urgent need for Federal legislation and action on this matter since a single National Facility could result in a $10 m saving by processing chemical wastes that are currently directed to overseas incinerators or destroyed in the ship "Vulcanus" that occasionally visits Australia. 109

10.7

D E P O S I T I O N A L ENVIRONMENTS OF THE L O C H I E L D E P O S I T Andrew Kremor The Electricity Trust of South A u s t r a l i a

The Lochiel Coal Deposit is in the northern part of the St. Vincent basin and is currently being assessed for use as a source of energy for electricity g e n e r a t i o n . To complement detailed geological investigations carried out on the d e p o s i t , a study of depositional environments and their related lithofacies has been u n d e r t a k e n . In the Lochiel a r e a , the Tertiary sequence has been divided into seven members based on lithology and geophysical borehole log r e s p o n s e . These have been defined from interpretation and correlation of over 2U0 boreholes which have been drilled at a spacing of less than one kilometre and these included 10 fully cored holes which provided a means of defining geophysical to lithological s i g n a t u r e s . To aid correlation of lithology and definition of depositional environment the members have been divided laterally into sedimentary facies and vertically into sedimentary c y c l e s . The correlations indicate that the lower part of the sequence was dominated by fluvio-lacustrine processes with the upper being lacustrine and estuarine. The distribution of facies within the area appears to be strongly related to localised structural f e a t u r e s . The basal Bumbunga Sand has been divided into three facies which represent a gradation of environment from fluvial channel through distributary and floodplain to p e a t s w a m p . This member is overlain by the Condowie Silt which consists of a cyclic sequence of lacustrine sediments and their laterally equivalent tloodplain and peatswamp d e p o s i t s . At the northern end of the a r e a , the top of the Condowie Silt has been eroded by the upward fining fluvial Darnleigh Park S a n d . Further to the n o r t h , this member is indistinguishable from the bumbunga S a n d . The Kooliata Coal Zone conformably overlies both the Condowie silt and the Darnleigh Park Sand and consists of three coal bearing horizons separated by well defined but thin interbeds of sand and s i l t . The coal zones have been divided into a coal facies and marginal sandy coal f a c i e s , these relate to specific sites in the peatswamp depositional e n v i r o n m e n t . The interbeds separating the coal bearing horizons are single lacustrine cycles which consist of facies relating to marginal s w a m p , shoreline and deeper w a t e r depositional e n v i r o n m e n t s . The Kooliata Coal Zone is overlain by the Warrindi Silt which is a highly variable lithology with lacustrine and lacustrine delta fill depositional environments producing a number of interdigitating f a c i e s . The sequence to the top of the Warrindi Silt is late upper Eocene and this is overlain by Oligocene marginal marine sediments ot the Tarella S i l t . This member is consistent in lithology and the top surface has been affected by regional tilting to the northwest and subsequent e r o s i o n .

110

10.12

FORAMINIFERAL BIOFACIES IN AN EVOLVING HOLOCENE EMBAYMENT; TOURVILLE BAY, SOUTH AUSTRALIA Karen Lablack^ and Antonio Belperio^ ^Geology Department, University of Adelaide ^Geological Survey of South Australia

Tourville Bay is a protected embayment on the west coast of South Australia created approximately 7000 years ago by postglacial drowning of a calcreted landscape. Active sedimentation under a stable sea level since then has largely infilled the bay with mixed biogenicsiliciclastic sediments. Spring tide range is about 2 m, and a tidal zonation of sedimentary environments and fades is well developed. Modern environments include subtidal seagrass meadows, bare intertidal sand flats, mangrove woodlands, cyanobacterial and samphire marshes, and gypseous supratidal flats. Organic carbon content of surficial sediments is highest in the mangrove woodland environment, which is also a zone where humic acids contribute to carbonate dissolution and dolomitization. Subsurface sediments are dominated by Posidonia australis seagrass bank lithofacies. Seagrass bank accumulation largely infilled the bay by 5000 yr B.P., and intertidal progradation has occurred across this surface. The biogenic contribution to the seagrass fades sediment has largely resulted from the in-situ accumulation of epiphytic and epibenthic fauna and flora, and includes foraminiferal, algal, mollusc and bryozoan remains. Mineralogy of the sediment is principally aragonite, calcite and low magnesian calcite, with total carbonate content typically approaching 90% by weight. Residual cellulose fibre from seagrass leaf sheaths contributes to stability and accumulation of this fades. Accumulation rate is of the order of 2 mm/year, whereas intertidal accumulation is an order of magnitude lower. Foraminiferal assemblages were examined from surficial sediments taken across the embayment. Three size fractions (1.0-0.5; 0.5-0.25 and 0.250.125 mm) were examined and the relative abundance of selected foraminiferal species were measured and compared. Relative abundance measurements may be used to delineate local sedimentary environments, and are consistent between size fractions. An example is Discorbis dimidiatus, which preferentially inhabits tidal sand flat and mangrove woodland environments. By contrast, Peneropolis planatus is most abundant in sediments of the subtidal Posidonia seagrass fades. Foraminiferal assemblages were also examined from samples taken at 20 cm intervals from a vibrocore that penetrated the entire Holocene sequence. Relative abundance measurements down-core show variations that reflect the original sedimentary environment (as deduced from lithological attributes) similar to that displayed by the surficial variations. Little intermixing of sediment between adjacent sedimentary environments appears to take place, although there is a consistent dilution of sedimentary components by reworked (Pleistocene) lithoskels. Comparison with foraminiferal data from elsewhere around the State indicates that particular species associations are locally rather than regionally significant, although the techniques used are applicable along other tidally influenced coasts.

Ill

1.9

METALLOGENY OF THE WILLYAMA SUPERGROUP, BROKEN HILL W.P. Laing^ and R.G. Barnes^ ^James Cook University of North Queensland, ^New South Wales Geological Survey

The approach toward a metallogeny of the Willyama Supergroup is made along two i n i t i a l l y separate paths. An integration of a l l regional data defines a number of nappe limbs which represent separate palaeogeographic e n t i t i e s , and the distribution of metallic deposits is then superimposed on this pattern. Five tectonostratigraphic domains have been delineated, corresponding to alternate limbs of recumbent nappes. Their mutual boundaries trend north to northeast, parallel to the generally colinear high-grade fold axes and stretching directions. From southeast to northwest they are: 1.

Broken H i l l domain of inverted stratigraphy;

Maybell domain of right-way-up stratigraphy;

S t i r l i n g Vale domain of inverted stratigraphy;

Mount Robe domain of inverted stratigraphy;

Kantappa domain of right-way-up stratigraphy.

After "unfolding", the nappe limbs r e f l e c t their present relative d i s t r i bution, with the Broken Hill limb at the base of an en-echelon nappe p i l e whose upper parts root from the west. The Willyama Supergroup is tectonically a failed Proterozoic r i f t orogen which subsequently became cratonised. The nappe p i l e perhaps formed a drape over closing "failed r i f t " basement, regional metamorphism subsequently producing a prograde pile in which the grade decreases progressivel y upward, from transitional granulite in the Broken H i l l limb to transitional greenschist/amphibol i te facies in the Kantappa limb. The presence of the recumbent isoclinal nappe pile soon after the onset of metamorphism produced isograds generally parallel to bedding. The foregoing division into nappe limbs has been achieved largely independently of metal occurrences. Superimposition of the l a t t e r data reveals a wealth of patterns of regional metal deposit distribution, with strong partitioning of many deposit types into one or two domains. Some metallogenic implications of the tectonic model are as follows: 1. The Broken Hill orebodies have been transported to their present position and do not l i e over whatever crustal feature may have fed them. 2. The Broken Hill orebodies interfingering rhyodacite.

were

hosted

clastic

sediments

and

3. The Maybell domain contains abundant small pods of base metal mineralisation which have been heavily explored and d r i l l e d in the past. This s t y l e , traditionally called "Broken Hill" type mineralisation, is associated with stratiform amphibolite and lesser acid metavolcanic of the Parnell Formation, stratigraphical 1 y lower than the Broken Hill orebodies in the Hores Gneiss. It is not associated with BIF and in detail is commonly not stratiform, but transgressive in local remobilised quartz+ gahnite veins. This amphibol i te-rel ated mineralisation d i f f e r s s i g n i f i cantly from the acid metavolcanic-rel ated Broken Hill type mineralisation, and here is termed Parnell type. 112

4. L i t t l e Broken H i l l , the area commonly ranked next i n Pb+Zn+Ag p o t e n t i a l to Broken H i l l , was o r i g i n a l l y partway between Broken H i l l and the Maybell domain to the west. L i t t l e Broken H i l l type m i n e r a l i s a t i o n ( g a r n e t - r i c h , presence of BIF, base metals i n both Hores Gneiss and P a r n e l l Formation) i s t r a n s i t i o n a l between Broken H i l l type and P a r n e l l type mineralisation. 5. S i g n i f i c a n t Cu m i n e r a l i s a t i o n i s r e s t r i c t e d to the Broken H i l l domain, where small s t r a t i f o r m Cu deposits are widespread i n the lower, v o l c a n i c dominated p a r t of the Willyama sequence, below the Broken H i l l Group. 6. The only Au m i n e r a l i s a t i o n , i n an otherwise Au-free p r o v i n c e , i s i n inverted domains: Diamond J u b i l e e and the S i l v e r King deposits (Mount Robe domain) and Panama Hat and ?Copper Blow (Broken H i l l domain). 7. W m i n e r a l i s a t i o n occurs i n a l l but the Kantappa domain, with i t s abundance decreasing s y s t e m a t i c a l l y from northwest to southeast. In the Mount Robe domain W occurs i n the Ettlewood C a l c - s i l i c a t e and i r r e g u l a r l y w i t h i n e q u i v a l e n t s of the Hores Gneiss. In the Maybell domain W occurs i n amphibolites of the P a r n e l l Formation (Corruga type d e p o s i t s ) . In the Broken H i l l domain W i s r e s t r i c t e d to the Mine area. 8. S t r a t i f o r m Sn occurs i n the mature sediment r i f t - c o v e r a t the top of the Willyama Supergroup i n the Kantappa domain. 9. The Ettlewood Cal c - s i l i c a t e occurs only i n the Mount Robe domain, whose i n f e r r e d provenance i n the 01 ary region to the west i s c o n s i s t e n t with the Ethiudna Cal c - s i l i c a t e i n the Willyama sequence a t 01 a r y . Their i n t e r p r e t a t i o n as shallow water carbonates suggests a s h e l f to the west. W m i n e r a l i s a t i o n i s known i n the Ethiudna Cal c - s i l i c a t e . Broken H i l l type m i n e r a l i s a t i o n as now d e f i n e d , occurs only i n the Broken H i l l domain. P a r n e l l type m i n e r a l i s a t i o n occurs l a r g e l y i n the Maybell domain but i s a l s o found i n the Mount Robe and Broken H i l l domains. The Hores Gneiss r e g i o n a l l y contains only rare m i n e r a l i s a t i o n . The Hores Gneiss shows a systematic f a c i e s v a r i a t i o n from a massive probable ash f l o w - u n i t i n the Mount Robe domain (Yanco Glen), through a mix of v o l c a n i c / sediment l a y e r s i n the Maybell domain, to predominantly sediment, i n the Broken H i l l domain. This progression i s i n c r e a s i n g l y d i s t a l from the i n f e r r e d acid v o l c a n i c c e n t r e , which a f t e r r e s t o r a t i o n of the Mount Robe nappe limb i n d i c a t e s a v o l c a n i c source to the (north)west near 01 a r y . The pre-nappe, western margin of the r i f t was between Mount Robe and Olary (the western l i m i t of s i g n i f i c a n t vol unes of basic magma), and the eastern margin may have been a t Broken H i l l (the apparent eastern l i m i t of Hores Gneiss a c i d v o l c a n i c s , and the possibl y mantle-tapping metal feeder zone). A d i s t i n c t i v e feature of the s t r a t i f o r m Pb+Zn+Ag metal logeny i s the d i s p a r i t y between the widespread low-grade P a r n e l l type m i n e r a l i s a t i o n and the r e l a t i v e l y rare high-grade Broken H i l l type m i n e r a l i s a t i o n . The Parnell type m i n e r a l i s a t i o n i s seen as an expression of "ambient" r i f t i n g - g e n e r a ted metal-bearing f l u i d s throughout the sedimentary trough. The Broken H i l l type m i n e r a l i s a t i o n i s d i s t a l to the probably coeval Hores Gneiss v o l c a n i c c e n t r e , but i s c l e a r l y proximal to the metal feeder system. The apparent " o n e - o f f " nature of the major metal e x h a l a t i o n suggests t h a t the genetic connection between ore and acid v o l c a n i c host rock i s l a r g e l y c h r o n o l o g i c a l : t h a t the same deep-seated process was r e s p o n s i b l e f o r each, on opposite sides of the r i f t . This metallogenic model, and the approach used to d e r i v e i t , has important i m p l i c a t i o n s f o r e x p l o r a t i o n i n the Willyama province and i n the other A u s t r a l i a n Middle P r o t e r o z o i c orogenic b e l t s of s i m i l a r t e c t o n i c s t y l e . 113

3.9 STRATIGRAPHIC RATIONALISATION OF THE EASTERN MOUNT ISA BLOCK, RECOGNITION OF KEY CORRELATIONS WITH GEORGETOWN AND BROKEN HILL BLOCKS IN AN EASTERN AUSTRALIAN FROTEROZOIC TERRAIN, AND THEIR METALLOGENIC IMPLICATIONS W.P. Laing and T.J. Beardsmore James cook University of North Queensland This paper presents preliminary new data and a new interprecation of the Middle Proterozoic eastern Mount Isa Block (MIB) which rationalises previously defined multiple stratigraphic units into a single sequence. This widespread homogeneous sequence bears striking similarities with the sequences of similar age in the Broken Hill Block (BHB) and the Georgetown Block (GB). The ramifications of the interpretation are explored at two levels, within the eastern MIB itself and between the MIB and the other Blocks. Current mapping around the Williams Batholith in the eastern MIB provides a subdivision of previously undifferentiated high-grade Soldiers Cap Group, and its correlation with the Mary Kathleen Group west of the Batholith in the Selwyn area. These two north-south trending belts lie on the flanks of the regional, north-south trending Selwyn Range Anticlinorium of Dp age, whose core is occupied by the Williams Batholith. The Kuridala Formation on the western and southern flanks of the Anticl inorium is divided into four stratigraphic units. Three correspond to the previous recognised threefold subdivision of the Soldiers Cap Group in the Cloncurry area, and one new unit stratigraphically underlies the others. A fifth unit, the Double Crossing Metamorphics, probably underlies this fourth unit: both are feldspathic metasedimentary packages. In the Soldiers Cap belt on the eastern and northern flanks of the Anticlinorium, correlatives of the latter new units occupy significant volumes, with a bedded feldspathic schist sequence (probably underlain) by a unit of migmatitic high-grade gneiss. This correlation replaces the Kuridala Formation and necessitates designation of two new stratigraphic units below the Soldiers Cap Group. The total area of "Soldiers Cap" metasedimentary facies in this rationalisation is of the order of 15,000 km , prior to unfolding of the Selwyn Range Anticlinorium. This major rationalisation of the lower part of the Mount Isa stratigraphic sequences produces close parallels with the sequence in the nearest exposures of similar Middle Proterozoic terrains, the GB and the BHB respectively 300 km to the east and 1200 km to the south. The new, lower two units (3-?5 km thick) in the Soldiers Cap belt represent immature, feldspathic, probably partly volcanically-derived sediments. They correspond to the feldspathic lower half of the Willyama Supergroup (Clevedale Migmatite to Thackaringa Group) and less certainly to parts of the presumed lower half of the Etheridge Group (Einasleigh Metamorphics). Missing at Soldiers Cap is an obvious equivalent to the middle unit in the Willyama Supergroup, the Broken Hill Group, which marks the transition from immature feldspathic sediments to mature quartz-rich sediments, from volcanic-dominated environment to non-volcanic environment. However rocks similar to the quartzofeldspathic (+/-qarnet) "Potosi" gneiss which forms two of the marker units in the Broken Hill Group, occur in several localities in the Soldiers Cap belt, and may represent an equivalent unit. At Georgetown the Einasleigh Metamorphics appear to embody a change from less- to more mature sediments, with a quartzofeldspathic leucogneiss (Ee2) near the change and calcareous rocks below it; these features may represent a Broken Hill Group equivalent in the GB, with the calcareous rocks equivalent to the Ettlewood Calc-silicate of the BHB. 114

The upper three units in the Soldiers Cap Group represent a relatively mature sediment package which correlates in detail with the upper part (Sundown and Paragon Groups) of the Willyama Supergroup of the BHB and with the upper Etheridge Group (Lane Creek Formation to Langdon River Mudstone) of the GB. These sequences, from 3 to 6 km thick, are each characterised by a lower unit (1-2 km) of partly turbiditic, non-carbonaceous, homogeneous, mature (aluminous) sediments, and an upper sequence (2-5 km) of variably carbonaceous, generally fine-grained sediments containing lenses and horizons of the following distinctive lithotypes: banded iron formation, impure dolomite, basal t/dolerite, and plagioclase-bearing quartzite. The uppermost unit characteristically contains pyrite. Critical elements of the tectonic fingerprint of each terrain are identical. The lower, feldspathic half of the sequence in each case) is at the highest metamorphic grade and in places occupies inverted parts of folds, ie probable F^ inverted nappe limbs. The metamorphism in each case is low-medium pressure/high temperature style, with sillimanite+Kfeldspar+ garnet assemblages in pelitic rocks and with limited production of migmatite. Direct physical connection between the two Queensland Blocks and the BHB cannot be proven. However a detailed study of the gravity data between the MIB and the GB, shows graphic evidence of a continuous, arcuate swing from NNE-trending anomalies on the margin of the Soldiers Cap belt to ENEtrending anomalies over the GB, strongly coincident with major anticlinoria and synclinoria in the latter area. In each Block the respective trends correspond with F^ fold orientations. The correspondence in one anomaly extending west from the GB is so close that one anticlinorium can be extrapolated under Mesozoic cover for 150 km, and drill-hole data shows predicted carbonaceous upper Etheridge Group rocks at this distance from the GB. The strati graphic correlation between the three Blocks is strong enough to warrant serious followup of its implications. Further work is planned to refine the Soldiers Cap stratigraphy to permit more detailed environmental comparisons, and geochronological sampling for age-of-sedimentation of the "Potosi" metavolcanics has commenced. Some metallogenic implications are already clear. The Soldiers Cap "Pegmont" style Pb+Zn stratiform mineralisation i'^ stratigraphically higher than Broken Hill style mineralisation, but the latter is probably correlative with the Georgetown "Mount Misery" style mineralisation. The 100 km-long strike length of Pegmont style occurrences is at the same stratigraphic level as the 100 km strike length of Selwyn/Mount Dore crosscutting Cu style mineralisation; each of these shows individual deposits of +10 million tonnes. Are the upper parts of the Broken Hill and Georgetown sequences as barren as they appear, and does the newly discovered lower part of the Soldiers Cap sequence contain Broken Hill style mineralisation? Does the lower Soldiers Cap feldspathic sequence contain Cu, Au and/or Co? Do the upper parts of the Soldiers Cap and Etheridge Groups contain stratiform W and Sn? These three grouped stratotectonic terrains differ in important ways, the differences include important features of their style of stratiform Pb+Zn deposits, from the Mount Isa terrain; that at least two of them appear to be physically connected suggests they might be segments of a single Eastern Australian Proterozoic terrain. This concept opens up many possible cross-correlations and fruitful implications, including a number of new exploration strategies for each terrain segment. 115

6.4

SEISMIC EVIDENCE FOR LATERAL DEEP CRUSTAL STRUCTURE IN CENTRAL AUSTRALIA Kurt Lambeck and Greg Burgess Research School of Earth Sciences, Australian National University, Canberra

Larabeck and Penney recorded travel times of teleseismic P waves across the central Australian basins and arches and concluded that the observed residuals were consistent with variations in depth to the Moho of up to 20 km. The average station spacing for the 700 km long line was about 30 km, too great to examine in detail the rapid variations in travel times recorded at some sites. A new line of closely spaced (about 10 km) seismic recorders has now been analysed. The north-south line extends from a point west of Gosse Bluff, across the northern Amadeus Basin, the southern Arunta Block and the Ngalia Basin to a point near the Ngalurbindi Hills south of Mt Allan. The recorded variations in travel times exceed 1 second over distances of a few tens of kilometers. These station anomalies are strongly dependent on the azimuth of the earthquake source region. The most rapid gradients occur for stations north of the northern margin of the Amadeus Basin with arrival times being early over the Arunta and late over the basins. Inversion of the travel times from different source regions (Japan and Marianas; Fiji-Tonga-Kermadec; Macquarie Ridge; Indian Ocean Ridge) results in the crustal model discussed.

116

4.16 ISOTOPIC AND FLUID INCLUSION STUDIES OF CAMBRIAN PALAEOENVIRONMENTS, EASTERN OFFICER BASIN, SOUTH AUSTRALIA I.B. Lambert^, T.H. Donnelly^, P.N. Southgate^, H. Etminan^, G. Weste^ ^Baas Becking Laboratory, Canberra ^Comalco, Adelaide Evaporation in arid Lower Cambrian epeiric sea environments resulted in deposition of a thick carbonate-evaporite sequence in the Officer Basin. Extensive red siltstones were then deposited in arid environments in the eastern portion of the basin. Middle Cambrian alkaline playa lake carbonates and siltstones formed in restricted areas on these red beds. This lacustrine sequence is characterised by calcite and dolomite-chert pseudomorphs after Na-carbonate minerals, by magadiite-type chert nodules, and by minor oil bleeds. Isotopic and fluid inclusion studies have been conducted to obtain further information on palaeoenvironmental conditions. The Sr isotope ratios of the carbonates and anhydrite/gypsum in the trilobite-bearing Lower Cambrian sequence are close to 0.709, typical of lower Cambrian mag^ne gglues. In contrast, the lacustrine carbonates have much higher Sr/ Sr ratios, around 0.722, suggesting that they formed from waters which incorporated radiogenic Sr from older felsic rocks. In general terms, the C- and 0-isotope data for Officer Basin carbonates do not display trends which readily distinguish marine from lacustrine sequences. This reflects the potential for waters in evaporative inland basins to evolve towards comparable 0-and C-isotope compositions to seawater. In detail, however, there are trends which^^ssist in elucidation of palaeoenvironmental conditions. The 6 C values of both the marine and the lacustrine carbonate strata are concentrated between 0 and -3°/oo5 indicating that no alj)gormally high concentrations of organic matter were buried. The 6 0 values for the marine carbonates are mainly between 20 and 23°/ooj with some sporadic lower values possibly reflecting late-stage recrystallisation by groundwaters. In the lacustrin^gstrata, micrite, small spotty pseudomorphs and carbonate crusts have 6 0 values maiiji^y between 24 and 28°/ooThese values indicate that considerable 0-enrichment occurred during evaporation of ground and surface waters entering the lake system. The larger pseudormorphs of sodiYg carbonate minerals are characterised by significantly lower 6 0 values, concentrated between 19 and 22.5°/oo and fluid inclusions with variable salinities and homogenisation temperatures up to ca. 110°C. These features imply that the trona and shortite were dissolved and pseudomorphed as a result of extensive influx of heated waters that mixed to varying degrees with the relatively saline interstitial brines. 34 Anhydrite and gypsum from the Lower Cambrian sequences have 6 S values concentrated between 30 and 36°/ooj the normal range for marine evaporites of this age. However, the sulfur isotope compositions of pyrite in the Lower Cambrian strata range widely, between areas.^^These are interpreted in terms of open marine environments ^^egative 6 S ranges) and closed-off bodies of seaw^jter (positive 6 S ranges). Pyrite in the lacustrine strata has 6 S values from -3 to 10°/oo # consistent with influx of non-marine waters after regression. 117

8.1

CAMBRIAN OIL REVISITED

R.A. Laws and D.I, Gravestock Geological Survey of South Australia Thick Cambrian sequences are widely distributed in South Australia. Attention was drawn to the petroleum potential of these rocks following oil shows in a water bore near Port Augusta in the 1950's. This discovery led directly to acquisition of the large petroleum exploration licences within which now lie the oil & gas fields of the Cooper and Eromanga Basins - fields which were initially found "accidentally" while drilling to the main Early Palaeozoic objective. Following the discovery of gas in the Permian at Gidgealpa in 1963, interest swung away from the Cambrian to concentrate on the younger rocks. A re-evaluation of the Cambrian has now commenced and the optimism of 30 years ago is re-awakening. The 2000m or so of sediments present in the Officer Basin underlying the Devonian largely comprise Cambrian to ?Late Proterozoic carbonates and clastics ranging from shallow marine through lagoonal, fluvial to aeolian in origin. This sequence probably extends southwards under the Maralinga Lands and Eucla Basin where evaporites, redbeds and cherty carbonates encountered in a number of drillholes are presumed to be of the same age. Oil shows in shallow core holes have been encountered in both areas within this sequence. Further south in the offshore Polda Trough over 1500m of halite and overlying redbeds are also presumed to be age equivalent. Salt diapirs and thick sediments are evident on seismic lines in the Arckaringa Basin region north of Coober Pedy may also be age equivalent, but remain to be drilled. Shallow marine platform and deeper slope carbonates overlain by a redbed sequence have been encountered in a broad swathe extending from Yorke Peninsula, north through the Flinders Ranges/Lake Frome region and are also known in the Gidgealpa area. This sequence includes algal/archaeocyathan reef-mounds recently studied in outcrop in the northern Flinders Ranges. Flysh is widespread in the eastern Mt Lofty Ranges, where granite intrusion, tectonism and metamorphism would have destroyed any petroleum. Deep water sediments possibly also occur northeast of the north Flinders Ranges and east of Gidgealpa. The hydrocarbon potential of the Cambrian is indicated by: - oil shows in shallow holes have been encountered over a broad area; - both actual & potential carbonate reservoirs are known, the former in vuggy dolomites on Yorke Peninsula (where the saline connate water indicates lack of flushing) and the latter in reefal (Flinders Ranges) and playa lake (Officer Basin) carbonates; - clastic reservoirs are widepsread in the Officer & Polda Basins as well as in the Flinders Ranges/Lake Frome area; - although the trap potential is uncertain due to lack of specific data, the regional data are such as to suggest that structural traps with the potential to contain petroleum, do occur. Industry' s recent interest has been shown by the award of new petroleum exploration licences in the Officer & Eucla Basins and plans by DelhiSantos to drill in the Arckaringa Basin. It is hoped that the next round of exploration will confirm that the Cambrian of South Australia has a high potential for petroleum. 118

4.7

ADELAIDEAN SEDIMENTATION ADJACENT TO THE ENORAMA DIAPIR, CENTRAL FLINDERS RANGES, SOUTH AUSTRALIA N.M. Lemon

Department of Geology and Geophysics, University of Adelaide Umberatana Group sediments outcrop around an elongate body of breccia known as the Enorama Diapir, just north of Oraparinna in the central Flinders Ranges of South Australia. These sediments are of late Proterozoic age and are part of the Adelaidean sequence deposited within the Adelaide Geosyncline. Sediments surrounding the diapir start with marine limestones and interbedded shales of the Etina Formation. These grade upward into the marine Enorama Shale, then the shales and shallow-water limestones of the Trezona Formation. Conditions became restricted, as did the area of deposition at the top of the Trezona, and erosion on a disconformity removed at least 150 metres of sediment in the central parts of the geosyncline. Continued subsidence in local areas, particularly around the Enorama and Oraparinna Diapirs, allowed deposition and preservation of an intertidal sequence at the top of the Trezona Formation, tentatively called the Yaltipena Member. The Trezona Formation is overlain by the Elatina Formation, a generally fluvioglacial unit with some glacial diamictites, possibly deposited in lacustrine conditions. Marine conditions returned with dolomites and shales of the Nuccaleena Formation capping the glacials. These mark the base of the Wilpena Group. The interbedding of shales and limestones, both within and between formations, reflects changes in the depositional water depth. Limestones show shallow-water features and the shales and silts a deeper-water environment down to and below storm wave base. The formation boundaries are very widespread and reflect basin-wide changes in sea level. Detailed mapping within the Etina and Trezona Formations shows a similar widespread distribution of the limestone-shale alternations, again reflecting basin-wide sea-level changes. These provide excellent time lines between which thickness and facies changes can be mapped. Most mappable units between time lines within the formations thicken toward the diapir over a distance of 6 kilometres. Units with suitable outcrop distribution show rapid thinning within 500 metres of the breccia. The deeper water shales show lenses 0-5 to 80 metres thick of coarse angular gravels developed within this zone close to the breccia. Shallowwater limestones show a far wider distribution of gravel derived from the breccia with gravel being most extensive at the level of the short hiatuses which occur within the bands of the upper part of the Etina Formation. Grainsize distribution within the upper part of the Etina Formation emphasizes these points. Shales at this level contain lenses of conglomerate swept into the basin by slumping and turbidity currents. These conglomerates are usually matrix supported, sub-angular, have a dolomitic matrix and appear as ''slump diamictites". Only the thickest beds extend more than 500m away from the breccia from which they were derived. At the level of the three limestone bands at the top of the Etina Formation, shallow-water stromatolites and beach sands dominate. These contain channelized, sub to well rounded, clast-supported conglomerates with a matrix of ooids, limestone intraclasts and sand. The bottom third of the Enorama Shale shows remarkable variation near the breccia with rapid thickening into a local sink with thick bands of con119

glomerate and slump diamictite. Many of these have a dolomitic matrix and there are some levels of dolomite and limestone which show remnant gypsum and anhydrite in sequence. Along the western margin of the breccia body at Mallee Water, the conglomerates and dolomites are steeply upturned against the breccia. Turbidites a little further up-section show bimodal flute directions, parallel to the margin of the breccia but I8O0 opposed. The Enorama Diapir does not appear to have been active during the deposition of the rest of the Enorama Shale and throughout most of the Trezona Formation, with no local input of detritus. However, there is an apparent paradox in the Trezona Formation with a much thicker section developed near the diapir in shallower water conditions. There is a considerable increase in the number and thickness of locally-derived intraclastic and oolitic limestones near the diapir, indicating a local high. This increase in thickness translates into a situation of deposition and preservation for the Yaltipena Member near the diapir compared to non-deposition and erosion away from it. Similarly, the Elatina Formation is better developed near the diapir, with evidence that some of the clasts in the diamictites near the top of the formation were derived locally from the breccia. All the features described above are readily explained if the Enorama Diapir is considered to have been an active diapiric intrusion during sedimentation. The geometry of reconstructed palinspastic sections thickening toward the diapir then rapidly thinning can be modelled in sand box experiments of syn-sedimentary diapirs. This also shows the dramatic upturning close to the diapir. The diapir appears to have behaved as an island remaining at or above sea level for a long period. Analogies to this situation can be seen around present-day salt diapirs in the Persian Gulf. These form low islands of breccia composed of rafts and inclusions carried to the surface by the rising salt. There is now no sign of the salt as it has been removed by dissolution down to depths of at least 50m. When sea level was low around the Enorama Diapir, a beach developed around the margins and pulses of diapiric activity caused well-rounded conglomerates to be washed away from the island along tidal channels. This debris was mixed with ooids and limestone intraclasts abraded from fringing stromatolite mounds. At higher sea levels, conglomerates were swept into the basin in a series of slumps triggered by pulses of activity in the diapir. At times, there may have been a lagoon over the diapir caused by the dissolution of the evaporitic component. This lagoon could have been isolated by a ring of the turned-up sediments around the margin so that hypersaline conditions from the underlying salts would have allowed the deposition of dolomite and evaporites. Diapiric activity breached the lagoon barrier and swept the gypsic and dolomitic muds into the basin along with breccia and minor beach deposits. With less diapiric uplift, the breccia may not have breached the sediments being deposited but just caused thinning over a local dome. The peripheral sinks around the diapir allow for local thickening and would channel any currents such as turbidites generated by diapir movement.

120

6.16

QUARTZ VEINING DURING FOLDING IN GREYWACKE SEQUENCES P.O. Lennox School of Applied Geology, University of N . S . W .

The temporal, geometrical, spatial and silica mass balance relationships between quartz veining and folding in greywacke sequences are at present poorly constrained, because previous studies have concentrated on geometrical relationships solely. Through an understanding of quartz veining in a simply folded terrane (Cape Liptrap, Victoria) compared with a multiply folded terrane (Bermagui, N.S.W.), it is possible to timetable the dominant stress field changes during folding and deformation in greywacke sequences. The quartz veined mesoscopic folds at Cape Liptrap outcrop within the Lower Devonian Liptrap Formation, a sequence of interbedded arenites and mudstones on the extreme eastern margin of the Melbourne Trough. The quartz veined mesoscopic folds at Bermagui outcrop in undifferentiated Ordovician greywacke and slate on the eastern margin of the Lachlan Fold Belt. Both sequences have been metamorphosed to lower greenschist facies conditions during deformation(s) associated with mesoscopic fold development and associated cleavage formation. In contrast to the single phase of deformation producing gently-plunging, upright folds 4-7m) at Cape Liptrap, the Bermagui exposures have been subjected by up to 5 phases of deformation producing variously, asymmetric, gently-plunging, upright to recumbent folds 2 - 5 m ) with axial surface cleavage, cleavage unrelated to any folds or kinks. The mesoscopic folds at Cape Liptrap exhibit incongruent parasitic folds 1-2 m) with an axial surface slaty cleavage in mudstones and a rough cleavage in arenites. The dominant cleavage, a differentiated crenulation cleavage at Bermagui which is axial planar to the first phase of folding appears stripey in outcrop in contrast to cross-cutting, millimetre to centimetre spaced crenulation cleavages which are axial surface to later fold phases. In both folded greywacke sequences it is possible to define definite quartz vein sets consisting of clusters of quartz veins of a similar size and orientation within the 0.5-1m thick, quartz-veined, fold - defining arenite b e d s . Invariably these sets are at a high angle to bedding, and consist of planar or semi-planar, planar to sigmoidal en echelon quartz veins. Whereas quartz veins at Cape Liptrap are usually fibrous and have formed by crack-seal deformation processes, the quartz veins at Bermagui are non-^fibrous and m a s s i v e . Quartz veins can best be subdivided into sets on the basis of their orientation in relation to the mesoscopic fold axes into three main sets (parallel, oblique or normal to the fold axis). The prolific quartz veining in folds at Fold Stack enabled identification of a consistent sequence of overlapping sets in the hinge zone and limbs of the mesoscopic f o l d s . The observed sequence of sets in the hinge zone corresponds to that predicted by theoretical models of quartz vein formation involving buckling of a competent bed in a thick incompetent m a t r i x . Overall the degree of quartz-^veining within folds at Bermagui is less than that at Cape Liptrap and hence a less comprehensive quartz vein sets history can be derived. Analysis of quartz veining from an area dominated by the first phase'of folding at Bermagui compared with an area containing coaxial refolding of the first phase of folding, indicates that the bulk of quartz veins formed during or after the first phase of folding and before the second phase of folding either parallel or normal to the fold axes. 121

Silica mass balance calculations for the quartz-^vein containing fold-^ defining arenite beds at Cape Liptrap indicate that advective mass transfer mechanisms must have been important during deformation. Oxygen isotope analyses and fluid inclusion studies are continuing on quartz vein specimens from Bermagui, so as to characterise the fluids and conditions during quartz vein development.

122

6«14

BASEMENT THRUSTS IN THE SOUTHERN ADAVALE BASIN J.H. Leven and D.M. Finlayson

Bureau of Mineral Resources, Geology and Geophysics, Canberra

Major basement thrusts have been delineated in the southern Adavale Basin and the northern Quilpie Trough using BMR and oil industry seismic data. Basement involved tectonics have played a major role in the formation of the Adavale Basin and the associated troughs and the subsequent development of the of the overlying basins in this region. This paper addresses the nature of these basement structures, and their influence on the tectonic development of the southern Adavale Basin region. The Adavale Basin is a concealed epi-cratonic basin of Devonian age, which underlies the more extensive Eromanga Basin in southwestern Queensland. The basin is comprised of a main depression and several isolated troughs, which are erosional remnants of a once more extensive depositional basin, which was deposited upon the Thompson Fold Belt. These troughs include the Barcoo Trough to the west, the Warrabin Trough to the southwest, the Quilpie Trough to the south, and che Cooladdi and Westgate troughs to the southeast of the main depression. The Devonian troughs all have a predominantly north-south structural axis, and have been preserved from later Carboniferous erosion by depression along steep monoclinal folds. The folding is interpreted as resulting from east-west compression, in which the Thompson Fold Belt as well as the Adavale Basin sediments deformed in a relatively ductile manner. This style of deformation is illustrated on the eastern side of the Cooladdi Trough, and on both the eastern and western sides the Quilpie Trough. Following the monoclinal folding, a later phase of tectonic activity produced major thrust faults with a predominantly east-west strike. The Quilpie Trough is truncated to the north by the Como Thrust, while the Cooladdi Trough has been isolated from the Blackwater Trough to the north by the Grenfield struture. The regional tectonic streTss responsible for both the Grenfield and the Como structures has been interpreted as a north-south compressional phase. In contrast with the ductile style of deformation associated with the previous east-west compressional phase, this strain was accommodated by thrust faulting in either the upper or middle crust. The seismic section from BMR Linai 10 shows the Como Fault with a throw of over 2000 metres, and a thrust p.'ane dipping north at approximately 70 degrees. A fore-thrust splay with a smaller dip and a throw of around 750 metres, disrupts the sediments of.the Quilpie Trough to the south of the main thrust. Reflection events within the basement beneath the Como fault align with the fault plane of this splay, and have been interpreted as reflections from the mylonized shear zone of this splay fault within the basement. This interpretation constrains the deeper geometry of the fault, and indicates that the thrust plane becomes less steeply dipping with increasing depth - possibly soling out in the mid-crustal region. BMR Line 10 shows that the basement block between the Como and Paradise Faults has undergone a rotation of approximately 7 degrees. This rotation is interpreted as resulting from the thrusting on the Como structure.

123

The Cooladdi Trough is truncated at its northern end b y the G r e n f i e l d S t r u c t u r e . The seismic section from PING 6 (Mesa A u s t . Ltd.,1982) shows this feature as a major thrust fault with a throw of around 1500 metres and a dip of approximately 50 degrees. On BMR Line 11, which also crosses this feature in a roughly parallel direction some 17 km to the e a s t , the G r e n f i e l d structure is seen to be an uplifted b a s e m e n t block w i t h dimenions of the order of 20 k m . Evidence from the deeper seismic data from Line 11 indicates that this basement uplift has resulted from a thrust developing in the mid-cristal region, and producing the upper crustal u p l i f t . Concurrently w i t h , or following this tectonic activity, regional u p l i f t and erosion resulted in the peneplanation of this area, with only the synclinal portions of the Adavale Basin being preserved as troughs. The efficiency of this peneplanation, the lack of local erosional p r o d u c t s , and paleomagnetic data suggest that an ice sheet was responsible for this Carboniferous erosion. Only minor thicknesses of sediments from the Cooper and Galilee b a s i n s w e r e p r e s e r v e d in the southern Adavale Basin region, and these w e r e deposited on the previous erosional surface which was relatively unstructured. The Eromanga Basin sediments were deposited during a p r o l o n g e d p e r i o d of regional subsidence. The isopach maps of the stratigraphic units in the Eromanga sequence show little v a r a t i o n in the thickness across the study region, indicating that no movement on these structures occurred during this p e r i o d . Tertiary rejuvenation of older thrust features has locally deformed the Eromanga Basin sequence, creating m i l d anticlinal structures. Over the Grenfield structure the observed u p l i f t is ^ 2 2 0 m e t r e s , while over the Como structure the uplift has b e e n ^75 metres. A component of the regional stress responsible for this rejuvenation must have b e e n north-south to have re-activated these features. The continued compaction of the thicker sections of D e v o n i a n sediments in the troughs has created mild synclinal structures in the Eromanga sequence which mimic the underlying b a s i n .

124

6.17

DETACHMENT FAULTING AND THE EVOLUTION OF PASSIVE CONTINENTAL MARGINS

G.S.

Lister, M.A. Etheridge and P.A. Symonds

Bureau of Mineral Resources, Geology and Geophysics, Canberra There is widespread acceptance of the lithospheric stretching model proposed by McKenzie to explain the crustal thinning, rifting and subsidence that predates and accompanies continental breakup, and leads to the development of passive continental margins. Evidence for such stretching of the continental lithosphere comes from data which demonstrates crustal thinning, and from normal fault geometries in the upper crust that require large extensions. Geophysical modelling of continental extension has been based almost entirely on symmetrical pure shear extension models. Such modelling, with variations induced by depthdependent strain or depth-dependent rheology allows limited prediction of the crustal thickness, subsidence histories and gravity profiles of extended terranes. Symmetric extension models, however, do not intrinsically predict the wide variation in gross continental margin architecture that is observed, nor the spatial variation of crustal thinning, or continental uplift. Features such as marginal plateaux, outer rises, detached ribbons of continental crust, and submerged continental fragments, remain largely unexplained, although sophisticated multilayer modelling does address some of these questions. Moreover, as pointed out by Bally, there is a notable absence of symmetrical rift structures in reflection seismic profiles through passive margins, and adjacent margins do not generally exhibit identical structures. We conclude, therefore, that symmetrical extension models have limited applicability, and that structural asymmetry may be a general feature of passive margin development. Structural asymmetry, on a range of scales, is a feature of many of the models recently proposed for continental extension in the Basin and Range Province of the western United States. These new models for continental extension are based on the operation of detachment faults, and/or shallowdipping crustal shear zones. It is logical to ask whether these mechanisms for continental extension are of more general application than in just the Basin and Range province. They may also operate in the period of continental extension which so often precedes continental breakup, and the final development of an ocean basin bounded by passive margins. In this paper we explore the consequences of this assumption, and we attempt to predict some aspects of the architecture of passive margins that would result from these inherently asymmetric models for continental extension. Detachment faulting leads to an inherent asymmetry of extensional structure, and if detachment faulting continues until breakup occurs complementary asymmetry of opposing passive margins will result. In the simplest case it should be possible to recognize upper-plate and lowerplate passive margins, and these will have contrasting structure, as well as different uplift/subsidence histories. Upper plate margins should be relatively devoid of structure, and margins younger than ca 100-200 Ma will be uplifted if simple underplating models apply. In contrast, the upper portions of a lower plate margin are highly structured. The basement to the post-extension sag basin on a lower-plate margin should consist of highly faulted and extended upper-plate remnants overlying the detachment fault, and hence will be, characterized by tilted fault blocks adjacent to half-graben filled with syn-rift sediments. These classical 125

passive margin rift basins are overlain by the gently dipping deposited during the post-extension, subsidence or sag phase of development.

strata margin

On a lower-plate margin, the detachment model predicts that the middle to lower crust bows upward as it is dragged out from underneath fracturing upper crustal rocks. This allows explanation of one of the more enigmatic features of passive margin development, namely the development of major rift basins inboard of an outer rise. It is difficult to explain why continental separation should not occur coincident with the rift basins, according to conventional models. This is not a problem for the detachment model, since the basement culmination occurs at the location where the crust is thinnest, and this would therefore be a preferred site for final continental separation. The basement culmination defined by the bowed up middle and lower crust could well define the outer rise observed on many passive margins, and in these cases the outer rise would be defined by metamorphic core complexes. We turn now to the origin of passive margin mountains. Some of the most important landforms on earth and found in the continental hinterland adjacent to passive margins, for example the Drakensburg in South Africa, the Eastern and Western Ghats in India, the spectacular Transantarctic Mountains, and finally, the Eastern Highlands in Australia. As pointed out by Oilier, great escarpments define major geomorphological phenomena in many of the above terranes. Oilier described the Great Dividing Range in eastern Australia as a gentle antiform in a planated land surface, bordered by a major morphotectonic feature, 2000-3000 km long, termed the Great (East Australian) Escarpment. This escarpment most likely had its origin in normal faults associated with opening of the Tasman Sea (at ca 80 M a ) in the south, and the opening of the Coral Sea Basin in the north. Oilier and Pain demonstrated that considerable scarp retreat has occurred since the time the passive margin formed, and much of the present day topographic relief in the Eastern Highlands of Australia is related to this feature. The planated land surface which was uplifted is probably greater than 100 M.Yr old. It could be argued that these mountain belts are eroded remnants of once even grander mountain chains as attempted by Lambeck, but this is difficult to substantiate. Topographic roughness has increased rather than decreased as would be expected if a process of gradual smoothing was going on. Subsequent to the Initial (1-2 km) uplift of pre-existing ancient peneplains, topographic roughness increases as rivers and glaciers incise their way through the uplifted land surfaces. Oilier suggests that the widespread occurrence of great escarpments on passive continental margins supports a genetic relation between rift and break-up, rather than an origin related to local variations of igneous, structural or climatic history. These arguments are not convincing, and there remains the intriguing possibility that the origin of passive margin mountains reflects the influence of a phenomenon fundamental to the process of continental extension. We suggest the initial uplift of passive margin mountains is the result of two factors. Firstly, there is thermal buoyancy induced as the result of lithospheric thinning under the relatively intact upper-plate margin. This may be as great as 3 k m . This uplift will decrease with time, as the thermal anomaly induced by extension slowly decays. Secondly, additional buoyancy is induced by igneous underplating caused by ponding of basaltic magmas at the crust-mantle interface. This uplift does not decay with time.

126

3.2 TECTONIC PROCESSES IN THE MOUNT ISA INLIER: THE SIGNIFICANCE OF TRANSFRESSIONAL STRIKE-SLIP FAULTING G.S. Lister, A. Thomas and J. Punn Bureau of Mineral Resources, Geology and Geophysics, Canberra Structural analysis in the Mount Isa inlier by 1983 had led to the conclusion that D^ was an event involving thrusting, and D2 was an event that involved substantial EW shortening, and the formation of kilometer scale upright folds. Major vertical shear zones also formed during this event. D3 was an event which was to us somewhat enigmatic. The last stage of deformation appeared to take place when the region was cut by numerous faults, but these faults seemed to be associated with only minor ductile deformation. However, a number of pre-D2 events must have taken place, and the tectonic evolution is considerably more complex than above. These early events must include two if not three phases of continental extension. The rifting events can be argued initially on the basis of bimodal igneous activity which took place at 1780 Ma, 1740 Ma and possible 1670 Ma. The Leichhardt River Fault Trough formed in Rift Phase I and accumulated 68 km of clastics, volcanogenic sediments and continental tholeiites. This was followed by Sag Phase I during which widespread deposition of quartzites took place, then a variety of chemical sediments. The rift edge can be recognized where the sag phase passes from unconformably relationships with the Kalkadoon Granite to conformable relations with the Myally sediments. The rift edge clearly acted as a structural weakness throughout the remainder of the tectonic history. The Quilalar Fault for example is a D3 strike-slip fault which cut into a D2 vertical ductile shear zone, and this shear zone occurs close to the inferred position of the western edge of the Leichhardt River Fault Trough. The rift was not symmetric, and was probably defined by one or more large half-graben. However, some features in the rift zone, such as the so-called Mount Gordon arch, do not necessarily relate to Rift Phase I, but to a younger rift event. The Mount Gordon arch may be no more than a large tilt block, planated before deposition of Sag Phase II, which includes the Mount Isa Group. T.H. Bell pointed to the existence of major thrusts in the Mount Isa inlier, and since this hypothesis has major implications, we decided that it had to be tested by examining in detail areas in which thrusting was thought to be important. One of the features which led to the recognition of thrusts in the Mount Isa area are so-called "ramp synclines". We went to the area around Lake Julius, NE of Mount Isa, where two of these synclines are exposed, and came to some surprising conclusions concerning their significance. The Lake Julius study demonstrated the existence of transpressional strike-slip faulting in the Mount Isa region. In the immediate Lake Julius area there is a NW to N-trending left-lateral strike slip system which is cut off by a younger, NE to N-trending right-lateral strike-slip fault sytem, termed the Mount Remarkable strike-slip fault system. These fault systems have characteristics similar to those recognized in many areas of major wrench faulting, for example along the San Andreas Fault in California. The Lake Julius strike-slip faults are transpressional, seemingly because the faults swing from a NW trending orientation to NS where they approach the inferred position of the eastern edge of the Leichhardt River Fault Trough. We can recognize master faults, anastomosing fault bounded compartments, and "flower" structures. What is 127

particularly interesting is that here we have an example of the effects of transpression exposed at a deeper level than is usually the case. The principal effect of transpression associated with the Lake Julius fault system was that pre-existing D2 folds were substantially tightened, and disrupted. This enables explanation of D3 folds with vertical extension directions. They are formed in transpressional compartments as the result of regional strike-slip faulting. What is surprising is that these brittle faults are associated with significant ductile deformation over large areas. The Quilalar Fault itself truncates tightened, distorted and distended folds which have occasionally suffered plastic deformation in excess of 80% shortening. One syncline, cored by Mount Isa shale, is narrowed from 1 km to less than 100 m, brittly truncating the adjacent anticline in the process. The point of this study was to test the thrusting hypothesis. We obtained excellent evidence in support of the existence of D^ thrusts. However the two ramp synclines we went to the Lake Julius area to study appear to hve had a rather different origin. We are forced to conclude that they formed as the result of thrusting associated with transpressional strike-slip faulting in D3 times. The D3 strike-slip fault appears to have run NS parallel to a D2 axial zone before running NW-SE to the next axial zone. Ongoing faulting led to transpression, which was relieved by thrusting of an elongate body of rock NW, removing the bend in the fault, and rotating and bending the thrust block 70-80® towards an EW orientation. Similar thrusting geometries have been recognized in other transpressional terrains. A number of possible examples of other transpressional systems have been recognized in the Mount Isa region, for example east of Lake Moondarra. Regionally, a common feature seems to be fault trends which switch from NE or NW to NS. Since NE-trending faults are right-lateral, and NW-trending faults are left-lateral, these NS segments should be transpressional. The Mount Isa Fault itself cuts a D2 shear zone, and may therefore bound one of the NW transpressional segments of a D3 strike-slip system. It is too eary to properly ascertain the regional significance of these results. It is disturbing that the structure of two of the so-called "ramp synclines" could be so reinterpreted, the more so because the geometry of these structures has much in common with other "ramp synclines". The evidence for early thrusting is hard to dispute, and we have found additional evidence to support the thrust hypothesis. However, whether or not the structures associated with "ramp synclines" are part of these thrust systems is a matter for further research. It is an important topic to clarify, since any shallow-dipping fault in transpressional zones will dilate, and may serve as a structural trap for mineralization. Gold prospects in the Lake Julius region occur near dilated thrusts filled with quartzihematite.

128

8.4

CORRELATION OF TERRESTRIAL PETROLEUM IN CHINA AND AUSTRALIA Luo Binjie^, Wang Youxiao^, Shen Ping^, Yang Xinhua^, Zheng Guodong and T.J, Mount^ ^Lanzhou Institute of Geology, Academla Slnlca, China ^Delhl Petroleum, Adelaide

Most oil and gas In China and Australia was formed In terrestrial sedimentary basins. Eleven oils were sampled from Middle Jurassic-Early Cretaceous strata. On the basis of mass—spectrometer and capillary chromatography analysis, they are paraffin-base crudes characterised by a high ratio of prlstane/phytane with heavy value. The compositions of samples are more or less alike; carbon numbers of principle peaks cover a wide range from C^Q to ^21 ^^^^ little Isoprenolds and the ratios of Pr/nC^y, Ph/Ci3 are 0.22-0.49, 0.04-0.09 respectively. Using ratios of Pr/Ph, Pr/nCi7 and Ph/nC^g, the samples can be divided Into three subgroups. The 6^-^C value of eleven oil samples Is from -23.53 °/oo to -26.4 o/oo. According to the ratios of Pr/Ph, Pr/nC^y and Ph/nC^g, Chinese terrestrial petroleum can be classified Into four types. Type I: the ratio of Pr/Ph Is near to 1 (0.87-1.39) with medium prlstane and phytane content. The ratios of Pr/nC^y and Ph/nC^g are 0.3-0.6 and 0.2-0.8 respectively. It Is generated In fresh-bracklsh lacustrine sediments. The majority of marine f a d e s oils distribute In this range. Type II: the ratio of Pr/Ph Is less than 0.8 (0.33-0.70), the phytane content Is especially high, usually with predominance of even carbon numbers or with predominant odd In the lower range and predominant even carbon numbers In higher ones when phytane Is regarded as a boundary. Type II Is formed In saline lacustrine environments and some of marine crude oils which are formed In saline environments probably have these characteristics. The ratio of Pr/Ph In type III Is more than 2.5 (3.0-4.28) with very low content of Isoprenolds and the ratios of Pr/nC^y and Ph/nC^s ^^^^ 0.35 and 0.2 respectively. It Is generated In rlver-lacustrlne or swamp deposits. The environment of forming type IV Is peaty-swamp or bog. The ratio of Pr/Ph spreads from 2.73 to 7.23 and the ratios of Pr/nC^y and Ph/nCi3 are 0.85-1.27 and 0.18-0.21 respectlvelv. Type III and type IV are similar In characteristics. The value or ^ ^^C In Chinese terrestrial petroleum distribute from -lO^/oo to -320/oo. The value of oils which are formed In salty-lacustrlne environments and In some of swamps Is more than -26^/oo. We can distinguish between fresh-bracklsh lacustrine and marine oils by using (-260/00) as a boundary value. This paper Introduces a new graph In which Is expressed the relationship between the ratio of Pr/Ph and the value of ^^^C for determining the environment of forming oil. The different regions of the graph represent different geochemlcal environments.

129

8.3

THE THERMAL HISTORY OF THE SOUTHERN BOWEN BASIN: AN APATITE FISSION TRACK ANALYSIS Susan J. Marshallsea Geology Department, University of Melbourne

Apatite fission track analysis ( A F T A ) , a powerful technique for detecting thermal events involving temperatures of around 60- 125®C, has been used to study the thermal history of the southern Bowen Basin. Unlike other thermal maturation indicators such as vitrinite reflectance which only give an indication of the maximum paleotemperatures reached, A F T A provides important information on the variation of paleotemperature through time. This study concentrates on the southern half of the Bowen Basin covering an area lying roughly between 23^ and 26® S. The Basin essentially contains a sequence of marine and non-marine Permian and Triassic sediments. Of interest in this study are the economically important coal measures of the Blackwater Group: 1. the Bandanna Formation, occurring on the western margin of the Basin, over the Springsure Shelf and the Denison Trough, 2. the Rangal Coal Measures of the central Bowen Basin, found in the north of the study area, 3. the Baralaba Coal Measures along the eastern margin of the Taroom Trough. Fission track ages and lengths were determined on apatites separated from outcrop and shallow well samples (< 500m) of the above three formations. These coal bearing units are essentially time equivalent and are considered to have a stratigraphic age of around 250 Myr. The samples of the formations under study contain a significant component of volcanogenic material suggesting that the majority of the apatites were derived from contemporaneous volcanism. The oldest apatite ages of around 240 Myr occur on the Springsure Shelf. The track length distributions in this area are typically narrow, with means of around 14 um and the standard deviations of the track length distributions are approximately 1.0 um. The apatite ages progressively decrease eastwards from the Springsure Shelf, through the Denison Trough and across the Comet Platform to ages of around 160 Myr. A similar decrease in apatite age is evident in the east of the study area, where ages decrease from around 220 Myr in the south to approximately 160 Myr in the north, near Baralaba. The mean track lengths of these samples are generally between 14 and 13 um with a few samples having means as short as 12 um. While the mean track lengths of these samples are very similiar, a subtle difference in the track length distributions is reflected in the standard deviation of these samples. Samples with ages of between less than 240 Myr and roughly 200 Myr have standard deviations of between approximately 1.2 and 1.6 um, whilst younger samples with ages between 200 and 160 Myr have standard deviations of between 1.6 and 2.0 um. In general, the youngest ages are found in the north of the study area, with minimum ages of around 100 Myr evident near Dingo. Samples with apatite ages of between 160-120 Myr have mean track lengths of usually around 13 um but with a few samples reaching 14 um. The standard deviations are generally in the region of 1.8 to 2.0 um, and at least one sample suggests a bimodal distribution, with the shorter component centring around 10-12 um and a longer component between 13-16 um. The youngest ages of near 100 Myr have distinctive mean track lengths of 13.5-14 um, standard deviations of between 1.2 to 1.4 um, with the majority of the tracks between 12 to 16 um, but with a few tracks of shorter lengths down to 5 um. 130

Extensive vitrinite reflectance data is available for the horizons investigated in this study, allowing a comparison of the fission track parameters and the values of R^max. A clear trend is evident. In regions such as the Springsure Shelf, where Romax values are low the apatite ages are old around 240-250 Myr. With increasing R^max the apatite ages steadily decrease until RQtr\ax values reach above 2.0 and the apatite ages are near 100 Myr. The apatite ages of samples from the Springsure Shelf are close to the stratigraphic age of the sediments and the track length parameters are indicative of rapidly cooled apatites, the age of which can be considered in terms of a definite event. These samples, are therefore interpreted as reflecting contemporaneous Permian volcanism, with the samples not having been subsequently buried to significant depths. The low vitrinite reflectance values of around 0.5 R^max on the Shelf reaffirm the idea that maximum paleotemperatures in this area have not exceeded 60^C. Vitrinite reflectance data indicates that samples of the Bandanna Formation, Rangal Coal Measures and the Baralaba Coal Measures in other parts of the study area have been subjected to higher maximum paleotemperatures. In the vicinity of Blackwater and Dingo the R^max values are above 2.0, so maximum paleotemperatures probably reached above 150^C. The young samples at Dingo have track length parameters indicative of rapid cooling from temperatures above 125®C. Therefore, the samples with ages intermediate between the young ages at Dingo and those on the Springsure Shelf are interpreted in terms of an admixture of two distinct components: an older, shorter component which has undergone shortening at temperatures of between 60125®C and a younger, longer component formed after subsequent cooling. As the degree of annealing increases the number of tracks in the older component is diminished so the apatite age and mean track length is reduced. Eventually, this older component is reduced to such an extent that the young, long component becomes dominant and so the mean track length and standard deviation increases, leading to a further reduction in apatite age. The track length distributions in samples with ages of around 100 Myr are not narrow enough, due to the presence of a few short tracks of less than 10 um,to actually date a time of rapid cooling. Therefore, the cooling in this region must have been initially slow, so that some tracks accumulated at temperatures between 60-125^C, followed by a more rapid cooling phase which must have occurred somewhat earlier than the age of the youngest samples, that is in the mid-early Cretaceous. It is not yet evident whether this cooling is associated with regional uplift, or some short lived Cretaceous thermal event associated with magmatism known in the north of the Bowen Basin. Most likely a combination of the two is responsible.

131

8.8

GEOCHEMISTRY OF A SUITE OF TESCHENITIC INTRUSIONS FROM THE SYDNEY-OJNNEDAH BASIN - EVIDENCE OF CRUSTAL TENSION D.J. Martin School of Earth Sciences, Macquarie University, Sydney

Numerous teschenitic intrusions in the depositional Sydney-Gunnedah basin have been intersected in drilling programmes carried out by the N.S.W. Department of Minerals and Energy, Coal Geology Division. Examples come from the Gunnedah basin, an area east of Kandos, the Goulburn River valley, the Hunter valley near Scone and the Southern Coalfield, together with Prospect Intrusion near Sydney. They were emplaced in the Permian fill of the foredeep developed between the cratonic Lachlan Fold Belt and the New England Orogen. The associated basaltic extrusives include the Jurassic Garrawilla Volcanics, Nombi Extrusives and Bulga Complex in the Gunnedah Basin, diatremes and associated Tertiary basaltic plugs and small flows near Kandos, and small Jurassic and Tertiary flows in the Southern Highlands. Many basaltic outcrops in the Gunnedah Basin, formerly thought to be Tertiary, are proving through K-Ar dating to be Jurassic in age also. On a plot of normative An against Differentiation Index, the intrusive suite shows a continuous variation from primary and near primary basanites (some containing spinel Iherzolite microxenoliths) through hawaiite and mugearite. Potassic rocks are rare, but a few trachydolerites and one tristanite have been found. The general trend is similar to that in the basaltic intrusives and extrusives of the Southern Highlands, the Monaro, eastern Victoria and Queensland. Mg numbers (100 Mg/(Mg + Fe of the teschenite chilled margins range from 70 to 37. Initial volatile content appears to have been lost to varying extent, commonly forming secondary carbonate in the adjacent country rocks. Chondrite-normalised transition element abundances form a tight array, with the exception of Ni and Cr, which show continuous variation from high to low. This suggests that differences between individual intrusions may be due to fractionation of olivine and pyroxene from compositionally similar magmas. The overall pattern corresponds with those examples from eastern Australia cited above, but with some regional differences. Ti/V ratios for the intrusions also overlap the range of the basalts mentioned above, varying between 60 and 120. The coherence of Ti and V in basalts from different tectonic settings is diagnostic and enables the identification of all the intrusions studied as continental intraplate basalts. A similar Ti/V for amphibole of inferred origin from metasomatised mantle lies in the same field. All the published (to March, 1985) K-Ar ages of alkaline rocks from the Sydney-Gunnedah basin and its environs, have been plotted on a generalised tectonic map of eastern New South Wales. All dated Late Permian, Triassic and Jurassic alkaline rocks in the region are emplaced within the Permian fill of the Sydney-Gunnedah foredeep. The sole exception known to the author is an alnoite found near Gloucester. Within the area of lineament analysis by CSIRO Division of Mineral Physics, most Jurassic alkaline intrusions in the Sydney district occur on NNE trending lineaments, while coastal alkaline dikes fan round a position about 30 km offshore. Jurassic alkaline rocks near Kandos are on NE and ENE trending lineaments. Igneous intersections in drillholes in the Gunnedah Basin are associated with NNE and NW lineations recognised in a N.S.W. Geological Survey study. 132

The close geochemical similarity of the Gunnedah Basin teschenitic intrusions to the local Jurassic extrusives, and the exclusively Jurassic age of the dated alkaline rocks in this area, strongly imply that the Gunnedah Basin intrusions are Jurassic also. Cretaceous basalts tend to occur along the coast, associated with the Coastal Lineament. Palaeocene-Eocene-Oligocene alkaline rocks (e.g. the Liverpool and Barrington volcanoes) trend parallel to NW or WNW lineaments, and commonly young towards the west. The WNW trend parallels the Lachlan River Lineament. In the Kandos area, some of the many diatremes are associated with Tertiary plugs and small flows, so that intrusions in the sedimentary rocks are most probably Tertiary. The majority of dated Miocene basalts are on the Lachlan Craton. In general, they also young westwards, and commonly occur at the intersection of WNW, NNE and ENE lineaments. These lineament trends are interpreted by the original authors as inherited from basement structures. The NNE lineaments result from sinistral wrench faulting, due to the action of a regional shear couple. Originally dextral, its reactivation as a sinistral shear couple must have predated or coincided with the emplacement of the Triassic-Jurassic alkaline rocks, probably during the break-up of Gondwanaland. The NW trend is a tensional direction within this stress field. The ENE and WNW lineaments coincide with the direction of transform and transcurrent faults developed during the opening of the Tasman Sea. Simple shear reactivation during Australia's northward separation from Antarctica after the opening of the Tasman Sea could explain the distribution of Tertiary alkaline rocks. Deep basement discontinuities on such a wide scale could well provide pathways for the rapid ascent of basanitic magma generated in the upper mantle. The geochemical similarities between the teschenites from different regions suggest a similar source and mechanism of formation. Supporting evidence for metasomatism of the mantle has been derived from REE studies, Nd/Sm and Rb/Sr isotope studies and oxygen isotope data. The geochemistry of amphibole/apatite xenoliths in a basanite dike in coastal N.S.W. suggests that mantle metasomatism by a volatile-charged, LIL-enriched liquid may be a necessary precursor to intraplate alkaline magmatism. Liquid inclusions in mantle xenoliths from Victoria and elsewhere are mainly CO2, which could metasomatise spinel Iherzolite mantle, or be released during periods of rifting. Others have found that kinematic and fluid dynamic constraints suggest CO2rich alkaline magma ascends rapidly in swarms of propagating magma-filled cracks, rather than as diapirs. Once at high crustal level, rapid magmadriven crack propagation is probably the mechanism of emplacement of sills. Exposures in Gunnedah Colliery workings and elsewhere show that magma advances in "fingers" (i.e. magma-filled cracks) which coalesce into a wedge. The association of alkaline rocks with crustal fractures in the Sydney-Gunnedah basin strongly implies that these mechanisms were operative. Recent work on xenoliths from south-eastern Australia by others has shown that the petrological mantle/crust boundary occurs at an unusually shallow depth (25 km to 35 km) in the area. This, together with the widespread occurrence of teschenite intrusions and the high number of primary basanites which occur, characterise the Sydney-Gunnedah Basin as an area of incipient crustal extension during the Jurassic and Tertiary, with deep fractures reactivated in a tensional tectonic regime. 133

14.3

AUSTRALIA'S NORTHERN TECTONIC BOUNDARY Kevin McCue

Bureau of Mineral Resources, Geology and Geophysics, Canberra Epicentres of recent shallow earthquakes in the region (0-12°S, 126°163°E) were plotted to locate the northern boundary of the Australian Plate and new and published fault plane solutions examined to determine its style of tectonism. The north eastern boundary with the Pacific Plate is a prominent subduction zone through the Solomon Islands to the triple junction at about 157°E) with the small Solomon Sea Plate. Limited fault mechanisms and geomagnetic lineations identify this part of the boundary across the Solomon Sea to the southern end of the Papuan Peninsula as a spreading centre. The central section through the island of New Guinea is a complex one. From the end of the spreading centre it follows the Papuan Peninsula with fault mechanisms characterised by both sinistral strike slip and thrust faulting. Near Lae it merges with the Northcoast zone through the Finisterre ranges as far as Madang. Fault plane solutions again indicate a combination of left lateral strike slip and thrust faulting along the strike of the seismicity. Near Madang the boundary is transformed 200 km to the southwest where it coincides with a broad zone of Holocene thrust faults along the southern margin of the Southern Highlands, parallel to but quite distinct from that along the New Guinea north coast. To about 140°E the shallow earthquake mechanisms are pure thrusts dipping at about 45° to the northeast and striking along the trend of the seismicity. The seismic zone bifurcates at (5°S, 140°E), one branch rejoining the northcoast zone, the other rotating to a more easterly direction along the southern edge of the central divide to Geelvink Bay. The thrust mechanisms here have a strong sinistral component due to the oblique direction of the compressive principal stress. To the west the boundary links across the Weber Basin to the Sunda Arc, either through the centre or around its southern margin west of the Tanimbar Islands where the last remnant of Australian oceanic crust is attempting to subduct beneath the Eurasian plate. The central link seems to be a continuation of the Irian Jaya segment' whilst fault plane solutions of earthquakes along the Tanimbar Islands sector have a strong meridional, left lateral component. Distinctive seismic gaps occur in the post-1960 epicentres. From east to west, the first occurs at the centre of the Solomon Sea spreading centre and is about 150 km wide. At its eastern end a weakly defined seismic zone branches off towards the southern tip of Bougainville while its western end is fault bound. The possible existence of an intervening subplate, the Woodlark plate, could provide a mechanism for the lack of seismicity. The central gap of about 100 km along the transform northcoast and Southern Highlands zones coincides with

134

between the the western

e d g e of t h e A u r e trough. A t the gap's northern end a coaxial e a r t h q u a k e l i n e a m e n t links t h r o u g h K a r k a r v o l c a n o to t h e B i s m a r c k Sea seismic l i n e a m e n t w h e r e t h e l a t t e r c h a n g e s s t y l e f r o m a c o n t i n u o u s to an en e c h e l o n f a u l t s y s t e m . There are also marked offsets in both n o r t h a n d s o u t h c o a s t l i n e s a t t h e e n d s of t h i s gap indicative of strong structural control. T h e w e s t e r n m o s t gap c e n t r e d on (6®S, 133°E) is a l s o a b o u t 150 km w i d e and stretches f r o m the T a n i m b a r I s l a n d s l i n e a m e n t to t h e S u n d a Arc a l o n g t h e e d g e of t h e A u s t r a l i a n c o n t i n e n t a l crust. More detailed s t r u c t u r a l a n a l y s e s m a y r e v e a l the n a t u r e of t h e s e g a p s b u t t h e y may be s i m p l y a r t i f a c t s of t h e s h o r t s a m p l e p e r i o d a n d so t h e l i k e l y s i t e s of n e a r - f u t u r e e a r t h q u a k e s . T h e d i s t i n c t i v e s t y l e s of t e c t o n i s m a l o n g A u s t r a l i a ' s c o m p l e x n o r t h e r n m a r g i n v a r y f r o m o c e a n i c - o c e a n i c s u b d u c t i o n in t h e n o r t h e a s t and northwest to c o n t i n e n t - c o n t i n e n t (?) c o l l i s i o n in t h e c e n t r e and seafloor spreading across the Solomon S e a . A number of tectonic breccias, hardly l a r g e e n o u g h to b e c a l l e d s u b p l a t e s , s e p a r a t e the Australian and Pacific plates throughout the Papua New Guinea region. Their e x i s t e n c e h e l p s e x p l a i n t h e d i v e r s i t y of f a u l t m e c h a n i s m s w h i c h are observed around the New Guinea island r e g i o n .

135

6.8

LOW PRESSURE TYPE METAMORPHISM AND TECTONIC EVOLUTION IN THE SOUTHEASTERN LACHLAN FOLD BELT K . G . M c Q u e e n , M.C* Brown and Graham Taylor C . A . E , School of Applied Science, Canberra

Recent mapping in Ordovician rocks south and southeast of Canberra has shown that regional metamorphic rocks of medium to high grade are more widespread than previously recognised. These rocks characteristically contain andalusite, cordierite and sillimanite indicating low pressure type metamorphism. The higher grade rocks define two parallel thermal a x e s . These axes trend approximately north-south, close to the faulted western margins of two major synclinorial zones containing Mid-Late Silurian felsic volcanic sequences (Cowra-Yass and Captain's Flat synclinorial zones). The metamorphic axes represent narrow zones of high heat flow now exposed by deep erosion of the marginal fault uplifts. They appear to be related to increased thermal gradients accompanying the m a j o r , Late Silurian v o l c a n i s m . This terrain has been extensively intruded by synkinematic and postkinematic Siluro-Devonian granitoids, which include S- and Itypes in the west and I-types to the e a s t . The presence of an anatectic S-type granitoid in the core of the well known Cooma Metamorphic Complex has been used as evidence for generation of some S-type magmas in areas of high heat flow and regional metamorphism. Studies in the recently discovered Cambalong Metamorphic Complex near Bombala, indicate that synkinematic intrusions with I-type characteristics are also intimately related to the regional metamorphism and possibly the underlying heat source. These intrusions show a range of compositions and appear to have been emplaced during or soon after the regional metamorphism. They also show evidence of a retrograde alteration apparent in the enclosed metasediments. Surrounding low grade metamorphic areas have been intruded by high level plutons with well developed contact metamorphic aureoles. Relationships between the Late Silurian volcanic sequences, SiluroDevonian instrusives and higher grade regional metamorphic zones suggest an extended and intense thermal event which was initially accompanied by rifting and felsic volcanism and which culminated with granitoid emplacement and cratonisation of this part of the Lachlan Fold Belt.

136

14.5

VARIATIONS IN THE SEISMICITY OF THE AUSTRALIAN P L A T E , 0 - 6 0 ® S , 90-165®E

Marion 0. Michael-Leiba Bureau of Mineral Resources, Geology and Geophysics, Canberra In the Australian Plate, 0-60®S, 90-165°E, during the period 1960-1984, the M J5.0 earthquakes associated with the Australian continent and its margins show significant clustering both in space and time. The temporal grouping is caused by foreshocks and aftershocks and the spatial clustering probably by zones of weakness in the continental crust. By contrast, the yearly number of oceanic events appears to follow a Poisson distribution, and the earthquakes tend not to be clustered in space. There were no M J5.0 oceanic foreshocks and aftershocks during the period 1960-1984. The mean number per year of M J5.0 events associated with the Australian c ontinent and its margins during the period 1960-1984 is 2.4 (or 2.0 if foreshocks and aftershocks are excluded) compared with 0.6 for oceanic events. Hence the mean yearly number of M J5.0 oceanic earthquakes is significantly less than the mean number associated with the Australian continent and its margins even when foreshocks and aftershocks are excluded. The mean yearly seismic energy release from events associated with the Australian continent and its margins is 9.6 x 10^^ joules. This is 5-6 times greater than that from oceanic earthquakes (1.7 x 10^^ joules). Neither the yearly number of events nor the seismic energy release show any periodic effects for periods of 2-12 years. However, the continental seismic energy release may be autocorrelated, an effect not caused by the presence of foreshocks and aftershocks.

137

4.8

EARLY PALAEOZOIC PLUTONISM IN THE PEAKE AND DENISON RANGES, S.A. Robert S. Morrison

Department of Geology and Geophysics, University of Adelaide The Peake and Denison Ranges are a series of north-south trending Adelaidean inliers 120kin long and 24km wide located approximately 1000km north of Adelaide. In the northern section of the largest inlier intrudes a suite of more than 30 plutons and associated dykes of Early Palaeozoic age. The largest pluton is 3km in diameter, but most are relatively small (0.5 - 1km). The granitoids intrude a sequence of Late Proterozoic (Burra Group) quartzites and siltstones, and a carbonate hosted "diapiric" breccia consisting of large angular clasts of Burra Group sediments. They occupy an area roughly corresponding to the intersection of the westerly trending Karari Fault Lineament with the Peake and Denison Ranges. The intrusive lithologies range from pyroxene - hornblende gabbro and monzogabbro to (quartz) monzonite and (quartz) syenite, albitite (alkali syenite), dolerite and lamprophyre dykes. Those plutons which display mineral zoning have gabbroic cores grading to leucocratic margins. Thin dykes or sills of biotite lamprophyre intrude or encircle these plutons. The intrusive relations show progressively more felsic magma intruding the parent host. Compositions of these rocks range from 51-72 wt.% Si02, 0.5-9 wt.% MgO, 3-11 wt.% Na20 and 0.2-9.2 wt.% K20, representing a mildly peralkaline to metaluminous suite of alkali-calcic affinity. Primary mineralogy consists of hornblende - biotite - clinopyroxene (diopsideferrosalite, aegirine-augite) - plagioclase (labradorite-albite) - alkali feldspar with varying quantities of magnetite - sphene - apatite - quartz +/- zircon. Post-emplacement metamorphism has resulted in partial metasomatic replacement of mafic components by actinolite - epidote - chlorite and felsic components by albite - muscovite - calcite. Such alteration has been attributed to the Cambro-Ordovician Delamerian Orogeny which metamorphosed the igneous suite to a lower greenschist facies. Potassium-argon dating of mineral assemblages have given figures rangeing from 680 to 470 Ma. The variety of dates reflect the effects of Delamerian metamorphism, with the oldest age probably indicating a more accurate age of emplacement. Contacts of plutons are commonly characterized by a 0.2-3.Om wide zone of leucocratic albitite consisting of over 90% albite plagioclase, but in the eastern section of the area of study, albitite occurs as thick sills or small plugs. Contact metamorphic aureoles are commonly absent. Distortion or brecciation of beds adjacent to contacts indicate an initial phase of magmatism of forceful emplacement of plutons into an active circulating meteoric water system which quickly disperse the latent heat of the intrusion and provided a mechanism for partial to complete albitization. The quartz monzonite plutons have no albitite, possess a small contact metamorphic aureole and intrude the overlying sediments without distortion or brecciation. Intrusive relations indicate that emplacement of these plutons was one of the last magmatic events. Two types of "diapiric" breccia are present; a finer grained "injection" breccia which post-dates a widespread "block" breccia. The presence of probable Delamerian intrusive clasts and the truncation of aplite dykes at injection breccia contacts indicates that the injection breccia postdates the emplacement of intrusives, whilst the block breccia predates emplacement. 138

Recent seismic surveys •conducted due west of the ranges are interpreted as indicating substantial salt diapiric activity within Cambro-Devonian trough carbonates. Such salt diapirs evolved from Mid Proterozoic evaporites of the Callanna beds and were periodically active until lithostatic equilibrium was reached. It is the subsequent collapse of diapirs which developed the earlier block breccia. The injection breccia formed as the result of either breccia remobilization along planes of greatest stress during Delamerian deformation or by carbonatitic activity or magma degassing. The diapiric breccia was in part controlled by structural weaknesses and may have provided the path of least resistance for rapid conduct of magma to near-surface conditions. Salt dissolving from such diapirs into the meteoric water system could have provided the solution required for subsequent albitization of intruding magma. The Karari Fault appears to have tapped magma in the lower crust providing conduit to upper crustal levels where it fractionated and intruded a wet sedimentary pile. The fault may have been extended far enough to provide magmatic carbonate to produce albitization and brecciation, but alternatively magmatic volatiles (C02, H20) migrating through carbonate beds may have been responsible for carbonate mobilization and subsequent recrystallization as breccia matrix. Deep crustal magma intruded into shallower levels could have resulted in partial melting of more siliceous material and the production of quartz monzonite plutons. This suite of slightly quartz saturated intrusives contains many characteristics of A-type granites including mildly peralkaline characteristics, highly contrasting mafic-felsic relations, gabbroic cognate xenoliths, abundant Fe-Ti-P minerals and late-stage lamprophyre dykes. It would be premature to claim the presence of a rift environment in the Peake and Denison Ranges during the Cambrian, but the strongly contrasting nature of these intrusions with those of the southern section of the Adelaide Geosyncline suggests a fundamentally different mode of petrogenesis.

139

1.8 THE GEOCHRONOLOGICAL AND GEOCHEMICAL EVOLUTION OF THE PROTEROZOIC LINCOLN COMPLEX, EYRE PENINSULA, SOUTH AUSTRALIA G.E. Mortimer, J.A. Cooper and R.L. Oliver Department of Geology and Geophysics, University of Adelaide Accretion of the Proterozoic Lincoln Complex of the Gawler orogenic domain occurred between at least ca.l850 Ma and ca.l650 Ma. Bimodal magmatic rocks predominate and granitoids and basaltic dyke rocks were emplaced episodically into an early, mainly orthogneissic and possibly supracrustal, layered sequence. Polyphase deformation and metamorphism declined during this period, from high-grade recumbent to low-grade upright styles. Locally subsolidus metamorphic assemblages in pyroxene-granitoid gneiss indicate early equilibration at 800O-900O C at >8 kbars. Garnet corona textures in some metamorphosed basic dykes suggest a later isobaric cooling history and possible anticlockwise P-T-t path. It is not clear to what extent Archaean high-grade gneissic rocks, exposed to the west, participated in the Proterozoic evolution of the domain, but it is suggested that the bulk of the Lincoln Complex constitutes new additions to the crust. The early Proterozoic orthogneissic sequence, the Massena Bay Gneisses, are mainly felsic possibly metavolcanic rocks, with subordinate intercalations of quartzite, calc-silicate and garnet-sillimanite gneisses. Rb-Sr total-rock data for the orthogneisses are disturbed and yield imprecise, updated age estimates. Calculated depleted mantle model ages of ca.2000 Ma allow the protoliths to have been derived from subcontinental lithosphere at about this time. The Sr-isotope data also allow the possibility that the orthogneiss protoliths may have contributed to the apparently coeval Hutchison Group to the west. A TR-biotite date suggests the tectonothermal evolution of the gneisses ceased by ca.l650 Ma. Batholithic intrusion of the Donington Granitoid Suite orthopyroxene-granitoids into the Massena Bay gneisses, prior to the first major deformation in the Lincoln Complex, was accompanied by extensive crystal fractionation attested by sympathetic bulk-rock and mineral compositional evolution. U-Pb zircon data yield a precise age of crystallisation of ca.l840 Ma, which is identical to Rb-Sr TR isochron ages for the granitoids. A range of IR (0.7043-0.7055), approximately correlated with Rb/Sr, developed within the magma chamber during protracted fractionation. Calculated depleted mantle model ages are ca.2000 Ma. The pyroxene-granitoids have been converted to hornblende-granitoid tectonite gneisses, with associated increases in Si, Rb/Sr, La/Sm and Th/Ba in a 5 km wide zone adjacent to the Kalinjala Mylonite Zone. Rb-Sr TR isochron data, have also been disturbed but isochron ages are not distinguishable from those of the pyroxenic protolith. Similar geochemical changes have also been noted in amphibolised mafic dykes in this zone. The Kalinjala Mylonite Zone apparently acted as a conduit for hydrous LIL-enriched retrogressive fluids at about the time of emplacement of the granitoids. Later mafic dykes, cutting the hydrated gneisses, were variably affected by these fluids. The dyke-like hornblende-granitoids and alkali feldspar granites of the Colbert Granitoid Suite were emplaced, prior to the last Major penetrative deformation in the Lincoln Complex, at ca.l755 Ma with an IR of ca.0.7087.

140

West of the Kalinjala Mylonite Zone, the late tectonic I-type granitoids of the Moody Granitoid Suite were emplaced into the Hutchison Group metasediments at ca.l710 Ma with an IR of ca.0.7070. These range from hornblende granodiorites to biotite adamellites. S-type leucocratic muscovite-biotite alkali feldspar granites are also associated with the I-type granitoids. Although not a component of the Lincoln Complex, the I-type granitoids were studied for comparative purposes, possibly being underlain by Archaean sialic crust. Geochemically many of the mafic dykes in the Lincoln Complex have much in common with the boninitic suite of active plate margins. In particular many have high Si, Mg, Ni, Cr and LREE but depleted Nb, Ti, V and Sr. Geochemical and Sr-isotopic models allow contamination of the mafic magmas by the enclosing granitoid gneisses or older Rb-rich crust. They do not, however, discount an origin by partial melting of depleted subcontinental lithospheric mantle re-enriched by a subduction-derived component. More certainly, uniform IR's throughout a suite of primitive to extensively fractionated noritic dykes do not favour contamination during emplacement, but displaces possible contamination to a well-mixed deeper crustal magma chamber. However, South Australian Early Proterozoic lower crustal xenoliths, sampled by kimberlites, are too low in Rb to be a suitable contaminant. An older, radiogenic, Rbenriched crustal or mantle component in the noritic (and other) dykes is thus favoured. The associated I-type granitoids show a wide compositional range from granodiorites to alkali feldspar granites. Their petrochemical evolution appears to be dominated by fractional crystallisation rather than restite control. They show little similarity to modern calc-alkaline granitoids but are enriched in (Fe/Fe+Mg), K/Na, and LIL elements. Regular geochemical changes with decreasing ^gSj transitional to A—type character, are present. In particular, overall levels of Zr, Nb, Y, P and LREE increase with time and HREE become more fractionated while Ni and Cr decrease. The granitoid and mafic rocks have similar-shaped mantle normalised elemental plots suggesting their origins are intimately related. Derivation of the granitoids by 15%-20% partial melting of a crustal source, geochemically analogous to the mafic dykes, satisfies geochemical and Sr-isotope models. Residues dt^jninaled by pyroxene and plagioclase are required for the earlier granitoids, with a little additional amphibole+garnet to fractionate the HREE in the later melts. Notwithstanding the residual mineralogical variations, the granitoid sources appear to become more trace element rich with time. An integrated scenario for accretion of the Lincoln Complex may involve initial Early Proterozoic extension and thinning of South Australian Archaean lithosphere over an upwelling convection cell. Subsidence led to deposition of both the Hutchison Group clastic and chemical sediments adjacent to the craton in the west, and the Massena Bay Gneisses protolith in the east. Partial melting of subcontinental lithosphere during rifting and subcrustal underplating by the resultant mafic magmas provided a fertile source of granitoid magmas (ca.lSSO Ma—ca.1750 Ma) inheriting their similar distinctive geochemical signature. Furthermore they provided a heat flux to the lower crust consistent with the metamorphic evolution of the complex. Subsequent tectonothermal evolution in the Lincoln Complex ceased by ca.l650 Ma.

141

1.12

THE GEOCHEMISTRY OF BASALTIC DYKES IN THE PROTEROZOIC LINCOLN COMPLEX, SOUTHEASTERN EYRE PENINSULA, SOUTH AUSTRALIA G.E. Mortj,mer, J.A. Cooper and R.L. Oliver

Department of Geology and Geophysics, University of Adelaide Formation of the silicic orthogneiss terrain of the Early to Middle Proterozoic Lincoln Complex of the Gawler orogenic domain in South Australia was accompanied by emplacement of mafic dykes. These dykes intrude ca.l840 Ma massive pyroxene granitoid gneisses and hornblende granitoid tectonite gneisses on wave-cut platforms along the southeastern coast of Eyre Peninsula. Approximately 20 km to the west, similar dykes cut an Archaean gneiss complex. The Lincoln Complex dykes thus afford the opportunity of studying both the long-term evolution of the South Australian subcontinental mantle and the possible interactions of Proterozoic mantle-derived melts with older crustal material. We have collected major and trace element data, as well as a few Sr-isotope data, on a suite of dykes from near Port Lincoln. In this study we noted a clear relationship between the mineralogy of the various dykes and that of the enclosing gneisses. In essence, although gradations are common, dykes cutting pyroxene granitoid gneiss are mafic granulites or unaltered dolerites, whereas dykes cutting amphibolised granitoid gneiss are amphibolites or amphibolised dolerites. The amphibolites are uniformly more siliceous than the other dykes. The prevalence of amphibolised and tectonised granitoid and basaltic rocks increases towards the Kalinjala Mylonite Zone which locally marks the western boundary of the Lincoln Complex. This zone apparently acted as a conduit for rock-altering fluids during its history. It is suggested that the dyke rocks reflect a major portracted cycle of mantle evolution, resulting in periodic emplacement of magma batches into the Lincoln Complex under a waning geothermal gradient. Depending on the magnitude of the differentials in temperature and hydration between the intruded magma and the host gneiss there was, with time, a declining effectiveness of both recrystallisation and amphibolisation. Mineralogical, textural and geochemical data for the unaltered dolerites reveal three distinct groups of dykes. The different groups are conviently termed, (1) norites, (2) gabbronorites, and (3) plagioclase-phyric dolerites. In many cases the geochemical distinctions can be traced through to the recrystallised dykes. The interpretations presented below, however, rest primarily on data from the pristine dolerites. The noritic dykes vary from aphric chilled margins to coarse-grained norites. Although uniformly moderately silica rich, they show a wide range in composition from 01- to Qz-normative with a constant Hy/Di ratio. Chilled margins show that the original liquids were highly magnesian with high (Mg/Mg+Fe), Ni and Cr contents. All the noritic dykes are LREE enriched with moderately fractionated HREE. Mantle-normalised incompatible element plots show enrichment in alkali elements and prominent depletions in Ti,Nb,Sr and V. Major and trace element modelling allows the compositional variation to be accounted for by closed-system fractional crystallisation of the observed phenocryst assemblages (opx+cpx+ol+plag). Sr-isotope data suggest the norites may have been emplaced ca.1500 Ma with an IR of ca.0.7050.

142

The plagioclase-phyric dykes are generally finer-grained than the norites. One specimen contains large, probably accumulative, olivine phenocrysts with disequilibrium reaction rims. Apart from this highly magnesian specimen, most of the phyric dykes have a restricted compositional range with moderate normative olivine contents. They are LREE enriched, but have almost unfractionated HREE. Normalised incompatible element plots show them to have lower overall trace element contents than the norites but similar prominent depletions in Ti, Nb, and V and enrichments in K, Rb, and Ba- Al/Ti and Ca/Ti ratios are higher than in chondrites. Modelling suggests crystal fractionation was dominated by assemblages rich in plag+cpx. One Sr-isotope analysis shows that the plagioclase-phyric dykes have lower IR ca.0.7029 at 1500 Ma. The gabbronorites are uniformly evolved with low MgO, (Mg/Mg+Fe), Ni, and Cr but have moderate normative olivine contents. They are LREE enriched but differ from both the previous groups in having depleted HREE and lower La/Sm ratios. Their normalised trace element patterns show no Ti depletions, but prominent Nb, Sr, and V depletions and K, Rb, and Ba enrichments. Modelled fractioning assemblages are dominated by cpx+plag. One Sr-isotope analysis shows that the gabbronorites had intermediate IR ca.0,7044 at 1500 Ma. Geochemically the norites and plagioclase-phyric dykes have the two-component signature of the boninitic suite of magmas. In particular the high MgO, Ni, and Cr but low Nb suggests a depleted mantle component whereas the high K, Rb, Ba, and LREE is more suggestive of an enriched source. These geochemical signatures are shared by many Proterozoic dyke suites and younger continental tholeiitic extrusives. Two main processes have been suggested to explain these conflicting characteristics. Metasomatic reenrichment of a previously depleted subcontinental lithosphere may be invoked. In subduction-related environments the added component from the wet subducted slab is selectively enriched in LIL- over HFS-elements. It has the further advantage of promoting relatively siliceous mantle-derived magmas. In continental areas an alternative explanation put forward involves contamination (possibly combined with fractional crystallisation) of an E- or N-type MORB-like parental magma by crustal material with the distinctive decoupled LIL/HFS geochemical signature. We have tested both of these geochemical alternatives by matching measured Sr contents and 87Sr/86Sr ratios of representative dykes with model mixes of estimated endmembers. In both cases, mixing of 1^-10% enriched component with a depleted remainder satisfies the data. However, the uniform IR and closely similar trace element patterns of the norite suite over an extensive range of fractionation suggest that no crust was assimilated during final emplacement and fractionation. Uniform contamination in a well-mixed deep crustal chamber prior to emplacement would require a Rb-rich component. South Australian lower crustal xenoliths (sampled by kimberlites) are too low in Rb. A partial melt of most easily fusible crustal material may be more appropriate. Most Phanerozoic high-Mg basalts are associated with continental rifting where they tend to appear early in sub-linear zones near or along the new continental margin. Enriched subcontinental mantle sources of alkali basalts and kimberlites do not show Nb depletions. This feature is restricted to subduetion-related environments. If the geochemical and isotope features of the Lincoln Complex dykes are derived from their mantle source this source must have been modified by earlier, subduetion-related, crustal generation processes. A calculated depleted-mantle model age of this possible source for the norites is ca.2000-2200 Ma.

143

10.2

QUATERNARY COASTAL AND MARINE AMINOSTRATIGRAPHY, SOUTH AUSTRALIA C.V. Murray-Wallace^ ^Department of Geology and Geophysics, University of Adelaide ^CSIRO Division of Soils, Adelaide

Confident age determination of stratigraphic sequences beyond the datable limits of radiocarbon methods has presented a significant problem in Quaternary studies. Although numerous dating methods are available, many are applicable only to specific time intervals and to particular mineralogical components of sediments. In recent years considerable attention has been paid to the application of time dependent protein diagenetic reactions to geochronometry. The most widely used of these reactions are amino acid racemisation and epimerisation. The protein of living organisms contains amino acids essentially in the L-configuration. After an organism dies however, this disequilibrium condition ceases and racemisation commences. In racemisation L-amino acids are reversibly converted into corresponding Diamine acids. The reaction continues until an equilibrium ratio is obtained (i.e., D/Ii=l). When this condition is reached the amino acids are said to be racemic. This state is generally obtained by the Miocene. As the racemisation reaction is time dependent, knowledge of the rate of racemisation and of the various rate controlling factors facilitates its application to geochronometry. Numerous marine and marginal marine lithostratigraphic units occur within the South Australian Gulf's region, and their origin relates to different Quaternary glacioeustatic sea levels. A variety of molluscan fauna from several of these stratigraphic units have been analysed using amino acid racemisation techniques. Amino acid racemisation (epimerisation) has been used as a basis for correlation of South Australian Mid and Late-Quaternary Coastal and Marine Formations (Aminozones), and in establishing a preliminary regional relative chronostratigraphy based on aminostratigraphic principles. Sediments of the Glanville Formation, a coastal/marine lithostratigraphic unit are represented by poorly consolidated muddy skeletal grainstones, and in places are locally pedogenically modified. The Glanville Formation was deposited during the last Interglacial, some 120 ka BP (Stage 5e of the oxygen isotope record). Correlation of the Glanville Formation parastratotype at Globe Derby Park with various hypostratotypes on the West Coast of South Australia and Northern Spencer Gulf has been established using amino acid racemisation, and demonstrates the utility of the technique in avoiding homotaxial interpretations. The extent of racemisation of a range of amino acids in the estuarine bivalve Anadara trapezia and the foraminifer Narginopora vertebralis supports the suggestion that these species are now no longer living in South Australian coastal waters. The presence of these species in Holocene 3ediments is the result of reworking. These species favour warmer climatic conditions and appear to have migrated from South Australian coastal waters following the Last Interglacial. Based on the extent of racemisation in Katelysia rhytiphora and Fulvia tenuicOBtata in sediments obtained from submarine cores from the Gulf St Vincent, interstadial marine strata younger than Last Interglacial (120 ka BP) but older than Holocene have been recognised at a water depth in the Gulf of 40 m. Preliminary work suggests an age of approximately 40 ka BP (Stage 2 of the marine oxygen isotope record). 144

A marine lithostratigraphic unit older than the Glanville Formation occurs at Redcliff, northern Spencer Gulf. The sediments are represented by poorly sorted clays with low calcium carbonate contents. Although they have similar characteristics to distal alluvial fan sediments, a former marine origin is attested by the presence of forminifera and occasional Anadara trapeza. These sediments have been informally referred to as the "Older Pleistocene marine beds", and have been tentatively equated with the Penultimate Interglacial, some 225 ka BP (Stage 7 of the marine oxygen isotope record). Amino acids in Anadara trapezia from the "Older Pleistocene marine beds" are consistently more extensively racemised suggesting an age greater than the Last Interglacial. Insufficient data are available at this stage however, to place an absolute age on these beds based on the extent of racemisation. Rates of racemisation are considerably slower in the foraminifer Marginopora vertebralis than a number of molluscan fauna studied. Besides demonstrating the species specific nature of amino acid racemisation, this displays the potential to date geological events of greater antiquity using this species owing to the slow racemisation rates. It may ultimately be possible for example, to establish a more precise absolute age of the Pliocene Hallett Cove Sandstone-

145

1«4

THE WESTERN GSEISS TERRAIN OF THE YILGARN BLOCK WESTERN AUSTRALIA John S. Myers Geological Survey of Western Australia

The western part of the Yllgarn Block comprises two major groups of high grade gneisses. The oldest group includes deformed and metamorphosed granites (3650 and 3500 Ma old), layered basic rocks (3750 Ma old) and siliceous metasedimentary rocks, which were metamorphosed in granulite facies about 3300 Ma ago. These rocks are best known from the north-west part of the Yilgarn where they are in tectonic contact with the granitegreenstone terrain, and suffered intense deformation and retrograde metamorphism in amphibolite facies together with the adjacent granitegreenstone terrain about 3000 - 2800 Ma ago. Similar rocks occur in the south-west Yilgarn but there the major part of the gneiss terrain consists of a distinctly younger group of rocks. This younger group comprises much less deformed granites and some basic rocks which are probably deeper, granulite facies, equivalents of the adjacent high-level 3000 - 2800 Ma old granite-greenstone terrain. In the south-west Yilgarn the high-grade gneisses were elevated and eroded prior to the intrusion of high-level granites about 2650 Ma ago. Mineral potential includes chromium and platinum group elements in fragmented layered basic intrusions in the older gneiss complex; gold in high-grade equivalents of 2800 Ma old greenstones in the younger gneiss complex; and gold and alumina in weathering products of low-grade 2650 Ma old granites and metavolcanic rocks.

146

3.8 SCAPOLITE PARAGENESIS IN CALC-SILICATES AT MARY KATHLEEN, N.W. QUEENSLAND: IMPLICATIONS FOR METAMORPHIC FLUID EVOLUTION N.H.S. Oliver and V.J. Wall Department of Earth Sciences, Monash University, Melbourne Scapolite is widely distributed and abundant in calc-silicates and metadolerites of the Mary Kathleen Fold Belt, a low pressure metamorphic belt reaching a peak grade of upper amphibolite facies. Electron microprobe analyses show that scapolites are variable in composition, but are commonly intermediate chloride-carbonate types. These data raise the possibility of widespread metasomatism by chloride-rich (and C02-bearing) regional metamorphic fluids. Scapolite-bearing assemblages can act as sensors of fluid salinities and CO2 contents. Hence, investigations of the scapolite bearing rocks can yield constraints on fluid compositions, evolution, and migration paths during metamorphism. In the main calc-silicate/metadolerite sequence, metadolerites range up to 1 X 10km in area. Scapolite typically comprises 40-50% (rarely up to 70%) of a given metadolerite sample. However, plagioclase may predominate in the cores of the largest bodies, and also on the boundaries where in contact with retrogressed calc-silicates. Scapolite may be completely absent in metadolerites in the vicinity of large high temperature retrograde calcite pods. Where present, it is found as granoblastic grains defining a foliation in part, as strained and recrystallized clumps up to 2cm across, as foliated veins, and in the most scapolitized bodies, also as coarse aggregates and sprays with crystals up to 6cm long. Coexisting phases include ubiquitous hornblende and sphene, commonly with accessory tourmaline, biotite, epidote and chalcopyrite. Diopsidic clinopyroxene and calcite are only abundant in the most intensely scapolitized dolerites. Metadolerite

scapolites are typically chloride-rich dipyres (e.g. modal data, bulk rock and scapolite compositions * are consistent with substantial addition of chloride and CO2 to the metadolerites during scapolitization. Most textures indicate scapolite growth synchronous with foliation formation (S2).

In the calc-silicates without abundant veining or retrograde alteration, scapolite occurs again as granoblastic grains, or large sieve-textured poikiloblasts, typically with calcite, and may show a strong preference for particular layers, commonly at mm to cm scales. In some units, it also locally occurs in lenses, veins and pods, generally deformed during the main deformation, D2* Scapolites in the calc-silicates range between dipyres of similar composition to those in the metadolerites, to Cl-bearing mizzonites (e.g. NaCa3Al5Si7024[Cl]o,2[C03lo.85 total range 25 to 80% meionite). NaCl:CaC03 ratios vary from layer to layer at the outcrop scale, particularly between major calc-silicate rock types. However, the variation is relatively restricted in particular layer types and usually at a specimen scale. The equilibrium plagioclase + calcite + NaCl = scapolite defines the composition of coexisting scapolite and plagioclase at calcite saturation. Estimates of fluid compositions require information on the compositions of the plagioclase and scapolite, and the activities of NaCl and CaCOg, At a given plagioclase composition, if calcite is present, the NaCl content of the coexisting scapolite is dependent on the NaCl activity 147

in the fluid. Although calcite does not coexist with scapolite in many of the metadolerite bodies sampled, scapolite compositions are similar in both calcite-bearing and calcite-poor parageneses. Whether or not the assemblage scapolite-plagioclase-calcite is present, the abundance of sodic, Cl-rich scapolite in the metadolerites indicates that large quantities of NaCl have been added, and the only feasible source is from saline fluids derived from evaporitic halite (and/or other salts) in the surrounding sediments. A rough estimate of fluid/rock ratio may be made, assuming moderately high salinities (around 30 molar %) and that around 10% of the available CI- in the fluid is fixed in scapolite. Minimum values of 5:1 fluid/rock are obtained using these assumptions, indicating large scales of fluid generation and migration. In the calc-silicates, many of the coexisting hydrous phases also contain CI (biotite up to 0.9 wt% CI, ferrohastingsitic amphibole up to 2% CI), which are certainly anomalously Cl-rich, and also indicate highly saline fluids. However, both scapolite compositions (NaCl/CaC03) and the compositions of coexisting hydrous silicates (Cl/OH, Fe/Mg) show wide variations at outcrop scales, but are within narrow ranges for particular layer types. The varying scapolite compositions at outcrop scales may reflect localized derivation of NaCl, different initial plagioclase compositions, or more complex factors due to fluid immiscibility (due to high salinity) or nonideal NaCl-CaC03 mixing in the scapolite. However, as many of the scapolite-bearing calc-silicate layers show low variance assemblages indicative of predominantly rock-buffering of fluid composition (in H2O-CO2 ^Istems), it is probable that the CI/CO3 ratio in the scapolites are at least partly internally controlled. This hypothesis generates problems when attempting to establish possible sources of migration Cl-rich fluids which have infiltrated in large volumes through the adjacent dolerites. It is apparent that NaCl has not been lost to a great degree from the pile prior to the higher temperature stages of the regional metamorphism (except perhaps in narrow zones of sediment adjacent to the dolerites). This suggests very limited flow of water at large scales prior to the metamorphism. During the metamorphism, widely variable fluid/rock ratios and complex fluid channelling are suggested by the distribution and composition of scapolites, and by the variance and mineral compositions in both calc-silicate and metadolerite assemblages.

148

2.4

THE MARY KATHLEEN DRANIDM-«EE DEPOSIT; A PRODUCT OF REGIONAL METAMORPHISM

N.H.S. Oliver^, V.J. Wall^ and P.J. Pearson^ ^Department of Earth Sciences, Monash University, Melbourne ^Department of Geology and Mineralogy, Queensland University The Mary Kathleen uranium and rare-earth orebody is hosted by calcic exoskarn in the Mary Kathleen Syncline. The original orebody held about 10 000 tonnes of U3O3 at a grade of 1.2 kg/tonne, as well as several percent REE, up to 7.6%, as noted in 1980 by Cruikshank and others. Debate in the literature has centred on timing of the ore with respect to granite intrusion and skarn formation. Our mapping and sampling in the pit, and isotopic studies of Page and of Mass and others, favour a model of syn- to late- metamorphic ore deposition, some 200 m.y. after granite intrusion, in contrast to syn- to post-tectonic granite intrusion, skarn formation and related uranium mineralization. In this paper, we discuss skarn formation and later modifications, including uranium mineralization. Several lines of evidence indicate that granite emplacement and primary skarn formation predated the regional metamorphism and deformation. a) Throughout the Mary Kathleen fold belt there is a strong spatial relationship between granites and endoskarns, exoskarns, pyroxene-rich hornfels and other unusual altered rocks. These associations form lenslike zones of weakly deformed rocks surrounded by strongly folded calcsilicates and metadolerites, indicative of pretectonic granite emplacement and skarn development. b) Zircon U-Pb ages (1740-1973 Ma) for the Burstall Granite as determined in 1983 by Page and Sm-Nd ages for banded skarn (1700±60 Ma determined by Maas and others) are substantially older than the 1620-1500 Ma dates for the main regional metamorphism and deformation discussed by Page. c) Skarns in the Mary Kathleen Syncline are locally cut by acid dykes (dated at 1730 Ma by Page), which Derrick in 1977 inferred on geochemical and petrographic grounds to represent late differentiates of the Burstall Granite magma. These and the radiometric ages are consonant with skarn bodies forming around the time of granite intrusion. The Mary Kathleen primary skarns formed by infiltration metasomatism of gabbro, limestone, and impure calcareous sediments by high temperature fluids associated with granite emplacement. d) The skarns have been modified during regional metamorphism and deformation. Near the margins of large skarn bodies, garnet±pyroxene lenses are found axial planar to F2 folds, and coarse garnet is observed in strain shadows abutting boundinaged garnet layers. In 1985, Maas and others also obtained Sm-Nd ages of 1580±50 Ma on garnet-rich skarns, these ages overlapping with regional metamorphism. The Mary Kathleen orebody is hosted by skarn (after calcareous conglomerate and gabbro) lying adjacent to the steeply dipping Mary Kathleen Shear. The shear forms the boundary between a regional low strain zone (the skarn and Burstall Granite) and the strongly deformed calc-silicates. High grade mineralization comprises uraninite- bearing veins and fracture networks, mainly west-dipping but forming an anastomosing array. The ore gangue includes ferrohastingsitic amphibole allanite - apatite ^ garnet - prehnite - sulphides - quartz. These overprint and replace the earlier garnet-pyroxene skarn mineralogy. On the basis of these structural and mineralogical overprinting relationships, the mineralization and alteration developed under amphibolite facies conditions. 1500 Ma U-Pb ages for uraninite, as 149

determined by Page, and the 1550 Ma whole-rock isochron on ore material of Maas and others confirm^ the timing of ore body formation as corresponding to late D2 or D3 regional deformations. The Mary Kathleen Shear exhibits a transposed, amphibolite facies marginal zone, and a retrograde core, indicating movement during D2 and at a later stage. The inferred sense of early movement on the shear is consistent with the orientation of the mineralized vein sets and suggests a genetic relationship. Other veins containing the assemblage calcite-andradite-hedenbergite-sulphides truncate ore veins and emanate from the hear. These veins may relate to the later movement on the shear. Thus the age relations, mineral assemblages and structural association of the Mary Kathleen orebody are consistent with its development during early retrograde regional metamorphism. 400-500®C fluid inclusion homogenisation temperatures in pyroxene, amphibole, apatite and garnet gangue minerals are, as noted in 1984 by Abeysinghe and others, concordant with this model. However, the latter authors concluded that mineralization formed during the retrograde stage of primary skarn formation. The inclusion densities reported are more compatible with fluid trapping under high pressures (3-4 kb, similar to those operating during the regional metamorphism), although some relatively low density inclusions were also recorded. Moreover it seems unlikely that fluid inclusions formed during primary skarn formation, at shallow depth, would survive the regional metamorphism. We would thus suggest that fluids trapped in inclusions were of regional metamorphic origin. Our studies have demonstrated that highly saline fluids, derived from evaporite dissolution, were widespread in the Mary Kathleen area during regional metamorphism, and indeed very high salinities are evident in fluid inclusions in regional mtamorphic rocks throughout the belt. The @-Nd modelling of Maas and coworkers is consonant with a 200 m.y. period between skarn formation and ore genesis, further supporting a regional metamorphic origin for the Mary Kathleen orebody. The primary skarns provided an appropriate environment (high a CO2, high pH) for the development of LREE-enriched minerals (allanite, monazite, apatite, etc.) through their interaction with the regional metamorphic fluids infiltrating through the fracture systems. Although the primary skarns may have been somewhat uranium-enriched, a local redistribution of uranium on the orebody scale cannot explain grades and tonnages involved. Uranium must have been introduced from a much larger volume of source rocks, in keeping with the @-Nd model of Maas and coworkers. As noted by other workers, transport of uranium in other than strongly acid fluids requires that the fluids be highly oxidized. The hypersaline metamorphic fluids involved in the Mary Kathleen system could have owed their inferred high ratio of S04^~+S03/H2S to evaporite dissolution and/or interaction with sulphate-bearing scapolite. Uraninite and sulphide precipitation resulted from partial reduction by Fe-bearing skarn minerals and probably increases in pH due to fluid-skarn interaction. The ingress of regional metamorphic fluids into the skarn body was related to enhanced fracture permeability. The ductility contrast between the massive garnetpyroxene skarn and its host rocks influenced the local stress field, producing dilational fracturing, the key control on fluid focussing involved in the Mary Kathleen uranium and other regional metamorphic mineralization. Other essential aspects of the genesis of this unique uranium orebody include the unusually oxidized regional metamorphic fluids and uranium rich source rocks such as the Burstall Granite suite.

150

HIGH PRESSURE, HIGH TEMPERATURE METAMORPHISM, NORTHERN EYRE PENINSULA R.L. Oliver and A.C. Purvis

Department of Geology and Geophysics, University of Adelaide Percussion drill hole ORP 1, situated at 200® and ca 16.25 km from Ooldea on the Transcontinental Railway 450 km west of Woomera, South Australia, penetrated, under six metres of Nullarbor Limestone cover, 90 metres of magnetite bearing metasedimentary gneisses including metamorphosed banded iron formations. The gneisses can be regarded as part of the northwestern fringe of the Gawler Craton. The nearest gneiss outcrop, in the vicinity of the Pidinga Lakes, 50 km southeast of ORP is mapped as Mulgathing Complex of 23002500 Ma. Samples from a number of drill hole intervals display a variety of high raetamorphic grade mineral assemblages. For example, the chips in ORP 1, 10-12m interval, are composed of essential orthopyroxene, clinopyroxene and plagioclase, with minor quartz, magnetite (5%) and hornblende. Cuttings from the 58-60 m interval contain assemblages as follows: (i) Spinel-biotite-sillimanite-apatite-plagioclase-quartz-magnetite banded iron formation (B.I.F.), with possible retrogressed cordierite. (ii) Spinel-hypersthene-plagioclase-quartz-magnetite (B.I.F.), with or without apatite and retrogressed cordierite. (iii)

Biotite-spinel-hypersthene-sillimanite-plagioclase-quartz-magnetite.

(iv)

Orthopyroxene-clinopyroxene-quartz-feldspar (sericitised).

The magntite commonly contains inclusions of spineal and may be rimmed by sillimanite or hypersthene. The 78-81m sample comprises chips with the following assemblages: (i) Spinel-biotite-apatite-sillimanite-plagioclase-magnetite quartz (B.I.F.). (ii) Plagioclase-magnetite-quartz (B.I.F.). (iii) Hypersthene-spinel-sillimanite-plagioclase-magnetite-quartz (B.I.F.). (iv) Biotite-sillimanite-quartz-orthoclase. (v) Hornblende-plagioclase-magnetite-quartz. (vi) Spinel-magnetite-sillimanite, enclosed in vermicular? quartzbiotite-hypersthene-sillimanite intergrowths. (vii) Sapphirine-quartz. The mineral assemblages outlined above clearly account for the marked magnetic anomaly (magnetic susceptibility 3300 x 16""^ cgs) on which the location of the drill hole was based. Evidence for the high grade of metamorphism alluded to above includes the association of hypersthene-sillimanite-quartz, occurring in assemblage (iii) of the drill hole interval 58-60 m and assembalges (iii) and (vi) of interval 78-81 m. Hensen and Green found, experimentally, that the PT stability field for this association is 900-1000°C at 9 Kb, an estimate substantiated by the work of Ellis. Indicative of even higher temperatures is the assemblage sapphirine plus quartz found in sample (vii) for interval 78-81 m. 151

Actual temperatures and pressures are likely to be slightly lower than those indicated by the diagnostic hypersthene-sillimanite-quartz and sapphirine-quartz because of the existence of other phases. The Fe^"*" content of the sapphirine and orthopyroxene, which would have a similar temperature and pressure lowering effect, is not known. Despite those influences, however, it is probable that unusually high granulite facies metamorphic temperatures and pressures of crystallisation are indicated by the mineral associations hypersthene-sillimanite-quartz and sapphirine-quartz. One of the few places elsewhere in the world where a high PT crystallisation environment is indicated by such mineral associations is Enderby Land, Antarctica, described, for example, by Sheraton and Ellis. It is noteworthy that the probed composition of sapphirine, spinel and garnet from ORP 1 are very similar to the composition of the same phases in the high grade Enderby Land rocks. Pyroxene compositions in the respective areas are slightly different, which perhaps accounts for the 800-850®C derived from ORP 1 coexisting pyroxene distribution coefficients compared with 900-920®C from the same coexisting phases in Enderby Land. In Enderby Land coexisting sapphirine-quartz has been formed over an area of 2500 km and sillimanite-orthopyroxene-quartz also occurs on a regional scale, but one can only speculate on the extent of the northern Eyre Peninsula high grade rocks. Studies by the South Australian Department of Mines and Energy of the northern Eyre Peninsula region reveal a variety of gneisses with mineral assemblages indicating, predominantly, middle to upper amphibolite metamorphic facies.

152

8.5

BASINS IN THE CENTRAL EROMANGA REGION AND THEIR PETROLEUM PROSPECTS V.L. Passmore Bureau of Mineral Resources, Geology and Geophysics, Canberra

Several thousand metres of terrestrial and marine sediments were deposited in three different phases over the central Eromanga region of southwestern Queensland during the Palaeozoic and Mesozoic. The preserved remnants of these sediments are contained within four stacked basins: the Eromanga, Cooper, Galilee, Adavale. The thickest sequence, up to 8.5 km of Devonian clastics and carbonates is in the eastern troughs of the Adavale Basin. Major deposition in the Cretaceous buried underlying source rocks beneath up to 2 km of sediment. Data from 51 wells sampled to determine source potential and maturity of rocks in each of the basins show that organic-rich source rocks are present in all the basins but in highly variable amounts. The development and thermal histories of the four basins are distinct from one another and have consequently produced variations in their potential for oil and gas. Significant variation in present geothermal gradients exists across the central Eromanga region. In general, source rocks in the western part of the region have been subjected to higher temperature and greater burial than those in the eastern part. Recent studies on geothermal gradients and organic maturation suggest that present high temperatures in the west are a late development. Nevertheless this combination of high temperature and deep burial in the west has resulted in prospective source rocks of the Eromanga and Cooper Basins being favorably placed within the oil window or wet gas zone to generate hydrocarbons. Source rocks farther east often lack sufficient depth of burial and temperature to have had significant generation of hydrocarbons yet. Burial and thermal trends in the Adavale Basin appear to have increased from west to east, a reversal of those which now prevail. Hydrocarbon potential in all the basins is very variable. Parts of the Eromanga and Cooper Basins are organically-rich and favorably placed for generation of hydrocarbons and are classified as having good potential. As recent drilling has shown the Eromanga Basin potential is largely for oil, while the Cooper Basin has a higher gas component. The potential of the Galilee and Adavale Basins is rated fair to poor. The better source rocks in the Galilee Basin are only now approaching the oil window. Those of the Adavale Basin have generally gone beyond the oil window.

153

10.3

PALAEOENVIRONMENTAL INTERPRETATION OF THE NGANKIPARI SAND IN THE ST. VINCENT BASIN, SOUTH OF ADELAIDE, SOUTH AUSTRALIA S.E. Phillips CSIRO Division of Soils, Adelaide

The Ngankipari Sand was deposited during the Late Pleistocene to Early Holocene along the eastern margin of the St Vincent Basin. This weakly consolidated, brown to red coloured, quartz-rich sand overlies a variety of strata varying in age from Pleistocene to Precambrian. In the thickest sequences there are several zones of carbonate accumulation. The resultant morphologically complex calcrete layers represent time breaks, thus sedimentation is considered to have been interrupted by periods of soil formation. Previously the Ngankipari Sand has been identified at Hallett Cove as calcarenite beach dune sediments correlated with the Bridgewater Formation of southwestern Victoria. Other palaeoenvironmental interpretations have attributed this lithostratigraphic unit to drift sand from coastal dunes, or to a sand of mixed, but unknown, origin. The present reconstruction of the palaeoenvironment is based on field observations, granulometric analyses and the interpretation of sand surface textures as observed in the scanning electron microscope. In thick exposures at Lonsdale and Hallett Cove, calcified insect pupal cases and roots, and the shells of land snails, indicate that deposition occurred in a terrestrial environment. However, there seems to be a complete lack of sedimentary structures. Carbonate incorporated in the sand may have been derived from several sources; the reworking of Tertiary limestones, calcareous sediments from local coastal regions or from local carbonate lakes. Granulometric characteristics suggest the deposit is an inland dune, yet the surface textures on the sand grains indicate a subaqueous environment of deposition. Derivation of the sand by aeolian reworking over short distances, or short periods of time, from the associated fluvial sands of the Ngaltinga Clay and Ochre Cove Formation may explain this anomoly. Sediments assigned to the formation also occur adjacent to the Onkaparinga River, but the depositional environment here is more difficult to interpret. At one locality there are several calcrete layers, yet at other sites only one hardpan occurs above calcareous sand. Granulometric characteristics at three different sites vary significantly. Surface textures on the sand grains include evidence of aeolian and possibly marine environments. Sands below the calcrete layer are probably derived from different sources. The mode of deposition includes both aeolian and fluvial processes. Proximity to the Onkaparinga River and the granulometric similarity with dunes in the Murray Basin suggest these sediments may be river source bordering dunes with locally variable sand supplies. Above the calcrete layer the sand has been reworked in modern times by winds from the southwest. In coastal exposures where the Ngankipari Sand is associated with lacustrine sediments, the sand may have been deposited as lunettes. In conclusion, multiple provenance is an important feature of the Ngankipari Sand. Episodic deposition occurred in a terrestrial environment dominated by aeolian activity, with the intervening periods represented by the formation of calcrete. The postulated inland dunes, river source bordering dunes and lunettes are restricted to the eastern margin of the St Vincent Basin. 154

1.5

GEOCHRONOLOGY OF THE WONGAN HILLS OTEENSTONE BELT, WESTERN GHEISS TERRAIN, YILGARN BLOCK, WESTERN AUSTRALIA R.T. Pidgeon and S.A. Wilde School of Physics and Geosciences, Western Australian Institute of Technology

The Wongan Hills Greenstone Belt is located approximately 150 kms northeast of Perth in the Western Gniess Terrain of the Archaean Yilgarn Block of Western Australia, The Western Gniess Terrain forms the western margin of the Yilgarn Block and consists of complexly deformed metasediments and ortho and paragneisses intruded by post-tectonic granitoids. The Western Gneiss Terrain is characterised by the almost complete absence of volcanic rocks in contrast to granite-greenstone provinces in the Yilgarn Block to the east. In the Western Gneiss Terrain volcanic rocks have only been identified at Tallering Peaks, Koolanooka Hills, Mount Saddleback and the Wongan Hills. The Wongan Hills Greenstone Belt consists mainly of mafic and felsic volcanic rocks, cherts and banded iron formations, with subordinate schists, gneisses and small ultramafic intrusions. The belt is bounded by Archaean migmatite and gneiss and is intruded by even-grained and porphyritic granitoids. The high metamorphic grade of the belt is similar to portions of the Jimperding Metamorphic Belt in the Western Gneiss Terrain to the south, but differs from the low grade rocks in greenstone belts of the Murchison Province. In carrying out a zircon U-Pb geochronological investigation of the Wongan Hills Greenstone Belt samples were taken from a concordant band of felsic porphyry within the mafic volcanics near the Mount Rupert homestead, a late porphyritic granite intrusion into the belt and a migmatite, which represents an earlier granitic event. Zircon U-Pb isotopic systems were determined on size and magnetic fractions from zircon populations from the above samples. Five data points from the felsic porphyry (W1) fall close to a single chord with concordia of 3048 + 25 Ma. Zircon data points from the migmatite (W7) also fall close to a single chord defining an upper intersection age of 2765 + 33 Ma. The zircon points from the porphyritic granite (W6) show a significant scatter about an chord which intersects concordia at 2620 + 80 Ma. The zircon U-Pb age of the porphyry of 3048 + 25 Ma is interpreted as the age of extrusion or emplacement of the porphyry and as such represents a time point within the developing volcanic sequence. The 2765 + 25 Ma age for the migmatite is significantly younger than the felsic porphyry and on this evidence the migmatites surrounding the belt cannot be considered as early basement rocks to the greenstone belt. The age of the undeformed porphyritic granite of 2620 + 80 Ma places a younger age limit on the metamorphic and deformational activity in the vicinity of the belt. The present geochronological results have important regional significance. For instance the age of volcanism at Wongan Hills is significantly older than volcanic activity in the Saddleback Greenstone Belt in the Westen Gneiss Terrain to the south which has been dated at 2650 - 2670 Ma. Also the age of the amphibolite facies metamorphism at Wongan Hills is younger than the 3048 + 25 Ma age of the volcanics and consequently cannot 155

be related to the high-grade metamorphic Metamorphic Belt dated at c 3200 Ma, The emplacement 2600 Ma granite

the

Jimperding

of the porphyritic granite is consistent ages reported in the Yilgarn Block.

with other

156

event

4.5

Sn-W M E T A L L O G E N Y IN THE D A M A R A P R O V I N C E , N A M I B I A

Franco Pirajno Dept. of Geology, Rhodes University, South Africa The Damara Province is one of the late Proterozoic Pan African orogenic belts. The Province consists of a northern coastal arm, a southern coastal arm and an intracontinental belt, which lies between the Congo Craton and the Kalahari Craton in the SW region of Southern Africa. Their meeting point is considered to be a triple junction from which South America and Southern Africa separated, leading to the break-up of a late Proterozoic supercontinent. The Damara Province, whose geodynamic evolution is thought to be the result of the opening and closing of a narrow ocean, is sub-divided into four major zones which are delineated on structural, metamorphic and tectonic characteristics. During the Jurassic and Cretaceous widespread magmatic activity took place in response to phases of opening of the South Atlantic, and the break-up of Gondwana. Magmatic products include mafic-intermediate lavas of the Karoo sequence, and ring-type complexes of alkaline and/or bimodal character. In the Northern and Central zones of the Province there are numerous Sn, Sn-W, and W deposits. These deposits occur in pegmatites, in hydrothermal quartz veins, and as replacement types, hosted in turbiditic rock sequences. Three major clusters or belts are recognised. 1. The Uis-Kohero belt contains syn-to post-tectonic Sn (Ta) bearing, generally unzoned, pegmatites. These are related to residual phases of biotite-bearing granitoid suites emplaced between 580 and 550 Ma. Sn (Ta) mineralisation however is associated with greisenization which appears to be a later event than the emplacement of the pegmatites. 2. To the SE of the above areas are a number of deposits which include W bearing greisens and Sn (Ta) in zoned pegmatites. The former are spatially and genetically related to a boron-rich granitic phase of a series of magmatic products, with typical bimodal chemistry, belonging to a late Karoo ring-type caldera structure (Erongo Complex). In the pegmatites, Sn (Ta) mineralisation occurs in their greisenised portions, and although this greisenisation appears to be spatially related to the Erongo Complex, its genetic links are as yet uncertain. 3. North of the Brandberg alkaline granite intrusion, also of late Karoo age, is a group of W-Sn and Sn (+ sulphides) deposits in hydrothermal quartz veins and replacement bodies. A substantial number of these deposits are located within or near the rims of small-to-medium-size circular features, which are clearly visible on Landsat imagery. Geological evidence indicates that these circular features may be the surface expression of intense de-gassing (volatile streaming) from deep magmatic centres, perhaps of similar

157

nature and age as the outcropping ring-type complexes. This process would form circular fractures along the flow path of the streaming volatiles, and be preceeded or accompanied by the hydrothermal event responsible for the Sn-W and Sn mineralisation. In conclusion the Sn-W mineralisation in the Damara Province, although hosted by late Proterozoic pegmatites and turbiditic f a d e s rocks, appears to be related to anorogenic magmatism connected with the Gondwana break-up during the Mesozoic. The emplacement of these intrusions is considered to be controlled by NE trending transform directions. The Sn-W metallogeny of the Damara Province has striking similarities with the Nigerian Sn province, where Pan African magmatism resulted in the emplacement of "older" granites and pegmatites. Later anorogenic magmatism gave rise to ring-complexes and high level productive greisen mineralisation.

158

1.1

BATTEN SUBGROUP, McARTHUR BASIN - MODERN ANALOGUES FOR AN EVOLVING ANCIENT LAKE K.A. Plumb

Division of Continental Geology, Bureau of Mineral Resources, Canberra The ca.-l650 Ma old Batten Subgroup of the McArthur Group, N.T. is a regressive carbonate-evaporite sequence up to 1200 m thick, deposited in the syndepositional half-graben Batten Trough. The sequence grades from sublittoral euxinic black shale at the base, through shoreline carbonates, to emergent and ephemeral-lacustrine facies at the top. In common with many Precambrian sequences, it displays the problem of differentiating between marginal-marine and lacustrine environments without fossil control. Direct comparison with modern analogues indicates a hypersaline-lacustrine model for the whole sequence, with parts of it showing particular similarities with some modern environments in South Australia. This model fits well with the lacustrine model proposed by Williams and Logan (1981) for the underlying Barney Creek Formation, the host to the H.Y.C. deposit. In the axial zone of the Batten Trough the subgroup overlies the Reward Dolomite and Barney Creek Formation with gradational contact, but around the basin margins the subgroup unconformably-overlies the older units with a basal conglomerate. The subgroup is everywhere overlain, with regional unconformity, by the Nathan Group. The following formal units are mapped, from the base up: Caranbirini Member (of Lynott Formation) - organic and pyrite-rich black shales; Hot Spring Member (of Lynott Formation) - dolomitic siltstones, dolostones, and stromatolites; Donnegan Member (of Lynott Formation) - dolomitic silts and fine sands with abundant cauliflower cherts after nodular anhydrite; Yalco Formation - cherty dolostones; Stretton Sandstone - fine-grained (playaf l a t ^ sands; Looking Glass Formation - regolithic chert after carbonates. All boundaries, except at the base of the Yalco Formation, are gradational. All units extend throughout the mapped extent of the subgroup and show similar thickness variations across the Batten Trough. It is inferred that the boiindaries between major units approximate broad time lines, rather than lateral facies relationships. The major units reflect basin-wide variations in the relationship between hydrological regime, sediment supply, and subsidence, controlled in turn by changes in climate and/or tectonic events. For example, remarkably precise analogies have previously been demonstrated by Muir, Lock, and von der Borch (1980), between one distinctive facies of the Yalco Formation and the ephemeral dolomite lakes of the Coorong. This facies requires a particular seasonally humid and dry climate and thus, in keeping with its non-gradational base, the formation is interpreted as reflecting a major climate change, and its base as an approximate time line. The immediately-under lying Donnegan Member is characterised by ubiquitous cauliflower cherts after anhydrite, intergrown with red dolomitic terriginous silts and sands in the manner of the continental sabkhas of Abu Dhabi in the Persian Gulf. The host sediments probably represent a distal sand-silt flat with intermittent ponding, as evidenced from waveripple bedforms alternating with thicker and plane-bedded units. The basic host sediments to the anhydrite are very similar to sands and silts in much of the overlying Yalco Formation, but the climate was clearly more arid. 159

Immediately imderlying the Donnegan Member, and locally interfingering with it, the upper Hot Springs Member is characterised by several distinctive one metre thick beds of chert (the "Upper Evaporite"). This chert pseudomorphs stratiform low-relief domal s t r o m a t o l i t e s , w h i c h i n turn e n c l o s e pseudomorphed intimately-intergrown masses after bladed gypsum and an asyet-unidentified fibrous t r i c l i n i c e v a p o r i t e , and both are cut by tepees. These beds are remarkably similar to the Pleistocene gypsum stromatolites of Marion Lake and again indicate ephemeral, but arid, lake environments. These modern analogues are all marginal-marine lakes. But what of the lower members of the sequence? The euxinic pyritic black shales of the Caranbirini Member are characterised by graded varve-like laminations with quartz-silt bases and carbonate-rich tops, and by slump folds and massive turbidite beds; all typical of offshore facies in modern l a k e s . Slump breccias r e l a t e to movements of the syndepositional Emu Fault. Thin cherty and leached " r e g o l i t h i c " layers within shale suggest sudden emergence and then resubmergence, without intervening transitional sequences, as the balance between sediment-supply and subsidence varied l o c a l l y . The gradation to the overlying Hot Springs Member is characterised by gradually increasing carbonate content and by abundant chert or spar-filled voids interpreted as probably after evaporitefilled syneresis cracks induced by increasing salinity. The main bulk of the overlying Hot Springs Member is characterised by stromatolitic carbonates and cherts, plane and ripple-laminated carbonateterriginous s i l t s and f i n e sands, plane-laminated carbonates, and local channel-form sands. Mud cracks, intraclast breccias, tepees, pseudomorphs after various forms of displacive gypsum, and leached exposure surfaces are common throughout. These features and the cycles which they commonly comprise are a l l typical of marginal l a c u s t r i n e sequences such as in the Green River Formation. The cycles i n the lower parts of the member are mostly typical of lake-margin carbonate flats and distal deltas, while high mud flat and ephemeral lake cycles increase upwards. Finally, similar and intense diagenesis characterises all of the subgroup; primary fabric is destroyed to varying degrees. Dolomite is the ubiquitous carbonate phase. C h e r t i f i c a t i o n is abundant in a l l marginal to emergent carbonates, commonly replacing almost all of the dolomite. Associated with the chert, pervasive potassium diagenesis converts much of the clay to submicroscopic K-feldspar; as much as 65% K-spar locally. This may provide an a l t e r n a t i v e explanation for other K-rich " t u f f i t e s " throughout the McArthur Group. The consistent stratigraphy of the Batten Subgroup can thus be explained by a hyper saline-lacustrine model, involving a progressive f i l l i n g of a tectonically-subsiding depression. The v e r t i c a l sequence of major facies relates to the basin-wide balance between sediment supply, subsidence, and hydrology, controlled in turn by c l i m a t i c and tectonic changes. Lateral interfingering and migration of facies is largely relevent only at the scale of i n d i v i d u a l cycles, at most a few metres thick. Comparison with modern environments in South Australia has been fundamental to critical elements of this model.

160

5.4

STRUCTURAL CONTROL OF GOLD MINERALIZATION IN ARCHAEAN SHEAR ZONES D.M. Ransom^ and M.G. Fotios^ ^Consultant, Adelaide ^Homestake Australia Ltd, Perth

The general relationships of the internal geometry of shear zones and their associated external structures have been established by J . G . Ramsay and his co-workers. While unanimity is lacking as to the details of the dynamics of some shear zone-associated structures, it is nevertheless possible by careful observation in the field to interpret the strain, displacements and rotations of the structures which predate, or are synchronous with the shear zone development. Ramsay subdivides shear zones on the basis of strain field into brittle, brittle-ductile and ductile types, each of which have characteristic arrays of of associated structures. Shear zones in the brittle field are common fault structures, and examples of those in the ductile field are the retrograde schist-zones of the Willyama Complex. In brittle-ductile shear zones, brittle structures such as veins and breccias coexist or interact with ductile structures such as lineation and schist— osity. These latter shear zones are efficient foci of solution movement and in the Archaean are common, perhaps the commonest, hosts to gold mineralization. The important structural features of shear zones are the shear zone boundaries, the internal foliation, lineation and vein arrays. Foliation, shear zone boundaries and certain classes of veins intersect in common lines, which are normal to lineation. Their assymetry, and the orientation of lineation indicate the direction of transport and the principal axes of internal strain. Gold orebody shape is commonly directly related to these structural elements, and orebody location is often controlled by the geometrical relationship of the shear zone and its' enclosing rocks. Four examples of auriferous Archaean shear zones are described. These are the Con Mine at Yellowknife in Canada, and the Triton and Rand Mines at Reedy»s, the Fraser's Mine at Southern Cross, and the Bellevue Mine at Sir Samuel in Western Australia. At Yellowknife, the shear zones of the Con Mine are moderately dipping zones of chlorite schist lying at a high angle to stratigraphy in a suite of essentially undeformed Archaean pillow basalts. Schistosity is more steeply dipping than the shear zone boundaries and lineation is steeply pitching. Veins post-date and pre-date schistosity. Gold occurs in quartz veins and veined zones of probably three generations, which exhibit micaceous, carbonate- and sulphide-rich selvedges. They are synchronous with deformation and dip steeper than the shear zone contacts and shallower than schistosity. Ore shoots within the quartz veins are parallel to lineation, and the ore zones are commonly restricted to kinks in the strike and dip of the shear zones, geometrically related to intersection of the shear zones with stratigraphy in the surrounding rocks. The ore veins are not en echelon extension fissures, although early and late tensile fracture veins are commonly observed. The symmetrical relationship of the auriferous quartz veins to the shear zone kinks suggests they are shear fractures. At Reedy»s, the Triton and Rand orebodies have been interpreted in the past as being biotite- and sulphide-rich sediments within a sequence of mafic volcanics. It has been shown recently that ore occurs within five or six 161

identifiable shear zones disposed In a low angle en echelon arrays probably within a larger shear zone structure of regional extent. Similar to Yellowknife* ore shoots In the Triton Mine are elongate parallel to llneatlon. Gold occurs associated with sulphide In rocks which are probably selvedges of fine quartz stockworks or tension vein arrays. These probably formed In response to the Initial movements of the shear zone and were subsequently transformed by deformation to finely foliated schists. The elongation of the shoots parallel to llneatlon results from subsequent deformation* their original shape being uncertain. Gold values decline as the shear zone passes Into a dolerlte from the mafic sequence and Is constrained on a large scale by regional disharmonic folding. Porphyrltic dykes have been Intruded into the shear zones late In the deformation history and are regarded as unrelated to gold deposition. At Southern Cross* structural control In the Fraser's Mine has been attributed to en echelon folding of a mineralized sediment unit. The ore zone In fact occurs In a shear zone which transects stratigraphy at a small angle and Is specifically related to Its* contacts with an ultramafic rock unit within a sequence of fine mafic tuffs and BIF. The ore zone as a whole pitches shallowly south* again parallel to llneatlon* and ore shoot geometry Is controlled partly by llneatlon and partly by weakly mineralized shear fractures which link separated zones of en echelon quartz tension veins. At Sir Samuel* the shear zones of the Bellevue Mine exhibit a relatively high angle between internal foliation and shear zone boundaries and weak development of llneatlon. In the old mine the ore shoots are elongate parallel to the foliation/shear zone boundary intersection* probably reflecting a low strain environment. The shapes of the ore shoots are complicated by lithological controls and their dismemberment by late unmineralized shear zones of similar sub-horizontal displacement. The overall control of the ore zone is unclear* but is probably constrained by a weakening of the shears on a sub-regional scale. Based on the foregoing examples* the controls of gold mineralization in shear zones can be generalized as follows: *

within the confines of the ore zone* ore shoots are related to the direction of transport within the shear zone and/or the principal strain axis. Where strain is large* ore shoots are usually parallel to llneatlon. Where strain is small* ore shoot elongation may be normal to the intersection of foliation and the shear zone boundary;

ore zones are usually defined as vein arrays of shear or tension fracture origin* and their mineralized selvedges. Commonly* original shape and geometry of these veins are obliterated by subsequent deformation. Gold deposition is invariably syn-deformational;

since shear zones are usually infinitely long structures relative to the gold orebodies which they enclose* factors other than the dynamics of their formation bear on the precise location of ore zones. In each of the above examples this control is related to the physical and geometric properties of the rocks which host the shear zone. Chemical controls of the shear zone host rocks may also be a factor* but in each of the examples described* the host rocks are completely normal representatives of Archaean mafic suites.

The management of Cominco Ltd* Homestake Australia Ltd* Metana Minerals NL* Golden Valley Mines NL* Spargos Exploration NL and Queen Margaret Gold Mines NL are acknowledged for permission to publish this paper. 162

3.5

ALUMINO-^GNESIAN CIAYS AS PRECURSORS OF CORDIERITE-ANTHOPHYLLITE ROCKS IN TEE ROSEBUD SYNCLINE (MT ISA INLIER) GEOCHEMICAL EVIDENCE AND FIELD RELATIONS J. Reinhardt Geology Dept. James Cook University of North Queensland

Cordierite- and anthophyllite-bearing assemblages are found in medium- to high-grade metamorphic rocks with bulk chemical compositions dominated by MgO, FeO, and Al^O^ (apart from SiO^ and H^O). Alkali and Ca contents are characteristically low. Cordierite-anthophyllite rocks are comparatively rare in metamorphic terrains, and most occurrences seem to be associated with metavolcanic rocks. Chloritic alteration of the volcanics and subsequent metamorphism is commonly regarded as the most likely process leading to the formation of cordierite- and anthophylliterich rocks. Particularly with respect to the use of such rocks in exploration for sulphide deposits, it must be pointed out that the rather unusual chemistry of cordierite-anthophyllite rocks may be the result of a range of different geological processes (e.g. synmetamorphic metasomatism, metamorphism of hydrothermally altered rocks, and processes involving partial melting, to name but a few). This has been conclusively demonstrated by various authors. Cordierite-rich rocks (including cordierite-anthophyllite assemblages) as well as chlorite- and talc-bearing high-grade assemblages are present in the Rosebud Syncline near Mary Kathleen (central Mt. Isa Inlier). These alumino-magnesian rocks occur as abundant lenses and layers in the Middle Proterozoic Corella Formation, a metasedimentary sequence which consists largely of calcsilicate rocks and pelitic schists. The study area lies entirely in the sillimanite zone, and lower-grade equivalents of the cordierite- and anthophyllite-rich rocks outside the syncline are not known to exist, despite the wide distribution of the Corella Formation in the eastern Mt. Isa Inlier. Smaller occurrences of cordierite-rich rocks of similar metamorphic grade are present in the Little Beauty Syncline just north of the study area. Although the original stratigraphic relations in the Rosebud Syncline are often obscured due to complex polyphase deformation, sedimentary structures such as bedding on a variety of scales are preserved in the high-Mg zones. Furthermore, gradational transitions into more common aluminous schists are frequently observed. The sum of field relations as well as the absence of any significant volcanic activity during the deposition of the sequence strongly suggests a sedimentary origin for the high-Mg rocks. The chemical compositions of the studied rocks exhibit a clear Mg-Al trend in an AFM plot, corresponding to the compositional trends of Ca- and alkali-poor clays deposited in hypersaline facies. A comparison with the compositions of cordierite-anthophyllite rocks derived from altered volcanics (as in volcanogenic massive sulphide deposits) reveals that their compositional field in an AFM diagram has only a small overlap with the field of evaporitic clays. The Mg/Fe ratio of the Rosebud Syncline rocks is significantly higher than the Mg/Fe ratio commonly found in hydrothermally altered volcanics. The trace elements show generally a good correlation with the ranges of trace elements observed in pelitic sediments. On the other hand, the comparison with trace element ranges of both acid volcanics and basalts shows a strong divergence for some elements. Even taking into account an 163

alteration model with depletion of the more mobile trace elements cannot satisfactorily explain the combined trace element patterns of the Rosebud Syncline rocks. Cr, Ni, and Co values are too high for altered acid volcanics to be considered as precursors, whereas the Nb contents are considerably lower than commonly observed in acid volcnics. Similary, Sc contents are too low, and Th contents are too high for rocks of basaltic descent. Moreover, the low TiO^ values are more characteristic of sedimentary rocks or acid volcanics than of basalts. All the evidence therefore points towards metamorphosed magnesian clays as precursors of the cordierite-anthophyllite (and related) rocks of the Rosebud Syncline. Unmetamorphosed equivalents have been found in sedimentary sequences, which were deposited in hypersaline environments, such as parts of the European Triassic and the Northwest-African Triassic. In some clays of evaporitic sequences, Mg-rich clay minerals like Mg-chlorite, palygorskite, sepiolite or corrensite are present in significant amounts. If carbonate (calcite,dolomite) and illite contents are low, compositions corresponding to cordierite-anthophyllite- and talc-bearing assembalges can be obtained. Increasing illite contents shift the compositions towards common pelitic schists, resulting in the disappearance of anthophyllite and the presence of aluminous minerals (e.g. aluminosiliates). If, on the other hand, larger amounts of Ca are present in the clays, calcsilicate assemblages form as soon as middle amphibolite facies conditions are reached. These variations in rock chemistry and mineralogy are indeed observed in the Rosebud Syncline. The interlayered calcsilicate rocks, aluminous schists and magnesian rock varieties represent that bandwidth of chemical variation, which is interpreted as reflecting deposition in an evaporitic basin. Hypersaline conditions are also indicated by the extraordinary abundance of Cl-rich scapolite in calcsilicate rocks and schists. Thus, it has been shown, that geochemical parameters can be successfully used to characterize cordierite-anthophyllite rocks of specific origin. It must be stressed that the field relations of such rocks are equally important for any interpretation. In this specific case, processes like synmetamorphic metasomatism or partial melting could be discarded, not so much as a result of the geochemical investigations, but from the mode of occurrence in the field. The possibility of discrimination between different types of cordierite-anthophyllite rocks by means of oxygen isotope geochemistry is currently examined.

164

2.1

THE OLYMPIC DAM COPPER-URANIUM-^OLD DEPOSIT D.E. Roberts

Roxby Management Services Pty Ltd, Adelaide

The Olympic Dam copper-uranium-gold deposit in South Australia was discovered in 1975. Probable ore reserves of 450 million tonnes of 2.5% copper, 0.08% uranium oxide, 0.6 g/t gold and 6.0 g/t silver are contained in a resource of 2,000 million tonnes of 1.6% copper, 0.06% uranium oxide and 0.6 g/t gold. The deposit also contains significant concentrations of iron, lanthanum and cerium and the deposit can be considered as a hematite-rich iron deposit containing intimately associated copper, uranium, gold and light rare earth elements. The middle Proterozoic terrain which hosts the Olympic Dam deposit is overlain unconformably by about 350m of younger Adelaidean (late Proterozoic) and Cambrian shelf sedimentary rocks (dolomitic shales, quartzites and dolomitic limestone). At Olympic Dam the terrain consists of an anorogenic, A-type, potassium feldspar granite containing a northwest trending graben in which the deposit is situated. Narrower elongate troughs within the graben contain thick sequences of coarse clastic sediments. A group of discordant rock units which consists of a variety of igneous dykes, massive iron-rich bodies and discordant polymict breccias has also been recognised. The discordant rock units transgress the three main sedimentary units and evidence indicating multiple events is common. Dip-slip and strike-slip faulting with steep attitudes is common throughout the deposit and structural zones control the distribution of discordant rock units and mineralization. A tectonic zone forms the northwest trending axis of the deposit and this zone contains narrower structural zones with various trends. The complex association of copper, uranium, rare earths, gold, silver and iron mineralization in the deposit makes it unique. Mineralized zones range from steeply dipping, discontinuous veins less than Im thick, through moderately dipping lenses 30m to 50m thick with strike and dip dimensions in the order of hundreds of metres, to flat-lying zones over 200m thick with areal extents of hundreds of thousands of square metres. Two types of copper mineralization have been defined on the basis of sulphide assemblage, rock association and stratigraphic position. Stratabound mineralization is confined to sedimentary units and is strongly zoned from pyrite at the base, through chalcopyrite to bornite and chalcocite at the top. The sulphides are evenly disseminated and constitute up to 15% of the rock. Copper grades are typically in the order of 2-3% with some narrower zones (20-40m) up to 5%. Transgressive mineralization occurs in a variety of rock types but is confined to the northwest trending tectonic zone. Sulphide assemblages are dominated by chalcocite and bornite but lateral and vertical zonation to chalcopyrite and pyrite is present. Higher grade zones (in excess of 5%) are intimately associated with discordant massive hematite bodies and polymict breccias. Disseminated blebby sulphides, veins and massive sulphide matrices are common textural associations. Uranium mineralization is closely associated with copper zones but locally uraniumrich, copper-poor, zones occur. The distribution and form of the uranium mineralization varies sympathetically with the type of copper mineralization. In stratabound copper zones uranium is evenly distributed with typical grades of 0.05 to 0.1% U3OQ whereas in transgressive copper zones uranium-rich veins and segregations are common with grades in excess of 0.2% U^OQ. In these areas complex textural associations of uraninite with bornite, chalcocite and chalcopyrite, are observed. Vein style and matrix uraninite often has an intimate association with chlorite. 165

Significant gold m i n e r a l i z a t i o n o c c u r s in intensely q u a r t z - s e r i c i t e altered zones which are spatially separate fronn c o p p e r - u r a n i u m m i n e r a l i z a t i o n . It is thought that the O l y m p i c D a m deposit was f o r m e d in an extensional continental e n v i r o n m e n t within a northwest trending tectonic zone. M i n e r a l i z a t i o n was deposited and continually modified by a very large evolving h y d r o t h e r m a l s y s t e m . E x t e n s i v e high level intrusive a c t i v i t y with probable alkaline affinities c o m p l e x l y interacted with structure and stratigraphy resulting in widespread transgressive c o p p e r - u r a n i u m - g o l d m i n e r a l i z a t i o n being superimposed on older stratabound m i n e r a l i z a t i o n in coarse c l a s t i c sediments. L i m i t e d fluid inclusion data indicate stratabound and transgressive m i n e r a l i z a t i o n were deposited f r o m fluids with similar c o m p o s i t i o n s (about 7 % N a C l equivalent) but at different t e m p e r a t u r e s (average 1 6 5 ° C for stratabound, 2 4 0 ° C for transgressive with some overlap in ranges). Sulphur isotope analyses indicate a c o m m o n source of sulphur with similar ^ ^ S values ( - 5 to - 8 /qq ^ ^^^ sulphides. The source of metals within the deposit is unknown.

166

11.4

THE MARLBOROUCT CHRYSOPRASE DEPOSITS A.D. Robertson Geological Survey of Queensland

Commercial deposits of chrysoprase are located in Marlborough Holdings on the upper slopes of a divide (South Slopeway; GR 910578 Marlborough 1:100 000 Sheet 8852) between Marlborough and Develin Creeks. These deposits occur near the southwestern margin of an irregularly shaped mass of ultramafic rocks covering an area of over 780 square kilometres. The alpine type ultramafics lie predominantly along faulted boundaries between major structural units and are considered to be remnants of Palaeozoic oceanic floor that has been thrust westward into its present position. The western margin of the ultramafics is overlain by Permian volcanics and sediments, the eastern margin is intrusive into metasediments. During the Tertiary, two periods of stability and deep weathering occurred in the Marlborough region. The first weathering phase favoured the development of a nickel-rich residual soil blanketing the ultramafic rocks. Deep weathering developed along permeable zones and part silification of the weathered profile resulted in the formation of the first generation of chrysoprase. Tectonic instability accompanied by faulting caused partial erosion of the soil profile and the deposition of fresh-water sediments in depressions on the ultramafic body. The second phase of deep weathering developed an iron-silica rich zone towards the base of the sedimentary sequence and a second generation of chrysoprase formed in the lateritic profile. The composite lateritic profile has been dissected by erosion. Piping processes have played a part in the destruction of the iron-silica rich zone and the zone now appears as isolated residual caps protecting the softer underlying strata. The chrysoprase bearing profile on South Slopeway can be divided into three main zones: (i) an iron-silica rich cap; (ii) the chrysoprase bearing zone; and (iii) the basal zone. The iron-silica rich cap can be further subdivided into (a) an iron-rich pisolite conglomerate and/or rubble and (b) lateritised sediments and the upper part of the residual fossil soil horizon. Silification within the cap is variable, resulting in the development of both box-work structures and massive jasper. The strongest silification occurs towards the base of the lateritised sediments. The chrysoprase bearing zone comprises the base of the fossil soil horizon and the saprolite horizon (of the lateritic profile). The base of this zone is defined by an increase in concentration of magnesite. Chrysoprase occurs as scattered irregular veins having neither vertical nor horizontal extent. Where magnesite increases in concentration, the percentage of chrysoprase rapidly decreases and its place is taken by opalite and chalcedony. Occasionally, nodular chrysoprase is found associated with increased concentrations of magnesite. The basal zone comprises partly weathered to fresh serpentinite accompanied in the upper parts by veins of magnesite, opalite, chalcedony and minor chrysoprase. Little chrysoprase of value is recovered from this zone. Maximum thicknesses recorded for the iron-silica cap and the chrysoprase bearing zone are 40m and 50m respectively.

167

11.1

PRECIOUS OPAL AND THE WEATHERED PROFILE AT COOBER PEDY R.S. Robertson and D.C. Scott Geological Survey of South Australia

The Goober Pedy Precious Stones Field, in the north of South Australia, is the largest producer of precious opal in Australia and the world. Geological investigations during and subsequent to a Government funded subsidised exploration program in 1981 have established that opal at Goober Pedy is a product of weathering processes and is localised within the weathered profile. Precious opal occurs in early Cretaceous marine claystone of the Bulldog Shale (Marree Sub Group) deposited on the western margin of the Great Australian (Eromanga) Basin. When fresh. Bulldog Shale is a dark grey, pyritic, silty or sandy claystone or shale with lenses of sand, fossiliferous cone-in-cone limestone and occasional boulders. Smectite is the main clay mineral. The opal fields extend along a 40—50 m high east—facing escarpment and gently west-sloping tableland called the Stuart Range. Throughout the Stuart Range, Bulldog Shale is deeply weathered and bleached. Weathering extends to depths in excess of 50 m. In opal bearing areas, the top 20 m of the weathered zone comprises bleached white or light mauve kaolinitic claystone, porous, lightweight and slightly silicified, above denser, darker-coloured claystone usually containing smectite as the major clay mineral. Precious opal is found anjrwhere in the upper bleached claystone (called 'sandstone' by miners) but the most productive part of the profile is usually a zone from about 5 m above to 1-2 m below the change to darker-colour denser claystone. In general, the deeply weathered profile and the opal-bearing zone follow the present land surface. Overprinting of weathering features is sometimes observed and there have been several phases of weathering. Precious and potch opal is found as irregular veins infilling cracks and joints and occasionally replacing fossil shells. Opal veins are usually located in subhorizontal features called 'levels' which are concentrations of weathering products such as gypsum, alunite, iron oxides and red brown tubules (termite burrows). 'Levels' probably represent old groundwater tables rather than bedding planes although their position may be influenced by original lithological variations. At Goober Pedy, opal is not localised in any particular stratigraphic horizon. The weathered zone is cut by steeply dipping structures called 'slides', thought to be faults mainly related to volume changes during weathering rather than tectonic movements. Fracturing during these volume changes may be the origin of the cavities in which opal and other weathering products were deposited. Several phases and styles of silicification are present within weathered claystone including the slight, pervasive silicification in normal bleached claystone and harder, more complete silicification known as 'blue ground'. 'Blue ground' is believed never to host opal but some miners contend that the margins of 'blue ground' are particularly favourable. Overlying weathered Bulldog Shale in most opalfield areas is a variable sequence of Pliocene(?)-Quaternary colluvial and alluvial sediments termed Russo Beds. These comprise red brown gypseous, sandy silt and clay containing numerous fragments of bleached claystone, variably silicified, and rounded silcrete boulders. The boulders are derived from older silicified sandstone, possibly of Miocene or Eocene age, deposited on the Stuart Range but now largely removed. Silicification of Russo Beds is 168

common, particularly towards the base of the unit, Silicification often present in the top 1-2 m of bleached claystone is thought to be related to this Russo Bed silicification. Opal is found in Russo Beds as scattered fragments derived from erosion of veins within Bulldog Shale. Opal at Goober Pedy has formed during weathering of the Bulldog Shale, probably by gradual concentration by evaporation or filtering of groundwater containing colloidal silica, trapped in open spaces. Silica was produced by the conversion of smectite clay to kaolinite and possibly by corrosion of quartz grains. Numerous types of silicification demonstrate the abundance and mobility of silica in the system but the rarity of precious opal is due to the special conditions needed to allow the accumulation of equal sized silica spheres and their packing in a regular array. The factors affecting opal formation are likely to be complex and subtle but some possible controls are suggested. Major vertical control, concentrating opal at the base of the bleached zone is likely to be the presence of permeability barriers. Although original lithological variations may have some influence, the principal barrier is provided by the weathering itself in the contrast between permeable upper bleached, porous claystone and lower impermeable partly weathered and fresh claystone. Suitable cavities were probably also concentrated in this zone due to fracturing during volume changes. Chemical factors, including pH, perhaps related to weathering of the pyritic Bulldog Shale, and the presence of minerals which could hasten or impede the precipitation of silica may also have influenced opal formation. Variations in the weathering environment, as reflected in subtle variations in the type of weathered profile, appear to be an important horizontal control on opal formation. Established opalfields and prospective areas found by drilling during the subsidised exploration program have a particular type of profile. Miners use this type of 'sandstone' as an exploration guide for new opal bearing areas. This variation may be due to the effect on groundwater movements of different palaeogeographic situations during weathering. Well-drained areas with relatively low water tables may have been favourable for opal formation whereas poorly-drained, high water table areas such as lakes and swamps and major drainage channels may have been unfavourable. A more localised control on opal formation may be provided by 'slides' which are believed by many miners to be an important indicator of favourable locations. 'Slides' were probably pathways for groundwater movement and hence could have influenced opal formation; however, the relationships appear not to be as clearcut as suggested by some miners. Timing of precious opal formation within the multiphase weathering and silicification process is not well established. An upper limit is provided by the Pliocene(?) Russo Beds which contain clasts of opal. The relationship between the deep, opal-bearing, weathered profile and present and past land surfaces may assist in providing a lower limit.

169

14.8

THE SEISMICITY OF QUEENSLAND AND NORTHEASTERN NEW SOUTH WALES 1866 THROUGH 1985 John M.W. Rynn

Department of Geology and Mineralogy, University of Queensland When considering the seismological nature of the Australian continent with the current available published information of either earthquake occurrences, epicentral distribution or ground motion parameters for seismic risk assessments, it appears that the seismic activity in the northeastern Australian region is extremely low. Recent studies have shown that this is not the case. Indeed, the level of activity is one of the highest within the Australian continent. The known seismic history spans 120 years with the earliest reported earthquakes being in 1866 on Cape York and 1975 near Warwick. A large proportion of the data is based on macroseismic observations principally because of the lack of seismograph stations in the region. The major stations were installed in Brisbane in 1937, Charters Towers in 1957 and Cooney Observatory (near Armidale) in 1974. In recent years, seven single component stations have been added and multi-station networks for reservoir induced seismicity studies about Wivenhoe (southern Queensland) and the Burdikin Falls (northern Queensland) dams implemented. Despite this increased instrumental coverage, an improved station distribution is still needed to provide more precise determinations of earthquake parameters for this region. A distinctive spatial distribution of earthquakes has now emerged. Earthquakes occur in both the terrestrial and marine terranes. They appear to be concentrated within the eastern parts of the Tasman Fold Belt system, with more intense pockets of activity in the northern, central and southern sectors in Queensland, the New England Tableland and along the Queensland-New South Wales border. Off the coast, epicentres have been located i n the Coral Sea Basin, along the Great Barrier Reef, in the Capricorn-Bunker groups of islands and associated with the Tasmantid seamounts (old Tasman Ridge). Many are confined to the continental shelf areas. The most active region is that defined as the Wide Bay-Burnett zone in central-eastern Queensland. An attempt has been made to define the epicentral distribution in terms of broad causal relationships with geological structures. The criteria involved include faults, major lineaments and volcanic provinces. Such relationships are considered on a broad regional sense rather than small-scale localised interpretations. This multi-disciplinary approach has allowed an earthquake zoning map to be produced wherein the region has been divided into nine zones. Little definitive information can be afforded on the temporal distribution of earthquakes over the 120 years. This is due to the bias in population distribution in the early years (macroseismic data) and the poor distribution of seismograph stations (instrumental data) in the region. Magnitude values are now available for all earthquakes. These have been calculated using the classical formulae for instrumental data (m , MS and ML) and for macroseismic data from both isoseismal maps (ML(l)f and the ML-Io relationship (ML(lo)). For those earthquakes where only isolated felt reports are available, maximum possible magnitudes (ML(E/M)) have been estimated for the ML-Io relationship and comparisons with other known earthquakes in the same area.

170

A considerable amount of macroseismic information is available for the region. More than 50 isoseismal maps have been produced and many more instances of felt reports are on file. Several aspects of the seismicity are worthy of note. Seventeen earthquakes with magnitudes greater than ML 5.0 are known to have occurred in the 120 year period. The largest event was the 1918 "Queensland'* earthquake (ML 6.2). The effects of this earthquake were experienced over 300000 sq. km. of southern Queensland and northeastern New South Wales. Several other earthquakes have effected areas of about 100000 sq. km. Current research involves the compilation of all known seismic data for the region which is being integrated towards the quantitative assessment of seismic risk for northeastern Australia. Analyses are directed towards the definition of parameters for all earthquakes, satistical studies of the data and the multidisciplinary approach to determine causal relationships between the seismicity and the geology/tectonics. The study is being undertaken in cooperation with geologists and civil engineers in the academic, governmental and counsulting communities. Funding is provided by a consortium of departments of the State Government of Queensland.

171

14.4

DETERMINATION OF EARTHQUAKE HYPOCENTRES A T REGIONAL RANGES M . S . Sarabridge and B.L.N. Kennett Research School of Earth Sciences, Australian National University, Canberra

A new method has been devised for the determination of earthquake epicentres at local to regional distance scales. This technique is based on a fully non-linear treatment of the inverse problem of estimating hypocentral parameters and origin time from observations of P and S travel times. An initial estimate of the bounds on epicentral parameters is determined from the arrival order of the P waves at the various stations in a network. From each pair of stations one geometrical constraint is obtained by considering the relative position of the stations in the arrival order. Combining all pairs of stations gives an epicentral region satisfying all or most geometrical constraints from which a set of epicentral bounds are chosen. Estimates of origin time and depth bounds are made with the use of P and (P~S) times. Alternatively in cases of little or poor quality S wave data an a priori depth bound may be used together with the P wave data to find a set of origin time bounds. The resulting set of hypocentral bounds serve as an initial region parameter space for the grid search procedure. This technique consists of several searches over regions of spatial parameters for given temporal parameters and a minimization of an error statistic along the temporal parameter axis. The search regions are progressively narrowed to increase computational efficiency and the algorithm avoids some numerical instabilities associated with matrix inversions. The method may be used with a variety of assumptions as to the appropriate error statistics on the observations, currently Gaussian statistics and a modification due to Jeffreys are being employed. The direct non-linear approach allows a direct estimation of confidence regions for the hypocentral parameters which is particularly helpful for studying the allowable range of depths. This new approach is being employed to re-examine the depth distribution of earthquakes in Southeastern Australia.

172

8.6

PETROGRAPHY AND DEPOSITIONAL ENVIRONMENT OF PERMIAN COAL, COLLIE BASIN, WESTERN AUSTRALIA Krishna K# Sappal

Department of Geology and Geophysics, Western Australian Institute of Technology The Collie Basin is the primary source of coal for electricity generations in Western Australia and is thus vital to the economy and future developments of the state. The basin is a small bilobate fault bounded intracratonic basin containing Early and Late Permian coal measure sequence. It is located 150 km S.S.E. of Perth and is subdivided into three sub-basins - Cardiff, Muja and Shotts. The basin covers an area of approximately 230km^ within a basement complex of Archaean granite and gneiss. Sedimentation in the Collie Basin began in early Permian times with the deposition of basal tillites followed by glacio fluvial sediments, which together comprise the Stockton Formation. The Collie Coal Measures, also of Permian age, conformably overlie the Stockton Formation. They form a succession of coal seams, sandstones, siltstones, shales, clays, grits and conglomerates. The estimated thickness of Permian sediments is 1300m in the Cardiff Sub-basin, 800m in the Muja Sub-basin and 600m in the Shotts Sub-basin. The coal measures are overlain unconformably by the Nakina Formation with a range of 4m to 35m thickness. It is composed of a sequence of poorly cemented sands, clays and grits of lacustrine and fluviatile origin of late Tertiary age. Along ridges where the Nakina Formation is thickest, a laterite unit of Pliocene age has developed. The petrographic analyses, in terms of lithotypes, macerals and mineral matter of nine coal seams (Ate, Bellona, Ceres, Diana, Eos, Flora, Galatea, Hebe and lona) from the Muja Sub-basin are presented in this paper, and a depositional environment for the Collie Coal Measures is suggested. It is well established that the physical and chemical variations in coal are a consequence of its environment of deposition and parent plant material, and are expressed in terms of type and rank. The type of coal is related to the parent plant material of the peat and its biochemical and geochemical alteration, and it is assessed in terms of petrographic analyses, lithotype, maceral and microlithotypes. Coal type is discussed in terms of macroscopic composition - bright, bright banded, banded, dull banded and dull, and in terms of maceral composition vitrinite (A/B), exinite (sporinite, cutinite, resinite and alginite) and inertinite (fusinite, semifusinite, macrinite and micrinite). The mineral matter associated with coal macerals is identified as clay, pyrite carbonate, quartz etc. The nine coal seams are predominantly characterised by banded, dull banded and dull coal types with very few bands of bright and bright banded coal. Three seams contain thin bands of carbonaceous shale and sandstone. The maceral analyses completed on individual subsections of seams, and average for whole seam, show that in all seams the vitrinite content ranges from. 25.3 percent to 64.5 percent, and most of the vitrinite is of type B. The variation of exinite is from 2.6 percent to 6.8 percent, and it is mostly composed of sporinite, resinite and cutinite. The inertinite content varies between 30.2 percent to 69.0 percent, and there is a dominance of semifusinite and inertodetrinite. Mineral matter is low in all seams with range between 1.1 percent to 3.9 percent and it consists mostly of clay found associated with inertinite and vitrinite B. Pyrite, when observed, usually occurs as discrete grains of 173

associated with inertinite and vitrinite B. The ratio of structured (fusinite and semifusinite) to detrital inertinite (inertodetrinite and micrinite) is referred to as the 'semifusinite ratio' and it is described as high (l.O - 0.8), m e d i u m (0.8 - 0.6), low (0.6 O.A) and very low (O.A - 0.0). The semifusinite ratio of Collie coal from the nine coal seams is low to medium. The coal types of the Collie Basin are related to the position of the water table in the b a s i n , and on the basis of the abundance of banded, dull banded and dull coal types, the water table postulated in the basin was m o s t l y fluctuating with occasional low water levels which accounts for m i n o r fusain band s in the coal. On the basis of m a c e r a l composition the coal from the Collie Basin contains high vitrinite and medium to low semifusinite ratios. The pattern of sedimentation in the basin was essentially braided fluvial with fluvio lacustrine and peat forming environments during periods of low sedimentation. The vitrinite reflects formation of peat from a woody vegetation within relatively stable and high ground water regimes. The higher content of inertodetrinite in the coal perhaps represents transported m a t e r i a l from the m a r g i n a l areas where greater fluctuations in ground water conditions would occur.

174

2.3

GEOCHEMISTRY AND MINERALOGY OF METASEDIMENTS, MOUNT DORE COPPER DEPOSIT K.M. Scott

CSIRO, Division of Mineral Physics and Mineralogy, NSW The Mount Dore copper deposit occurs 90 km south of Cloncurry, in the Mount Elliot Member of the Kuridala Formation, adjacent to and dipping beneath the Mount Dore Granite. The host rocks consist of siltstone, shale (often carbonaceous), fine-grained quartzite, schist and breccia (both tectonic and sedimentary) which rapidly lens out, making correlation between drill holes difficult. A massive clean quartzite jEorms the footwall. Lower amphibolite grade regional metamorphism, characterized by the development of scapolite and andalusite, affects the sediments. The Mount Dore Granite which forms zne hanging wall to the mineralization is non-foliated, medium to coarse-grained and partly porphyritic. Granite emplacement is post-metamorphic and controlled by faulting, resulting in brecciation of both the granite and the immediate host sediments. Silicification of the sediments occurs close to the granite. Although weathering generally persists to about 80 m within the prospect, in breccia bands it can occur at depths greater than 450 m, whereas carbonaceous shales to the south of the prospect show negligible oxidation below a few centimetres. However, because of its location in the breccia bands, ore often occurs as secondary copper minerals, particularly chrysocolla. Sulfides generally only occur in the deeper western portion of the prospect, topographically beneath the Mount Dore Granite. The mineralization has a strike length of 1 km and may be 340m wide and 130m thick in its northern portion but is much narrower and thinner in the more poorly defined southern portion. It displays metallogenic zonation from Cu + Zn > Cu + Co + Zn Cu from north to south. Carbonaceous shale with an aggregate thickness greater than 50m defines a coherent area 600 x 300m in the northern and central parts of the deposit. This area, which hosts about 60% of the mineralization, is bounded by more chloritic and dolomitic sediments. The chloritic sediments are enriched in Fe and Mg relative to the carbonaceous sediments and may be derived from volcanics to the east. Three different environments of deposition of the carbonaceous and chloritic sediments can be envisaged. Organic material can be considered as forming as a mud bank not affected by incoming easterly-derived material i.e. carbonaceous and chloritic sediments are coeval. Alternatively if the organic matter accumulated in a local depression, the chloritic material could represent either marginal authigenic growth or turbiditic activity in non-quiet periods. An apparent inverse relationship between carbonaceous and chloritic sediments seems to preclude the latter possibility but the former two cannot be separated on current information. As thick carbonaceous sequences occur to the north and south of the prospect, the Kuridula Formation may contain a series of such mud bands or sub-basins with poor development of organic matter in between. Feldspars associated with the deposit vary from albite and orthoclase in the extreme north to orthoclase which may contain more than 8% Ba in the south. Microcline is subsidiary to orthoclase in the mineralized sections but more abundant in barren holes. The albite in the northern part of the deposit is primary i.e. unrelated to granitic alteration. 175

Copper tends to occur in carbonaceous samples throughout the deposit but the highest grades generally occur within weathered or brecclated rocks. Such rocks also have elevated Co, Zn and possibly Mo and P contents. Sulfur Isotopes show that the pyrlte and chalcopyrlte are not In equilibrium and that the footwall pyrlte » llVoo) Is Isotoplcally much heavier than that from the ore zone - 2.9®/oo). High Ag, Co and N1 contents also distinguish ore zone pyrites from those In the foot and hanging walls. The low S Isotoplc ratios could reflect a S contribution from the granite but the low (< 150 ppm) Cu content of pyrlte from the granite Indicates that Cu Is unlikely to be granite-derived. Thus the most likely genesis of the deposit Is by remoblllzatlon of Cu from the carbonaceous shales during brecclatlon and slllclfIcatlon of the sediments adjacent to the Mount Dore Granite.

176

3.1 MAQIATISM, MANTLE METASOMATISM AND TECTONICS IN A PROTEROZOIC MOBILE BELT, EASTERN ARDNTA BLOCK, CENTRAL AUSTRALIA Warwick Sivell Department of Geology and Geophysics, University of Adelaide

In the Harts Range, central Australia, basement gneiss complexes of the Strangways Metamorphic Complex, including the Oonagalabi and Entia gneiss complexes, are structurally overlain by a supracrustal cover sequence consisting of the Irindina supracrustal assemblage and the Harts Range metaigneous complex (HRMC). Mafic-ultramafic granulites and amphibolites are interlayered with the predominantly "quartzofeldspathic basement gneisses on a meso- and macroscopic scale. The contiguous HRMC consists mainly of metabasaltic amphibolite, leucoamphibolite, anorthositic gneiss and ultramafic rocks. Within the Oonagalabi gneiss complex, extremely fractionated basaltic to ferrobasaltic amphibolites and transitional granulite-amphibolites comprise two geochemically and spatially distinct mafic tholeiitic suites (deformed sills/dykes). The metatholeiites are characterised by high to very high FeO, TiO^ and PÔ^ contents and variable depletion in CaO and AlÔ^. A more primitive metanorite with high Cr and Ni levels constitutes a third petrographically distinct metabasite type. High-grade (granulite fades) metamorphism and, in particular, subsequent limited (amphibolite fades) retrogression appear to have affected only abundances of certain highly mobile elements (e.g., KÔ) in these rocks which otherwise preserve remarkably well their pristine 'igneous major and trace element signatures. Despite similar Zr/Nb ratios, the rocks from the three Oonagalabi suites show different degrees of enrichment in LREE and other immobile elements. The data suggests that the primary basaltic liquids were derived by different degrees (15-30%) of partial melting from essentially similar undepleted source regions. Clinopyroxene in the residual mantle assemblage exerted the main control on the composition of the segregating melt at the lower degrees of melting. Basaltic amphibolites comprising mafic layers within the Entia gneiss complex show geochemical trends (from Mg-rich to more Fe, Ca and Al-rich compositions) mainly indicative of a metacumulus origin. Petrogenetic modelling indicates that the Entia amphibolites may be analogues of the large amounts of (clinopyroxene-rich) abstracted cumulates required to generate the basaltic to ferrobasaltic spatially and temporally associated Oonagalabi metabasites. Associated relatively iron-enriched Entia amphibolites are less accumulative and overlap in composition with the least differentiated Oonagalabi metabasites. They show compositional affinities with typical continental tholeiites. Both the Oonagalabi ferrobasaltic and the (metacumulate) Entia amphibolites imply extensive low-pressure crystal fractionation of mantle-derived magma during long residence times in shallow-level, infrequently-replenished differentiating tholeiitic sills and/or magma chambers in an intracontinental rift environment. in contrast to the Oonagalabi metatholeiites, the mafic Entia rocks suffered variable metasomatic enrichment in LREE and mobile LIL-elements (K, Rb and Ba) during high-grade (chiefly amphibolite fades) metamorphism at high P^ However, constant ratios of immobile high-field strength (HFS-) 2 elements (e.g.,Zr/Nb) indicate melting of a relatively undepleted mantle source similar to the source for the Oonagalabi basalts. Geochemical data for metabasalts from other undifferentiated 177

e a s t e r n A r u n t a b a s e m e n t g n e i s s terranes confirm t h a t a common h o m o g e n e o u s mantle w i t h some chondritic characters y i e l d e d m a n y of the b a s e m e n t tholeiite s e q u e n c e s . V o l u m i n o u s metabasaltic amphibolites a n d r e l a t e d r o c k s comprise geochemically distinct lower a n d u p p e r HRMC suites w i t h i n the early Proterozoic supracrustal cover sequence. The lower m e t a b a s i t e s e x h i b i t LREE d e p l e t i o n , low R b , Ba a n d K contents a n d h i g h Zr/Nb ratios (Zr/Nb=65) a n d display tholeiitic differentiation f e a t u r e s . They show some similarities to a b y s s a l tholeiites (N-MORB) p r o d u c e d a t m o d e r n spreading r i d g e s . The u p p e r metab a s i t e s are e n r i c h e d in L R E E , R b , Ba a n d K a n d h a v e l o w e r Zr/Nb ratios (Zr/ Nb=36). These m e t a b a s a l t s p o s s e s s close g e o c h e m i c a l affinities w i t h typical continental tholeiites. G e o c h e m i c a l a n d field data indicate t h a t the HRMC m e t a b a s i t e s were formed in an intracratonic r i f t e n v i r o n m e n t . The trace e l e m e n t characteristics imply strongly d e p l e t e d a n d h e t e r o g e n e o u s source r e g i o n ( s ) . Y o u n g e r norite intrusives in the c o v e r sequence p r e serve in p a r t a chondritic incompatible e l e m e n t s i g n a t u r e . Variable, selective a n d progressive L I L - e l e m e n t re-enrichments of the p r e v i o u s l y d e p l e t e d HRMC m a n t l e source(s) necessitate a convergence of m a n t l e metasomatism a n d f u n d a m e n t a l tectonic e v e n t s t h a t initiated/maintained HRMC magmatism. The sequence of metasomatic a n d magma-generating e v e n t s p r o p o s e d to account for the t e m p o r a l g e o c h e m i c a l v a r i a t i o n of tholeiitic m a g m a s in the A r u n t a Block is compatible w i t h a tectonic m o d e l for the ensialic e v o l u t i o n of the Proterozoic mobile b e l t involving (1) early a d d i t i o n of a n essentially chondritic to e n r i c h e d basaltic component to p r i m i t i v e "relatively-mafic" c o n t i n e n t a l crust; (2) incompatible e l e m e n t depletion of u p p e r m a n t l e during e x t r a c t i o n of mafic c o n t i n e n t a l u n d e r p l a t e a n d / o r "cratonising" basement granitoid magmas; (3) shallow e m p l a c e m e n t of d e p l e t e d m a n t l e b e n e a t h a b r o a d zone of a rifting b a s i n a n d large-scale c r u s t a l contamination (+ m a n t l e metasomatism) of v o l u m i n o u s b a s a l t i c m a g m a s ; (4) spontaneous s u b c r u s t a l delamination along a t h e r m a l l y - w e a k e n e d crust-mantle boundary; a n d (5) subsequent orogenic d e f o r m a t i o n v i a crust-restacking a n d A subduction resembling m o d e r n Cordilleran-style c o l l i s i o n a l t e c t o n i c s .

178

8.7 PKTROGENESIS OF GYMPIE GROUP VOLCANICS AND THEIR RELATION TO EARLY PERMIAN VOLCANISM IN EASTERN AUSTRALIA AND NEW ZEALAND

Warwick Sivell^ and Bruce Waterhouse^ ^Department of Geology and Geophysics, University of Adelaide ^Department of Geology and Mineralogy, University of Queensland In the Gympie area, south-east Queensland, predominantly shallow marine sediments and volcanics of the Early Permian Gympie Group overlie a basement of folded and metamorphosed Devonian and Carboniferous strata (Yarraman. North and South d'Aguilar Blocks) including fine-grained, deep-water sediments and basalts, flysch and marine shelf sediments, as well as allied Permian greenschists (basic metavolcanics and some quartzofeldspathic and pelitic schists). Separated from these rocks by a discontinuous serpentinite belt, the Gympie Group represents a tectonically anomalous lithostratigraphic association not present elsewhere in eastern Australia. Mafic phenocryst-rich basaltic to basaltic andesitic tuff-breccias, agglomerates and subordinate lavas, together with sparsely quartz- and plagiophyric dacitic tuffs and glassy flows, comprise the essentially bimodal submarine volcanic sequence of the two lowermost formations of the Gympie Group, namely the Highbury Volcanics and the Rammutt Formation. Lithological associations in these formations indicate that the Gympie Group accumulated in an island arc environment. Geochemical data imply three distinct crystallisation intervals for the main volcanic series. These are (1) initial olivine (± Cr-spinel) + clinopyroxene removal causing rapid MgO, CaO, Ni and Cr depletion in the ascending mantle-derived magma; (2) extensive clinopyroxene-dominated olivine + clinopyroxene + plagioclase + magnetite fractionation giving rise to tholeiitic FeO, Ti02, AI2O3, Nb and P enrichment in the basalt-basaltic andesite range; and (3) plagioclase (dominant) + augite + hypersthene + magnetite + apatite crystallisation resulting in decrease of AI2O3, Ti02f FeOt/ V, P, Nb and Sr levels, enhanced compatibilities of Cr, Ni and Y, and increasing Si02 incompatibility in the majority of the erupted dacites. Identical degrees of rare earth element (REE) fractionation for the basalts and most dacites suggests their comagmatic origin and precludes significant amphibole or garnet involvement in the genesis of these rocks. Either amphibole fractionation or crustal contamination may have enhanced the abundance of light rare earth elements (LREE), Ba and Sr relative to other incompatible elements in a small proportion of the silicic magmas. Volcanological constraints probably prevented eruption of intermediate andesites. Low Mg-numbers, Ni, Cr, REE and high field strength cation(HFSC) abundances for the least-accumulative Gympie Group basalts, along with high Ba/La ratios, and low Zr/Y, Ti/Y and Nb/Zr, are features typical of island arc tholeiites. The Gympie suite shows close geochemical, mineralogical and lithostratigraphic affinities with some magmatic arc components of the Permian volcanic arc in the Rangitata Orogen of New Zealand. The metabasites are similar to basic pyroclastics and lavas of the Brook Street Volcanics, as well as primitive basalts from the basal Takitimu sequence, in South Island. Together with rocks from these New Zealand terranes, the Gympie suite is thought to comprise the remnants of a magmatic arc mobile belt which extended (prior to formation of the Tasman Sea) along the margin of Gondwana. The association represents mainly an immature submarine tholeiitic stage of arc development. 179

4.15 LITHOFACIES AND CYCLIC SEQUENCES IN AN ALKALINE LAKE SEQUENCE OF LOWER TO MIDDLE CAMBRIAN AGE FROM THE OFFICER BASIN P.N. Southgate^ and R. Henry^ ^Baas Becking Geoblological Laboratory, Canberra ^Comalco Exploration, Adelaide Seven fully cored drill holes have been used to delineate lithofacies and cyclic sequences in an alkaline playa sequence of Lower to Middle Cambrian age from the Observatory Hill Beds of the Officer Basin. By grouping together rocks of like lithology that contain similar styles of layering and sedimentary fabrics it has been possible to delineate 5 lithofacies. From lake centre to lake edge the lithofacies are: (1) Lake facies - black and fissile dolomite mudstones dotted with evaporite pseudomorphs. These sediments were deposited in small lakes or saline ponds. (2) Saline mudflat facies - laminated, dolomitic and silty mudstones extensively disrupted by evaporite pseudomorphs, many of which occur along former sheet cracks and desiccation cracks. Chert nodules, stromatolitic crusts and intraclast gravels are locally abundant. These sediments accumulated in brineclogged semi-emerged to emerged flats. (3) Dry mudflat facies irregular and discontinuously laminated silty dolomitic mudstones containing desiccation cracks and sheet cracks, stromatolite, sinter and dolomite crusts, intraclast gravels and teepees. Evaporite pseudomorphs are rare to absent. These sediments accumulated from sheet flood deposits on emerged low-relief flats. (4) Sand flat facies desiccated, cross bedded and laminated fine to coarse grained dolomitic quartz sands with interbedded dolomitic siltstone and mudstone. These sheet flood depos its mark the zone where large quantities of terrigenous silts and sands were deposited and therefore prevented from travelling any further into the lake basin. A fifth lithofacies, the mud pond facies is restricted to the upper parts of all seven cores. Dominated by terrigenous clays and quartz silt, rocks of this facies contain large dewatering structures that are filled with sand and mud-clasts. These sediments accumulated in ponds and small lakes and as such mark the termination of evaporative conditions. The predictable ordering of sedimentary structures and lithologies, both within and between each of the lithofacies, produces a series of sedimentary cycles. Large scale cycles of tens of metres thickness record the sequential dominance of one lithofacies over another. These cycles record the controlling influence of basin tectonics and climate on both the formation and termination of the lake. Superimposed on these long term variations are a series of smaller cycles of tens of centimetres thickness. Consisting of either the same or adjacent lithofacies these sequences record minor oscillations in the position of the strandl ine. These small scale cycles provide a detailed account of the interaction between sediments deposited during sheetfloods with those that accumulated in a gradually expanding or contracting lake.

180

6.12

STRUCTURE OF THE KERGUELEN PLATEAU

H.M.J. Stagg, D.C. Ramsay, C.J. Pigram, P.J. Hill, H.L. Davies, J.B. Colwell and M.F. Coffin Division of Marine Geosciences and Petroleum Geology, Bureau of Mineral Resources, Geology and Geophysics, Canberra

The Kerguelen Plateau in the south-central Indian Ocean about 4000 km southwest of Perth persists as one of the most enigmatic features of the seafloor, its crustal nature and evolution still a subject of speculation. The plateau lies on the Antarctic Plate,and stretches for approximately 2500 km as a basement depth anomaly in a northwest-southeast direction. The width of the feature ranges for 200 to 500 km, and it stands two to four km above the adjacent seafloor. Through analysis of 5600+ km of 48-channel seismic reflection data plus gravimetric, magnetic, and bathymetric data acquired over the plateau by the Bureau of Mineral Resources R/V Rig Seismic in early 1985 combined with the free air gravity field derived from SEASAT radar altimeter data and all previous marine geophysical data, we have redefined the structure of the plateau. The salient results of our geophysical investigations include a division of the plateau province on morphological grounds into three distinct sectors (northern southern, and eastern); the identification of a linear seamount chain between Kerguelen and Heard Islands; the discovery of a close analogue to the plateau's conjugate feature (Broken Ridge); the delineation of major sedimentary basins, both faulted and unfaulted; the discovery of a block of deep, presumably oceanic crust to the northeast of the southern plateau; and documentation of deformation characteristic of an extensional tectonic regime occurring through much of the plateau's history. Geological sampling undertaken by the R/V Rig Seismic over the central southern part of the plateau using free-fall grabs recovered a variety of rocks including volcanics, granitic rocks and quartzose metasediments. Unfortunately some question exists as to whether some of these rocks are ^ situ. Overall, we conclude that the plateau province amalgamation of tectonic elements. The northern sector, dominated by magmatism and small basins, is interpreted as a hotspot swell supplemented by hotspot volcanism. The southern sector, of more subdued relief, contains few igneous intrusions, is affected by significant faulting, is the site of a major sedimentary basin, (the Raggatt Basin) and probably includes continental rocks. The eastern sector, a system of fault-bounded ridges, basins, horsts, and grabens, exhibits a structural style similar to that of the main plateau, but probably shares a common genesis with its conjugate features. Broken Ridge and Diamantina Zone.

181

15.3

DISCOVERY OF SHATTER CONES AT THE LAWN HILL CIRCULAR STRUCTURE, NORTHWESTERN QUEENSLAND Alastair Stewart

Bureau of Mineral Resources, Geology and Geophysics, Canberra

The Lawn Hill circular structure is located 250 km north-northwest of Mount Isa, and comprises an annular outcrop of closely folded and brecciated Cambrian limestone and chert about 20 km across, and a core about 6 km across of poorly exposed fractured Proterozoic siltstone, rhyolite, and tuff of the Lawn Hill Formation. Reconnaissance geological mapping in 1976-77 by I.P.Sweet and L.J.Hutton was unable to show how the structure originated, but following a suggestion by J.Ferguson in 1980, G.M. Derrick, L.J.Hutton, and I.H.Wilson searched the structure later that year for evidence of meteoritic impact. Although they noted 'vaguely radiating striations on curved fractures', these were not identified as shatter cones. Derrick and others' investigation indicated that the central part of the structure had not been uplifted, and they concluded that this appeared to exclude the possibility of the structure originating by impact or explosion. Mechanisms for the brecciation of the Cambrian limestone, such as brecciation accompanying faulting or tight folding, solution collapse, and debris-flow of calcarenite on a fore-reef slope or on a collapsed platform margin following earthquake or gravitational collapse were suggested, but the origin of the structure remained unknown. On 17 July 1985, Mr Ken Mitchell, senior field assistant with the Bureau of Mineral Resources, visited the Lawn Hill circular structure, and recognised well formed shatter cones in the Proterozoic siltstone in the central core of the structure. In August 1985 I spent two days with Mr Mitchell examining the area. The shatter cones are present over a large area, and most occur as colluvial float material, although a few in situ cones were found. The cones are a few centimetres across, and display the characteristic sharp narrow striations radiating from the apex. Parasitic cones lie on the flanks of larger cones, and their striations form the horsetail pattern typical of nested shatter cones. The apices of the few In situ cones point away from the centre of the circular structure. Large ferruginous concretions in the Proterozoic siltstone exhibit cone-incone structure; the radiating striations on these cones are linked by small cross-striations, which are typical of cone-in-cone structure but do not occur on the shatter cones. The two types of cone are quite distinct from each other. In the Cambrian limestone annulus, limestone breccia commonly forms crosscutting dykes, sills, and lenses, as well as interlayers in the limestone beds. Near the inner margin of the annulus, Cambrian chert breccia forms scattered outcrops overlying the brecciated limestone, and consists of a mixture of angular clasts of silicified siltstone together with some irregularly concave clasts of the same rock type, suggestive of fall-back breccia containing softened 'bombs'. The combination of annular shape, shatter cones, large-scale brecciation, and the mobile nature of the limestone and chert breccias indicate that the Lawn Hill circular structure originated by hypervelocity energy release - a detonation - either by impact of an extraterrestrial body, or by expansion of an intraterrestrial magmatic fluid. The time of formation of the Lawn Hill cryptoexplosion structure can be fixed no closely than mid-Cambrian to late Tertiary. 182

5.5

MEGAKINKS AS POTENTIAL GOLD TARGETS Michael P. Stubley Macquarie University, Sydney

The Mid-Palaeozoic sedimentary rocks of the Lachlan Fold Belt, southeastern New South Wales, are multiply deformed and have a strong, meridional tectonic grain. Several distinct zones, up to 10 km wide or more, have been rotated away from the meridional trend. Due to the regionally abrupt nature of the rotations and the resulting kink-like geometry, these structures are termed 'megakinks'. The precise location and geometry of the inferred megakinks is, at present, unclear. Trend variations range from 20 to 60 degrees across megakink boundaries. A continent-wide, north-south compression has been proposed for their formation. Structural analysis of the southern margin of the sinistral Narooma Megakink Band, including the Mystery Bay Kink Zone, is reported. The monotonous Ordovician greywacke-pelite succession exposed south of Mystery Bay has fold vergence indicating a major, upright anticline to the south-west and is faulted against an older chert, slate and basic volcanic succession (Wagonga Formation). The major fault may be the seaward extension of the dextral, Mid-Devonian Tantawangalo Fault, displacing the Wagonga Formation southwestwards to its present position at Morunna Point. The Cretaceous Dromedary Complex crops out as numerous dykes within the study area, as Montague Island, and to the west where it delineates the poorly-defined megakink boundary. Neglecting the Mt Dromedary monzonite, the intrusions approximate the trace of the proposed Tantawangalo Fault extension. Upright, meridional, tight to isoclinal F^ folds dominate within the greywacke-pelite succession, except where locally refolded by coaxial F2 folds with eastward, moderately dipping axial planes. Locally, a near-vertical crenulation cleavage, S3, is developed at high angles to S-j* A 600 m-wide zone of conjugate kink bands (Mystery Bay Kink Zone) abuts the southern-most Wagonga Formation, with individual kinks being transitional to S3 in places. Orientations of the mesoscopic kink bands and S3 suggest a similar stress configuration and are believed to result from the same deformation, with S3 possibly slightly older. All structures have been subsequently rotated approximately 75® counterclockwise to the north about a near-vertical axis within a 900 m zone, including 55® within 350 m. Structures are noncontinuous through this zone, with discrete rotations greater than 15® marked by faults. This is consistent with observations of coastal outcrop within the Bermagui Megakink Band. From relationships elsewhere, the rotation is tentatively assigned to the middle Carboniferous. Sinistral kink bands are more numerous and generally larger than dextral ones. From conjugate relationships, the kink bands have a triclinic symmetry with respect to S-^, with a^ subhorizontal and slightly counterclockwise to the foliation and 02 parallel to the inferred megakink rotation axis. Conversely, the dextral Bermagui Megakink Band is dominated by dextral mesoscopic kinks and o^ is oriented slightly clockwise to the foliation. Estimates of shortening caused by mesoscopic kink folding range up to 8 percent. Outcrop-scale kinks are very variable in geometry and size, with widths from several millimetres to metres. Their vertical extent is unknown. The migration model for kink formation is rejected for the Mystery Bay kinks, as the kink planes do not generally bisect external and internal folial directions, although the process may contribute to their modification. The geometry is also inconsistent with the idealised 183

rotation model/ although a rotational component is clearly evident. A distinctive feature of many of the kink bands is the incorporation of smaller kinks or crenulations into the larger structures. Most intrakink folds appear synchronous with the larger folding, although some may be older. In detail, kink planes may exhibit an irregular and serrated appearance defined by slightly oblique, intrakink fold terminations. Unlike the situation in idealised kink folding, slip along the foliation is not entirely confined between the kink boundaries. This results in variable widths, rotation angles, and the formation of prismatic voids parallel to the kink fold axis. Similarly oriented prismatic voids also develop in response to fracturing accompanying rotation. Voids formed from both processes are usually barren, although quartz filling is evident in some instances. Early stages of rotation necessitate the separation of intrakink folia, commonly preserved as quartz veinlets. At least three orders of magnitude of kink folding are evident in the Mystery Bay Kink Zone. Possibly this zone is parasitic upon the Narooma Megakink Band. Inferred principal stress directions and age constraints are consistent for all orders, with megakinking as the final stage of a progressive deformation. If this assumption is valid, microstructures associated with outcrop-scale kinking may be mirrored at a larger scale. Large prismatic voids may provide vertical conduits for hydrothermal fluids and sites for deposition of gold. Concentration of faults along megakink boundaries would also be conducive to gold deposition. A correlation between LANDSAT lineaments and megakink boundaries has been noted previously. Comparison of gold occurrences in the Ordovician sedimentary rocks and inferred megakink boundaries reveals a moderate correlation, most prominent between the latitudes of Murrah and Narooma, and along the northern boundary of the Bateman's Bay Megakink Band. Future recognition of smaller mesokink bands may increase the correlation. Recognition of faults is often difficult in areas of sparse outcrop or monotonous sequences. However, the position of a megakink boundary is easily detected during field mapping and may delineate potential gold deposits. Other precious and base metal deposits may exhibit a similar pattern. Potential targets should concentrate along megakink boundaries, although complimentary structures may also be expected anywhere within the band.

184

2.8

RECOGNITION OF TIDAL ENVIRONMENTS IN THE ABNER AND BESSIE CREEK SANDSTONES» McARTHDR BASIN, NORTBDERN TERRITORY I.P. Sweet

Division of Continental Geology, Bureau of Mineral Resources, Canberra

The Abner and B e s s i e Creek S a n d s t o n e s l i e w i t h i n t h e l o w e r p a r t of t h e Roper Group, a p r e d o m i n a n t l y s i l i c i c l a s t i c s e d i m e n t a r y s u c c e s s i o n up t o 5 km t h i c k w h i c h c o n s t i t u t e s t h e y o u n g e s t c o m p o n e n t of t h e m i d d l e P r o t e r o z o i c M c A r t h u r B a s i n . BMR r e s e a r c h on t h e Roper Group h a s b e e n d i r e c t e d a t e s t a b l i s h i n g i t s petroleum potential. Organic-rich shales are present at s e v e r a l s t r a t i g r a p h i c l e v e l s i n t h e b a s i n , and i n 1985 l i v e o i l was r e c o v e r e d f r o m t h e V e l k e r r i F o r m a t i o n i n t h e u p p e r Roper G r o u p . B e c a u s e of t h e i r f r i a b i l i t y and h i g h p o r o s i t y i n many o u t c r o p s , t h e Abner and B e s s i e Creek S a n d s t o n e s a r e b e i n g a s s e s s e d as p o t e n t i a l h y d r o c a r b o n r e s e r v o i r s . The A b n e r S a n d s t o n e t h i c k e n s f r o m 35 m i n t h e e a s t ( n e a r B o r r o l o o l a ) t o a r o u n d 700 m i n t h e s o u t h w e s t , a n d r a n g e s f r o m 200 t o 4-00 m t h r o u g h m o s t of t h e n o r t h w e s t e r n p a r t of t h e b a s i n . F i v e d i s t i n c t f a c i e s a s s e m b l a g e s h a v e been r e c o g n i s e d ; f a c i e s a s s e m b l a g e 3 d o m i n a t e s t h e Abner S a n d s t o n e , and i s d e s c r i b e d i n d e t a i l . The o t h e r f o u r a s s e m b l a g e s c o n t a i n a v a r i e t y of s t r u c t u r e s i n d i c a t i n g v e r y s h a l l o w w a t e r t o e m e r g e n t c o n d i t i o n s and a r e n o t d e s c r i b e d f u r t h e r . A s s e m b l a g e 3 i s r e m a r k a b l y u n i f o r m i n most o u t c r o p s . I t i s f i n e to mediumg r a i n e d w i t h some c o a r s e i n t e r v a l s , and c r o s s - b e d d e d on a 0.1 0.3 m s c a l e , o c c a s i o n a l l y to 0.5 m and r a r e l y more. Trough crOSs-bedding predominates, but planar t a b u l a r sets are present. Although well bedded, bedding plane partings are p o o r l y d e v e l o p e d , and a r e c o m m o n l y m a r k e d by f a i n t , p o o r l y p r e s e r v e d wave r i p p l e s . C l a y e y p a r t i n g s and m u d f l a k e s a r e r a r e . Pa1 a e o c u r r e n t s a r e m o s t l y to t h e n o r t h - w e s t , b u t i n v i r t u a l l y a l l o u t c r o p s e v i d e n c e f o r minor t r a n s p o r t i n a l m o s t the opposite d i r e c t i o n is preserved. F a c i e s a s s e m b l a g e 3 of t h e A b n e r S a n d s t o n e t y p i f i e s t h e p r o b l e m s of e n v i r o n m e n t a l a n a l y s i s of P r o t e r o z o i c q u a r t z a r e n i t e s - a r e t h e r o c k s m a r i n e or f l u v i a l ? The a l m o s t u n i m o d a l p a l a e o c u r r e n t s , and t h e u n i f o r m c r o s s - b e d d i n g c o u l d i n d i c a t e a b r a i d e d a l l u v i a l system. However, t h e r e are t h r e e main r e a s o n s why I b e l i e v e t h a t t h e a s s e m b l a g e was d e p o s i t e d i n a m a r i n e , p r o b a b l y t i d e d o m i n a t e d , e n v i r o n m e n t . (1) The o c c u r r e n c e of w a v e r i p p l e m a r k s . T h e s e a r e c o n s i s t e n t w i t h d e p o s i t i o n i n a m a r i n e o r l a c u s t r i n e e n v i r o n m e n t . (2) C r o s s b e d s i n d i r e c t i o n s o t h e r t h a n t o w a r d s t h e main mode o c c u r i n intimate association with a l l other cross beds, suggesting r e g u l a r r e v e r s a l s of c u r r e n t s . A t i d a l r e g i m e i s t h e m o s t l o g i c a l means of a c h i e v i n g t h i s . (3) The p l a n a r n a t u r e of most b e d d i n g s u r f a c e s ; i e t h e l a c k of c h a n n e l i n g , s c o u r s , e r o s i o n a l b a s e s and l a g d e p o s i t s . The f i r s t two c h a r a c t e r i s t i c s a r e taken to ind i c a t e a t i d e dominated marine environment, while t h e t h i r d i s i n t e r p r e t e d a s an i n d i c a t o r of a s u b t i d a l ( t i d a l s h e l f ) e n v i r o n m e n t . W e l l d o c u m e n t e d modern s h e l v e s such as t h e N o r t h Sea c o n t a i n e x t e n s i v e a r e a s of r e l a t i v e l y l a r g e b e d f o r m s 185

s u c h 8LS s a n d w a v e s j b u t "the s c a l e oT c r o s s —bedding i n "the Abner S a n d s t o n e s u g g e s t s m a i n l y m e g a r i p p l e s or s m a l l s a n d w a v e s . The b e s t a n a l o g y a p p e a r s t o b e t h a t of s a n d r i b b o n s , w h i c h o c c u r i n a r e a s of l o w e r c u r r e n t v e l o c i t i e s t h a n t h e s a n d w a v e s . F a d e s a s s e m b l a g e 3 i s t h e r e f o r e i n t e r p r e t e d as r e p r e s e n t i n g an open s h e l f e n v i r o n m e n t , p r o b a b l y of r a t h e r u n i f o r m w a t e r d e p t h and t i d a l c u r r e n t v e l o c i t y , on w h i c h v a s t f i e l d s of m e g a r i p p l e s d e v e l o p e d . The t i m e - v e l o c i t y p a t t e r n of t i d a l c u r r e n t s was probably h i g h l y asymmetrical, r e s u l t i n g in n e a r l y unimodal palaeocurrents. The B e s s i e Creek S a n d s t o n e i s s i m i l a r t o F a c i e s a s s e m b l a g e 3 of t h e A b n e r S a n d s t o n e e x c e p t t h a t muddy p a r t i n g s and m u d f l a k e s a r e m o r e common. I t a l s o i s i n t e r p r e t e d as a s u b t i d a l , s h e l f d e p o s i t . F a c i e s assemblage 3 appears to r e s u l t from a minor t r a n s g r e s s i v e - r e g r e s s i v e p h a s e d u r i n g Abner t i m e , w h e r e a s t h e B e s s i e Creek S a n d s t o n e i s p a r t of a s i n g l e r e g r e s s i v e c y c l e .

186

11.2

MINERALOGICAL FORMS OF SILICA IN SILCRETES Medard Thiry

Ecole NationaXe Superieure des Mines de Paris, France

The paragenesis and the sequences of deposition of silica have been studied in 3 examples of silicification from the zone of weathering and pedogenesis in the Paris Basin and the Central Massif of France. These siliceous accumulations show many variations in facies or habits of silica which relate to each other in a systematic and ordered fashion as a result of successive recrystallization. The final stage is the most stable form of silica, quartz. 1.

Mineral sequences

The initial silica facies are most frequently opal with formed either by dehydration of gel deposits (mostly in nodular forms) or direct precipitation of opal (notably as limpid zoned opal). The opal varies in structural state from amorphous materials to partially-ordered opal-CT, in which there is a disordered array of alternating cristobalite and tridymite rings. From this material, recrystallization leads to better and better structurally organised quartz: fibrous quartz develops from limpid zoned opal, and microcrystalline quartz from granular opal without clear organisation. The fibrous varieties appear before the microcrystalline quartz, the partly disordered helicoidal forms before the three-dimensionally ordered forms. However, after the formation of microcrystalline quartz facies, some dissolution and recrystallization phases may occur. Under these conditions, the most microcrystalline facies are dissolved while the bigger crystals are preserved and often overgrown. Ultimately the large euhedral quartz crystals commonly present in voids are formed. 2.

Structural order and solubility

The mineral sequences described above are regulated by the relative solubility of each of the silica facies, and this solubility is itself determined both by the chemical purity and the structural state of each silica form. The initial opaline silica facies records conditions when the activity of Si in solution is much higher than appropritate for the kinetics of quartz crystallisation. Under such conditions there is a proliferation of nucleii with admixed foreign ions, and the silica precipitated is amorphous. Percolation of solutions through the system effectively removes foreign ions by leaching and results in deposition of new silica which is more structurally organised into opal-CT. Subsequently such conditions may lead to the formation of microcrystalline fibrous chalcedony in which the fibres are preferentially aligned in one direction, or subgrain quartz with mosaic structure. Ultimately phases of dissolution and leaching of foreign ions will allow the growth of microcrystalline quartz, the overgrowth of grains, and finally the crystallisation of the large quartz crystals in the voids. Each of these new silica facies is the result of dissolution of some of the first silica deposits followed by reprecipitation under a new set of environmental conditions.

187

Circulation of solutions and recrystallisation stages

All stages of dissolution and recrystallisation are dependent on percolation of solutions fed by meteoric waters. Initially, dilute solutions leach foreign ions, purify the millieu, and permit structural reorganization to more crystalline phases. At the same time the solutions become more concentrated in Si by means of preferential dissolution of disordered phases to provide solutions from which more ordered and less soluble phases will crystallise. And so on. There is also an effect according to the rate of percolation of solutions through the silica facies which have variable porosity. There may be some zones with rapid circulation in which dissolution reactions are dominant, some zones in which progressive recrystallisation of quartz is favoured, and other zones with slow circulation in which the solutions attain saturation in Si and no transformations occur. Dynamics of surficial silica accumulations in silcretes The relative interaction of water percolation, dissolution and recrystallisation explains the specific orientation patterns with respect to the distribution of silica facies observed within a number of surficial silica accumulations or silcretes. The vertical pattern in mature profiles consists of dissolution features and quartz overgrowths at the top of the profile and deposits of opaline silica at the base. A lateral distribution is also evident with dissolution and recrystallisation features upstream, but deposits of opal in nodules and laminae downstream in a more confined environment in which there is also carbonate and sulfate. The interference between meteoric percolations through silica-rich source material with variable porosity controls the size of the silica accumulations which make up siliceous duricrusts or silcretes.

188

1.14

THE DEPOSITIONAL HISTORY OF THE 2.0 Gk WYLOO GROUP, SOUTHERN PILBARA, WESTERN AUSTRALIA Alan

Thome

Geological Survey of Western Australia The 12-14km thick Wyloo Group comprises a mixed suite of sedimentary rocks with minor felsic and mafic volcanics. These rock types were deposited in an arcuate basin developed along the southwestern margin of the Pilbara Craton. The depositional history of the Wyloo Group records a change from terrestrial and shallow marine to 'deep-water' sedimentation. During this evolution, the distribution of sedimentary facies was strongly influenced by local and regional tectonic activity along the southern Pilbara margin. During early Wyloo Group deposition, the influence of tectonism was greatest in the southwest where local uplift gave rise to a series of fandeltas around the eastern closure of the Wyloo Dome. Elsewhere along the southern and western Pilbara margin this period was characterized by tectonic stability and tidally-influenced shoreline sedimentation. By the time the middle Wyloo Group was laid down tectonism along the entire southern edge of the Pilbara gave rise to widespread fan-delta growth. Late Wyloo Group deposition was contemporaneous with a period of craton margin rifting and subsidence, the effects of which were three fold: 1)

The reduction deposition.

terrigenous

supply

led

widespread

dolomite

Localised extrusion of basaltic magma occured the Wyloo Dome.

Downwarp and faulting led to the formation of a deep craton margin basin - the Ashburton Trough.

in the area north of

The infill of the Ashburton Trough was the result of a regional compression which caused uplift and erosion of the Sylvania Dome and led to the development of submarine-fan systems in the area further west. In the early stages of uplift, sand and mud derived from the Sylvania Dome was shed longitudinally into the southeast half of the Ashburton Trough and distributed via a system of braided channels and depositional lobes. A subsequent decrease in the sand supply led to the development of a muddy fan system in which coarse-grained sediment was transported in a few major channels which were separated by extensive areas of mudstone deposition. Further west along the Pilbara margin a second submarine-fan system was developed in the vicinity of the Wyloo Dome. Here, sediment derived from the western Hamersley Basin was distributed by a feeder channel system into the area northwest of the Wyioo Dome. A third submarine-fan complex, built up of sediment derived from an area which lay to the south of the Ashburton Trough, was developed during the final phase of Wyloo Group deposition. This fan system was located to the southeast of the Wyloo Dome. The history of Wyloo Group sedimentation is interpreted as a response to crustal extenstion followed by continental crustal collision along the southern Pilbara margin. 189

10.8

KINERAL SANDS IN AUSTRALIA - A REVIEW R. Towner

Bureau of Mineral Resources, Geology and Geophysics, Canberra Australia is the world's leading producer and exporter of rutile, ilmenite, zircon and monazite. In 1984, Australia produced 47% of the world's output of rutile concentrates, 30% of ilmenite concentrates, 59% of zircon concentrates, and about 60% of the world's monazite. Commercial production of mineral sands concentrates in Australia began on the east coast in 1934, on the southwest coast of Western Australia in 1956, and in the Eneabba area, north of Perth in 1974. To the end of 1983, domestic production of saleable-grade concentrates totalled 6.99 Mt of rutile, 8.78 Mt of zircon, 14.44 Mt of ilmenite, and 159 000 t of monazite. The bulk of production is exported and only a small proportion is consumed in Australia; the total value of mineral sands exports to date, in current dollars, is about $1900 million. Australian mineral sands placer deposits are of two broad types, namely 'strand' (or beach) deposits concentrated by sea action, and 'dune' deposits formed by wind action. In some areas both dune and strand deposits are superimposed. Concentrations of heavy minerals occur as seams, either on present-day beaches formed by storm waves, or in ancient strand-lines located some distance inland. Individual seams can sometimes be traced for several kilometres; seam thicknesses range from a few centimetres to several metres. Lower-grade accumulations of heavy minerals generally occur in aeolian dunes particularly in southern Queensland. Mineral sands operations are presently restricted to three regions, namely southern Queensland-northern New South Wales, the Bunbury-Capel area, WA, and the Eneabba region, WA. Heavy mineral (HM) suites vary from region to region. In the Bunbury-Capel area the HM suite generally comprises 75-90% ilmenite, 5-10% zircon and less than 2% rutile. At Eneabba characteristic HM suite proportions are 50-60% ilmenite, up to 20% zircon, and up to 12% rutile. East coast suites generally comprise up to 35% ilmenite, 15-40% zircon, and 20-40% rutile. East coast ilmenite has a relatively high chromium content which renders it unsuitable for pigment manufacture. Over time, mining grades have declined markedly. During the early mining days, mining grades averaged about 10% rutile and zircon combined (some seams contained up to 50% rutile plus zircon, in situ). Presently, grades as low as 0.2% combined rutile and zircon are being worked. Notwithstanding, the viability of operations has been sustained by ongoing development of improved and more cost effective mining methods and higher mineral prices. Hand recovery methods of the early 1940s have given way to highly mechanised bulk-mining operations. The various mining techniques presently used include dredging, hydraulicing, front-end loader/feeder operations, scrapers, and bucket-wheel excavators. These advances increased production capacities from 30 tph in the 1950s to 2800 tph for presently-operating dredges bulkmining high level dune deposits. The mineral sand ores are generally separated into concentrates and tailings at the mine site by a wet concentration process. Wet concentrators use gravity techniques to exploit the difference in specific gravities between the heavy minerals fraction (s.g. more than 4.2) and the gangue minerals fraction (s.g. 2.7-3.7). The types of equipment used in this process include the Reichert cone (unit capacities in the range 65-100 t/h),spirals, sluices

190

and t r a y s . O v e r t i m e , Reichert cones have become the basic primary means of separation. A f t e r c o n c e n t r a t i o n , the i n d i v i d u a l minerals in a HM concentrate are separated by methods based on differences in their magnetic and e l e c t r i c a l properties. Magnetic techniques separate ilmenite (magnetic) from rutile (non-magnetic), and electrostatic methods separate rutile (good c o n d u c t o r ) from zircon and monazite (poor c o n d u c t o r s ) . BMR assessed Australia's economic demonstrated resources of m i n e r a l sands in 1984 as: ilmenite - 39.81 M t , rutile - 7.9 M t , zircon - 11.48 M t , monazite 0.217 M t , a l l of w h i c h are located in the three mining r e g i o n s . H o w e v e r , about 40-45% of rutile and zircon resources on the east coast are unavailable fpr mining because of e n v i r o n m e n t a l and alternate land use considerations. The n u m b e r of operating establishments has declined over the last 15 years from 27 (employing about 3000 p e o p l e ) in 1970-71, to 16(employing just under 1700) in 1983-84; the 16 operations are owned by 7 c o m p a n i e s . Approximately 90% of w o r l d production of rutile and ilmenite is used to manufacture titanium dioxide (TiO ) pigment for use in p a i n t , p a p e r and p l a s t i c s . The remaining 10% is used in the manufacture of welding rod coatings and f l u x e s , as w e l l as for manufacturing titanium m e t a l . Some 40% of world production of zircon is consumed in the foundry industry, 35% in refractory b r i c k s , and 20% in c e r a m i c s . A s m a l l amount of zircon is used to produce zirconium m e t a l . Monazite is used to produce rare earth oxides which are used in polishing p o w d e r s , c a t a l y s t s , m i s c h m e t a l , and a s individual oxides/metals. O v e r recent y e a r s , world demand for TiO^ has increased to about 2-5 M t / y e a r , for zircon to about 650 000 t / y e a r , and for monazite to about 30 000 t/lyear. Projections of demand for TiO^ and zircon vary w i d e l y , but a n n u a l increases of about 2% for both a p p e a r to be r e a l i s t i c . C o n s e q u e n t l y , Australian producers should have steadily growing markets for their raw m a t e r i a l products notwithstanding that competition from South Africa (titanium s l a g , rutile and zircon) and Sierra Leone (rutile) is likely to increase. The level of domestic processing of m i n e r a l sands w i l l increase substantially over the next few y e a r s , especially for ilmenite and zircon. Australian capacity to produce beneficiated ilmenite (also called synthetic rutile) is presently 60 000 t/year of product but by 1987 this w i l l have increased to about 270 000 t / y e a r . The manufacture of zirconia f o r use in new ceramics such as partially stabilised zirconia w i l l be the m a j o r direction of further processing of zircon in A u s t r a l i a . The production of zirconium sponge and tubing for nuclear hardware is unlikely to be economic before the year 2 0 0 0 , if at a l l , because of an o v e r - s u p p l y . Plans for the processing of monazite into high-purity rare earth oxides are currently at feasibility s t a g e . Considering likely developments for the industry, and production levels, Australia's economic demonstrated resources of m i n e r a l sands w i l l be substantially depleted by the turn of the c e n t u r y . Indications of new discoveries are not too e n c o u r a g i n g . In the medium term deposits at Jurien Bay are expected to be mined again in 1986-87. Other areas w i t h some p o t e n t i a l include King S o u n d , W A , the Murray B a s i n , V i c , as w e l l as various s m a l l u n m i n e d , known areas in southwest W e s t e r n A u s t r a l i a . In the long t e r m , interest is likely to shift to hard rock titaniferous magnetite deposits in Western Australia and South A u s t r a l i a .

191

10.11

OLIGO-^IIOCENE CLIMATES IN SOUTHEASTERN AUSTRALIA: INTIMATIONS FROM MURRAY BASIN PALYNOLOGY Elizabeth M. Truswell

Bureau of Mineral Resources, Geology and Geophysics Canberra Palynological analyses of the marine Geera Clay and Renmark Group in the western Murray Basin have yielded diverse assemblages of pollen and spores. These provide the basis for speculative interpretation of climatic change in southeastern Australia during the Late Oligocene and Early Miocene. The Oakvale - 1 bore has served as a palynological reference section for the region, but the events first recognized in that borehole

have now been

idciiL i f i cd in oLhcr cored

sequences in the

area. In the Oakvale section, a quantitative zonation based on the frequency of major taxa has been statistically calculated, and has allowed a division into two major zones to be recognized. The younger of the two zones has been further divided into four subzones. The basal Zone II, of mid to Late Oligocene age, and incorporating the Olney Formation and the lower Geera Clay, is characterised by high Myrtaceae and high Nothofagus brassi type pollen values. However, ^ brassi at the site has lower values than it does in coeval sites in southeastern Australia. These assemblages are considered to reflect evergreen rainforest with abundant myrtaceous trees associated with the parent trees of N. brassi. A climatic regime of high, year-round precipitation may have supported this forest type. In the Late Oligocene, the vegetation changed. ^ brassi became reduced in importance, and Araucaria became more prominent. This may reflect a drier type of rainforest growing under a slightly seasonal moisture regime. Today, Araucaria in Australia is most extensively developed in southeast Queensland, under rainfalls of 800-1400 mm per year. The moisture regime under which it flourishes today is a mildly seasonal one, with drier months following the summer maximum. The climatic boundary identified in the western Murray Basin thus appears to mark a reduction in precipitation achieved by a transition from a uniform to a slightly seasonal rainfall pattern. The precipitation decline seems to have migrated coastwards with time. In the eastern Murray Basin the reduction in precipitation occurred later, in the Early Miocene. In the Gippsland Basin there is no sustained drop in precipitation - as judged by Nothofagus brassi values - until the latest Miocene. The migration of this precipitation boundary can be reconciled with mechanisms for climatic change involving northward motion of the zone of sub-tropical high pressure.

192

5.6

MAGNETIC AND TfiERMAL LINEARS: SOME ECONOMIC IMPLICATIONS FOR GOLD MINERALIZATION IN THE YILGARN BLOCK, WESTERN AUSTRALIA David H. Tucker^ and Peter Wilson^

bureau of Mineral Resources, Geology and Geophysics, Canberra ^CSIRO, Division of Minerals and Geochemistry, Perth A spatial association between some of the known gold and base metal sulphide deposits and dominant dykes within the eastern goldfields of the Archaean Yilgarn Block has been identified. Many of the dominant dykes have a high length/width ratio which has enabled them to be traced with airborne magnetic and remote sensing thermal responses for lengths of 100-500 kilometres or more. Remotely sensed thermal data has been received from the NOAA-AVHRR satellite series which are near polar-orbiting spacecraft at an altitude between 87O and 930 km with an orbital period of 102 minutes, resulting in 14.1 orbits/day. The two satellites operational at present, NOAA-7 and NOAA-9 have approximate equator crossing times of 0330 hrs and I53O hrs. The AVHRR is a five-channel multispectral scanner with an instantaneous field of view of I.3 milliradians resulting in a ground resolution of 1.1 km at nadir and 5-5 km near the scan limit. The swath width of the scanner is approximately 2900 km wide divided into 2048 pixels per scan line. The wide scan provides contiguous global coverage at the equator on a daily basis, whilst at higher altitudes, for example Australia, considerable swath-edge overlap occurs. The AVHRR has five spectral channels: the visible (0.55-0.68), the near infra-red (0.725-1.10), the mid infra-red (3*55-3-93) and the two thermal infra-red windows (10.3-11.3 and 11.5-12.5). Three day and night images, summer, spring and winter were compared to assess the effect of annual variations in insolation on the detectability and thermal detail of surface and near-surface features, particularly the extensive linear zones. Bureau of Mineral Resources (BMR) airborne magnetic data from low level survey and regional interpretative data has been processed to produce high quality large area pixel image maps, where the total magnetic intensity data were processed to an approximate 450 m cell size. Both the magnetic and thermal data grids have been reconfigured to a Lambert Conformal projection for suitable comparison to maps of 1:1,000,000 sheet area. This study has highlighted differences between geologically mapped dyke swarms and those detected by airborne magnetic and thermal response data. In particular, they indicate greater strike length for individual dykes, bifurcation and multiple branching of near.surface linear features. A combination of geophysics and geologic field work indicates that some dykes can have a wide (up to 1500 m) enclosing and accompanying fracture system with three primary strike directions - 150°, 230°, 270°. The intrusive dyke and accompanying parallel fractures do not offset the Archaean greenstone terrain. The dyke contacts are sharp with a fine grained, chilled marginal phase. Contact metamorphic effects near the margins are minor for narrow dykes due to the rapid cooling effects. Larger dykes have associated thermal aureoles in which fusion of country rock (pyrometamorphism) has been observed. More extensive thermal aureoles are associated with xenolith-rich dykes which are considered to be zones developed near the tops of intrusions which have reached the present land surface. 193

The fracturing associated with the dykes is believed zo be produced at the time of rapid emplacement of dyke material in an extensive tensional regime. The two remaining conjugate fracture systems appear to localize mineralization when the dyke is in contact with Archaean mafic volcanics and associated sediments. A mineralizing process can be proposed whereby fluids circulate in extensive open fractures driven by the heat capacity of the intrusion and mineralization derived from hydrothermal solutions and country rock would be reconcentrated into favourable host material.

194

3.3

THE METAMORPHIC AND TECTONIC DEVELOPMENT OF THE SOUTHEASTERN MARGIN OF THE PILBARA CRATON» WESTERN AUSTRALIA: EVIDENCE FROM THE SYLVANIA INLIER Ian M. Tyler Geological Survey of Western Australia

The Sylvania Inlier comprising pre-2800 Ma granite-greenstone terrain in the southern margin of the Pilbara Craton occurs to the southeast of the 2800 Ma to 2200 Ma Hamersley Basin. The greenstones together with pos.t-tectonic granites and numerous NNE trending mafic dykes which cut them have undergone static metamorphism transitional from greenschist to epidote-amphibolite facies, which increases southwards across the inlier. The dykes probably represent a feeder system to mafic volcanics of the overlying Fortescue Group. Metamorphic mineral assemblages in the Fortescue and Hamersley Group rocks adjacent to the inlier indicate pumpellyite-actinolite and lowermost greenschist facJes conditions. These are attributed to burial metamorphism and are part of a zonal pattern related to a southwards increase in stratigraphic thickness and metamorphic grade of the Fortescue and Hamersley Groups. They are regarded as synchronous with assemblages observed in the dykes and granite-greenstone terrain of the inlier. The Fortescue and Hamersley Groups to the north of the Inlier show an early episode of thrusting followed by a deformation which produced a fold belt comprising three structural zones. From south to north these zones comprise: a)

zone

large-scale

tight

folds,

overturned

the

north;

b) A zone of reverse faulting; c) A zone of open, upright folding. Contacts between the fold belt and the inlier are faulted, however basement may be exposed as anticlinal cores north of these faults. The folding pattern can be interpreted in terms of large scale thrusting with the inlier representing a basement massif emplaced into its cover of Fortescue and Hamersley groups. This is consistent with the observed contrast in metamorphic grade. A model for the development of the southeastern margin of the Pilbara Craton involves the establishment of the Hamersley Basin during Fortescue Group times in a tensional environment, subsidence being controlled by listric norma] faulting in a marginal rift system. Maximum development of the basin corresponds to peak burial metamorphism with epidoteamphibol ite facies assemblages developed in the basement. Subsequent compression, probably at the end of Wyloo Group times, has exploited the already established east-west normal faults as reverse faults and thrusts, jutaposing medium-grade basement against very low - to low-grade cover. Uplift, with a vertical displacement of between 4 and 6 km based on the metamorphic grade contrast, took place on several fault zones within the inlier as well as the main basement/cover faults.

195

10.13

URANIUM SERIES DATING OF INSULAR PHOSPHORITES H.H. Veeh

School of Earth Sciences, Flinders University, Adelaide It is now generally accepted that insular phosphorites are the result of chemical reaction between derivatives of seabird guano and the substrate rock, usually carbonate, but details of this process and the specific environmental conditions which promote it are not well defined. Since the net accumulation of m o d e m guano deposits appears to be largely restricted to islands located in areas of intensified organic productivity, frequently associated with upwelling, and reduced rainfall, the distribution pattern of ancient insular phosphorite deposits may be used to advantage in the reconstruction of paleoceanography (upwelling) and paleoclimate (aridity) over the ocean, provided the time of their formation is known. Although many of the smaller deposits on low atolls and coral cays have evidently been formed within the last 6000 years, the age of the major deposits on high islands is presently known only in the most general terms, and absolute dating would be desirable. The relatively high concentration (up to 90 ppm) of uranium in island phosphate rocks, where it is closely associated with the mineral apatite, have invited the application of uranium series disequilibrium dating methods. The success of this effort critically depends on the validity of the following assumptions. (1) Only uranium, but none of its decay products ^^^Th and ^^^Pa enter the apatite component of the phosphorite at the time of its formation. (2) The apatite remains a closed system with respect to uranium and its decay product. (3) The age of the apatite is closely related to the deposition of the phosphate by the action of seabirds. The data obtained so far for phosphorites from different locations and ages tend to support at least the first two of these assumptions. Uranium-series ages of phosphorites on Holocene coral cays and Pleistocene terraces are consistent with independently established ages of the associated substrate rocks and with the notion that ^^^Th and ^^^VSL contained in the apatite arose there by radioactive decay of an initially pure uranium parent fraction. The uranium-series isotopes in phosphorites from high islands, including Nauru, Ocean Island, Makatea, Bellona and Christmas Island (Indian Ocean), with very few notable exceptions, are in radioactive equilibrium, indicating a minimum age of 200,000 years for these deposits. An important consideration in the present context is the depositional environment of the apatite, because it reflects the initial conditions of uranium-decay series systematics. The source and pathway of uranium, in particular, is critical for a correct interpretation of the uranium isotopic data. If it is assumed that the uranium of primary phosphorites on high islands was derived from seawater = 1.14) at the time of their formation near sealevel, i.e. prior to uplift, the emplacement of phosphate on these islands must have begun more than 1 million years ago to be consistent with the ^^Û/^^Û data (23Û/238U = 1^00). The antiquity of major phosphorite deposits on elevated atolls suggested by the ^^Û/^^Û data can be tested if the rate of emergence is known. Using Christmas Island as an example, the extrapolated mean uplift rate of 0.14 mm/year, based on the 120,000 year age of the 20 m reef terrace, would place the time of emergence of the central plateau, where most of the phosphate ore is located, at approx. 1.5 million years B.P.. This is not inconsistent with the uranium-series age limits placed on these deposits. 196

4.1 CONCEPTS OF SEISMIC STRATIGRAPHIC ANALYSIS APPLIED TO LATE PROTEROZOIC WILPENA ®OUP, ADELAIDE (XOSYNCLINE, SOUTH AUSTRALIA C.C. von der Borch^, N. Christie-Blick^ and A.E. Grady^ ^School of Earth Sciences, Flinders University, Adelaide ^Lamont-Doherty Geological Observatory, Palisades, New York, U.S.A. Seismic stratigraphic analysis of Phanerozoic strata is a proven tool, both for subsurface chronostratigraphic correlation and facies prediction. The technique takes advantage of the often dense coverage of processed multichannel seismic profiles that exists in regions prospective for hydrocarbons. These profiles provide three-dimensional geometric overviews of large-scale heirarchical transgressive-regressive units, defined as depositional sequences. These sequences are separated from each other by usually strong seismic reflectors termed sequence boundaries, identified as unconformities or their correlative conformities. Sequence boundaries can usually be identified and correlated over widespread regions, have been tied-in to land-based stratigraphic sections and drill core and are of chronostratigraphic significance. They are often represented by distinct, sharply-defined shifts in vertical facies associations, assumed to be caused by major downward migrations of coastal onlap. These migrations in turn may be due to eustatic sealevel cycles or widespread plate-margin tectonism. This paper explores the application of some of the stratigraphic, chronostratigraphic and geometric concepts of seismic stratigraphic analysis, to detailed measured sections through outcropping late Proterozoic strata of the Adelaide Geosyncline in South Australia. Although actual seismic profiles are non-existent, it has been possible to describe four depositional sequences in 'Vertical stratigraphic sections through the several-kilometre-thick Wilpena Group. Sharp vertical facies-shifts have been described and proposed as actual sequence boundaries, with which future studies will seek chronostratigraphic correlation in regions of outcropping "Adelaidean" strata remote from The Adelaide Geosyncline. The four Wilpena Group Sequences are composed of the Nuccaleena Dolomite, Brachina Formation and ABC Range Quartzite (Sequence 1); the Bunyeroo Formation (Sequence 2); the Wonoka Formation and Bonney Sandstone/Billy Springs Beds (Sequence 3); and the Rawnsley Quartzite (Sequence 4). Major submarine canyon erosion has been identified to have occurred in response to a lowering of coastal onlap at the sequence boundary separating either Sequence 1 and 2 or 2 and 3. A major rise in coastal onlap has been identified within Sequence 3 (thick coastal facies sandstones and mudstones of the Bonney Sandstone). Finally, a distinct upsection shift in sedimentary style, from terrigenous to carbonate deposition, has been noted from Sequence 2 to Sequence 3, possibly related to a major change in climate. All of these unique "fingerprints" will be used in future studies which will seek widespread sequence correlation of late Proterozoic Strata.

197

6.10

CALCULATIONS ON A MODEL FOR LITHOSPHERIC EXTENSION DUE TO A LOW-ANGLE DETACHMENT ZONE Herman Voorhoeve^ and Gregory Houseman^

^Geology Department, Australian National University, Canberra ^Research School of Earth Sciences, Australian National University A uniform-sense simple shear model has been proposed by Wernicke in 1981, to explain extension of continental lithosphere in the Basin and Range province of the USA. This model, and other related models, which involve low-angle detachment zones through the lithosphere have also been proposed to explain observations of rifted continental margins such as the Red Sea, or the southeast coast of Australia. The clear asymmetry between upper and lower plates in these extensional models indicates that there will be important differences in the thermal evolution of the two plates, and thus in the heatflow and subsidence histories. In order to develop further an understanding of these differences we will present calculations of the temporal and spatial variation of surface heat flux and subsidence for a simplified analog of the simple shear model, for comparison with the pure shear model of McKenzie proposed in 1978. By assuming that horizontal gradients are negligible, the energy equation is reduced to the one—dimensional thermal diffusion problem. Fourier series solutions to this equation are then obtained with an initial geotherm which depends on the amount of extension and depth to the detachment zone (which is assumed to be initially planar, cutting through the entire lithosphere at a low angle). Initial results indicate the time-scale for evolution of the thermal profile is about lOOMa. This is consistent with the assumption that the duration of extension is much less than the time for thermal adjustment. Following McKenzie, we calculate subsidence assuming a homogeneous crustal layer overlying homogeneous mantle with a temperature-dependent density and a constant coefficient of thermal expansion. The calculated heat flux and subsidence histories may be useful in estimating thermal maturation in sedimentary basins which can be attributed to the mechanism discussed.

198

6.1

FIRST DEEP SEISMIC PROFILE ACROSS TASMAN FOLD BELT CHEEPIE TO BEENLEI6H - QUEENSLAND

K.D. Wake-Dysterl, Wright^, M.J. Sexton^, D.W. Johnstone^, R.W. Day^, C.G. Murray^, 0. Dixon^ and J.M.W. Rynn^ ^Bureau of Mineral Resources, Geology and Geophysics, Canberra ^Geological Survey of Queensland University of Queensland

During 1984, the Bureau of Mineral Resources, Geology and Geophysics (BMR) in co-operation with the Geological Survey of Queensland (GSQ) obtained 820 km of six-fold CDP reflection data recorded to 20 s recording time, along a line extending from Cheepie in southwestern Queensland to near Beenleigh south of Brisbane. The work was an extension of a major multidisciplinary study of the structural and depositional history of the central Eromanga Basin, which was commenced in 1980. In addition to the long seismic recordings which were made in an attempt to record deep crustal reflection events, gravity observations were taken at 500 m intervals along the seismic traverse, and an aeromagnetic profile was flown over the traverse. The seismic studies form the basis of a lithosphere transect of the type proposed for Australian Continental Reflection Profiling (ACORP) studies. The transect crosses parts of the Thomson and New England Fold Belts of the Tasman Fold Belt System that lie beneath cratonic cover of the Jurassic to Cretaceous Eromanga, Surat, and Clarence-Moreton Basins. From west to east, structural elements traversed include the Eromanga Basin, the Devonian to Carboniferous Adavale Basin, the early Palaeozoic Nebine Ridge, the Surat Basin, the Permian to Triassic Bowen Basin, the northern extension of the Palaeozoic Texas Block, the Triassic Esk Trough and Ipswich Basin, and the late Palaeozoic Beenleigh Block. Preliminary seismic sections produced by BMR reveal encouraging results. Zones of deep seismic reflection events have been recorded from several parts of the traverse. These deep reflections occur at varying depths, and their interpretation will not be possible until the seismic data are stacked into a continuous profile. However, their existence indicates that the profile may add greatly to present understanding of the structural style and tectonic evolution of the Palaeozoic rocks of the Tasman Fold Belt System in this region. In the younger sedimentary basins, the profile should prove a useful aid to correlation and structural interpretation, thereby assisting petroleum and coal exploration. On a broader scale, integration of data from the seismic profile and related geophysical and geological investigations, with that from the Wivenhoe Dam seismic surveillance project, could fundamentally advance our understanding of the geological evolution of the Australian continent.

199

17.3

MINING THE PAST: THE USE OF HISTORICAL RECORDS FOR QUEENSLAND MINING HISTORY Janice Wegner

History Department, James Cook University of North Queensland This paper is concerned with the interpretation of historical records h'kely to be found for Queensland mining fields between 1860 and 1920, including geological reports. It is illustrated by examples from the history of the Croydon Goldfield, North Queensland. Initial discoveries of mining fields and the subsequent rushes were characterised by several factors which make the historical records for them very unreliable. The prospectors and miners rarely had scientific or technical training. The rushes were usually remote and in country unsuitable for closer settlement. They suffered from a number of drawbacks, such as unreliable transport, high prices, and poor living conditions. Milling and mining methods were therefore primitive' requiring high returns per ton. These conditions ensured that even fields which paid expenses or ''wages'' were quickly abandoned for other rushes. Official records reflect the impermanance of these rushes. Administration was often inadequate, resulting in unreliable production records. The administrators also had no scientific or technical training, so their opinions could rely too heavily on those of the miners. Geological surveys are useful according to the state of the science at that time, and are usually descriptions of the orebodies as exposed in the mines. Newspaper reports were often published to "boom" the field. If the rush did not "duffer out'', the next stage of the field's history began once the mines went below the water table and encountered more complex ores. Increasing depth and new kinds of ores required capital for machinery, which was provided by local partnerships and companies. This process of attracting capital did not occur automatically; fields at this stage were in danger of declining. A decline could be arrested or reversed by discoveries of new orebodies, either on the surface or at depth, increases in metal prices, or the introduction of new mining or ore treatment technology. The most famous was the cyanide process of extracting gold and silver from tailings, which revitalised several Queensland goldfields in the 1890's. The use of the best - or most suitable - technology available could prolong the life of afield. However the introduction of such technology was often to impress share-holders rather than increase efficiency. Closer examination of such instances shows overspending on capital improvement before a mine was proved. Planning and prospecting were usually inadequate and mines were often over-capitalised. Even the physical evidence such as the remains of mills, then, can be misleading. Generally, miners appeared to be too conservative to adopt new technical advances or give them a fair trial, though in many cases they appear justified by the fact that most of these advances were made for massive orebodies, not the patchy reefs and contact deposits being mined in Queensland. Hence miners relied on past experience, which led them into mis-conceptions such as the belief that ich orebodies in reefs would always recur at depth. :ompany records for the boom periods, such as prospectuses and annual reports, and newspaper eports are usually entertainingly misleading, because share-dealing was a gambling mania shared by all classes. Official records are more reliable because of better administrative controls and the regular visits of Government technical staff. However, opinions expressed by the Warden are still likely to be heavily influenced by the business and mineowning establishments. Declining fields were marked by such factors as formerly good mines being abandoned, the gradual disappearance of companies and particularly, a sudden preference for the opinions of trained geologists over the advice of the "practical" man for new prospecting efforts. Unfortunately the assistance of geologists could be nullified by a tendency to accept advice best suited to the miners' own hopes. The failure of prospecting resulted in a reversion to rush conditions, with small miners using primitive methods. Since 1920 changes in mining and ore treatment methods have favoured massive deposits like Mt. Morgan and Mt. Isa, or the "new" minerals such as iDauxite and uranium. Further exploitation of the deposits being mined between 1860 and 1920 is dependent on the same factors as before: changes in prices and new technology, though now aided by great advances in the science of geology. 200

2.6

McARTHUR BASIN STRUCTURE

Peter Wellman and Ken A. Plumb Bureau of Mineral Resources, Geology and Geophysics, Canberra

The present geological model for the McArthur Basin proposes that the Proterozoic sediments are up to 4 km thick over widespread shelves, and 4-10 km thick in the central Batten Trough and possibly to the southwest. BMR seismic refraction and magnetotelluric surveys have confirmed this model along an east-west traverse across the southern part of the basin, and identified the 10 km deep Betaloo Sub-Basin in the southwest. Gravity and magnetic anomalies are used here to study the structures basin-wide. The geological model has been effectively confirmed. Thickness variation of the preserved McArthur Basin sediments has been mapped by estimating depths to magnetic bodies, and the nature of the thick sediment has been determined from their combined gravity and magnetic signatures. The margins of the zones of thick sediment generally correspond to steep gravity and/or magnetic anomaly gradients. Magnetic bodies within the basin succession complicate mapping of depth to basement. Selection of anomalies depends on geological constraints from surface geology. All techniques consistently define a broad zone of overall thick sediments widening from 50 km wide at latitude 1 2 t o 250 km wide at 17°S, and bounded by shallower shelves ( 4 km) to the east and west. Separate north-south troughs cut by northwest transverse highs may be defined within this zone. The Batten Trough lies as postulated geologically, divided by an extension of the Urapunga Tectonic Ridge. Its thickest and widest development is concealed beneath the Gulf of Carpentaria and adjacent coastal plain, between Roper River and Groote Eylandt. The concealed Beetaloo Subbasin is outlined as a zone of thick Roper Group. A new totally concealed trough of thick carbonate rocks (McArthur Group equivalents) is suggested in the southwest of the area. Both of these troughs trend into the Tomkinson Creek Beds, north of Tennant Creek. A major problem in understanding the McArthur Basin is determining the cause of the 10 km local subsidence and 3 km of regional subsidence. The presence of troughs of thick cover, with adjacent shelves of thin cover, makes it very unlikely that the cause of the subsidence is overthrusting, whilst thermal cooling without extension is unlikely because it would give a •saucer' shaped basin, with no central linear trough. The cause is more likely to be crustal extension and rifting.

201

The area of McArthur Basin with thick sediment widening southwards from a 50 km width at the northern Trough at 12°N, ro about 270 km width in the south about could be explained if the subsidence causing the thick from extension during counter-clockwise rotation about near ll^N and I S V E .

shows an irregular end of the Batten 17®N. This widening sediments resulted a pole of rotation

The dykes of the McArthur Basin have been mapped by Tucker and Boyd using aeromagnetic anomalies. The dykes are probably mostly of Roper Group age, but it is not known which dykes are co-genetic. Most of the dykes are approximately radial to the same pole as the block rotation referred to above. Hence this dyke pattern is consistent with having been emplaced in the same tensional stress field as that inferred for the block rotation. This rotation and extension is consistent with a dominant pattern of northwest-striking sinistral strike-slip faults (transforms?), and secondary northerly trending block faults (Plumb & others, 1980). The various grabens and troughs have subsided at different rates, and so this somewhat simple model requires the intermittent operation of rotational movements over a period of some 200 Ma.

202

4.14 STRONTIUM ISOTOPIC STODIKS OF BARITES FROM THE CAMBRIAN MT READ VOLCANICS, TASMANIA D.J. Whitfordl, b.L. Gulsonl, M.J. Korschl and M. Solomon ICSIRO Division of Mineral Physics and Mineralogy, North Ryde 2bmR Division of Petrology and Geochemistry, Canberra Barite is a very common gangue mineral in volcanogenic massive sulfide denoslts and is regarded as an important indicator in exploration. In the S i a n m . 4 a d Volcanics of western Tasmania, barite is an Important colpon^nt in the syngenetlc sulfide deposits such as Que River and Rosebery and also occurs associated with generally uneconomic vein-style mineralization, at least some of which is thought to be of Devonian age. Barite Is characterised by a very low Rb/Sr ratio and high Sr abundance. The measured S^Sr/S^Sr ratio is therefore generally very close to the Initial ratio when the barite first crystallised and hence should reflect that of the hydrothermal fluid from which it precipitated. ^Consequently the isotoplc composition of Sr extracted from barite can, on this Jasls be used to make Inferences about the origin of hydrothermal fluids and perhaps the source(s) of related ore metals. Most stratiform barltes, many of which are associated gf^^.VC^ratJis massive sulfides, are characterized by a narrow range of °'Sr/ Sr ratios ? r o f ca. 0 709 io ca. 0.711. The lower end of this range overlaps the field inferred for Cambrian seawater (0.7090-0.7094). Vein barltes have a much wider range of Isotoplc compositions from values îîl^'&ycV^/sRr massive stratiform deposits to more radiogenic compositions with Sr/ Sr ratios up to 0.717. The initial ®^Sr/®^Sr ratio of the intermediate to silicic volcanic hosts S mineralization is difficult to determine due to alteration related to submarine volcanlsm, mineralization, and regional metamorphism. At Que and Rb/Sr ratios measured in relict calcic clinopyroxene ohenocrvsts have been used to calculate the initial ratio of the local ^ n S r i J S (ca 0.706), assuming an age of 540 ma. Jerlved from a w h o l e rock ?b-Pb age. It is not clear if such initial ^ h t / ^ H v ratios can be applied t ^ t h e volcanic belt as a whole, particularly in the south snicic rocks predominate. It is significant however, that the calculated initial 8^Sr/86sr ratio of the host rocks is higher than that inferred for the Cambrian mantle. Such a ratio implies that a component of old continental crust was Involved during magma genesis, a conclusion in accord with whole-rock and sulfide Pb Isotope studies. The isotoplc differences between stratiform and vein barltes could be related to initial variations in fluid compositions and/or differing " s ^ o n L s to deformation and regional metamorphism. Barltes are generally recrystalllzed and the possibility exists that it has isotopically r^SSubrated during metamorphism. Such a reequlllbratlon is Perhaps more likely for vein-style, rather than massive stratlfom requiring unreasonably large water rock ratios. The initial Sr/ Sr ^^tio defined by rocks from Que River, that have ^ P f isotopically during the Devonian metamorphism and defomatlon, is ca. 0.7097, too low to explain the ratios in most of the However, requllibration may explain ^h® observation of S r / S r ratios In vein barltes lower than those measured in nearby stratiform barite. veins wiiJ radiogênirsr preserved in barltes could be related to Devonian Suionisi L d mineralization, known to have high isotopes may therefore have some exploration applications in characterizing and distinguishing barite occurrences. 203

Assuming that the Sr in the stratiform barites is a direct reflection of the hydrothermal fluid from which it initially precipitated, then the Sr is not simply a mixture of that derived from the local volcanic pile and seawater. Similarly, a juvenile magmatic component and seawater alone cannot explain the observed Sr isotopic composition of the barites. The isotopic data require an additional radiogenic component. The nature of the radiogenic component is difficult to define but is most likely to be old continental material. Fluids responsible for ore deposition could have tapped into older continental crust underlying the volcanics. Such old crustal material could represent a significant source of metals.

204

1.15

WALLROCK ALTERATION ASSOCIATED WITH UNCONFORMITY-RELATED URANIUM DEPOSITS, NORTHERN TERRITORY A.R. Wilde, V.J. Wall and M.S. Bloom Dept. Earth Sciences, Monash University, Melbourne

The processes involved in formation of unconformity-related uranium deposits have hitherto been poorly understood. In this paper hydrothermal alteration associated with the uranium deposits of Nabarlek and Koongarra (N.T.. Australia) is described. Temporal and spatial variation of various alteration parageneses are used to constrain aspects of fluid chemistry and make inferences on mineralising processes. The Nabarlek and Koongarra unconformity-related uranium deposits are situated within the Alligator Rivers Uranium Field, Northern Territory. Host rocks are Lower Proterozoic (>2000 m.a.) schist and amphibolite of the Cahill Formation. These rocks are overlain by gently-dipping alluvial Kombolgie sandstone 0 1 6 5 0 m.a.). The deposits are characterised by extensive hydrothermal alteration envelopes, symmetrically disposed about reverse fault zones, typically extending 2 km from ore. Inner and outer alteration zones are recognised. The latter is characterised by pseudomorphous replacement of metamorphic phases, by a stable assemblage of quartz, chlorite, rutile, sericite and pyrite (or magnetite). Schist shows replacement of biotite by chlorite several kilometers from ore. In contrast pristine amphibolite survives to within 250 m of the ore zone, while marble is silicified within 100 m of it. Volume change in schist and amphibolite was minimal with loss of Ca. Na and Sr and gain in K and Rb. (due to conversion of feldspar to sericite). Quartz deposition (with or without pyrite and chalcopyrite) occurred in fault zones along strike from the deposits and in overlying sandstone. The inner halo is a zone of marked quartz depletion, extending uptp 50 m from mineralised fault zones. Quartz was replaced by a fine-grained assemblage of variable proportions of magnesian chlorite, phengitic-mica and hematite, with residual phases such as rutile, muscovite-mica and apatite. Nabarlek is distinguished from Koongarra by the presence of intense phengitic-mica alteration adjacent to the mineralised fault. Alteration involved considerable mass transfer and volume loss (desilification), estimated at 20Z for Nabarlek and 5Z for Koongarra. Bulk chemical changes involved increase of Mg, Al, Fe, Ti. Cu, Co, Ni, Ba, Rb, Li, Fe' /Fe^ and at Nabarlek, K. Uraninite and brannerite deposition postdates this intense alteration; occurring as reverse fault breccia matrix, in associated veins, or disseminated adjacent to these features. Gangue is mainly chlorite with minor phengitic-mica, rutile, and chalcopyrite. Dolomite and later calcite veins cut primary ore. It is proposed that the primary mineralisation, at Nabarlek, occurred at circa 1590 m.a., on the basis of a Nd-Sm whole-rock isochron and reinterpretation of existing K-Ar and Rb-Sr data. At Nabarlek and to a lesser extent at Koongarra. primary ore was overprinted by an illitic-mica and hematite alteration, resulting in the alteration of uraninite to coffinite and incongruent dissolution of chlorite. It is proposed that this event occurred at 900 m.a.

205

During t h e e a r l y ( q u a r t z - d e p o s i t i o n a l ) e v e n t , t e m p e r a t u r e s w e r e in e x c e s s of 200*C (fluid inclusion data). T h i s is c o n s i s t e n t w i t h f i s s i o n t r a c k dating of z i r c o n , w h i c h has an a n n e a l l i n g t e m p e r a t u r e in e x c e s s of 2 0 0 * C . During the m i n e r a l i s a t i o n p h a s e , t e m p e r a t u r e s of at l e a s t 160*C are suggested by a p p l i c a t i o n of the c h l o r i t e s o l i d - s o l u t i o n m o d e l of W a l s h e . F l u i d i n c l u s i o n d a t a on l a t e c a l c i t e v e i n s shows t h a t a f t e r m i n e r a l i s a t i o n , t e m p e r a t u r e s w e r e in the r a n g e 1 7 0 - 5 0 * C . Fission-track data suggests that a superincumbent pile of upto 5 km o f s e d i m e n t s o r i g i n a l l y o v e r l a y the d e p o s i t s , i n d i c a t i n g a l i t h o s t a t i c p r e s s u r e of c i r c a 1 k b . F l u c t u a t i n g f l u i d f O , . d u r i n g f o r m a t i o n of the d e p o s i t s is i n d i c a t e d by the s p a t i a l a n d t e m p o r a l d i s t r i b u t i o n of s u l p h i d e and o x i d e m i n e r a l s . Early (outer halo) alteration occurred with pyrite, chalcopyrite or magnetite stable. C l o s e r to the o r e b o d i e s , the a b u n d a n c e of h e m a t i t e , i n d i c a t e s a rise in f l u i d fOj. Ore deposition, h o w e v e r , occurred with pyrite again stable. This is c o n s i s t e n t w i t h reduction being the m e c h a n i s m of precipitation. The pH of the s o l u t i o n s at the t i m e o f m i n e r a l i s a t i o n is e s t i m a t e d to be <5.0 at 2 0 0 * C . Mineralisation is i n f e r r e d to be the r e s u l t of i n f l u x of a hot ( > 2 0 0 * C ) . oxidised ( i . e . in e q u i l i b r i u m w i t h h e m a t i t e and c a p a b l e of t r a n s p o r t i n g uranium), silica-undersaturated fluid ( p r o b a b l y , t h e r e f o r e of s u r f a c e o r near-surface origin), into reducing basement rocks along reverse fault zones. As t h e f l u i d became quartz-saturated (possibly a r e s u l t of cooling). quartz deposition occurred in r e v e r s e fault channelways. restricting a c c e s s of l a t e r f l u i d s . F l u i d p e n e t r a t i n g the b a s e m e n t r o c k s became r e d u c e d , p r o d u c i n g a l t e r a t i o n of the o u t e r h a l o . Back-circulation of r e d u c e d fluid into t h e f a u l t z o n e s c a u s e d r e d u c t i o n of the i n c o m i n g oxidised f l u i d s and p r e c i p i t a t i o n of u r a n i u m . Reduction was particularly effective in t h e v i c i n i t y of g r a p h i t i c o r f e r r o u s i r o n - r i c h r o c k s ( e . g . amphibolite). The r e m o v a l of l a r g e v o l u m e s of s i l i c a f r o m the i n n e r a l t e r a t i o n z o n e ( e s p e c i a l l y at N a b a r l e k ) i n f e r s f l u i d / r o c k r a t i o s in e x c e s s of 1 0 0 0 : 1 .

206

5.3

POTENTIALLt ECONOMIC PALAEODRAINAGE SYSTEMS OF CENTRAL AND WESTERN AUSTRALIA USING NOAA-AVHRR IMAGERY Peter Wilson CSIRO, Division of Minerals and Geochemistry, Perth

The potential of NOAA-AVHRR day and night imagery as a tool to assist in geological exploration and mapping has been demonstrated by the highlighting and reconstruction of structurally controlled palaeodrainage networks within the Cainozoic sediments of Australia's central and western sedimentary basins. These include the Phanerozoic Canning and Officer Basins. The unique regional perspective offered by the 1.1 km spatial resolution and the thermal infra-red and visible wavelengths has revealed an intricate nature of exposed and buried palaeodrainage networks. The NOAA satellite series are near polar-orbiting spacecraft at an altitude between 870 and 930 km with an orbital period of 102 minutes^ resulting in 14.1 orbits/day. The two satellites operational at present^ NOAA-7 and NOAA-9 have approximate equator crossing times of 0330 hrs and 1530 hrs. The AVHRR is a five-channel multispectral scanner with an instantaneous field of view of 1.3 milliradians resulting in a ground resolution of 1.1 km at nadir and 5-5 km near the scan limit. The swath width of the scanner is approximately 2900 km wide divided into 2048 pixels per scan line. The wide scan provides contiguous global coverage at the equator on a daily basis, whilst at higher altitudes, for example Australia, considerable swath-edge overlap occurs. The AVHRR has five spectral channels: the visible

(0.55-0.68), the near infra-red ( 0 . 7 2 5 - 1 . 1 0 ) , the mid infra-red (3-55-3.93)

and' the two thermal infra-red windows (10.3-11.3 and 11.5-12.5). Three night images, summer, spring and winter were compared to assess the effect of annual variations in insolation on the detectability and thermal detail of surface and near-surface features. Differences in reflectance, emissivity and thermal inertia of materials and the variable physical and temporal conditions under which the materials are heated were important considerations when assessing the thermal response as a function of seasonal change. The Officer Basin as described by other researchers is essentially a deep elongated depression containing a gently folded sedimentary sequence of Phanerozoic rocks unconformably overlying a thick Proterozoic sequence. The basin is located between the Yilgarn and Musgrave Blocks in the Gibson Desert and Great Victoria Desert regions of Western Australia and South Australia. The Throssell and Baker palaeodrainage systems are largely intact and still form prominent valleys in the undulating lateritic plains. An AVHRR day-pass reflected image shows only minor physiographic detail, although the extent of ferruginization within the Gibson Desert is clearly highlighted. The dense network of first, second and third order streams, individual catchments, drainage divides and textures can be recognised on the nightthermal image. Also areas of gravel pavement that have been obscured and buried by encroaching aeolian sand cannot be observed on either Landsat or aerial photos but can be identified on the thermal imagery as relatively warm zones. The night-thermal image of the Officer Basin has also unmasked apparent structural control of the major trunk-valleys. A prominent east-west trending lineament appears to control the sharp change in

207

direction of the Throssell palaeoriver in the vicinity of Lake Yeo from southwest to due east. Displacement to its alignment is disrupted at the southern end of the Westwood fault. This lineament forms a structural control to the Miocene shoreline which is now the extension of the Eucla Basin. A northerly extension to the Westwood fault is also apparent as is a suite of NW-SE trending lineaments which influence the shape and alignment of the Baker and Throssell palaeorivers. In the northwestern sector of the basin a sudden change in image texture and the presence of relatively short semi-circular linears and tangential lineaments is indicative of concentric bedding-plane faults. This is consistent with the presence of diapiric structures. Several circular structures displaying drainage anomalies are prominent within the lateritized plain. An apparent deltaic feature at the confluence of the Throssell and Baker palaeorivers in the Jubilee Lakes region of the Eucla Basin is a possible source of mineralization transported from sources in the respective catchments. West of the Jubilee Lakes is located the Neale Plateau^ a low-level wave-cut platform formed in front of a fault-controlled feature which may once have been sea-filled. A littoral long-shore drift along the Miocene shoreline may have transported minerals from the Jubilee Lakes delta region westwards to the vicinity of the Neale Plateau. A reduction in velocity could have occurred as the current turneo north around the remnant headland thereby causing a drop in load and the formation of a mineral placer deposit. Prominent relict drainage'systems within the Eastern Goldfields region of the Yilgarn Blocks drain east-southeast towards the Eucla Basin. A broad relatively warm, linear feature occurs along the Officer-Eucla boundary. Although it has minimal surface expression, magnetic and gravity data verify that it is a barrier of shallow basement east of a deep sub-basin containing sediments exceeding 500 metres in depth. The AVHRR night-thermal image positively highlights the existence of buried palaeovalleys. These show as meandering, structurally controlled extensions of the relict drainage chains within the Yilgarn Block. They appear as major palaeochannels incised into the aeolian covered, duricrusted peneplain. The palaeoriver in which Lake Rason is located, is the only outlet through the shallow basement barrier to the Eucla Basin. A deltaic feature in the vicinity of Plumridge Lake is a possible source of mineralization, especially as the transported sediments originated within the gold-rich Eastern Goldfields. The sub-basin also has enormous potential as a source of placer gold, silver and uranium deposits. Mineral deposition is extremely probable as the sub-basin is the site of the palaeorivers' dispersing sectors. A continuum of deposition could have occurred during two periods of changing sea—levels. One, in a quiet period when the ocean depth over the sub-basin was very shallow and dispersion and settlement of the bed-load occurred within the sub-basin; and the second, after total withdrawal of the sea, but when the palaeorivers were still actively flowing. There can be no doubt of the potential of the palaeodrainage systems described as economically significant environments for the control and/or location of mineral and energy resources. Their potential as sources of gold, diamond and placer deposits within the alignment of the palaeochannels must be recognized, especially when their catchments are located within mineralized Precambrian provinces. The broad synoptic view and repeatability of day and night coverage by the NOAA satellite series have added a new dimension to assessing the mineral potential of arid regions where geologic knowledge and access are very limited.

208

6.13

THE INTERPRETATION OF EXPANDING SPREAD REFLECTION PROFILES RECORDED IN SOUTHERN QUEENSLAND C. Wright

Bureau of Mineral Resources, Geology and Geophysics, Canberra

Four expanding expanding spread reflection profiles have been recorded in southern Queensland coincident with BMR's regional GDP reflection lines. The purpose of these profiles is to obtain better estimates of seismic velocities throughout the crust and to study the change in reflection character as a function ot angle of incidence, especially for possible reflections from structure in the lower crust. Two of the expanding the spreads recorded in the Eromanga Basin to offsets of 2 5 km involved recording of 13 shots. However, for the two remaining expanding spreads recorded across the Nebine Ridge and within the Surat Basin, the degree of data redundancy was increased at offset distances greater than 10 km so that 19 shots were recorded with a maximum offset of 2 5 km. The shots of the expanding spread recorded across the Nebine Ridge, together with many of the routine GDP profiling shots, were also recorded by eight seismic refraction instruments deployed at 1 km intervals at each end of the expanding spread. The objective of this experiment was to compare the records at the low frequencies recorded by conventional seismometers with the higher frequency records obtained from arrays of geophones, and to assess the value of the refraction equipment for seismic velocity measurements at shallow depths. The refracted arrivals from the Nebine Ridge expanding spread have been used to derive a laterally varying P wave velocity model after correcting for weathering and elevation differences below the recording stations (conventional statics).The near-surface structure below the expanding spread consists of a wedge-shaped veneer of sediments with an average P wave velocity of 2.27 km/s and with thicknesses of about 350 m and 900 m at the eastern and western extremities respectively. The topography of the underlying basement is well resolved by the data. The velocities immediately below the sediments cannot be accurately determined, but an excellent fit to the data is obtained with a velocity of 5.23 km/s which increases to 5.68 km/s at a depth of 1.7 km. The main function of the modelling is to enable the effects of inhomogeneities in the uppermost regions of the earth to be removed prior to the processing and interpretation of deep crustal reflections from both the expanding spread and the coincident GDP profile, thus providing the basis for more effective large scale static corrections.

209

6.7

THE CENTRAL AUSTRALIAN SEISMIC EXPERIMENT, 1985

C. Wright, B.R. Goleby, C.D.N. Collins and D.M. Finlayson Bureau of Mineral Resources, Geology and Geophysics, Canberra

The major objective of the Central Australian seismic experiment is to investigate the structural evolution of the lithosphere in the Central Australian region with emphasis on the formation of the Ngalia and Amadeus Basins and the evaluation of their petroleum p r o s p e c t i v i t y . The Wiso, N g a l i a , Amadeus and Officer Basins are separated by regions of exposed Proterozoic crust known as a r c h e s . Some of the largest differences in gravity anomalies seen on a n y continent exist between the basins and arches. T h e interpretation of these anomalies in conjunction w i t h the results of the seismic profiling w i l l have important bearing on the ideas on the formation of the intracontinental basin s t r u c t u r e s . The BMR Explosion Seismology Section has recorded a regional north-south reflection line of about 380 km in length extending from Napperby Creek south of the Reynolds Ranges to a location about 10 km east of Curtin Springs. In addition to the routine CDP recording, two orthogonal expanding spreads with maximum offsets of 36 km have been recorded on the Southern Arunta B l o c k . T h e main purpose of these expanding spreads is to obtain better velocity control throughout the crust and to obtain a limited amount of three-dimensional c o v e r a g e . T h e shots of the expanding spreads have also been recorded by a set of 16 refraction recorders deployed to give detailed three-dimensional coverage and enable the application of tomographic techniques to image the underlying structure. An additional north-south line of 40 km length w a s recorded to the west of Mount H a y and crosses the northern margin of the Redbank zone; it has been designed to enable the mapping of any steeply dipping fault structures into the middle and lower c r u s t . Eight shots recorded at offsets pf 21-25 km w e r e also recorded on this additional line to assist in velocity control and fault location. A wide angle reflection/refraction line of 300 km length w a s recorded along an east-west route on the southern Arunta Block that crosses the main reflection line at the point of intersection of the orthogonal expanding s p r e a d s . T h e purpose of the w i d e angle reflection/refraction line is to enable regional seismic velocities to b e determined parallel to the strike of the Southern Arunta B l o c k , where the lateral heterogeneities are expected to b e m i n i m a l .

210

ABSTRACTS OF SGGMP PAPERS

211

THE STRUCTURAL ROLE OF FLUORINE IN MAGMATIC SYSTEMS WITH APPLICATION TO THE GENESIS OF LAMPROITES S.F.Foley, W.R.Taylor and D.H.Green Geology Department, University of Tasmania, Hobart. The effect of fluorine on phase relationships and melt structure was investigated in the system kalsilite - forsterite - silica [Ks-Fo-Qz]. This system was chosen as the potassic analogue of the base of the basalt tetrahedron and is thus suitable for modelling ultrapotassic rocks. Experiments were run at 28kb for direct comparability with the results of Green, Gupta and Taylor (1984), who studied the Ks-Fo-Qz system in volatile-free conditions and with H2O and CO2. Fluorine was added as MgF2 by direct substitution for oxygen (i.e. using the exchange vector Ks-Fo-Qz with 4 mol% ^^^ fluorine) contains a large liquidus field for fluorphlogopite, and the thermal stability of mica is increased by 300^C relative to the water-saturated system. Fluorine expands the phase volume of enstatite relative to forsterite, so that the peritectic point PHL+EN+FO+L, a model for melting of a phlogopite-bearing harzburgite, lies in the silica-undersaturated field. This contrasts with the silica-rich position of the same peritectic point in the H20-saturated system. Experimental micas have excess Si ( >6 cations per 22 oxygens) and are Al-deficient (<2) in the F-bearing system. Natural micas from ultrapotassic rocks share these characteristics which appear to be due to fluorine: no excess Si or Al-deficiency is seen in the KS-F0-QZ-H2O system. Expansion of the enstatite phase volume relative to that of forsterite is frequently taken to indicate polymerisation of the melt. This polymerising action of fluorine contradicts the popular assumption that fluorine dissolves in magmatic systems by formation of Si-F bonds. The mechanism of fluorine dissolution was therefore investigated by infrared spectroscopy. A composition close to the PHL+EN+FO+L peritectic point was chosen for infrared study as an analogue of melts produced by partial melting in the mantle. Glasses were prepared at 1 atm because high pressure run products could not be used due to the abundance of quench crystals. Spectra were obtained in the regions 4000-400 cm"l (mid IR) and 500-100 cm"^ (far IR). The use of these regions permits study of absorption bands characteristic of vibrations in the aluminosilicate network (mid IR) and those of network modifying cations, especially uni- and divalent (far IR). Difference spectra obtained by subtracting the digital spectra of fluorine-free glasses from those of fluorine-bearing glasses revealed substantial modifications of melt structure due to fluorine. The difference spectra for the mid IR region show a shift in the high frequency region to higher wavenumbers due to addition of at least two absorption bands at approximately 1100 and 1200 cm~^, and removal of absorption near 950 cm""^. Studies of simpler silicate and aluminosilicate glasses have demonstrated that a shift to higher frequencies may be due to an increase in the degree of polymerisation of silicate anions or an increase in the Si/(Si+Al) ratio of the silicate network. The effect of fluorine on the far IR spectra is pronounced; bonding Interactions with all the network modifying cations K, Mg and A1 are seen even at very low fluorine contents where there is no noticeable difference in the mid IR region. The far IR absorption bands are assigned with reference to simple fluoride mineral spectra. The spectroscopic data demonstrate that fluorine 212

dissolves by complexing with network modifying cations. Also, tetrahedral KAIO2 groups are removed from the aluminosilicate network causing polymerisation and an increase in the Si/(Si+Al) ratio of the network. In most natural melts fluorine will be less abundant than H2O so that HF will be the dominant species, and the dissolution can be represented by KAIIV02 + 4 HF

Al^^Fg + KF + 2 H2O

2 (KAl^Vsi^Og) + 4 HF

(KAl^^SijOg.Si02) + KF + Al^^Fg + 2 H2O

The polymerising action of fluorine will thus be masked by the depolymerising action of water which is released by HF dissolution. In the presence of mixed H2CH-HF fluids, the forsterite+enstatite phase boundary in Ks-Fo-Qz can be expected to indicate overall depolymerisation. It is this masking effect which has led to previous suggestions that fluorine depolymerises melts by forming Si-F bonds causing breakup of the melt structure: the use of direct substitution has permitted resolution of the problem in this study. Dingwell and Mysen (1985) made viscosity measurements on albite melts to which fluorine was added by direct substitution for oxygen, and found substantial decreases in viscosity due to fluorine. It is important to note that "polymerisation" discussed here refers to the aluminosilicate network, whereas viscosity is a measure of the overall structure of the melt. The polymerisation state of the silicate network, represented by the forsterite+enstatite phase boundary, is more useful for considering the composition of melts produced in the mantle, because the phase volumes of silicate minerals reflect their relative stabilities as residual mantle phases. In the case of an H20+HF-bearing system, the stability of olivine is increased, which will result in a relatively silica-rich partial melt. The viscosity, however, is more useful in considering separation of this partial melt. A reduction in viscosity will increase the flow rate of a small melt fraction in the mantle, making melt extraction more likely. H2O is also known to decrease viscosity in silicate melts, and H2O+HF mixtures reduce the viscosity more than H2O alone. Lamproites include a range of compositions which have high Mg-number, Ni and Cr, and carry mantle—derived ultramafic nodules, and thus appear to represent little modified mantle-derived liquids. These range in Si02 content from about 40 wt% to at least 51 wt%. The generation of the more silica-rich liquids in the mantle will be assisted by H2O and HF, and requires a high H2O/CO2 ratio. This volatile mixture is indicated by chemical analyses of the lamproites. The range of lamproite primary compositions can be explained by melting in a reduced environment in which CH^, and not CO2, will be the dominant carbon species. Methane also depolymerises silicate melts by a dissolution mechanism which releases H2O in a similar manner to that of HF. Lamproites may thus result from a C02-poor environment which need not be poor in carbon. Oxidation to currently observed oxidation states during emplacement of the magmas can be attributed to dissociation of less than 0.1 wt% H2O driven by diffusive H2 loss. An increase in the pressure of melting will cause melts to become less silica-rich. The lower silica olivine lamproites could therefore be derived by melting of a similar source to leucite lamproites in a similar CH^-H20-HF-rich environment, but at greater depth. Diamonds are commonest in the olivine lamproites of the Kimberley region of Western Australia, which is compatible with a greater depth of origin. 213

EXPERIMENTAL INVESTIGATION OF THE ROLE OF AMPHIBOLE IN THE EVOLUTION OF ISLAND ARC VOLCANICS, SOLOMON ISLANDS T.H. Green and N.J. Pearson School of Earth Sciences, Macquarie University, Sydney

A basalt from an island arc volcanic province. New Georgia, Solomon Islands, provides a relatively rare example of evidence for a role of amphibole crystallization in the fractionation of the magma. Additionally, the common occurrence of amphibole-rich mafic inclusions in Pliocene-Recent andesitic-dacitic volcanics in the nearby Guadalcanal province points to a link, via amphibole fractionation, between basaltic and more evolved members of this particular calc-alkaline suite. Compositionally, the basalt is calcalkaline (mg 64) . It contains common zoned phenocrysts of olivine (mg = 76-81), clinopyroxene (mg 73-86), plagioclase (^^^q-SS^ and microphenocrysts of magnetite. Rare, resorbed, magnetite-rimmed pnenocrysts of amphibole (pargasite, n^ 70-72) also occur. Mafic inclusions from Savo Island and Guadalcanal are dominated by amphibole (pargasitic hornblende to ) but (pargasite to ferroan pargasite), also contain subordinate olivine (mg = 79), clinopyroxene (mg 73-89) and plagioclase (Ancc^go)- The microstructure of these inclusions varies from interlocking ana poikilitic, cumulate to recrystallized, equidimensional mosaic. Phase relations in the basalt have been determined between 5 and 15kb and 950-1100°C, for varied water contents and oxygen fugacity and HM buffers). The near-liquidus phases change from olivine-clinopyroxene dominated at 5kb (5-10% H O ) , + plagioclase (2% HÔ) to olivine-clinopyroxene-amphibole at lOkb (2-10% HÔ) to olivine-clinopyroxene-garnetamphibole at 15kb. Amphibole did not occur at 1050®C where clinopyroxenegarnet (15kb) orthopyroxene-clinopyroxene (lOkb) or olivine-clinopyroxene (5kb) formed. Matching of the experimentally determined crystallization sequence with the observed phenocrysts suggests that the most likely conditions for producing the natural assemblage from a basalt magma similar to the bulk composition of the New Georgia basalt are 5-lOkb, 1000°C, 2 -5% wt. HÔ and fO^ <HM. Absence of orthopyroxene and garnet as phenocrysts provides an important constraint. Apart from decreasing mg^ with decreasing temperature (T) (or decreasing H2O content, constant T), the compositional variation (corresponding to that noted above from inclusions) in the experimentally obtained amphibole did not show any significant correlation with physical conditions. Hence closer matching of the conditions of crystallization is not possible, simply using amphibole compositions. Application of the results indicates that a hydrous basalt parent magma, derived by partial melting of a fluid-modified mantle wedge above the Benioff Zone, rises and undergoes fractionation at 20-30km depth. This depth coincides with the approximate crustal thickness in the region, and reflects the effect of a crustal "density filter", and suggests that at this stage of island arc evolution, the crust may be progressively thickened via addition of an amphibole-rich mafic component at its base, and a complementary andesitic component at the surface, as proposed for the Sunda Arc, Indonesia (Foden, 1983).

214

HIGH PRESSURE STUDIES OF MAFIC DYKES: IMPLICATION FOR THE CRDSTAL HISTORY OF THE EAST ANTARCTIC S H I E L D . S . M . Kuehner and D . H . Green Geology Department, University of Tasmania, H o b a r t . Hundreds of m a f i c d y k e s h a v e b e e n e p i s o d i c a l l y e m p l a c e d in c h e m i c a l l y d i s t i n c t s w a r m s w i t h i n the A r c h a e a n c r u s t of the East Antarctic Shield. Their emplacements are bound in time by tectono-thermal metamorphic e v e n t s at -^2500 m . y . ( 7 - 1 0 k b , 6 5 0 - 7 5 0 ^ 0 ) and at --1100 m . y . (3-5 k b , 570-670 C) (Ellis, 1983; Sandiford and W i l s o n , 1983). C h e m i c a l l y i d e n t i c a l h i g h M g t h o l e i i t e d y k e s w e r e emplaced in both Enderby Land and the Vestfold Hills (1000 k m a p a r t ) a t '"2400 m . y . ( S h e r a t o n a n d C o l l e r s o n , 1 9 8 3 ) . In t h e V e s t f o l d H i l l s , these dykes are extremely well preserved. Chilled margins often contain castellated opx + unaltered oliv in a groundmass of s w a l l o w t a i l e d , h o l l o w pigeonite. A large suite of tholeiite basalts was emplaced in the Vestfold Hills at ~1360 m . y . and is chemically similar to the ~ 1 1 9 0 m . y . d y k e s of E n d e r b y L a n d (Sheraton and Collerson, 1983). The Vestfold Hills dykes are c h a r a c t e r i s t i c a l l y p l a g + c p x + o l i v p h y r i c , w h i l e t h e Enderby Land dykes lack olivine, and are opx + cpx + plag phyric. Only one sample from the Vestfold Hills c o n t a i n s o p x p h e n o c r y s t s ( r a r e ) , a n d w a s chosen for study. Our goal in undertaking an experimental study of samples selected f r o m t h e s e s u i t e s w a s to d e t e r m i n e the p r e s s u r e a t w h i c h t h e chilled margin mineral assemblages were stable, and thus provide additional constraints on the uplift history of the East Antarctic Shield. T h e h i g h M g tholeiite studied (Mg# = 65) has oliv as the 1 atm liquidus at 1280 C , and is followed by o p x , c p x , plag between 1180 -1150 C . A b o v e 7 . 5 k b , 1280 C , opx replaces oliv at the liquidus. At 7.5 k b , the composition of- the experimental opx matches the most Mg-rich opx values in the n a t u r a l r o c k (Mg# = 8 4 , 5% W o ) . N a t u r a l r o c k o p x is z o n e d to Mg// 7 8 , and the o b s e r v e d o l i v is in e q u i l i b r i u m w i t h t h i s l a t e r c o m p o s i t i o n . These o b s e r v a t i o n s i n d i c a t e o p x is the l i q u i d u s p h a s e in t h e h i g h Mg d y k e s , followed by olivine after a small crystallization i n t e r v a l . T h i s i m p l i e s d y k e e m p l a c e m e n t a t pressures of '"8 k b . The tholeiitic basalt (Mg// = 44) has oliv as a liquidus phase at 1 a t m , 1165^C to 8 k b , 1185 C , a b o v e w h i c h o p x + c p x a p p a r e n t l y c o - p r e c i p i t a t e at the liquidus. Plag crystallizes with oliv at 1 atm from 1150^C to llOO^C, followed by c p x . The a s s e m b l a g e o l i v + p l a g + cpx only exists at pressures less than 5.5 k b in the studied composition. At 4 k b , 1120^C, opx crystallizes, 35^ below the onset of cpx c r y s t a l l i z a t i o n . Comparison between natural and synthetic mineral compositions is hampered by the replacement of oliv in the natural rock by opx + m a g s y m p l e c t i t e s , a n d in the e x p e r i m e n t a l r u n s o p x + cpx intergrowths. The results of our study indicate that both the Vestfold Hills a n d E n d e r b y L a n d w e r e a t p r e s s u r e s of '"8 k b at '"2400 m . y . , consistent with previously determined conditions of peak m e t a m o r p h i s m a t 2 5 0 0 m . y . in t h e s e a r e a s . T h e o p x + c p x + p l a g p h e n o c r y s t a s s e m b l a g e of the 1 1 9 0 m . y . d y k e s in Enderby Land also indicate emplacement pressures >8 k b , and implies Enderby L a n d r e m a i n e d n e a r the b a s e of the A n t a r c t i c crust for nearly 2000 m . y . ('"3100 m.y.-1200 m . y . ) . Roughly 14 km of u p l i f t is r e q u i r e d b e t w e e n t h e t i m e of d y k e e m p l a c e m e n t a t 1190 m . y . and metamorphism at 1100 m . y . The Vestfold Hills record a slightly different history in that the o l i v + p l a g + c p x + ( o p x ) phenocryst assemblages require emplacement pressures <5 kb at 1360 m . y . It is likely that this terrain has undergone slow u p l i f t f r o m 8 kb to <5 kb over a period of 1000 m . y .

215

METAMORPHISM AND MELTING IN THE EASTERN HARTS RANGE, NORTHERN TERRITORY R.L. Oliver, J.D. Foden and S.J. Sullivan Dept of Geology and Geophysics, University of Adelaide The Entia Dome is a basement gneiss complex of early Proterozoic age and is part of the eastern Arunta Inlier. Rocks of the Entia Dome belong to the Entia Gneiss Complex which is composed of complexly deformed quartzofeldspathic gneisses, amphibolites, pelitic gneisses, calc silicate gneisses and a range of orthogneiss intrusive rocks ranging in composition from quartz diorite to granite. On the basis of observations throughout the dome, the peak of metamorphism appears to have occurred after the intrusion and first deformation of the major granite bodies and involved the widespread development of kyanite in conjunction with local melting of quartzo-feldspathic and pelitic gneisses to yield migmatite. These migmatites are of compositions close to minima in the Q-Ab-Or system; temperatures and pressures at the peak of metamorphism were respectively about 700°C and 7 kbar. Particularly in the pelitic gneisses, melting has proceeded partly by the breakdown of muscovite to produce a restite which contains an aluminosilicate (kyanite). The complete elimination of melts in the OrAb-Q system from these assemblages results in the formation of the refractory assemblage biotite-kyanite-quartz (+/- cordierite, garnet, gedrite, corundum, staurolite). These dominantly biotite-rich schists are widespread throughout the entia Dome and paragenetic relationships involving aluminous phases in these provide useful information about the pro- and retrograde paths of metamorphism in the east Arunta Block. In this paper we describe the petrology of one specific occurrence of this biotite schist lithology from the N.E. corner of the Entia Dome. This lies approximately 10 km to the north of Inkamulla Bore. This locality was the subject of previous investigation by Dobos (1978) and by Green and Vernon (1974). Here early kyanite-gedrite assemblages are successively replaced by first cordierite (+/- corundum) giving the assemblage kyanite - gedrite - cordierite and then by staurolite to give the assemblage kyanite - gedrite - staurolite. It is suggested that the sequence of metamorphic assemblages described above implies a polybaric cooling path, best interpreted as a segment of a clockwise P-T-time path. As calculated maximum temperatures are no higher than 750°C it is thus not possible for the more mafic granitic rocks in the Entia Gneiss Complex (quartz diorite to granodiorite) to have been produced by local crustal fusion. This implies a further 10 to 20 km of additional crust beneath the Entia Dome and an overall crustal thickness at that time in excess of 50 km.

216

ROLE OF SUPERHEATED MAQIA IN THE FORMATION OF ORBICULAR GRANITOIDS R.H. Vernon School of Earth Sciences, Macquarie University, Sydney Orbicular structures in granitoid plutons typically consist of cores (enclaves) of various rock-types, around which are concentric shells of rhythmically precipitated mafic and felsic layers dominated by radial and tangential arrangements of elongate crystals. The orbs thus contrast strongly with the enclosing granitoids, which have typical granitic microstructures with randomly distributed crystals. Orbicular shells and comblayering have similar features, and both require absence or paucity of nuclei in the magma. This permits a significant degree of undercooling in the plutonic environment and forces crystallization to occur only on available solids, namely the walls of the magma chamber or enclaves, if present. Experiments have shown that superheating is an effective mechanism for destroying nuclei, and so orbicular granitoids may have crystallized from formerly superheated magma. Effective superheating is most probably caused by injection of water into a felsic or intermediate magma. The rarity of orbicular granitoids implies that superheating situations are also rare, most granitic magmas carrying sufficient nuclei to permit normal crystallization. Some orbs in granitoids show evidence of synmagmatic deformation-and disintegration.

217

ABSTRACTS OF TECHNICAL POSTERS

218

BMR EARTH-SCIENCE RESEARCH J.S. Adkins Bureau of Mineral Resources, Geology and Geophysics, Canberra The synthesis of existing knowledge, and the development, by strategic research, of the framework within which successful exploration programs can be mounted, is the principal aim of Commonwealth government-sponsored research in the geological sciences. The series of posters presented here focus on various research activities of the Bureau of Mineral Resources, Geology and Geophysica (BMR). Essential characteristics of BMR's research are: 1.

It is national interest research, the results of which should be in the public domain.

It is strategic, rather than tactical. than that undertaken by industry.

It is designed to ensure that Australia is in the forefront internationally in the understanding of the occurrence of its mineral resources and of its resource potential.

It is on a longer time-scale

The posters cover several important research topics including Continental margins research (geological and geophysical studies of Australia's vast and largely unexplored offshore area) Proterozoic basins (and the hydrocarbon potential of basins such as the McArthur Basin) Earthquake risk in Australia Petroleum and mineral genesis in sediments Studies on the controls of the concentration and distribution of mineralisation in the Mount Isa Province Hydrogeology research Geological applications of remote sensing Late Palaeozoic/Mesozoic basin studies (including detailed analysis of the Clarence-Moreton Basin) Airborne geophysical coverage of Australia Murray Basin Research.

219

A VIDEO PRESENTATION ON THE GEOLOGICAL SETTING OF ANCIENT DETRITAL ZIRCONS WITHIN THE JACK HILLS METAMORPHIC BELT, W.A. John L. Baxter, Simon A. Wilde, Robert T. Pidgeon and Lindsay B, Collins Western Australian lastitute of Technology, Perth The discovery that metasediments in the Jack Hills Metaraorphic Belt contained very ancient detrital zircons, comparable in age (c 4,1 Ga) to those in the Narryer Metamorphic Belt, has inspired a detailed Ftudy of the belt. As a progress report, a video has been prepared describing aspects of the geology of the Jack Hills Metamorphic Belt. The Jack Hills Metamorphic Belt is situated in a major shear zone at the northern margin of the Yilgarn Block within the Western Gneiss Terrain. It contains a low grade suite of metasediments which have been intruded by mafic and ultramafic units. The distribution of rock units in the belt is complicated by the development of folds and shear zones, but appears to consist of a lower para-amphibolite unit overlain by semi-pelitic schists. These pass into a fining-upward sequence of fluvial sandstone and conglomerate; the apparent top of the succession is banded iron formation. The metamorphic grade of the sequence is upper greenschist facies which is in marked contrast to the granulite facies of the Narryer Metamorphic Belt to the west. Within the sedimentary succession there are a number of weakly deformed sequences from which detailed sedimentological data have been obtained. The sedimentary sequence appears to represent a transition from below wave base, with a substantial sediment input (the pelitic and semi-pelitic schists), to a shoreface and an aggrading sandy alluvial plain (sandstones and conglomerates). There was then a retreat below wave base, with low sediment supply and chemical precipitation (phyllite and banded iron formation). It is from within one of the conglomerate units that the ancient detrital zirons have been obtained. Archaean to Proterozoic deformation events have affected the Jack Hills Metamorphic Belt. Five phases of deformation have been recognised. These are :D^ Isoclinal folds associated with a strong mineral elongation lineation. D2 Open to tight minor folds with a prominent axial surface fabric producing an intersection lineation, indicating that a regional anticline is the principal geometric control on the belt. D3 Gentle to open sub-horizontal folds. D^ Shear zones that cut the belt and the enclosing granitoids. These form conjugate planes of failure, and may form shear folds in adiacent units. D. Localised kinking and buckling. The relationships between deformation, metamorphism and granitoid emplacement are complex. There appear to be two ages of granitoid in the district based on U-Pb determinations on zircons; c 3.5 Ga has been obtained from sheared gneissic granitoid, whilst £ 2.75 Ga is recorded from undeformed granite. No definite age for the supracrustal sequence has been determined, although a Sm-Nd age (T-chur) of 3.68 Ga was obtained from a probable metavolcanic. The presence of old zircons in conglomerate within the supracrustals, as reported by Dr's Compston and Pidgeon in "Nature", indicates a primitive Archaean contribution to these rocks similar to that contributing to the metasediments at Mt. Narryer. These two areas contain the oldest crystals so far identified on Earth. 220

STURTIAN GLACIATION, IRONSTONE DEPOSITION AND PENECONTEMPORANEOUS FAULTING, NORTHERN YEDNALUE ANTICLINE, CENTRAL FLINDERS RANGES, SOUTH AUSTRALIA G. Circosta, D. New and V.A. Gostin Geology Department, University of Adelaide, S.A. The Sturtian sequence rests on a major unconformity cut across a tilted and faulted sequence of Burra Group dolomites, siltstones and quartzites. Sturtian sediments thicken over a distance of 12 km from a few tens of metres in the southwest to 1 600 metres in the northeast, and form the southwestern edge of a deep Sturtian sedimentary basin with most of the sediment apparently being derived from the west. The earliest Sturtian deposits of the Pualco Formation ("Tillite") consist mainly of siltstones, some sandstones and lenticular diamictites containing striated clasts. These glacigene diamictites increase in abundance towards the western source. This was succeeded by a probable lacustrine environment which gave rise to mixed chemical and clastic, iron-rich and siliceous sediments of the Holowilena Ironstone. Penecontemporaneous erosion and redeposition formed various calcareous and ferruginous sandstones with jasper and other fragments indicating early selective lithification. In the west, basinward downfaulting along preexisting faults was accompanied by removal of sediments, and preservation of a small syncline of Pualco Formation deposits against a fault, while in the east, deposition continued with a conformable sequence of the Wilyerpa Formation. Minor faulting persisted during deposition of the lower part of the Wilyerpa Formation when ferruginous, dolomitic siliceous silt matrix diamictites were deposited by mass flow mechanisms down the palaeoslope. These diamictites wedge-out eastwards and are replaced by laminated siltstones, some graded sandstones, and rare conglomerates. The major part of the Wilyerpa Formation consists of laminated siltstones with varying proportions of ice-rafted megaclasts. Some massive sandstones were deposited probably by mass flow mechanisms. A 90 metre long sandstone dyke originating in the Pualco Formation passes upward into the basal Wilyerpa Formation, showing that part of the early Sturtian sequence was still unlithified, and that only a short time interval was involved in the local unconformities. The Wilyerpa Formation wedges out south-westward against the basin edge where it is unconformably overlain by shales of the widespread Tapley Hill Formation, whereas in the east, a conformable sequence is present. All tectonic activity had apparently ceased in the area during the deposition of the Tapley Hill Formation.

221

DEPOSITIONAL TECTONICS OF A SYNSEDIMENTARY GRABEN WILKAWILLINA GORGE, SOUTH AUSTRALIA J.D.A. Clarke Flinders University of South Australia, Adelaide INTRODUCTION The late Precambrian to early Cambrian sediments of the Flinders Ranges of South Australia contain many examples of diapirically controlled sedimentation. The Bunkers Graben lies on the eastern margin of the Flinders Ranges and is a good example of diapirically generated graben formation in the carbonate dominated early Cambrian Hawker Group. Detailed mapping of the structure has revealed both the episodic nature of graben subsidence and the control that this had on depositional facies. STRATIGRAPHY The Hawker Group within the Bunkers Graben is comprised of five main units. The Wilkawillina Limestone (peritidal-shallow marine carbonates 230-381 m thick, on the south margin developed into a small platform interfingering with deeper water facies in the graben centre). The Parara Limestone (slope facies 60-802 m thick, pinches out on platform). The Bunkers Sandstone (shallow marine tidal sand deposit, occurs only within the graben, thickness 0-319 m and onlaps platform to south.) The Oraparinna Shale (basinal-lagoonal shale, pinches out in southern platform, 103-500 m thick). The Edeowie Limestone is thin (6-15 m) supratidal dolomite. PALAEOGEOGRAPHY Structural evidence suggests that the graben was elongate along a northeast-southwest axis. Palaeoccurrents within the Bunkers Sandstone indicate a bimodal current flow parallel to this axis. The south westerly currents on the north flank of the graben and the north easterly currents on the south flank indicate an anti-clockwise tidal circulation. This configuration indicates that the graben was open to the north east, and closed to the south west, possibly by the rising diapir. TECTONIC HISTORY Tl. represents the closing phases of deposition of the Wilkawillina Limestone. Subsidence is low, and greatest slightly south of the graben axis. Subsidence also occurred during deposition of the preceding Pound Subgroup, as indicated by synsedimentary micro-grabens, but was likewise low. Scarp-derived clasts of Pound Subgroup occur in the Wilkawillina Limestone along the southern boundary fault. T2: there has been a major increase in subsidence, centred to the north of the graben axis. This, coupled with a major transgression, has resulted in the deposition of the deeper water slope facies of the Parara Limestone. Along the southern margin, reduced subsidence and a near-shore locality has resulted in the growth of a small carbonate platform. T3: subsidence remains rapid, but the centre is beginning to move south. A regression has resulted in the exposure of the platform and the deposition of the Bunkers Sandstone. T4: due to a rise in sea level the Oraparinna Shale is deposited in the graben, interfingering with the biohermal facies on the rejuvenated platform. Subsidence is now greatest well to the south of the axis of the graben. Further scarp-derived breccias were deposited along the southern margin of the graben. T5: after a further low in sea level and the deposition of the peritidal Edeowie Limestone, sedimentation of the clastic Billy Creek Formation was now taking place. Subsidence was now non-existent. T6t During the Ordovician Delamerian Orogeny the graben was folded into a north easterly plunging syncline, accompanied by reversal of many of the boundary faults. 222

LARGE SCALE SLUMPING IN THE UMBERATANA GROUP, WILLOURAN RANGES R.P. Coats^ and R. Dalgarno^ ^Formerly Geological Survey of South Australia, Adelaide ^Consultant Geologist During regional mapping for the SA Geological Survey in the 1960's,one of us (RPC) noted extensive examples of sedimentary breccia in large scale erosional scours at the base of the Amberoona Formation in the Willouran Ranges. Helicopter sampling for Anaconda Aust Inc. in 1966, together with field visits for SADME in 1980 gave opportunity for rapid examination and photography by R.D. Mapping for thesis presentation by Murrell provided a record of a series of these slumps in a structural basin near the intersection of the Bungarider and South Hill Faults. The Umberatana Group in the Willouran Ranges is marked by unconformity representing early or pre-Sturtian fault block movements on the series of structures paralleling the Norwest Fault. Angular truncation of the Burra Group by the Sturtian glacials occurs locally adjacent to faults in the three synclines preserved between the South Hill Fault and Willouran Hill in the southeast corner of CURDIMURKA 1:250 000 sheet (unpub). The palaeogeography of the Sturtian in the northern Flinders Ranges would imply both eastern and western sources for the Sturtian glacio-complexes with a deeper water facies dominant in the axial trough of the COPLEY Sheet area. Northwest of West Mount Hut to beyond Mt Nor'West on the western side of the Willouran Ranges, the Amberoona Formation erodes the Tapley Hill Formation in a broad scour approximately 15 km in width. Locally near Mt. Nor'West iself, the Amberoona Slump rests on the upper part of the Sturtian glacials. Directional structures observed in this area indicate an easterly trend of slump transport. The features resembles broad slumps and scour features on low angle slopes off the present delta area of the Mississippi. In the three synclines of Marinoan preserved within the Willouran Ranges, (viz. adjacent to the South Hill Fault; in the Kingston Dam synclinal structure; in the faulted repetition to the north along Mirra Creek) multiple slump units occur over several hundred metres vertically in the basal part of the Amberoona Formation. Near the South Hill Fault some 5 km south of Chintapanna Dam, six major debris slumps are recorded by Murrell eroding to the level of the Tindeplina Shale Member of the Tapley Hill Formation. The most easterly record of disturbance in the Amberoona Formation is in the vicinity of North Well on the Willouran Creek (MARREE), and extending north and south some 10 km along strike (Forbes, pers. comm.). Evidence for such slope instability in the west near the Torrens Hinge Zone is afforded by disconformity between Amberoona Formation and Sturtian tillite near Lake Pidleeominna. The Amberoona slumps may have been triggered by seismic activity in unconsolidated sediments, generated by movements on the fundamental NW to N-S faults such as the Torrens Hinge Zone, Bungarider and East Willouran Faults.

223

^OUNDWATER RESOURCES OF TBE BAROSSA VALLEY, SOUTH AUSTRALIA M.A. Cobb Water Search, Angaston South Australia The Barossa Valley is a north-south fault controlled arcuate and asymmetric depression, deepest on its eastern side, and infilled with up to 140 metres of sediment. The sediments range from the Tertiary basal clean sands and gravels up through carbonaceous and non-carbonaceous clays and sands to Quaternary clays and silts with discontinuous ferruginous gravel layers. Rapid lateral and vertical facies changes makes the definition of unique aquifers difficult. However, for the sedimentary sequence, three aquifers can be generalised consisting of a basal gravel/sand, a middle sand and an upper discontinuous gravel. The water table is found in a range of sediments depending on location. Water quality in each of the aquifers is variable, with the basal and middle aquifers containing the better quality water (1000-3000 mg/L). The upper gravels are somewhat saline whereas the water table shows the widest range in salinity from less than 1000 mg/L up to 14 000 mg/L. Despite the marginal quality of the groundwaters, they are extracted for irrigation of vines and vegetables. In 1979/80, groundwater withdrawal was estimated at 1.6 x 10^ m^ and a tentative water budget showed a near balance. Observations to that date revealed that water levels in all aquifers appeared to recover each year. Similarly at that time there was no conclusive evidence that groundwater salinities were rising on a regional scale. However, the overall area of crops irrigated within the Barossa Valley continued to increase with groundwater withdrawals supplying most of this additional irrigation water. This increase is now being reflected in declining water levels in most aquifers and an indication of regional increases in salinities (although water quality observations have only been made for a relatively short time). Formal water resource management policies may be required to ensure the availability of groundwater of useable quality for the future. The thirty-odd wineries and distilleries within the Barossa Valley produce 450 000 m^ of waste annually. Present practice relies heavily on extended storage of wastewater in anaerobic lagoons with authorised discharge to rivers during periods of high river flow. There is some minor use of the wastewater for irrigation of vines and pasture adjacent to the wineries. It is of interest to note that this volume of wastewater is equivalent to a significant portion of the groundwater extracted for irrigation. Whilst the discharge of wastewaters to streams has a marked short term effect on the quality of their waters, such action does not impinge on the local groundwaters. However, drainage from the Valley catchments does contribute to recharge of the Northern Adelaide Plains groundwater system further downstream. Studies are underway to evaluate alternative methods of winery waste treatment and disposal. The use of the wastewaters for irrigation to replace some of the pumped groundwaters is not being considered as a serious option.

224

JOHANNES MENGE (1788-1852) - SOUTH AUSTRALIA'S FIRST GEOLOGIST B.J. Cooper^ D.W. Corbett^ and P.A. Rogers^ ^South Australian Department of Mines and Energy University of Adelaide With South Australia celebrating its 150th anniversary in 1986, it seems appropriate to remember Johannes Menge, the State's first resident geologist. Menge was born at Steinau, Germany in 1788 and was largely self educated. Settling in London in 1830, he was appointed "Mine and Quarry Agent and Geologist" to the South Australian Company before formal establishment of the Colony. Menge arrived in South Australia in January 1837 being among the first German settlers in a State later noted for its German heritage and two years before the arrival of his famous contemporary in New South Wales, W.B. Clarke. Of necessity, Menge became an explorer in the little known bush of South Australia ranging widely and often alone from Kangaroo Island to the Flinders Ranges and east to the Murray River. As early as 1838, Menge recognised the mineral potential of the Adelaide Geosyncline and began using his reputation to stimulate mineral search. He foresaw the spectacular copper discoveries, which were to take place at Kapunda and Burra during the following decade, and even went so far as to propose a Mining College and an independent Mining Association to promote exploration. After Menge left employment with the South Australian Company in 1838 he became an independent spokesman on geological matters. As early as 1841, he had ambitions of becoming "Colonial geologist", but given the economic situation at the time, this never eventuated. In 1839, it seems that Menge was the person or one of several German Colonists who found the fertile lands of the Barossa Valley. Later, he predicted its potential for growing vines! About the same time, Menge discovered Australia's first known opal deposits in the same area. Menge's publications on South Australian geology are limited. A series of 10 articles entitled "The Geology of South Australia" appeared in The South Australian Register, a local newspaper during 1841. No scientific journals were then published in South Australia. Another important work is his Mineral Kingdom of South Australia, a separate pamphlet, describing 500 mineral and rock specimens, also published in 1841. Among South Australia's early immigrants, Johannes Menge was a legendary figure. He was one of the few German intellectuals, being fluent in numerous languages and highly knowledgeable in religion and philosophy, in addition to geology. Colonists even recalled him 50 years after his death. Within the geological profession, Menge probably lacks due recognition. His eccentric habits coupled with a geological philosophy that was out of date during his lifetime contribute to this situation. The departure from South Australia of virtually all geologists as a consequence of the Gold Rushes also meant that South Australian geology needed to be reborn in the 1870s. Like many others Menge left South Australia for the Victorian Goldfields in 1852. He died 10 months after his arrival at Forest Creek (Castlemaine). In 1859, William Cawthorne published an account of Menge's unusual life. It was the first biography of an Australian geologist. 225

EDIACARAN EXTENSIONAL FAULTS, MOUNT FROME R. Dalgarno Consultant Geologist, Adelaide Unconformity exposed along the east flank of the Mount Frome Diapir has been mapped in outline but detailed stratigraphic and sedimentological studies should be pursued. The region presents an opportunity to study details of progressive unconformable onlap of the Ediacaran sequence from Black Oak Well in the south, continuously for 5 km to the north. The Wonoka Formation is overlapped with slight angular discordance by the Bonney Sandstone in two areas 1 km N and 3-4 km NNW of Black Oak Well. The total Precambrian sequence is locally reduced to less than 100 m thickness and shows considerable variation along strike. Basal conglomerates of the Bonney Sandstone resting on the diapir illustrate penecontemporaneous extensional faulting which is illustrated by the poster. Growth faults range in throw from a few metres to the order of 100 metres with bedding plane splays resulting in zero displacement within the overlying Cambrian limestone sequence. Aligned clasts within the breccia body below the unconformity are consistent with either later viscous flow or penecontemporaneous decollement planes which would have permitted the low angle, rotational detachments of the cover. Pull-apart features observed are prograded by the Bonney Sandstone resulting in lenses of conglomerate which thicken into the faults and display ductile rollovers and flexural folds characteristic of growth structures. Some brittle deformation involves rupture of the basal conglomerate lenses by breccia. Detailed studies may reveal that this represents ductile piercement by a "mud diapir" following synsedimentary faulting. Dr T. Mount, on a field visit to the area during 1985 with R. Dalgarno and Y. Bone, observed halite casted siltstone fragments within the breccias at the base of the Bonney Sandstone at G.R. 56250N 29100E some 750 m NNW of Black Oak Well. Their lithological similarity to clasts in the Blinman and Arkaba Diapirs suggests stripping of a similar source within the Frome Structure during the Ediacaran. More detailed study of the conglomerates in this region should provide valuable sedimentological information. The history of tectonic activity at the Mt Frome Structure is traced from conglomerate bands and spectacular slump folds in the Brachina Formation on the west flank of the dome (GR 57500N, 25800E). Later piercement or diapiric intrusion is demonstrated by a sharp intrusive contact of carbonate breccias in a classic exposure at GR 56900N, 27200E. These localities have been recorded as Geological Monuments due to their significance in the understanding of the Flinders Range diapiric province.

226

MIDDLE PROTEROZOIC SEDIMENTS OF THE GAWLER CRATON S.J. Daly, A.J. Parker and R.B. Flint Geological Survey of South Australia, Adelaide The oldest sediments which postdate the Hutchison Group outcrop in the southeastern portion of the Gawler Craton, at Moonabie Range and Whyalla. The Moonabie Formation was deposited in shallow, probably discontinuous basins with associated abundant volcanic activity. Sediments are predominantly quartz sandstone with local heavy mineral bands and massive volcanoclastic grit. The grits contain abundant glassy quartz, chert fragments and angular pebbles and cobbles of basalt to rhyolite. At Moonabie Range these sediments are interbedded with vesicular basalt, porphyritic dacite and tuffaceous rhyolite and rhyodacite (ca 1740Ma). Gentle folding or tilting of the Moonabie Formation occurred prior to the deposition of the Corunna Conglomerate which outcrops discontinuously in the southern Gawler Craton at Talia, Uno, Corunna and in Blue Range and Moonabie Range. The basal conglomerate facies contains cobbles and pebbles of local crystalline basement, acid volcanics and mylonite and is overlain by ripple-marked and mudcracked siltstones with interbedded thin rhyolite or rhyodacite tuffs, probably part of the lower Gawler Range Volcanics. At Corunna a thick alluvial fan conglomerate, the Red Conglomerate Member, overlies the siltstone facies to form the prominent cliffs of Corunna Range. The Red Conglomerate was eroded and overlain by crossbedded coarse sandstone and conglomerate. The youngest unit seen at Corunna is a well sorted, crossbedded, quartz sandstone, possibly deposited in a marine environment, interbedded with greenish siltstones. At Uno a similar sequence is overlain by at least 300 m of laminated carbonaceous siltstone and sandstone, locally carbonate-rich with inferred altered tuffaceous interbeds. The Corunna Conglomerate has been gently folded and is intruded by rhyolite dykes and plugs and, in the Moonabie Range, by the Charleston Granite (ca 1580Ma). The Tarcoola Formation, which outcrops in the Tarcoola-Kingoonya area, has been subdivided into three members. The lowest unit, the Peela Conglomerate Member, was probably deposited in a shallow fluvial environment and contains abundant volcanic and crystalline basement clasts, thin rhyolite tuffs and basalt flows. Overlying the conglomerate facies are claystones and quartzites, the Fabian Quartzite Member, which contains rare mudcracks, load casts, thin carbonates and stromatolites. These bony quartzites are the most commonly seen outcrop and may have been deposited in a shallow marine environment. In the upper part of the section the quartzite intertongues with black and greenish carbonaceous clayey siltstones, the Sullivan Shale Member. This unit outcrops poorly and is best seen underground at the Tarcoola Blocks Mine or in SADME Wilgena 1 drillhole where it is over 713 m thick. The siltstones contain very fine tuffaceous debris and lithic fragments which vary in composition between rhyodacite and basalt, although the debris is now highly altered. Wilgena 1 also intersected basaltic sills indicating probable sea floor vents. The Sullivan Shale was deposited in a marine or lacustrine environment and is strikingly similar to the Uno Shale Member of the Corunna Conglomerate The Tarcoola Formation and interbedded volcanics, considered a correlative of the lower Gawler Range Volcanics, have been folded, subsequently overlain by the upper Gawler Range Volcanics, and finally intruded by granite of the Hiltaba Suite and dykes of the Wiltabbie Volcanics, the latest volcanic event in the Craton. 227

WONOKA FORMATION SEDIMENTATION, RELATIVE SEA-LEVEL, AND BASIN ANALYSIS IN THE NORTHERN FLINDERS RANGES, SOUTH AUSTRALIA P.A. Dibona and C.C. von der Borch Flinders University of S.A., Adelaide The late Proterozoic Wonoka Formation (Wonoka) in the northern Flinders Ranges, South Australia, represents a major transgressive-regressive cycle which includes the overlying Bonney Sandstone Member of the Pound Quartzite. In interpreting this cycle we have found that relative time surfaces, associated with certain "stratigraphic events", can possibly be located and correlated basinwide. Our analysis based on seismic-stratigraphic concepts utilizes unconformities and periods of non-deposition or low sedimentation, condensed sections (CS), as time surfaces. These unconformities and condensed sections (CS), are caused by relative sea-level changes; a combination of subsidence, eustatics, sedimentation rates, tectonics, and sediment supply.Based on a regional study of the Wonoka in the northern Flinders Ranges we chose two localities for testing the possibility of correlation of time surfaces. Using these two localities, one of which is a unique exposure of a complete shelf to slope transition, the other a representative stratigraphic section, we were able to propose two time surfaces within the Wonoka. The first time surface is at the base of the Wonoka overlying the Bunyeroo Formation. At this contact there is a thin dolostone, which can be traced basin-wide, and associated shallow-water carbonates. Above this sequence the Wonoka grades upwards into calcite-bearing mudstones and siltstones with interbedded carbonate turbidites. This fine-grained sequence is then gradationally overlain by storm/current deposited sandy limestones. This succession, from above the basal dolostone, represents a major transgression basin-wide. Submarine canyon infilling and slope-shelf facies progradation is associated with this transgression. The seond time surface we define occurs at a thin, widespread, glauconitebearing, carbonate/siliclastic unit that occurs within the Wonoka. This unit is found over a large area of the Wonoka paleo-shelf. At one locality this same unit can be traced across the shelf to the shelfbreak and onto the slope. On the slope this unit was deposited as debris flows which overlie a major slope truncation. We interpret this unit as a condensed section which formed during starved sedimentation conditions. This event occurred basin-wide and may be a useful time surface for correlation of events from the shelf to slope environments. Above this unit is a quartzite, variable in thickness across the basin, with sedimentary structures indicative of a shallow—water environment. Overlying the quartzite the facies are similar to the storm/current sandy limestones below in the Wonoka. This "return" to outer—shelf facies may reflect either a short-term relative sea-level rise or the beginning of the Wonoka regression. The latter could be marked by very high siliciclastic sedimentation rates as the quartzite was deposited and then a return to normal sedimentation rates. In either situation the overall regressive cycle continues throughout the upper Wonoka to a gradational contact with the shallow-water-deposited Bonney Sands. In studying the overall facies succession within the Wonoka a major transgressive-recessive cycle is inferred with variations in this "first-order" cycle caused by fluctuations in sedimentation rates, tectonics, and eustatic changes. However, within this major cycle, certain stratigraphic events (time surfces) associated with changes of relative sea-level can be correlated from shelf to slope environments across the basin. 228

MOUNT PAINTER BRECCIAS J.F. Drexel and R.B. Major South Australian Department of Mines and Energy The Mount Painter Block, in the northern Flinders Ranges in South Australia, is about 550 km north of Adelaide. The lower to middle Proterozoic Mount Painter Complex basement is composed of metasedimentary gneisses and schists (Radium Creek Metamorphics), granites and amphibolite dykes. The Complex was folded and metamorphosed prior to deposition of the late Proterozoic Adelaidean System. The Complex and Adelaidean sediments were folded and metamorphosed by the Cambro-Ordovician Delamerian Orogeny, and intruded by pegmatites c.460 Ma. Large volumes of breccia occur along a broad wedge-shaped northeasterly trending zone from Radium Ridge to Hematite Valley, a distance of 20 km. The breccias are of several types and origins. The most abundant are massive granitic breccias which resulted from pervasive fracturing of the crystalline basement by hydrothermal fluids. Included in this type are irregular and discontinuous pods of clastic sediments and uranium-bearing hematitic layers and dykes that show both sedimentary and intrusive characteristics. Numerous linear and generally northeasterly trending fault breccias cut through the basement, and many merge into the margins of the massive granitic breccias. Potash metasomatism affected the basement prior to and during brecciation and has completely replaced the original rock in some areas. The best examples of potash metasomatism are found within fault breccias where they merge with the massive granitic breccias. The faults were presumably fluid feeders to the massively brecciated areas. All varieties of breccia are found overlain or intruded by quartz-hematite rocks of the Mount Gee unit. Estimation of the age of massive brecciation is based on: the breccias, and particularly the included pods of fine to mediumgrained sediment, show no evidence of metamorphism by the Delamerian Orogeny which affected other rocks on and around the Mount Painter Block a dyke-like body of hematitic breccia within granitic breccia contains clasts of pegmatite which has been equated to the 460 Ma Arkaroola Pegmatite monazite and samarskite from the hematitic breccias have been dated at 440±50 Ma and 400±50 Ma respectively. The evidence indicates that the main phase of hydrothermal activity and brecciation probably occurred in the late Ordovician. Relatively passive hematite-quartz-uraninite fluids were emplaced along more porous zones of the granitic breccias during and after their formation. The occurrence of a kaolinised palaeosurface on basement below the quartzhematite Mount Gee unit on Mount Gee and Mount Painter suggests that a second period of hydrothermal-hot spring activity may have occurred as recently as the Tertiary.

229

GYPSUM IN SOUTH AUSTRALIA E.A. Dubowski and L.C. Barnes South Australian Department of Mines and Energy Gypsum is widespread in South Australia in lake or dune deposits. There are two types of lake deposit, coastal salinas in which deposition began 6 000 years ago and in a few places continues today, and inland playa lakes where deposition began 18 000 years ago. Coastal salinas are Holocene groundwater lakes in interdunal corridors of an extensive Pleistocene dune system. These corridors flooded following the rise in sea level at the end of the last glacial stage and within resulting brine lakes, gypsum deposition followed one or two patterns. In lakes where salinity changed rapidly, e.g. Streaky Bay, laminated gypsarenite was deposited in 1-2 mm thick beds of sand-sized euhedral gypsum prisms with laminae of aragonite pelletoids. In larger deeper lakes little affected by input of meotoric waters, poorly layered domes of gypsum, about 0.5-1 m across were deposited in the deeper parts. Selenite domes pass upwards into laminated selenite formed when selenite crystals grew upwards with long axes of the crystals at right angles to bedding. Selenite crystals up to 2 m long enclose aragonite laminae. As the lake tilled, each freshening by meteoric water increasingly controlled sedimentation, growth of selenite crystals ceased and laminated selenite gave way to laminated gypsarenite. Eventually, the brine pond became ephemeral, chemical sedimentation was intermittent and surface gypsarenite was subjected to mechanical breakdown. In inland lakes, sedimentation was similar but domal and laminated selenite is not present, laminated gypsarenite comprises layers of subhedral to anhedral gypsum crystals with clay and aragonite laminae. In coastal salinas and inland lakes, the upper part of laminated gypsarenite has been subjected to aeolian reworking forming cross—bedded gypsarenite dunes, generally on the eastern side of the lake. Vegetationcovered dunes are capped by gypsite, formed by rapid dissolution, and redeposition of gypsum in the zone of soil moisture. Lake deposits are the principal suppliers to the plaster and cement industries. However, gypsum from coastal salinas has to be washed or allowed to leach to remove salt. Annual output from the main lake deposits is; Lake MacDonnell - 750 OOOt, reserves exceed 500 million t. New Lake, KI - 96 OOOt, worked out by 1986 Blanchetown - 40 OOOt from an inland playa. Dunes are generally lower in salt impurity and used mainly in agriculture, although Lake Fowler produced 9 500t in 1984 for cement manufacture. Dunes are also worked at Morgan, Cooke's Plain, Rotten Lake, Buckleboo and Bumbunga.

230

SEDIMENTOLOGY OF EARLY CAMBRIAN MARINE CARBONATES, NORTBDEASTERN OFFICER BASIN John N. Dunster Comalco Aluminium Limited, Glenside The 'Ouldburra Formation' is a newly recognized unit in the Northeastern Officer Basin, South Australia. The Formation reaches a maximum thickness of 1 300 m. It consists of mixed carbonate/siliciclastics, marine carbonates and evaporites which were deposited during a series of transgressions and regressions of an epeiric sea and on an intermittently emergent flanking sabkha. Though only sparsely fossiliferous, an Early Cambrian age has been established for the Formation. The 'Ouldburra Formation' is underlain by the Murnaroo Sandstone overlain, sometimes disconformably, by the 'Oongudinna Formation'.

and

The basal 'Ouldburra Formation' is typified by halite and sandstone grading up to stacked sand/silt/mudstone sets which become increasingly calcareous. These were deposited in small isolated salinas or playa lakes on a mixed siliciclastic/carbonate mudstone tidal flat. These units are overlain by a thick sequence of calcareous and dolomitic carbonates with minor and sporadic clastics and gypsum/anhydrite interbeds. This package is the dominant and most widespread unit in the 'Ouldburra Formation' and often displays cyclicity of sedimentation. Deposition occurred during a series of transgressions and regressions of restricted shallow marine waters over a flanking carbonate sabkha. Ooid shoals, carbonate mud mounds and domed algal bioherms were developed offshore. The top of the Formation is typically an interdigitation of laminated carbonate mudstones and 'red bed' siltstones with abundant nodular gypsum and bedded 'chicken-wire' anhydrite. These units represent a final regression and were deposited on a sabkha which contained playa lakes and evaporite pans.

231

CEOLOGY OF THE LATE PRECAMBRIAN BRACHINA SUBGROUP AT HALLETT COVE, SOOTH AUSTRALIA I.A. Dyson School of Earth Sciences, Flinders University of S.A., Adelaide The Brachina Subgroup is the lower of two upward-coarsening cycles within the late Precambrian (Adelaidean) Wilpena Group in the Flinders Ranges of south Australia. This succession of doniinantly terrigenous clastics was deposited within the Adelaide Geosyncline and rests conformably above what have been interpreted as glacio-fluvial and glacio-lacustrine sediments of the Reynella Siltstone at the top of the Umberatana Group. The top of the Brachina Subgroup is marked by the Precambrian-Cambrian u n c o n f o r m i t y 25 km to the south of Hal let Cove at Sellick Hill Extensive outcrop of the Brachina Subgroup occurs along the shore platform and coastal cliffs between Hallett Cove and Port Stanvac. The base of the Brachina Subgroup is defined by the Seacliff Sandstone which IS interbedded with numerous thin dolomites. This unit is interpreted to have been deposited in a shallow shelf to proximal slope environment. It overlies the Reynella Siltstone with a sharp, apparently conformable contact. The Reynella Siltstone is characterised by broad fluvial channels, pebbly mudflows, possible pedogenic carbonates, replaced evapontes and stromatolites, interbedded with massive red-brown siltstones. It is interpreted to represent a distal alluvial fan sequence, p e l o w e r m o s t d o l o m i t e , near the base of the Seacliff Sandstone was deposited in an interpreted intertidal-supratidal environment. It is considered to represent the lateral equivalent of the Nuccaleena Formation which crops out in the Flinders R a n g e s . At the top of the Seacliff Sandstone is a unit of thinly interbedded dolomite and shale less than 10m thick, which grades into the interbedded shales and fine sandstones of the Brachina S u b g r o u p . This dolomitic unit is interpreted to be partly diagenetic and partly detrital in origin. The overlying shales, siltstones and sandstones of the Brachina Subgroup are interpreted as turbidites. The sandstones become thicker and more a b u n d a n t up section w h e r e the sequence is c h a r a c t e r i s e d by hummocky cross-stratification (HCS). The abundance of HCS and wave ripples suggests this was storm-dominated environment. Higher in the section interbedded shales and sandstones are associated with sand wave complexes, HCS and swaley cross-stratification suggesting shallowing into a tide- and storm-dominated environment above fairweather wave base. The turbidite and HCS dominated unit passes upward into a predominantly sandstone sequence correlated with the ABC Range Quartzite of the Flinders Ranges. This sandstone is well exposed at Port S t a n v a c . The ABC Range Q u a r t z i t e represents deposition in a wave-dominated inner shelf and deltaic setting. The Brachina Subgroup at Hallett Cove represents a cycle of transgressive and regressive sedimentation. The upward transgressive transition from the d o m m a n t l y subaerial environment of the Reynella Siltstone through to the shallow water Nuccaleena Formation and shelf-slope environment of the Seacliff Sandstone followed the termination of late Proterozoic glaciation in the Adelaide Geosyncline. Rapid increase in sediment influx resulted in an upward-coarsening and thickening sequence of interbedded shales and sandstones. Continued progradation and aggradation finally resulted in deposition of the ABC Range Quartzite. 232

OVERTHRUST TERRANES IN THE LACHLAN FOLD BELT^ SOUrHEASTERN AUSTRALIA C. L. Fergusson, D.R. Gray and R. A. F. Cas Dept. Earth S c i e n c e s , Monash U n i v e r s i t y , Melbourne Most P a l a e o z o i c rocks o f the Lachlan Fold B e l t in s o u t h e a s t e r n A u s t r a l i a group i n t o t h r e e t e c t o n o - s t r a t i g r a p h i c t e r r a n e s ; the B a l l a r a t Terrane in t h e s o u t h w e s t , t h e Melbourne T e r r a n e i n t h e c e n t r e , and t h e Benambra T e r r a n e in t h e e a s t and n o r t h . These developed a d j a c e n t t o the ancient Gondwanaland c o n t i n e n t a l margin and were amalgamated in the Middle Devonian by m a j o r o v e r t h r u s t i n g . There was d e x t r a l displacement o f at l e a s t s e v e r a l hundred k i l o m e t r e s between the Benambra Terrane and the Melbourne Terrane. The B a l l a r a t and Melbourne Terranes c o n s i s t mainly o f q u a r t z - r i c h t u r b i d i t e sequences d e p o s i t e d in a c o n t i n e n t a l margin sediment prism that had an e n s i m a t i c basement and l a y a d j a c e n t t o a c r a t o n t o t h e s o u t h w e s t . Both t e r r a n e s are dominated by u p r i g h t , s h a l l o w p l u n g i n g , c h e v i o n f o l d s t h a t h a v e a c c o m o d a t e d between 50% and 70% s h o r t e n i n g . W i t h i n t h e B a l l a r a t Terrane g r a n i t e ages i n d i c a t e that d e f o r m a t i o n i s d i a c h r o n o u s and y o u n g s f r o m w e s t t o e a s t . By t h e M i d d l e Devonian t h e d e f o r m a t i o n e x t e n d e d eastwards t o the Benambra Terrane and was r e s p o n s i b l e f o r major o v e r t h r u s t ing of the Ballara and M e l b o u r n e T e r r a n e s and a m a l g a m a t i o n o f a l l three t e r r a n e s . The Benambra T e r r a n e has a widespread Ordovician q u a r t z - r i c h f l y s c h , and t h e i r metamorphosed e q u i v a l e n t s , w i t h a b e l t o f O r d o v i c i a n m a f i c i n t e r m e d i a t e v o l c a n i c s , v o l c a n i c l a s t i c s and limestone in eastern N.S.W. These are o v e r l a i n by S i l u r i a n t o e a r l y Middle Devonian s u b a e r i a l s i l i c i c v o l c a n i c s , s h a l l o w marine carbonates and c l a s t i c s , deep-marine t u r b i d i t e f i l l e d troughs and c o n t i n e n t a l c l a s t i c sequences. Regional d e x t r a l shear in t h e S i l u r i a n t o M i d d l e D e v o n i a n , e v i d e n c e d by e a s t - w e s t f o l d s o f Early S i l u r i a n age and e l o n g a t e n o r t h e r l y trending m i d - S i l u r i a n t o Early Devonian troughs, had a s i g n i f i c a n t i n f l u e n c e on the Benambra Terrane. The Middle Devonian o v e r t h r u s t i n g and f o l d i n g event marks a major change in t e c t o n i c s t y l e o f c e n t r a l V i c t o r i a from s t r i k e - s l i p f a u l t i n g t o major e a s t west compression. This c o i n c i d e s with a gap in magmatic a c t i v i t y throughout t h e L a c h l a n F o l d B e l t and a change in r e g i o n a l f a c i e s p a t t e r n s from v o l canism, shallow^ and deep-marine sedimentation t o mainly f l u v i a t i l e o v e r l a p s e q u e n c e s and a b e l t o f magmatism i n c e n t r a l V i c t o r i a . Overthrusting i s probably r e l a t e d t o rapid c o n v e r g e n c e a l o n g an i n f e r r e d s u b d u c t i o n z o n e east o f the Benambra Terrane.

233

PETROGENESIS OF THE COBAW BATHOLITH, CENTRAL VICTORIA Stephen J. Fermio School of Applied Geology, S.A. Institue of Technology, Adelaide The Cobaw Batholith, situated in central Victoria, is a high level compound batholith with a narrow contact aureole. The batholith has an areal extent of 520km^ and is comprised of both S and I~type granitoids that were emplaced after the Tabberabberan Orogeny approx. 360 m y ago. The S-type granitoid is a garnet and cordierite bearing adamellite which almost completely encloses the I-type granitoids in a ring like fashion. The adamellite is the only S-type granitoid in the complex and is relatively homogenous in composition. It contains sporadic, gneissic metasedimentary xenoliths up to Im in length which contain cordierite, ilmenite and sillimanite. These xenoliths are interpreted as representing restite since they are unlike any of the surrounding sedimentary country rocks, in terms of their metamorphic grade and refractory composition. Other xenoliths are very rare. The three most volumetrically significant I-type granitoids comprise two thirds of the batholith and all are relatively silicic (68-72wt % Si02), hornblende bearing granodiorites. On the basis of cross cutting relationships these granodiorites are inferred to have postdated emplacement of the S-type adamellite. The three I-types are cogenetic and compositions vary from slightly metaluminous (0.61% Di) to slightly peraluminous (1.30% C). Computer modelling of this trend towards increasingly peraluminous compositions within the I-type granodiorites, using analysed whole rock and mineral compositions, suggests that the removal of 8% plagioclase (Ani^g) , 3% biotite and 1% hornblende can produce the observed differenciation trend. This extract assemblage is compatible with the observed crystallization sequence for the granodiorites and Sr, Rb and Ba trace element abundances along the trend. A filter pressing mechanism is favoured over a crystal sinking mechanism due to the high viscosity of intermediatefelsic magmas. Xenoliths are more abundant in the peraluminous granodiorites and make up approx. 1% of the total rock volume. The dark, fine grained ovoid tonalitic-dioritic variety are the most common. They vary between 5—20cm in size and the major phases include hornblende, biotite, clinopyroxene, quartz and plagioclase in a variety of combinations. These xenoliths occasionally exhibit amphibole rich margins, acicular skeletal apatite crystals and radiating textures all of which are consistent with the rapid cooling of a magma of tonalitic-dioritic composition enclosed within cooler granodioritic magma. Whole rock compositions of these xenoliths fall well off the differentiation trends defined by the I-type granodiorites on variation diagrams, indicating that they are compositionally quite distinct to the granodiorites. The CaO and Sr contents of the S-type adamellite are too high to have been derived from the Ordovician or Silurian metasediments presently exposed in the Lachlan Fold Belt, and therefore less chemically mature sediments such as those present in the Precambrian successions near the margins of the LFB, probably underlie the Ordovician in central Victoria. The source of the I-type granodiorites in the Cobaw Batholith is probably related to the presence of large volumes of lower crustal intermediate rock which could have been underplated during the Cambrian and/or Siluro-Devonian magmatic events in the region. Partial melting of such underplated material could have been induced by the intervention of a basaltic plume derived from the upper mantle. Incomplete mixing of the I-type granitoid melt, produced by the interaction of this plume and the underplated material, possibly remains in the form of the rapidly cooled tonalitic-dioritic xenoliths. The resulting upwelling of intermediate magma induced partial melting in the overlying Precambrian metasedimentary stratum causing both S and I-type melts to rise synchronously to form the Cobaw Batholith. 234

A D E L A I D E A N SEDIMENTS OF THE PEAKE A N D DENISON RANGES R . B . Flint South Australian Department of Mines and Energy The Peake and Denison Ranges consist of four Precambrian inliers in the Eromanga B a s i n . Adelaidean sedimentation occurred in a north-westerly extension of the Adelaide G e o s y n c l i n e . A great thickness of dominantly paralic sediments of the Callanna G r o u p , Burra Group and Umberatana Group were deposited in a steadily subsiding fault-bounded t r o u g h . Initial rifting of the geosyncline resulted in deposition of the Callanna Group consisting of t h i n , basal conglomeratic sandstones and red m u d s t o n e s , stromatolitic and oolitic d o l o m i t e s , and basic volcanics (Cadlareena V o l c a n i c s ) . The latter unit is more than 750 m thick and consists dominantly of amygdaloical basalts and coarser-grained dolerites w i t h minor p y r o c l a s t i c s . Igneous textures are p r e s e r v e d , however the volcanics have been extensively altered; the present mineralogy is albitic p l a g i o c l a s e , o r t h o c l a s e , chlorite, a c t i n o l i t e , e p i d o t e , c a l c i t e , hematite and q u a r t z . Faulting and diapirism caused extensive disruption of the Callanna Group above the Cadlareena V o l c a n i c s . The upper sequence consists of thinbedded sandstones (often with m u d c r a c k s , ripple-marks and halite casts on shale l a m i n a e ) , f l a g g y , buff d o l o m i t e s , dolomitic s i l t s t o n e s , s h a l e s , sandstones and a r k o s e s . Gypsum p s e u d o m o r p h s , halite casts and cauliflower cherts are common. These c y c l i c , hypersaline sand-shale-carbonate sediments were deposited in a r i f t e d , intracratonic basin and their immense thickness (up to 17 k m ) attests to considerable subsidence during deposition. Continued basal subsidence and paralic sedimentation during the Torrensian period produced over 10 km of Burra Group s e d i m e n t s , a thicker and more arenaceous sequence than elsewhere in the g e o s y n c l i n e . Dominant rock types are orthoquartzites and sandy siltstones (with clay-galls and ripple-marks) of the Mount Margaret Quartzite, and dolomites (conglomeratic and stromatolitic) and arenites of the Skillogalee D o l o m i t e . Other lithologies include black c h e r t s , magnesite conglomerates and m i c r o f o s s i l i f e r o u s , stromatolitic d o l o m i t e s . Sturtian-Marinoan sediments of the Umberatana Group disconformably overlie the Burra G r o u p . The glacial Calthorinna Tillite consists of 650 m of diamictites (with striated erratics), conglomeratic dolomites, and arenites. A marine tcaasgression followed the glacial phase with deposition of laminated silty shales and dolomLtlc siltstones (Tapley Hill Formation). The sequence is capped by regressive marine carbonates which are overlain by reddish-brown to green siltstones and s h a l e s , sandstones and dolomites (Willochra Subgroup), the youngest Adelaidean beds e x p o s e d . DiaplrLsm has affected all Adelaidean units to form both b r o a d , irregular zones of disruption containing d i s o r i e n t a t e d , rafted blocks of varying size and lithology, including Ordovician m o n z o n i t e - s y e n i t e s , within a carbonate m a t r i x , and narrow bands of carbonate breccia intruded as s i l l s , plugs or dykes along Delamerian faults and anticlinal hinge z o n e s .

235

GEOLOGY OF THE WILLOURAN RANGES Bryan G . Forbes Geological Survey of South Australia, Adelaide

The Willouran Ranges are a northwestern extension of the Flinders Ranges in arid northern South A u s t r a l i a , southwest of M a r r e e , and represent part of the Adelaide Geosyncline. The poster shows only part of the Willouran Ranges and represents a stage in preparation of the CURDIMURKA 1:250 000 geological m a p "by the S . A . Geological Survey. Information presented on the m a p is based on air photo interpretation but also incorporates field data from H o r w i t z , W e b b , M u r r e l l , Coats, Dalgarno, P r e i s s , Utah Development Company and other explorers, P a r k e r , B e l p e r i o , K r i e g , Forbes and others. Oldest rocks depicted are the Adelaidean (late Proterozoic) Black Knob Marble and Noranda Volcanics which are known only in tectonically disturbed zones. The type section of the Curdimurka Subgroup (upper Callanna Group) is well exposed southeast and southwest of "Callanna" homestead where it reaches k TOO m in thickness and where Rowlands and others described evaporitic features suggestive of a playa or sabkha depositional environment. Contacts w i t h the overlying Burra Group appear everywhere to be faulted. The contact of the Burra Group with the Rischbieth structural complex is interpreted (with earlier workers) also as a fault representing syndepositional movement of an intrusive megabreccia during and after deposition of the Burra Group. Parker has described overprinting of folds and thrusts within the inlier and suggests significant tectonism prior to or duri^ig early sedimentation of the Burra Group. Movement of intrusive breccias appears to have partly controlled deposition of the Burra Group elsewhere and continued at least until after deposition of the Tapley Hill Formation, as evidenced east of "Callanna" b y a small tongue of breccia intruding the Tapley Hill Formation and by disturbance of the Amberoona Formation near the Bungarider F a u l t . Major units of the Burra Group are Emeroo Subgroup, mainly clastic up to k 000 m in thickness; Skillogalee Dolomite, partly magnesite-rich up to 3 TOO m ; and Myrtle Springs Formation mainly clastic up to h UOO m . In some areas the base of the unconformably overlying Umberatana Group (over T 000 m ) comprises a diamictite tentatively correlated with the Bolla Bollana Tillite. Other features of interest in the Umberatana Group are an unconformable pebbly arkose (?Serle Conglomerate) near the Boorloo Mine and erosional relationships of the Amberoona Formation, particularly near the Bungarider F a u l t . The youngest Adelaidean unit near "Callanna" is the Ulupa Siltstone of the Wilpena Group (total thickness about 2 000 m overall). Although the Cambro-Ordovician Delamerian Orogeny was the major foldforming deformation in the region, northwesterly trending fracture systems such as the Norwest and Bungarider Faults and some other structural features were probably operative during the Adelaidean.

236

UNDERGROUND WATER RESOURCES OF THE ADELAIDE METROPOLITAN AREA WITH THE LATEST UNDERSTANDING OF RECHARGE MECHANISM N.Z. Gerges Geological Survey of South Australia Groundwater occurs as both pressure and non pressure water within Tertiary to Recent sediments of the Adelaide area. The main groundwater supply is obtained from the Tertiary sediments from three main confined aquifers designated A, B and C in order of increasing depths. For aquifers A and B, salinity is less than 1 000 mgL""^, and large quantities are being pumped for use in industry, schools and recreation grounds. Extraction for 1984 has been estimated at 6 000-8 000 Ml year"^. Aquifer C is too saline for general use. On six occasions since 1915, it has been necessary to augment the Metropolitan Water Supply from groundwater sources. The last occasion was in the 1967-1968 summer, where 9 500-10 700 ML"^ was pumped into the distribution system during a seven month period. Re-interpretation of old data together with information obtained from recent drilling contribute significantly to the understanding of the recharge mechanism. It shows that surface waters do not contribute significantly to the Tertiary confined aquifer recharge mechanism. Major recharge occurs from bedrock through to a deeply buried clastic almost non-marine sediments. The theory is supported by evidence from salinity distribution and stratification in Quaternary and Tertiary sediments, the pre-pumping potentiometric surface, and to a lesser extent isotopic analysis.

237

LARGE RECUMBENT FOLDS IN WEEKEROO SCHISTS, OLARY DISTRICT, S.A. A.E. Grady Flinders University of S.A., Adelaide iVell preserved bedding and associated sedimentary structures in a sequence of Lower to Middle Proterozoic quartzo-feldspathic and micaceous schists (Weekeroo Schists) within the Willyama Complex, Olary District, South Australia, allow the delineation of separate zones of upward facing and downward facing middle and late generation folds. These zones (at least 2 km wide) are interpreted as outcropping limbs of large, recumbent, early phase folds. Folds of this type are well known from the Willyama Complex in the Broken Hill Block, western N.S.W,, but have not previously been reported from the Olary District. The Willyama Complex, in the Olary District, has been subjected to five deformation phases. Abundant evidence on all scales supports phases D2, D3, Di^ and D5. The earliest phase, Dj, has long been invoked to explain a widespread, early, layer parallel foliation and rare early, isoclinal mesoscopic folds. Mesoscopic and macroscopic fold facing evidence from the Weekeroo Schists supports the interpreted Di phase and strongly suggests that Fx folds included some which were large and recumbent. In the Olary District, the Willyama Complex occurs within a number of inliers surrounded by Upper Proterozoic (Adelaidean) sedimentary sequences. The Weekeroo Schists have prominent occurrence along the northern edge of the westernmost, Weekeroo Inlier, which is structurally divided into Western, Central and Eastern parts. The northern zone of the Eastern Weekeroo Inlier displays extensive evidence of downward facing post-Di folds and is therefore interpreted to represent part of the inverted limb of a recumbent Fi fold. In contrast, the northern zone of the Western Weekeroo Inlier displays only upward facing post-Di folds, representing part of an upright limb of an Fi fold. The location and orientation of Fi fold hinges remain to be delineated. The intervening Fi fold hinge could occur within the central Weekeroo Inlier where some unusual steeply plunging F3 folds occur. Extensive detailed investigations are required in order to test this hypothesis.

238

ARCHAEOCYATHAN DISTRIBUTION ON THE L O W E R CAMBRIAN CARBONATE S H E L F , FLINDERS R A N G E S , SOUTH A U S T R A L I A ^D.I. Gravestock and ^ N . P . James ^South Australian Department of Mines and E n e r g y , A d e l a i d e , S . A . ^Memorial University of N e w f o u n d l a n d , St J o h n s , N e w f o u n d l a n d , Canada A r c h a e o c y a t h s , characteristic of the Lower Cambrian carbonate shelf fauna In the Flinders R a n g e s , dominate the oldest Faunal Assemblages I and I I . Assemblage I archaeocyaths became established over and between oold shoals and over stromatolltlc carbonate mud f l a t s . Isolated bloherms built from lime m u d , the calcareous alga Renalels and a r c h a e o c y a t h s , are Interbedded with calcarenltes and calclrudltes, and the 14 known archaeocyathan species occur In both f a d e s . Upper Assemblage I has scarce archaeocyaths In a nodular lime mudstone with Irregular 'clotted' (?algal) f a b r i c . Llthlflcatlon and reworking of the upper sequence are locally evident and no Assemblage I archaeocyaths occur In younger u n i t s . Assemblage II at Wllkawllllna Gorge occurs In cross-bedded gralnstones Interbedded with and overlain by Isolated algal-archaeocyathan b l o h e r m s , the latter comprising up to 30 per cent of the f r a m e w o r k . Archaeocyaths are moderately diverse with 24 species evenly divided between lower and upper Assemblage I I . By contrast. In the Mt Scott r a n g e , richly fosslllferous mud mounds- pass laterally Into fossll-poor lime m u d s t o n e , and upward Into bloherms dominated by R e n a l c l s , which In turn are overlain by thinly bedded calcarenltes and calclrudltes. The main mud mound In lower Assemblage II contains numerous diverse archaeocyaths (40 species), abundant sponge s p i c u l e s , brachlopods and other f o s s i l s . The overlying bloherms In upper Assemblage II have a framework dominated by Renalcls with archaeocyaths (4 species) rarely comprising more than 20 per c e n t , while archaeocyaths In the overlying calcarenltes (Initially 16 species) decline In abundance and diversity upwards and small shelly fossils predominate. Despite this sensitivity to lateral and vertical fades changes, archaeocyaths have proved useful for local correlation at species level and for International correlation with Siberia at genus l e v e l . The oldest archaeocyaths are particularly significant since the first shelly trlloblte remains have not been found below Upper Assemblage I I , and the value of other numerous small shelly fossils for correlation Is still being assessed (these Include m o l l u s c s , b r a c h l o p o d s , coelosclerltophores and s p o n g e s ) . Above Assemblage I I , deeper water slope carbonates predominate (Parara L i m e s t o n e ) , but at certain localities persistent shelf f a d e s have abundant a r c h a e o c y a t h s , trllobltes and small shelly f o s s i l s , hence an effective blostratlgraphlc zonatlon can be achieved with various combinations of the total f a u n a .

239

STROMATOLITES IN UPWARD-SHALLOWING SEQUENCES Kathleen Grey and Alan M. Thorne Geological Survey of Western Australia, Perth Upward-shallowing sequences in the 2,0 Ga Duck Creek Dolomite (Wyloo Group, Ashburton Fold Belt), Western Australia, contain the following facies types: intraclast grainstone (transgressive barrier); domical stromatolite, small branching-columnar stromatolite, large branching columnar stromatolite (low-intertidal and subtidal lagoon); laminated dolomite (mid-intertidal); tepees, disrupted domes and cuspate stromatolites (high-intertidal and supratidal). Two facies associations have been recognized (Preferred Sequences 1 and 2). Columnar stromatolites in Preferred Sequence 2 reflect the more extensive development of the lagoonal fades. The small branching-columnar, niched Pilbaria perplexa, and the larger Pilbaria cf* perplexa, characterize low-intertidal and subtidal lagoons. The microdigitate Asperia ashburtonia occurs in high intertidal or supratidal settings. At Lake Clifton (southwest Western Australia), Recent stromatolites occur in a coastal lacustrine environment. They show a broad morphological resemblance to fossil pilbariform and asperiform stromatolites, but are taxonomically distinct. The modern equivalents of the asperiform stromatolites grow in a sub-aerial environment, while the pilbariform types occur in a sub-aquaeous setting. Throughout the world, pilbariform and asperiform stromatolites occur in stratigraphic units of differing ages, and provide an opportunity to examine the relative roles of evolution and environment in determining morphological variation. If stromatolite morphology were controlled solely by environmental factors, taxonomic differences would never be as drastic as reported and identical ubiquitous species would be anticipated at time-horizons where environmental constraints were similar. This is not the case for asperiform and pilbariform taxa which have been formally described. Radiometric datings are poor for many of the stratigraphic units concerned: nevertheless, when the most probable ages are considered, different taxa have been erected for both types of stromatolite from units of different ages. Moreover, identical taxa have been recognized in units of approximately the same age. The occurrence of stromatolites in upward-shallowing sequences provides an important model for studying relationships between stromatolite form and depositional environment. The present data suggest that when environmental conditions are known, and when systematic descriptions of a sufficiently refined level are available, time-related changes in stromatolite morphology may be detected and used as a basis for stromatolite biostratigraphy.

240

EARLY CAMBRIAN SYNDEPOSITIONAL TECTONICS AT WIRREALPA, S.A. P.G. Haslett BHP Minerals, Adelaide, S.A. At Wirrealpa, Cambrian sequences are exposed around an area of probable Callanna Bed equivalents and widespread breccias and megabreccias. This central zone of Proterozoic rocks has previously been considered to be of diapiric origin, intruded as a plastic breccia mass carrying blocks of Callanna lithologies from depth to their present position. The contacts between the Cambrian and Adelaidean sequences are complex and variable. Detailed mapping has confirmed that a NW-SE trending breccia "limb" north of Old Wirrealpa Spring represents a narrow zone of active syndepositional tectonism in Early Cambrian times. Across this "limb" major changes in sedimentary facies, thickness and diagenetic alteration can be demonstrated in time equivalent Early Cambrian carbonate and clastic sequences. Sequences NE of the "limb" have generally been deposited in deeper water environments, are thicker, and show lateral facies changes from ill-sorted non-carbonate megabreccias adjacent to the "limb" passing gradationally into better sorted, more carbonate-rich and finer grained sequences distally. The equivalent age sequences to the SW of the "limb" are massive shallow water platform carbonates overlying poorly bedded clastic breccias. Extensive erosion and karst development has occurred in response to periodic uplift and exposure of this sequence prior to the middle Early Cambrian. Many of the areas of breccia/megabreccia previously considered to be of intrusive diapiric origin can be shown to be of Cambrian age and of sedimentary origin. Limited mapping within exposed Callanna Bed sequences suggests that very significant portions are generally intact. Clearly there has been significant uplift and exposure of Callanna-type sequences in the Early Cambrian, particularly across relatively narrow hinge zones like the one referred to above. Repeated syndepositional movements have caused very complex facies relationships in nearby contemporaneous sediments, as well as complex structural relationships in underlying Cambrian breccias and older sequences. There is little evidence of substantial nearby exposure of Adelaidean sequences at Wirrealpa after the middle Early Cambrian although minor pebble beds indicative of local instability are still present in the Middle Cambrian Billy Creek Formation. Thick lithic sandstones of the Narinna Greywacke at Wirrealpa also reflect more distal exposure of polymict sources in the general area in the Early Cambrian. A precise understanding of the nature of the Cambrian syndepositional tectonism is difficult to achieve. Diapirism has been postulated as one possible mechanism by which older sequences have been emplaced at the Cambrian depositional surface. High angle block faulting, with associated talus breccia deposition, and Early Cambrian low-angle thrusting, also bear consideration. The Cambrian/Proterozoic contact relationships should be of considerable significance in any attempt to determine the nature of this tectonism. The Delamerian folding and later faulting, along with the generally poor exposure, has meant that a study of these contacts has not yet presented any clear-cut answer. Very little drill-hole data is available to clearly determine the nature and orientation, at depth, of the contacts.

241

MARALINGA SOUTH AUSTRALIA - AN IDEAL DISPOSAL SITE FOR AUSTRALIA'S INTRACTABLE WASTES Michael J. Knight School of Applied Geology, University of N.S.W., Sydney Australia has quantities of intractable waste containing stable compounds that are difficult to dispose of currently in an environmentally acceptable manner. The main wastes in this category are industrial organo-chlorine compounds (e.g. PCB, Dioxin) and some radioactive wastes. There are about 8 000 tonnes of organo-chlorine compounds stored in Australia and the generation rate is nearly 950 tonnes/year. Low to intermediate level radioactive wastes from institutional (e.g. medicine) and industrial sources are being generated at about 2 cm^/person/year. In aggregate, the stored and generted radioactive waste to 1995 could be placed in a single trench 5 m deep, 5 m wide and 40 m long. There have been strong moves in the last five years to dispose of the intractable chemical wastes by high temperature (=1200®C) incineration. However, the three attempts to site a burner (Sydney, Melbourne and Broken Hill) have met severe opposition and have, to date, failed to establish a facility. Maralinga is an ideal site to locate the National Facility required for both chemical and radioactive wastes. There are two possible disposal strategy options: Option A consists of an incinerator and two secure landfills and Option B; one deep (550 m) geological repository excavated in Archean diorite which is overlain by 300 m of chocolate shale. Groundwater at Maralinga is mainly located in sandstone and limestone units of the upper 150 m zone above the shale. Groundwater salinities are generally high and in some areas approach sea water (30 000 ppm or more). The area is remote, seismically stable, pastorally very poor and has very low rainfall and nearby there are soils rich in lime that could neutralize any acidic fallout if needed. The site has been contaminated in part by the past British Atomic Weapons testing program. One advantage of the deep geological repository option is the possibility of providing a safer site for long-life elements that need to be cleaned up from the contaminated ground surface. Royalties could be paid to South Australia by states that use the facility and there would be some employment generated. The site is well serviced by a sealed road from a siding on the Trans-Australia railway. Rail transport from most significant generation points in Australia is safe and inexpensive (about $50/tonne). The capital cost for either option will be $10 - 20 m. Unit disposal costs amount to about $300/tonne for a mined cavity isolation and $500 1000/tonne for incineration (chemical wastes). A secure landfill would cost $150 000 - 300 000 (capital cost) with a disposal cost of $1030/tonne There is an urgent need for Federal legislation and action on this matter since a single National Facility could result in a $10 m saving by processing chemical wastes that are currently directed to overseas incinerators or destroyed in the ship "Vulcanus" that occasionally visits Australia.

242

MANUAL OF STRUCTURAL TECHNIQUES IN OUTCROP AND DRILLCORE FOR THE PRACTISING EXPLORATIONIST W.P. Laing^ and R. L. Hammond^ ^James Cook University of North Queensland, Townsville The authors have compiled a comprehensive Manual of techniques available for analysing structures in hardrock t e r r a i n s , in which a large proportion of e x p l o r a t i o n i s t s find themselves working. The Manual concentrates on making " s t r u c t u r e " a usable, "non-threatening" tool for the e x p l o r a t i o n i s t . I t incorporates only enough i n i t i a l structural theory as i s necessary to understand the practical techniques. The Manual c o n s i s t s of two principal sections: structural techniques in outcrop, and structural techniques in d r i l l core. The former are l a r g e l y "standard" techniques, although in a f i e l d so admittedly unfamiliar to the "non-structural" e x p l o r a t i o n i s t , one hesitates to say that anything i s standard: i t i s for precisely this reason that the Manual should f i l l a large vacuum in practical geology. The main contribution of the book i s in the area of d r i l l c o r e a n a l y s i s . Much of this material i s new, and draws upon the senior a u t h o r ' s decade of experience in exploration and d r i l l c o r e l o g g i n g . The authors develop two conceptual tools crucial to an understanding of the geometry of structures in d r i l l c o r e : 1. The f o l i a t i o n e l l i p s e in d r i l l c o r e . The very useful properties of this e l l i p s e , and of the types of l i n e a t i o n s that can be developed on i t , are analysed and explained in simple terms. 2. The e f f i c i e n t use of o n e ' s hands in reconstructing f o l i a t i o n s and l i n e a t i o n s i n space. This "eye-opener" in many cases obviates the need for stereonet construction. A range of techniques i s then presented, s t a r t i n g with the basic cone/small c i r c l e construction for any f o l i a t i o n in non-oriented d r i l l c o r e , and progressing to constructions for bedding/cleavage r e l a t i o n s h i p s , the determination of fold axis orientation, vergence c a l c u l a t i o n , p o s i t i o n around a f o l d , and other structural parameters. The Manual i s above all designed to be " u s e r - f r i e n d l y " . I t assumes v i r t u a l l y no experience in structural a n a l y s i s . The emphasis throughout i s on visual techniques which do not necessarily involve stereonet computations, although the l a t t e r quantitative procedures are included. The Manual will contain the following features, aimed at making l i f e easier for the f i e l d and coreyard e x p l o r a t i o n i s t : (a)

A size designed to f i t into a f i e l d pouch or a large pocket;

(b)

Cover to be (hard) waterproof paperback;

(c)

Spiral

binding;

(d) Each main section stiffened page;

to be flagged by a protruding

labelled

tag on a

(e) Each technique will be described on a new page, with a s i n g l e "openpage" format; explanatory text on the left-hand page and accompanying diagrams on the right-hand page. (f) The text for each technique will be l a i d out under a series of headings. One of the f i r s t w i l l be "Information you need before you s t a r t " , which will help the user to rapidly decide whether i t i s possible to use that technique. 243

P.G. Lennox School of Applied Geology, U.N.S.W., Sydney Synthetic phlogopite-quartz mixtures have been deformed in a gas medium deformation apparatus at high pore fluid pressures at temperatures up to 600OC, confining pressures up to 300 MPa and strain rates of lO""** to 10""® sec*"^. One of the experimental arrangements permitted fluid flow from one end of the specimen to the other during deformation. In some experiments where the rock analogue was subjected to pore fluid pressures approaching the confining pressure voids (elongaged normal to the shortening direction) were initiated and filled with fibrous mineral. In many runs, even at pore fluid pressures approaching the confining pressure and in specimens containing a high weight percentage sand, the rock analogue does not microcrack, but clogs, leading to the fluid flow around the outside of the specimen. Microfabrics include a dominant planar alignment of the platy mineral component normal to the shortening direction. T.E.M. studies showed this microfabric consisted of grossly aligned submicron to micron sized mica plates whose degree of alignment increased at higher degrees of shortening. This dominant microfabric formed by a combination of primarily "rotation and solution effects and secondarily stress-induced growth and aggregation mechanisms and to an even lesser degree shearing. The secondary microfabric consists of submicron kink band boundaries surrounding elongated micron-sized blebs of kinked mica, which are oriented almost to the dominant microfabric forming a conjugate oblique planar microfabric. The regularity of this conjugate planar microfabric inspite of the compositional heterogeneity of the specimen (quartz grains in a mica matrix) probably reflects the effect of the overall strain experienced by the specimen. This conjugate planar may be formed by kinking of a layered medium by shortening normal to layering or growth of conjugate kink zones by shortening parallel to the foliation. Microstructures include strain shadows in the lee of quartz grains, decamicron kinking of the synthetic phlogopite dominant fabric with axial surface spacing reflecting average quartz grain size control and minor quartz grain scalloping due either to spalling off of tabular grain fragments during deformation or solution processes. Quartz stringers consisting of aligned quartz and quartz overgrowths on rare quartz grains in some highly strained specimens may reflect solution processes. Conjugate shear zones whose obtuse bisector is the shortening direction developed at high strains. New submicron-sized mica plates developed in some of these shear zones at a low angle to the zone. This result is similar to that observed under very different conditions in shearing experiments in clay. Microstructural studies indicate platy preferred orientation is parallel to the foliation and hence a shearing mechanism is unlikely to have been significant in producing the observed microfabrics and microstructures.

244

GEOLOGICAL FEATURES OF SCIENTIFIC INTEREST IN AUSTRALIA South Australia E,M. McBrlar Department of Geology, The University of Adelaide Sites displaying geological features of special scientific interest are fundamentally important for teaching and research. Those identified by the Geological Society in South Australia are called 'geological monuments'. They are rare and irreplaceable. A number of criteria are used to select the sites, including that the geology exemplified may be representative or rare, and that the geological monuments, collectively, will represent the geological history of the State with minimum duplication. Some geological monuments are of international significance, such as those covering the Precambrian fauna at Ediacara and the modern carbonate sedimentation in the Coorong. Efforts to list the sites and to protect them are receiving increasing support from the geological community. The South Australian Subcommittee began work under its first grant from the Australian Heritage Commission in 1975. Sites were suggested by local geologists, and the Subcommittee, assisted by invited experts, determined priorities for investigation, which were carried out by the Subcommittee and consultants employed under the grants received. Five volumes have now been prepared, with 220 geological monuments covering most of the State, and fifty five sites now on the Register of the National Estate. Currently a small grant is being used to fill gaps in the records and nominate further sites to the Register. A further grant has been applied for to continue site surveys, especially in the north and northwest of the state, nominate further sites to the Register, and allow the preparation of master microfiche of the existing five volumes, so that information will be available to decision makers in Local Government, mining exploration companies and elsewhere. National project E.B. Joyce Department of Geology, University of Melbourne At the Geological Convention in Canberra in February 1983 the Federal Committee for Geological Monuments met formally for the first time, and following a motion passed at the meeting, a letter of application was sent to the Australian Heritage Commission requesting a grant to begin a review and report on geological and geomorphological features of national and international significance for the whole of Australia. A grant of $5 000 was approved early in 1984, and at a Workshop on Geological Conservation held in Canberra in May 1984 attended by representatives of all Divisional Subcommittees a preliminary document prepared by the Federal Convenor was tabled. Following discussion the meeting decided a methodology for the survey, assembled a preliminary list of features, and prepared a programme to complete the report. Grants to the Society by the Australian Heritage Commission total over $200 000 for the past decade, and it is expected that about 20 reports will eventually be prepared. It is appropriate that one of the last of this series will be an overview of geological conservation in Australia. 245

BARITE DEPOSITS OF THE ORAPARINNA DIAPIR W.S. McCallum South Australian Department of Mines and Energy, Adelaide Much of Australia' s barite production comes from several deposits in the Oraparinna Diapir area, central Flinders Ranges. The diapir extends about 7 km east-west by 9 km north-south within a broad domal structure. The core comprises rafts of Willouran sedimentary and igneous rock, breccia derived therefrom, and rafts of displaced Sturtian sediment. Enclosing Sturtian shale, siltstone, quartzite and tillite dip away from the diapir, being steeply upturned on some margins. Barite lodes are within Sturtian to Marinoan Umberatana Group sediments along the southern and eastern diapir margins, and in Marinoan Wilpena Group in the overlying Bunkers Graben to the northeast. All deposits are in moderately to steeply dipping fracture zones either along diapir/country rock contacts, or radiating out from the diapir. Barite deposited from saline solutions emanating from the diapir, sulphate being derived from connate water within enclosing sediments; barium may have been derived from igneous rocks or evaporites associated with the diapir, or by dissolution of feldspar within sediments. In the Bunkers Graben, at Oraparinna Mine, barite is emplaced in Brachina Subgroup siltstone along a fault system subparallel to graben bounding faults. Five lodes of white, pure barite up to 6 m across have produced about 270 OOOt of barite since 1940. Annual output is 3 000 to 5 OOOt of industrial grade product, i.e. 90% of Australian requirements. South of Oraparinna Mine, at Turley Lode at the base of the graben, numerous narrow barite veins have infilled fissures subparallel to and radiating from the nearby country rock/diapir contact. Matthews Lode is emplaced adjacent to the eastern fault boundary of the graben. On the eastern margin of the diapir, at Southern and Bowering Lodes barite is dismembered veins within disrupted host rock immediately adjacent to the diapir, and within displaced host surrounded by diapiric matrix. Faulting and brecciation of barite has produced red iron staining, resulting in material suitable only for oil drilling. At McRaes Lodes, industrial grade barite infills northerly dipping fractures in Sturtian Wilyerpa Formation adjacent to, but not extending into, the diapir. Along the southern margin, several thin, siliceous barite veins strike south and southeast through Wilyerpa and Tapley Hill Formations. Small production is recorded from Howard, Hall and Vincent Lodes. At Linke's Lode, the largest producer in Australia, barite veins within and adjacent to the margin of a raft of Tapley Hill Formation extend for 450 m, and in places are cut by diapiric material indicating diapiric movement after barite emplacement. Barite is brecciated, generally ferruginous, oil drilling grade. Similar veins at Bairstow and Tom Hill Lodes are within another raft of Tapley Hill Formation.

246

MAGNESITE IN THE ADELAIDE OEOSYNCLINE W.S. McCallum and L.C. Barnes South Australian Department of Mines and Energy In South Australia, there are three types of magnesite deposits. The most important economically are sedimentary with magnesite interbedded with dolomite and dolomitic silt. These deposits are mostly in Skillogalee Dolomite within Adelaidean Burra Group, with several small deposits in underlying River Wakefield Subgroup. Magnesite is recorded in Skillogalee Dolomite from Torrens Gorge near Adelaide north to Witchelina in the northern Flinders Ranges. Greatest development is in the Copley-Witchelina area where in a section several hundred metres thick there are more than 60 magnesite beds up to 1 m thick. Magnesite formed as a chemical precipitate in very shallow marginal lagoons subject to periodic sea level fluctuations, with addition of alkaline continental water. Sedimentary magnesite is commonly pelletal. During periods of low sea-level, magnesite mud dried and dessicated, and fragments were rolled up, then redeposited in the next influx of water. Pellets vary in size from fine to very coarse, and both fining and coarsening upwards sequences are observed. Pelletal magnesite is commonly poorly sorted with little or no visible layering. The matrix can be magnesite or dolomite, or clastic material introduced during reworking. Magnesite interbeds generally have sharp contacts with adjacent dolomite, which includes chert and stromatolitic dolomite. Grade varies widely depending upon amount of silica and dolomite impurity, ranging up to 47% MgO. Sedimentary deposits near Orroroo, in Port Germein Gorge and at Mundallio near Port Augusta have been worked on a small scale but most production has come from Copley. Initial mining methods were simple, blocks being 'harvested' from outcrop. In the early 1980's, small quarries near Myrtle Springs produced 500 to 1 OOOt annually. In 1984/85, 30 000 tonnes of magnesite (greater than all prior S.A. production) was shipped to Queensland Alumina for use as a filter medium to purify treatment water. Replacement deposits, formed when magnesite replaced dolomite of Balcanoona Formation in Umberatana Group, are found at Balcanoona and Mount Fitton, northern Flinders Ranges. At Balcanoona in Weetootla Gorge, inferred reserves total 20 million tonnes of coarsely crystalline magnesite averaging 41 to 44% MgO. There has been no production from replacement deposits. Thin surficial residual deposits, similar to calcrete, have developed on magne.sium-rich dolomite of varying ages. These deposits are generally pure, often 45-47% MgO, but all are small. Near Roberts town in the MidNorth, residual magnesite developed on Skillogalee Dolomite is traceable for several kilometres and numerous small pits have been worked. Residual deposits developed over Callanna Group dolomite have been mined at Paratoo.

247

THE LEIGH CREEK-FREELING HEIGHTS STRUCTURAL CORRIDOR E.S.T. O'Driscoll Western Mining Corporation, Adelaide, S.A. Investigations Into the regional structural of the Adelaide Geosyncllne have shown a number of continuous linear zones of structural disturbance crossing the geosyncllne and extending Into surrounding areas. These zones are observed as linear discontinuities In Integrated gravity, aeromagnetlc, topographic, geologic and remote sensing data. These discontinuities are associated with unusual rock types and mineral deposits of various ages and may represent foundational features of the Adelaide Geosyncllne which have been Intermittently reactivated. One such corridor, here named the Leigh Creek - Freellng Heights structural corridor (LC - FH) was detected In a LANDSAT lineament study. LC - FH Is approximately 18 km wide and trends east - northeasterly for about 150 km across the geosyncllne. The structure appears to be part of a much larger feature corresponding to alignment of granite culminations and structural highs from Eyre Peninsula to Tlbooburra and on Into Queensland through the Eulo and Neeblne Ridges. Near Mount Painter the high level British Empire Granite and sodlc granites Intrusive Into Adelaldean rocks lie within the corridor while to the east the trend Is associated with Devonian and younger Intruslves. LC-FH Is not readily apparent on COPLEY, but several geological features lie within or on the margins of the corridor. The early to middle Proterozolc Mount Painter Inller lies within the corridor as do uranium deposits at Mount Painter and Beverley. Two concentrations of amphlbollte dykes In the basement In the Hamilton Creek area and south of Mount Painter are roughly coincident with the northern and southern limits of the corridor. LC - FH broadly defines a south to north shelf to basin transition for the Sturtlan and Marlnoan. Dlamlctlte In Marlnoan glacials Is not present south of the corridor and shelf fades of Wllpena Group, Including ABC Range Quartzlte, are not present to the north. Submarine canyons In Wonoka Formation at Patsy Springs on the southern margin, and Fortress Hill on the northern boundary may be related to the corridor. Near Mount Scott a fault associated with conglomerates In Pound Subgroup and fades changes In Cambrian sediments parallels the southern margin of the corridor. The projection of LC - FH onto the Stuart Shelf coincides with an Inferred graben extending ENE from Woomera within which there Is thickening of Pandurra Formation. The distribution patterns of Tapley Hill Formation and Whyalla Sandstone suggest localised troughs coincident with the corridor which also broadly defines the southern limit of Cambrian limestone. Leigh Creek Coal Field lies entirely within the corridor. Trlasslc coal basins are aligned north-south apparently coincident with a deep basement structure crossing the corridor.

248

MAFIC DYKE SWARMS OF AUSTRALIA A . J . Parker^, P . W . Baillie^, D . M . Boyd^, M . Freeman^, M . P . McClenaghan^,. C . Murray^, J . S . Myers^, B . A . Pietsch^, R . C . Rickwood^, and D . M . Tucker^ South Australian Geological Survey, Parkside Tasmanian Geological Survey, Rosny Park University of Adelaide, Adelaide Northern Territory Geological Survey, Darwin Geological Survey of Queensland, Brisbane Geological Survey of Western Australia, Perth University of New South W a l e s , Kensington Bureau of Mineral Resources, Canberra Mafic dykes have been relatively poorly studied in Australia even though they represent major lithological components in all principal tectonic provinces ranging in age from Archaean to R e c e n t . With few exceptions Australian dyke swarms have been emplaced into continental crustal rocks and therefore record crustal tectonic processes typical of cratonic b l o c k s . The data upon which this poster is based are presented on a simplified 1:2.5 million scale tectonic map of A u s t r a l i a . The data have been derived from published and unpublished geological and geophysical maps produced mainly by state geological surveys, the Bureau of Mineral Resources and various universities. The poster is presented in two parts: the principal map portrays outcropping or confirmed dyke swarms whereas concealed dykes, such as the Gairdner Dyke Swarm of the Stuart Shelf (S. Aust.), are shown on the clear film overlay. The latter has been derived from geophysical interpretation of aeromagnetic m a p s . Amongst the most striking mafic dyke swarms of Australia are the very long, w i d e , E-W dykes of the Yilgarn B l o c k . These are the Widgiemooltha Suite of dykes dated at c a . 2420 Ma and of which the largest is about 585 km long and up to 3 km w i d e . During the Proterozoic, mafic dykes were intruded into Archaean and Proterozoic host rocks throughout the Western, North and Central Australian Orogenic Provinces. In the Yilgarn and Pilbara Blocks dyke swarms indicate that the blocks were essentially semirigid cratonic blocks that have not been significantly tilted or deformed since the A r c h a e a n . The dykes are particularly abundant around, and subparallel to, block margins and are predominantly of Middle Proterozoic age c a . 1000-1400 M a . Dykes of this age are also prominent in other provinces and form extensive swarms in the Pine Creek Geosyncline, Gawler Craton, Musgrave Block and, to a lesser extent, Mt Isa Inlier. Early Proterozoic dykes c a . 1900-1640 Ma are common in the North Australian Orogenic Province, particularly in the Mt Isa region where they represent feeders to mafic volcanic sequences. In the Tasman Fold Belt mafic dykes have been relatively poorly recorded mainly because they are smaller and less frequent than in Precambrian terrains. Nevertheless there are numerous small swarms particularly common in, or associated w i t h . Palaeozoic granites and metavolcanic sequences. Some may be issociated with gold mineralization. Finally, throughout the eastern coastal regions of Australia, there are swarms of Mesozoic and Tertiary dykes trending generally oblique to the coast. These are common in the Sydney Basin, the Gippsland Basin and in Tasmania.

249

THE W A 6 G A W A G 6 A TO BATEMANS B A Y , N . S . W . TRANSECT OF THE LACHLAN OROGEN: PROFILE 5 , INTERNATIONAL GEODYNANICS PROJECT (WG9)

M . J. Rickard and K. A . W . Crook Department of Geology, Australian National University, Canberra, Australia A newly published l:250,000-scale strip map with structural and geophysical profiles, and time-space plots will be presented as a poster. The timespace plots Indicate a coherence in the structural pattern and geological history of the various fault-bounded blocks across this sector of the Lachlan Orogen. Major deformations occurred in the Early Silurian, the Early Devonian and the Carboniferous. In the west complexly folded Ordovician turbidites and metamorphics are intruded by mid-Silurian S-type granites. The junction with the western side of the Tumut Trough is a wide belt of shearing along the Gilmore Fault Zone. In places to the north and south of this transect, lenses of ultramafics occur. The Tumut Trough is filled with Silurian flysch and bimodal volcanics followed unconformably by Early Devonian parallc sediments, ignimbrites and I-type granite. The basal unit of mafic amphibolites and ophiolitic volcanics (Silurian or older?) abuts a linear ultramaflc belt defining the eastern edge of the trough. Closure of the trough at the beginning of the Devonian may have occurred by the collapse of a small ocean-basin rift; alternatively, the trough may represent a larger Silurian back-arc basin closed by eastwards subduction of the trough beneath a west-facing volcanic arc (Goobarragandra Block). Eastwards the horsts and grabens of the Canberra-Yass Zone expose widespread felsic volcanics and shallow-water sediments of Silurian (S-type) and Devonian (I-type) age overlying Ordovician and locally Early Silurian quartz-rich flysch. Ordovician volcanics occur in this zone to the south at Kiandra. The eastern boundary of this zone is the S-I line; here the Silurian volcanics and sediments are of deeper water f a d e s in the Captains Flat Trough and the margin of the Murrumbidgee batholith is strongly sheared. A long narrow Cooma-type metamorphic complex buttresses the Cullerin Horst. The Monaro Zone to the east is similar except that all of the felsic magmatic rocks have I-type chemistry (Silurian volcanics and Early Devonian granites). Ordovician rocks in the Canberra and Monaro Zones vary considerably in deformational style, (simple upright folds to poly-deformed rocks with multiple cleavages). Areas with contrasting style are elongate N-S, and are juxtaposed abruptly E-W, and the fanning of Sj from W-dipping north of Batemans Bay, where melange is present, to E-dipping further west, is consistent with proposals that these zones form part of an Ordovician convergent margin accretionary prism. The Comerong rift is possibly situated on a geosuture. The structures in the adjacent Ordovician turbidites overturn to downward facing in places against the rift margins. Basal bimodal volcanics and small A-type granite plutons flank the rift. Mid-Upper Devonian molassic sediments (Lambie f a d e s ) are preserved in synclinal inliers in the Comerong, Minuma Range and Wee Jasper regions and as a gently folded sheet at The Rock, SW of Wagga. In the latter, basement faulting has created crosscutting monoclines in the cover sandstone parallel to SE-trendlng wrench faults. All the structural evidence (volcanics, faults and folds) points to E-W lateral movements - extension followed by compression. There is no evidence for longitudinal movements except locally on diagonal wrench faults. 250

WILLIAMSTOWN KAOLIN - SILLIHANITE - MICA DEPOSITS I.J. Townsend Geological Survey of South Australia Williams town mineral deposits are located 60 km by road northeast of Adelaide and 4 km southeast of the rural village of Williamstown in the eastern Mount Lofty Ranges. Williamstown is the only current Australian source of sillimanite and a major producer of mica and high alumina refractory clay. NGM Ltd. operate Private Mine 13 under agreement with the landowner producing from two open cut mines, Mount Crawford, the main mine about 90 m deep, and Springfield 300 m to the south which is almost 20 m deep. Two abandoned mines. Warren and Reservoir are on the same property. Mount Lofty Ranges comprise an extensive sequence of sandstone, siltstone, shale and dolomite of Adelaidean to Early Cambrian age with several inliers of older Barossa Complex rocks (c.l500 m. years). The mineral deposits are adjacent to the east of the Warren Inlier in a fault bounded block of Barossa Complex muscovite and biotite - quartz schist and sillimanite gneiss, surrounded by basal Adelaidean sediments all dipping steeply eastwards. Pipe-like segregations of sillimanite, kyanite, rutile and quartz have developed and late stage hydrothermal activity associated with pegmatite intrusion has altered much sillimanite to kaolin, and kyanite to green muscovite. Shearing has produced muscovite zones marginal to and within the main ore bodies. At Mount Crawford, an elliptical orebody 50-70 m across dips steeply easterly and plunges northerly at 70 degrees. Kaolin and kaolinised sillimanite contains scattered masses and pods of unreplaced sillimanite. In the central and southern part of the orebody unreplaced sillimanite comprises about 10% of the orebody. The northeasterly part of the orebody contains less sillimanite and more mica. A major fault on the eastern side of the quarry has brought weathered and bleached Adelaidean sediments into contact with Barossa Complex. At Springfield, two contorted zones of kaolin-mica up to 4 m wide are hosted by folded biotite schist. Mining since 1906 has produced nearly 340 000 tonnes of saleable ore comprising 270 000 tonnes of refractory clay, 36 000 tonnes of mica and 30 000 tonnes of sillimanite. White kaolinised sillimanite containing 40-48% AI2O3 accounts for more than 80% of total output. Main use is in refractories and is exported to U.K. and Japan. Annual production approximates 7 000 tonnes. White clay with 35-45% AI2O3 containing some mica is milled for use as filler in paint, plastics, rubber, paper, glass, cosmetics and ink. Annual production approximates 2 000 tonnes. About 600 tonnes of pale yellow to cream, massive, tough sillimanite rock with 50-60% AI2O3 is produced annually for use in high temperature refractories. Fine grained, pale green muscovite is used as filler in plasterboard, paint, welding rods, as a non-adhesive agent in foundry moulds and as decorative glitter. Production has risen to over 3 000 tonnes per year and demand is increasing.

251

BASEMENT BLOCKS IN UMBERATANA GROUP - OLARY R.G. Wiltshire School of Applied Geology, S.A. Institute of Technology, Adelaide, S.A. Within the Umberatana Group in the MacDonald Corridor north of Olary there are several blocks of Willyama Complex which have previously been interpreted as basement inliers, however field evidence suggests that they may be ice rafted blocks deposited from icebergs during the Sturtian glaciation. The Willyama Complex blocks consist of granite, gneiss and migmatite and have approximately elliptical outcrop shapes with sizes ranging from 50 m X 100 m to 200 m x 800 m. The blocks lie in the upper part of the Pualco Tillite (Sturtian glacial sequence) and lower part of the overlying Benda Siltstone and have their long axes parallel to bedding. The contacts of the blocks with the enclosing sedimentary rocks are concordant but unconformable and in places thin (1 to 2 m) conglomerate beds are developed along the contacts. Pacing in the surrounding sedimentary rocks is consistently to the east on both sides of the basement blocks. Campana and King in 1958 interpreted one of these blocks as the core of an anticline with an overturned western limb. This is contrary to the facing evidence and also the blocks do not occur at the base of the Adelaidean sequnce. Pitt suggested in 1971 that the blocks were fault-bounded inliers but there is no evidence of major faulting at or near the contacts of the blocks. Forbes and Pitt in 1980 interpreted some of the blocks as coarse granite-clast conglomerate, but some of these have continuous outcrop of massive granite. The evidence supporting the interpretation that the blocks were ice rafted during the Sturtian glaciation are: (i)

the stratigraphic position of the blocks in the Pualco Tillite and Benda Siltstone, both of which contain dropstones

(ii)

the constant facing of the enclosing sedimentary rocks across the blocks

(iii)

the lack of faulting adjacent to the blocks

(iv)

the gradation from dropstones 20 cm across through erratics 2 m across to blocks 200 m and 800 m across.

252

AUTHOR INDEX LEGEND 11.4 KN SG P 167

Session n u m b e r , technical paper Keynote paper SGGMP paper Technical poster Page number

253

ADKINS, J.S. ALLEN, R.L. ANDREW, A.S. BAILLE, P.W. BARNES, L.C. BARNES, R.G. BARNETT, S.R. BAXTER, J.L. BEARDSMORE, T.J. BELPERIO, A.P. BENBOW, M.C. BLISSETT, A.H. BLOOM, M.S. BOND, G.C. BONE, Y. BOYD, D.M. BROWN, M.C. BRUCE, J.R. BUICK, I.S. BURGESS, G. BURLINSON, K.G. CAMERON, R.G. CANN, J.H. CARR, G.R. CARTHEW, S.J. CAS, R.A.F. CHALMERS, R.O. CHIVAS, A.R. CHRISTIE-BLICK, N. CIRCOSTA, G. CLARKE, J.D.A. COATS, R.P. COBB, M.A. COFFIN, M.F. COLLINS, C.D.N. COLLINS, L.B. CQLWELL, J.B. COOPER, B.J. COOPER, J.A.

CORBETT, D.W. CREELMAN, R.A. CROOK, K,A.W.

P 219 8.11 23

FITZGERALD, M.J. FLINT, D.J. FLINT, R.B. FODEN, J.D. FOLEY, S.F. FORBES, B.G. FOTIOS, M.G. FREEMAN, M.

P 249 P 230, P 247 1.9 112 12.1 27 P 220 3.9 114 4.3 29, 10.1 30,

10.12 111

GAULL, B.A. GERGES, N.Z. GLEN, R.A. GOLEBY, B.R. GORTER, J.D. GOSTIN, V.A.

10.9 31 1.10 67 1.15 205 KN 14 2.9 33 P 249 6.8 136 5.2 43 3.6 77 6.4 116 5.1 35 8.9 37 10.4 39 2.2 41 5.2 43 P 233 17.1 45 10.5 47 KN 14, 4.1 197 P 221 P 222 P 223 P 224

GRADY, A.E. GRAVESTOCK,D.I. GRAY, D.R. GREEN, D.H. GREEN, T.H. GREENHALGH, S.A. GREY, K. GULSON, B.L.

6.12 181 6.2 58, 6.7 210 P 220

6.12 181 P 225 1.6 102, 1.7 69, 1.8 140, 1.12 142, 1.13 48 P 225 4.9 50 P 250 4.11 52, P 223, P 226 3.7 53 P 227 6.15 54 6.15 54

DALGARNO, R. DALLMEYER, R.D. DALY, S.J. DASHLOOTY, S.A. DAVIDSON, G.J. DAVIES, H.L. DAY, R.W. De DECKKER, P. DENHAM, D. DENMEAD, A.K. DENMEAD, E.N. DiBONA, P.A. DIXON, 0. DONNOLLY,T.H. DREXEL, J.F. DRUMMOND, B.J. DUBOWSKI, E.A. DUNN, J. DUNSTER, J.N. DYSON, I.A.

6.1 199 10.5 47 14.1 55 17.2 56 17.2 56 P 228 6.1 199 4.16 117 P 229 6.2 58, 14.6 59 2.5 75, P 230 3.2 127 P 231 P 232

EICKHOFF, K.H. ELLIOTT, P.J. ELSTON, W.E. ETHERIDGE, M.A. ETMINAN, H.

4.12 61 1.3 63 15.1 64 6.17 125 4.16 117, 8.10 66

FANNING, C.M.

1.7 69, 1.10 67, 4.2 71 P 233

FERGUSSON, C.L.

FERMIO, S.J. FINLAYSON, D.M.

2.2.41, 10.10 25

6.12 181

14.2 79 12.2 80, P 237 6.3 82 6.7 210 4.13 83 10.1 30, 10.4 39, 15.2 84, P 221 4.1 197, P 238 8.1 118, P 239 P 233 SG 212, SG 215 SG 214 6.6 87, 14.7 -85 4.4 89, P 240 4.14 203

HABERMEHL, M.A. HAINES, P.W. HAMMOND, R.L. HANCOCK, S. HASLETT, P.G. HAYDON, R.C. HELLSTEN, K.J. HENRY, R. HIGGINS, M.L. HILL, P.J. HLADKY, G. HOUSEMAN, G. HUNT, J.W.

10.6 90, 12.3 91 4.6 92 P 243 12.4 94 P 241 1.1 96 1.11 97 4.15 180 1.11 97

JAMES, N.P. JAMES, P.R. JENKINS, R.J.F. JOHNSON, G.I. JOHNSTONE, D.W. JONES, J.B. JOYCE, E.B.

P 239 1.13 48 4.10 101 1.6 102 6.1 199 11.3 104 P 245

KENNETT, B.L.N. KINNY, P. KNIGHT, M.J. KOMINZ, M.A. KORSCH, M.J. KREMOR, A. KUEHNER, S.M.

6.5 106, 14.4 172 1.2 107 12.5 109, P 242 KN 14 4.14 203 10.7 110 SG 215

LABLACK, K. LAING, W.P. LAMBECK, K. LAMBERT, I.B. LAWS, R.A. LEMON, N.M. LENNOX, P.G. LEVEN, J.H. LISTER, G.S. LUDWIG, K.R.

254

P 234 6.7 210, 6.11 73, 6.14 123 11.3 104 2.5 75 1.10 67, P 227, P 235 3.6 77, SG 216 SG 212 4.2 71, P 236 5.4 161 P 249

6.12 181 4.9 50 6.9 99, 6.10 198

8.2 100

10.12 111 1.9 112, 3.9 114, P 243 6.4 116 4.16 117, 8.10 66

8.1 118

LUO, B.

4.7 119 6.16 121, P 244 6.11 73, 6.14 123 3.2 127, 6.17 125 1.7 69, 1.10 67, 4.2 71 8.4 129

MACKIE, A.W. MAJOR, R.B. MARSHALLSEA, S.J. MARTIN, D.J. McBRIAR, E.M. MCCAFFREY, R.

5.1 35 P 229 8.3 130 8.8 132 P 245 14.1 55

McCALLUM, W.S. McCLENAGHAN, M.P. McCONACHY, G.W. McCUE, K. McDOUGALL, R. MCQUEEN, K.G. MICHAEL-LEIBA, M.O. MORRISON, R.S. MORTIMER, G.E. MOUNT, T.J. MURRAY, C.G. MURRAY-WALLACE, C.V. MYERS, J.S.

P 246, P P 249 1 . 1 96 1 4 . 3 134 1 4 . 7 85 6 . 8 136 14.2 79, 4 . 8 138 1.8 140, 1 . 1 3 48 8 . 4 129 6 . 1 199, 1 0 . 2 144 1.4 146,

247

NATION, R. NEW, D.

1 4 . 7 85 P 221

O'DRISCOLL, E.S.T. OLIVER, N.H.S. OLIVER, R.L.

P 248 2 . 4 1 4 9 , 3 . 8 147 1.7 69, 1.8 140, 1.12 142, 3.4 151, SG 216

PARHAM, R.T. PARKER, A.J. PASSMORE, V.L. PEARSON, N.J. PEARSON, P.J. PHILLIPS, S.E. PIDGEON, R.T. PIETSCH, B.A. PIGRAM, C.J. PIRAJNO, F. PLUMB, K.A. PREISS, W.V. PURVIS, A.C.

1 4 . 7 85 1 . 1 0 6 7 , P 2 2 7 , P 249 8 . 5 153 SG 214 2 . 4 149 1 0 . 3 154 1 . 5 1 5 5 , P 220 P 249 6 . 1 2 181 4 . 5 157 2 . 6 2 0 1 , 2 . 7 159 4 . 2 71 3 . 4 151

RAMSAY, D.C. RANSOM, D.M. REINHARDT, J. RICKARD, M.J. RICKWOOD, R.C. ROBERTS, D.E. ROBERTSON, A.D. ROBERTSON, R.S. ROGERS, P.A. ROKIAH ESA MOHAMED RYNN, J.M.W.

6.12 181

14.5

137

1.12

142,

P 249

14.8

170

2 . 3 175 6 . 1 199 1 0 . 5 47 8 . 4 129 6.6 87, 14.7 3.1 177, 8 . 7

85 179

TAPLEY, D. TAYLOR, G. TAYLOR, G.F. TAYLOR, W.R. THIRY, M. THOMAS, A. THORNE, A.M.

6 . 6 87 6 . 8 136 1 0 . 1 0 25 SG 212 1 1 . 2 187 3 . 2 127 1.14 189,

11.1 168

8.2 100 KN 16 4.14 4.15

VANDENBERG, A.H.M. VEEH, H.H. VERNON, R.H. VON DER BORCH, C.C. VOORHOEVE, H.

SG 2 1 7

WAKE-DYSTER, K.D. WALL, V.J.

11.1 68 P 225 1 4 . 6 59 6 . 1 199, 1 4 . 4 172 8 . 6 173 KN 20

1 0 . 8 190 P 251 1 0 . 1 1 192 5 . 6 193 P 249 1 5 . 1 64 3 . 3 195 6 . 3 82 1 0 . 1 3 196 4 . 1 1 9 7 , P 228 6 . 1 0 198

P 249

5 . 4 161 3 . 5 163 P 250 P 249 2 . 1 165 1 1 . 4 167

SAMBRIDGE, M.S. SAPPAL, K.K. SAWKINS, F.J. SCOTT, D.C. SCOTT, K.M. SEXTON, M.J. SHELLEY, J.M.G. SHEN, P. SINGH, R. SIVELL, W. SMYTH, M. SNEIDER, R.M. SOLOMON, M. SOUTHGATE, P.N. STAGG, H.M.J. STEWART, A.J. STUBLEY, M.P. SULLIVAN, S.J. SWEET, I.P. SYMONDS, P.A.

TOWNER, R. TOWNSEND, I.J. TRUSWELL, E.M. TUCKER, D.H. TUCKER, D.M. TWIST, D. TYLER, I.M.

203 180,

4.16

117

6.12 181 15.3 18.2 5 . 5 183 SG 216 2 . 8 185 6 . 1 7 125

P 240

255

WANG, Y. WATERHOUSE, J.B. WEGNER, J.H. WELLMAN, P. WESTE, G. WHITFORD, D.J. WILDE, A.R. WILDE, S.A. WILLIAMS, G.E. WILSON, P. WILTSHIRE, R.G. WRIGHT, C.

6 . 1 199 1.15 205, 3 . 8 147 8 . 4 129 8 . 7 179 1 7 . 3 200

2.4

149,

2.6 201 4 . 1 6 117 4 . 1 4 203 1 . 1 5 205 1 . 5 1 5 5 , P 2Z0 1 5 . 2 84 5 . 3 2 0 7 , 5 . 6 193 P 252 6 . 1 199, 6 . 7 210, 6 . 1 3 209

YANG, X. YPMA, P.

8.4 2.9

129 33

ZHENG, G.

8.4

129